CMB
Understanding CMB
The cosmic microwave background is the oldest light an instrument can detect, emitted roughly 380,000 years after the Big Bang when the universe cooled enough for protons and electrons to combine and photons to travel freely. Cosmic expansion has stretched that light by a factor of about 1,100, redshifting what was once a hot glow into the microwave band. To a radio engineer the CMB looks like a uniform, unpolarized (to first order) thermal noise background present in every direction, with a power spectral density set entirely by its physical temperature through the Johnson-Nyquist relation that underlies all thermal radiation.
The 1965 discovery was itself an RF measurement problem. Penzias and Wilson, working with a sensitive horn-reflector antenna at 4.08 GHz, found an excess antenna temperature of about 3.5 K that persisted after they accounted for the receiver, the atmosphere, the ground, and even pigeon droppings in the horn. That irreducible noise floor was the CMB. The lesson endures: any radio astronomy or remote-sensing system that pushes toward a few-kelvin system temperature must treat the CMB as a real, calculable contribution to the antenna temperature rather than as instrument noise.
Spectrum, Peak, and Receivers
Because the CMB is a blackbody, its spectrum is fixed by one number, the temperature. The spectral radiance per unit frequency peaks near 160 GHz, while the photon-number and per-wavelength peaks land elsewhere, a reminder to be explicit about which spectral variable is used. Experiments therefore build receivers that straddle the peak, typically from about 30 GHz to 300 GHz. At the low end, cryogenic HEMT low-noise amplifiers dominate; above roughly 100 GHz, superconducting transition-edge sensor bolometers and kinetic inductance detectors take over because amplifier noise becomes prohibitive. Calibration against a known blackbody load and careful control of atmospheric water vapor, which both emits and absorbs in these bands, are central to every CMB measurement.
CMB Key Relations
Bν(T) = (2hν³/c²) · 1 / (ehν/kT − 1)
Spectral peak (Wien, frequency form):
νpeak ≈ 58.8 GHz/K × T ≈ 160 GHz at 2.725 K
Temperature vs redshift:
T(z) = T0(1 + z), T0 = 2.725 K
Where h = Planck constant, k = Boltzmann constant, c = speed of light, ν = frequency, T = temperature, z = redshift. Example: at the last-scattering redshift z ≈ 1100, T ≈ 3000 K, a visible-light glow since stretched into the microwave band.
Landmark CMB Measurements
| Instrument | Year | Frequency coverage | Key result | Detector type |
|---|---|---|---|---|
| Penzias & Wilson | 1965 | 4.08 GHz | Discovery, ~3 K excess | Maser receiver |
| COBE FIRAS | 1990 | 60 GHz to 600 GHz | Blackbody confirmed to 2.725 K | Polarizing FTS bolometer |
| COBE DMR | 1992 | 31, 53, 90 GHz | First anisotropy detection | Differential radiometer |
| WMAP | 2003 | 23 GHz to 94 GHz | Precision anisotropy map | HEMT radiometer |
| Planck | 2013 | 30 GHz to 857 GHz | High-resolution all-sky map | HEMT + bolometer |
Frequently Asked Questions
What is the CMB in RF and microwave terms?
It is a near-perfect 2.725 K blackbody radiation field filling the whole sky, which to a receiver looks like a faint, spectrally smooth thermal noise background. Its brightness follows the Planck law, peaking near 160 GHz. Penzias and Wilson found it in 1965 as an irreducible ~3 K antenna-temperature excess at 4.08 GHz.
Why does the CMB peak near 160 GHz?
A blackbody peak scales linearly with temperature; at 2.725 K the per-frequency radiance peaks near 160 GHz (about 58.8 GHz per kelvin). That places the strongest signal in the millimeter-wave band shared with radio astronomy and atmospheric sounders, so experiments build receivers spanning roughly 30 to 300 GHz.
How is the CMB measured?
With cryogenic radiometers and bolometers reaching a few kelvin system noise. Because anisotropies are only ~18 microkelvin out of 2.725 K, instruments use differential or chopped measurements, stable references, and long integration. COBE FIRAS confirmed the blackbody shape; WMAP and Planck mapped the anisotropy with HEMT amplifiers and TES bolometers.