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 Tmax 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(Tmax)]1/3, where g*s(Tmax) is the effective number of entropy degrees of freedom in the primordial plasma at Tmax. We present a state-of-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.

Challenges and opportunities of gravitational-wave searches at MHz to GHz frequencies

“…The hot thermal plasma of the early Universe acts as a source of GWs, which, similarly to the relic photons of the CMB, peak in the $ 100 GHz range today. The spectrum of this signal is determined by the particle content and the maximum temperature T max reached by the thermal plasma in the Universe history (Ghiglieri and Laine 2015;Ghiglieri et al 2020;Ringwald et al 2021). Ignoring the dependence on the number of relativistic degrees of freedom, the energy density in GWs per logarithmic frequency interval can then be written as…”

“…Therefore the detection of the cosmic gravitational microwave background corresponding to T max [ 10 16 GeV would rule out slow-roll inflation as a viable pre hot Big Bang scenario. Note that since at leading order X GW;0 ðf Þ scales linearly with T max and the peak frequency depends on g Ãs ðT max Þ, the detection of the peak of the cosmic gravitational microwave background would determine both T max and g Ãs ðT max Þ, see Ringwald et al (2021) for more details.…”

Universal 1020 Hz stochastic gravitational waves from photon spheres of black holes

“…At present, however, it would be fair to say that the existence of these sources are not guaranteed. There is a more plausible source of highfrequency waves, namely those emitted by thermal fluctuations in the hot plasma, first proposed in [17] and further developed recently in [18,19]. These lead to peak emissions around 80 GHz, with an amplitude that scales with the reheating temperature, reaching Ω gw ∼ 10 −10 for grand unified theory-scale temperatures.…”

“…In order to boost the study of high-frequency gravitational wave detectors, the existence of guaranteed sources which emit high-frequency gravitational waves is essential. So far, various high-frequency gravitational wave sources have been proposed [4,[9][10][11][12][13][14][15][16][17][18][19][20]. In particular, primordial black holes (PBHs) [21] are a possible source of highfrequency gravitational waves [22][23][24][25].…”

“…In particular, primordial black holes (PBHs) [21] are a possible source of highfrequency gravitational waves [22][23][24][25]. PBHs evaporated before the big bang nucleosynthesis produced stochastic background around 10 15 ∼ 10 19 Hz with its typical density parameter Ω gw ∼ 10 −7.5 . At present, however, it would be fair to say that the existence of these sources are not guaranteed.…”