An unexpected resource could be helping the universe glow more than it should: ScienceAlert

An unexpected resource could be helping the universe glow more than it should: ScienceAlert

As the New Horizons probe reached the outer darkness of the solar system, beyond Pluto, its instruments picked up on something strange.

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

The strange thing was the amount of light. There was considerably more than scientists thought there should be – twice as muchin reality.

Now, in a new paper, scientists provide a possible explanation for the optical light excess: a by-product of an otherwise undetectable interaction of dark matter.

“The results of this work,” write a team of researchers led by astrophysicist José Luis Bernal of Johns Hopkins University, “offering a possible explanation for the cosmic optical background excess allowed by independent observational constraints, which may answer one of the longest-standing unknowns in cosmology: the nature of dark matter .”

We have a lot of 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 in concentrated spots than there should be.

For example, galaxies spin faster than they should under the gravity generated by the mass of visible matter.

The curvature of space-time around massive objects is greater than it should be if we calculated the curvature of space only from the amount of glowing material.

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

And there’s a lot of it. About 80 percent of the matter in the universe is dark matter.

There are some hypotheses about what it could be. One of the candidates is the axionwhich 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 while most models say it is not necessary.

It turns out that axions in a certain mass range should also behave exactly as we expect dark matter to behave. And there could be a way to detect them – because axions are theoretically expected to decay into photon pairs in the presence of a strong magnetic field.

Several experiments look for sources of these photons, but they should also be streaming through space in large numbers.

The difficulty is separating them from all 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 may be the best tool for the job so far. It is far from Earth and the Sun, and LORRI is much more sensitive than instruments attached to the more distant Voyager probes launched 40 years earlier.

Scientists have assumed that the excess detected by New Horizons is the product attributed to stars and galaxies we cannot see. And that option is still on the table. Bernal and his team’s work 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 electron volts could produce the observed signal under certain conditions.

That’s incredibly light for a particle, which is usually measured in mega-electron volts. But with recent estimates placing the hypothetical piece of matter at a fraction of a single electron volt, these numbers would be require axions to be relatively sturdy.

It is impossible to determine which explanation is the correct one based on the current data. However, by narrowing down the masses of the axions that could be responsible for the excess, the researchers have laid the groundwork for future searches for these puzzling particles.

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

“In addition, the ultraviolet instrument onboard New Horizons (which will have better sensitivity and examine a different range of the spectrum) and future studies of very high energy gamma-ray attenuation will also test this hypothesis and extend the search for dark matter to a wider range of frequencies.”

The research has been published in Physical assessment letters.

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