One of the biggest unsolved mysteries in physics is the fact that 84% of the matter that makes up the known universe is completely invisible to us and seems not to interact with the matter that we see around us on a day-to-day basis.
In cosmology, this gap in our knowledge could be considered something of a failure. However, there is still much we know about dark matter, even if we do not quite know what it is.
What Do We Know About Dark Matter?
We know that the gravitational influence of this ‘dark matter’ literally holds galaxies together, so it does interact with gravity. We know it does not interact with photons or baryonic matter. So, despite not knowing what actually makes up dark matter, scientists have taken these characteristics and developed several robust models to highlight leading candidates for dark matter candidates. This shifts the impetus on experiments that confirm or dismiss these particles as suspects.
In a new paper, published in the journal Science and funded by US Department of Energy’s Early Career Research Program, Leinweber Center for Theoretical Physics at U-M, and the Miller Institute for Basic Research in Science at UC Berkeley, researchers focused their investigation on one of the current leading candidates for dark matter particles — the sterile neutrino.
The authors focus on the fact that if the hypothesized sterile neutrino — a massive cousin of the neutrino — does exist then it should decay into other particles, including photons. Physicists have determined that these photons should have a characteristic energy of 3.5 keV. This means that as dark matter is found throughout the Universe, we should also be able to detect emissions of the energy — corresponding to X-rays — wherever it is decaying.
An unexplained emission line at 3.5 keV has been detected from galaxies and galaxy clusters surrounding our own Milky Way which has been speculated to be a result of the decay of sterile neutrinos. Rather than focus on these distant objects, the researchers from the University of Michigan and the Berkley Center for Theoretical Physics decided that they would look for this 3.5keV emission line — and, therefore, dark matter decay — closer to home.
The team turned their attention to 20 years' worth of data collected within the Milky Way by the XMM-Newton Space Telescope and searched for the anomalous X-ray emission. If this is the result of dark matter decay, it should be present in the Milky Way.
The extraordinary amount of observations collected by the XMM-Newton and utilized by the team was made possible through the collaboration between the European Space Agency and Dornier Satellitensysteme, part of Daimler Chrysler Aerospace, Friedrichshafen, Germany.
The company led an industrial consortium involving a further 46 companies from 14 European countries, plus one more located in the United States. The X-ray mirror modules aboard the XMM-Newton, the largest space telescope ever built in Europe, were created by Media Lario, Como.
As the Milky Way sits in a halo of dark matter, any observations XMM-Newton makes should be staring through this halo. There should be evidence of dark matter decay in the form of the 3.5 keV X-ray emission line in data supplied by the space telescope. The team’s numerical calculations indicate that the Milky Way should actually be a brighter source of this radiation than distant galaxies and galaxy clusters.
The team was also able to up the sensitivity of their investigation massively by looking at the darkest part of the Milky Way. The bright signal in the X-ray band of the electromagnetic spectrum that the team was expecting to find was not present in the data from XMM-Newton.
This gives a strong indication that the 3.5 keV emission line found in distant galaxies is not due to the decay of dark matter, and further, that dark matter is very likely not comprised of sterile neutrinos.
Not only does the team’s findings throw serious doubt on sterile neutrinos as the major component of dark matter, they believe that the sensitivity of their analysis was so great that further examination could rule out several other candidates. Other physicists are not quite as sure and have critiqued the team’s results, sparking an intense debate in the scientific community that is likely to rage on for some considerable time.
Amongst the undeniable disappointment in being unable to provide support for the sterile neutrino/ dark matter connection and the eschewing controversy, the team has undoubtedly provided cosmologists with an entirely new way to search for dark matter.
They have also demonstrated that the XMM-Newton space telescope, a fantastic piece of engineering that required over 1000 engineers and 150 scientists to create, is still delivering important results. A testament to the ingenuity of space technology and a vital key in the ongoing hunt for dark matter.
References and Further Reading
Dessert. C, Safdi. B. R, Rodd. N.L, ‘The dark matter interpretation of the 3.5-keV line is inconsistent with blank-sky observations,’ Science, (2020), https://science.sciencemag.org/cgi/doi/10.1126/science.aaw3772
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