The universe caught suppressing cosmic structure growth

As the universe evolves, scientists expect large cosmic structures to grow at a certain rate: dense regions such as galaxy clusters would grow denser, while the void of space would grow emptier.

But University of Michigan researchers have discovered that the rate at which these large structures grow is slower than predicted by Einstein's Theory of General Relativity.

They also showed that as dark energy accelerates the universe's global expansion, the suppression of the cosmic structure growth that the researchers see in their data is even more prominent than what the theory predicts. Their results are published in Physical Review Letters.

Galaxies are threaded throughout our universe like a giant cosmic spider web. Their distribution is not random. Instead, they tend to cluster together. In fact, the whole cosmic web started out as tiny clumps of matter in the early universe, which gradually grew into individual galaxies, and eventually galaxy clusters and filaments.

"Throughout the cosmic time, an initially small clump of mass attracts and accumulates more and more matter from its local region through gravitational interaction. As the region becomes denser and denser, it eventually collapses under its own gravity," said Minh Nguyen, lead author of the study and postdoctoral research fellow in the U-M Department of Physics.

"So as they collapse, the clumps grow denser. That is what we mean by growth. It's like a fabric loom where one-, two- and three-dimensional collapses look like a sheet, a filament and a node. The reality is a mixture of all three cases, and you have galaxies living along the filaments while galaxy clusters -- groups of thousands of galaxies, the most massive objects in our universe bounded by gravity -- sit at the nodes."

The universe is not only made of matter. It also likely contains a mysterious component called dark energy. Dark energy accelerates the expansion of the universe on a global scale. As dark energy accelerates the expansion of the universe, it has the opposite effect on large structures.

"If gravity acts like an amplifier enhancing matter perturbations to grow into large-scale structure, then dark energy acts like an attenuator damping these perturbations and slowing the growth of structure," Nguyen said. "By examining how cosmic structure has been clustering and growing, we can try to understand the nature of gravity and dark energy."

Nguyen, U-M physics professor Dragan Huterer and U-M graduate student Yuewei Wen examined the temporal growth of large-scale structure throughout cosmic time using several cosmological probes.

First, the team used what's called the cosmic microwave background. The cosmic microwave background, or CMB, is composed of photons emitted just after the Big Bang. These photons provide a snapshot of the very early universe. As the photons travel to our telescopes, their path can become distorted, or gravitationally lensed, by large-scale structure along the way. Examining them, the researchers can infer how structure and matter between us and the cosmic microwave background are distributed.

Nguyen and colleagues took advantage of a similar phenomenon with weak gravitational lensing of galaxy shapes. Light from background galaxies is distorted through gravitational interactions with foreground matter and galaxies. The cosmologists then decode these distortions to determine how the intervening matter is distributed.

"Crucially, as the CMB and background galaxies are located at different distances from us and our telescopes, galaxy weak gravitational lensing typically probes matter distributions at a later time compared to what is probed by CMB weak gravitational lensing," Nguyen said.

To track the growth of structure to an even later time, the researchers further used motions of galaxies in the local universe. As galaxies fall into the gravity wells of the underlying cosmic structures, their motions directly track structure growth.

"The difference in these growth rates that we have potentially discovered becomes more prominent as we approach the present day," Nguyen said. "These different probes individually and collectively indicate a growth suppression. Either we are missing some systematic errors in each of these probes, or we are missing some new, late-time physics in our standard model."

The findings potentially address the so-called S8 tension in cosmology. S8 is a parameter that describes the growth of structure. The tension arises when scientists use two different methods to determine the value of S8, and they do not agree. The first method, using photons from the cosmic microwave background, indicates a higher S8 value than the value inferred from galaxy weak gravitational lensing and galaxy clustering measurements.

Neither of these probes measures the growth of structure today. Instead, they probe structure at earlier times, then extrapolate those measurements to present time, assuming the standard model. Cosmic microwave background probes structure in the early universe, while galaxy weak gravitational lensing and clustering probe structure in the late universe.

The researchers' findings of a late-time suppression of growth would bring the two S8 values into perfect agreement, according to Nguyen.

"We were surprised with the high statistical significance of the anomalous growth suppression," Huterer said. "Honestly, I feel like the universe is trying to tell us something. It is now the job of us cosmologists to interpret these findings.

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Cosmic ray protons reveal new spectral structures at high energies

Discovered in 1912, cosmic rays have been studied extensively and our current understanding of them is compiled into what is called the Standard Model. Recently, this understanding has been challenged by the detection of unexpected spectral structures in the cosmic ray proton energy spectrum. Now, scientists take this further with high-statistics and low-uncertainty measurement of these protons over a broader energy range using the CALorimetric Electron Telescope, confirming the presence of such structures.

Cosmic rays constitute high-energy protons and atomic nuclei that originate from stars (both within our galaxy and from other galaxies) and are accelerated by supernovae and other high-energy astrophysical objects. Our current understanding of the Galactic cosmic ray energy spectrum suggests that it follows a power-law dependence, in that the spectral index of protons detected within a certain energy range goes down by power law as energy increases. But recent observations made using magnetic spectrometers for low energy levels and calorimeters for high energy levels has hinted at a deviation from this power-law variation, with the spectral index of protons becoming larger around an energy of few hundred GeV at energies up to 10 TeV. Following this "spectral hardening," characterized by a smaller absolute value of the spectral index, a "spectral softening" has been detected above 10 TeV using the CALorimetric Electron Telescope (CALET), a space telescope installed at the International Space Station. However, better measurements with high statistics and low uncertainty need to be performed over a broad energy spectrum for the confirmation of these spectral structures.

This is exactly what a team of international researchers led by Associate Professor Kazuyoshi Kobayashi from Waseda University in Japan set out to do. "With the data collected by CALET over roughly 6.2 years, we have put forth a detailed spectral structure of the cosmic ray protons. The novelty of our data lies in the high-statistics measurement over a broader energy range of 50 GeV to 60 TeV," elaborates Kobayashi. The findings of their study, which included contributions from Professor Emeritus Shoji Torii from Waseda University (PI, or Principal Investigator, of CALET project) and Professor Pier Simone Marrocchesi from University of Siena in Italy, was published in the journal Physical Review Letters on 1 September 2022.

The new observations confirmed the presence of spectral hardening and softening below and above 10 TeV, suggesting that the proton energy spectrum is not consistent with a single power law variation for the entire range. Moreover, the spectral softening starting at around 10 TeV is consistent with a previous measurement reported by the Dark Matter Particle Explorer (DAMPE) space telescope. Interestingly enough, the transition by spectral softening was found to be sharper than that by spectral hardening.

The variations and the uncertainty in the new CALET data were controlled using Monte Carlo simulations. The statistics was improved by a factor of around 2.2 and the spectral hardening feature was confirmed with a higher significance of more than 20 sigmas.

Talking about the significance of this research, Kobayashi remarks, "This result will significantly contribute to our understanding of cosmic ray acceleration by supernovae and the propagation mechanism of cosmic rays. The next step would be to extend our measurement of the proton spectra to even higher energies with reduced systematic uncertainties. This should be accompanied by a shift in the theoretical understanding to accommodate the new observations."

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Astronomers risk misinterpreting planetary signals in James Webb data

NASA's James Webb Space Telescope is revealing the universe with spectacular, unprecedented clarity. The observatory's ultrasharp infrared vision has cut through the cosmic dust to illuminate some of the earliest structures in the universe, along with previously obscured stellar nurseries and spinning galaxies lying hundreds of millions of light years away.

In addition to seeing farther into the universe than ever before, Webb will capture the most comprehensive view of objects in our own galaxy -- namely, some of the 5,000 planets that have been discovered in the Milky Way. Astronomers are harnessing the telescope's light-parsing precision to decode the atmospheres surrounding some of these nearby worlds. The properties of their atmospheres could give clues to how a planet formed and whether it harbors signs of life.

But a new MIT study suggests that the tools astronomers typically use to decode light-based signals may not be good enough to accurately interpret the new telescope's data. Specifically, opacity models -- the tools that model how light interacts with matter as a function of the matter's properties -- may need significant retuning in order to match the precision of Webb's data, the researchers say.

If these models are not refined? The researchers predict that properties of planetary atmospheres, such as their temperature, pressure, and elemental composition, could be off by an order of magnitude.

"There is a scientifically significant difference between a compound like water being present at 5 percent versus 25 percent, which current models cannot differentiate," says study co-leader Julien de Wit, assistant professor in MIT's Department of Earth, Atmospheric, and Planetary Sciences (EAPS).

"Currently, the model we use to decrypt spectral information is not up to par with the precision and quality of data we have from the James Webb telescope," adds EAPS graduate student Prajwal Niraula. "We need to up our game and tackle together the opacity problem."

De Wit, Niraula, and their colleagues have published their study in Nature Astronomy. Co-authors include spectroscopy experts Iouli Gordon, Robert Hargreaves, Clara Sousa-Silva, and Roman Kochanov of the Harvard-Smithsonian Center for Astrophysics.

Leveling up

Opacity is a measure of how easily photons pass through a material. Photons of certain wavelengths can pass straight through a material, be absorbed, or be reflected back out depending on whether and how they interact with certain molecules within a material. This interaction also depends on a material's temperature and pressure.

An opacity model works on the basis of various assumptions of how light interacts with matter. Astronomers use opacity models to derive certain properties of a material, given the spectrum of light that the material emits. In the context of explanets, an opacity model can decode the type and abundance of chemicals in a planet's atmosphere, based on the light from the planet that a telescope captures.

De Wit says that the current state-of-the-art opacity model, which he likens to a classical language translation tool, has done a decent job of decoding spectral data taken by instruments such as those on the Hubble Space Telescope.

"So far, this Rosetta Stone has been doing OK," de Wit says. "But now that we're going to the next level with Webb's precision, our translation process will prevent us from catching important subtleties, such as those making the difference between a planet being habitable or not."

Light, perturbed

He and his colleagues make this point in their study, in which they put the most commonly used opacity model to the test. The team looked to see what atmospheric properties the model would derive if it were tweaked to assume certain limitations in our understanding of how light and matter interact. The researchers created eight such "perturbed" models. They then fed each model, including the real version, "synthetic spectra" -- patterns of light that were simulated by the group and similar to the precision that the James Webb telescope would see.

They found that, based on the same light spectra, each perturbed model produced wide-ranging predictions for the properties of a planet's atmosphere. Based on their analysis, the team concludes that, if existing opacity models are applied to light spectra taken by the Webb telescope, they will hit an "accuracy wall." That is, they won't be sensitive enough to tell whether a planet has an atmospheric temperature of 300 Kelvin or 600 Kelvin, or whether a certain gas takes up 5 percent or 25 percent of an atmospheric layer.

"That difference matters in order for us to constrain planetary formation mechanisms and reliably identify biosignatures," Niraula says.

The team also found that every model also produced a "good fit" with the data, meaning, even though a perturbed model produced a chemical composition that the researchers knew to be incorrect, it also generated a light spectrum from that chemical composition that was close enough to, or "fit" with the original spectrum.

"We found that there are enough parameters to tweak, even with a wrong model, to still get a good fit, meaning you wouldn't know that your model is wrong and what it's telling you is wrong," de Wit explains.

He and his colleagues raise some ideas for how to improve existing opacity models, including the need for more laboratory measurements and theoretical calculations to refine the models' assumptions of how light and various molecules interact, as well as collaborations across disciplines, and in particular, between astronomy and spectroscopy.

Read more at Science Daily

The very first structures in the Universe

The very first moments of the Universe can be reconstructed mathematically even though they cannot be observed directly. Physicists from the Universities of Göttingen and Auckland (New Zealand) have greatly improved the ability of complex computer simulations to describe this early epoch. They discovered that a complex network of structures can form in the first trillionth of a second after the Big Bang. The behaviour of these objects mimics the distribution of galaxies in today's Universe. In contrast to today, however, these primordial structures are microscopically small. Typical clumps have masses of only a few grams and fit into volumes much smaller than present-day elementary particles. The results of the study have been published in the journal Physical Review D.

The researchers were able to observe the development of regions of higher density that are held together by their own gravity. "The physical space represented by our simulation would fit into a single proton a million times over," says Professor Jens Niemeyer, head of the Astrophysical Cosmology Group at the University of Göttingen. "It is probably the largest simulation of the smallest area of the Universe that has been carried out so far." These simulations make it possible to calculate more precise predictions for the properties of these vestiges from the very beginnings of the Universe.

Although the computer-simulated structures would be very short-lived and eventually "vaporise" into standard elementary particles, traces of this extreme early phase may be detectable in future experiments. "The formation of such structures, as well as their movements and interactions, must have generated a background noise of gravitational waves," says Benedikt Eggemeier, a PhD student in Niemeyer's group and first author of the study. "With the help of our simulations, we can calculate the strength of this gravitational wave signal, which might be measurable in the future."

It is also conceivable that tiny black holes could form if these structures undergo runaway collapse. If this happens they could have observable consequences today, or form part of the mysterious dark matter in the Universe. "On the other hand," says Professor Easther, "If the simulations predict black holes form, and we don't see them, then we will have found a new way to test models of the infant Universe."

From Science Daily

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