Ultralocality and the robustness of slow contraction to cosmic initial conditions

Anna Ijjas (Albert Einstein Institute, Max Planck Institute for Gravitational Physics)

I will discuss the detailed process by which slow contraction smooths and flattens the universe using an improved numerical relativity code that accepts initial conditions with non-perturbative deviations from homogeneity and isotropy along two independent spatial directions. Contrary to common descriptions of the early universe, I will show that the geometry first rapidly converges to an inhomogeneous, spatially-curved and anisotropic ultralocal state in which all spatial gradient contributions to the equations of motion decrease as an exponential in time to negligible values. This is followed by a second stage in which the geometry converges to a homogeneous, spatially flat and isotropic spacetime. In particular, the decay appears to follow the same history whether the entire spacetime or only parts of it are smoothed by the end of slow contraction.

Meeting ID:  854 3210 3337      Password: 959078

Observing the Inner Shadow of a Black Hole: A Direct View of the Event Horizon

Andrew Chael (Princeton University)

Simulated images of a black hole surrounded by optically thin emission typically display two main features: acentral brightness depression and a narrow, bright “photon ring” consisting of strongly lensed images superposedon top of the direct emission. The photon ring closely tracks a theoretical curve on the image plane correspondingto light rays that asymptote to unstably bound photon orbits around the black hole. This critical curve has asize and shape that are purely governed by the Kerr geometry; in contrast, the size, shape, and depth of theobserved brightness depression all depend on the details of the emission region. For instance, images of sphericalaccretion models display a distinctive dark region—the “black hole shadow”—that completely fills the photonring. By contrast, in models of equatorial disks extending to the black hole’s event horizon, the darkest regionin the image is restricted to a much smaller area—aninner shadow—whose edge lies near the direct lensedimage of the equatorial horizon. Using both semi-analytic models and general relativistic magnetohydrodynamic(GRMHD) simulations, we demonstrate that the photon ring and inner shadow may be simultaneously visible insubmillimeter images of M87, where magnetically arrested disk (MAD) simulations predict that the emissionarises in a thin region near the equatorial plane. We show that the relative size, shape, and centroid of thephoton ring and inner shadow can be used to estimate the black hole mass and spin, breaking degeneracies inmeasurements of these quantities that rely on the photon ring alone. Both features may be accessible to directobservation via high-dynamic-range images with a next-generation Event Horizon Telescope.

Meeting ID:  854 3210 3337      Password: 959078