Dark Photon Dark Matter in the Presence of Inhomogeneous Structure. (arXiv:2003.13698v1 [astro-ph.CO])
<a href="http://arxiv.org/find/astro-ph/1/au:+Witte_S/0/1/0/all/0/1">Samuel J. Witte</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Rosauro_Alcaraz_S/0/1/0/all/0/1">Salvador Rosauro-Alcaraz</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+McDermott_S/0/1/0/all/0/1">Samuel D. McDermott</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Poulin_V/0/1/0/all/0/1">Vivian Poulin</a>
Dark photon dark matter will resonantly convert into visible photons when the
dark photon mass is equal to the plasma frequency of the ambient medium. In
cosmological contexts, this transition leads to an extremely efficient, albeit
short-lived, heating of the surrounding gas. Existing work in this field has
been predominantly focused on understanding the implications of these resonant
transitions in the limit that the plasma frequency of the Universe can be
treated as being perfectly homogeneous, i.e. neglecting inhomogeneities in the
electron number density. In this work we focus on the implications of heating
from dark photon dark matter in the presence of inhomogeneous structure (which
is particularly relevant for dark photons with masses in the range $10^{-15}
lesssim m_{A’} < 10^{-12}$ eV), emphasizing both the importance of
inhomogeneous energy injection, as well as the sensitivity of cosmological
observations to the inhomogeneities themselves. More specifically, we derive
modified constraints on dark photon dark matter from the Ly-$alpha$ forest,
and show that the presence of inhomogeneities allows one to extend constraints
to masses outside of the range that would be obtainable in the homogeneous
limit, while only slightly relaxing their strength. We then project sensitivity
for near-future cosmological surveys that are hoping to measure the 21cm
transition in neutral hydrogen prior to reionization, and demonstrate that
these experiments will be extremely useful in improving sensitivity to masses
near $10^{-14}$ eV, potentially by several orders of magnitude. Finally, we
discuss implications for both reionization and early star formation, and show
that probes which are inherently sensitive to the inhomogeneous state of the
Universe could resolve signatures unique to the light dark photon dark matter
scenario, and thus offer a fantastic potential for a positive detection.
Dark photon dark matter will resonantly convert into visible photons when the
dark photon mass is equal to the plasma frequency of the ambient medium. In
cosmological contexts, this transition leads to an extremely efficient, albeit
short-lived, heating of the surrounding gas. Existing work in this field has
been predominantly focused on understanding the implications of these resonant
transitions in the limit that the plasma frequency of the Universe can be
treated as being perfectly homogeneous, i.e. neglecting inhomogeneities in the
electron number density. In this work we focus on the implications of heating
from dark photon dark matter in the presence of inhomogeneous structure (which
is particularly relevant for dark photons with masses in the range $10^{-15}
lesssim m_{A’} < 10^{-12}$ eV), emphasizing both the importance of
inhomogeneous energy injection, as well as the sensitivity of cosmological
observations to the inhomogeneities themselves. More specifically, we derive
modified constraints on dark photon dark matter from the Ly-$alpha$ forest,
and show that the presence of inhomogeneities allows one to extend constraints
to masses outside of the range that would be obtainable in the homogeneous
limit, while only slightly relaxing their strength. We then project sensitivity
for near-future cosmological surveys that are hoping to measure the 21cm
transition in neutral hydrogen prior to reionization, and demonstrate that
these experiments will be extremely useful in improving sensitivity to masses
near $10^{-14}$ eV, potentially by several orders of magnitude. Finally, we
discuss implications for both reionization and early star formation, and show
that probes which are inherently sensitive to the inhomogeneous state of the
Universe could resolve signatures unique to the light dark photon dark matter
scenario, and thus offer a fantastic potential for a positive detection.
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