| Abstract: Gravitational waves have opened a new window into the early universe. Due to their small interactions, gravitational waves can travel effectively freely to us, carrying information from early stages of the universe, when temperatures and densities were much larger than what can be achieved in laboratories. In this talk, I will first show that for gravitational wave sources that only operate during a small finite time, the low frequency spectral shape of the waves is fixed by causality and, hence, independent of details of the source. Furthermore, their spectral shape encodes information about the equation of state of the universe, and also about the presence of free-streaming radiation. This makes gravitational waves a powerful tool to study early universe cosmology. In the second part of the talk, I will explore a number of motivated scenarios beyond the standard model that could be tested with future gravitational wave instruments, if we detect a sufficiently strong stochastic gravitational wave signal. |