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 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.

早期宇宙的热等离子体中产生的随机引力波是有保证的背景。其每个对数频率间隔的能量密度与标准热大爆炸时代开始时原始等离子体达到的最高温度 T _{max成比例。}它在微波范围内达到峰值，大约为 80 GHz [106.75/g _* s(T _max )] ^1/3，其中 g _* s(T _{max ) 是 T max}处原始等离子体中熵自由度的有效数. 我们为通用模型提供了该宇宙引力微波背景 (CGMB) 的最新预测，并对标准模型 (SM) 及其几个扩展的情况进行了计算。在最小扩展方面，我们考虑中微子最小 SM (νMSM) 和 SM-Axion-Seesaw-Higgs 门户暴胀模型 (SMASH)，它们提供了包括暴胀在内的完整且一致的宇宙学历史。作为 SM 的非最小扩展的示例，我们考虑最小超对称标准模型 (MSSM)。此外，我们讨论了在实验室实验中探测CGMB的当前上限和前景，从而测量了热大爆炸开始时的最高温度和有效自由度数。