First results from DESI make the most precise measurement of our expanding universe

With 5,000 tiny robots in a mountaintop telescope, researchers can look 11 billion years into the past. The light from far-flung objects in space is just now reaching the Dark Energy Spectroscopic Instrument (DESI), enabling us to map our cosmos as it was in its youth and trace its growth to what we see today. Understanding how our universe has evolved is tied to how it ends, and to one of the biggest mysteries in physics: dark energy, the unknown ingredient causing our universe to expand faster and faster.

To study dark energy's effects over the past 11 billion years, DESI has created the largest 3D map of our cosmos ever constructed, with the most precise measurements to date. This is the first time scientists have measured the expansion history of the young universe with a precision better than 1%, giving us our best view yet of how the universe evolved. Researchers shared the analysis of their first year of collected data in multiple papers that will be posted today on the arXiv and in talks at the American Physical Society meeting in the United States and the Rencontres de Moriond in Italy.

"We're incredibly proud of the data, which have produced world-leading cosmology results and are the first to come out of the new generation of dark energy experiments," said Michael Levi, DESI director and a scientist at the Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab), which manages the project. "So far, we're seeing basic agreement with our best model of the universe, but we're also seeing some potentially interesting differences that could indicate that dark energy is evolving with time. Those may or may not go away with more data, so we're excited to start analyzing our three-year dataset soon."

Our leading model of the universe is known as Lambda CDM. It includes both a weakly interacting type of matter (cold dark matter, or CDM) and dark energy (Lambda). Both matter and dark energy shape how the universe expands -- but in opposing ways. Matter and dark matter slow the expansion down, while dark energy speeds it up. The amount of each influences how our universe evolves. This model does a good job of describing results from previous experiments and how the universe looks throughout time.

However, when DESI's first-year results are combined with data from other studies, there are some subtle differences with what Lambda CDM would predict. As DESI gathers more information during its five-year survey, these early results will become more precise, shedding light on whether the data are pointing to different explanations for the results we observe or the need to update our model. More data will also improve DESI's other early results, which weigh in on the Hubble constant (a measure of how fast the universe is expanding today) and the mass of particles called neutrinos.

"No spectroscopic experiment has had this much data before, and we're continuing to gather data from more than a million galaxies every month," said Nathalie Palanque-Delabrouille, a Berkeley Lab scientist and co-spokesperson for the experiment. "It's astonishing that with only our first year of data, we can already measure the expansion history of our universe at seven different slices of cosmic time, each with a precision of 1 to 3%. The team put in a tremendous amount of work to account for instrumental and theoretical modeling intricacies, which gives us confidence in the robustness of our first results."

DESI's overall precision on the expansion history across all 11 billion years is 0.5%, and the most distant epoch, covering 8-11 billion years in the past, has a record-setting precision of 0.82%. That measurement of our young universe is incredibly difficult to make. Yet within one year, DESI has become twice as powerful at measuring the expansion history at these early times as its predecessor (the Sloan Digital Sky Survey's BOSS/eBOSS), which took more than a decade.

"We are delighted to see cosmology results from DESI's first year of operations," said Gina Rameika, associate director for High Energy Physics at DOE. "DESI continues to amaze us with its stellar performance and is already shaping our understanding of the universe."

Traveling back in time

DESI is an international collaboration of more than 900 researchers from over 70 institutions around the world. The instrument was constructed and is operated with funding from the DOE Office of Science, and sits atop the U.S. National Science Foundation's Nicholas U. Mayall 4-meter Telescope at Kitt Peak National Observatory, a program of NSF's NOIRLab.

Looking at DESI's map, it's easy to see the underlying structure of the universe: strands of galaxies clustered together, separated by voids with fewer objects. Our very early universe, well beyond DESI's view, was quite different: a hot, dense soup of subatomic particles moving too fast to form stable matter like the atoms we know today. Among those particles were hydrogen and helium nuclei, collectively called baryons.

Tiny fluctuations in this early ionized plasma caused pressure waves, moving the baryons into a pattern of ripples that is similar to what you'd see if you tossed a handful of gravel into a pond. As the universe expanded and cooled, neutral atoms formed and the pressure waves stopped, freezing the ripples in three dimensions and increasing clustering of future galaxies in the dense areas. Billions of years later, we can still see this faint pattern of 3D ripples, or bubbles, in the characteristic separation of galaxies -- a feature called Baryon Acoustic Oscillations (BAOs).

Researchers use the BAO measurements as a cosmic ruler. By measuring the apparent size of these bubbles, they can determine distances to the matter responsible for this extremely faint pattern on the sky. Mapping the BAO bubbles both near and far lets researchers slice the data into chunks, measuring how fast the universe was expanding at each time in its past and modeling how dark energy affects that expansion.

"We've measured the expansion history over this huge range of cosmic time with a precision that surpasses all of the previous BAO surveys combined," said Hee-Jong Seo, a professor at Ohio University and the co-leader of DESI's BAO analysis. "We're very excited to learn how these new measurements will improve and alter our understanding of the cosmos. Humans have a timeless fascination with our universe, wanting to know both what it is made of and what will happen to it."

Using galaxies to measure the expansion history and better understand dark energy is one technique, but it can only reach so far. At a certain point, light from typical galaxies is too faint, so researchers turn to quasars, extremely distant, bright galactic cores with black holes at their centers. Light from quasars is absorbed as it passes through intergalactic clouds of gas, enabling researchers to map the pockets of dense matter and use them the same way they use galaxies -- a technique known as using the "Lyman-alpha forest."

"We use quasars as a backlight to basically see the shadow of the intervening gas between the quasars and us," said Andreu Font-Ribera, a scientist at the Institute for High Energy Physics (IFAE) in Spain who co-leads DESI's Lyman-alpha forest analysis. "It lets us look out further to when the universe was very young. It's a really hard measurement to do, and very cool to see it succeed."

Researchers used 450,000 quasars, the largest set ever collected for these Lyman-alpha forest measurements, to extend their BAO measurements all the way out to 11 billion years in the past. By the end of the survey, DESI plans to map 3 million quasars and 37 million galaxies.

State-of-the-art science

DESI is the first spectroscopic experiment to perform a fully "blinded analysis," which conceals the true result from the scientists to avoid any subconscious confirmation bias. Researchers work in the dark with modified data, writing the code to analyze their findings. Once everything is finalized, they apply their analysis to the original data to reveal the actual answer.

"The way we did the analysis gives us confidence in our results, and particularly in showing that the Lyman-alpha forest is a powerful tool for measuring the universe's expansion," said Julien Guy, a scientist at Berkeley Lab and the co-lead for processing information from DESI's spectrographs. "The dataset we are collecting is exceptional, as is the rate at which we are gathering it. This is the most precise measurement I have ever done in my life."

DESI's data will be used to complement future sky surveys such as the Vera C. Rubin Observatory and Nancy Grace Roman Space Telescope, and to prepare for a potential upgrade to DESI (DESI-II) that was recommended in a recent report by the U.S. Particle Physics Project Prioritization Panel.

"We are in the golden era of cosmology, with large-scale surveys ongoing and about to be started, and new techniques being developed to make the best use of these datasets," said Arnaud de Mattia, a researcher with the French Alternative Energies and Atomic Energy Commission (CEA) and co-leader of DESI's group interpreting the cosmological data. "We're all really motivated to see whether new data will confirm the features we saw in our first-year sample and build a better understanding of the dynamics of our universe."

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New research suggests that our universe has no dark matter

The current theoretical model for the composition of the universe is that it's made of 'normal matter,' 'dark energy' and 'dark matter.' A new uOttawa study challenges this.

A University of Ottawa study published today challenges the current model of the universe by showing that, in fact, it has no room for dark matter.

In cosmology, the term "dark matter" describes all that appears not to interact with light or the electromagnetic field, or that can only be explained through gravitational force.

We can't see it, nor do we know what it's made of, but it helps us understand how galaxies, planets and stars behave.

Rajendra Gupta, a physics professor at the Faculty of Science, used a combination of the covarying coupling constants (CCC) and "tired light" (TL) theories (the CCC+TL model) to reach this conclusion.

This model combines two ideas -- about how the forces of nature decrease over cosmic time and about light losing energy when it travels a long distance.

It's been tested and has been shown to match up with several observations, such as about how galaxies are spread out and how light from the early universe has evolved.

This discovery challenges the prevailing understanding of the universe, which suggests that roughly 27% of it is composed of dark matter and less than 5% of ordinary matter, remaining being the dark energy.

Challenging the need for dark matter in the universe

"The study's findings confirm that our previous work ("JWST early Universe observations and ΛCDM cosmology") about the age of the universe being 26.7billionyears has allowed us to discover that the universe does not require dark matter to exist," explains Gupta.

"In standard cosmology, the accelerated expansion of the universe is said to be caused by dark energy but is in fact due to the weakening forces of nature as it expands, not due to dark energy."

"Redshifts" refer to when light is shifted toward the red part of the spectrum.

The researcher analyzed data from recent papers on the distribution of galaxies at low redshifts and the angular size of the sound horizon in the literature at high redshift.

"There are several papers that question the existence of dark matter, but mine is the first one, to my knowledge, that eliminates its cosmological existence while being consistent with key cosmological observations that we have had time to confirm," says Gupta.

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Final supernova results from Dark Energy Survey offer unique insights into the expansion of the universe

In 1998, astrophysicists discovered that the universe is expanding at an accelerating rate, attributed to a mysterious entity called dark energy that makes up about 70% of our universe. While foreshadowed by earlier measurements, the discovery was somewhat of a surprise; at the time, astrophysicists agreed that the universe's expansion should be slowing down because of gravity.

This revolutionary discovery, which astrophysicists achieved with observations of specific kinds of exploding stars, called type Ia (read "type one-A") supernovae, was recognized with the Nobel Prize in Physics in 2011.

Now, 25 years after the initial discovery, the scientists working on the Dark Energy Survey have released the results of an unprecedented analysis using the same technique to further probe the mysteries of dark energy and the expansion of the universe. They placed the strongest constraints on the expansion of the universe ever obtained with the DES supernova survey.

In a presentation at the 243rd meeting of the American Astronomical Society on Jan. 8 and in a paper submitted to the Astrophysical Journal in January titled, "The Dark Energy Survey: Cosmology results with ~1500 new high-redshift type Ia supernovae using the full 5-year dataset," DES astrophysicists report results that are consistent with the now-standard cosmological model of a universe with an accelerated expansion. Yet, the findings are not definitive enough to rule out a possibly more complex model.

Taking a unique approach to analysis

The Dark Energy Survey is an international collaboration comprising more than 400 astrophysicists, astronomers and cosmologists from over 25 institutions led by members from the U.S. Department of Energy's Fermi National Accelerator Laboratory. DES mapped an area almost one-eighth the entire sky using the Dark Energy Camera, a 570-megapixel digital camera built by Fermilab and funded by the DOE Office of Science. It was mounted on the Víctor M. Blanco Telescope at the National Science Foundation's Cerro Tololo Inter-American Observatory, a Program of NSF's NOIRLab in 2012. DES scientists took data for 758 nights across six years.

To understand the nature of dark energy and measure the expansion rate of the universe, DES scientists perform analyses with four different techniques, including the supernova technique used in 1998.

This technique requires data from type Ia supernovae, which occur when an extremely dense dead star, known as a white dwarf, reaches a critical mass and explodes. Since the critical mass is nearly the same for all white dwarfs, all type Ia supernovae have approximately the same actual brightness and any remaining variations can be calibrated out. So, when astrophysicists compare the apparent brightnesses of two type Ia supernovae as seen from Earth, they can determine their relative distances from us.

Astrophysicists trace out the history of cosmic expansion with large samples of supernovae spanning a wide range of distances. For each supernova, they combine its distance with a measurement of its redshift -- how quickly it is moving away from Earth due to the expansion of the universe. They can use that history to determine whether the dark energy density has remained constant or changed over time.

"As the universe expands, the matter density goes down," said DES director and spokesperson Rich Kron, who is a Fermilab and University of Chicago scientist. "But if the dark energy density is a constant, that means the total proportion of dark energy must be increasing as the volume increases."

The culmination of a decade of effort

The standard cosmological model is ΛCDM, or Lambda Cold Dark Matter, or Lambda Cold Dark Matter, a model based on the dark energy density being constant over cosmic time. It tells us how the universe evolves, using just a few features, such as the density of matter, type of matter and behavior of dark energy. The supernova method constrains two of these features very well: matter density and a quantity called w, which indicates whether the dark energy density is constant or not.

According to the standard cosmological model, the density of dark energy in the universe is constant, which means it doesn't dilute as the universe expands. If this is true, the parameter represented by the letter w should equal -1.

When the DES collaboration internally unveiled their supernova results, it was a culmination of a decade's worth of effort and an emotional time for many of the astrophysicists involved. "I was shaking," said Tamara Davis, a professor at the University of Queensland in Australia and co-convener of DES's supernova working group. "It was definitely an exciting moment."

The results found w = -0.80 +/- 0.18 using supernovae alone. Combined with complementary data from the European Space Agency's Planck telescope, w reaches -1 within the error bars. "w is tantalizingly not exactly on -1, but close enough that it's consistent with -1," said Davis. "A more complex model might be needed. Dark energy may indeed vary with time."

To come to a definitive conclusion, scientists will need more data. But DES won't be able to provide that; the survey stopped taking data in January 2019. The supernova team, led by many Ph.D. students and postdoctoral fellows, will soon have extracted all they can from the DES observations.

"More than 30 people have been involved in this analysis, and it is the culmination of almost 10 years of work," said Maria Vincenzi, a research fellow at Duke University who co-led the cosmological analysis of the DES supernova sample. "Some of us started working on this project when we were barely at the beginning of our Ph.D., and we are now starting faculty positions. So, the DES Collaboration contributed to the growth and professional development of an entire generation of cosmologists."

Pioneering a new approach

This final DES supernova analysis made many improvements upon DES's first supernova result released in 2018 that used just 207 supernovae and three years of data.

For the 2018 analysis, DES scientists combined data about the spectrum of each supernova to determine their redshifts and to classify them as type Ia or not. They then used images taken with different filters to identify the flux at the peak of the light curve -- a method called photometry. But spectra are hard to acquire, requiring lots of observing time on the largest telescopes, which will be impractical for future dark energy surveys like the Legacy Survey of Space and Time, LSST, to be conducted at the Vera C. Rubin Observatory, operated jointly by NSF's NOIRLab and DOE's SLAC National Accelerator Laboratory.

The new study pioneers a new approach to use photometry -- with an unprecedented four filters -- to find the supernovae, classify them and measure their light curves. Follow-up spectroscopy of the host galaxy with the Anglo-Australian Telescope provided precise redshifts for every supernova. The use of the additional filters also enabled data that is more precise than previous surveys and is a major advancement compared to the Nobel-winning supernovae samples, which only used one or two filters.

DES researchers used advanced machine-learning techniques to aid in supernova classification. Among the data from about two million distant observed galaxies, DES found several thousand supernovae. Scientists ultimately used 1,499 type Ia supernovae with high-quality data, making it the largest, deepest supernova sample from a single telescope ever compiled. In 1998, the Nobel-winning astronomers used just 52 supernovae to determine that the universe is expanding at an accelerating rate. "It's a really massive scale-up from 25 years ago," said Davis.

There are minor drawbacks of the new photometric approach compared to spectroscopy: Since the supernovae do not have spectra, there is greater uncertainty in classification. However, the much larger sample size enabled by the photometric approach more than makes up for this.

The innovative techniques DES pioneered will shape and further drive future astrophysical analyses. Projects like Rubin's LSST and NASA's Nancy Grace Roman Space Telescope will pick up where DES left off. "We're pioneering these techniques that will be directly beneficial for the next generation of supernova surveys," said Kron.

"This new supernova result is exciting because this means we can really tie a bow on it and hand it out to the community and say, 'This is our best attempt at explaining how the universe is working,'" said Dillon Brout, an assistant professor at Boston University who co-led the cosmological analysis of the DES Supernova sample with Vincenzi. "These constraints will now be the gold standard in supernova cosmology for quite some time."

Even with more advanced dark energy experiments forthcoming, DES scientists emphasized the importance of having theoretical models to explain dark energy in addition to their experimental observations. "All of this is really unknown territory," said Kron. "We do not have a theory that puts dark energy into a framework that relates to other physics that we do understand. For the time being, we in DES are working to constrain how dark energy works in practice with the expectation that, later on, some theories can be falsified."

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DESI early data release holds nearly two million objects

The universe is big, and it's getting bigger. To study dark energy, the mysterious force behind the accelerating expansion of our universe, scientists are using the Dark Energy Spectroscopic Instrument (DESI) to map more than 40 million galaxies, quasars, and stars. Today, the collaboration publicly released its first batch of data, with nearly 2 million objects for researchers to explore.

The 80-terabyte data set comes from 2,480 exposures taken over six months during the experiment's "survey validation" phase in 2020 and 2021. In this period between turning the instrument on and beginning the official science run, researchers made sure their plan for using the telescope would meet their science goals -- for example, by checking how long it took to observe galaxies of different brightness, and by validating the selection of stars and galaxies to observe.

"The fact that DESI works so well, and that the amount of science-grade data it took during survey validation is comparable to previous completed sky surveys, is a monumental achievement," said Nathalie Palanque-Delabrouille, co-spokesperson for DESI and a scientist at the Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab), which manages the experiment. "This milestone shows that DESI is a unique spectroscopic factory whose data will not only allow the study of dark energy but will also be coveted by the whole scientific community to address other topics, such as dark matter, gravitational lensing, and galactic morphology."

Today the collaboration also published a set of papers related to the early data release, which include early measurements of galaxy clustering, studies of rare objects, and descriptions of the instrument and survey operations. The new papers build on DESI's first measurement of the cosmological distance scale that was published in April, which used the first two months of routine survey data (not included in the early data release) and also showed DESI's ability to accomplish its design goals.

DESI uses 5,000 robotic positioners to move optical fibers that capture light from objects millions or billions of light-years away. It is the most powerful multi-object survey spectrograph in the world, able to measure light from more than 100,000 galaxies in one night. That light tells researchers how far away an object is, building a 3D cosmic map.

"Survey validation was very important for DESI because it allowed us -- before starting the main survey -- to adjust our selection of all the objects, including stars, bright galaxies, luminous red galaxies, emission line galaxies, and quasars," said Christophe Yeche, a scientist with the French Alternative Energies and Atomic Energy Commission (CEA) who co-leads the target selection group. "We've been able to optimize our selection and confirm our observation strategy."

As the universe expands, it stretches light's wavelength, making it redder -- a characteristic known as redshift. The further away the galaxy, the bigger the redshift. DESI specializes in collecting redshifts that can then be used to solve some of astrophysics' biggest puzzles: what dark energy is and how it has changed throughout the universe's history.

While DESI's primary goal is understanding dark energy, much of the data can also be used in other astronomical studies. For example, the early data release contains detailed images from some well-known areas of the sky, such as the Hubble Deep Field.

"There are some well-trodden spots where we've drilled down into the sky," said Stephen Bailey, a scientist at Berkeley Lab who leads data management for DESI. "We've taken valuable spectroscopic images in areas that are of interest to the rest of the community, and we're hoping that other people will take this data and do additional science with it."

Two interesting finds have already surfaced: Evidence of a mass migration of stars into the Andromeda galaxy, and incredibly distant quasars, the extremely bright and active supermassive black holes sometimes found at the center of galaxies.

"We observed some areas at very high depth. People have looked at that data and discovered very high redshift quasars, which are still so rare that basically any discovery of them is useful," said Anthony Kremin, a postdoctoral researcher at Berkeley Lab who led the data processing for the early data release. "Those high-redshift quasars are usually found with very large telescopes, so the fact that DESI -- a smaller, 4-meter survey instrument -- could compete with those larger, dedicated observatories was an achievement we are pretty proud of and demonstrates the exceptional throughput of the instrument."

Survey validation was also a chance to test the process of transforming raw data from DESI's ten spectrometers (which split a galaxy's light into different colors) into useful information.

"If you looked at them, the images coming directly from the camera would look like nonsense -- like lines on a weird, fuzzy image," said Laurie Stephey, a data architect at the National Energy Research Scientific Computing Center (NERSC), the supercomputer that processes DESI's data. "The magic happens in the processing and the software being able to decode the data. It's exciting that we have the technology to make that data accessible to the research community and that we can support this big question of 'what is dark energy?'"

DESI's early data was a unique project for NERSC. All of the experiment's code, including the computational heavy lifting, is written in the programming language Python rather than the traditional C++ or Fortran.

"That was the first time that using pure Python was shown to be a feasible approach for a major experiment at NERSC, and since then, Python has become increasingly common in our user workload," Stephey said.

The DESI early data release is now available to access for free through NERSC.

There is plenty of data yet to come from the experiment. DESI is currently two years into its five-year run and ahead of schedule on its quest to collect more than 40 million redshifts. The survey has already catalogued more than 26 million astronomical objects in its science run, and is adding more than a million per month.

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The bubbling universe: A previously unknown phase transition in the early universe

Think of bringing a pot of water to the boil: As the temperature reaches the boiling point, bubbles form in the water, burst and evaporate as the water boils. This continues until there is no more water changing phase from liquid to steam.

This is roughly the idea of what happened in the very early universe, right after the Big Bang, 13.7 billion years ago.

The idea comes from particle physicists Martin S. Sloth from the Center for Cosmology and Particle Physics Phenomenology at University of Southern Denmark and Florian Niedermann from the Nordic Institute for Theoretical Physics (NORDITA) in Stockholm. Niedermann is a previous postdoc in Sloth's research group. In this new scientific article, they present an even stronger basis for their idea.

Many bubbles crashing into each other

"One must imagine that bubbles arose in various places in the early universe. They got bigger and they started crashing into each other. In the end, there was a complicated state of colliding bubbles, which released energy and eventually evaporated," said Martin S. Sloth.

The background for their theory of phase changes in a bubbling universe is a highly interesting problem with calculating the so-called Hubble constant; a value for how fast the universe is expanding. Sloth and Niedermann believe that the bubbling universe plays a role here.

The Hubble constant can be calculated very reliably by, for example, analyzing cosmic background radiation or by measuring how fast a galaxy or an exploding star is moving away from us. According to Sloth and Niedermann, both methods are not only reliable, but also scientifically recognized. The problem is that the two methods do not lead to the same Hubble constant. Physicists call this problem "the Hubble tension."

Is there something wrong with our picture of the early universe?

"In science, you have to be able to reach the same result by using different methods, so here we have a problem. Why don't we get the same result when we are so confident about both methods?" said Florian Niedermann.

Sloth and Niedermann believe they have found a way to get the same Hubble constant, regardless of which method is used. The path starts with a phase transition and a bubbling universe -- and thus an early, bubbling universe is connected to "the Hubble tension."

"If we assume that these methods are reliable -- and we think they are -- then maybe the methods are not the problem. Maybe we need to look at the starting point, the basis, that we apply the methods to. Maybe this basis is wrong."

An unknown dark energy

The basis for the methods is the so-called Standard Model, which assumes that there was a lot of radiation and matter, both normal and dark, in the early universe, and that these were the dominant forms of energy. The radiation and the normal matter were compressed in a dark, hot and dense plasma; the state of the universe in the first 380,000 years after Big Bang.

When you base your calculations on the Standard Model, you arrive at different results for how fast the universe is expanding -- and thus different Hubble constants.

But maybe a new form of dark energy was at play in the early universe? Sloth and Niedermann think so.

If you introduce the idea that a new form of dark energy in the early universe suddenly began to bubble and undergo a phase transition, the calculations agree. In their model, Sloth and Niedermann arrive at the same Hubble constant when using both measurement methods. They call this idea New Early Dark Energy -- NEDE.

Change from one phase to another -- like water to steam

Sloth and Niedermann believe that this new, dark energy underwent a phase transition when the universe expanded, shortly before it changed from the dense and hot plasma state to the universe we know today.

- This means that the dark energy in the early universe underwent a phase transition, just as water can change phase between frozen, liquid and steam. In the process, the energy bubbles eventually collided with other bubbles and along the way released energy, said Niedermann.

"It could have lasted anything from an insanely short time -- perhaps just the time it takes two particles to collide -- to 300,000 years. We don't know, but that is something we are working to find out," added Sloth.

Do we need new physics?

So, the phase transition model is based on the fact that the universe does not behave as the Standard Model tells us. It may sound a little scientifically crazy to suggest that something is wrong with our fundamental understanding of the universe; that you can just propose the existence of hitherto unknown forces or particles to solve the Hubble tension.

"But if we trust the observations and calculations, we must accept that our current model of the universe cannot explain the data, and then we must improve the model. Not by discarding it and its success so far, but by elaborating on it and making it more detailed so that it can explain the new and better data," said Martin S. Sloth, adding:

"It appears that a phase transition in the dark energy is the missing element in the current Standard Model to explain the differing measurements of the universe's expansion rate.

How fast is the universe expanding?

The Hubble constant is a value for how fast the universe is expanding.

In Martin S. Sloth and Florian Niedermann's model, the Hubble constant is 72. Approximately. After all, large distances are being calculated, so we must allow for uncertainty of a few decimals.

What does 72 mean? It means 72 km per second per Megaparsec. Megaparsecs are a measure of the distance between, for example, two galaxies, and one megaparsec is 30,000,000,000,000,000,000 km. For every megaparsec between us and, for example, a galaxy, the galaxy moves away from us at 72 km per second.

When you measure the distance to galaxies by supernovas, you get a Hubble constant of approx. 73 (km/s)/megaparsec. But when measuring on the first light particles (the cosmic background radiation), the Hubble constant is 67.4 (km/s)/megaparsec.

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Billions of celestial objects revealed in gargantuan survey of the Milky Way

Astronomers have released a gargantuan survey of the galactic plane of the Milky Way. The new dataset contains a staggering 3.32 billion celestial objects -- arguably the largest such catalog so far. The data for this unprecedented survey were taken with the Dark Energy Camera, built by the US Department of Energy, at the NSF's Cerro Tololo Inter-American Observatory in Chile, a Program of NOIRLab.

The Milky Way Galaxy contains hundreds of billions of stars, glimmering star-forming regions, and towering dark clouds of dust and gas. Imaging and cataloging these objects for study is a herculean task, but a newly released astronomical dataset known as the second data release of the Dark Energy Camera Plane Survey (DECaPS2) reveals a staggering number of these objects in unprecedented detail. The DECaPS2 survey, which took two years to complete and produced more than 10 terabytes of data from 21,400 individual exposures, identified approximately 3.32 billion objects -- arguably the largest such catalog compiled to date. Astronomers and the public can explore the dataset here.

This unprecedented collection was captured by the Dark Energy Camera (DECam) instrument on the Víctor M. Blanco 4-meter Telescope at Cerro Tololo Inter-American Observatory (CTIO), a Program of NSF's NOIRLab. CTIO is a constellation of international astronomical telescopes perched atop Cerro Tololo in Chile at an altitude of 2200 meters (7200 feet). CTIO's lofty vantage point gives astronomers an unrivaled view of the southern celestial hemisphere, which allowed DECam to capture the southern Galactic plane in such detail.

DECaPS2 is a survey of the plane of the Milky Way as seen from the southern sky taken at optical and near-infrared wavelengths. The first trove of data from DECaPS was released in 2017, and with the addition of the new data release, the survey now covers 6.5% of the night sky and spans a staggering 130 degrees in length. While it might sound modest, this equates to 13,000 times the angular area of the full Moon.

The DECaPS2 dataset is available to the entire scientific community and is hosted by NOIRLab's Astro Data Lab, which is part of the Community Science and Data Center. Interactive access to the imaging with panning/zooming inside of a web-browser is available from the Legacy Survey Viewer, the World Wide Telescope and Aladin.

Most of the stars and dust in the Milky Way are located in its disk -- the bright band stretching across this image -- in which the spiral arms lie. While this profusion of stars and dust makes for beautiful images, it also makes the Galactic plane challenging to observe. The dark tendrils of dust seen threading through this image absorb starlight and blot out fainter stars entirely, and the light from diffuse nebulae interferes with any attempts to measure the brightness of individual objects. Another challenge arises from the sheer number of stars, which can overlap in the image and make it difficult to disentangle individual stars from their neighbors.

Despite the challenges, astronomers delved into the Galactic plane to gain a better understanding of our Milky Way. By observing at near-infrared wavelengths, they were able to peer past much of the light-absorbing dust. The researchers also used an innovative data-processing approach, which allowed them to better predict the background behind each star. This helped to mitigate the effects of nebulae and crowded star fields on such large astronomical images, ensuring that the final catalog of processed data is more accurate.

"One of the main reasons for the success of DECaPS2 is that we simply pointed at a region with an extraordinarily high density of stars and were careful about identifying sources that appear nearly on top of each other," said Andrew Saydjari, a graduate student at Harvard University, researcher at the Center for Astrophysics | Harvard & Smithsonian and lead author of the paper. "Doing so allowed us to produce the largest such catalog ever from a single camera, in terms of the number of objects observed."

"When combined with images from Pan-STARRS 1, DECaPS2 completes a 360-degree panoramic view of the Milky Way's disk and additionally reaches much fainter stars," said Edward Schlafly, a researcher at the AURA-managed Space Telescope Science Institute and a co-author of the paper describing DECaPS2 published in the Astrophysical Journal Supplement. "With this new survey, we can map the three-dimensional structure of the Milky Way's stars and dust in unprecedented detail."

"Since my work on the Sloan Digital Sky Survey two decades ago, I have been looking for a way to make better measurements on top of complex backgrounds," said Douglas Finkbeiner, a professor at the Center for Astrophysics, co-author of the paper, and principal investigator behind the project. "This work has achieved that and more!"

"This is quite a technical feat. Imagine a group photo of over three billion people and every single individual is recognizable!" says Debra Fischer, division director of Astronomical Sciences at NSF. "Astronomers will be poring over this detailed portrait of more than three billion stars in the Milky Way for decades to come. This is a fantastic example of what partnerships across federal agencies can achieve."

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The most precise accounting yet of dark energy and dark matter

Astrophysicists have performed a powerful new analysis that places the most precise limits yet on the composition and evolution of the universe. With this analysis, dubbed Pantheon+, cosmologists find themselves at a crossroads.

Pantheon+ convincingly finds that the cosmos is composed of about two-thirds dark energy and one-third matter -- mostly in the form of dark matter -- and is expanding at an accelerating pace over the last several billion years. However, Pantheon+ also cements a major disagreement over the pace of that expansion that has yet to be solved.

By putting prevailing modern cosmological theories, known as the Standard Model of Cosmology, on even firmer evidentiary and statistical footing, Pantheon+ further closes the door on alternative frameworks accounting for dark energy and dark matter. Both are bedrocks of the Standard Model of Cosmology but have yet to be directly detected and rank among the model's biggest mysteries. Following through on the results of Pantheon+, researchers can now pursue more precise observational tests and hone explanations for the ostensible cosmos.

"With these Pantheon+ results, we are able to put the most precise constraints on the dynamics and history of the universe to date," says Dillon Brout, an Einstein Fellow at the Center for Astrophysics | Harvard & Smithsonian. "We've combed over the data and can now say with more confidence than ever before how the universe has evolved over the eons and that the current best theories for dark energy and dark matter hold strong."

Brout is the lead author of a series of papers describing the new Pantheon+ analysis, published jointly today in a special issue of The Astrophysical Journal.

Pantheon+ is based on the largest dataset of its kind, comprising more than 1,500 stellar explosions called Type Ia supernovae. These bright blasts occur when white dwarf stars -- remnants of stars like our Sun -- accumulate too much mass and undergo a runaway thermonuclear reaction. Because Type Ia supernovae outshine entire galaxies, the stellar detonations can be glimpsed at distances exceeding 10 billion light years, or back through about three-quarters of the universe's total age. Given that the supernovae blaze with nearly uniform intrinsic brightnesses, scientists can use the explosions' apparent brightness, which diminishes with distance, along with redshift measurements as markers of time and space. That information, in turn, reveals how fast the universe expands during different epochs, which is then used to test theories of the fundamental components of the universe.

The breakthrough discovery in 1998 of the universe's accelerating growth was thanks to a study of Type Ia supernovae in this manner. Scientists attribute the expansion to an invisible energy, therefore monikered dark energy, inherent to the fabric of the universe itself. Subsequent decades of work have continued to compile ever-larger datasets, revealing supernovae across an even wider range of space and time, and Pantheon+ has now brought them together into the most statistically robust analysis to date.

"In many ways, this latest Pantheon+ analysis is a culmination of more than two decades' worth of diligent efforts by observers and theorists worldwide in deciphering the essence of the cosmos," says Adam Riess, one of the winners of the 2011 Nobel Prize in Physics for the discovery of the accelerating expansion of the universe and the Bloomberg Distinguished Professor at Johns Hopkins University (JHU) and the Space Telescope Science Institute in Baltimore, Maryland. Riess is also an alum of Harvard University, holding a PhD in astrophysics.

Brout's own career in cosmology traces back to his undergraduate years at JHU, where he was taught and advised by Riess. There Brout worked with then-PhD-student and Riess-advisee Dan Scolnic, who is now an assistant professor of physics at Duke University and another co-author on the new series of papers.

Several years ago, Scolnic developed the original Pantheon analysis of approximately 1,000 supernovae.

Now, Brout and Scolnic and their new Pantheon+ team have added some 50 percent more supernovae data points in Pantheon+, coupled with improvements in analysis techniques and addressing potential sources of error, which ultimately has yielded twice the precision of the original Pantheon.

"This leap in both the dataset quality and in our understanding of the physics that underpin it would not have been possible without a stellar team of students and collaborators working diligently to improve every facet of the analysis," says Brout.

Taking the data as a whole, the new analysis holds that 66.2 percent of the universe manifests as dark energy, with the remaining 33.8 percent being a combination of dark matter and matter. To arrive at even more comprehensive understanding of the constituent components of the universe at different epochs, Brout and colleagues combined Pantheon+ with other strongly evidenced, independent and complementary measures of the large-scale structure of the universe and with measurements from the earliest light in the universe, the cosmic microwave background.

Another key Pantheon+ result relates to one of the paramount goals of modern cosmology: nailing down the current expansion rate of the universe, known as the Hubble constant. Pooling the Pantheon+ sample with data from the SH0ES (Supernova H0 for the Equation of State) collaboration, led by Riess, results in the most stringent local measurement of the current expansion rate of the universe.

Pantheon+ and SH0ES together find a Hubble constant of 73.4 kilometers per second per megaparsec with only 1.3% uncertainty. Stated another way, for every megaparsec, or 3.26 million light years, the analysis estimates that in the nearby universe, space itself is expanding at more than 160,000 miles per hour.

However, observations from an entirely different epoch of the universe's history predict a different story. Measurements of the universe's earliest light, the cosmic microwave background, when combined with the current Standard Model of Cosmology, consistently peg the Hubble constant at a rate that is significantly less than observations taken via Type Ia supernovae and other astrophysical markers. This sizable discrepancy between the two methodologies has been termed the Hubble tension.

The new Pantheon+ and SH0ES datasets heighten this Hubble tension. In fact, the tension has now passed the important 5-sigma threshold (about one-in-a-million odds of arising due to random chance) that physicists use to distinguish between possible statistical flukes and something that must accordingly be understood. Reaching this new statistical level highlights the challenge for both theorists and astrophysicists to try and explain the Hubble constant discrepancy.

"We thought it would be possible to find clues to a novel solution to these problems in our dataset, but instead we're finding that our data rules out many of these options and that the profound discrepancies remain as stubborn as ever," says Brout.

The Pantheon+ results could help point to where the solution to the Hubble tension lies. "Many recent theories have begun pointing to exotic new physics in the very early universe, however such unverified theories must withstand the scientific process and the Hubble tension continues to be a major challenge," says Brout.

Overall, Pantheon+ offers scientists a comprehensive lookback through much of cosmic history. The earliest, most distant supernovae in the dataset gleam forth from 10.7 billion light years away, meaning from when the universe was roughly a quarter of its current age. In that earlier era, dark matter and its associated gravity held the universe's expansion rate in check. Such state of affairs changed dramatically over the next several billion years as the influence of dark energy overwhelmed that of dark matter. Dark energy has since flung the contents of the cosmos ever-farther apart and at an ever-increasing rate.

"With this combined Pantheon+ dataset, we get a precise view of the universe from the time when it was dominated by dark matter to when the universe became dominated by dark energy," says Brout. "This dataset is a unique opportunity to see dark energy turn on and drive the evolution of the cosmos on the grandest scales up through present time."

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Hubble reaches new milestone in mystery of universe's expansion rate

Completing a nearly 30-year marathon, NASA's Hubble Space Telescope has calibrated more than 40 "milepost markers" of space and time to help scientists precisely measure the expansion rate of the universe -- a quest with a plot twist.

Pursuit of the universe's expansion rate began in the 1920s with measurements by astronomers Edwin P. Hubble and Georges Lemaître. In 1998, this led to the discovery of "dark energy," a mysterious repulsive force accelerating the universe's expansion. In recent years, thanks to data from Hubble and other telescopes, astronomers found another twist: a discrepancy between the expansion rate as measured in the local universe compared to independent observations from right after the big bang, which predict a different expansion value.

The cause of this discrepancy remains a mystery. But Hubble data, encompassing a variety of cosmic objects that serve as distance markers, support the idea that something weird is going on, possibly involving brand new physics.

"You are getting the most precise measure of the expansion rate for the universe from the gold standard of telescopes and cosmic mile markers," said Nobel Laureate Adam Riess of the Space Telescope Science Institute (STScI) and the Johns Hopkins University in Baltimore, Maryland.

Riess leads a scientific collaboration investigating the universe's expansion rate called SH0ES, which stands for Supernova, H0, for the Equation of State of Dark Energy. "This is what the Hubble Space Telescope was built to do, using the best techniques we know to do it. This is likely Hubble's magnum opus, because it would take another 30 years of Hubble's life to even double this sample size," Riess said.

Riess's team's paper, to be published in the Special Focus issue of The Astrophysical Journal reports on completing the biggest and likely last major update on the Hubble constant. The new results more than double the prior sample of cosmic distance markers. His team also reanalyzed all of the prior data, with the whole dataset now including over 1,000 Hubble orbits.

When NASA conceived of a large space telescope in the 1970s, one of the primary justifications for the expense and extraordinary technical effort was to be able to resolve Cepheids, stars that brighten and dim periodically, seen inside our Milky Way and external galaxies. Cepheids have long been the gold standard of cosmic mile markers since their utility was discovered by astronomer Henrietta Swan Leavitt in 1912. To calculate much greater distances, astronomers use exploding stars called Type Ia supernovae.

Combined, these objects built a "cosmic distance ladder" across the universe and are essential to measuring the expansion rate of the universe, called the Hubble constant after Edwin Hubble. That value is critical to estimating the age of the universe and provides a basic test of our understanding of the universe.

Starting right after Hubble's launch in 1990, the first set of observations of Cepheid stars to refine the Hubble constant was undertaken by two teams: the HST Key Project led by Wendy Freedman, Robert Kennicutt, Jeremy Mould, and Marc Aaronson, and another by Allan Sandage and collaborators, that used Cepheids as milepost markers to refine the distance measurement to nearby galaxies. By the early 2000s the teams declared "mission accomplished" by reaching an accuracy of 10 percent for the Hubble constant, 72 plus or minus 8 kilometers per second per megaparsec.

In 2005 and again in 2009, the addition of powerful new cameras onboard the Hubble telescope launched "Generation 2" of the Hubble constant research as teams set out to refine the value to an accuracy of just one percent. This was inaugurated by the SH0ES program. Several teams of astronomers using Hubble, including SH0ES, have converged on a Hubble constant value of 73 plus or minus 1 kilometer per second per megaparsec. While other approaches have been used to investigate the Hubble constant question, different teams have come up with values close to the same number.

The SH0ES team includes long-time leaders Dr. Wenlong Yuan of Johns Hopkins University, Dr. Lucas Macri of Texas A&M University, Dr. Stefano Casertano of STScI, and Dr. Dan Scolnic of Duke University. The project was designed to bracket the universe by matching the precision of the Hubble constant inferred from studying the cosmic microwave background radiation leftover from the dawn of the universe.

"The Hubble constant is a very special number. It can be used to thread a needle from the past to the present for an end-to-end test of our understanding of the universe. This took a phenomenal amount of detailed work," said Dr. Licia Verde, a cosmologist at ICREA and the ICC-University of Barcelona, speaking about the SH0ES team's work.

The team measured 42 of the supernova milepost markers with Hubble. Because they are seen exploding at a rate of about one per year, Hubble has, for all practical purposes, logged as many supernovae as possible for measuring the universe's expansion. Riess said, "We have a complete sample of all the supernovae accessible to the Hubble telescope seen in the last 40 years." Like the lyrics from the song "Kansas City," from the Broadway musical Oklahoma, Hubble has "gone about as fur as it c'n go!"

Weird Physics?

The expansion rate of the universe was predicted to be slower than what Hubble actually sees. By combining the Standard Cosmological Model of the Universe and measurements by the European Space Agency's Planck mission (which observed the relic cosmic microwave background from 13.8 billion years ago), astronomers predict a lower value for the Hubble constant: 67.5 plus or minus 0.5 kilometers per second per megaparsec, compared to the SH0ES team's estimate of 73.

Given the large Hubble sample size, there is only a one-in-a-million chance astronomers are wrong due to an unlucky draw, said Riess, a common threshold for taking a problem seriously in physics. This finding is untangling what was becoming a nice and tidy picture of the universe's dynamical evolution. Astronomers are at a loss for an explanation of the disconnect between the expansion rate of the local universe versus the primeval universe, but the answer might involve additional physics of the universe.

Such confounding findings have made life more exciting for cosmologists like Riess. Thirty years ago they started out to measure the Hubble constant to benchmark the universe, but now it has become something even more interesting. "Actually, I don't care what the expansion value is specifically, but I like to use it to learn about the universe," Riess added.

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Dark energy: Neutron stars will tell us if it's only an illusion

A huge amount of mysterious dark energy is necessary to explain cosmological phenomena, such as the accelerated expansion of the Universe, with Einstein's theory. But what if dark energy was just an illusion and general relativity itself had to be modified? A new SISSA study, published in Physical Review Letters, offers a new approach to answer this question. Thanks to huge computational and mathematical effort, scientists produced the first simulation ever of merging binary neutron stars in theories beyond general relativity that reproduce a dark- energy like behavior on cosmological scales. This allows the comparison of Einstein's theory and modified versions of it, and, with sufficiently accurate data, may solve the dark energy mystery.

For about 100 years now, general relativity has been very successful at describing gravity on a variety of regimes, passing all experimental tests on Earth and the solar system. However, to explain cosmological observations such as the observed accelerated expansion of the Universe, we need to introduce dark components, such as dark matter and dark energy, which still remain a mystery.

Enrico Barausse, astrophysicist at SISSA (Scuola Internazionale Superiore di Studi Avanzati) and principal investigator of the ERC grant GRAMS (GRavity from Astrophysical to Microscopic Scales) questions whether dark energy is real or, instead, it may be interpreted as a breakdown of our understanding of gravity. "The existence of dark energy could be just an illusion," he says, "the accelerated expansion of the Universe might be caused by some yet unknown modifications of general relativity, a sort of 'dark gravity'."

The merger of neutron stars offers a unique situation to test this hypothesis because gravity around them is pushed to the extreme. "Neutron stars are the densest stars that exist, typically only 10 kilometers in radius, but with a mass between one or two times the mass of our Sun," explains the scientist. "This makes gravity and the spacetime around them extreme, allowing for abundant production of gravitational waves when two of them collide. We can use the data acquired during such events to study the workings of gravity and test Einstein's theory in a new window."

In this study, published in Physical Review Letters, SISSA scientists in collaboration with physicists from Universitat de les Illes Balears in Palma de Mallorca, produced the first simulation of merging binary neutron stars in theories of modified gravity relevant for cosmology: "This type of simulations is extremely challenging," clarifies Miguel Bezares, first author of the paper, "because of the highly non-linear nature of the problem. It requires a huge computational effort -months of run in supercomputers -- that was made possible also by the agreement between SISSA and CINECA consortium as well as novel mathematical formulations that we developed. These represented major roadblocks for many years till our first simulation."

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Have we detected dark energy? Scientists say it’s a possibility

Dark energy, the mysterious force that causes the universe to accelerate, may have been responsible for unexpected results from the XENON1T experiment, deep below Italy's Apennine Mountains.

A new study, led by researchers at the University of Cambridge and reported in the journal Physical Review D, suggests that some unexplained results from the XENON1T experiment in Italy may have been caused by dark energy, and not the dark matter the experiment was designed to detect.

They constructed a physical model to help explain the results, which may have originated from dark energy particles produced in a region of the Sun with strong magnetic fields, although future experiments will be required to confirm this explanation. The researchers say their study could be an important step toward the direct detection of dark energy.

Everything our eyes can see in the skies and in our everyday world -- from tiny moons to massive galaxies, from ants to blue whales -- makes up less than five percent of the universe. The rest is dark. About 27% is dark matter -- the invisible force holding galaxies and the cosmic web together -- while 68% is dark energy, which causes the universe to expand at an accelerated rate.

"Despite both components being invisible, we know a lot more about dark matter, since its existence was suggested as early as the 1920s, while dark energy wasn't discovered until 1998," said Dr Sunny Vagnozzi from Cambridge's Kavli Institute for Cosmology, the paper's first author. "Large-scale experiments like XENON1T have been designed to directly detect dark matter, by searching for signs of dark matter 'hitting' ordinary matter, but dark energy is even more elusive."

To detect dark energy, scientists generally look for gravitational interactions: the way gravity pulls objects around. And on the largest scales, the gravitational effect of dark energy is repulsive, pulling things away from each other and making the Universe's expansion accelerate.

About a year ago, the XENON1T experiment reported an unexpected signal, or excess, over the expected background. "These sorts of excesses are often flukes, but once in a while they can also lead to fundamental discoveries," said Dr Luca Visinelli, a researcher at Frascati National Laboratories in Italy, a co-author of the study. "We explored a model in which this signal could be attributable to dark energy, rather than the dark matter the experiment was originally devised to detect."

At the time, the most popular explanation for the excess were axions -- hypothetical, extremely light particles -- produced in the Sun. However, this explanation does not stand up to observations, since the amount of axions that would be required to explain the XENON1T signal would drastically alter the evolution of stars much heavier than the Sun, in conflict with what we observe.

We are far from fully understanding what dark energy is, but most physical models for dark energy would lead to the existence of a so-called fifth force. There are four fundamental forces in the universe, and anything that can't be explained by one of these forces is sometimes referred to as the result of an unknown fifth force.

However, we know that Einstein's theory of gravity works extremely well in the local universe. Therefore, any fifth force associated to dark energy is unwanted and must be 'hidden' or 'screened' when it comes to small scales, and can only operate on the largest scales where Einstein's theory of gravity fails to explain the acceleration of the Universe. To hide the fifth force, many models for dark energy are equipped with so-called screening mechanisms, which dynamically hide the fifth force.

Vagnozzi and his co-authors constructed a physical model, which used a type of screening mechanism known as chameleon screening, to show that dark energy particles produced in the Sun's strong magnetic fields could explain the XENON1T excess.

"Our chameleon screening shuts down the production of dark energy particles in very dense objects, avoiding the problems faced by solar axions," said Vagnozzi. "It also allows us to decouple what happens in the local very dense Universe from what happens on the largest scales, where the density is extremely low."

The researchers used their model to show what would happen in the detector if the dark energy was produced in a particular region of the Sun, called the tachocline, where the magnetic fields are particularly strong.

"It was really surprising that this excess could in principle have been caused by dark energy rather than dark matter," said Vagnozzi. "When things click together like that, it's really special."

Their calculations suggest that experiments like XENON1T, which are designed to detect dark matter, could also be used to detect dark energy. However, the original excess still needs to be convincingly confirmed. "We first need to know that this wasn't simply a fluke," said Visinelli. "If XENON1T actually saw something, you'd expect to see a similar excess again in future experiments, but this time with a much stronger signal."

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Hunting dark energy with gravity resonance spectroscopy

Dark Energy is widely believed to be the driving force behind the universe's accelerating expansion, and several theories have now been proposed to explain its elusive nature. However, these theories predict that its influence on quantum scales must be vanishingly small, and experiments so far have not been accurate enough to either verify or discredit them. In new research published in EPJ ST, a team led by Hartmut Abele at TU Wien in Austria demonstrate a robust experimental technique for studying one such theory, using ultra-cold neutrons. Named 'Gravity Resonance Spectroscopy' (GRS), their approach could bring researchers a step closer to understanding one of the greatest mysteries in cosmology.

Previously, phenomena named 'scalar symmetron fields' have been proposed as a potential candidate for Dark Energy. If they exist, these fields will be far weaker than gravity -- currently the weakest fundamental force known to physics. Therefore, by searching for extremely subtle anomalies in the behaviours of quantum particles trapped in gravitational fields, researchers could prove the existence of these fields experimentally. Within a gravitational field, ultra-cold neutrons can assume several discrete quantum states, which vary depending on the strength of the field. Through GRS, these neutrons are made to transition to higher-energy quantum states by the finely tuned mechanical oscillations of a near-perfect mirror. Any shifts from the expected values for the energy differences between these states could then indicate the influence of Dark Energy.

In their study, Abele's team designed and demonstrated a GRS experiment named 'qBOUNCE,' which they based around a technique named Ramsey spectroscopy. This involved causing neutrons in an ultra-cold beam to transition to higher-energy quantum states -- before scattering away any unwanted states, and picking up the remaining neutrons in a detector. Through precise measurements of the energy differences between particular states, the researchers could place far more stringent bounds on the parameters of scalar symmetron fields. Their technique now paves the way for even more precise searches for Dark Energy in future research.

From Science Daily

Dark energy survey releases most precise look at the universe's evolution

In 29 new scientific papers, the Dark Energy Survey examines the largest-ever maps of galaxy distribution and shapes, extending more than 7 billion light-years across the Universe. The extraordinarily precise analysis, which includes data from the survey's first three years, contributes to the most powerful test of the current best model of the Universe, the standard cosmological model. However, hints remain from earlier DES data and other experiments that matter in the Universe today is a few percent less clumpy than predicted.

New results from the Dark Energy Survey (DES) use the largest-ever sample of galaxies observed over nearly one-eighth of the sky to produce the most precise measurements to date of the Universe's composition and growth.

DES images the night sky using the 570-megapixel Dark Energy Camera on the National Science Foundation's Víctor M. Blanco 4-meter Telescope at Cerro Tololo Inter-American Observatory (CTIO) in Chile, a Program of NSF's NOIRLab. One of the most powerful digital cameras in the world, the Dark Energy Camera was designed specifically for DES. It was funded by the Department of Energy (DOE) and was built and tested at DOE's Fermilab.

Over the course of six years, from 2013 to 2019, DES used 30% of the time on the Blanco Telescope and surveyed 5000 square degrees -- almost one-eighth of the entire sky -- in 758 nights of observation, cataloging hundreds of millions of objects. The results announced today draw on data from the first three years -- 226 million galaxies observed over 345 nights -- to create the largest and most precise maps yet of the distribution of galaxies in the Universe at relatively recent epochs. The DES data were processed at the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign.

"NOIRLab is a proud host for and member of the DES collaboration," said Steve Heathcote, CTIO Associate Director. "Both during and after the survey, the Dark Energy Camera has been a popular choice for community and Chilean astronomers."

At present the Dark Energy Camera is used for programs covering a huge range of science including cosmology. The Dark Energy Camera science archive, including DES Data Release 2 on which these results are based, is curated by the Community Science and Data Center (CSDC), a Program of NSF's NOIRLab. CSDC provides software systems, user services, and development initiatives to connect and support the scientific missions of NOIRLab's telescopes, including the Blanco telescope at CTIO.

Since DES studied nearby galaxies as well as those billions of light-years away, its maps provide both a snapshot of the current large-scale structure of the Universe and a view of how that structure has evolved over the past 7 billion years.

Ordinary matter makes up only about 5% of the Universe. Dark energy, which cosmologists hypothesize drives the accelerating expansion of the Universe by counteracting the force of gravity, accounts for about 70%. The last 25% is dark matter, whose gravitational influence binds galaxies together. Both dark matter and dark energy remain invisible. DES seeks to illuminate their nature by studying how the competition between them shapes the large-scale structure of the Universe over cosmic time.

To quantify the distribution of dark matter and the effect of dark energy, DES relied mainly on two phenomena. First, on large scales galaxies are not distributed randomly throughout space but rather form a weblike structure that is due to the gravity of dark matter. DES measured how this cosmic web has evolved over the history of the Universe. The galaxy clustering that forms the cosmic web in turn revealed regions with a higher density of dark matter.

Second, DES detected the signature of dark matter through weak gravitational lensing. As light from a distant galaxy travels through space, the gravity of both ordinary and dark matter in the foreground can bend its path, as if through a lens, resulting in a distorted image of the galaxy as seen from Earth. By studying how the apparent shapes of distant galaxies are aligned with each other and with the positions of nearby galaxies along the line of sight, DES scientists were able to infer the clumpiness of the dark matter in the Universe.

To test cosmologists' current model of the Universe, DES scientists compared their results with measurements from the European Space Agency's orbiting Planck observatory. Planck used light known as the cosmic microwave background to peer back to the early Universe, just 400,000 years after the Big Bang. The Planck data give a precise view of the Universe 13 billion years ago, and the standard cosmological model predicts how the dark matter should evolve to the present.

Combined with earlier results DES provides the most powerful test of the current best model of the Universe to date, and the results are consistent with the predictions of the standard model of cosmology. However, hints remain from DES and several previous galaxy surveys that the Universe today is a few percent less clumpy than predicted.

Ten regions of the sky were chosen as "deep fields" that the Dark Energy Camera imaged repeatedly throughout the survey. Stacking those images together allowed the scientists to glimpse more distant galaxies. The team then used the redshift information from the deep fields to calibrate the rest of the survey region. This and other advancements in measurements and modeling, coupled with a threefold increase in data compared to the first year, enabled the team to pin down the density and clumpiness of the Universe with unprecedented precision.

DES concluded its observations of the night sky in 2019. With the experience gained from analyzing the first half of the data, the team is now prepared to handle the complete dataset. The final DES analysis is expected to paint an even more precise picture of the dark matter and dark energy in the Universe.

The DES collaboration consists of over 400 scientists from 25 institutions in seven countries.

"The collaboration is remarkably young. It's tilted strongly in the direction of postdocs and graduate students who are doing a huge amount of this work," said DES Director and spokesperson Rich Kron, who is a Fermilab and University of Chicago scientist. "That's really gratifying. A new generation of cosmologists are being trained using the Dark Energy Survey."

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Supernovae twins open up new possibilities for precision cosmology

Cosmologists have found a way to double the accuracy of measuring distances to supernova explosions -- one of their tried-and-true tools for studying the mysterious dark energy that is making the universe expand faster and faster. The results from the Nearby Supernova Factory (SNfactory) collaboration, led by Greg Aldering of the Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab), will enable scientists to study dark energy with greatly improved precision and accuracy, and provide a powerful crosscheck of the technique across vast distances and time. The findings will also be central to major upcoming cosmology experiments that will use new ground and space telescopes to test alternative explanations of dark energy.

Two papers published in The Astrophysical Journal report these findings, with Kyle Boone as lead author. Currently a postdoctoral fellow at the University of Washington, Boone is a former graduate student of Nobel Laureate Saul Perlmutter, the Berkeley Lab senior scientist and UC Berkeley professor who led one of the teams that originally discovered dark energy. Perlmutter was also a co-author on both studies.

Supernovae were used in 1998 to make the startling discovery that the expansion of the universe is speeding up, rather than slowing down as had been expected. This acceleration -- attributed to the dark energy that makes up two-thirds of all the energy in the universe -- has since been confirmed by a variety of independent techniques as well as with more detailed studies of supernovae.

The discovery of dark energy relied on using a particular class of supernovae, Type Ia. These supernovae always explode with nearly the same intrinsic maximum brightness. Because the observed maximum brightness of the supernova is used to infer its distance, the small remaining variations in the intrinsic maximum brightness limited the precision with which dark energy could be tested. Despite 20 years of improvements by many groups, supernovae studies of dark energy have until now remained limited by these variations.

Quadrupling the number of supernovae

The new results announced by the SNfactory come from a multi-year study devoted entirely to increasing the precision of cosmological measurements made with supernovae. Measurement of dark energy requires comparisons of the maximum brightnesses of distant supernovae billions of light-years away with those of nearby supernovae "only" 300 million light-years away. The team studied hundreds of such nearby supernovae in exquisite detail. Each supernova was measured a number of times, at intervals of a few days. Each measurement examined the spectrum of the supernova, recording its intensity across the wavelength range of visible light. An instrument custom-made for this investigation, the SuperNova Integral Field Spectrometer, installed at the University of Hawaii 2.2-meter telescope at Maunakea, was used to measure the spectra.

"We've long had this idea that if the physics of the explosion of two supernovae were the same, their maximum brightnesses would be the same. Using the Nearby Supernova Factory spectra as a kind of CAT scan through the supernova explosion, we could test this idea," said Perlmutter.

Indeed, several years ago, physicist Hannah Fakhouri, then a graduate student working with Perlmutter, made a discovery key to today's results. Looking at a multitude of spectra taken by the SNfactory, she found that in quite a number of instances, the spectra from two different supernovae looked very nearly identical. Among the 50 or so supernovae, some were virtually identical twins. When the wiggly spectra of a pair of twins were superimposed, to the eye there was just a single track. The current analysis builds on this observation to model the behavior of supernovae in the period near the time of their maximum brightness.

The new work nearly quadruples the number of supernovae used in the analysis. This made the sample large enough to apply machine-learning techniques to identify these twins, leading to the discovery that Type Ia supernova spectra vary in only three ways. The intrinsic brightnesses of the supernovae also depend primarily on these three observed differences, making it possible to measure supernova distances to the remarkable accuracy of about 3%.

Just as important, this new method does not suffer from the biases that have beset previous methods, seen when comparing supernovae found in different types of galaxies. Since nearby galaxies are somewhat different than distant ones, there was a serious concern that such dependence would produce false readings in the dark energy measurement. Now this concern can be greatly reduced by measuring distant supernovae with this new technique.

In describing this work, Boone noted, "Conventional measurement of supernova distances uses light curves -- images taken in several colors as a supernova brightens and fades. Instead, we used a spectrum of each supernova. These are so much more detailed, and with machine-learning techniques it then became possible to discern the complex behavior that was key to measuring more accurate distances."

The results from Boone's papers will benefit two upcoming major experiments. The first experiment will be at the 8.4-meter Rubin Observatory, under construction in Chile, with its Legacy Survey of Space and Time, a joint project of the Department of Energy and the National Science Foundation. The second is NASA's forthcoming Nancy Grace Roman Space Telescope. These telescopes will measure thousands of supernovae to further improve the measurement of dark energy. They will be able to compare their results with measurements made using complementary techniques.

Aldering, also a co-author on the papers, observed that "not only is this distance measurement technique more accurate, it only requires a single spectrum, taken when a supernova is brightest and thus easiest to observe -- a game changer!" Having a variety of techniques is particularly valuable in this field where preconceptions have turned out to be wrong and the need for independent verification is high.

The SNfactory collaboration includes Berkeley Lab, the Laboratory for Nuclear Physics and High Energy at Sorbonne University, the Center for Astronomical Research of Lyon, the Institute of Physics of the 2 Infinities at the University Claude Bernard, Yale University, Germany's Humboldt University, the Max Planck Institute for Astrophysics, China's Tsinghua University, the Center for Particle Physics of Marseille, and Clermont Auvergne University.

This work was supported by the Department of Energy's Office of Science, NASA's Astrophysics Division, the Gordon and Betty Moore Foundation, the French National Institute of Nuclear and Particle Physics and the National Institute for Earth Sciences and Astronomy of the French National Centre for Scientific Research, the German Research Foundation and German Aerospace Center, the European Research Council, Tsinghua University, and the National Natural Science Foundation of China.

Additional background

In 1998, two competing groups studying supernovae, the Supernova Cosmology Project and the High-z Supernova Search team, both announced they had found evidence that, contrary to expectations, the expansion of the universe was not slowing but becoming faster and faster. Dark energy is the term used to describe the cause of the acceleration. The 2011 Nobel Prize was awarded to leaders of the two teams: Saul Perlmutter of Berkeley Lab and UC Berkeley, leader of the Supernova Cosmology Project, and to Brian Schmidt of the Australian National University and Adam Riess of Johns Hopkins University, from the High-z team.

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Dark Energy Survey physicists open new window into dark energy

An analysis by Dark Energy Survey physicists yields more precise estimates of the average density of matter as well as its propensity to clump together -- two key parameters that help physicists probe the nature of dark matter and dark energy.

The universe is expanding at an ever-increasing rate, and while no one is sure why, researchers with the Dark Energy Survey (DES) at least had a strategy for figuring it out: They would combine measurements of the distribution of matter, galaxies and galaxy clusters to better understand what's going on.

Reaching that goal turned out to be pretty tricky, but now a team led by researchers at the Department of Energy's SLAC National Accelerator Laboratory, Stanford University and the University of Arizona have come up with a solution. Their analysis, published April 6 in Physical Review Letters, yields more precise estimates of the average density of matter as well as its propensity to clump together -- two key parameters that help physicists probe the nature of dark matter and dark energy, the mysterious substances that make up the vast majority of the universe.

"It is one of the best constraints from one of the best data sets to date," says Chun-Hao To, a lead author on the new paper and a graduate student at SLAC and Stanford working with Kavli Institute for Particle Astrophysics and Cosmology Director Risa Wechsler.

An early goal

When DES set out in 2013 to map an eighth of the sky, the goal was to gather four kinds of data: the distances to certain types of supernovae, or exploding stars; the distribution of matter in the universe; the distribution of galaxies; and the distribution of galaxy clusters. Each tells researchers something about how the universe has evolved over time.

Ideally, scientists would put all four data sources together to improve their estimates, but there's a snag: The distributions of matter, galaxies, and galaxy clusters are all closely related. If researchers don't take these relationships into account, they will end up "double counting," placing too much weight on some data and not enough on others, To says.

To avoid mishandling all this information, To, University of Arizona astrophysicist Elisabeth Krause and colleagues have developed a new model that could properly account for the connections in the distributions of all three quantities: matter, galaxies, and galaxy clusters. In doing so, they were able to produce the first-ever analysis to properly combine all these disparate data sets in order to learn about dark matter and dark energy.

Improving estimates

Adding that model into the DES analysis has two effects, To says. First, measurements of the distributions of matter, galaxies and galaxy clusters tend to introduce different kinds of errors. Combining all three measurements makes it easier to identify any such errors, making the analysis more robust. Second, the three measurements differ in how sensitive they are to the average density of matter and its clumpiness. As a result, combining all three can improve the precision with which the DES can measure dark matter and dark energy.

In the new paper, To, Krause and colleagues applied their new methods to the first year of DES data and sharpened the precision of previous estimates for matter's density and clumpiness.

Now that the team can incorporate matter, galaxies and galaxy clusters simultaneously in their analysis, adding in supernova data will be relatively straightforward, since that kind of data is not as closely related with the other three, To says.

"The immediate next step," he says, "is to apply the machinery to DES Year 3 data, which has three times larger coverage of the sky." This is not as simple as it sounds: While the basic idea is the same, the new data will require additional efforts to improve the model to keep up with the higher quality of the newer data, To says.

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New study sews doubt about the composition of 70 percent of our universe

 Until now, researchers have believed that dark energy accounted for nearly 70 percent of the ever-accelerating, expanding universe.

For many years, this mechanism has been associated with the so-called cosmological constant, developed by Einstein in 1917, that refers to an unknown repellent cosmic power.

But because the cosmological constant -- known as dark energy -- cannot be measured directly, numerous researchers, including Einstein, have doubted its existence -- without being able to suggest a viable alternative.

Until now. In a new study by researchers at the University of Copenhagen, a model was tested that replaces dark energy with a dark matter in the form of magnetic forces.

"If what we discovered is accurate, it would upend our belief that what we thought made up 70 percent of the universe does not actually exist. We have removed dark energy from the equation and added in a few more properties for dark matter. This appears to have the same effect upon the universe's expansion as dark energy," explains Steen Harle Hansen, an associate professor at the Niels Bohr Institute's DARK Cosmology Centre.

The universe expands no differently without dark energy

The usual understanding of how the universe's energy is distributed is that it consists of five percent normal matter, 25 percent dark matter and 70 percent dark energy.

In the UCPH researchers' new model, the 25 percent share of dark matter is accorded special qualities that make the 70 percent of dark energy redundant.

"We don't know much about dark matter other than that it is a heavy and slow particle. But then we wondered -- what if dark matter had some quality that was analogous to magnetism in it? We know that as normal particles move around, they create magnetism. And, magnets attract or repel other magnets -- so what if that's what's going on in the universe? That this constant expansion of dark matter is occurring thanks to some sort of magnetic force?" asks Steen Hansen.

Computer model tests dark matter with a type of magnetic energy

Hansen's question served as the foundation for the new computer model, where researchers included everything that they know about the universe -- including gravity, the speed of the universe's expansion and X, the unknown force that expands the universe.

"We developed a model that worked from the assumption that dark matter particles have a type of magnetic force and investigated what effect this force would have on the universe. It turns out that it would have exactly the same effect on the speed of the university's expansion as we know from dark energy," explains Steen Hansen.

However, there remains much about this mechanism that has yet to be understood by the researchers.

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Measurements of pulsar acceleration reveal Milky Way's dark side

 

Milky Way in the night sky.

It is well known that the expansion of the universe is accelerating due to a mysterious dark energy. Within galaxies, stars also experience an acceleration, though this is due to some combination of dark matter and the stellar density. In a new study to be published in Astrophysical Journal Letters, researchers have now obtained the first direct measurement of the average acceleration taking place within our home galaxy, the Milky Way.

Led by Sukanya Chakrabarti at the Institute for Advanced Study with collaborators from Rochester Institute of Technology, University of Rochester, and University of Wisconsin-Milwaukee, the team used pulsar data to clock the radial and vertical accelerations of stars within and outside of the galactic plane. Based on these new high-precision measurements and the known amount of visible matter in the galaxy, researchers were then able to calculate the Milky Way's dark matter density without making the usual assumption that the galaxy is in a steady-state.

"Our analysis not only gives us the first measurement of the tiny accelerations experienced by stars in the galaxy, but also opens up the possibility of extending this work to understand the nature of dark matter, and ultimately dark energy on larger scales," stated Chakrabarti, the paper's lead author and a current Member and IBM Einstein Fellow at the Institute for Advanced Study.

Stars hurtle through the galaxy at hundreds of kilometers per second, yet this study indicates that the change in their velocities is occurring at a literal snail's pace -- a few centimeters per second, which is about the same speed as a crawling baby. To detect this subtle motion the research team relied on the ultraprecise time-keeping ability of pulsars that are widely distributed throughout the galactic plane and halo -- a diffuse spherical region that surrounds the galaxy.

"By exploiting the unique properties of pulsars, we were able to measure very small accelerations in the Galaxy. Our work opens a new window in galactic dynamics," said co-author Philip Chang of the University of Wisconsin-Milwaukee.

Extending outwards approximately 300,000 light years from the galactic center, the halo may provide important hints to understanding dark matter, which accounts for about 90 percent of the galaxy's mass and is highly concentrated above and below the star-dense galactic plane. Stellar motion in this particular region -- a primary focus of this study -- can be influenced by dark matter. Utilizing the local density measurements obtained through this study, researchers will now have a better idea of how and where to look for dark matter.

While previous studies assumed a state of galactic equilibrium to calculate average mass density, this research is based on the natural, non-equilibrium state of the galaxy. One might analogize this to the difference between the surface of a pond before and after a stone is tossed in. By accounting for the "ripples" the team was able to obtain a more accurate picture of reality. Though in this case, rather than stones, the Milky Way is influenced by a turbulent history of galactic mergers and continues to be perturbed by external dwarf galaxies like the Small and Large Magellanic Clouds. As a result, stars do not have flat orbits and tend to follow a path similar to that of a warped vinyl record, crossing above and below the galactic plane. One of the key factors that enabled this direct observational approach was the use of pulsar data compiled from international collaborations, including NANOGrav (North American Nanohertz Observatory for Gravitational Waves) that has obtained data from the Green Bank and Arecibo telescopes.

This landmark paper expands upon the work of Jan H. Oort (1932); John Bahcall (1984); Kuijken & Gilmore (1989); Holmberg & Flynn (2000); Jo Bovy & Scott Tremaine (2012) to calculate the average mass density in the galactic plane (Oort limit) and local dark matter density. IAS scholars including Oort, Bahcall, Bovy, Tremaine, and Chakrabarti have played an important role in advancing this area of research.

"For centuries astronomers have measured the positions and speeds of stars, but these provide only a snapshot of the complex dynamical behavior of the Milky Way galaxy," stated Scott Tremaine, Professor Emeritus at the Institute for Advanced Study. "The accelerations measured by Chakrabarti and her collaborators are directly caused by the gravitational forces from the matter in the galaxy, both visible and dark, and thereby provide a new and promising window on the distribution and the composition of the matter in the galaxy and the universe."

This particular paper will enable a wide variety of future studies. Accurate measurements of accelerations will also soon be possible using the complementary radial velocity method that Chakrabarti developed earlier this year, which measures the change in the velocity of stars with high precision. This work will also enable more detailed simulations of the Milky Way, improve constraints on general relativity, and provide clues in the search for dark matter. Extensions of this method may ultimately allow us to directly measure the cosmic acceleration as well.

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Researchers predict location of novel candidate for mysterious dark energy

 Astronomers have known for two decades that the expansion of the universe is accelerating, but the physics of this expansion remains a mystery. Now, a team of researchers at the University of Hawaiʻi at Mānoa have made a novel prediction -- the dark energy responsible for this accelerating growth comes from a vast sea of compact objects spread throughout the voids between galaxies. This conclusion is part of a new study published in The Astrophysical Journal.

In the mid-1960s, physicists first suggested that stellar collapse should not form true black holes, but should instead form Generic Objects of Dark Energy (GEODEs). Unlike black holes, GEODEs do not 'break' Einstein's equations with singularities. Instead, a spinning layer surrounds a core of dark energy. Viewed from the outside, GEODEs and black holes appear mostly the same, even when the "sounds" of their collisions are measured by gravitational wave observatories.

Because GEODEs mimic black holes, it was assumed they moved through space the same way as black holes. "This becomes a problem if you want to explain the accelerating expansion of the universe," said UH Mānoa Department of Physics and Astronomy research fellow Kevin Croker, lead author of the study. "Even though we proved last year that GEODEs, in principle, could provide the necessary dark energy, you need lots of old and massive GEODEs. If they moved like black holes, staying close to visible matter, galaxies like our own Milky Way would have been disrupted."

Croker collaborated with UH Mānoa Department of Physics and Astronomy graduate student Jack Runburg, and Duncan Farrah, a faculty member at the UH Institute for Astronomy and the Physics and Astronomy department, to investigate how GEODEs move through space. The researchers found that the spinning layer around each GEODE determines how they move relative to each other. If their outer layers spin slowly, GEODEs clump more rapidly than black holes. This is because GEODEs gain mass from the growth of the universe itself. For GEODEs with layers that spin near the speed of light, however, the gain in mass becomes dominated by a different effect and the GEODEs begin to repel each other. "The dependence on spin was really quite unexpected," said Farrah. "If confirmed by observation, it would be an entirely new class of phenomenon."

The team solved Einstein's equations under the assumption that many of the oldest stars, which were born when the universe was less than 2 percent of its current age, formed GEODEs when they died. As these ancient GEODEs fed on other stars and abundant interstellar gas, they began to spin very rapidly. Once spinning quickly enough, the GEODEs' mutual repulsion caused most of them to 'socially distance' into regions that would eventually become the empty voids between present-day galaxies.

This study supports the position that GEODEs can solve the dark energy problem while remaining in harmony with different observations across vast distances. GEODEs stay away from present-day galaxies, so they do not disrupt delicate star pairs counted within the Milky Way. The number of ancient GEODEs required to solve the dark energy problem is consistent with the number of ancient stars. GEODEs do not disrupt the measured distribution of galaxies in space because they separate away from luminous matter before it forms present-day galaxies. Finally, GEODEs do not directly affect the gentle ripples in the afterglow of the Big Bang, because they are born from dead stars hundreds of millions of years after the release of this cosmic background radiation.

The researchers were cautiously optimistic about their results. "It was thought that, without a direct detection of something different than a Kerr [Black Hole] signature from LIGO-Virgo [gravitational wave observatories], you'd never be able to tell that GEODEs existed," said Farrah. Croker added, "but now that we have a clearer understanding of how Einstein's equations link big and small, we've been able to make contact with data from many communities, and a coherent picture is beginning to form."

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New test of dark energy and expansion from cosmic structures

Starry night sky

A new paper has shown how large structures in the distribution of galaxies in the Universe provide the most precise tests of dark energy and cosmic expansion yet.

The study uses a new method based on a combination of cosmic voids -- large expanding bubbles of space containing very few galaxies -- and the faint imprint of sound waves in the very early Universe, known as baryon acoustic oscillations (BAO), that can be seen in the distribution of galaxies. This provides a precise ruler to measure the direct effects of dark energy driving the accelerated expansion of the Universe.

This new method gives much more precise results than the technique based on the observation of exploding massive stars, or supernovae, which has long been the standard method for measuring the direct effects of dark energy.

The research was led by the University of Portsmouth, and is published in Physical Review Letters.

The study makes use of data from over a million galaxies and quasars gathered over more than a decade of operations by the Sloan Digital Sky Survey.

The results confirm the model of a cosmological constant dark energy and spatially flat Universe to unprecedented accuracy, and strongly disfavour recent suggestions of positive spatial curvature inferred from measurements of the cosmic microwave background (CMB) by the Planck satellite.

Lead author Dr Seshadri Nadathur, research fellow at the University's Institute of Cosmology and Gravitation (ICG), said: "This result shows the power of galaxy surveys to pin down the amount of dark energy and how it evolved over the last billion years. We're making really precise measurements now and the data is going to get even better with new surveys coming online very soon."

Dr Florian Beutler, a senior research fellow at the ICG, who was also involved in the work, said that the study also reported a new precise measurement of the Hubble constant, the value of which has recently been the subject of intense debate among astronomers.

He said: "We see tentative evidence that data from relatively nearby voids and BAO favour the high Hubble rate seen from other low-redshift methods, but including data from more distant quasar absorption lines brings it in better agreement with the value inferred from Planck CMB data."

From Science Daily