13.2 Cosmic microwave background radiation

The cosmic microwave background (CMB) is a faint glow of radiation permeating the universe. Discovered by accident in 1964, it provides crucial evidence for the Big Bang theory and offers insights into the early universe's conditions.

CMB data reveals tiny temperature fluctuations across the sky, which are key to understanding cosmic structure formation. 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 isotropy consistent with expanding universe from hot, dense state
• Blackbody spectrum matches predictions of early universe models
• Temperature aligns with theoretical calculations for cosmic expansion (approximately 2.7 Kelvin)
• Slight anisotropies indicate quantum fluctuations 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 acoustic oscillations in primordial plasma
• Angular scale 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 (baryonic matter, dark matter, dark energy)
• Power spectrum 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
• Baryon density derived from relative heights of acoustic peaks in CMB power spectrum
• Dark matter density 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 inflationary models of early universe (exponential expansion)