Relicta

Relicta is a retired fire. She once glowed at about 3000 K; now she sits at 2.7 K, and she is still a perfect blackbody.

For its first few hundred thousand years the universe was a hot plasma, and light could not travel far before scattering off free electrons. As it cooled to about 3000 K, electrons and nuclei joined into neutral atoms and the universe turned transparent. That moment, decoupling or last scattering, came at a redshift z of about 1090 (Planck 2018: 1089.92). The light released then has been travelling ever since.

As space expanded, the wavelengths of that light stretched with it by a factor of 1 + z, so its temperature fell in the same proportion, the first line on this page. Photons stretch; atoms, galaxies and Relicta do not. Stretching a blackbody gives a cooler blackbody, so the shape of Planck’s curve, the third line, has never changed. Today it peaks at about 160 GHz, in the microwaves. On a wavelength axis the peak falls at 1.06 mm, a different point, not c divided by 160 GHz.

In 1965 Arno Penzias and Robert Wilson at Bell Labs found a stubborn excess hiss in their horn antenna, the same in every direction, equivalent to about 3 K. They shared in the 1978 Nobel Prize. In 1990 the first spectrum from NASA’s COBE satellite, measured by John Mather’s team, matched a blackbody almost perfectly; Mather and George Smoot shared the 2006 Nobel Prize. Dale Fixsen’s 2009 value for today’s temperature is 2.72548 K, the second line.

Her old photo and her new cardigan show the same curve with a different scale. Retirement, not decay.

I was 3000 K once. Now I’m 2.7 K.