The cosmic microwave background (CMB) is like a baby picture of our universe. It's the leftover heat from the Big Bang, giving us a glimpse of what things were like when the cosmos was just 380,000 years old.

By studying the CMB, scientists have learned a ton about our universe's shape, age, and makeup. It's helped confirm ideas like cosmic inflation and revealed the surprising fact that most of the universe is made of stuff we can't even see.

The Cosmic Microwave Background

Origin of cosmic microwave background

• Remnant heat from the Big Bang which occurred ~13.8 billion years ago
• ~380,000 years after the Big Bang, universe cooled enough for protons and electrons to combine forming neutral hydrogen atoms (recombination)
  • Before recombination, universe was opaque due to constant scattering of photons by free electrons
• At recombination, photons could travel freely through universe without being scattered making universe transparent
  • These photons have been traveling through expanding universe ever since becoming redshifted to microwave wavelengths
• Provides strong evidence for Big Bang theory and helps constrain various cosmological parameters (Hubble constant, dark matter density)

Characteristics of cosmic background radiation

• Nearly perfect black body spectrum with temperature of 2.725 K (-270.425℃ or -454.765℉)
  • Temperature is almost uniform in all directions with very small fluctuations
  • Represents a state of thermal equilibrium in the early universe
• Highly isotropic meaning it appears nearly the same in all directions (north, south, east, west)
  • Isotropy supports idea of homogeneous universe on large scales and the cosmological principle
• Tiny temperature fluctuations on order of 1 part in 100,000 are present
  • Fluctuations correspond to regions of slightly higher or lower density in early universe
  • Over time, these density fluctuations grew through gravitational attraction leading to formation of galaxies and large-scale structures we observe today (clusters, superclusters, filaments, voids)

Evidence for flat universe

• Observed angular size of CMB fluctuations can be used to determine geometry of universe
• If universe were positively curved (closed), fluctuations would appear larger than expected
• If universe were negatively curved (open), fluctuations would appear smaller than expected
• Observations of CMB, particularly by WMAP and Planck satellites, have shown angular size of fluctuations is consistent with flat universe
  • In flat universe, sum of angles in triangle equals 180° and parallel lines remain parallel
• Flatness of universe, as supported by CMB observations, agrees with predictions of cosmic inflation - period of rapid expansion in early universe

Insights into universe's age and composition

• Temperature of CMB, combined with expansion rate of universe (Hubble constant), allows astronomers to estimate age of universe
  • Current estimates based on CMB data put age of universe at ~13.8 billion years
• Relative heights of peaks in CMB power spectrum (plot of strength of temperature fluctuations at different angular scales) provide information about composition of universe
  • Ratio of heights of second and first peaks is sensitive to amount of ordinary (baryonic) matter
  • Third peak is sensitive to amount of dark matter
• CMB data, combined with other observations, has led to current understanding of universe's composition:
  1. ~5% ordinary matter (protons, neutrons, electrons)
  2. ~27% dark matter
  3. ~68% dark energy
• Helps constrain Hubble constant and number of light particle species (neutrinos) in early universe

Analysis of CMB Data

• CMB is a form of electromagnetic radiation, allowing for detailed study using various instruments
• Angular power spectrum of CMB reveals information about early universe structure
  • Peaks in the spectrum correspond to acoustic oscillations in the primordial plasma
• Analysis of CMB data supports models of cosmic inflation and provides evidence for the cosmological principle