# Dark matter as an intergalactic heat source

Physics 15, 180

Spectra taken from the quasars indicate that the intergalactic gas may have been heated by a form of dark matter called dark photons.

Dense gaseous clouds across the universe absorb light from distant quasars, producing absorption lines in the quasars’ spectra. A new study shows that the larger-than-expected width of these streaks from nearby gas clouds could be caused by a form of dark matter called dark photons [1]. These particles can heat the clouds, causing the absorption lines to widen. Other explanations for the expansion have been proposed – based on more traditional heating sources – but if the photon-dark mechanism is at work, it could also be causing heating in low-density clouds from earlier eras of the universe. The researchers are already planning to test this prediction.

When viewing the spectrum of a distant quasar, astronomers often notice absorption lines coming from the intervening gas clouds. The most prominent absorption line is the Lyman alpha line for hydrogen. In fact, some spectra of quasars contain a “forest” of Lyman alpha lines, each coming from a cloud at a different distance (or different eras) from our galaxy. By examining the widths, depths, and other details of the line shapes, researchers can extract information about the density, temperature, and other features of the clouds. This information can be compared with the results of cosmological simulations that attempt to reproduce the agglomeration of matter in galaxies and other large-scale structures.

Comparisons between forest data and simulations generally showed good agreement, but discrepancy appears for relatively close gas clouds. Observations show that these so-called low redshift clouds produce broader absorption lines than would be expected in the simulations. “This could be an indication of a particular candidate for dark matter, the so-called dark photon,” says Andrea Caputo of CERN in Switzerland. “This dark photon can inject some energy and heat up the gas, [which makes] The lines are a little wider, in better agreement with the data.”

To investigate how this energy injection works, Caputo and his colleagues ran cosmological simulations using dark photons. The dark photon theory posits that particles can spontaneously transform into ordinary photons with some small probabilities, but this conversion can be enhanced when dark photons enter an ionized gas that meets a resonance condition. The condition amounts to the gas having a certain density, which is determined by the mass of the dark photon. If the intergalactic cloud had this density, ordinary photons from the resonance conversion would heat the gas.

Caputo asserts that cloud density changes over time, so the resonance state will only be achieved for a certain period of time. This time-dependent heating is unique to dark photons, since other proposed types of heat-producing dark matter, such as those that decay or annihilate, would be expected to be “on” all the time. However, models of continuous heating are constrained by other cosmological observations, such as the cosmic microwave background, which do not show signs of unexplained heating.

Caputo and colleagues’ simulation suggests that dark photons have a very small mass of about 10−14 Volt /c2 (about 1019 times smaller than the mass of an electron) will resonate into photons in low redshift Lyman-alpha clouds. This conversion would inject between 5 and 7 electron volts of energy per hydrogen atom in the gas, enough to explain the observations.

In addition, the team speculates that the dark photon heating may have occurred at a higher redshift, but only in so-called low-density clouds, which in the past had a higher density—potentially high enough to satisfy the resonance condition. The team is currently running simulations to see if this predicted heating would match observations of high redshift clouds.

However, exotic dark matter physics models may not be needed to explain the Lyman alpha data, says astrophysicist Blakeslee Burckhardt of Rutgers University in New Jersey. Dark photons are an exciting possibility, she says, but researchers have yet to rule out more traditional sources of heating, such as jets of supermassive black holes at the centers of galaxies, known as AGNs.

Sam Witt, a cosmologist from the University of Amsterdam, agrees that the dark photon explanation is more speculative than other scenarios, but he thinks the researchers have made a convincing case with testable predictions. “If future studies exclude traditional astrophysical explanations, it is necessary to consider the possibility that we may be observing the first non-gravitational imprint of dark matter,” he says.

– Michael Sherber

Michael Sherber Corresponding Editor for Physics Journal Based in Lyon, France.

## References

1. Bolton youth et al.Low-Lymann redshift comparison.
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Forest Observations to Hydrodynamic Simulations Using Photon Dark Matter Synthesis,” Phys. Rev. Litt. 129211102 (2022).

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