Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Harri on 18/11/2018 19:28:11
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As the title says, if the universe were to slow down and perhaps even reverse direction, what would happen to light that was and is now travelling towards the edge of the universe?
As I understand it, the universe is expanding faster than the speed of light.
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As the title says, if the universe were to slow down and perhaps even reverse direction, what would happen to light that was and is now travelling towards the edge of the universe?
As I understand it, the universe is expanding faster than the speed of light.
Well for one, there is no posited 'edge of the universe'. When it is said that it is expanding faster than light, it just means that objects a sufficiently large distance from us are increasing their proper distance at a rate greater than light speed. That the expansion is accelerating means that the boundary where that difference takes place (the Hubble sphere) is growing at less than light speed.
If the expansion were to be slowing down (or just be steady), there would be no event horizon. The event horizon is a distance beyond which no signal can reach us ever, and no signal from us can ever reach them. The event horizon is currently about 16 billion light years out, not much further than the Hubble radius of about 14 BLY.
If the expansion were to actually turn into a contraction, the Hubble radius would represent where things are sufficiently distant to be coming at us at greater than light speed. Not sure how that would work mathematically.
As I understand it, the universe is expanding faster than the speed of light.
The expansion of the universe isn't a speed that is measured in distance per second. It doubles its dimensions every 2x amount of time. You can't technically double that.
Anyway, if the expansion rate were half since the beginning, it would today have the visible dimensions it does now: at age 13.8 billion years, the stuff 13.8 billion light years away will be going away at light speed, just like it is now (sort of, this is assuming the rate is constant, which it isn't).
But there would be twice as much matter between here and 13.8 billion light years away.
Maybe. Matter is energy, and half the expansion rate is half the energy, so half the matter would be generated, so it perhaps would look just like it does now. It all depends on what you decide to hold constant while messing with the expansion rate.
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A nice question.
Let's assume space contracts, we still have light propagating at 'c'. The cosmological blue and redshift should show us something, presuming it is correct. But then again, we need some frame of reference noticing it. We could also use distances, as we already do, between stellar objects. It depends on the rate too I would guess.
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" the stuff 13.8 billion light years away will be going away at light speed, just like it is now (sort of, this is assuming the rate is constant, which it isn't). " That might be a truth of sorts Halc, but the cosmological blue and redshift should show a change. measuring closer to us. So should so called 'fixed candles'. That's stars of a defined magnitude. As for us :) watching galaxies at a speed faster than light? I don't think we would see them in that case as they should be 'infinitely red shifted' to us.