Thread by @starstrickenSF

why is the sky dark at night?

the answer is actually pretty complex & relies on some of the coolest parts of our Universe—the expansion of space-time, the speed of light, & the Big Bang.

let‘s dig in.

first, let’s break down the question. why would we expect the sky to be bright?

let’s say the Universe is uniformly filled with stars. that means that whenever you look up, no matter which way, you’ll ALWAYS see something bright.

now, you COULD say that distant stars are fainter. brightness decreases with distance as 1/r², so maybe they’re just too dim.

but here’s the thing: the further out you go, the more stars you get! that # increases w/ distance as r², so those effects just cancel each other out!

let’s think about this more deeply.

remember that light has a constant speed. the farther out we look into space, the further back in time we see. we see objects *as they were* when they emitted that light.

why does that matter?

when astronomers measured the distances to nearby galaxies, they found that the galaxies were actually moving *away* from us. in other words, they found that the Universe is *expanding*.

if the Universe is expanding, there must be some finite time billions of years ago when it BEGAN.

that’s the Big Bang.

(side note: we can actually measure the age of the Universe by measuring the rate of the expansion of the Universe!)

so, if the Universe began at some finite time 13.7 billion years ago, we are only seeing the light that has had TIME to reach us.

light from extremely distant stars and galaxies—the first stars and galaxies in the Universe—just hasn’t reached us yet.

let’s summarize what we know so far:

because the Universe has a finite AGE, the light we receive on Earth is limited. light from outside the observable universe just hasn’t reached us yet.

we know some other cool stuff about the expansion of the Universe. light from distant stars is *redshifted*.

the wavelength of light is literally shifted towards red parts of the EM spectrum, like the infrared. for more on redshift:

https://twitter.com/starstrickensf/status/1174831846460280832?s=21

that means that light from distant stars and galaxies—the era after the Big Bang when light started seeping through the cosmos—*is* detectable...but not as visible light.

it’s redshifted to the microwave part of the spectrum.

you know that static on your (old) TV?

about 1% is microwave radiation leftover from the Big Bang.

wow.

let’s summarize that second point.

because the Universe is expanding and light gets redshifted, we only see a small fraction of the actual light distributed throughout the Universe. the rest is in the form of microwave “static.”

let me make one more point.

what if interstellar clouds and dust just soak up all the light?

if there’s enough starlight absorbed, it would actually heat up the cloud and re-radiate the light. but if the light comes from far away, it’ll get re-emitted as non-visible light.

in other words, re-emitted light would still be in the redder parts of the spectrum, invisible to the human eye. so that could be another factor to explain our problem.

in sum, the night sky is dark bc:

1) the Universe has a finite age. light from stars outside the observable universe hasn’t had time to reach us.

2) the Universe is expanding. we can’t see the wavelength the most distant stars are radiating in.*

*some dust can make this worse

disclaimer: this was one of the most difficult threads on the Universe that I’ve ever written. cosmology is really really hard and even us astronomers have a hard time understanding it.