Abstract
We show that if dark energy evolves in time, its dynamical component could be dominated by a bath of dark radiation. Within current constraints this radiation could have up to $\sim 10^4$ times more energy density than the cosmic microwave background. We demonstrate particular models in which a rolling scalar field generates different forms of dark radiation such as hidden photons, milli-charged particles and even Standard Model neutrinos. We find the leading effect on the late-time cosmological expansion history depends on a single parameter beyond $\Lambda$CDM, namely the temperature of the dark radiation today. Cosmological observations of this modified expansion rate could provide a striking signature of this scenario. The dark radiation itself could even be directly detectable in laboratory experiments, suggesting a broader experimental program into the nature of dark energy.
Highlights
A variety of cosmological measurements have established that the Universe is currently undergoing an accelerated expansion driven by dark energy
We show that if dark energy evolves in time, its dynamical component could be dominated by a bath of dark radiation
How can the dark energy couple to the standard model, and what kinds of signals can these interactions lead to? A systematic identification of these could lead to a robust program of laboratory based probes of dark energy, similar to the effort that is currently underway to identify the nature of dark matter
Summary
A variety of cosmological measurements have established that the Universe is currently undergoing an accelerated expansion driven by dark energy. A cosmological constant is the simplest proposal for dark energy, and such a constant cannot be probed in laboratory experiments This is theory prejudice —the experimental nature of physics requires observational probes to establish if the dark energy is a constant or if it has a dynamical component, e.g., a kinetic energy. This kinetic energy can couple to the standard model through nongravitational interactions and could potentially be probed in the laboratory. In the rest of this section, we present minimal models that can generate different particle contents in the background dark radiation
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