Between September 2017 and June 2018, three separate groups of astronomers—all working independently from one another, besides their shared access to the same universal research—managed to locate and identify all of the remaining (non-dark) matter in the universe. That sounds huge, right? And also unfathomable?
Accounting for all matter the universe is pretty mind boggling, but it’s not nearly as impossible as it sounds. So let’s break down what it actually means.
The Big Bang has been the leading cosmological model for the birth of the universe since around the 1940s. If we start from the idea that everything began from the same spot with a single act of nucleosynthesis, and we know that matter can neither be created nor destroyed, then it stands to reason that there would a finite amount of matter in the universe. Which means that we could theoretically estimate how much stuff there is scattered throughout the great abyss. Scientists managed to confirm some of these numbers by studying the light from the residual radiation leftover from the Big Bang (also known as the cosmic microwave background), and eventually figured out how many particles of “ordinary” or baryonic matter—liquids, solids, plasmas, and gases—should be available throughout the universe.
Over the decades, different groups of astronomers tried to count up all the observable matter they could find—stars, clouds, gases, planets, and so on—in order to account for the expected quantity of matter, but until recently, all of them had fallen short of the estimated total. As of 2014, the collective work of astronomers had still only managed to inventory about 70 percent of the ordinary matter that should have been there.
Also, that ordinary matter only actually comprises about 15 percent of the total mass of matter in the universe; the rest of it is is believed to be made up of mysterious dark matter.
Even on a massive cosmological scale, this was still disappointing. Either these scientists were missing something or the theory of Big Bang Nucelosynthesis that they had based their work around was fundamentally flawed. The only way to know for certain was to take a closer look at the warm-hot intergalactic medium, also known as the WHIM.
Imagine the universe at the moment of its inception as one of those bags of cheap fake spiderwebs you buy at Halloween: a little wad of tangled strands that you can compress even tighter with your fist, or stretch across the bushes or basement or wherever else you lay your haunted happenings. As you stretch it out—that’s our Big Bang, in this metaphor—the strands of webbing slowly get thinner, less dense, and more translucent, until all that’s left is an impossibly thin latticework of white tendrils that are all-but invisible to the eye.
That all-but-invisible net of too-thin webbing? That’s the WHIM. Except instead of artificial spiderwebs, it’s hot cosmic gas.
It’s one thing to know that there’s low-density matter spread out in the WHIM; it’s another thing entirely to observe it for yourself and confirm that it exists.
“It was clear from the early days of cosmological simulations that many of the baryons would be in a hot, diffuse form—not in galaxies,” Ian McCarthy, an astrophysicist at Liverpool John Moores University, told Quanta, which first reported the discovery.
Unfortunately, the distortions of light they expected to find in the WHIM (indicating the presence of electrons in hot, ionized gas) were still too faint to register on modern research equipment—at least, until the last year, when three international research groups found new ways of seeing, all while working independently from one another.
As one of the teams, based at the University of Edinburgh wrote in a study on arXiv, “the fact that two independent studies using two different catalogues achieve similar conclusions provides strong evidence for the detection of gas filaments.” (At the time of this writing, their research is still under peer review, so the scientists declined to answer questions from Motherboard.)
The Edinburgh researchers, led by Anna de Graaff, took existing models of interacting galaxy pairs, then zoomed in and stacked a million color-shifted images on top of one another to amplify the faintest presence of any matter that fell within the redshift range. If we go back to our store-bought spiderweb metaphor, it’s like taking a photo of that single stretched-out-to-the-point-of-nigh-invisibility strand, stripping out the rest of the photo, then layering it on top of itself enough times that thinnest string appears more clearly. A million layers of nothing would still appear as nothing; but a million layers of something, however faint, would show up on a screen.
And that’s precisely what happened for the Edinburgh team, just as they predicted. Even more remarkable was that the quantity of matter they observed was equal to that missing gap in public research.
Under the leadership of Hideki Tanimura, researchers at the Institute of Space Astrophysics took a similar approach—which yielded similar results. “The measurement [of electron pressure in the WHIM] is challenging due to the morphology of the source and the relative weakness of the signal,” they explained in a paper in ArXiv. “The discrepancy may hint to the presence of diffuse intercluster gas in the supercluster.”
Three teams of independent researchers reached the same conclusions about the missing matter, all around the same time
Using their own method of stacking color-shifted data, Tanimura’s team found comparable evidence to the Edinburgh research group—enough to conclude, with reasonable confidence, that they had located the very missing matter they’d been looking for, just like the team at Edinburgh.
But reasonable confidence is not enough for science. “One always worries about ‘weak signals’ that are the result of combining large numbers of data,” Michael Shull, an astronomer at the University of Colorado at Boulder, told Quanta. “As is sometimes found in opinion polls, one can get erroneous results when one has outliers or biases in the distribution that skew the statistics.”
That’s where the third team of researchers comes in, a collective group from multiple institutions led by Fabian Nicastro from the National Institute of Physics in Rome. Nicastro’s group had also realized the need to amplify the faint signals from the sparse and distant in the WHIM. Rather than relying on stacked imaging models, however, these researchers looked to a distant quasar as a lighthouse, to see what was reflected in the cobweb mists of the WHIM. Usually, when astronomers use this method of observing the universe, they search for traces of light-absorbing hydrogen. But that doesn’t work in the heat of the WHIM, so Nicastro’s team had to find another way. They relied instead on oxygen particles, which is less populous in the WHIM, but doesn’t get stripped down and ionized to the point that it can’t retain light like hydrogen does.
Once they identified the amount of oxygen between the Earth and their target quasar, Nicastro’s team extrapolated their findings to span the whole known universe—and sure enough, they landed on the same 30 percent of matter that they knew should have been there all along.
That’s how three teams of independent researchers reached the same conclusions about the missing matter, all around the same time. So what does that mean in a larger, astronomical context? The matter has always been there; and we’ve already spent years expecting it would be there, precisely where we found it. In a puzzle as large as the universe itself, we not only predicted all of the matter that could have possibly been out there, but we managed to account for it all, too. That’s pretty wild! And it moves us that much closer to confirming our beliefs about our cosmological origins—which brings us one spacewalk-sized step towards a larger understanding of the universe.