When the New Horizons probe reached the outer darkness of the solar system, past Pluto, its instruments picked up something strange.
Very, very dimly, the space between the stars shone with optical light. This was not in itself unexpected; this light is called the cosmic optical background, a faint luminescence from all light sources in the Universe outside our galaxy.
The weird part was the amount of light. There were many more than scientists thought there should be – twice as many, in fact.
Now, in a new paper, scientists expose a possible explanation for excess optical light: a byproduct of an otherwise undetectable dark matter interaction.
“The results of this work,” write a team of researchers led by astrophysicist José Luis Bernal of Johns Hopkins University, “provide a potential explanation for the excess cosmic optical background that is allowed by observational constraints independent, and which can answer one of the oldest unknowns of cosmology: the nature of dark matter.”
We have many questions about the Universe, but dark matter is among the trickiest. It’s the name we give to a mysterious mass in the Universe. responsible for providing much more gravity in concentrated places than there should be.
Galaxies, for example, spin faster than they should under the gravity generated by the mass of visible matter.
The curvature of spacetime around massive objects is greater than it would be if we calculated the warping of space based solely on the amount of glowing matter.
But whatever creates this effect, we cannot detect it directly. The only way we know it’s there is that we just can’t explain the extra gravity.
And there are many. About 80% of the matter in the Universe is dark matter.
There are a few guesses as to what it could be. One of the candidates is the axion, which belongs to a hypothetical class of particles first conceptualized in the 1970s to answer the question of why strong atomic forces follow what is called charge-parity symmetry then that most models say they don’t need it.
It turns out that axions in a specific mass range should also behave exactly as we expect dark matter to do. And there might be a way to detect them, because theoretically, axions are expected to decay into photon pairs in the presence of a strong magnetic field.
Several experiments are looking for sources of these photons, but they are also expected to travel through space in excessive numbers.
The difficulty is to separate them from all the other light sources in the Universe, and this is where the cosmic optical background comes in.
The background itself is very difficult to detect because it is so faint. The Long Range Reconnaissance Imager (LORRI) aboard New Horizons is probably the best tool for the job yet. It’s far from Earth and the Sun, and LORRI is far more sensitive than the instruments attached to the more distant Voyager probes launched 40 years earlier.
Scientists assumed that the excess detected by New Horizons was the product attributed to stars and galaxies that we cannot see. And that option is still on the table. Bernal and his team’s job was to assess whether axion-like dark matter could possibly be responsible for the extra light.
They performed mathematical modeling and determined that axions with masses between 8 and 20 electronvolts could produce the observed signal under certain conditions.
That’s incredibly light for a particle, which tends to be measured in megaelectronvolts. But with recent estimates putting the hypothetical chunk of matter at a fraction of a single electronvolt, those numbers would require axions to be relatively beefy.
It is impossible to say which explanation is correct based on current data alone. However, by narrowing down the masses of the axions that might be responsible for the excess, the researchers laid the groundwork for future research into these enigmatic particles.
“If the excess comes from the decay of dark matter into a photon line, there will be a significant signal in the next line intensity mapping measurements,” the researchers write.
“Additionally, the ultraviolet instrument on board New Horizons (which will have better sensitivity and will probe a different range of the spectrum) and future very high energy gamma ray attenuation studies will also test this hypothesis and expand the search for dark matter at a wider frequency range.”
The research has been published in Physical examination letters.
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