Thesis defence

Making maps of the polarised cosmic microwave background with next generation ground-based experiments

Simon Biquard

September 26, 2025

Outline

TBW

The expanding Universe

Introduction

• The Universe is expanding
• Universe was smaller in the past
• Hot Big Bang theory: hotter, denser, “smaller”
• How do we know that?

Galaxies

• Milky Way x 100,000 light years
• Many other galaxies! ~10^11 in observable Universe
• Distant galaxies are moving away
• How do we know?

Redshift I

• Like Doppler effect of sound
• The farther away, the faster a galaxy recedes from us
• Expected if all galaxies move away from each other
• i.e. If Universe expands in all directions

Redshift II

• Spring analogy!
• Distance & redshift -> use z to talk about cosmic distances
• Light has a finite speed -> use z to talk about time
• High z is far away in space and in time

Looking at the Big Bang: the CMB

• In cosmology, we can see the history of the Universe
  • Moon ~1.3s away/ago
  • Nearby stars ~ 10-100 years
  • Other galaxies -> millions/billions of years
• Can we look so far into the past that we see the Big Bang?

Primordial Plasma

• Hot Big Bang: all matter and radiation compressed to an unimaginable degree → denser & hotter
• In this state, ordinary matter (atoms) cannot exist
• It is a hot plasma

Cosmic History

• “Inside-out star”
• BBN: nuclear reactions everywhere, nuclei forming
• Photons bouncing off protons & electrons, cannot travel very far
• Time passes: moving from core of the star to its diffuse edge
• Eventually (380,000 years), the first atoms form
• Photons can travel freely, like the light of the star shining out into space

The CMB

• From our perspective, CMB photons seem to come from a spherical surface: the LSS
• This is the farthest away that we can see into the cosmos
• Visible spectrum (3000K) at the time
• Today microwave (2.7K) because of the expansion
• Blackbody spectrum: confirms Big Bang theory!

Observing the CMB

TBW

Cosmic inflation

• CMB: triumph of Hot Big Bang theory?
• Also a sign that cosmic history must be more complex → inflation

The Horizon Problem

• Horizon problem: why is the CMB so homogeneous?
• Points at same temperature must have been in contact
• Light from 2 opposite points “meets” for the first time right now when reaching us

Inflation

• Most widely accepted solution: inflation
• Expansion fast enough to take a causally connected region of space and blow it up to a size bigger than our observable Universe
• Like enlarging a single pixel of an image

Successes of Inflation

• Seeds of cosmic structure
• Still not proven
• Smoking gun: B modes