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 backgroundfaint luminescence from all light sources in the universe outside our galaxy.
The strange part was the amount of light. There was significantly more than scientists thought there should be – double morein fact.
Now, in a new paper, scientists lay out a possible explanation for the optical excess of light: a byproduct of an otherwise undetectable interaction of dark matter.
“The results of this work,” write a team of researchers led by astrophysicist Jose Luis Bernal of Johns Hopkins University, “provide a potential explanation for the excess of the cosmic optical background allowed by independent observational constraints, and this may answer one of cosmology’s longest-standing unknowns: the nature of dark matter .”
We have many questions about the universe, but dark matter is among the most vexing. This is 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, rotate 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 calculate the curvature of space based on the amount of luminous material alone.
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 that extra gravity.
And there are many. Approximately 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 actionwhich belongs to a hypothetical class of particles first conceptualized in the 1970s to solve the question of why the strong atomic force follows something called charge-parity symmetry when most models say it is not necessary.
As it turns out, axions in a certain mass range should also behave exactly as we expect from dark matter. And there may be a way to detect them—because axions are theoretically expected to decay into pairs of photons in the presence of a strong magnetic field.
Several experiments are looking for sources of these photons, but they too must be spreading out into space in excess.
The difficulty is to separate them from all the other light sources in the universe, and this is where the cosmic optical background comes into play.
The background itself is very hard to detect as it is so faint. The Long Range Reconnaissance Instrument (LORRI) aboard New Horizons probably is the best job tool ever. It is far from Earth and the Sun, and LORRI is much more sensitive than the instruments attached to the more distant Voyager probes launched 40 years earlier.
Scientists have hypothesized 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 job was to assess whether axion-like dark matter might be responsible for the extra light.
They performed mathematical modeling and found 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 that is usually measured in megaelectronvolts. But with recent estimates that put the hypothetical piece of matter at a fraction of an electronvolt, those numbers would require the axions to be relatively strong.
It is impossible to say which explanation is correct based solely on the current data. However, by narrowing down the axion masses 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 dark matter decay to a photon line, there will be a significant signal in upcoming line intensity mapping measurements,” the researchers write.
“Furthermore, the ultraviolet instrument on board New Horizons (which will have better sensitivity and probe a different range of the spectrum) and future studies of the decay of very high-energy gamma rays will also test this hypothesis and extend the search for dark matter to a wider range of frequencies.”
The study was published in Physical examination letters.