🚀Astrophysics II Unit 13 – Early Universe: Inflation and CMB

Key Concepts and Theories

• Big Bang theory proposes the universe began from a singularity and expanded and cooled over time
• Cosmic inflation theory suggests a brief period of exponential expansion in the early universe, solving horizon and flatness problems
• Cosmic microwave background (CMB) radiation is the oldest light in the universe, a remnant from the early stages of the universe
• Baryogenesis explains the observed matter-antimatter asymmetry in the universe
  • Sakharov conditions: baryon number violation, C and CP symmetry violation, and interactions out of thermal equilibrium
• Nucleosynthesis describes the formation of light elements (hydrogen, helium, and traces of lithium) in the early universe
• Dark matter and dark energy are hypothesized to explain observations of galaxy rotation curves and accelerating expansion of the universe
• Standard Model of particle physics describes elementary particles and their interactions
  • Includes quarks, leptons, and force-carrying bosons (photons, gluons, W and Z bosons)

Timeline of Early Universe Events

• Planck epoch ($t < 10^{-43}$ seconds): Quantum gravity effects dominate, physics is uncertain
• Grand unification epoch ($10^{-43}$ seconds $< t < 10^{-36}$ seconds): Strong, weak, and electromagnetic forces are unified
• Inflationary epoch ($10^{-36}$ seconds $< t < 10^{-32}$ seconds): Universe undergoes exponential expansion
• Electroweak epoch ($10^{-32}$ seconds $< t < 10^{-12}$ seconds): Electromagnetic and weak forces separate, quarks and leptons form
• Quark epoch ($10^{-12}$ seconds $< t < 10^{-6}$ seconds): Quarks combine to form hadrons (protons and neutrons)
• Hadron epoch ($10^{-6}$ seconds $< t < 1$ second): Hadrons and antihadrons annihilate, leaving a small excess of matter
• Lepton epoch ($1$ second $< t < 10$ seconds): Leptons and antileptons annihilate, neutrinos decouple from matter
• Nucleosynthesis ($10$ seconds $< t < 20$ minutes): Light elements (hydrogen, helium, and traces of lithium) form

Cosmic Inflation Theory

• Proposed by Alan Guth in 1980 to explain the observed flatness and homogeneity of the universe
• Suggests a brief period of exponential expansion in the early universe, driven by a hypothetical scalar field called the inflaton
• Solves the horizon problem by explaining how distant regions of the universe can have similar properties
  • Regions that appear disconnected were in causal contact before inflation
• Addresses the flatness problem by driving the universe towards a flat geometry
  • Any initial curvature is smoothed out by the rapid expansion
• Predicts a nearly scale-invariant spectrum of primordial density fluctuations, which seed the formation of large-scale structures
• Inflationary models include slow-roll inflation, chaotic inflation, and eternal inflation
• Observational evidence for inflation includes the flatness of the universe and the near scale-invariance of the CMB power spectrum

Cosmic Microwave Background (CMB)

• The oldest light in the universe, a remnant from the early stages of the universe
• Discovered by Arno Penzias and Robert Wilson in 1965
• Corresponds to a time when the universe became transparent to photons, about 380,000 years after the Big Bang
• Has a nearly perfect black-body spectrum with a temperature of 2.725 K
• Exhibits tiny temperature fluctuations ($\Delta T/T \sim 10^{-5}$) that correspond to primordial density fluctuations
  • These fluctuations are the seeds of large-scale structure formation
• Polarization of the CMB provides information about the early universe and the formation of the first stars
  • E-mode polarization arises from density fluctuations, while B-mode polarization can be a signature of primordial gravitational waves
• Studied by various experiments, including COBE, WMAP, and Planck satellites

Observational Evidence

• Hubble's law: Galaxies are receding from us with velocities proportional to their distance, indicating an expanding universe
• Abundance of light elements: Primordial nucleosynthesis predictions match observed abundances of hydrogen, helium, and lithium
• Cosmic microwave background (CMB) radiation: A nearly perfect black-body spectrum with tiny temperature fluctuations
  • CMB power spectrum is consistent with predictions of inflation and a flat universe
• Large-scale structure: Distribution of galaxies and clusters follows the pattern of primordial density fluctuations
• Type Ia supernovae: Used as standard candles to measure cosmic distances, revealing the accelerating expansion of the universe
• Gravitational lensing: Distortion of light by massive objects provides evidence for dark matter and constrains cosmological parameters
• Baryon acoustic oscillations (BAO): Imprint of sound waves in the early universe on the distribution of galaxies

Mathematical Models and Equations

• Friedmann equations describe the expansion of the universe in the context of general relativity
  • $\left(\frac{\dot{a}}{a}\right)^2 = \frac{8\pi G}{3}\rho - \frac{kc^2}{a^2}$ (first Friedmann equation)
  • $\frac{\ddot{a}}{a} = -\frac{4\pi G}{3}\left(\rho + \frac{3p}{c^2}\right)$ (second Friedmann equation)
• Hubble's law relates the recessional velocity of a galaxy to its distance: $v = H_0 d$
• Cosmological parameters: Hubble constant ($H_0$), matter density ($\Omega_m$), dark energy density ($\Omega_\Lambda$), curvature density ($\Omega_k$)
• Inflation models: Slow-roll inflation described by the inflaton field and its potential $V(\phi)$
  • Slow-roll parameters: $\epsilon = \frac{1}{2}\left(\frac{V'}{V}\right)^2$ and $\eta = \frac{V''}{V}$
• CMB power spectrum: Characterized by angular power spectra $C_\ell$ for temperature and polarization fluctuations
• Einstein field equations: $G_{\mu\nu} = \frac{8\pi G}{c^4} T_{\mu\nu}$, relating the curvature of spacetime to the energy-momentum tensor

Current Research and Open Questions

• Nature of dark matter and dark energy: Identifying the particles or fields responsible for these phenomena
• Inflationary model selection: Distinguishing between various inflationary models using observational data
• Primordial gravitational waves: Searching for B-mode polarization in the CMB as evidence of inflation
• Neutrino masses and their role in the early universe: Investigating the impact of massive neutrinos on structure formation
• Baryogenesis mechanisms: Exploring theories that explain the observed matter-antimatter asymmetry
• Quantum gravity and the Planck epoch: Developing a theory of quantum gravity to describe the earliest stages of the universe
• Multiverse and eternal inflation: Investigating the possibility of multiple universes and their observational consequences
• Precision measurements of cosmological parameters: Refining estimates of $H_0$, $\Omega_m$, $\Omega_\Lambda$, and other parameters

Applications and Implications

• Understanding the origin and evolution of the universe: Insights into the fundamental laws of physics and cosmology
• Constraining particle physics models: Early universe conditions provide unique tests for theories beyond the Standard Model
• Formation and evolution of galaxies and large-scale structures: Studying the growth of density fluctuations and the role of dark matter
• Cosmological simulations: Modeling the evolution of the universe and comparing with observational data
• Gravitational wave astronomy: Using primordial gravitational waves to probe the inflationary epoch
• Dark energy and the ultimate fate of the universe: Investigating the nature of dark energy and its impact on cosmic expansion
• Anthropic reasoning and fine-tuning: Exploring the apparent fine-tuning of cosmological parameters for the emergence of life
• Philosophical and theological implications: Reflecting on the nature of reality, the role of science, and the place of humanity in the cosmos