The cosmos plays hide-and-seek. Sometimes, though, even when astronomers have a hunch for where their prey might hide, it can take them decades of searching to confirm it. The case of the universe’s missing matter — a case that appears to now be closed, as I reported earlier this month — is one such instance. To me, it is a fascinating tale in which clever cosmological models drew a treasure map that took 20 years to explore.
Scientists knew back in the 1980s that they could observe only a fraction of the atomic matter — or baryons — in the universe. (Today we know that all baryons taken together are thought to make up about 5 percent of the universe — the rest is dark energy and dark matter.) They knew that if they counted up all the stuff they could see in the universe — stars and galaxies, for the most part — the bulk of the baryons would be missing.
But exactly how much missing matter there was, and where it might be hiding, were questions that started to sharpen in the 1990s. Around that time, astronomer David Tytler of the University of California, San Diego, came up with a way to measure the amount of deuterium in the light of distant quasars — the bright cores of galaxies with active black holes at their center — using the new spectrograph at the Keck telescope in Hawaii. Tytler’s data helped researchers understand just how many baryons were missing in today’s universe once all the visible stars and gas were accounted for: a whopping 90 percent.
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