🪐Intro to Astronomy Unit 29 – The Big Bang

What's the Big Bang?

• Cosmological model explaining the origin and evolution of the universe
• Universe began as an infinitely hot, dense point called a singularity approximately 13.8 billion years ago
• Rapid expansion and cooling of the universe occurred in the first fraction of a second after the Big Bang
• As the universe expanded and cooled, fundamental particles such as quarks and electrons formed
  • These particles combined to form protons and neutrons
• Universe continued to expand and cool, allowing atoms to form (primarily hydrogen and helium)
• Over billions of years, gravity caused matter to clump together, forming stars, galaxies, and larger structures
• Big Bang theory does not explain the cause of the initial singularity or what existed before

Timeline of the Universe

• Planck Epoch (0 to 10^-43 seconds): The earliest stage of the universe, governed by quantum gravity
• Grand Unification Epoch (10^-43 to 10^-36 seconds): Fundamental forces (except gravity) were unified
• Inflationary Epoch (10^-36 to 10^-32 seconds): Rapid exponential expansion of the universe
• Electroweak Epoch (10^-32 to 10^-12 seconds): Electromagnetic and weak nuclear forces separated
• Quark Epoch (10^-12 to 10^-6 seconds): Quarks combined to form hadrons (protons and neutrons)
• Hadron Epoch (10^-6 to 1 second): Hadrons were the dominant form of matter
• Lepton Epoch (1 to 10 seconds): Leptons (electrons, neutrinos) were the dominant form of matter
• Photon Epoch (10 seconds to 380,000 years): Photons were the dominant form of energy
  • Universe was a hot, dense plasma of electrons, protons, and photons
• Recombination (380,000 years): Electrons combined with protons to form neutral atoms (primarily hydrogen)
  • Universe became transparent to light, releasing the Cosmic Microwave Background (CMB) radiation
• Dark Ages (380,000 to 400 million years): No stars or galaxies had formed yet
• Reionization (400 million to 1 billion years): First stars and galaxies formed, reionizing the universe
• Structure Formation (1 billion years to present): Formation and evolution of galaxies, stars, and planets

Key Evidence for the Big Bang

• Hubble's Law and cosmic expansion: Galaxies are moving away from each other, with more distant galaxies receding faster
  • Indicates the universe is expanding and was once much smaller and denser
• Cosmic Microwave Background (CMB) radiation: Remnant heat from the early universe, observed as uniform background radiation
  • CMB has a nearly perfect black-body spectrum, consistent with Big Bang predictions
• Abundance of light elements: Big Bang nucleosynthesis explains the observed abundances of hydrogen (~75%) and helium (~25%)
  • Heavier elements formed later in stars through stellar nucleosynthesis
• Large-scale structure of the universe: Distribution of galaxies and galaxy clusters follows a "cosmic web" pattern
  • Consistent with predictions of structure formation from initial density fluctuations in the early universe
• Age of the oldest stars: Oldest observed stars are approximately 13.2 billion years old
  • Consistent with the estimated age of the universe based on the Big Bang model
• Redshift of distant galaxies: Light from distant galaxies is shifted towards longer (redder) wavelengths
  • Indicates the universe is expanding, as predicted by the Big Bang theory

Major Players and Discoveries

• Edwin Hubble (1920s): Discovered the expansion of the universe through observations of distant galaxies
  • Hubble's Law relates a galaxy's distance to its recessional velocity
• Georges Lemaître (1920s-1930s): Proposed the idea of the "primeval atom" and the expanding universe
  • Derived Hubble's Law from Einstein's equations before Hubble's observations
• George Gamow, Ralph Alpher, and Robert Herman (1940s): Developed the theory of Big Bang nucleosynthesis
  • Predicted the existence of the Cosmic Microwave Background (CMB) radiation
• Arno Penzias and Robert Wilson (1965): Accidentally discovered the CMB while working on a radio antenna
  • Confirmed the predictions of Gamow, Alpher, and Herman
• Stephen Hawking and Roger Penrose (1960s-1970s): Developed the theory of singularities in general relativity
  • Showed that the Big Bang singularity is a prediction of Einstein's equations
• Alan Guth (1980s): Proposed the theory of cosmic inflation
  • Explained the observed flatness and uniformity of the universe
• Saul Perlmutter, Brian Schmidt, and Adam Riess (1990s): Discovered the accelerating expansion of the universe
  • Led to the concept of dark energy, which makes up ~68% of the universe's energy density

Cosmic Microwave Background

• Remnant heat from the early universe, observed as a nearly uniform background of microwave radiation
• Discovered in 1965 by Arno Penzias and Robert Wilson, confirming predictions by Gamow, Alpher, and Herman
• CMB has a temperature of 2.725 Kelvin and a nearly perfect black-body spectrum
  • Consistent with predictions of the Big Bang theory
• Tiny fluctuations (anisotropies) in the CMB correspond to density fluctuations in the early universe
  • Seeds for the formation of galaxies and large-scale structures
• Studied by satellites such as COBE (1989), WMAP (2001), and Planck (2009)
  • Provided precise measurements of the CMB temperature, anisotropies, and polarization
• CMB observations support key aspects of the Big Bang model, including the age and composition of the universe
• Polarization of the CMB can provide evidence for cosmic inflation and gravitational waves in the early universe

Structure Formation

• Tiny density fluctuations in the early universe, seen as anisotropies in the CMB, served as seeds for structure formation
• Regions with slightly higher density had stronger gravitational attraction, causing matter to accumulate
• Dark matter, which makes up ~27% of the universe's energy density, played a crucial role in structure formation
  • Dark matter halos formed first, providing gravitational wells for baryonic matter (gas) to fall into
• Gas cooled and condensed within dark matter halos, forming the first stars and galaxies
  • First stars (Population III) were massive, short-lived, and contributed to reionization of the universe
• Galaxies formed and evolved through mergers and interactions, leading to the diverse types observed today
  • Spiral galaxies (Milky Way), elliptical galaxies (M87), and irregular galaxies (Large Magellanic Cloud)
• Galaxies are organized into larger structures, such as galaxy clusters (Virgo Cluster) and superclusters (Laniakea Supercluster)
• Large-scale structure of the universe resembles a "cosmic web" of filaments, walls, and voids
  • Consistent with predictions from the Cold Dark Matter (CDM) model of structure formation

Unanswered Questions

• What caused the initial singularity and triggered the Big Bang?
  • Theories such as cosmic inflation and quantum fluctuations attempt to address this question
• What happened before the Big Bang, and is the concept of "before" meaningful in this context?
• What is the nature of dark matter and dark energy, which together make up ~95% of the universe's energy density?
  • Candidates for dark matter include weakly interacting massive particles (WIMPs) and axions
• How did the universe come to have such a high degree of flatness and uniformity?
  • Cosmic inflation provides a possible explanation, but the details are still speculative
• What is the ultimate fate of the universe?
  • Will it expand forever (Big Freeze), collapse back on itself (Big Crunch), or reach a stable state (Big Bounce)?
• Are there other universes beyond our own (multiverse), and can we ever detect them?
• How did the first stars and galaxies form, and what were their properties?
  • Upcoming telescopes like James Webb Space Telescope (JWST) may provide insights into this early epoch
• What is the nature of gravity and spacetime at the quantum scale?
  • Theories of quantum gravity, such as string theory and loop quantum gravity, attempt to unify general relativity and quantum mechanics

Cool Big Bang Facts

• The Big Bang was not an explosion in space, but rather an expansion of space itself
• The universe has no center or edge; every point in the universe is expanding away from every other point
• The observable universe has a diameter of about 93 billion light-years, but the entire universe may be infinite in size
• The universe is estimated to be 13.8 billion years old, with an uncertainty of only 0.1 billion years
• The CMB is the oldest light in the universe, dating back to about 380,000 years after the Big Bang
  • Before this time, the universe was opaque to light due to the high density of charged particles
• The Big Bang produced a large number of neutrinos, which are nearly massless particles that rarely interact with matter
  • These "relic neutrinos" are estimated to have a density of about 330 million per cubic meter
• The expansion of the universe is accelerating due to the effects of dark energy
  • This was discovered in the late 1990s by observing distant supernovae, leading to a Nobel Prize in 2011
• If the universe had expanded slightly slower or faster in the early stages, stars, galaxies, and life might not have been able to form
  • This "fine-tuning" problem is one motivation for the idea of a multiverse with varying physical constants