A new possible explanation for the Hubble tension

The universe is expanding. How fast it does so is described by the so-called Hubble-Lemaitre constant. But there is a dispute about how big this constant actually is: Different measurement methods provide contradictory values. This so-called "Hubble tension" poses a puzzle for cosmologists. Researchers from the Universities of Bonn and St. Andrews are now proposing a new solution: Using an alternative theory of gravity, the discrepancy in the measured values can be easily explained -- the Hubble tension disappears. The study has now been published in the Monthly Notices of the Royal Astronomical Society (MNRAS).

The expansion of the universe causes the galaxies to move away from each other.

The speed at which they do this is proportional to the distance between them.

For instance, if galaxy A is twice as far away from Earth as galaxy B, its distance from us also grows twice as fast.

The US astronomer Edwin Hubble was one of the first to recognize this connection.

In order to calculate how fast two galaxies are moving away from each other, it is therefore necessary to know how far apart they are.

However, this also requires a constant by which this distance must be multiplied.

This is the so-called Hubble-Lemaitre constant, a fundamental parameter in cosmology.

Its value can be determined, for example, by looking at the very distant regions of the universe.

This gives a speed of almost 244,000 kilometers per hour per megaparsec distance (one megaparsec is just over three million light years).

244.000 kilometers per hour per megaparsec -- or 264,000?

"But you can also look at celestial bodies that are much closer to us -- so-called category 1a supernovae, which are a certain type of exploding star," explains Prof.

Dr. Pavel Kroupa from the Helmholtz Institute of Radiation and Nuclear Physics at the University of Bonn.

It is possible to determine the distance of a 1a supernova to Earth very precisely.

We also know that shining objects change color when they move away from us -- and the faster they move, the stronger the change.

This is similar to an ambulance, whose siren sounds deeper as it moves away from us.

If we now calculate the speed of the 1a supernovae from their color shift and correlate this with their distance, we arrive at a different value for the Hubble-Lemaitre constant -- namely just under 264,000 kilometers per hour per megaparsec distance.

"The universe therefore appears to be expanding faster in our vicinity -- that is, up to a distance of around three billion light years -- than in its entirety," says Kroupa.

"And that shouldn't really be the case."

However, there has recently been an observation that could explain this.

According to this, the Earth is located in a region of space where there is relatively little matter -- comparable to an air bubble in a cake.

The density of matter is higher around the bubble. Gravitational forces emanate from this surrounding matter, which pull the galaxies in the bubble towards the edges of the cavity.

"That's why they are moving away from us faster than would actually be expected," explains Dr. Indranil Banik from St. Andrews University.

The deviations could therefore simply be explained by a local "under-density."

In fact, another research group recently measured the average speed of a large number of galaxies that are 600 million light years away from us. "It was found that these galaxies are moving away from us four times faster than the standard model of cosmology allows," explains Sergij Mazurenko from Kroupa's research group, who was involved in the current study.

Bubble in the dough of the universe

This is because the standard model does not provide for such under-densities or "bubbles" -- they should not actually exist.

Instead, matter should be evenly distributed in space. If this were the case, however, it would be difficult to explain which forces propel the galaxies to their high speed.

"The standard model is based on a theory of the nature of gravity put forward by Albert Einstein," says Kroupa.

"However, the gravitational forces may behave differently than Einstein expected." The working groups from the Universities of Bonn and St. Andrews have used a modified theory of gravity in a computer simulation.

This "modified Newtonian dynamics" (abbreviation: MOND) was proposed four decades ago by the Israeli physicist Prof.

Dr. Mordehai Milgrom. It is still considered an outsider theory today.

"In our calculations, however, MOND does accurately predict the existence of such bubbles," says Kroupa.

If one were to assume that gravity actually behaves according to Milgrom's assumptions, the Hubble tension would disappear: There would actually only be one constant for the expansion of the universe, and the observed deviations would be due to irregularities in the distribution of matter.

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HETDEX reveals galaxy gold mine in first large survey

Astronomers have barely scratched the surface of mapping the nearly endless stars and galaxies of the heavens. Using supercomputers, researchers with The University of Texas at Austin have has now revealed the locations of more than 200,000 new astronomical objects. Their goal is to map even more and use that knowledge to predict the ultimate fate of the universe.

The Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) has scanned the dark skies of the Davis Mountains in West Texas since 2017 with a keen eye towards capturing spectroscopic data on Lyman-alpha frequency light from neutral hydrogen emission in galaxies over 10 billion light years away. These galaxies emit a signature wavelength of light from hydrogen that signals to astronomers the intense creation of new stars.

The HETDEX collaboration involves a large team including astronomers, engineers, technicians, and graduate students from six academic institutions in the United States and Germany.

For the first time, the researchers have cataloged astronomical objects -- mapping over 51,863 Lyman-alpha-emitting galaxies at high redshift; 123,891 star forming galaxies at lower redshift; 5,274 non-emission line galaxies at low redshift; and 4,976 active galactic nuclei (AGN) -- bright spots that signal the presence of black holes.

The paper describing the catalog is published February 2023 in The Astrophysical Journal.

"We've just exploded in terms of the number of redshifts cataloged for the first time," said study co-author Erin Mentuch Cooper, a research scientist at The University of Texas at Austin (UT Austin). Cooper is the data manager for the HETDEX project.

"There is a gold mine of astronomy exploration in the HETDEX catalog. That's what I love about it," said study co-author Karl Gebhardt, the Herman and Joan Suit Professor in Astronomy, College of Natural Sciences, UT Austin. Gebhardt is project scientist and principal investigator of HETDEX.

A star's redshift tells astronomers how fast a star is moving away from the Earth because its frequency, akin to its color, gets lower as it moves away, much like the horn of a train as it passes by.

The faster a star moves away, the farther away it is. That relationship between speed and distance, called Hubble's Law, can pin down a galaxy's location and allows astronomers to create a 3D map of over 200,000 stars and galaxies with HETDEX.

"This is only a small percentage of what we will find, but it's a good start. Ultimately, HETDEX aims to map one million red-shifted galaxies," Cooper said.

HETDEX is unique from previous large sky surveys because it's a non-targeted survey, blanketing the sky and collecting spectra from the 35,000 fiber optic cables of the Visible Integral Field Replicable Unit Spectrograph (VIRUS).

VIRUS takes starlight from distant galaxies and splits the light into its component colors like a prism does. HETDEX tiles the sky, collecting 35,000 spectra in a moon-sized swath of sky and moving from spot to spot. It collects about 500-600 hours of observations each year for its survey data.

"We have the largest spectroscopic instrument on the planet, and we're doing one of the longest surveys in terms of time," Gebhardt said. "To analyze this data, we need the fastest computer we can get our hands on, and that's where TACC comes in. TACC does all the data storage and all the data analysis for this giant survey."

The data from the telescope goes straight to the TACC Corral data storage system via high speed lines at 100 Gigabits/second.

"TACC has worked hard with us to streamline our system, and it's just working fantastically. We can process years of data in a couple of days, maybe a week of time on TACC systems. And we do it multiple times because we keep adjusting and improving our methods," Gebhardt added.

HETDEX used the Maverick and Stampede2 supercomputers of the Texas Advanced Computing Center, a leading academic supercomputing center at UT Austin. Stampede2 is funded by the National Science Foundation as a shared resource for thousands of scientists across the US. They helped process and analyze about 60 terabytes of image data on TACC's Corral system.

What's more, Cooper and colleagues have worked with TACC to create JupyterHub public access to the data.

"Anyone with any academic credentials can get a TACC account and go on through a web browser to access our data. We're going to let them access all of our data. This is just the catalog right now. But, the future is going to offer a legacy potential of the science from HETDEX. TACC is helping setting that up," Cooper said.

One interesting highlight from the catalog is the identification of an active galactic nuclei (AGN) with strong Lyman-alpha light emission. Gebhadt co-authored a studyled by UT Austin astronomy post-doctoral researcher Chenxu Liu, published November 2022 in The Astrophysical Journal. It presents intriguing evidence of a black hole without a surrounding host galaxy.

"This is what I call 'naked black holes,'" Gebhardt said. "Nothing confirmed yet, but we suspect these could be out there. Only a survey like HETDEX will be able to find these."

The science generated from HETDEX adds to the bigger picture of understanding the expansion of the entire universe, unexpectedly growing much faster than expected based on precise observations from the Hubble Space Telescope in 2019 of supernovae that act like a cosmic yardstick.

The Holy Grail for HETDEX is an accurate measure of the Universe expansion rate 10 billion years ago that will reveal the physical model for dark energy.

Astronomers are at odds over how to explain the measure of the current expansion rate. Understanding it could require a modification in the theory of gravity, or a change in the fundamental Big Bang theory. It might be the handiwork of an undiscovered particle.

A precise value of the expansion rate early in the Universe can be compared to the expansion rate today. This comparison can determine if the Universe will continue to expand forever, or will someday collapse on itself many billions of years from now.

"The whole point of the HETDEX project is to measure the expansion of the universe," Gebhardt said.

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Ghostly 'mirror world' might be cause of cosmic controversy

New research suggests an unseen 'mirror world' of particles that interacts with our world only via gravity that might be the key to solving a major puzzle in cosmology today -- the Hubble constant problem.

The Hubble constant is the rate of expansion of the universe today. Predictions for this rate -- from cosmology's standard model -- are significantly slower than the rate found by our most precise local measurements. This discrepancy is one that many cosmologists have been trying to solve by changing our current cosmological model. The challenge is to do so without ruining the agreement between standard model predictions and many other cosmological phenomena, such as the cosmic microwave background. Determining whether such a cosmological scenario exists is the question that researchers, including Francis-Yan Cyr-Racine, assistant professor in the Department of Physics and Astronomy at The University of New Mexico, Fei Ge and Lloyd Knox at the University of California, Davis have been trying to answer.

According to NASA, cosmology is the scientific study of the large-scale properties of the universe as a whole. Cosmologists study concepts such as dark matter, and dark energy and whether there is one universe or many, sometimes called a multiverse. Cosmology entails the entire universe from birth to death with mysteries and intrigue at every turn.

Now, Cyr-Racine, Ge, and Knox have discovered a previously unnoticed mathematical property of cosmological models which could, in principle, allow for a faster expansion rate while hardly changing the most precisely tested other predictions of the standard cosmological model. They found that a uniform scaling of the gravitational free-fall rates and photon-electron scattering rate leaves most dimensionless cosmological observables nearly invariant.

"Basically, we point out that a lot of the observations we do in cosmology have an inherent symmetry under rescaling the universe as a whole. This might provide a way to understand why there appears to be a discrepancy between different measurements of the Universe's expansion rate."

The research, titled "Symmetry of Cosmological Observables, a Mirror World Dark Sector, and the Hubble Constant" was published recently in Physical Review Letters.

This result opens a new approach to reconciling cosmic microwave background and large-scale structure observations with high values of the Hubble constant H0: Find a cosmological model in which the scaling transformation can be realized without violating any measurements of quantities not protected by the symmetry. This work has opened a new path toward resolving what has proved to be a challenging problem. Further model building might bring consistency with the two constraints not yet satisfied: the inferred primordial abundances of deuterium and helium.

If the universe is somehow exploiting this symmetry researchers are led to an extremely interesting conclusion: that there exists a mirror universe very similar to ours but invisible to us except through gravitational impact on our world. Such "mirror world" dark sector would allow for an effective scaling of the gravitational free-fall rates while respecting the precisely measured mean photon density today.

"In practice, this scaling symmetry could only be realized by including a mirror world in the model -- a parallel universe with new particles that are all copies of known particles," said Cyr-Racine. "The mirror world idea first arose in the 1990s but has not previously been recognized as a potential solution to the Hubble constant problem.

"This might seem crazy at face value, but such mirror worlds have a large physics literature in a completely different context since they can help solve important problem in particle physics," explains Cyr-Racine. "Our work allows us to link, for the first time, this large literature to an important problem in cosmology."

In addition to searching for missing ingredients in our current cosmological model, researchers are also wondering whether this Hubble constant discrepancy could be caused in part by measurement errors. While it remains a possibility, it is important to note that the discrepancy has become more and more significant as higher quality data have been included in the analyses, suggesting that the data might not be at fault.

"It went from two and a half Sigma, to three, and three and a half to four Sigma. By now, we are pretty much at the five-Sigma level," said Cyr-Racine. "That's the key number which makes this a real problem because you have two measurements of the same thing, which if you have a consistent picture of the universe should just be completely consistent with each other, but they differ by a very statistically significant amount."

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