An unexpected source may be helping the universe shine brighter than it should: ScienceAlert

When the New Horizons probe reached the outer darkness of the Solar System, beyond Pluto, its instruments detected something strange.

Very, very faintly, the space between the stars glowed with optical light. This in itself was not unexpected; this light is called cosmic optical backgrounda faint luminescence from all light sources in the Universe outside our galaxy.

The strange part was the amount of light. There was much more to it than scientists thought there should be: the doubleIn fact.

Now, in a new paper, scientists present a possible explanation for the excess optical light: a byproduct of an otherwise undetectable interaction of dark matter.

“The results of this work”, write a team of researchers led by Johns Hopkins University astrophysicist José Luis Bernal, “offer a potential explanation for the cosmic optical background excess that independent observational constraints allow, and that may answer one of the oldest unknowns of the Cosmology: The Nature of Dark Matter.”

We have many questions about the Universe, but dark matter is one of the most vexing. It is the name we give to a mysterious mass in the Universe responsible for providing much more gravity at concentrated points than it should.

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 should be if we calculated the warping of space based on the amount of bright material alone.

But whatever is creating this effect, we can’t detect it directly. The only way we know it’s there is that we can’t explain this extra gravity.

And there are a lot of them. Approximately 80% of the matter in the Universe is dark matter.

There are a few hypotheses as to what it could be. One of the candidates is the actionwhich belongs to a hypothetical class of particles first conceptualized in the 1970s to solve the question of why strong atomic forces follow something called charge parity symmetry when most models say you don’t have to.

As a result, axions in a specific mass range should also behave exactly as we expect dark matter to. 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 would also have to go through excess space.

The difficulty is separating them from all the other light sources in the Universe, and that’s where the cosmic optical background comes in.

The background is very difficult to detect because it is very faint. The Long Range Reconnaissance Imager (LORRI) aboard New Horizons is possibly the the best tool for the job yet. It’s far from Earth and the Sun, and LORRI is much more sensitive than the instruments attached to the more distant Voyager probes that were launched 40 years earlier.

Scientists have assumed that the excess detected by New Horizons is the product attributed to stars and galaxies that we cannot see. And that option is still very much on the table. Bernal and his team’s work was to assess whether axion-like dark matter could be responsible for the extra light.

They performed mathematical modeling and determined that axions with masses between 8 and 20 electron volts could produce the observed signal under certain conditions.

This is incredibly light for a particle, which tends to be measured in megaelectronvolts. But with recent estimates that put the hypothetical piece of matter at a fraction of a single electronvolt, those numbers would be they require axions to be relatively robust.

It is impossible to say which explanation is correct based solely on the current data. However, by reducing the masses of the axions that could be responsible for the excess, the researchers have laid the groundwork for future searches for these enigmatic particles.

“If the excess arises from the decay of dark matter into a photon line, there will be an important signal in upcoming line intensity mapping measurements,” write the researchers.

“Furthermore, the ultraviolet instrument on board New Horizons (which will have better sensitivity and probe a different range of the spectrum) and future high-energy gamma-ray attenuation studies will also test this hypothesis and expand the search for dark matter at a wider range of frequencies.”

The research was published in Physical review letters.