Particle physics from cosmic backgrounds
Most striking evidence beyond the Standard Model so far comes from cosmological observations, most notably from studying the cosmic microwave background (CMB). This background radiation is the oldest electromagnetic radiation in the Universe dating back to the so-called recombination epoch in roughly 370,000-year-old Universe when protons and electrons formed electrically neutral hydrogen atoms. The precise observations of the fluctuations in the CMB can be used, for example, to deduce the energy budget of the Universe, and the most recent observations show that 27% consists of dark matter that gravitates but does not interact electromagnetically. On the other hand, the recent observations of gravitational-wave (GW) signals from the mergers of massive astrophysical objects were a milestone for the start of an era of truly multi-messenger astronomy. The next generation of space-based GW observatories like LISA will be able to explore the thermal history of the Universe beyond the CMB by observing the stochastic GW background reaching to first moments after the Big Bang.
In these lectures, we will discuss both CMB and stochastic GW background and study how these cosmological backgrounds can be used as complementary probes on the fundamental interactions not restricted to the reach of collider experiments.