Photons are the fundamental particle of light. Senors in our retina respond to photons emitted by distant starts when we look up at the night sky. How much light has been emitted by all the stars in the universe since its birth? The answer, as reported in the journal Science by a team of astrophysicists, is 4 x 10⁸⁴ photons as written in scientific notation – that is a 4 followed by 84 zeroes!
Our universe is old and vast. It is roughly 13.7 billion years old and is thought to contain over a billion-trillion stars – way too many to measure directly. Instead, this team of scientists used the Fermi Gamma-ray Space Telescope to indirectly measure the photon density of the universe at different time points in its history. Gamma-rays are a high-energy form of electromagnetic radiation. Starlight photons can collide with gamma-rays producing a pair of electrons and reducing the energy of the gamma-rays in the process. More collisions occur when the density of photons is high so looking at a source of gamma-rays and measuring the energy lost to photon collisions is a way to measure the photon density. Gamma-ray sources with different ages then provide the photon density at different time periods.
The team studied 739 blazars as their sources for gamma-rays. Blazars are thought to be powered by super massive black holes. The accretion of matter into the black hole shoots jets of gamma-rays perpendicular to the spin axis of the black hole. A blazar is positioned so a jet is pointed directly at the Earth. The blazars in this study ranged in distance from 200 million light-years to 11.6 billion light-years which corresponds to different ages and time-periods.
In addition to the total number of photons, the study also found that the rate of star formation rose in the first two billion years of the universe, peaked roughly 10 billion years ago, and has been declining since then.
Despite all those starlight photons, far more photons were made in the Big Bang and are now part of the so-called cosmic microwave background radiation – there are estimated to be 1089 CMB photons!