Astrophysics Lunch

Leo Stein will talk about: Why binary pulsars can't help, and we need gravitational waves 

There are many reasons to think general relativity requires corrections at some yet-unknown curvature length scale—from effective field theory to the straightforward desire to test theory with observation. The simplest "gravitational" corrections are long-ranged scalar fields, but these generically lead to wildly differing phenomenology, like scalar "hair" and the presence of dipolar gravitational radiation! The lore, then, is that scalar-tensor theories are strongly constrained from weak-field (Solar System and binary pulsar) tests. This lore turns out to be wrong when the theory is built from certain curvature invariants related to topological properties. These topologically-related theories hide their corrections from weak-field tests: we prove that certain scalar "hairs" are forbidden, suppressing dipole radiation, so the potential bounds on these theories are weaker. Stronger constraints would come from a pulsar-black-hole binary or from gravitational wave measurements. 

David Chernoff will talk about: Prospects for astrophysical detection of cosmic superstrings