The sky still glows from its first light
A nearly isotropic microwave background of 2.725 K fills the sky and is the leftover radiation from recombination.
Established3 min read
Penzias and Wilson found excess antenna noise at 4,080 MHz in 1965. It did not go away when they cleaned the pigeon droppings from the horn. Dicke’s group at Princeton recognised it as the predicted afterglow of a hot early universe.
COBE, WMAP, and Planck turned that hiss into a map. The spectrum is a blackbody to extraordinary precision. The anisotropies are one part in 100,000, and their statistics are the main dataset of modern cosmology.
The glow is not starlight, not zodiacal dust, not a local accident. It is the oldest light we can see, redshifted into microwaves by the expansion of space.
Caveats
- Foregrounds (our galaxy, dust) must be modelled and subtracted.
- The map is of the last-scattering surface, not of time zero.
Sources
- 1Penzias A. A., Wilson R. W.. A Measurement of Excess Antenna Temperature at 4080 Mc/s. Astrophysical Journal 142, 1965.
- 2Planck Collaboration. Planck 2018 results. I. Overview and the cosmological legacy of Planck. Astronomy & Astrophysics 641, A1, 2020.