12 January 2022

RTG Colloquium - Online (orig. University Bielefeld)

10:30- 15:00 CET


10:30 - 11:30 Christos Tsagas (Aristotle University of Thessaloniki): Cosmic Acceleration in 'Tilted' Universes

Abstract: Tilted cosmological models are spacetimes that allow for two families of observers, moving relative to each other with finite peculiar velocities. By construction, tilted cosmologies provide a more realistic description of the actual universe, where bulk peculiar flows appear to be the norm rather than the exception. Given that relative-motion effects are known to interfere with the way the associated observers interpret their data and understand the world they live in, it is worth investigating the theoretical implications of such large-scale peculiar motions for cosmology. In this talk we consider the implications of bulk peculiar flows for cosmic acceleration and more specifically for the deceleration parameter of the universe. Applying relativistic cosmological perturbation theory to a tilted almost-Friedmann universe, we consider the mean kinematics of typical galaxies, like our Milky Way,  which move with respect to the smooth Hubble flow. We find that observers residing in these galaxies can assign very different values to their deceleration parameters, entirely because of their relative motion. In fact, some observers may even experience apparent accelerated expansion, while the host universe is actually decelerating. Although the accelerating effect is a local artefact of relative motion, the affected scales can be large enough to create the false impression of a recent global event.

11:30 - 14:00 Break

14:00 - 15:00 Alberto Sesana (University of Milan-Bicicca): The Importance of GW Background Detection for SMBHB Population Studies

Abstract: The recent detection of gravitational waves (GWs) marked the opening of a completely new window on the Universe. At nHz frequencies, pulsar timing arrays (PTAs) promise to detect the signal coming from the cosmological population of supermassive black hole binaries (SMBHBs) within the next few years. PTAs will help probing the high mass end of the SMBHB mass function, contributing to our understanding of the cosmic evolution of SMBHs and the physics governing their pairing in binaries and subsequent dynamics. After reviewing the astrophysics of SMBHBs, I will describe the recent detection of a common red signal in PTA dataand discuss its possible implications for SMBHB astrophysics.

More details will follows soon.

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