Where Is The Universe Hiding Its Missing Mass?

Astronomers have spent years in search of something that sounds like it would be hard to miss – around a third of the “normal” matter in the Universe. New results from NASA’s Chandra X-ray Observatory may help locate this elusive expanse of missing matter. Scientists have conducted independent, well-established observations to confidently calculate how much normal matter in terms of hydrogen, helium, and other elements existed just after the Big Bang. In the time-lapse between the first few minutes and the first billion years or so, a chunk of the normal matter became cosmic dust, gas, and objects like stars and planets that telescopes can see in the present-day Universe.

The problem arises when astronomers add up the mass of all the normal matter in the present-day Universe about a third of it can’t be found. This missing matter is distinct from the still-mysterious dark matter.


One hypothesis is that the missing mass congregated into gigantic strands or filaments of warm (temperature less than 100,000 Kelvin) and hot (temperature greater than 100,000 Kelvin) gas in the intergalactic space. Astronomers call these filaments the “warm-hot intergalactic medium” or WHIM. As they are invisible to optical light telescopes, only some of the warm gas in filaments has been detected in ultraviolet light.

Applying a new technique, researchers have uncovered new and strong evidence about the hot component of the WHIM based on data from Chandra and other telescopes.

Orsolya Kovacs of the Center for Astrophysics in Harvard & Smithsonian (CfA) in Cambridge, Massachusetts, said, “If we find this missing mass, we can solve one of the biggest conundrums in astrophysics. Where did the universe stash so much of its matter that makes up stuff like stars and planets and us?”

Astronomers used Chandra telescope to look for and study such filaments of warm gas lying along the path to a quasar located about 3.5 billion light-years from Earth and is a bright source of X-rays powered by a rapidly growing supermassive black hole. If the WHIM’s hot gas component is related to these filaments, some of the X-rays from the quasar would be absorbed by that hot gas. Thus, they watched out for a signature of hot gas imprinted in the quasar’s X-ray light detected by Chandra.

A major challenge of this method is that the signal of absorption by the WHIM is much weaker as compared to the total amount of X-rays released by the quasar. While searching the entire spectrum of X-rays at different wavelengths, it is tough to distinguish such weak absorption features that are actual signals of the WHIM, from random fluctuations.

Kovacs and her team overcame this problem by focusing their pursuit only on certain parts of the X-ray light spectrum to reduce the likelihood of false positives. They first identified galaxies near the line of sight to the quasar that is positioned at the same distance from Earth as regions of warm gas detected from ultraviolet data. Using this technique they identified 17 likely filaments between the quasar and us and found their distances.

As the expansion of the universe stretches out light as it travels, any absorption of X-rays by matter in these filaments will be shifted to redder wavelengths. The extent of the shifts depend on the known distances to the filament, so the team knew exactly where to search in the spectrum for absorption from the WHIM.

Akos Bogdan, a co-author also from CfA, said, “Our technique is similar in principle to how you might conduct an efficient search for animals in the vast plains of Africa. We know that animals need to drink, so it makes sense to search around watering holes first.”

Although narrowing down their search helped, the researchers also had to resolve the problem of the faintness of the X-ray absorption. For this, they boosted the signal by adding spectra together from 17 filaments, transforming a 5.5-day-long observation into the equivalent of almost 100 days’ worth of data. With this technique, they detected oxygen with parameters suggesting that it was in a gaseous form with a temperature of about one million degrees Kelvin.

By extrapolating these observations of oxygen to the full set of elements, and the observed region to the local universe, the researchers reported in a paper published in the Astrophysical Journal on February 13, 2019, that they can account for the complete amount of missing matter.

Co-author Randall Smith, also of CfA, said, “We were thrilled that we were able to track down some of this missing matter. 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.”

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