By Brian Santo, contributing writer
Ever lost your phone and your spouse said, “Have you looked in the couch?” and of course you looked in the couch, it was the first place you looked, and it wasn’t there, and the spouse looks between the cushions in the couch, and of course they find your phone there? Now imagine being an astronomer and you’ve misplaced roughly 85% of the mass in the universe only to have someone else discover that most of it was right there in the sofa all along. Now where are the car keys?
Two sets of researchers simultaneously determined that roughly half of the so-called dark matter in the universe slipped into the space between the galaxies, pretty much where scientists originally expected that it might be and looked for it but missed it anyway.
Physicists and astronomers have observed how the universe is behaving. They know, for example, that the universe is expanding and at what rate. They’ve detected everything that’s visible in the universe — stars and galaxies, dust and planets, black holes and nebulae — and estimated their accumulated mass. They have observed how galaxies behave.
They constructed a mathematical model of the universe, and the short story is that the math didn’t work. The formula was almost certainly sound, but given the evidence, the value for one factor — mass — had to be wrong. By a lot. All of the visible matter in the universe together adds up to roughly 5% of the mass that the universe should have for galaxies to behave as they do.
The model says that the remaining 95% has to be some energy but mostly ordinary baryonic matter: neutrons, protons, electrons.
Scientists’ favored hypothesis was that the mass is there, we just can’t see it with instruments that detect signals anywhere along the electromagnetic spectrum — not gamma rays, not low-frequency radio, and certainly not visible light. Perhaps the missing baryonic material was in the form of exceedingly diffuse gas, which would explain why it was eluding detection.
This has been conjecture for decades, but now a group of researchers at the University of Edinburgh in Scotland and a separate international team (Canada, England, South Africa) working with the Institute of Space Astrophysics in France have found roughly half of the dark matter that the model says should be out there. And as expected, it’s in the form of filaments of diffuse, low-temperature gas that stretch between galaxies.
Because the missing mass is dark matter and not directly observable, both teams had to find other evidence of its existence. Both relied on a phenomenon called the Sunyaev-Zel’dovich (SZ) effect. A 1999 paper from Physics Reports explains that the effect “causes a change in the apparent brightness of the Cosmic Microwave Background Radiation toward a cluster of galaxies or any other reservoir of hot plasma.”
We know that galaxies are surrounded by halos of gaseous matter. The Edinburgh group surveyed galactic halos and measured the SZ effect. Based on their results, they believe that these halos might hold as much as 30% of the missing matter, as they explain in the paper (the summary in this paper is by far the more readable of the two for non-cosmologists).
The international group measured the SZ effect between roughly 260,000 pairs of luminous red galaxies, looking not only at halos but also the gas filaments that they expected would stretch between each pair of LRGs. This material may represent as much as half of the missing matter, according to their paper .
Hideki Tanimura, one of the members of the international team and a professor at the University of British Columbia told New Scientist that he’d read the Edinburgh paper and said that there was no real disagreement between their findings on how much of the missing matter has been found.
“We expect some differences because we are looking at filaments at different distances,” he said. “If this factor is included, our findings are very consistent with the other group.”
Now to find the other half, which both teams speculate might be in gaseous filaments that we have yet to pinpoint and measure.
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