Nonsingular Bounce Cosmology & Perturbation Theory-Effective Field Theory Perspective by Yi-Fu Cai

PROGRAM: PHYSICS OF THE EARLY UNIVERSE - AN ONLINE PRECURSOR
ORGANIZERS: Robert Brandenberger (McGill University, Montreal, Canada), Jerome Martin (Institut d'Astrophysique de Paris, France), Subodh Patil
(Instituut-Lorentz for Theoretical Physics, Leiden, Netherlands) and L Sriramkumar (Indian Institute of Technology Madras, India)
DATE: 31 August 2020 to 03 September 2020
VENUE: Online

The aims of the School were to expose graduate students and post-doctoral fellows to essential ideas related to the physics of the early universe. The School was, to begin with lectures on basic cosmological perturbation theory and alternative paradigms for the generation of the primordial perturbations. These were to be followed by lectures on the relation between models of the early universe and particle physics as well as the comparison of these models with the cosmological data. We had also planned to cover emerging topics such as the non-trivial effects of gauge fields during inflation and reheating as well as their imprints on observable quantities.

Models of the early universe that have been constructed so far are like toy models, with no definitive understanding of the fundamental physics that operate at high energies. Importantly, these simple constructions do not seem to provide answers to key conceptual questions such as the nature of the cosmological constant and the need to have a unified description of the early universe, which is consistent from the point of view of ultraviolet physics. While the School was aimed at raising a new generation of theorists to grapple with such problems, the Workshop had hoped to engender new ideas in these directions.

We have planned an online precursor which in a certain way mimics our original plan. We have requested the lecturers we had invited for the School to give broader talks at the level of colloquia and we have requested the invited speakers for the Workshop to give shorter technical talks.

We have planned a program with adequate time in between the talks to encourage interactions.


Eligibility criteria: The meeting will be open to Ph.D. students, post-doctoral fellows, and faculty working in the area. We also intend to permit senior Master's and advanced undergraduate students to participate in the program, depending on the demand.

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0:00:00 Nonsingular bounce cosmology and the perturbation theory from the perspective of effective field theory
0:00:10 ICTS
0:02:18 Lesson from inflationary cosmology
0:06:20 Outline
0:07:24 Ekpyrotic model
0:09:18 Nonsingular cosmologies
0:10:51 Challenges of nonsingular bounce cosmologies
0:11:02 Challenges: Background
0:16:47 Challenges: Perturbation
0:17:19 Towards nonsingular bounce by a single scalar field
0:17:36 Nonsingular bounce with single scalar
0:20:50 Equations of Motion
0:21:21 Sketch
0:22:01 Background solution
0:24:31 Comments:
0:25:25 Cosmological perturbations in nonsingular bounce
0:25:29 Sketch Plots
0:28:33 Setup of Perturbations
0:29:00 Curvature Perturbations
0:29:26 Gradient instability near the bounce
0:30:19 Numerical Estimates
0:30:37 Evade the theoretical No-Go with the EFT
0:31:08 DHOST Bounce
0:32:52 Enhancement on IR modes through the bounce
0:32:54 An observational No-Go theorem
0:33:00 Summary