From the April 2012 issue

I’ve read about background radiation fields that fill the universe. Is the infrared radiation as ancient as the microwave, and are there other cosmic fields of radiation besides these two?

Nicholas Weeks, Moorpark, California
By | Published: April 23, 2012 | Last updated on May 18, 2023
When scientists are observing the cosmic infrared background, they’re seeing the universe as it was at an age of about 1 million years. Credit: ESA/Planck consortium
When scientists talk about radiation, they are referring to photons, which are massless particles. According to quantum mechanics, the energy of a photon is directly proportional to its frequency (i.e. the frequency of the radiation) and inversely proportional to the wavelength at which it is observed. So, the shorter the wavelength, the higher the frequency, and the longer the wavelength, the lower the frequency.

Radiation spans a huge range of wavelengths, and thus photons can have a correspondingly large range of energies. From short wavelengths to long (and thus high to low frequencies), these ranges are gamma ray, X-ray, ultraviolet, visible, infrared, microwave, and radio. Astrophysical objects emit some wavelengths more than others, depending on what they are. Low-mass stars radiate in infrared and visible, whereas supernova remnants emit X-rays and visible.
The cosmic background radiation currently emits mainly at microwave wavelengths; its radiation intensity peaks at a wavelength of approximately 2 millimeters (about 0.1 inch). However, photons stretch as the universe expands, and their wavelengths get longer (energy decreases) as the expanding universe gets older. Therefore, the cosmic background radiation used to have a shorter wavelength (higher energy) when the universe was younger and smaller. When the universe was 1 million years old, the cosmic background was mostly infrared radiation.

In addition to the cosmic microwave background radiation left over from the early universe, today we observe photons emitted from stars and galaxies in the cosmos at all frequencies, including gamma ray, visible, infrared, and radio. This emission from stars and galaxies is typically brighter, and thus more easily observed, than that from the background radiation. — Dragan Huterer, University of Michigan, Ann Arbor