Gas clouds across the universe are known to absorb light produced by distant massive celestial objects, called quasars. This light manifests as Lyman’s so-called alpha forest, a dense structure composed of absorption lines that can be observed using spectroscopy tools.
Over the past few decades, astrophysicists have assessed the value of these absorption lines as a tool to better understand the universe and the relationships between cosmological objects. Lyman’s alpha forest could also potentially contribute to the ongoing search for dark matter, providing an additional tool to test theoretical predictions and models.
Researchers from the University of Nottingham, Tel Aviv University, New York University and the Institute for Fundamental Physics of the Universe in Trieste recently compared observations of the Lyman alpha forest to low redshift to hydrodynamic simulations of the intergalactic medium and dark matter composed of photons, a renowned dark matter candidate.
Their article, published in Physical examination letters (LRP), builds on previous work by some members of their team, which compared simulations of the intergalactic medium (IGM) with Lyman-alpha forest measurements collected by the Cosmic Origins Spectrograph (COS) aboard the Hubble Space Telescope.
“In our analyses, we found that the simulation predicted linewidths that were too narrow compared to the COS results, suggesting that there may be additional non-canonical heating sources occurring at low redshifts. “said Hongwan Liu, Matteo Viel, Andrea Caputo and James. Bolton, the researchers who conducted the study, told Phys.org via email.
“We explored several models of dark matter that could act as this heating source. Building on the experience of two of the authors with dark photons in a previous paper published in LRPwe finally realized that heating dark matter from dark photons could work.”
Based on their previous observations, Liu, Viel, Caputo, and Bolton decided to modify a hydrodynamic simulation of the IGM (i.e., a sparse cloud of hydrogen that exists in the spaces between galaxies). In their new simulation, they included the effects of heat that the models predict would be produced by dark matter from dark photons.
“In regions of space where the mass of the dark photon matches the effective plasma mass of the photon, conversions of dark photons to photons can occur,” explained Liu, Viel, Caputo and Bolton. “The converted photons are then rapidly absorbed by the IGM in these regions, heating the gas. The amount of energy transferred from dark matter to the gas can be calculated theoretically.”
The researchers added this estimated energy transfer between dark photons and intergalactic clouds to their simulations. This eventually allowed them to reach a series of simulated absorption linewidths, which they could compare to actual observations of the Lyman-alpha forest collected by COS.
“Overall, we have shown that the Lyman-alpha forest is extremely useful for understanding dark matter patterns where energy can be converted from dark matter into heating,” said Liu, Viel, Caputo and Bolton. . “I think our study will encourage physicists interested in dark matter to pay more attention to the Lyman-alpha forest.”
Overall, the comparison between the COS measurements and the hydrodynamic simulations carried out by this team of researchers suggests that dark photons could in fact be a source of heat in intergalactic gas clouds. Their findings could thus be the first clue to the existence of dark matter that is not observed through its gravitational effects.
While this is a fascinating possibility, Liu, Viel, Caputo, and Bolton have yet to rule out other possible theoretical explanations. They hope that their study will inspire other teams to probe the properties of IGM in the early universe in the same way.
“A particularly interesting consequence of dark photon heating is that underdense regions of the IGM are heated earlier than overdense regions,” said Liu, Viel, Caputo and Bolton. “This can lead to underdense regions being hotter than overdense regions, which is contrary to standard expectations. There are indications that IGM exhibits this behavior at high redshifts. If so, this could be another important piece of evidence in favor of the dark heating of dark matter by photons.”
James S. Bolton et al, Comparison of Low Redshift Lyman-α Forest Observations to Hydrodynamic Simulations with Dark Photon Dark Matter, Physical examination letters (2022). DOI: 10.1103/PhysRevLett.129.211102
James S Bolton et al, Limits on non-canonical heating and turbulence in the low redshift Lyman α intergalactic forest medium, Royal Astronomical Society Monthly Notices (2022). DOI: 10.1093/mnras/stac862
Andrea Caputo et al, Dark Photon Oscillations in Our Inhomogeneous Universe, Physical examination letters (2020). DOI: 10.1103/PhysRevLett.125.221303
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