The Cosmic Optical Background (COB) is the visible light emitted by all sources outside the Milky Way. This faint glimmer of light, which can only be observed using very precise and sophisticated telescopes, could help astrophysics learn more about the origins of the universe and what lies beyond our galaxy.
Last year, physicists working at different institutes across the United States released the most accurate COB measurements collected to date, collected by the New Horizons spacecraft, an interplanetary space probe launched by NASA over ten years old. These measurements suggest that the COB is twice as bright as theoretical predictions.
Researchers at Johns Hopkins University recently conducted a theoretical study exploring the possibility that this observed excess light could be caused by the decay of a hypothetical type of dark matter particle, called axions. In their article published in Physical examination lettersthey showed that axions with masses between 8 and 20 eV could potentially explain the excess COB flux measured by the New Horizons team.
“Marc Postman is a colleague across the street who is an incredible observational cosmologist, and so when his article with Todd Lauer and the New Horizons team came out, I noticed it and I read,” Marc Kamionkowski, one of the researchers who carried out the study, told Phys.org.
“The measurement they collected is a great example of cleverly repurposing a powerful astronomical observatory for purposes different from those for which it was designed. We sent this incredible little spacecraft to Pluto years ago, and it did everything it was supposed to do, but it had no brakes, and just keeps getting further and further away from the sun without much to do Marc and Todd realized it could be used to detect – for the very first time – the cosmic background of optical photons from all unresolved galaxies in the universe, and it did.”
After reading the paper by Lauer and his colleagues, Kamionkowski realized that if the excess they measured was in fact attributed to axion decay, it could potentially be confirmed using available cosmological data. Specifically, this excess would be detected with high signal-to-noise ratio during SPHEREx, a planned two-year NASA mission that will send a near-infrared space observatory into space to collect potentially valuable new measurements.
“Our calculations are embarrassingly simple, because these are the kinds of calculations that we and tons of other people have been doing for years,” Kamionkowski explained. “The idea that the two-photon decay of an axion could lead to a cosmic signal already existed when I was a graduate student more than 30 years ago. Our job is simply to add the photons of all those produced by the decay of an axion over time, a simple We had to factor in some cosmic redshift factors correctly, but that’s a homework problem in a typical cosmology class.
Taken together, the calculations performed by Kamionkowski and colleagues highlight the possibility of confirming or disproving the link between axion dark matter decay and recently observed COB excess using future measurements. intensity mapping (LIM) data to be collected by NASA’s SPHEREx satellite.
SPHEREx is scheduled to launch in 2025, collecting near-infrared measurement signals from around 450 million galaxies. The Johns Hopkins researchers previously published a follow-up paper, in which they explored the consistency of the axion decay scenario with existing constraints at the gamma ray COB.
“NASA’s Fermi Telescope has obtained the energy spectra of gamma rays from more than 800 blazars, and the most energetic gamma rays can be attenuated by the production of electron-positron pairs via scattering with COB photons,” added Kamionkowski.
“In our new study, we modeled the expected attenuation of this COB scattering and by comparing with the Fermi data, we were able to place an upper bound on the COB background of dark matter decay, which was still consistent with excess COB deducted from New Horizons.
“My student Gabriela Sato-Polito also worked with Dan Grin (Haverford College) looking for deep VLT images of several high redshift clusters for dark matter decay lines. These measurements should allow us to probing some, but not all, of the parameter space for dark matter decay consistent with New Horizons excess.”
José Luis Bernal et al, Cosmic Optical Background Excess, Dark Matter and Line Intensity Mapping, Physical examination letters (2022). DOI: 10.1103/PhysRevLett.129.231301
Tod R. Lauer et al, New Horizons Observations of the Cosmic Optical Background, The Astrophysical Journal (2021). DOI: 10.3847/1538-4357/abc881
José Luis Bernal et al, Search for dark matter with γ-ray attenuation, arXiv (2022). DOI: 10.48550/arxiv.2208.13794
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