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### Condensed Matter Seminars

### Particle / Cerca Seminars

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Massless particles such as photons and gravitons do not travel solely on the null cone in a generic curved spacetime. They propagate at all speeds equal to and less than c. This fact does not appear to be well appreciated in cosmology, and its consequences deserve to be worked out to ensure we are interpreting observations correctly. A rather dramatic (and hypothetical) example would be the following: suppose a significant fraction of photons from a distant supernova travels slower than c, then we may be mislead into thinking the SN is dimmer than it actually is, because some of the light has not arrived yet. Motivated by these considerations, Glenn Starkman and I have initiated a program to understand the properties of light in our inhomogeneous universe from first principles. The central object that elucidates the causal structure of how physical signals propagate through spacetime is the retarded Green’s function, which I will review at a pedagogical level. I hope to convince the audience that this off-the-light-cone effect is real in our universe, and I will describe concrete steps we are taking to extract its possible observational co