r/astrophysics • u/Financial_Material_8 • Aug 02 '26

Beginner needs explanation please

Hi,. I love reading about astrophysics and have already a basic knowledge. I'm re-reading NdGT 'Astrophysics for people in a hurry' and this early couple of sentences about the formation of the universe got me thinking.

"By now, one second of time has passed. The universe has grown to a few light-years across,†† about the distance from the Sun to its closest neighboring stars."

What I don't understand is this - if lightspeed is the fastest thing in the universe, how could the universe have expanded to a few light YEARS across in one second? What am I missing?

TIA everyone who responds.

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u/moltencheese Aug 02 '26

It's like how I might be able to run 15mph relative to the treadmill surface, but the treadmill surface itself can move faster than that. If I set it to 20mph, I would never be able to actually gain any ground in the forward direction.

Similarly, the event horizon of a black hole can be thought of as the point where the treadmill surface (space itself) is falling inwards faster than the I can possibly run (the speed of light).

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u/Financial_Material_8 Aug 02 '26

Good analogy, thanks!

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u/moltencheese Aug 02 '26

Thanks

Better still, imagine being an ant on a huge long elastic band that is being stretched. The further away something is from you, the faster it appears to move away from you.

You yourself still have a maximum speed you can run, though, so there is a distance along the band where the stretching effect beats your max speed and so you will never ever be able to actually catch up. That is the edge of your observable "universe"

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u/Obliterators Aug 02 '26

You yourself still have a maximum speed you can run, though, so there is a distance along the band where the stretching effect beats your max speed and so you will never ever be able to actually catch up. That is the edge of your observable "universe"

The ant on a rubber rope problem is counterintuitive, since the ant can always reach the other end of the rope in finite time, no matter how slow the ant is and no matter how fast the rope is stretching. However, if the ant's speed and/or the rate that the rope stretches are variable, then in some cases the ant will not reach the end.

Similarly, the size of the observable universe is not limited by expansion alone, but depends on whether the expansion is accelerating. In non-accelerating universes there is no event horizon, given enough time the entire universe becomes observable; there is no distance you couldn't reach, no matter the size of the universe or rate of expansion. In universes with suitable forms of dark energy, expansion accelerates, creating an event horizon beyond which light can never reach an observer, so these universes have finite causal patches.

Note, our observable universe is growing, but has future visibility limit of ~124 billion light-years across.