Tuesdays 10:30 - 11:30 | Fridays 11:30 - 12:30
Showing votes from 2015-10-09 12:30 to 2015-10-13 11:30 | Next meeting is Friday Jul 3rd, 11:30 am.
We point out a surprising consequence of the usually assumed initial conditions for cosmological perturbations. Namely, a scale-invariant spectrum of Gaussian, linear, adiabatic, scalar, growing mode perturbations not only creates acoustic oscillations, of the kind observed in great detail on large scales today, it also leads to the production of shock waves in the radiation fluid of the very early universe. At very early epochs, $1$ GeV$<T<10^{7}$ GeV, assuming standard model physics, viscous damping is negligible and nonlinear effects turn acoustic waves into shocks after $\sim 10^4$ oscillations. The resulting scale-invariant network of shocks provides a natural mechanism for creating significant departures from local thermal equilibrium as well as primordial vorticity and gravitational waves.
Isolated objects in asymptotically flat spacetimes in general relativity are characterized by their conserved charges associated with the Bondi-Metzner-Sachs (BMS) group. These charges include total energy, linear momentum, intrinsic angular momentum and center-of-mass location, and in addition an infinite number of supermomentum charges associated with supertranslations. Recently it has been suggested that the BMS symmetry algebra should be enlarged to include an infinite number of additional symmetries known as superrotations. We compute the corresponding "super center-of-mass'' charges and show that they are finite and well defined. The supermomentum charges are associated with ordinary gravitational wave memory, and the super center-of-mass charges are associated with total (ordinary plus null) gravitational wave memory, in the terminology of Bieri and Garfinkle. Some of these charges can give rise to black hole hair, as described by Strominger and Zhiboedov. We clarify how this hair evades the no hair theorems.