Gravitational Waves as a Big Bang Thermometer

Abstract

There is a guaranteed background of stochastic gravitational waves produced in the thermal plasma in the early universe. Its energy density per logarithmic frequency interval scales with the maximum temperature T max which the primordial plasma attained at the beginning of the standard hot big bang era. It peaks in the microwave range, at around 80 GHz [106.75/g * s (T max )] 1/3 , where g * s (T max ) is the effective number of entropy degrees of freedom in the primordial plasma at T max . We present a stateof-the-art prediction of this Cosmic Gravitational Microwave Background (CGMB) for general models, and carry out calculations for the case of the Standard Model (SM) as well as for several of its extensions. On the side of minimal extensions we consider the Neutrino Minimal SM (νMSM) and the SM -Axion -Seesaw -Higgs portal inflation model (SMASH), which provide a complete and consistent cosmological history including inflation. As an example of a non-minimal extension of the SM we consider the Minimal Supersymmetric Standard Model (MSSM). Furthermore, we discuss the current upper limits and the prospects to detect the CGMB in laboratory experiments and thus measure the maximum temperature and the effective number of degrees of freedom at the beginning of the hot big bang.

Tachyonic Preheating in Palatini $R^2$ Inflation

“…Currently, the most sensitive probe of such primordial GW signals is the number of effective relativistic degrees of freedom N eff during Big Bang nucleosynthesis [79]. However, a GW background at 3-30GHz frequency range can also leave an imprint on the CMB Rayleigh-Jeans tail and may thus be probed via future 21cm physics experiments [80,81].…”

Tachyonic Preheating in Plateau Inflation

“…These numbers are compatible with the lattice studies [30,31] performed for similar models. Such frequencies are not observable with nearfuture gravitational-wave interferometers [52][53][54], but may be probed through the effects on big bang nucleosynthesis [55][56][57] or 21 cm measurements [57,58]. Finally, let us take a closer look at the applicability of the linear theory for small φ 0 .…”

Chiral gravitational waves from thermalized neutrinos in the early Universe

“…This is caused primarily by one additional factor of k/ T in its power spectrum and secondarily by the further suppression due to the smallness of ξ. The parity-even gravitational spectrum due to hydrodynamic fluctuations of the primordial plasma has been previously obtained from the emission rate of gravitons based on a perturbative Standard Model calculation [30,31,46]. Another frequently used measure to quantify the strength of a gravitational wave signal is the power spectral density S h (f ) [47], which has the dimension of Hz −1 and is related to…”