The Universe is full of radiation, so why is the sky dark at night?
At present, the cosmic mass budget contains a thousand times more energy per unit volume in the mass of ordinary matter than in the relic radiation left over from the Big-Bang, the so-called the cosmic microwave background. If efficient engines were to convert the rest mass (M) of all ordinary matter to pure radiation energy (E) right now through the relation E=Mc², then the result would be a radiation background that is a thousand times brighter than the relic glow of the Big-Bang. This glow accounts for a percent of the white-noise (snow) that is visible in old television sets.
But as it turns out, the natural engines for converting matter to radiation are inefficient. Black holes at all masses can reach a conversion efficiency of 42% if they spin at nearly the speed of light. However, the existing populations of stella-mass and supermassive black holes digested only ~0.01% of all ordinary matter throughout cosmic history. Stars, on the other hand, processed about 10% of all ordinary matter but converted only 0.05% of their total rest mass to radiation. Coincidentally, black holes and stars contributed nearly equally to the cosmic radiation budget over cosmic history. Both types of engines convert about one part in twenty thousand of the ordinary-matter mass to radiation energy, resulting in a cumulative radiation background (after proper accounting for cosmological redshift) that is only ~5% of the cosmic microwave background.
About half of the radiation produced by stars and black holes is absorbed by surrounding dust and reemitted as heat. The dust particles are typically warmed to a surface temperature that is about ten times hotter than the microwave background temperature of 2.73 degrees Kelvin. Whereas the peak brightness of the microwave background is at a wavelength of ~2 millimeters, the dust emission peaks at an infrared wavelength of ~0.2 millimeters which is ~400 times longer than the wavelength of visible light emitted by the Sun. Altogether, the cosmic radiation background includes peaks of comparable brightness in the infrared and optical bands.
Black holes also emit high-energy radiation in the X-ray and gamma-ray bands, at wavelengths that are more than a thousand times shorter than those of visible light. However, this short-wavelength radiation carries an energy density that is less than a percent of the optical and infrared backgrounds.
The total flux of cosmic radiation, dominated by the microwave background, amounts to 6 micro-watts per square meter on Earth. For comparison, the Sun delivers 1.4 kilo-watts per square meter on the day side of Earth. Life-as-we-know-it relies on Earth being warmed by sunlight because the cosmos delivers only 4 billionths of the solar radiation flux. As a result, astrobiologists focus on close-in environments of stars, the so-called habitable zones, where liquid water on an Earth-like planet can give rise to the chemistry of life-as-we-know-it. The night sky appears dark because the diffuse cosmic background of visible light amounts to only 2 billionths of the solar flux.
How significant is the flux from nearby stars within our Milky-Way galaxy? The nearest Sun-like stars in the Alpha Centauri A & B system deliver about 30 trillionth of the solar flux on Earth because they are 274,000 times farther away than the Sun. The thickness of Milky-Way’s disk of stars is hundreds of times larger than the distance to Alpha Centauri. Flux declines inversely with distance squared and the number of stars out to that thickness scales as distance cubed. Beyond that distance, the number of stars scales as distance squared because the disk is two-dimensional. In total, the Milky Way contributes of order a thousand times more than Alpha Centauri, of order 30 billionths of the Solar flux. The night sky would have been as bright as day only if the Milky-Way extended to the scale of the cosmic horizon, billions of light years.
The situation was more favorable in the early Universe. In 2014, I published a single-authored paper titled “The Habitable Epoch of the Early Universe”, in which I pointed out that between 10 to 17 million years after the Big-Bang, the microwave background temperature was in the range of 277–373 degrees Kelvin, about room temperature. This cosmic radiation bath would have warmed planets like the Earth to a habitable temperature, even if they did not reside close to stars. Unfortunately, the standard cosmological model leads to the formation of the first stars or planets only 50–100 million years after the Big-Bang, when the cosmic radiation temperature was freezing cold.
However, if life will be found by NASA’s Dragonfly mission in the methane and ethane oceans of Saturn’s moon Titan — where the temperature is about 94 degrees Kelvin, then this finding would imply that life could have started ~100 million years after the Big Bang, on rocks far from stars. I published this insight in a 2022 paper.
Extraterrestrial life could have blossomed everywhere if we lived at a time when the cosmic optical background is a billion times larger. In that case, the night sky would have appeared as bright as day, with the chemistry of life in liquid water not restricted to the habitable zone around stars. As much as astrobiologists might regret not living in such a universe, we are all grateful that our cosmos is what it is, as it allows us to sleep in darkness every 24 hours, when the Earth blocks the Sun.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
