8.4 Big Bang theory

The Big Bang theory is our best explanation for how the universe began. It posits that everything started from an incredibly hot, dense point about 13.8 billion years ago. Since then, the universe has been expanding and cooling, forming galaxies, stars, and planets.

Evidence for the Big Bang includes the expansion of the universe, cosmic microwave background radiation, and the abundance of light elements. The theory also describes the early stages of the universe, from the Planck epoch to the formation of atoms, providing a timeline for cosmic evolution.

Origins of the Big Bang theory

• The Big Bang theory is the prevailing cosmological model explaining the origin and evolution of the universe
• Developed in the early 20th century based on observations of distant galaxies and the expansion of the universe
• Key contributors include Georges Lemaître, who proposed the "primeval atom" hypothesis, and George Gamow, who developed the theory further

Key evidence for the Big Bang

Expansion of the universe

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• Observations of distant galaxies show they are moving away from us, with more distant galaxies receding faster (Hubble's law)
• The expansion of the universe implies it was smaller, denser, and hotter in the past
• The expansion rate is determined by the Hubble constant, currently estimated at ~70 km/s/Mpc

Cosmic microwave background radiation

• The CMB is the remnant heat from the early stages of the universe, redshifted to microwave wavelengths due to cosmic expansion
• Discovered by Arno Penzias and Robert Wilson in 1965, providing strong evidence for the Big Bang model
• The CMB has a nearly perfect black body spectrum at a temperature of 2.7 K, with small anisotropies reflecting early density fluctuations

Abundance of light elements

• The Big Bang nucleosynthesis predicts the relative abundances of light elements (hydrogen, helium, and traces of lithium) formed in the early universe
• Observed abundances of these elements in the oldest stars and galaxies closely match the predictions of the Big Bang model
• Heavier elements are formed later in the life cycles of stars and during supernovae explosions

Timeline of the Big Bang

Planck epoch

• The earliest stage of the universe, from 0 to approximately $10^{-43}$ seconds after the Big Bang
• At this time, the universe is thought to be a quantum foam of space-time, with all four fundamental forces unified
• The physics of this epoch is not yet well understood, as it requires a theory of quantum gravity

Grand unification epoch

• Occurs between $10^{-43}$ and $10^{-36}$ seconds after the Big Bang
• During this epoch, three of the four fundamental forces (electromagnetic, weak, and strong nuclear forces) are unified as the electronuclear force
• The universe undergoes a phase transition, causing the separation of the strong nuclear force from the electronuclear force

Inflationary epoch

• Takes place between $10^{-36}$ and $10^{-32}$ seconds after the Big Bang
• The universe undergoes a period of exponential expansion, driven by a hypothetical scalar field called the inflaton
• Inflation solves several problems in the standard Big Bang model, such as the horizon and flatness problems

Electroweak epoch

• Occurs between $10^{-32}$ and $10^{-12}$ seconds after the Big Bang
• The electromagnetic and weak nuclear forces are still unified as the electroweak force
• The universe continues to cool and expand, and the Higgs field acquires a non-zero value, breaking electroweak symmetry

Quark epoch

• Takes place between $10^{-12}$ and $10^{-6}$ seconds after the Big Bang
• Quarks and gluons are the dominant particles in the universe, forming a quark-gluon plasma
• As the universe cools, quarks begin to combine to form hadrons (protons and neutrons)

Hadron epoch

• Occurs between $10^{-6}$ and 1 second after the Big Bang
• Hadrons (protons and neutrons) become the dominant particles in the universe
• The universe continues to cool and expand, allowing for the formation of light atomic nuclei (deuterium, helium-3, and helium-4)

Lepton epoch

• Takes place between 1 and 10 seconds after the Big Bang
• Leptons (electrons, positrons, neutrinos, and antineutrinos) are the dominant particles in the universe
• As the universe cools, electron-positron pairs annihilate, leaving a small excess of electrons

Photon epoch

• Begins approximately 10 seconds after the Big Bang and lasts until about 380,000 years later
• Photons are the dominant particles in the universe, interacting frequently with charged particles (electrons and protons)
• The universe is opaque due to the constant scattering of photons by charged particles

Stages of the early universe

Baryogenesis

• The process by which an excess of matter (baryons) over antimatter is generated in the early universe
• Requires three conditions: baryon number violation, C and CP symmetry violation, and interactions out of thermal equilibrium
• The exact mechanism of baryogenesis is still unknown and is an active area of research

Nucleosynthesis

• The formation of light atomic nuclei (deuterium, helium-3, helium-4, and traces of lithium) in the early universe, starting about 3 minutes after the Big Bang
• The relative abundances of these light elements depend on the density of protons and neutrons in the early universe
• Big Bang nucleosynthesis predictions match the observed abundances of light elements in the oldest stars and galaxies

Recombination and decoupling

• Recombination occurs about 380,000 years after the Big Bang, when the universe has cooled sufficiently for electrons and protons to form neutral hydrogen atoms
• Decoupling is the process by which photons stop frequently interacting with matter and begin to travel freely through the universe
• The cosmic microwave background radiation originates from the time of recombination and decoupling

Cosmic inflation

Solving horizon and flatness problems

• Inflation solves the horizon problem by proposing that the universe underwent exponential expansion, allowing regions that were once in causal contact to be separated by vast distances
• The flatness problem is solved by inflation, as exponential expansion drives the curvature of the universe towards zero, resulting in a nearly flat geometry
• Inflation predicts that the universe should be very close to spatially flat, which is supported by observations of the cosmic microwave background

Quantum fluctuations and density perturbations

• During inflation, quantum fluctuations in the inflaton field are stretched to macroscopic scales, becoming the seeds for structure formation in the universe
• These quantum fluctuations lead to small density perturbations in the early universe, which grow over time due to gravitational instability
• The resulting density fluctuations are responsible for the formation of galaxies, clusters, and large-scale structure in the universe

Fate of the universe

Open vs closed universe

• The fate of the universe depends on its geometry and the amount of matter and energy it contains
• An open universe has negative curvature and will expand forever, with the expansion rate approaching a constant value
• A closed universe has positive curvature and will eventually stop expanding and collapse back on itself in a "Big Crunch"

Heat death and Big Freeze

• In an expanding universe, the heat death scenario occurs when the universe reaches a state of maximum entropy, with no usable energy remaining
• The Big Freeze is a scenario in which the universe continues to expand and cool indefinitely, with all matter eventually decaying into low-energy photons and leptons
• Both scenarios result in a cold, dark, and lifeless universe

Big Rip and phantom energy

• The Big Rip is a hypothetical scenario in which the expansion of the universe accelerates so rapidly that it tears apart all structures, down to atoms and subatomic particles
• This scenario is driven by a form of dark energy called phantom energy, which has a negative pressure greater in magnitude than its energy density
• The Big Rip is considered a more extreme and less likely fate for the universe compared to the heat death or Big Freeze scenarios

Challenges and alternatives to the Big Bang theory

Horizon problem

• The horizon problem arises from the observation that distant regions of the universe, which should not have been in causal contact, have nearly the same temperature and density
• This suggests that these regions were once in thermal equilibrium, but there is insufficient time in the standard Big Bang model for this to occur
• Cosmic inflation provides a solution to the horizon problem by proposing a period of exponential expansion in the early universe

Flatness problem

• The flatness problem refers to the observation that the universe appears to be very close to spatially flat, which requires a precise balance between the expansion rate and the matter/energy density
• In the standard Big Bang model, any initial curvature should have grown over time, making a flat universe highly unlikely without fine-tuning
• Cosmic inflation solves the flatness problem by driving the curvature of the universe towards zero during the inflationary epoch

Magnetic monopole problem

• Grand Unified Theories (GUTs) predict the existence of magnetic monopoles, hypothetical particles with a single magnetic pole (either north or south)
• If magnetic monopoles were produced in the early universe, they should be abundant today, but none have been observed
• Cosmic inflation provides a solution by diluting the density of magnetic monopoles to undetectable levels

Steady State theory

• The Steady State theory, proposed by Fred Hoyle, Hermann Bondi, and Thomas Gold, suggests that the universe has no beginning or end, and maintains a constant average density
• To maintain a constant density, the theory proposes the continuous creation of matter as the universe expands
• The discovery of the cosmic microwave background radiation and the observed evolution of galaxies over cosmic time have largely discredited the Steady State theory

Oscillating universe models

• Oscillating or cyclic universe models propose that the universe undergoes an endless series of expansions and contractions, with each cycle beginning with a Big Bang and ending with a Big Crunch
• These models attempt to avoid the problem of the initial singularity and provide an infinite timeline for the universe
• However, oscillating models face challenges such as the increasing entropy in each cycle and the need for a mechanism to trigger the bounce from contraction to expansion

Philosophical and religious implications

Creation ex nihilo

• The Big Bang theory implies that the universe had a beginning, which raises philosophical and religious questions about the origin of the universe
• Creation ex nihilo, or creation out of nothing, is the belief that the universe was created by a divine being or power from no pre-existing matter or energy
• The Big Bang theory is sometimes seen as compatible with the idea of creation ex nihilo, as it describes the universe originating from an initial singularity

Anthropic principle

• The anthropic principle is the philosophical consideration that observations of the universe must be compatible with the conscious and sapient life that observes it
• The weak anthropic principle states that the universe's ostensible fine-tuning is the result of selection bias, as only in a universe capable of eventually supporting life will there be living beings to observe it
• The strong anthropic principle suggests that the universe must have those properties which allow life to develop within it at some point in its history

Fine-tuning of universal constants

• The laws of physics and the values of fundamental constants appear to be fine-tuned to allow for the existence of complex structures and life
• Examples of fine-tuning include the strength of the fundamental forces, the mass of elementary particles, and the initial conditions of the universe
• Some argue that this fine-tuning suggests the presence of a divine creator or the existence of multiple universes (the multiverse hypothesis), while others propose anthropic explanations or the possibility of a deeper theory that explains these apparent coincidences