Image: NASA via Shutterstock
Between September 2017 and June 2018, three separate groups of astronomers – all working independently of each other, in addition to their shared access to the same universal search – succeeded in locating and identifying all the remaining (non-dark) matter in the universe. Sounds huge, right? And also unfathomable?
Accounting for all the matter in the universe is mind-boggling enough, but it’s not as impossible as it seems. So let’s break down what that actually means.
The Big Bang has been the primary cosmological model for the birth of the universe since around the 1940s. If we start from the idea that everything started in the same place 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 be a finite quantity of matter in the universe. Which means we could theoretically estimate the amount of stuff scattered across the great abyss. Scientists were able to confirm some of these numbers by studying the residual radiation light from the Big Bang (also known as the Cosmic Microwave Background) and finally 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 have tried to count all the observable matter they could find – stars, clouds, gases, planets, etc. – in order to account for the amount of material expected, but until recently all were less than the estimated total. As of 2014, the collective work of astronomers had only managed to inventory about 70% of the ordinary matter that should have been there.
Moreover, this ordinary matter is actually only about 15% of the total mass of matter in the universe; the rest is thought to be mysterious dark matter.
Even on a massive cosmological scale, it was still disappointing. Either these scientists were missing something, or the Big Bang theory of nucleosynthesis on which they based their work was fundamentally flawed. The only way to know for sure was to take a closer look at the hot-hot intergalactic medium, also known as WHIM.
Imagine the universe at the moment of its creation like one of those bags of cheap fake cobwebs you buy on Halloween: a small bundle of tangled strands that you can squeeze even tighter with your fist, or stretch to through the bushes or the basement or anywhere else. lay down your haunted events. As you stretch it – it’s our Big Bang, in this metaphor – the strands of webbing slowly become thinner, less dense and more translucent, until there is only one Incredibly thin trellis of white tendrils that are almost invisible to the eye.
This almost invisible net of straps that are too thin? It’s the whim. Except that instead of artificial cobwebs, it’s hot cosmic gas.
It is one thing to know that there is low density matter scattered throughout the WHIM; it is quite another to observe it for oneself and confirm that it exists.
“From the earliest days of cosmological simulations, it was clear that many baryons would be in hot, diffuse form, and not in galaxies,” Ian McCarthy, an astrophysicist at Liverpool John Moores University, told Quanta, who was the first to report the discovery.
Unfortunately, the light distortions they expected to find in the WHIM (indicating the presence of electrons in a hot, ionized gas) were still too faint to register on modern research equipment – at least until last year, when three international research groups discovered new ways of seeing, while working independently of each other.
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 catalogs reached similar conclusions provides strong evidence for the detection of filaments gas”. (As of this writing, their research is still under peer review, so the scientists declined to answer Motherboard’s questions.)
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 each other to amplify the faintest presence of any material in the redshift range. If we go back to our store-bought spider web metaphor, it’s like taking a photo of that single strand stretched to the point of near-invisibility, removing the rest of the photo, and then superimposing it on top by itself enough times for the thinnest string to appear more clearly. A million layers of nothing would still appear as nothing; but a million layers of something, however faint, would appear on a screen.
And that is precisely what happened for the Edinburgh team, just as they predicted. What’s even more remarkable is that the amount of matter they observed was equal to this missing gap in public research.
Led by Hideki Tanimura, researchers at the Institute for Space Astrophysics took a similar approach, which yielded similar results. “The measure [of electron pressure in the WHIM] is difficult due to the morphology of the source and the relative weakness of the signal,” they explained in an article in ArXiv. “The gap may hint at the presence of diffuse intercluster gas within the supercluster.”
Three teams of independent researchers came to the same conclusions about the missing matter, around the same time
Using their own color-shifted data stacking method, Tanimura’s team found evidence comparable to that of the Edinburgh research group – enough to conclude, with reasonable confidence, that they had located the material. missing they were looking for, just like the Edinburgh team.
But reasonable confidence is not enough for science. “There’s always a concern about ‘weak signals’ resulting from the combination of a lot of data,” Michael Shull, an astronomer at the University of Colorado at Boulder, told Quanta. “As sometimes found in opinion polls, you can get erroneous results when you have outliers or biases in the distribution that skew the statistics.”
This is where the third team of researchers comes in, a collective multi-institution group led by Fabian Nicastro of the National Institute of Physics in Rome. Nicastro’s group had also realized the need to amplify the weak signals from the sparse and distant in the WHIM. Rather than relying on stacked imagery patterns, however, these researchers looked to a distant quasar like a beacon, to see what was reflected in the cobweb-like mists of WHIM. Usually, when astronomers use this method of observing the universe, they look for traces of light-absorbing hydrogen. But it doesn’t work in the heat of WHIM, so Nicastro’s team had to find another way. Instead, they relied on oxygen particles, which are less populated in the WHIM, but aren’t stripped and ionized to the point that they can’t hold light like hydrogen does.
Once they identified the amount of oxygen between Earth and their target quasar, Nicastro’s team extrapolated their findings to cover the entire known universe – and sure enough, they landed on the same 30% of matter they knew they should have been there all along.
This is how three teams of independent researchers came to the same conclusions about the missing matter, at about the same time. So what does this mean in a larger astronomical context? The question has always been there; and we have already spent years expecting it to be there, precisely where we found it. In a puzzle as vast as the universe itself, we not only predicted all the matter that could have existed, but we also managed to account for it. It’s pretty wild! And it brings us that much closer to confirming our beliefs about our cosmological origins, bringing us one spacewalk-sized step toward a broader understanding of the universe.