Latest news with #LambdaColdDarkMatter
The Hindu
01-05-2025
- Science
- The Hindu
S8 tension: cosmologists can't agree on how clumpy the universe is
Cosmology is in for exciting times, going by the latest research that suggests the key to revealing the fundamental nature of the universe lies in finding out how clumpy it is. Accepted theory has it that after the universe was explosively born in a 'Big Bang' in the void some 13.8 billion years ago, it expanded, engendering galaxies, star clusters, solar systems, and planets. When scientists looked at the cosmic microwave background (CMB) — the radiation left over from the Big Bang itself — they saw an absolutely smooth glow across the sky. The early universe must have been remarkably uniform, they concluded, with only small variations in density (of about one part in 100,000 when it was 380,000 years old). Primordial fluctuations How did matter in the universe get to be so lumpy today after starting out so evenly? The 'lumps' we see in the universe arose from different chunks of matter like galaxies and dark matter — a hypothetical, invisible form of matter that doesn't interact with light or electromagnetic radiation and which makes up a significant portion of the universe — being pulled together by gravitational forces. Over the years, cosmologists have tried to map the overall spread of matter through the early universe. In the standard cosmological model, called the Lambda Cold Dark Matter (ΛCDM) model, dark matter and dark energy — the mysterious force that drives the expansion of the universe — comprise about 95% of the universe. The interplay between these components influences how the primordial fluctuations evolved into the large-scale structures that we observe today. Cosmologists use the term Sigma 8, or S8, to quantify the matter around us. This matter is made up of baryonic particles, such as protons and neutrons, that bunch up in different regions of space. The value of S8 is calculated by studying various regions of the universe. Each region is defined by an astronomical length scale of approximately 26 million light-years. Within these regions, cosmologists count the number of galaxies and other cosmic structures, such as galactic clusters and filaments, to assess the distribution of matter. A higher value for S8 indicates more clustering with a greater amount of matter clumped together, while a lower value indicates a more uniform distribution of matter. A problem arose when cosmologists used different ways to measure the value of S8 and came up with different estimates. This lack of agreement has come to be called the 'S8 tension' in astrophysics. Cosmic-shear surveys Astronomers have conducted galaxy surveys to determine the value of S8 . One method involves measuring the distortion in the shape of galaxies as seen from the earth: an effect known as cosmic shear. These distortions occur when starlight passes through a galactic cluster and is bent and amplified by gravitational forces, much like a magnifying glass does. Astronomers use this gravitational lensing to study indistinct epochs in the evolution of the universe. Cosmic-shear surveys help to map the diffusion of matter, including dark matter, in the universe so cosmologists can deduce the amplitude of matter fluctuations as quantified by S8. The results of the latest such survey were recently published in the journal Physics by an international team of researchers from the University of Tokyo. They used the Hyper Suprime-Cam (HSC) — a camera installed on the Subaru Telescope in Hawaii — to collect data and came up with a value of 0.747 for S8, which tallies with the values found by previous surveys. 'The Subaru HSC survey is one of the deepest wide area surveys of the sky,' Surhud S. More, a co-author of the study and professor of astrophysics at the Inter-University Centre for Astronomy and Astrophysics in Pune, wrote in an email. He added that the researchers probed matter's distribution using the gravitational lensing effect down to small scales. 'We were able to show that any movement of ordinary matter, such as gas within the large-scale structure of the universe, will not be sufficient to explain the smaller value of the clumpiness which had been found in our previous study.' In other words, the discrepancy in S8 has to do with the dark matter and dark energy that pervades the cosmos. While this reaffirms that all is well with the ΛCDM model, it does not dispel the S8 tension itself: studies like this were based on gravitational lensing to determine the value of S8 to be 0.747, which does not agree with the higher value predicted by data from the CMB. Relic radiation Cosmologists consider the CMB to be a better tool to look back in space and time. They have known for a long time that the surge of primordial matter in the CMB holds clues to the universe's origins in the form of 'ripples' generated by the expanding universe. These ripples resulted in lumps and bumps — future star clusters and galaxies — in the otherwise uniform fabric of space. These telltale galactic signatures were detected in 1992 by NASA's Cosmic Background Explorer satellite. But with the S8 tension persisting, the ΛCDM model looks to be in need of modification — unless some as yet undiscovered systematics could affect such a conclusion. As Prof. More said, 'One of the main difficulties in using deep surveys such as Subaru HSC is our lack of understanding of how fast the galaxies in these surveys are actually receding from us, quantified by the redshift [increase in wavelength] of certain lines in their spectrum. As the millions of galaxies used in these analyses are faint, one cannot analyse the spectrum of light of these galaxies to determine this redshift. This constitutes one of the major uncertainties that still remains unresolved before we start entirely doubting the standard theory of cosmology.' A new view Last year, data from the Dark Energy Spectroscopic Instrument in Arizona in the US suggested that the push of dark energy — represented by the cosmological constant lambda in the ΛCDM model — is weakening and that the universe may actually be decelerating over time. The possibility of dark energy getting weaker means that the pace of expansion of the universe will eventually slow down and may, at some point, even turn negative. In that case, it is not inconceivable that the universe will collapse in on itself in a 'big crunch'. In any case, the task of updating the ΛCDM model will become easier when the Rubin Legacy Survey of Space and Time (LSST) begins operating later this year. The LSST will launch from the Vera C. Rubin Observatory being built in northern Chile, using its camera — the largest ever built — to peer back in space and time like never before. Who knows what answers this unparalleled wide-field astronomical survey of the universe, wider and deeper than all previous surveys combined, will provide to questions we can't even imagine now about the mysteries of the universe… Prakash Chandra is a science writer.
Yahoo
20-03-2025
- Science
- Yahoo
Dark energy is even stranger than we thought, new 3D map of the universe suggests. 'What a time to be alive!' (video)
When you buy through links on our articles, Future and its syndication partners may earn a commission. New results from the Dark Energy Spectroscopic Instrument (DESI) suggest that the unknown force accelerating the expansion of the universe isn't what we believed it to be. This hints that our best theory of the universe's evolution, the standard model of cosmology, could be wrong. The newly released DESI data comes from its first three years of observations collected as the instrument, mounted on the Nicholas U. Mayall 4-meter Telescope at Kitt Peak National Observatory, continues to build the largest 3D map of the universe ever created. By the time DESI completes its five-year mission next year, the instrument will have measured the light from an estimated 50 million galaxies and black hole-powered quasars, in addition to the starlight of over 10 million stars. It is the capability of DESI to capture light from 5,000 galaxies simultaneously that makes it the ideal instrument to conduct a survey large enough to investigate the properties of dark energy. This new analysis focuses on data from the first three years of DESI observations, encompassing nearly 15 million of the best-measured galaxies and quasars. "The universe never ceases to amaze and surprise us," DESI Project Scientist Arjun Dey said in a statement. "By revealing the evolving textures of the fabric of our universe as never before, DESI and the Mayall telescope are changing our very understanding of the future of our universe and nature itself." Dark energy is the placeholder name given to whatever aspect of the universe is causing the fabric of spacetime to inflate faster and faster, constantly pushing galaxies apart more rapidly. It is thought to account for around 70% of the universe's matter and energy. The mysterious "stuff" called dark matter makes up another 25%, and ordinary matter comprising stars, planets, moons, our bodies and the cat next door accounts for just 5%. Essentially, everything we understand about the universe, including all of chemistry and biology is wrapped up in that 5%! The current "best guess" at the identity of dark energy is the cosmological constant, the vacuum energy of energy space, which is baked into the pie we call the standard model of cosmology or the Lambda Cold Dark Matter (LCDM) model. However, this model is built on the presumption that dark energy, represented by the Greek letter lambda (Λ), is constant over time. Vacuum energy describes the density of particles popping in and out of existence. While "something" appearing from "nothing" sounds crazy, you can think of it as the universe having an overdraft facility. Pairs of virtual particles are allow to "borrow" some energy from the cosmos to come into existence as long as they pay it back by meeting and annihilating each other. When taken in isolation, the DESI findings don't actually challenge the picture of dark energy developed in the LCDM model. It is when the DESI data is compared with other measurements of the cosmos that problems with the cosmological constant start to manifest. DESI is hinting, and not for the first time, that dark energy isn't constant but is changing over time. Specifically, this accelerating "push" seems to be weakening. These measurements include our observations of a "fossil" light left over from an event that happened shortly after the Big Bang called the "last scattering," when the universe had expanded and cooled enough to allow electrons to bond with protons and form the first neutral atoms. The disappearance of free electrons suddenly allowed photons, the particles that make up light, to travel freely. In other words, it was as if a universal fog had lifted, and the cosmos became transparent. This first light is referred to as the "cosmic microwave background" or "CMB," and it can still be observed today. Tiny variations or "wrinkles" were "frozen into" the CMB by fluctuations in the density of matter in the early universe called baryon acoustic oscillations (BAO). As the cosmos continued to expand, so too did these wrinkles. Thus, BAO wrinkles can act as a standard measuring stick of the expansion of the universe, with their size varying at different cosmic times. This variation arises as a result of how fast the universe was expanding at those times. Thus, measuring the BAO reveals the strength of dark energy throughout the history of the cosmos, and DESI can do this more precisely than any other instrument. Changes in dark energy itself were also hinted at when DESI data was compared with observations of type Ia supernovas, cosmic explosions that occur when white dwarf stars "overfeed" on a companion star. This stolen material piles up on the surface of the stellar remnant until a thermonuclear runaway is triggered. Type Ia supernovae are so uniform in terms of their light output that astronomers can use them as "standard candles" for measuring cosmic distances. In fact, type Ia supernovas were integral to the discovery that the expansion of the universe is accelerating, the genesis of dark energy, back in 1998. These distance measurements are possible because of a phenomenon called "redshift," which occurs when the wavelength of traveling light is stretched as it crosses the expanding universe. The longer the light has traveled, the more extreme the shift toward the long wavelength "red end" of the electromagnetic spectrum. That means measuring the redshift of a very well-known and consistent source of light, a standard candle, can give distance measurements. DESI data can also be combined with observations of an effect called "gravitational lensing," the distortion of light from distant galaxies by foreground objects of great mass to show the signature of evolving dark energy. The evolution of dark energy isn't robust enough to be considered a "discovery" just yet, but different combinations of the data with other observations are pushing this concept toward what is considered the "gold standard" in physics for such a determination. In addition to unveiling these latest dark energy results on Wednesday (March 19), the DESI collaboration also announced that its Data Release 1 (DR1) is now available for anyone to explore through the National Energy Research Scientific Computing Center (NERSC). DR1 contains information regarding 18.7 million cosmic objects, including roughly 4 million stars, 13.1 million galaxies, and 1.6 million quasars. Luz Ángela García Peñaloza, a former DESI team member and a cosmologist at the Universidad ECCI in Colombia, is just one scientist who is thrilled with the new DESI results and the fact that DR1 is now available to the general astronomical community. told "I am also really excited to find out DESI has released redshift information of about 19 million galaxies and quasars. We've increased the number of identified galaxies by an order of magnitude in less than 10 years!" García Peñaloza said. "The most fascinating result of all is that different sets of observations, a combination of BAO from DESI with CMB data from Planck, and the three main sets of luminosity distances of type Ia supernovas are making a stronger case for an evolving dark energy model, disfavoring the cosmological constant. "This is getting more and more consistent with other independent cosmological tests that seem to be opening a window of opportunity for new ways to explore and study dark energy and the accelerated expansion of the universe." Related Stories: — 'Mind-blowing' dark energy instrument results show Einstein was right about gravity — again — In a way, and the dark universe grew up together — Dark energy could be getting weaker, suggesting the universe will end in a 'Big Crunch' The availability of the DR1 data means astronomers outside the DESI collaboration can now dive into this vast dataset collected between May 2021 and June 2022. "Our results are fertile ground for our theory colleagues as they look at new and existing models, and we're excited to see what they come up with," DESI director Michael Levi, a scientist at Berkeley Lab, said. "Whatever the nature of dark energy is, it will shape the future of our universe. "It's pretty remarkable that we can look up at the sky with our telescopes and try to answer one of the biggest questions that humanity has ever asked.' Meanwhile, the DESI collaboration is preparing to begin additional analyses of the new dataset to extract even more findings as DESI itself continues collecting data during its fourth year of operations. "Just amazing," García Peñaloza concluded. "What a time to be alive and to be a cosmologist!" The DESI data is discussed in a series of papers available here.
