13.2 Cosmic microwave background radiation
2 min read•july 25, 2024
The (CMB) is a faint glow of radiation permeating the universe. Discovered by accident in 1964, it provides crucial evidence for the and offers insights into the early universe's conditions.
CMB data reveals tiny across the sky, which are key to understanding cosmic . These fluctuations help scientists determine the universe's composition, age, and geometry, making the CMB a cornerstone of modern cosmology.
Discovery and Properties of the CMB
Discovery of cosmic microwave background
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- Arno Penzias and Robert Wilson accidentally discovered CMB in 1964 using Holmdel Horn Antenna at Bell Labs
- Detected persistent background noise in radio signals initially thought to be interference
- Noise turned out to be uniform microwave radiation from all directions in space
- Nobel Prize in Physics awarded to Penzias and Wilson in 1978 for their discovery
CMB evidence for Big Bang
- Uniformity and consistent with expanding universe from hot, dense state
- matches predictions of early universe models
- Temperature aligns with theoretical calculations for cosmic expansion (approximately 2.7 Kelvin)
- Slight anisotropies indicate in early universe
- CMB data helps constrain age of universe to about 13.8 billion years
- Observations support Big Bang nucleosynthesis predictions for light element abundances (hydrogen, helium, lithium)
Significance of CMB temperature fluctuations
- Tiny variations in temperature across sky, magnitude about 1 part in 100,000
- Caused by quantum fluctuations in early universe and in primordial plasma
- of fluctuations related to size of universe at recombination
- Seeds for structure formation in universe led to galaxies and galaxy clusters
- Provide information about composition of early universe (, , )
- analysis reveals peaks corresponding to different physical processes (acoustic oscillations, Silk damping)
CMB in cosmological parameters
- Flatness of universe indicated by CMB data suggests nearly zero curvature
- derived from relative heights of in CMB power spectrum
- inferred from overall shape of CMB power spectrum
- Dark energy properties constrained by combining CMB data with other observations (supernovae, galaxy clustering)
- Hubble constant independently estimated using CMB measurements
- Age of universe calculated using CMB data and other cosmological observations
- CMB observations support of early universe (exponential expansion)
Key Terms to Review (18)
Acoustic oscillations: Acoustic oscillations refer to the periodic sound waves produced in a medium, which can also describe the density fluctuations that occurred in the early universe, particularly in the primordial plasma before recombination. These oscillations played a crucial role in creating the temperature and density variations observed in the cosmic microwave background radiation, as they led to the formation of the structures we see in the universe today.
Acoustic Peaks: Acoustic peaks refer to the distinct oscillation patterns seen in the power spectrum of cosmic microwave background (CMB) radiation, which provide critical information about the early universe. These peaks arise from sound waves propagating through the hot plasma of the early universe before the formation of the cosmic microwave background, leading to variations in temperature and density that we can observe today. The positions and amplitudes of these peaks help cosmologists understand fundamental properties of the universe, including its geometry, composition, and expansion history.
Angular scale: Angular scale is a measure of the apparent size of an object in the sky, expressed in angular units such as degrees, arcminutes, or arcseconds. It relates to how much of the sky an object occupies as seen from a specific point, allowing astronomers to understand and describe the size of celestial objects and the separation between them. Angular scale is especially important in observing distant astronomical phenomena like cosmic microwave background radiation, which spans large areas of the sky.
Baryon density: Baryon density refers to the number of baryons, which include protons and neutrons, per unit volume in the universe. This measurement is crucial for understanding the amount of normal matter present, as baryons make up most of the visible matter in stars, galaxies, and interstellar gas. It is an important factor in cosmology, particularly in explaining the large-scale structure of the universe and how it evolved over time.
Baryonic Matter: Baryonic matter refers to the form of matter that makes up stars, planets, and living organisms, primarily composed of baryons such as protons and neutrons. This type of matter constitutes a small fraction of the total mass-energy content of the universe, as most of it is made up of dark matter and dark energy. Baryonic matter interacts with electromagnetic forces, allowing it to form atoms and, consequently, the visible structures we observe in the universe.
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.
Blackbody spectrum: A blackbody spectrum is the characteristic distribution of electromagnetic radiation emitted by a perfect blackbody, an idealized physical body that absorbs all incident radiation, regardless of frequency or angle. This spectrum is solely determined by the temperature of the blackbody and follows Planck's law, which describes how energy is radiated at different wavelengths. Understanding the blackbody spectrum is essential for interpreting various astronomical phenomena, including the cosmic microwave background radiation.
Cosmic microwave background: The cosmic microwave background (CMB) is the remnant radiation from the Big Bang, filling the universe with a nearly uniform glow of microwave radiation. It serves as a snapshot of the universe when it was just 380,000 years old, providing vital clues about its early conditions, structure, and expansion. The CMB plays a crucial role in understanding the universe's constituents, its expansion over time, and influences our comprehension of dark matter and dark energy.
Cosmic Microwave Background Radiation: Cosmic Microwave Background (CMB) radiation is the afterglow of the Big Bang, a nearly uniform background radiation that fills the universe. This relic radiation provides crucial evidence for the Big Bang theory and offers insights into the early universe's conditions, structure formation, and the universe's expansion. Its uniformity and slight temperature fluctuations are key to understanding cosmic evolution and the distribution of matter in the universe.
Dark energy: Dark energy is a mysterious force that is driving the accelerated expansion of the universe. It constitutes about 68% of the total energy density of the universe and affects how galaxies, galaxy clusters, and large-scale structures behave over cosmic timescales. This concept connects to many aspects of astrophysics, including the formation and evolution of the universe, the cosmic microwave background radiation, and our understanding of Hubble's law.
Dark Matter: Dark matter is a mysterious and invisible form of matter that does not emit, absorb, or reflect light, making it undetectable by traditional astronomical methods. Despite being unseen, it makes up about 27% of the universe's total mass-energy content and plays a crucial role in the structure and evolution of galaxies, influencing gravitational interactions in the cosmos.
Dark matter density: Dark matter density refers to the amount of dark matter present in a given volume of space. This invisible substance is thought to make up about 27% of the universe's total mass-energy content, influencing the gravitational effects that shape galaxies and larger structures. Understanding dark matter density is crucial for interpreting observations of cosmic microwave background radiation and for studying the expansion of the universe, as it plays a key role in the formation and evolution of cosmic structures.
Inflationary Models: Inflationary models are theoretical frameworks that describe a rapid expansion of the universe in its earliest moments, right after the Big Bang. These models suggest that this exponential growth solved several major cosmological problems, including the uniformity of the cosmic microwave background radiation and the flatness of the universe. By proposing that the universe underwent a brief period of extreme inflation, these models provide insights into the large-scale structure and evolution of the cosmos.
Isotropy: Isotropy refers to the property of being the same in all directions. In cosmology, it is crucial for understanding the uniformity of the universe at a large scale, particularly as observed in the cosmic microwave background radiation, which is remarkably uniform across the sky. This uniformity suggests that, on a cosmic scale, the universe has a consistent structure and distribution of matter.
Power Spectrum: The power spectrum is a tool used in astrophysics to describe the distribution of power or intensity across different frequencies or scales. In the context of cosmic microwave background radiation, it reveals how fluctuations in temperature are distributed across various angular scales, providing insights into the early universe's structure and evolution.
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.
Structure formation: Structure formation refers to the process by which matter in the universe evolves from small density fluctuations in the early universe to the large-scale structures we observe today, such as galaxies, galaxy clusters, and superclusters. This process is heavily influenced by gravitational forces and the distribution of dark matter, shaping the cosmic web. Understanding structure formation is crucial to explaining how cosmic structures evolved and how they relate to phenomena like cosmic microwave background radiation, dark matter evidence, and potential dark matter candidates.
Temperature fluctuations: Temperature fluctuations refer to the small variations in temperature that can be observed in a specific region of the universe, particularly in the context of the cosmic microwave background radiation. These fluctuations are crucial as they provide insights into the density variations of matter in the early universe, influencing the formation of cosmic structures such as galaxies. Understanding these temperature changes helps scientists decipher the conditions that existed shortly after the Big Bang and how they led to the universe's large-scale structure today.