🌌Cosmology Unit 3 – The Big Bang Theory and Supporting Evidence

What's the Big Bang Theory?

• Cosmological model explaining the origin and evolution of the universe
• Proposes the universe began as an extremely hot, dense point ~13.8 billion years ago
• Initial singularity rapidly expanded and cooled, allowing matter and energy to form
• Universe continues to expand today, with galaxies moving away from each other
• Accounts for observed phenomena like cosmic microwave background radiation and abundance of light elements
• Developed throughout the 20th century based on Einstein's theory of general relativity and observations by Hubble and others
• Does not address the initial cause of the expansion or what preceded the Big Bang

Key Players and Discoveries

• Albert Einstein developed the theory of general relativity (1915), laying the foundation for modern cosmology
• Alexander Friedmann derived solutions to Einstein's equations that suggested an expanding universe (1922)
• Georges Lemaître independently proposed the idea of an expanding universe and the "primeval atom" (1927)
  • Lemaître's work was the first to connect the recession of galaxies with the origin of the universe
• Edwin Hubble discovered the relationship between a galaxy's distance and its redshift, known as Hubble's law (1929)
  • Hubble's observations provided evidence for an expanding universe
• George Gamow, Ralph Alpher, and Robert Herman predicted the existence of cosmic microwave background radiation (1948)
• Arno Penzias and Robert Wilson accidentally discovered the cosmic microwave background (1965), providing crucial evidence for the Big Bang theory

Timeline of the Universe

• Planck Epoch (0 to 10^-43 seconds): The earliest stage, dominated by quantum gravity effects
• Grand Unification Epoch (10^-43 to 10^-36 seconds): Strong, weak, and electromagnetic forces were unified
• Inflationary Epoch (10^-36 to 10^-32 seconds): Brief period of exponential expansion
  • Inflation explains the observed flatness, homogeneity, and isotropy of the universe
• Electroweak Epoch (10^-32 to 10^-12 seconds): Electromagnetic and weak forces separated, quarks formed
• Quark Epoch (10^-12 to 10^-6 seconds): Quarks combined to form hadrons (protons and neutrons)
• Hadron Epoch (10^-6 to 1 second): Hadrons dominated the universe, neutrinos decoupled
• Lepton Epoch (1 to 10 seconds): Leptons (electrons, positrons) dominated, neutrinos continued to interact
• Photon Epoch (10 seconds to 380,000 years): Photons dominated, nuclei formed (nucleosynthesis)
• Matter Domination (380,000 years to present): Matter began to dominate, atoms formed (recombination), cosmic microwave background released

Evidence Supporting the Big Bang

• Hubble's law and the expansion of the universe
  • Galaxies are moving away from each other, with more distant galaxies receding faster
• Cosmic microwave background radiation
  • Remnant heat from the early universe, uniformly distributed in all directions
• Abundance of light elements (hydrogen, helium, lithium)
  • Primordial nucleosynthesis in the early universe produced these elements in predicted ratios
• Large-scale structure of the universe
  • Distribution of galaxies and galaxy clusters consistent with predictions from the Big Bang model
• Redshift of distant galaxies
  • Light from distant galaxies is shifted towards longer (redder) wavelengths due to the expansion of space
• Age of the oldest stars and galaxies
  • Estimated ages are consistent with the timeline of the Big Bang model

Cosmic Microwave Background

• Remnant radiation from the early universe, ~380,000 years after the Big Bang
• Released when the universe cooled enough for electrons and protons to form neutral atoms (recombination)
  • Before recombination, photons were constantly scattered by free electrons, making the universe opaque
• Nearly uniform in all directions, with a temperature of ~2.7 Kelvin
• Discovered by Arno Penzias and Robert Wilson in 1965
  • Detected as a faint "noise" in their radio antenna, confirmed to be the predicted cosmic microwave background
• Provides a "snapshot" of the universe at the time of recombination
• Tiny fluctuations in the temperature of the CMB correspond to density variations in the early universe
  • These density variations seeded the formation of galaxies and large-scale structures

Expansion of the Universe

• Galaxies are moving away from each other, with more distant galaxies receding faster (Hubble's law)
• Expansion rate is determined by the Hubble constant, currently estimated at ~70 km/s/Mpc
• Expansion is uniform on large scales, but not uniform on smaller scales due to gravitational interactions
• Expansion is accelerating, likely driven by dark energy
  • Dark energy is a hypothetical form of energy that permeates all of space and exerts a negative pressure
• Redshift of distant galaxies is a consequence of the expansion of space
  • As space expands, the wavelength of light traveling through it is stretched, shifting it towards the red end of the spectrum
• The ultimate fate of the universe depends on the nature of dark energy and the total matter-energy content
  • Possibilities include eternal expansion, a "Big Freeze," or a "Big Rip"

Challenges and Alternative Theories

• Horizon problem: Regions of the universe that should not have been in causal contact have nearly identical temperatures
  • Inflation provides a solution by proposing a brief period of exponential expansion in the early universe
• Flatness problem: The universe appears to be nearly spatially flat, requiring fine-tuned initial conditions
  • Inflation drives the universe towards flatness, explaining the observed geometry
• Magnetic monopoles: Grand Unified Theories predict the existence of magnetic monopoles, which have not been observed
  • Inflation dilutes the density of magnetic monopoles, making them extremely rare
• Steady State theory: An alternative to the Big Bang, proposing a universe that is eternally expanding and creating new matter
  • Largely discredited by the discovery of the cosmic microwave background and the observed evolution of galaxies
• Oscillating universe models: Propose a universe that undergoes cycles of expansion and contraction
  • Challenges include the observed acceleration of the expansion and the second law of thermodynamics

Implications and Future Research

• The Big Bang theory provides a framework for understanding the history and evolution of the universe
• Ongoing research aims to refine our understanding of the universe's composition, including dark matter and dark energy
  • Dark matter is thought to make up ~27% of the universe, but its nature remains unknown
  • Dark energy, which makes up ~68% of the universe, is driving the accelerated expansion
• Studying the cosmic microwave background can provide insights into the early universe and the physics of high energies
  • Experiments like the Planck satellite have provided detailed measurements of the CMB
• Gravitational waves, predicted by the theory of inflation, could offer a window into the earliest moments of the universe
  • Detection of primordial gravitational waves would provide strong evidence for inflation
• Future telescopes, such as the James Webb Space Telescope, will allow us to observe the earliest galaxies and stars
  • These observations will test our understanding of the timeline of the universe and the formation of cosmic structures
• Unifying quantum mechanics and general relativity remains a major challenge in cosmology
  • A theory of quantum gravity is needed to describe the earliest stages of the universe and the nature of singularities