[sigmoid10]

Light is indeed redshifted as it climbs through a gravitational well, but it is also blueshifted again when it travels down the gravitational well of our own galaxy. For distant galaxies, the total effect is negligible compared to the doppler redshift due to their velocity. Also, when you look in any direction in the sky, the redshift increases with distance, irrespective of galaxy or cluster masses. So the further they are away, the faster they are receding from us. This is also exactly what the Hubble constant measures.

[tdfirth]

Cool, thanks for the reply! I hadn't considered the blueshifting part, that makes a lot of sense though.

I also hadn't heard of Hubble's constant, which I am now learning about :). Very mysterious indeed.

[arcastroe]

> the further [galaxies] are away, the faster they are receding from us

I've heard this repeated many times as a surprising discovery. Why is this surprising? Let's say you have some number of objects moving at different speeds (relative to Earth). After enough time, the objects that move fastest should be farthest away, by the simple definition of speed!

What am I missing?

[luma]

Substitute "speed" (really, "velocity") with "acceleration" and your statement is true. So, you're pretty close in your reasoning - if everything is accelerating at the same rate, then things further away from us should be moving away from us faster.

So here's the problem: maintaining a constant velocity requires no outside force, but acceleration does. So, we're seeing everything accelerate away from us.... but why? Where does that force come from?

[vlovich123]

Amateur so I’m sure this is a stupid idea. What if they’re not accelerating away from us but space time itself is growing (because of the original Big Bang) and taking everything else along for the ride on top of the original exploding outward, resulting in acceleration of acceleration? Probably it’s growth rate would be decreasing which might be observable in the derivative of the derivative of acceleration? If that were happening, I wonder if virtual particles have some link to that process.

My reasoning here is that everything is accelerating from everything else since our position in the galaxy isn’t particularly special right? Then the only way for that really to be true would be for space time itself to be growing.

[sigmoid10]

That's sort of what's happening. If you imagine space as a rubber sheet with the galaxies pinned to certain places, something is pulling the sheet apart, making it seem like the galaxies are accelerating away from each other. We know the math behind this mechanism thanks to general relativity (it only needs a certain energy density for empty space), but we don't know what causes it. So we just call it "dark energy." Without it, the original expansion of the universe caused by the big bang should be slowing down due to gravity. Dark energy causes it to accelerate instead.

[ItsMonkk]

Another amateur, but I think it's more likely the reverse. As we travel through time, us, and the atoms we are made of, shrink(or another way to look at it is 'being consumed', or using up its energy). And we shrink according to how fast we travel through time, and we move through time based on mass. The relative shrinking causes gravity. This would result in that same change in 'acceleration' that we observe, but really it's not that anything is moving, everything is simply changing sizes.

This must be wrong because it feels obvious and testable, simply make 2 satellites, send one to space, then confirm they are the same size when it gets back. And yet I can't find evidence either way.

[867-5309]

>acceleration of acceleration

maybe this is where the c² in the famous equation comes from

>everything is accelerating from everything else

but what about galactic collisions? we are told the big bang did not happen from one single point but instead everywhere at once

[arcastroe]

Ah thank you :).

One more question: How do we know that each galaxy is actually accelerating and not moving at a constant speed?

I'm seeing some circular logic. As I understand it, we think they are accelerating _because_ we see that galaxies that are farther away are moving faster away from us.

But we just agreed that we expect to see this same observation even if each individual galaxy maintains constant velocity. Apologies. It's a bit late. I'm sorry if I'm missing something incredibly obvious.

[adgjlsfhk1]

between 1970 and now, if you look at the same far away galaxies, they are receding faster now than they were in 1970.

[koheripbal]

Wait... Is this correct? Do you have a source? This is the first that I've heard of this being directly measurable.

[arcastroe]

^_^ thank you

[donohoe]

Dumb question: is it actually accelerating or are we slowing down?

[ByteJockey]

Possibly dumb answer: If everything else was moving at a constant velocity and we were slowing down, wouldn't the things on the other side of us appear to be accelerating towards us?

My understanding is that basically everything is moving away from us (presumably our reputation precedes us).

[cecilpl2]

They are all accelerating away from us, in every direction.

[albert_e]

do you mean slowing down in how we experience time?

[kadoban]

That seems like it would itself need an explanation. Why would the initial conditions be objects starting at the ~same place but moving with random velocity?

The CMB would also need explanation and I'm thinking the curves of galaxy speeds wouldn't match unless you tweak the initial speeds way too much, but I'm not sure how to formulate that right.