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| An artist’s impression of the cosmic web. It looks like a vast cobweb-like structure or mostly purple and some orange filaments on a black background. (Image credit: Volker Springel (Max Planck Institute for Astrophysics) et al.) |
The cosmic web is a gigantic network of crisscrossing celestial superhighways
that connects nearly everything in the universe.
Scientists have made one of the most precise maps of the universe's matter,
and it shows that something may be missing in our best model of the cosmos.
Created by pooling data from two telescopes that observe different types of
light, the new map revealed that the universe is less "clumpy" than previous
models predicted — a potential sign that the vast cosmic web that connects
galaxies is less understood than scientists thought.
According to our current understanding, the cosmic web is a gigantic network
of crisscrossing celestial superhighways paved with hydrogen gas and dark
matter. Taking shape in the chaotic aftermath of the Big Bang, the web's
tendrils formed as clumps from the roiling broth of the young
universe; where multiple strands of the web intersected, galaxies
eventually formed. But the new map, published Jan. 31 as three separate
studies in the journal Physical Review D, shows that in many parts of the
universe, matter is less clumped together and more evenly spread out than
theory predicts it should be.
"It seems like there are slightly less fluctuations in the current universe
than we would predict assuming our standard cosmological model anchored to the
early universe," co-author Eric Baxter, an astrophysicist at the University of
Hawaii, said in a
statement.
Spinning the cosmic web
According to the standard model of cosmology, the universe began taking
shape after the Big Bang, when the young cosmos swarmed with particles of
both matter and antimatter, which popped into existence only to annihilate
each other upon contact. Most of the universe's building blocks wiped
themselves out this way, but the rapidly expanding fabric of space-time,
along with some quantum fluctuations, meant that some pockets of the
primordial plasma survived here and there.
The force of gravity soon compressed these plasma pockets in on themselves,
heating the matter as it was squeezed closer together to such an extent that
sound waves traveling at half the speed of light (called baryon acoustic
oscillations) rippled outward from the plasma clumps. These ripples pushed
away the matter that hadn't already been drawn into the center of a clump,
where it came to rest as a halo around it. At that point, most of the
universe's matter was distributed as a series of thin films surrounding
countless cosmic voids, like a nest of soap bubbles in a sink.
Once this matter, primarily hydrogen and helium, had sufficiently cooled, it
clotted further to birth the first stars, which, in turn, forged heavier and
heavier elements through nuclear fusion.
To map out how the cosmic web was spun, the researchers combined
observations taken with the Dark Energy Survey in Chile — which scanned the
sky in the near-ultraviolet, visible and near-infrared frequencies from 2013
to 2019 — and the South Pole Telescope, which is located in Antarctica and
studies the microwave emissions that make up the cosmic microwave background
— the oldest light in the universe.
Though they look at different wavelengths of light, both telescopes use a
technique called gravitational lensing to map the clumping of matter.
Gravitational lensing occurs when a massive object sits between our
telescopes and its source; the more that light coming from a given pocket of
space appears warped, the more matter there is in that space. This makes
gravitational lensing an excellent tool for tracking both normal matter and
its mysterious cousin dark matter, which, despite making up 85% of the
universe, doesn't interact with light except by distorting it with gravity.
With this approach, the researchers used data from both telescopes to
pinpoint the location of matter and weed out errors from one telescope's
data set by comparing it to the other's.
"It functions like a cross-check, so it becomes a much more robust
measurement than if you just used one or the other," co-lead author
Chihway Chang, an astrophysicist at the University of Chicago, said in the statement.
The cosmic matter map the researchers produced closely fitted our
understanding of how the universe evolved, except for a key discrepancy: It
was more evenly distributed and less clumped than the standard model of
cosmology would suggest.
Two possibilities exist to explain this discrepancy. The first is that we're
simply looking at the universe too imprecisely, and that the apparent
deviation from the model will disappear as we get better tools to peer at
the cosmos with. The second, and more significant, possibility is that our
cosmological model is missing some seriously big physics. Finding out which
one is true will take more cross-surveys and mappings, as well as a deeper
understanding of the cosmological constraints that bind the universe's soap
suds.
"There is no known physical explanation for this discrepancy," the
researchers wrote in one of the studies. "Cross-correlations between surveys
… will enable significantly more powerful cross-correlation studies that
will deliver some of the most precise and accurate cosmological constraints,
and that will allow us to continue stress-testing the [standard
cosmological] model."
Reference:
Y. Omori et al, Joint analysis of Dark Energy Survey Year 3 data and CMB
lensing from SPT and Planck . I. Construction of CMB lensing maps and
modeling choices, Physical Review D (2023).
DOI: 10.1103/PhysRevD.107.023529
C. Chang et al, Joint analysis of Dark Energy Survey Year 3 data and CMB
lensing from SPT and Planck . II. Cross-correlation measurements and
cosmological constraints, Physical Review D (2023).
DOI: 10.1103/PhysRevD.107.023530
T. M. C. Abbott et al, Joint analysis of Dark Energy Survey Year 3 data and
CMB lensing from SPT and Planck . III. Combined cosmological constraints,
Physical Review D (2023).
DOI: 10.1103/PhysRevD.107.023531