Dark matter as an intergalactic heat source

Dark matter as an intergalactic heat source

Physics 15, 180

The quasar spectra suggest that the intergalactic gas may have been heated by a form of dark matter called dark photons.

KG Lee/Max Planck Institute for Astronomy and C. Stark/UC Berkeley

Cloudy forecast. Light from distant quasars travels through the Universe towards Earth and is imprinted with the absorption signatures of hydrogen gas it encounters along the way. These absorption lines suggest anomalous heating that could be explained by dark matter.Cloudy forecast. Light from distant quasars travels through the Universe towards Earth and is imprinted with the absorption signatures of hydrogen gas it encounters along the way. These absorption lines suggest abnormal heating that could be explained… Show more

Dense gas clouds across the Universe absorb light from distant quasars, producing absorption lines in quasar spectra. A new study shows that the larger-than-expected widths of these lines from nearby gas clouds could result from a form of dark matter called dark photons. [1]. These particles could heat the clouds, leading to a broadening of the absorption lines. Other explanations for the broadening – based on more conventional heating sources – have been proposed, but if the dark photon mechanism is at work, it could also cause warming in the low clouds. density of previous epochs of the Universe. The researchers are already planning to test this prediction.

When viewing the spectrum of a distant quasar, astronomers often observe absorption lines coming from the clouds of gas in between. The most important absorption line is the Lyman-alpha line of hydrogen. Indeed, some quasar spectra have a “forest” of Lyman-alpha lines, each coming from a cloud at a different distance from our Galaxy (or at different times). By examining the widths, depths and other details of the lines’ shapes, researchers can extract information about cloud density, temperature and other characteristics. This information can be compared to the results of cosmological simulations that attempt to reproduce the clumping together of matter in galaxies and other large-scale structures.

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

P. Gaikwad/Kavli Institute of Cosmology, Cambridge

See the trees. Light from a distant quasar passes through regions of dense (purple) gas in the intergalactic medium. The gas absorbs light at specific frequencies, leading to a “forest” of absorption lines in quasar spectra (green).

To explore how this injection of energy might work, Caputo and his colleagues performed cosmic simulations with dark photons. Dark photon theory assumes that particles can spontaneously convert to normal photons with low probability, but this conversion can be enhanced when dark photons enter an ionized gas satisfying a resonant condition. The condition comes down to the gas having a certain density, which is determined by the mass of the dark photon. If an intergalactic cloud has this density, then ordinary photons generated by resonance conversion will heat the gas.

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

Simulations by Caputo and his colleagues suggest that dark photons with an extremely small mass of around 10−14 eV/vs2 (around 1019 times smaller than the mass of the electron) would resonate into photons in low redshift Lyman-alpha clouds. This conversion would inject between 5 and 7 eV of energy per hydrogen atom in the gas, enough to account for the observations.

Additionally, the team predicts that dark photon heating could have occurred at a higher redshift, but only in so-called subdense clouds, which in the past had higher densities, potentially high enough to meet the resonance condition. The team is currently running simulations to see if this predicted warming agrees with observations of high redshift clouds.

However, exotic models of dark matter physics may not be necessary to explain the Lyman-alpha data, says astrophysicist Blakesley Burkhart of Rutgers University in New Jersey. She says black photons are an exciting possibility, but researchers have yet to rule out more conventional heating sources, such as jets from supermassive black holes at the centers of galaxies, known as active galactic nuclei.

Sam Witte, 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 compelling case with testable predictions. “If future studies rule out conventional astrophysical explanations, it is imperative to consider the possibility that we may observe the first non-gravitational imprint of dark matter,” he says.

–Michael Schirber

Michael Schirber is corresponding editor for physics review based in Lyon, France.


  1. J.S. Bolton et al.“Low Redshift Lyman Comparison

    from forest observations to hydrodynamic simulations with dark photon dark matter”, Phys. Rev. Lett. 129211102 (2022).


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