A giant 'wall' of galaxies has been found to extend across the universe
Albert Einstein's equations that he devised to explain the workings of gravity in the physical Universe at the beginning of the 20th century still hold sway.
In one of the largest tests of general relativity to date, a large team of astronomers has mapped the distribution of nearly 6 million galaxies over the 11 billion years of the universe's history.
The way gravity binds these galaxies together along the strands of the cosmic web against the outward pull of the Universe's expansion, and the way the web evolves over time, is exactly in line with the predictions made by Einstein's famous theory.
Perhaps, it is the largest test of general relativity to date, covering much of the 13.8 billion year history of the Universe – meaning the theory holds up on very large and very small scales.
The findings have been submitted for publication, and are available in three new articles uploaded to arXiv prior to peer review.
“General relativity has been tested very well on the scale of solar systems, but we also needed to check that our assumptions work on much larger scales,” says cosmologist Pauline Zarrouk of the French National Center for Scientific Research.
“Studying the rate at which galaxies are formed allows us to test our theories directly and, so far, is consistent with what is predicted by general relativity on cosmological scales.”
Gravity is fundamental to how the Universe works. We don't know what it is or what it is, it's just that hard things tend to attract other things in abundance; that the force of attraction is directly proportional to mass; and that it changes the geometry of space-time around the mass.
It also acts as the glue that holds the Universe together. Vast strands of gravitational fields produced by dark matter stretch across the Universe in a web-like fashion; and many things in the Universe are distributed in the threads and nodes of this cosmic web.
It is predictable and measurable and, so far, best constrained and explained by the theory of general relativity. But finding flaws in this theory may reveal solutions to some surprising problems, such as the irreconcilable differences between quantum mechanics and classical physics. So scientists keep probing it to see what the contents of the Universe look like if general relativity means it should, at all scales.
This brings us to the Lawrence Berkeley National Laboratory led by the Dark Energy Spectroscopic Instrument (DESI), a major international collaboration currently working to map the visible Universe to reveal its greatest secrets. Effective from 2019; the new results are based on a detailed and extended analysis of just the first year of data obtained by the instrument.
The DESI Collaboration used that data to conduct a painstaking study of 5.7 million galaxies and quasars throughout Earth's history, mapping their growth, evolution, and distribution in the cosmic web since the beginning of the Universe 11 billion years ago.
This simulation shows how gravity changes the distribution of galaxies in the Universe. (Claire Lamman and Michael Rashkovetskyi/DESI collaboration)
They used the theory of general relativity to predict the growth and distribution of the cosmic web, and found that the Universe we live in behaves as relativity predicts, on a large cosmic scale. Add more gravity, or remove some, and the Universe will never look the same.
The result follows a paper earlier this year that measured the rate of expansion of the universe based on the cosmic remnants of acoustic waves that intensify when the atomic fog that filled the early atmosphere is removed. The DESI Collaboration hopes that ongoing efforts will continue to shed light on the evolution of the Universe and the mysterious forces that drive it.
“This is the first time that DESI has observed the growth of cosmic structure,” said physicist Dragan Huterer of the University of Michigan. “We demonstrate an excellent new ability to probe modified gravity and improve constraints on dark energy models. And it's just the tip of the iceberg.”
DESI is at the Mayall Telescope in Arizona, seen here during the 2023 Geminid shower. (KPNO/NOIRLab/NSF/AURA/R. Sparks)
The results also put constraints on the upper limit of the mass of the neutrino, a particle that is so 'ghostly' that we could not measure it accurately.
Research is ongoing, as is the work of the Collaboration. Researchers are currently analyzing data from DESI's first three years of operation. By the time the instrument completes its mission, it will have collected data on more than 40 million galaxies and quasars.
Among the biggest hopes is that it will help to reveal the nature of dark matter, something mysterious that cannot be seen something is responsible for generating more gravity in the Universe; and dark energy, the unseen mystery something is responsible for driving the accelerating dynamic expansion of the Universe.
“Dark matter makes up about a quarter of the Universe, and dark energy makes up the other 70 percent, and we don't really know which one,” said physicist Mark Maus of Lawrence Berkeley National Laboratory and the University of California Berkeley.
“The idea that we can take pictures of the Universe and tackle these big, fundamental questions is amazing.”
The group's papers are now available on the arXiv preprint server.
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