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Discovered a light source in the universe that tells us more about dark matter

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When the New Horizons spacecraft reached the darkness outside the Solar System, beyond Pluto, its instruments detected something strange. Very, very faintly, the space between the stars shone with optical light. This was not unexpected: this light is called cosmic optical background (COB) radiation, a faint glow of all light sources in the Universe outside our galaxy. The strange thing was the amount of light, considerably more than scientists thought there should be: double, in fact.

 

Now, in new work, scientists provide a possible explanation for the excess of optical light – a byproduct of an otherwise undetectable interaction of dark matter.

“The results of this work”, writes the group of researchers led by astrophysicist José Luis Bernal of Johns Hopkins University, “provide a potential explanation for the excess of cosmic optical background radiation that could answer one of the oldest unknowns of cosmology : the nature of dark matter ”.

Dark matter is the name we give to the mysterious mass in the Universe, still inscrutable, which, unlike known matter, would not emit electromagnetic radiation (energy propagation in a magnetic field) and would currently be detectable only indirectly, through its gravitational effects. Galaxies, for example, rotate faster than they should under the gravity generated by the mass of visible matter. But whatever creates this effect, we can’t detect it directly. The only way to know it’s there is just because we can’t calculate this extra gravity. And there’s a lot of it. About 80% of the matter in the universe is dark matter .

 

A fraction of an electron volt

There are some speculations about what it could be. One candidate is the axion , which belongs to a hypothetical class of particles, first conceptualized in the 1970s. As we have seen, axions  in a given mass range should also behave exactly as we expect dark matter to do. And there may be a way to detect them: in theory, axions should decay into photon pairs in the presence of a strong magnetic field. The difficulty lies in separating them from all other light sources in the Universe, and this is where the cosmic optical background radiation comes into play.

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Cosmic radiation itself is very difficult to detect because it is weak. The Long Range Reconnaissance Imager (LORRI) aboard New Horizons is perhaps the best tool for this job. Scientists have speculated that the excess of light, detected by New Horizons, is the product attributed to stars and galaxies we can’t see. And this option is still very likely. The work of Bernal and his team was to evaluate whether axion-type dark matter could be responsible for the extra light. They conducted mathematical modeling and determined that axions with masses between 8 and 20 electron volts could produce the observed signal under certain conditions. This range (between 8 and 20 electron volts) is considered incredibly light massfor a particle which tends to be measured in mega electron volts. The answer given by astrophysicists is that the hypothetical piece of matter revealed is a fraction of a single electron volt.

It is impossible to tell which explanation is correct based on current data alone. However, by narrowing down the masses of the axions that may be responsible for the excess light, the researchers have laid the groundwork for future searches for these enigmatic particles. “If the excess light results from the decay of dark matter into a photon line, there will be a significant signal in upcoming light source intensity mapping measurements,” the researchers write.

 

 

  • Cosmic Optical Background Excess, Dark Matter, and Line-Intensity Mapping (journals.aps.org)
 
 

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