Why the universe doesn’t light up the night sky more
In a universe of billions of billions of stars, why isn’t the sky always bright?
In a universe of billions of billions of stars, why isn’t the sky always bright?
In 1823, amateur astronomer Heinrich Wilhelm Olbers looked up at the night sky and wondered how, in an infinite universe of infinite age, the night sky was so dark. Surely, such a universe must have a star or galaxy at every point and be ablaze with light. Yet, it was not. Although he was far from the first to ask this question and it likely goes as far back as the 6th century monk Cosmas Indicopleustes, the idea has since then been called Olbers Paradox.
The problem is simply one of geometry. While each two dimensional patch of sky scales as distance squared, the volume of that region of sky from here to a given distance scales as distance cubed. Thus, if the universe were infinite, you would see more and more galaxies in the same patch of sky the further out you looked. Thus even as the light of each individual galaxy diminishes, the number of galaxies more than makes up for it.
A pernicious misconception proposes that Einstein’s theory solves Olbers’ Paradox. Einstein’s theory tells us that because the universe is expanding, light traveling across the void is stretched out. As it stretches, its wavelength gets longer. This is called “redshift” since, in the visible spectrum, it means the light is shifting towards the red side of the spectrum. Despite the name, light of any wavelength can redshift.
The reasoning for why redshift makes the night sky darker is that a lot of the visible light has been stretched into the infrared where we can’t see it.
So is this true? If we were to somehow artificially blueshift the light back into its proper place, would the heavens be ablaze like a thousand suns?
No.
While it is true that the universe is brighter in the infrared, it hardly accounts for the darkness of the sky. Indeed, if you look at the expected brightness of our expanding universe versus a completely static one, ours is about 40% as bright, meaning that redshift accounts for about 60% (give or take 10%) of the darkness.
The amount of the brightness at all wavelengths, however, is determined by the age of the universe. For an infinite age, steady-state universe, redshift could not account for the blackness of the sky because it only cuts the amount that is there. If you cut an infinite amount by 60%, you still get an infinite amount. Thus, any scientific evaluation of Olbers paradox must either accept a finite aged universe or introduce some new physics to account for the total amount of light.
While it was considered common knowledge that the universe was infinite and unchanging in Olbers time, evidence gathered throughout the 20th and 21st centuries have shown again again that our universe has a finite age of about 13.8 Gyr.
A finite age to the universe is the first step in solving Olbers Paradox. Indeed, the luminosity of the night sky at all wavelengths over time, called, to throw some technical jargon at you, the bolometric intensity of the Extragalactic Background Light (EBL), scales with the age of the universe. You can, in fact, just from this fact and the observed brightness of the sky estimate that the universe is about 13 Gyr old, not bad for such a simple model. Expansion simply deepens the shade of an already black sky.
Another contributor after the finite age is how long light sources themselves last. If you modeled the universe as having a constant amount of light per galaxy, you would find that after 280 Gyr (280 billions years) the night sky would be as intense as your living room. In practice, that can’t happen because galaxies brightest stars burn out in considerably less time. Thus, the darkness of the night sky has as much to do with the lifecycles of stars and galaxies and the light they produce as it has to do with cosmology.
If you look at light production from the perspective of the evolution of galaxies, you again get that a static, but finite aged universe would be about 2–3 times brighter in the visible spectrum at night than an expanding one. But again, that is a factor based on the age of the universe itself.
What about dust and other things blocking light from view? It turns out that they play a role but not the one you’d think. Effects like the absorption of light by dust, rather than blocking light, actually just shift it into other wavelengths. Optical light is absorbed but re-emitted as infrared. This is true of anything that blocks light, even you. Eventually it will re-radiate it. If you look at all wavelengths of light, the age of the universe is still the determining factor.
To sum up, this means that the total amount of light over all wavelengths in the sky scales with the age of the universe. How much light is added at any given time is determined by how much light output galaxies have in various wavelengths and over how much time from the formation of their first stars to dying out. How much light we can see is then determined by how much of that is in the visible spectrum originally and then cut by a factor of 2–3 by the redshifting of light caused by the expansion of the universe. (Some ultraviolet light is also shifted into the visible spectrum, but not as much as is shifted out of it.)
This means that, over all, the night sky will continue to get brighter as billions of years pass but will probably never reach the same intensity as your living room because galaxies have finite lifetimes.
Overduin, James Martin, and Paul S. Wesson. “Dark matter and background light.” Physics reports 402.5–6 (2004): 267–406.