An illustration of the Millennium simulation, which uses supercomputers to formulate how the key components of the Universe would have evolved over cosmic time. Illustration: Springel et al. (2005); Spectrum: NASA/CXC/CfA/Kovács et al.
There’s a puzzle that has been taxing astronomers for many years: Where is all of the matter in the universe? You might think that it would be hard to miss, but observations of the universe have turned up only around two thirds of the regular matter that is known to exist due to mathematical models. So where is the other third?
The matter in question is normal matter, referring to elements like hydrogen and helium which were created in the first few minutes following the Big Bang. Over the first one billion years of the history of the universe, this matter was spread out and gradually became cosmic dust, gas, stars, and planets. Scientists have calculated how much of this matter must have existed immediately after the Big Bang and found that about a third of it cannot be accounted for in current models of the universe. This is a separate issue from the question of dark matter, which is a different type of matter that effects the movements of galaxies.
Astronomers believe the missing normal matter could have formed into huge strands of hot gas out in the reaches of space, which would be invisible to most telescopes but could be detected using ultraviolet light. These gas strands are known as the “warm-hot intergalactic medium” or WHIM. Now researchers using data from NASA’s Chandra telescope believe that they have found evidence of the WHIM.
Chandra was used to search for filaments of warm gas near to a quasar, which produces X-rays from its supermassive black hole. The scientists were able to see that some of the X-rays were absorbed by the hot gas, which allowed them to find a “signature” of hot gas as the X-rays traveled 3.5 billion light-years to us. Because the universe is expanding, X-rays are stretched as they travel, meaning that the rays absorbed by matter are shifted towards the red end of the spectrum. The researchers calculated how much shift should have occurred in the distance between them and the quasar, and this information told them where in the spectrum to look for absorption by the WHIM.
“We were thrilled that we were able to track down some of this missing matter,” co-author of the paper Randall Smith of the Center for Astrophysics, Harvard & Smithsonian, said in a statement. “In the future we can apply this same method to other quasar data to confirm that this long-standing mystery has at last been cracked.”
The findings are published in The Astrophysical Journal.
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