An unexpected source may be helping the universe glow more than it should

When the New Horizons probe reached the outer darkness of the solar system, past Pluto, its instruments picked up something strange.

Very dim, the interstellar space glowed with optical light. This in itself was not unexpected. This light is called the cosmic optical background, and it is a faint luminosity from all sources of light in the universe outside our galaxy.

The strange part was the amount of light. There were a lot more than the scientists thought there should be—twice as many, in fact.

Now, in a new paper, scientists lay out a possible explanation for the excess optical light: a by-product of an 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, offer a possible explanation for the extra cosmic optical background allowed by independent observational limitations, and may answer one of the most ancient unknowns in cosmology: the nature of matter. dark. “

We have many questions about the universe, but dark matter is among the most troubling. It’s the name we give to a mysterious block in the universe responsible for providing much more attractiveness in concentrated spots than it should be.

Galaxies, for example, rotate faster than they should under the gravity created by the mass of visible matter.

The curvature of space-time around massive objects is greater than it should be if we calculate the warp of space based solely on the amount of glowing matter.

But whatever this effect may be, we cannot detect it directly. The only way we know is that we can’t account for this extra gravity.

And there is a lot of it. Almost 80% of the matter in the universe is dark matter.

There are some hypotheses about what that could be. One candidate is axions, which belong to a hypothetical class of particles first conceived in the 1970s to solve the question of why strong atomic forces follow something called parity-of-charge symmetry when most models say they don’t.

As it turns out, axions in a certain mass range should behave exactly as we would expect them to from dark matter. And there might be a way to detect them — because, in theory, axions would be expected to decay into pairs of photons in the presence of a strong magnetic field.

Many experiments are looking for the sources of these photons, but they must also be streaming through space in excessive numbers.

The difficulty is separating it from all other sources of light in the universe, and this is where the cosmic optical background comes in.

It’s very hard to spot the background itself because it’s so faded. The Long Range Reconnaissance Imager (LORRI) aboard New Horizons is probably the best tool for the job yet. It is farther from the Earth and the Sun, and LORRI is much more sensitive than the instruments attached to the more distant Voyager probes launched 40 years ago.

Scientists hypothesized that the excess New Horizons detected is the product attributed to stars and galaxies that we cannot see. 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 eV can 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 putting the hypothetical piece of matter at a fraction of an electronvolt, those numbers would require axons to be relatively fat.

It is impossible to know the correct interpretation based on current data alone. However, by narrowing down the axon masses that could be responsible for the increase, the researchers laid the foundations for future searches for these enigmatic particles.

“If the excess originates from dark matter decay into the photon line, it will be an important signal in upcoming mapping measurements of the line’s intensity,” the researchers write.

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

Research published in Physical review letters.

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