The universe is a little hotter than it should be. “Dark photons” may be to blame for this.

Observations show that the intergalactic gas in our universe is a little hotter than it should be.

Recently, a group of astrophysicists used sophisticated computer simulations to come up with a radical solution: An exotic form of dark matter known as “dark photons” could heat up this place.

These strange particles should be carriers of a new, fifth force of nature that normal matter is not subject to, but sometimes these dark photons can change their identity, becoming ordinary photons, providing a source of heat.

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Feeling of neutrality

We could find such dark photons by observing intergalactic gas using what is known as the Lyman-alpha forest. When we observe light from a distant bright object such as a quasar (luminous objects powered by black holes at the centers of distant galaxies), a number of gaps appear in the smooth spectrum of light from that distant object.

And here’s why: this light has to travel through billions of light-years of gas to get to us. Sometimes this light passes through a relatively dense clump of neutral hydrogen, a type of hydrogen made up of one proton and one neutron, that permeates gas clouds throughout the universe.

Most of this light will pass unaffected, but light at a very specific wavelength will be absorbed. This wavelength corresponds to the energy difference required to move an electron from its first to its second energy level within hydrogen atoms.

When astronomers look at the light emanating from this object, it appears otherwise unremarkable, except for a break in the wavelength of this particular energy transition, known as the Lyman-alpha line.

Light from a distant object will pass through numerous clouds and clumps of neutral hydrogen. The expansion of the Universe causes the slits to redshift towards different wavelengths, with a new slit appearing at a different wavelength depending on the distance to specific gas clouds. The end result of this is a “forest”: a series of lines and gaps in the spectrum.

It’s getting hot in here

These Lyman-alpha spans can also be used to measure the temperature of each gas cloud. If neutral hydrogen were perfectly still, the gap would look like an incredibly thin line. But if individual molecules are moving, then the gap will increase due to the kinetic energy of these molecules. The hotter the gas, the greater the kinetic energy of the molecules and the wider the gap.

In an article published in November in the journal Physical Review Letters (will open in a new tab), a group of astrophysicists pointed out that when using this method, it seems that the clouds of gas that scatter between galaxies are too hot. Computer simulations of the evolution of these gas clouds predict that they will be slightly colder than we observe, and so it is possible that something is heating these clouds, which is not currently accounted for in our astrophysical models.

The authors of the study argue that one possible explanation for this discrepancy is the presence of “dark photons” in our universe. It’s a very hypothetical form of dark matter, a mysterious, invisible substance that accounts for roughly 80% of the entire mass of the universe, but doesn’t seem to interact with light.

Because astronomers do not currently understand the nature of dark matter, the field is wide open to what it might be. In this model, dark matter does not consist of invisible particles (such as a phantom version of electrons), but of a new type of force carrier, that is, a type of particles that mediate interactions between other particles.

Warm and fluffy darkness

The familiar photon is the carrier of the force of electromagnetism – it is he who creates electricity, magnetism and light. Dark photons would be carriers of a new force of nature that does not operate on a normal scale in normal scenarios (for example, in our laboratories or inside the solar system, where we would otherwise already observe it).

According to the authors of the study, dark photons will still have a small mass, and therefore they can still make up dark matter. In addition, since they are force carriers, they can also interact with each other and with other potential dark matter particles. In the models investigated by the team of astrophysicists, dark photons are capable of another trick: they can turn into a normal photon from time to time.

From a physics point of view, dark photons can “mix” with ordinary photons, very rarely changing their identities. When they do, the newly created photon continues to do what normal photons always do: heat things up. The researchers performed the first-ever simulation of the evolution of the universe, including the effects of these insidious shape-shifting dark photons. They found that a certain combination of the dark photon’s mass and the probability of becoming a normal photon could explain the heating discrepancy.

This result is very far from the win-win case of the existence of dark photons. A number of possibilities could also explain the results of Lyman-alpha, such as inaccurate observations or poor understanding of (normal) astrophysical heating between galaxies. But this is an intriguing clue, and the results can be used as a springboard to further explore the viability of this exotic idea.

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