Institute of Cosmology and Gravitation

Tays Miranda (Jyväskylä) and Emmanuel Frion (Helsinki)

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Event Details

This session will consist of two short talks:

Tays Miranda (University of Jyväskylä)

Title: The stochastic formalism and contracting cosmologies

Abstract: In this talk I will show how quantum field fluctuations give rise to stochastic noise in a cosmological model described by a scalar field with an exponential potential. In particular, I compute these quantum fluctuations in an inflating or contracting cosmology and discuss how they perturb the equation of state on large scales, which can lead to a quantum instability of the classical collapse solution, below the Planck scale in the case of a pressureless collapse. Also I will comment on the issues for the implementation of the stochastic formalism, beyond the inflationary scenario, related to gauge corrections to the amplitude of the stochastic noise and the validity of the separate universe approach.

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Emmanuel Frion (Helsinki Institute of Physics)
Title: Testing the Equivalence Principle with Black Hole Shadows and Photon Rings
Abstract: I will present how the expansion of the Universe can modify the size of a black hole shadow, and I will comment about its potential applications as an almost model-independent probe of the matter content of the Universe and of the value of the Hubble rate today. The expected time variation of a black hole shadow is however very small, which is beyond the current resolution of the Event Horizon Telescope and its near-future improvements. I will argue that photon rings could be better targets in the future. I also propose to use the smallness of the shadow drift to probe a time variation of the gravitational constant, which I illustrate with the data of M87*.

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