Abstract
Many extensions of Standard Model (SM) include a dark sector which can interact with the SM sector via a light mediator. We explore the possibilities to probe such a dark sector by studying the distortion of the CMB spectrum from the blackbody shape due to the elastic scatterings between the dark matter and baryons through a hidden light mediator. We in particular focus on the model where the dark sector gauge boson kinetically mixes with the SM and present the future experimental prospect for a PIXIE-like experiment along with its comparison to the existing bounds from complementary terrestrial experiments.
Highlights
The energy spectrum of the cosmic microwave background (CMB) follows the most perfect blackbody spectrum ever observed
For z 5 × 104, even the kinetic equilibrium cannot be maintained due to the inefficient Compton scatterings and the spectrum distortion can be characterized by the Compton yparameter which is given by the line of sight integral of electron pressure [2]
The attempt to measure potential CMB spectral distortion has been made by the Far Infrared Absolute Spectrophotometer (FIRAS) instrument aboard the COBE satellite [3] two decades ago, leading to the upper bounds |μ| 10−4 and |y| 10−5
Summary
The energy spectrum of the cosmic microwave background (CMB) follows the most perfect blackbody spectrum ever observed. The CMB spectral distortion can, for instance, be induced by the energy injection into the background plasma in many non-standard cosmological scenarios [5]. The. Coulomb interactions of non-relativistic plasma consisting of baryons with photons can extract energy from the CMB and maintain the kinetic equilibrium. The temperature of baryons follows that of photons and decreases inversely proportional to the scale factor of the Universe, Tb Tγ ∼ 1/a, instead of 1/a2 for the decoupled non-relativistic matter. This extraction of energy from the CMB results in the μ-distortion of the order of μ −3 × 10−9.
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