Glossary

12.4 Big Bang Theory and Cosmic Microwave Background

5 min read•august 16, 2024

The explains the universe's origin and evolution, starting from a hot, dense state 13.8 billion years ago. It covers key concepts like , , and the formation of galaxies, providing a framework for understanding our universe's structure and composition.

The serves as crucial evidence for the Big Bang, offering a snapshot of the early universe. This ancient light reveals temperature fluctuations that led to cosmic structures, while its uniformity supports the idea of cosmic inflation in the universe's first moments.

The Big Bang Theory

Fundamental Concepts and Key Features

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  • Big Bang theory proposes universe began from extremely hot, dense state approximately 13.8 billion years ago and has been expanding ever since
  • Suggests all matter, energy, space, and time originated from singular point called which rapidly expanded in cosmic explosion
  • Inflation, period of rapid exponential expansion, explains uniformity and flatness of observable universe
  • Predicts formation of light elements (hydrogen, helium, trace amounts of lithium) during first few minutes after Big Bang through process known as Big Bang nucleosynthesis
  • Accounts for observed abundance of elements in universe and large-scale structure of galaxies and galaxy clusters
  • Incorporates concept of to explain accelerating expansion of universe observed in recent decades
    • Dark energy counteracts gravity's attractive force on cosmic scales
    • Makes up approximately 68% of universe's total energy content

Theoretical Implications and Consequences

  • Implies universe has finite age rather than existing eternally
  • Suggests all regions of observable universe were once in causal contact despite current vast separations
  • Predicts existence of cosmic microwave background radiation as relic of early universe
  • Explains observed of distant galaxies as result of cosmic expansion
  • Provides framework for understanding evolution of cosmic structures from initial quantum fluctuations
  • Addresses Olbers' paradox by explaining darkness of night sky through finite age and expansion of universe

Evidence for the Big Bang

Observational Support

  • Expansion of universe evidenced by redshift of distant galaxies supports Big Bang theory's prediction of expanding universe
  • Cosmic microwave background (CMB) radiation discovered in 1964 by Penzias and Wilson represents afterglow of early universe
  • Observed abundance of light elements (hydrogen and helium) in universe closely matches predictions of Big Bang nucleosynthesis
  • Large-scale structure of universe, including distribution of galaxies and galaxy clusters, aligns with predictions of Big Bang model and subsequent cosmic evolution
  • Darkness of night sky, known as Olbers' paradox, explained by finite age and expansion of universe as predicted by Big Bang theory
  • , detected in recent years, provide additional support for inflationary period in early universe

Detailed Analysis of Evidence

  • CMB radiation exhibits nearly perfect blackbody spectrum at temperature of 2.7 K
  • describes linear relationship between galactic distance and recessional velocity
  • Primordial abundance of deuterium serves as sensitive probe of Big Bang nucleosynthesis
  • in galaxy distribution corroborate predictions of early universe dynamics
  • Lyman-alpha forest in quasar spectra traces distribution of neutral hydrogen consistent with Big Bang
  • Existence of high-redshift quasars and galaxies confirms early structure formation as predicted by theory

The Universe's Timeline

Early Epochs and Fundamental Forces

  • Planck era (0 to 10−4310^{-43} seconds) represents earliest known period where current physics breaks down and quantum gravity effects dominate
  • Grand Unification era (10−4310^{-43} to 10−3610^{-36} seconds) characterized by separation of gravity from other fundamental forces
  • Inflationary epoch (10−3610^{-36} to 10−3210^{-32} seconds) involves rapid exponential expansion of universe, smoothing out irregularities and setting stage for structure formation
  • Electroweak era (10−3210^{-32} to 10−1210^{-12} seconds) sees separation of strong nuclear force from electroweak force
  • Quark confinement (10−1210^{-12} to 10−610^{-6} seconds) occurs as quarks combine to form hadrons, including protons and neutrons

Later Stages and Structure Formation

  • Big Bang nucleosynthesis (3 minutes to 20 minutes) results in formation of light elements, primarily hydrogen (75%) and helium (25%)
  • Recombination era (380,000 years) marks formation of neutral atoms and release of cosmic microwave background radiation
  • Dark Ages (380,000 years to 400 million years) characterized by cooling and expansion of universe before first stars form
  • Reionization (400 million to 1 billion years) begins as first stars and galaxies ionize surrounding neutral hydrogen
  • Galaxy formation and evolution (1 billion years to present) involves hierarchical clustering and merging of cosmic structures
  • Solar System formation (4.6 billion years ago) occurs within Milky Way galaxy
  • Present day (13.8 billion years) continues to see accelerating expansion of universe

Cosmic Microwave Background Radiation

Characteristics and Significance

  • Cosmic microwave background (CMB) radiation provides snapshot of universe at time of recombination, approximately 380,000 years after Big Bang
  • Near-perfect blackbody spectrum of CMB supports hot, dense early universe predicted by Big Bang theory
  • Tiny temperature fluctuations in CMB (about 1 part in 100,000) reveal density variations in early universe that led to formation of large-scale structures
  • Angular size of temperature fluctuations provides information about geometry and curvature of universe, supporting idea of flat universe
  • Polarization patterns in CMB, particularly B-mode polarization, could provide evidence for gravitational waves from inflationary period
  • Uniformity of CMB across sky supports concept of cosmic inflation, explaining how regions of universe that appear causally disconnected could have once been in thermal equilibrium

Analysis and Cosmological Implications

  • Detailed analysis of CMB power spectrum allows cosmologists to determine fundamental parameters of universe
    • Age of universe (13.8 billion years)
    • Composition (4.9% ordinary matter, 26.8% dark matter, 68.3% dark energy)
    • Rate of expansion (Hubble constant H0≈67.4H_0 \approx 67.4 km/s/Mpc)
  • provide evidence for acoustic oscillations in early universe plasma
  • Sunyaev-Zel'dovich effect in CMB reveals presence of hot gas in galaxy clusters
  • CMB lensing effects constrain distribution of dark matter on large scales
  • Absence of large-scale non-Gaussianity in CMB supports simple inflationary models
  • Precise measurements of CMB spectrum constrain possible deviations from standard cosmological model (Λ\LambdaCDM)

Key Terms to Review (20)

Baryon acoustic oscillations: Baryon acoustic oscillations refer to the regular, periodic fluctuations in the density of visible baryonic matter (normal matter) in the universe caused by sound waves in the hot plasma of the early universe. These oscillations played a crucial role in the formation of the large-scale structure of the universe and are imprinted in the cosmic microwave background radiation as well as in the distribution of galaxies. Understanding these oscillations helps shed light on fundamental aspects of cosmology, including the nature of dark matter and dark energy.
Big bang theory: The big bang theory is the prevailing cosmological model explaining the origin and evolution of the universe, suggesting it began as a singular point approximately 13.8 billion years ago and has since been expanding. This expansion is evidenced by the observed redshift of distant galaxies and aligns with key concepts such as the cosmic microwave background radiation, which serves as a remnant heat signature from the early universe.
Cmb anisotropies: CMB anisotropies refer to the small fluctuations in temperature and density observed in the Cosmic Microwave Background radiation, which is the afterglow of the Big Bang. These anisotropies provide critical insights into the early universe's conditions, helping scientists understand the formation of cosmic structures and the universe's overall geometry.
Cosmic inflation: Cosmic inflation is a theory that suggests a rapid expansion of the universe occurred within the first few moments after the Big Bang, leading to the vast and uniform cosmos we observe today. This exponential growth explains the large-scale structure of the universe and solves several puzzles related to the Big Bang theory, such as the horizon and flatness problems.
Cosmic microwave background radiation: Cosmic microwave background radiation (CMB) is the afterglow of the Big Bang, representing a uniform field of microwave radiation that fills the universe and is a critical piece of evidence for the Big Bang theory. This radiation, which is remarkably uniform in all directions, provides insights into the early universe's conditions and supports the existence of dark matter and dark energy as it reveals the universe's large-scale structure and evolution.
Cosmological redshift: Cosmological redshift refers to the phenomenon where light from distant galaxies is stretched to longer wavelengths due to the expansion of the universe. As the universe expands, the space between objects also expands, causing light traveling through this space to be stretched, resulting in a shift toward the red end of the spectrum. This redshift provides crucial evidence for the Big Bang Theory and helps us understand the dynamics of the universe, including its rate of expansion and the nature of cosmic microwave background radiation.
Cosmology: Cosmology is the scientific study of the universe's origin, evolution, structure, and eventual fate. It connects various phenomena such as the Big Bang Theory and the Cosmic Microwave Background, which are fundamental in understanding how the universe began and expanded over time. This field encompasses a wide range of topics, including the formation of galaxies, the distribution of cosmic structures, and the nature of dark energy and dark matter.
Dark energy: Dark energy is a mysterious form of energy that makes up about 68% of the universe and is believed to be responsible for the accelerated expansion of the universe. It plays a crucial role in understanding how the cosmos behaves, particularly when considering observations related to the movement of galaxies and the cosmic microwave background radiation. By influencing the dynamics of cosmic expansion, dark energy ties into the framework of both the Big Bang Theory and our understanding of gravity on cosmic scales.
Expansion of the universe: The expansion of the universe refers to the phenomenon where galaxies are moving away from each other, indicating that the universe itself is increasing in size. This concept is rooted in observational evidence, such as the redshift of light from distant galaxies and the cosmic microwave background radiation, which provides a glimpse into the early state of the universe after the Big Bang.
Flatness problem: The flatness problem refers to the question of why the universe appears to be so close to geometrically flat, despite the fact that any deviation from flatness would have drastically changed its evolution. This situation is puzzling in the context of the Big Bang, as the early universe would have had a wide range of possible curvatures, yet we observe a universe that is remarkably flat today. The implications of this issue lead to significant discussions around inflationary theory and the Cosmic Microwave Background.
Georges Lemaître: Georges Lemaître was a Belgian priest and astrophysicist who is best known for proposing the theory of the expansion of the universe, leading to the formulation of what is now known as the Big Bang theory. His groundbreaking work laid the foundation for modern cosmology, connecting religious beliefs with scientific inquiry and suggesting that the universe began from a 'primeval atom' that exploded into existence.
Gravitational waves: Gravitational waves are ripples in the fabric of spacetime caused by the acceleration of massive objects, such as merging black holes or neutron stars. These waves transport energy across the universe and provide a new way to observe cosmic events, connecting directly to phenomena like the Big Bang and the Cosmic Microwave Background (CMB). The detection of gravitational waves has opened up a new field of astronomy, allowing scientists to study events that were previously undetectable through traditional electromagnetic observations.
Hubble's Law: Hubble's Law is a fundamental principle in cosmology that describes the observation that the farther away a galaxy is, the faster it is receding from us. This relationship between distance and velocity supports the idea of an expanding universe, directly linking to the Big Bang Theory and the cosmic microwave background radiation as remnants of that initial explosion.
Multiverse theory: Multiverse theory proposes the existence of multiple, possibly infinite, universes beyond our own, each with different physical laws and constants. This concept arises from various interpretations of quantum mechanics and cosmological models, suggesting that our universe may be just one of many, with some potentially having different properties or outcomes than what we observe.
Nucleosynthesis: Nucleosynthesis is the process by which new atomic nuclei are created from existing nucleons (protons and neutrons), which primarily occurs in stars and during the early moments of the universe. This process is essential for understanding the formation of elements, their abundance in the universe, and how these elements contribute to the development of stars, galaxies, and planetary systems.
Planck Satellite: The Planck Satellite was a space observatory launched by the European Space Agency in 2009 to measure the cosmic microwave background (CMB) radiation across the entire sky with high precision. It provided crucial insights into the early universe, supporting and refining the Big Bang Theory by mapping the temperature fluctuations in the CMB, which are remnants of the hot, dense state that existed shortly after the universe's birth. Its data has significantly advanced our understanding of cosmology, dark matter, and the overall structure of the universe.
Redshift: Redshift is a phenomenon where light from an object moving away from an observer shifts towards the red end of the spectrum. This effect indicates that the object is receding, and it plays a crucial role in understanding the expansion of the universe and the behavior of light from distant celestial objects.
Singularity: A singularity is a point in space-time where the gravitational field becomes infinitely strong, leading to a breakdown of the laws of physics as we currently understand them. In essence, it represents a boundary beyond which our conventional understanding of space and time fails. Singularities are commonly associated with black holes and the initial state of the universe during the Big Bang, where density and temperature reach infinite values.
Stephen Hawking: Stephen Hawking was a renowned theoretical physicist and cosmologist known for his groundbreaking work on black holes, general relativity, and the nature of the universe. His research provided significant insights into gravitational collapse and proposed theories about the origins of the universe, which are foundational to our understanding of cosmic phenomena like black holes and the Big Bang. Hawking's ability to communicate complex ideas about the cosmos, despite his battle with ALS, has made him an inspirational figure in both science and popular culture.
Thermodynamics of the early universe: The thermodynamics of the early universe refers to the study of temperature, energy, and the laws governing physical processes in the extremely hot and dense conditions present shortly after the Big Bang. Understanding this concept is crucial for grasping how matter and radiation interacted during the universe's infancy, leading to the formation of structures and the Cosmic Microwave Background radiation we observe today.
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