13.1 Fundamentals of Big Bang cosmology
2 min read•july 25, 2024
The , supported by , , and light element abundance, explains our universe's origins. , a rapid expansion in the early universe, solved major cosmological problems and seeded .
The universe's evolution spans epochs from the Planck era to the present day, each marked by distinct physical processes. The , asserting the universe's homogeneity and isotropy, simplifies models and aligns with observations from galaxy surveys and background radiation.
Observational Evidence and Theoretical Foundations
Evidence for Big Bang theory
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- Hubble's Law and expanding universe demonstrated of distant galaxies correlates with distance, indicating universal expansion
- Cosmic Microwave Background Radiation (CMBR) discovered by Penzias and Wilson exhibits uniform temperature ~2.7 K, remnant heat from early universe
- Abundance of light elements produced during matches observed ratios of hydrogen, helium, and lithium in the cosmos
Cosmic inflation in early universe
- Rapid exponential expansion of space-time occurred ~ seconds after Big Bang, lasted ~ seconds
- Solved major cosmological problems addressed horizon problem (uniformity of CMB), flatness problem (spatial geometry), magnetic monopole problem (lack of observation)
- during inflation provided seeds for large-scale structure formation (galaxy clusters, superclusters)
Universe Evolution and Cosmological Principles
Universe evolution since Big Bang
- (0 to s): all fundamental forces unified
- ( to s): strong force separates from electroweak force
- ( to s): rapid expansion of universe
- ( to s): quarks and gluons form quark-gluon plasma
- ( to 1 s): quarks combine to form hadrons (protons, neutrons)
- (1 to 10 s): leptons dominate mass of universe
- (3 to 20 minutes): formation of light elements (hydrogen, helium, lithium)
- and photon decoupling (380,000 years): neutral atoms form, CMB released
- (380,000 years to 150 million years): universe becomes transparent, no stars yet
- (150 million to 1 billion years): first stars and galaxies form, ionize surrounding gas
- Present day (13.8 billion years): large-scale structure, galaxies, stars, and planets
Cosmological principle and implications
- Homogeneity of universe means matter distributed uniformly on large scales (galaxy superclusters)
- Isotropy of universe indicates universe looks same in all directions (CMB uniformity)
- No special location or direction in universe refutes Earth-centric models
- Implications for cosmological models simplify mathematical descriptions, allow use of
- Observational support comes from large-scale galaxy surveys, uniformity of cosmic microwave background radiation
Key Terms to Review (20)
Big Bang Theory: The Big Bang Theory is the leading explanation for the origin of the universe, suggesting that it began as an infinitely small, hot, and dense point about 13.8 billion years ago and has been expanding ever since. This theory not only describes the birth of the universe but also connects to key concepts like cosmic expansion, the formation of galaxies, and the evolution of the cosmos over time, shaping our understanding of fundamental astrophysical principles and historical perspectives on astronomy.
Cosmic inflation: Cosmic inflation is a theory that proposes a rapid expansion of the universe at an exponential rate during the first moments after the Big Bang. This expansion explains several key features of the universe, such as its large-scale uniformity and the distribution of galaxies, addressing issues like the horizon problem and flatness problem. The theory suggests that tiny quantum fluctuations in the early universe could have been stretched to cosmic scales, leading to the structure we observe today.
Cosmic microwave background radiation: Cosmic microwave background radiation (CMB) is the afterglow of the Big Bang, a faint glow of microwave radiation that fills the universe and can be detected in all directions. This radiation is a remnant from when the universe was just about 380,000 years old, providing critical evidence for the Big Bang theory and giving insights into the early universe's temperature and density fluctuations.
Cosmological Principle: The cosmological principle is the assumption that the universe is homogeneous and isotropic when viewed on a large enough scale. This means that, on average, the distribution of matter and energy is uniform throughout the cosmos, and the laws of physics apply equally everywhere. This principle underpins many models of the universe, including the Big Bang theory, suggesting that our observations of the universe should not be biased by our specific location within it.
Dark Ages: The term 'Dark Ages' refers to a period in the history of the universe that spans from the end of recombination to the formation of the first stars, roughly between 380,000 years and 1 billion years after the Big Bang. During this time, the universe was dominated by neutral hydrogen and helium, resulting in a lack of visible light and structure, leading to the characterization of this epoch as 'dark.' This phase is significant as it represents a critical transition from an opaque universe to one filled with light and cosmic structures.
Friedmann-Lemaître-Robertson-Walker Metric: The Friedmann-Lemaître-Robertson-Walker (FLRW) metric is a solution to Einstein's field equations in general relativity that describes a homogeneous and isotropic expanding or contracting universe. This metric is foundational in Big Bang cosmology, as it provides a framework for understanding the dynamics of the universe's expansion over time, linking the geometry of space with the distribution of matter and energy.
Grand unification epoch: The grand unification epoch is a phase in the early universe, occurring roughly between 10^-43 seconds and 10^-36 seconds after the Big Bang, where the three fundamental forces of electromagnetism, weak nuclear force, and strong nuclear force are believed to have been unified into a single force. This epoch represents a significant moment in the evolution of the universe, highlighting the potential for these forces to behave as one under extreme conditions before the universe cooled and expanded, allowing them to separate into distinct forces we observe today.
Hadron epoch: The hadron epoch is a phase in the early universe that lasted from approximately 10 microseconds to 1 second after the Big Bang, during which temperatures were high enough for quarks to combine and form hadrons, such as protons and neutrons. This period was crucial for the formation of the building blocks of atomic nuclei, paving the way for the subsequent epochs in cosmic evolution.
Hubble's Law: Hubble's Law states that the recessional velocity of galaxies is directly proportional to their distance from us, which implies that the universe is expanding. This foundational observation connects the distribution of galaxies in clusters and larger structures to the overall expansion and evolution of the cosmos as described by Big Bang cosmology.
Inflationary epoch: The inflationary epoch refers to a rapid exponential expansion of the universe that occurred just after the Big Bang, lasting for a tiny fraction of a second. This period is crucial in explaining the large-scale uniformity and flatness of the universe we observe today, as well as addressing some critical issues like the horizon and flatness problems.
Large-scale structures: Large-scale structures refer to the vast formations of galaxies, galaxy clusters, and superclusters that make up the universe's large-scale cosmic web. These structures are essential for understanding the distribution of matter and the evolution of the universe, revealing how gravity influences the arrangement of cosmic matter over billions of years.
Lepton Epoch: The lepton epoch refers to a significant phase in the early universe's history, lasting from about 10$^{-12}$ seconds to 10$^{-6}$ seconds after the Big Bang, during which leptons, such as electrons and neutrinos, dominated the particle interactions. In this period, the universe was extremely hot and dense, allowing for the creation of various elementary particles, while the electromagnetic force and weak nuclear force were not yet distinct. Understanding this epoch is crucial for grasping how the universe evolved from a hot plasma to a state where atoms could eventually form.
Nucleosynthesis: Nucleosynthesis is the process by which elements are formed through nuclear reactions, particularly in stars and during the early moments of the universe. This process is crucial for understanding how the elements we see today were created, especially hydrogen, helium, and trace amounts of lithium and other light elements, which originated during the Big Bang. The study of nucleosynthesis helps connect the formation of these elements with stellar evolution and the lifecycle of galaxies.
Planck epoch: The Planck epoch refers to the earliest stage of the universe's history, lasting from the moment of the Big Bang up to about 10^{-43} seconds after it. During this time, the universe was incredibly hot and dense, and the known laws of physics, particularly general relativity and quantum mechanics, were not yet fully unified, leading to conditions where traditional physics breaks down.
Primordial nucleosynthesis: Primordial nucleosynthesis refers to the process that occurred in the early universe, during the first few minutes after the Big Bang, where the light elements such as hydrogen, helium, and small amounts of lithium and beryllium were formed. This event is crucial for understanding the chemical composition of the early universe and provides insight into the conditions that existed shortly after the Big Bang.
Quantum fluctuations: Quantum fluctuations refer to temporary changes in the energy levels of a quantum system that occur due to the uncertainty principle, leading to spontaneous creation and annihilation of particle-antiparticle pairs. These fluctuations are fundamental to understanding the behavior of particles at the quantum level and play a crucial role in the early universe's evolution, particularly during the rapid expansion after the Big Bang and the formation of the cosmic microwave background radiation.
Quark epoch: The quark epoch is a period in the early universe that lasted from approximately 10^{-12} to 10^{-6} seconds after the Big Bang, during which the universe was hot and dense enough for quarks to exist freely before combining to form protons and neutrons. This epoch is significant because it marks the era when the fundamental building blocks of matter were being formed, laying the groundwork for the eventual creation of atomic nuclei.
Recombination: Recombination refers to the process in which electrons combine with protons to form neutral hydrogen atoms as the universe cooled after the Big Bang. This crucial event occurred approximately 380,000 years after the Big Bang, marking a transition from a hot, dense plasma state to a cooler, more transparent universe. As recombination happened, photons could travel freely through space, leading to the decoupling of matter and radiation and setting the stage for the formation of cosmic structures.
Redshift: Redshift refers to the phenomenon where light from an object is shifted to longer wavelengths, making it appear more red than it actually is. This effect occurs when an object moves away from the observer, and it plays a crucial role in understanding the universe's expansion and the motion of celestial bodies.
Reionization: Reionization is the process that occurred in the early universe when the neutral hydrogen atoms that filled space were ionized, creating a hot, ionized plasma. This significant event transformed the universe from a mostly neutral state to one filled with free electrons and protons, allowing light from the first stars and galaxies to travel freely through space. It plays a crucial role in understanding the evolution of the cosmos and the formation of structures within it.