Scientists Find Universe’s Missing Atoms

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For years, scientists have been trying to answer the big questions: Why rainbows are so awesome, what makes cats so… weird, and just where the hell is most of the “Stuff” in the universe. We might not have made any progress on those first two, but a team of researchers have managed to find more than half of the “missing” atoms in the universe.

These atoms were described as missing or absent because physicists have known for some time roughly how much matter came into being shortly after the big bang. According to their models, 70% of the universe is dark energy with another 23% as dark matter. That’s a little weird, though, given that we have yet to detect or precisely characterize either, but alas. The remainder of the stuff in creation exists either as regular ol’ energy and… well… stuff. You, me, my cat, the sun. That’s all what’s known as Baryonic matter. It only accounts for 4.6% for everything we know of, but there’s a problem there. Of that sliver of all that is, scientists can only detect half. Or at least they could. New work by a team at the University of Edinburgh, though, might clear things up.

“This is embarrassing, as you can imagine,” astronomer Renyue Cen of Princeton University told Science Mag. “Not only do we have most of matter which is dark, and most of energy which is still darker; but of the 5% which is normal atoms, most are missing.”

The best explanation for the missing stuff was that it had been stretched across the universe in super-thin strands of hot filaments that span between galaxies and clusters. These strands are known as WHIMs or Warm-Hot Intergalactic matter. They’re hot enough to glow in the upper bands of radiation — namely X-rays — but they’re so thin that they’ve been almost impossible to detect.

To sort it out, the team used the Cosmic Microwave Background Radiation — a signal left over from the Big Bang, and one of our largest pieces of evidence for the event — to backlight, in a sense, the WHIMs. They compared a million pairs of galaxies, separated by relatively narrow distances, and then stitched all the images together, looking for traces of the WHIMs.

This works because photons from the CMB can strike electrons in the WHIMs, which are like the connective tissue of the universe. When that happens, sometimes, the photons will gain energy. This is known as the Sunyaev-Zel’dovich (SZ) effect. The new signal is faint and extremely weak, but it’s still measurable by modern instruments.

By looking for evidence of the SZ effect, cosmologists claim to have located a good chunk of the missing mass — as much as 30%. Subsequent experiments, led by the next generation of super-powerful X-Ray telescopes, should be enough to give us more direct evidence, and round out the models. But it’s a great indication that we’re on the right track. It’s also a little unnerving when you think about it. Most of the stuff that is, doesn’t exist in any form that’s recognizable to us. And when you think about what percentage of the total mass that we can observe is in stars or black holes, it’s really weird to remember that the overwhelming majority of the universe is bizarre and empty.

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