Soft Gravitons, Medium Consequences, Hard Truths

In this talk, I will revisit a question raised by Steven Weinberg in one of his last papers, namely whether mutual transparency of matter and gravitons is affected by the presence of a gravitational wave background. We will see that a naive calculation could lead us to conclude that low-frequency gravitational waves could have dramatic consequences for metastable relics. These apparent consequences are a mirage arising from trusting leading order pertubation theory. To avoid this pitfall, we formulate a new version of Weinberg's soft graviton theorm in a way that accounts for the graviton medium. Ultimately, we will see that mutual transparency of matter and gravitational radiation is ensured, and is a consequence of the infrared safety of gravity.