It would be silly to think we completely understand our universe, given how small the Earth is compared to the vastness of the cosmos. But from here on our tiny planet, it appears that much of the universe is missing. And I'm not just talking about dark matter. Regular stuff seems to be missing, too.
The cosmic web model of the universe. Graphic: Andrew Pontzen/Fabio Governato (Wikimedia Commons)
Astronomy fans probably know that as far as humans can tell, the universe is composed mostly of some mysterious, unexplained energy called dark energy that pushes it apart. The remaining piece, about a quarter, is dark matter, another unexplained thing that seems to build the universe's skeleton. Just four per cent is the regular matter that we can see: Stars, planets, and interstellar and intergalactic gas.
But the observed amount of this regular matter still falls perhaps a third short of the amount of stuff that physicists think should exist based on their models of the universe.
Specifically, scientists are looking for baryons, particles made from quarks that make up the nuclei inside of atoms. Scientists claimed to have found this missing stuff before. Notably, two papers published last year reported the discovery of a filament of hot gas connecting galaxies. A 2014 paper found extra stuff in the halos surrounding galaxies like the Milky Way, which may account for the missing matter in these types of objects.
Another group of scientists now says it has found the true source of the missing stuff, following an intense search using a space-based telescope.
"We were granted a lot of time," astrophysicist Fabrizio Nicastro from the Italian National Institute of Astrophysics told Gizmodo. "And we got this detection."
Nicastro's team says that "the missing baryons have been found", in a paper published today in Nature. What they really did was observe the light coming from one very bright source several billion light-years away. As this light approaches Earth, it stretches due to the expanding universe, so longer wavelengths indicate older light. At two distances, they observed what looked like a filter made of positively-charged oxygen atoms that absorbed the light.
This filter would be made by the "WHIM", or warm-hot intergalactic medium - hot gas in between galaxies. That, says Nicastro, would be the source of the missing matter; this gas would be the missing baryons.
The observation took a lot of time - it was the longest observation performed by the European Space Agency's XMM-Newton space telescope, over 20 days in two sessions.
Assistant professor Jessica Rosenberg from George Mason University thought the paper was certainly interesting. "It's a tantalising result," she said. But extrapolating a single source of light with two absorptions to account for all of the missing matter "is a bit of a leap, in my opinion," she said.
Others were harsher. "This is one detection, and so deriving a cosmic mean density of hot gas is a bold thing to do - and not necessarily in a good way," said Jason Tumlinson, astronomer at the Space Telescope Science Institute. "Usually you would need a larger sample to be confident in the derived mass budget."
Rosenberg thought the study was worthwhile, given the significant investment of time. But she said that the journal Nature has a tendency to oversell astronomy results, something also suggested by a few astronomers on Twitter. She surmised that readers might not be excited about the results unless scientists claim to have solved one of the universe's problems. Those kinds of paradigm-shifting papers come infrequently.
This might call to mind another recent controversy, in which a Nature paper claimed to have found a galaxy that was "lacking dark matter". But others thought there wasn't enough data to make such a claim - similar to what is occurring with this paper.
Hunting for the WHIM will continue regardless. Nicastro told Gizmodo that existing and future missions with better instrumentation will continue to make observations, and that seven other sources of light have been selected to look for the gas.
As usual, science is a process. And if you make bold claims, people will inevitably push back.