Cosmic Microwave Background
The Cosmic Microwave Background (CMB) is the thermal electromagnetic radiation permeating the entire observable universe, representing the afterglow of light released approximately 380,000 years after the Big Bang when the universe cooled enough for protons and electrons to combine into neutral hydrogen atoms. It is observed today as a nearly uniform blackbody radiation at a temperature of approximately 2.725 K, with tiny temperature fluctuations of about 1 part in 100,000 that encode the seeds of large-scale cosmic structure. The CMB is considered one of the strongest pieces of evidence for the Big Bang model and provides precise measurements of cosmological parameters.
Key Formula
T_CMB ≈ 2.725 K (present-day temperature)
LaTeX: T_{\text{CMB}} \approx 2.725\,\text{K}
| Symbol | Meaning | Unit |
|---|---|---|
| T_CMB | Present temperature of the CMB | Kelvin (K) |
Worked Example
Problem
The CMB temperature at recombination was approximately 3000 K. Using Wien's displacement law, calculate the peak wavelength of CMB photons at recombination and compare it to the present-day peak wavelength at 2.725 K.
Solution
Wien's displacement law: λ_max = b / T, where b = 2.898 × 10⁻³ m·K. At recombination (T = 3000 K): λ_max = 2.898 × 10⁻³ / 3000 = 9.66 × 10⁻⁷ m = 966 nm (near-infrared) At present (T = 2.725 K): λ_max = 2.898 × 10⁻³ / 2.725 = 1.063 × 10⁻³ m = 1.063 mm (microwave) Ratio: 1.063 mm / 966 nm ≈ 1100, consistent with the redshift factor z ≈ 1100.
Answer
Peak wavelength at recombination ≈ 966 nm; at present ≈ 1.063 mm; redshift factor z ≈ 1100.
CMB Properties and Measurement Missions
| Property / Mission | Value / Year | Key Contribution |
|---|---|---|
| Temperature | 2.7255 ± 0.0006 K | Blackbody spectrum peak |
| Discovered by | Penzias & Wilson, 1965 | Nobel Prize 1978 |
| COBE satellite | 1989–1993 | Confirmed blackbody spectrum |
| WMAP satellite | 2001–2010 | High-resolution anisotropy map |
| Planck satellite | 2009–2018 | Precise cosmological parameters |
| Anisotropy level | ~10⁻⁵ K | Seeds of galaxy formation |
Interactive Tools
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Related Terms
Big Bang Theory
The Big Bang Theory is the prevailing cosmological model describing the origin and evolution of the universe, proposing that all matter, energy, space, and time originated from an extremely hot, dense singularity approximately 13.8 billion years ago. The rapid expansion from this primordial state led to cooling, the formation of fundamental particles, and eventually atoms, stars, and galaxies. Evidence supporting this model includes the observed expansion of the universe, the cosmic microwave background radiation, and the abundance of light elements like hydrogen and helium.
Cosmic Inflation
Cosmic inflation is a theoretical period of exponentially rapid expansion that the universe underwent approximately 10⁻³⁶ to 10⁻³² seconds after the Big Bang, during which it expanded by a factor of at least 10²⁶ in linear size. Proposed by Alan Guth in 1980 and refined by Andrei Linde and others, inflation elegantly resolves three major cosmological problems: the horizon problem (why the CMB is so uniform), the flatness problem (why the universe appears spatially flat), and the magnetic monopole problem. Quantum fluctuations during inflation are thought to be the seeds of all large-scale structure, stretched to cosmological scales.
Cosmological Redshift
Cosmological redshift is the increase in the wavelength of photons as they travel through an expanding universe, caused by the stretching of space itself rather than by the relative motion of source and observer (Doppler effect). Quantified by the redshift parameter z = (λ_observed − λ_emitted) / λ_emitted, it is directly related to the expansion factor of the universe: 1 + z = a(t_now) / a(t_emit). Cosmological redshift allows astronomers to determine the distance and lookback time to distant galaxies and serves as a primary tool for mapping the large-scale structure of the universe.
"Cosmic" derives from Greek "kosmos" (universe, order). "Microwave" refers to the electromagnetic spectrum range (1 mm – 1 m wavelength). "Background" indicates it is a pervasive, isotropic signal rather than from a specific source. The term was established after Penzias and Wilson's 1965 discovery.