CMB Temperature Radio Approx

Cosmic Microwave Background (CMB) Temperature

The Cosmic Microwave Background is leftover radiation from the early universe, and its temperature is remarkably uniform across the sky at T = 2.7255 K (COBE/FIRAS measured it in 1989, and Planck refined the figure in 2013). No blackbody spectrum in nature comes closer to perfect, and it peaks around 160 GHz in the microwave band.

Arno Penzias and Robert Wilson stumbled onto it in 1964 at Bell Labs, work that earned them the Nobel Prize in 1978 and pinned down the Big Bang model. Buried in the signal are faint anisotropies of order ΔT/T ≈ 10⁻⁵, the seeds from which every cosmic structure grew. COBE, WMAP, Planck, SPT and ACT have each mapped them more sharply than the last.

Applications

The CMB feeds into estimating cosmological parameters (H₀, Ωₘ, ΩΛ, nₛ), testing inflation through B-mode polarization, putting limits on neutrino masses, calibrating radio telescopes, studying baryon acoustic oscillations (BAO), and probing reionization via the kinetic Sunyaev–Zel'dovich effect.

FAQ

Why is the CMB temperature 2.7255 K exactly? Since recombination at z≈1100, when T sat around 3000 K, the universe has expanded by roughly a factor of 1100 and the photon gas cooled adiabatically along with it, down to what we measure now.

What frequency band dominates the CMB signal? Intensity peaks near 160 GHz, a wavelength of about 1.9 mm. That is why most experiments work between 30 GHz and 300 GHz, enough to trace the blackbody curve and tell the foregrounds apart.

How is CMB used to measure the universe's age? Fit the angular power spectrum of the temperature anisotropies to a ΛCDM model and out come the Hubble constant and an age of 13.8 billion years.