The Cartesian Cafe

Richard Easther is a scientist, teacher, and communicator. He has been a Professor of Physics at the University of Auckland for over the last 10 years and was previously a professor of physics at Yale University. As a scientist, Richard covers ground that crosses particle physics, cosmology, astrophysics and astronomy, and in particular, focuses on the physics of the very early universe and the ways in which the universe changes between the Big Bang and the present day.

In this episode, Richard and I discuss the details of cosmology at large, both technically and historically. We dive into Einstein's equations from general relativity and see what implications they have for an expanding universe alongside a discussion of the cast of characters involved in 20th century cosmology (Einstein, Hubble, Friedmann, Lemaitre, and others). We also discuss inflation, gravitational waves, the story behind Brian Keating's book Losing the Nobel Prize, and the current state of experiments and cosmology as a field.

Patreon: https://www.patreon.com/timothynguyen

Originally published on May 3, 2022 on YouTube: https://youtu.be/DiXyZgukRmE

Timestamps:

• 00:00:00 : Introduction
• 00:02:42 : Astronomy must have been one of the earliest sciences
• 00:03:57 : Eric Weinstein and Geometric Unity
• 00:13:47 : Outline of podcast
• 00:15:10 : Brian Keating, Losing the Nobel Prize, Geometric Unity
• 00:16:38 : Big Bang and General Relativity
• 00:21:07 : Einstein's equations
• 00:26:27 : Einstein and Hilbert
• 00:27:47 : Schwarzschild solution (typo in video)
• 00:33:07 : Hubble
• 00:35:54 : One galaxy versus infinitely many
• 00:36:16 : Olbers' paradox
• 00:39:55 : Friedmann and FRLW metric
• 00:41:53 : Friedmann metric was audacious?
• 00:46:05 : Friedmann equation
• 00:48:36 : How to start a fight in physics: West coast vs East coast metric and sign conventions.
• 00:50:05 : Flat vs spherical vs hyperbolic space
• 00:51:40 : Stress energy tensor terms
• 00:54:15 : Conservation laws and stress energy tensor
• 00:58:28 : Acceleration of the universe
• 01:05:12 : Derivation of a(t) ~ t^2/3 from preceding computations
• 01:05:37 : a = 0 is the Big Bang. How seriously can we take this?
• 01:07:09 : Lemaitre
• 01:11:51 : Was Hubble's observation of an expanding universe in 1929 a fresh observation?
• 01:13:45 : Without Einstein, no General Relativity?
• 01:14:45 : Two questions: General Relativity vs Quantum Mechanics and how to understand time and universe's expansion velocity (which can exceed the speed of light!)
• 01:17:58 : How much of the universe is observable
• 01:24:54 : Planck length
• 01:26:33 : Physics down to the Big Bang singularity
• 01:28:07 : Density of photons vs matter
• 01:33:41 : Inflation and Alan Guth
• 01:36:49 : No magnetic monopoles?
• 01:38:30 : Constant density requires negative pressure
• 01:42:42 : Is negative pressure contrived?
• 01:49:29 : Marrying General Relativity and Quantum Mechanics
• 01:51:58 : Symmetry breaking
• 01:53:50 : How to corroborate inflation?
• 01:56:21 : Sabine Hossenfelder's criticisms
• 02:00:19 : Gravitational waves
• 02:01:31 : LIGO
• 02:04:13 : CMB (Cosmic Microwave Background)
• 02:11:27 : Relationship between detecting gravitational waves and inflation
• 02:16:37 : BICEP2
• 02:19:06 : Brian Keating's Losing the Nobel Prize and the problem of dust
• 02:24:40 : BICEP3
• 02:26:26 : Wrap up: current state of cosmology

Notes:

• Easther's blogpost on Eric Weinstein: http://excursionset.com/blog/2013/5/25/trainwrecks-i-have-seen
• Vice article on Eric Weinstein and Geometric Unity:
  https://www.vice.com/en/article/z3xbz4/eric-weinstein-says-he-solved-the-universes-mysteries-scientists-disagree

Further learning:

• Matts Roos. "Introduction to Cosmology"
• Barbara Ryden. "Introduction to Cosmology"
• Our Cosmic Mistake About Gravitational Waves: https://www.youtube.com/watch?v=O0D-COVodzY