Why is it Dark at Night?
Why Physics Stack Exchange recommends it
Physics Stack Exchange answerers point people to it as a clear primer on Olbers' paradox and why the expanding, finite-age universe leaves the night sky dark.
Recommended by 3 people across 3 different months, 2013 to 2025.
What it is
A short MinutePhysics explainer by Henry Reich on Olbers' paradox: why the night sky is dark even though an infinite universe of stars should make it bright. It walks through the resolution involving the finite age of the universe and its expansion.
What people said
Recommends OmnipresentAbsence · 2013-02 · Physics Stack Exchange · on “How was the universe created?”
The most widely supported theory for the creation of the universe is the Big Bang Theory [LINK] . It states that the universe was created about 13.7 billion years ago from a singularity (a very, very hot point with an infinitely small volume and infinitely large density) and it has been expanding ever since (so getting less dense and less hot). Click here [LINK] for a timeline since the Big Bang. The 4 main arguments supporting this theory are sometimes called the 4 pillars of the Big Bang Theory , click here [LINK] for an article regarding those core arguments. Also, this video [this video] regarding 'Olbers' Paradox' might be of interest to you too, it's actually one of the most blatantly obvio
Recommends yanbo · 2013-10 · Physics Stack Exchange · on “The inner workings of the Olbers paradox”
There is one thing that you have failed to consider: the expansion of the universe. Since the expansion of the universe is "faster" the further away it is from us, there will come a point where the expansion is faster than the speed of light. (This actually happens, I'll add sources if I have the time to do so.) So, the light from stars at or greater than that distance would not be able to reach us. Secondly, since infinite time has not elapsed, not all light have reached us. Edit: great video to answer your question [this video]
Recommends Void · 2014-07 · Physics Stack Exchange · on “If the universe didn't expand faster than light, would our nights brighter like day?”
I like this video [this video] by MinutePhysics on this topic. It can clarify things as a primer. When you state that the universe expanded at a speed higher than the speed of light, you have to stop and ask what is actually meant by such a statement. What is moving with respect to what? In standard cosmology, we describe the universe expansion by the Hubble rate $H(t)$. $H(t)$ is however not a speed, but a "rate" $[H]=time^{-1}$. The "apparent speed" (corresponding roughly to redshift) between two points at a distance $d$ in the rest frame of the universe is then $$v = H(t)d$$ For any given Hubble rate, we can find a $d$ for which the "apparent speed" is formally larger than the speed of light,
Cites mmesser314 · 2025-06 · Physics Stack Exchange · on “How bright is the night sky?”
Olbers' paradox [LINK] says that the night sky should be bright if the universe is infinite and full of stars. If you look in any direction, eventually you will come to a star. All directions should be emitting light. Minute Physics has a description of the paradox and the resolution that is hard to improve on. Why is it Dark at Night? [this video] . Don Lincoln made a $2019$ video describing why we expect the universe is at least $250$ times bigger than the visible universe. How far is the edge of the universe? [LINK] Measurements since then improved that number to $500$ . The universe might be infinite. Distant parts might be much the same as the part we can see. Or not. You might imagine waiti