Cosmic Microwave Background
The cosmic microwave background is the faint, nearly uniform glow of relic radiation released when the universe first became transparent, the oldest light we can observe and a cornerstone of the hot Big Bang model.
Definition
The cosmic microwave background is the thermal radiation left over from the hot, dense early universe, emitted when electrons and protons combined into neutral atoms about 380,000 years after the Big Bang, now observed as a 2.7-kelvin blackbody filling all of space.
Scope
This area covers the discovery and near-perfect blackbody spectrum of the cosmic microwave background, the physics of recombination and the surface of last scattering from which it originates, the tiny temperature anisotropies and their angular power spectrum that encode the contents and geometry of the universe, and the faint polarization that probes the early universe and the epoch of reionization.
Sub-topics
Core questions
- Why does the universe glow with nearly uniform microwave radiation?
- What event released the cosmic microwave background and when?
- How do its tiny anisotropies reveal the composition and geometry of the universe?
- What does its polarization tell us about the early universe?
Key concepts
- Blackbody spectrum
- Last scattering surface
- Recombination
- Temperature anisotropy
- Acoustic peaks
- Angular power spectrum
- Polarization
Key theories
- Relic blackbody radiation
- The hot early universe was filled with radiation in thermal equilibrium that, after decoupling from matter, persists today as a near-perfect blackbody spectrum cooled by expansion to a few kelvin.
- Acoustic peaks
- Sound waves in the primordial plasma imprint a characteristic series of peaks in the angular power spectrum of the temperature anisotropies, whose positions and heights tightly constrain cosmological parameters.
Clinical relevance
The cosmic microwave background is the single most powerful observational pillar of cosmology: its blackbody spectrum confirms the hot Big Bang, its anisotropies measure the densities of ordinary matter, dark matter, and dark energy with percent-level precision, and its statistics test inflation and the geometry of space.
History
Predicted as relic radiation by Gamow's group in the 1940s and rediscovered theoretically by Dicke and Peebles, the background was found serendipitously by Penzias and Wilson in 1965; COBE confirmed its blackbody spectrum and detected anisotropies in the 1990s, and WMAP and Planck mapped it with the precision that established the concordance cosmology.
Debates
- Large-scale anomalies
- Several unexpected features at the largest angular scales, such as a low quadrupole and apparent alignments, have prompted debate over whether they reflect statistical flukes, foreground contamination, or new physics beyond the standard model.
Key figures
- Arno Penzias
- Robert Wilson
- Robert Dicke
- James Peebles
- George Smoot
Related topics
Seminal works
- penzias1965
- dicke1965
Frequently asked questions
- Why is the cosmic microwave background in the microwave band?
- It was emitted as visible and infrared light from a roughly 3,000-kelvin plasma, but the expansion of the universe has stretched its wavelengths by a factor of about a thousand, redshifting it into the microwave region and cooling it to about 2.7 kelvin.
- Can we see anything older than the cosmic microwave background?
- Not in light: before recombination the universe was opaque to photons, so the cosmic microwave background is the earliest electromagnetic signal observable; reaching earlier epochs requires other messengers such as neutrinos or gravitational waves.