Talks

The problem of time is often discussed as an obstacle in the canonical quantisation of gravitational systems: general covariance means there is no preferred time parameter with respect to which evolution could be defined. We can instead characterise dynamics in relational terms by defining one degree of freedom to play the role of an internal clock for the other variables; this leads to a multiple choice problem of which variable should play the role of clock. I will review recent results obtained in a quantum cosmological model with three dynamical degrees of freedom: a volume or scale factor variable for the geometry, a massless scalar matter field, and a perfect fluid. Each of these variables can be used as a clock for the other two. We obtain three different theories which, if we require them to have unitary time evolution with respect to the given clock, make very different statements about the fate of the Universe. Only one resolves the classical singularity, and only one leads to a quantum recollapse of the Universe at large volume. Nonclassical behaviour arises whenever a classical solution terminates in finite time so that reflecting boundary conditions are needed to make the theory unitary. We discuss general implications for a canonical quantisation of gravitational systems.

Zoom Link: https://pitp.zoom.us/j/95556457739?pwd=S0dZUVYwTTBOaFNpNGcrc2ladHZ5QT09

In this talk, I will present how gravitational-wave (GW) measurements can be utilized to search for ultralight bosons and primordial black holes (PBHs), which can be the keys to some of the unsolved questions about our Universe. In the first half, I will discuss the current constraints on excluding the mass of ultralight bosons around ~10^-13 eV, based on the mass-spin measurements of BBHs from the second GW catalog (GWTC-2). These constraints are driven by the phenomenology called “superradiance”, which extracts the angular momentum of a rapidly spinning black hole to form a slowly spinning black hole-boson cloud system. In the second half, I will illustrate how the distance measurements of very high-redshift BBHs made by the next-generation GW detectors may provide cleaner hints or constraints for the existence of PBHs in the future.

Zoom Link: https://pitp.zoom.us/j/95012157554?pwd=UWdBZ1FOTSt2dVlBaS81MjRLbVBvUT09