A gargantuan effort by a huge international team of scientists has just provided us with the most accurate map of all matter in the Universe obtained to date.
By combining data from two major surveys, the international collaboration has revealed where the Universe does and doesn’t keep all of its junk – not just the normal matter that makes up planets, stars, dust, black holes, galaxies, but dark matter, too: the mysterious invisible mass generating more gravity than normal matter can account for.
The resulting map, showing where matter has collected over the 13.8 billion year lifespan of the Universe, will be a valuable reference for scientists seeking to understand how the Universe has evolved.
Indeed, the results already show that matter is not quite distributed as we thought, suggesting that there could be something missing in the current standard model of cosmology.
According to current models, at the time of the Big Bang, all matter in the Universe condensed into a singularity: a single point of infinite density and extreme heat that suddenly burst and spewed out quarks that rapidly combined to form a soup of protons, neutrons and nuclei. Hydrogen and helium atoms appeared a few hundred thousand years later; from these, the entire universe was created.
How those early atoms spread out, cooled, clumped together, formed stars, rocks and dust, is detective work based on how the Universe around us looks today. And one of the main clues that we’ve used is where all the stuff is now, because then scientists can go back and figure out how it got there.
But you can’t see everything. In fact, most of the matter in the Universe – about 75% – is completely invisible to our current detection methods.
We only detected it indirectly, because it creates stronger gravitational fields than there should be just based on the amount of normal matter. This manifests itself in phenomena such as galaxies spinning faster than they should, and a little quirk of the Universe we call gravitational lensing.
When something in the Universe has sufficient mass – say, a cluster of thousands of galaxies – the gravitational field around it becomes strong enough to influence the curvature of spacetime itself.
This means that any light that passes through this region of space does so along a curved path, resulting in distorted and magnified light. These lenses are also stronger than they should be if they were only created by normal matter.
To map matter in the Universe, the researchers compared gravitational lensing data collected by two different surveys – the Dark Energy Survey, which collected data in near-ultraviolet, visible and dark wavelengths. near infrared; and the South Pole Telescope, which collects data on the cosmic microwave background, the faint traces of radiation left by the Big Bang.
By comparing these two sets of data taken by two different instruments, researchers can be much more certain of their results.
“It works like a cross-check, so it becomes a much more robust measure than if you just used one or the other,” says astrophysicist Chihway Chang from the University of Chicago, who was the lead author. of one of the three articles describing the work.
The main authors of the other two articles are physicist Yuuki Omori of the Kavli Institute for Cosmological Physics and the University of Chicago, and telescope scientist Tim Abbott of NOIRLab’s Cerro Tololo Inter-American Observatory.
The resulting map, based on the positions of the galaxies, the galaxy lens and the cosmic microwave background lens, can then be extrapolated to infer the distribution of matter in the Universe.
This map can then be compared to models and simulations of the evolution of the Universe to see if the observed distribution of matter matches the theory.
The researchers made comparisons and found that their map mostly matched current models. But not quite. There were some very slight differences between observation and prediction; the distribution of matter, the researchers say, is less clumped, more evenly spaced than the models predict.
This suggests that our cosmological models could use an adjustment.
That’s hardly a surprise – there are some discrepancies between cosmological observation and theory that seem to suggest we’re missing a trick or two somewhere; and the team’s findings are consistent with previous work – but the more accurate and complete our data, the more likely we are to resolve these discrepancies.
There is more work to do; the results are not yet certain. Adding additional surveys will help refine the map and validate (or negate) the team’s findings.
And, of course, the map itself will help other scientists conduct their own investigations into the mysterious and murky history of the Universe.
The research has been published in Physical examination D. All three articles are available on the arXiv preprint server and can be found here, here, and here.