Question about Expanding Universe

[Chanakya]

Two questions, actually:

(1) Sure, the universe's expanding. But then we're part of that expansion, right? That is, our bodies too are expanding, aren't they? The distances/space between our cells, and therefore the size of our bodies, and also the size of our telescopes and whatnot? Does that not kinda sorta cancel out, then? That is, vaster spaces will obviously expand vastly more, but the proportionality of it? (Well, clearly that isn't how that works, I guess. We wouldn't even be talking about an expanding universe otherwise. But I was wondering why not.)

(2) Does everything expand? Even at the teeny-tiny quantum scale? Or is it only at the 'regular' scale, where QM doesn't directly come in? (So that, as far as our bodies, is it only our the distance/space between our cells that's expanding along with the expanding universe; or would that expansion apply also to inter-molecular space/distances, maybe even sub-atomic space/distances?

 

[Ziggurat]

Two questions, actually:

(1) Sure, the universe's expanding. But then we're part of that expansion, right? That is, our bodies too are expanding, aren't they?

No. The forces that bind our body together, the molecular bonds, keep them from expanding along with space.

The distances/space between our cells, and therefore the size of our bodies, and also the size of our telescopes and whatnot? Does that not kinda sorta cancel out, then?

No, since bound objects don't expand with the universe.

If the expansion rate is sufficiently fast, then binding forces might be insufficient to keep an object from expanding. That's one hypothesized future state, the "Big Rip", but that's not happening right now or any time soon even if that theory is correct. Our bodies, and even individual galaxies, are sufficiently bound to keep from expanding along with the universe right now.

(2) Does everything expand? Even at the teeny-tiny quantum scale?

The way scale enters into it is that at smaller scales, less binding force is required to counter the expansion. But it's not a quantum/non-quantum thing. And again, even galaxies are sufficiently bound to not expand with the universe.

 

[theprestige]

even galaxies are sufficiently bound to not expand with the universe.

Okay so follow-up question that just occurred to me: Space "wants to" put more distance between stars in the Milky Way. But the galaxy is a gravitationally-cohesive whole, so the stars don't actually move farther apart.

But is the tension between gravitational attraction and spacetime expansion observable? When a star is swinging around the core of the galaxy in its gravity-determined orbit, is that orbit detectably perturbed at all by the efforts of spacetime to widen its orbital track?

 

[Ziggurat]

Okay so follow-up question that just occurred to me: Space "wants to" put more distance between stars in the Milky Way. But the galaxy is a gravitationally-cohesive whole, so the stars don't actually move farther apart.

But is the tension between gravitational attraction and spacetime expansion observable?

In principle if your measurements were sensitive enough, it should be. That is, the observed acceleration of stars around the center of the milky way should be somewhat reduced by the pull from expansion.

In practice, we don't have anywhere near the required sensitivity. First off, expansion even at the scale of a galaxy is still pretty damn small, so it's a minor perturbation to begin with. Second, we don't have anywhere near good enough measurements of the mass distribution of galaxies to pull that off. We have to infer how much ordinary matter is there based on how much light it gives off, and that's not very accurate. Plus, we have to infer most of the mass (dark matter) from motion anyways. And even if we did know the mass perfectly, our measurements of velocities are probably not accurate enough either. And I don't think there's any way we can hope to close that gap in the foreseeable future.

 

[Ziggurat]

So let's crunch some numbers. Hubble's constant is roughly 70 km/s/Mpc. Let's use Andromeda, since we can see that from the outside. Andromeda has a radius of about 23 kpc, so that translates to an expansion rate of 70 km/s/Mpc * 23 kpc = 1.6 km/s at the edge.

Figuring out the required acceleration to counter this is a bit tricky, but we can skip doing this directly by noting the rotation rate. At the edge, the rotation rate for Andromeda is around 200 km/s. That's the orbital velocity that stars at the edge have around the center. The expansion rate is on the order of 1% of this. That's not nothing, and again, with really high precision measurements, maybe we could see this. But alas, we can't do high precision measurements of the mass distribution of Andromeda, because most of it is dark matter which we have to infer from the motion, we can't get it independently and then use it to accurately prediction the motion and find the perturbation from expansion.

 

[Mike Helland]

So let's crunch some numbers. Hubble's constant is roughly 70 km/s/Mpc. Let's use Andromeda, since we can see that from the outside. Andromeda has a radius of about 23 kpc, so that translates to an expansion rate of 70 km/s/Mpc * 23 kpc = 1.6 km/s at the edge.

Figuring out the required acceleration to counter this is a bit tricky, but we can skip doing this directly by noting the rotation rate. At the edge, the rotation rate for Andromeda is around 200 km/s. That's the orbital velocity that stars at the edge have around the center. The expansion rate is on the order of 1% of this. That's not nothing, and again, with really high precision measurements, maybe we could see this. But alas, we can't do high precision measurements of the mass distribution of Andromeda, because most of it is dark matter which we have to infer from the motion, we can't get it independently and then use it to accurately prediction the motion and find the perturbation from expansion.

We've known that galaxies exist for less than 100 years.

It takes like 200,000 years for a galaxy to rotate.

How certain can we be about how fast things are moving? That seems like a rather large extrapolation.

 

[Chanakya]

No. The forces that bind our body together, the molecular bonds, keep them from expanding along with space.



No, since bound objects don't expand with the universe.

If the expansion rate is sufficiently fast, then binding forces might be insufficient to keep an object from expanding. That's one hypothesized future state, the "Big Rip", but that's not happening right now or any time soon even if that theory is correct. Our bodies, and even individual galaxies, are sufficiently bound to keep from expanding along with the universe right now.

Oh, ok.

So, while the Milky Way itself isn't "expanding", because it's bound up closely enough to 'resist' the expansion; but the distance between Milky Way and Andromeda is indeed expanding. Is that right?

Sure, that explains it, simply and clearly. Thanks.

The way scale enters into it is that at smaller scales, less binding force is required to counter the expansion. But it's not a quantum/non-quantum thing. And again, even galaxies are sufficiently bound to not expand with the universe.

Ok, got that.

 

[Ziggurat]

Oh, ok.

So, while the Milky Way itself isn't "expanding", because it's bound up closely enough to 'resist' the expansion; but the distance between Milky Way and Andromeda is indeed expanding. Is that right?

Getting much closer, but still not quite right. Andromeda and the Milky Way are actually still gravitationally bound to each other, and in fact will eventually collide (we're talking 4.5 billion year time scale, don't panic). Galaxy clusters and even super clusters can still be gravitationally bound. But yes, when you get to large enough scales, galaxies aren't bound to each other anymore and they will fly apart.

 

[Ziggurat]

We've known that galaxies exist for less than 100 years.

It takes like 200,000 years for a galaxy to rotate.

How certain can we be about how fast things are moving? That seems like a rather large extrapolation.

No, it's not a large extrapolation at all. Differential red shift measurements from opposite sides of a galaxy tell us the orbital velocity quite directly. We don't need to watch them make a full circle.

The only assumption we're making here is that the structure of a galaxy is fairly stable, that all those stars didn't just coincidentally form that shape before either collapsing or flying off into the void. And given that we don't see galaxies doing this, I think it's a pretty damn safe assumption.

 

[Chanakya]

Getting much closer, but still not quite right. Andromeda and the Milky Way are actually still gravitationally bound to each other, and in fact will eventually collide (we're talking 4.5 billion year time scale, don't panic). Galaxy clusters and even super clusters can still be gravitationally bound. But yes, when you get to large enough scales, galaxies aren't bound to each other anymore and they will fly apart.

Ah right. No, I was aware of that, that we (as in MW) will collide with Andromeda, but hadn't quite connected the dots I guess, as far as what we're talking about here.

Ok, so not just galaxies but galaxy clusters also are "bound", then, and therefore don't expand away. Got that.

 

[Ziggurat]

Ah right. No, I was aware of that, that we (as in MW) will collide with Andromeda, but hadn't quite connected the dots I guess, as far as what we're talking about here.

Ok, so not just galaxies but galaxy clusters also are "bound", then, and therefore don't expand away. Got that.

Yup.

Unless there's a Big Rip (not a given), in which case everything will eventually expand away, starting with the largest structures and working its way down eventually to our very constituent atoms.

 

[Mike Helland]

No, it's not a large extrapolation at all. Differential red shift measurements from opposite sides of a galaxy tell us the orbital velocity quite directly. We don't need to watch them make a full circle.

The only assumption we're making here is that the structure of a galaxy is fairly stable, that all those stars didn't just coincidentally form that shape before either collapsing or flying off into the void. And given that we don't see galaxies doing this, I think it's a pretty damn safe assumption.

We don't see galaxies flinging off stars?

 

[Ziggurat]

We don't see galaxies flinging off stars?

Not in any appreciable numbers, unless they're being ripped apart by interactions with other galaxies.

When we do these galaxy rotation curve measurements, we aren't measuring individual star velocities. A few individual stars with very different velocities wouldn't make a difference. We're measuring the aggregate velocity of a whole bunch of stuff. And either that's the orbital velocity of that aggregate, or the galaxy isn't stable. There isn't any ambiguity about that. And it's not plausible that all these galaxies we measure are all wildly unstable, either about to collapse or explode.

 

[Mike Helland]

Not in any appreciable numbers

Wouldn't it take like 10,000 years of observations to know that?

Again. We learned what a galaxy is 94 years ago.

They look like they are moving fast. So let's say they are.

They look like they are being flung out. Maybe they are.

 

[Ziggurat]

Wouldn't it take like 10,000 years of observations to know that?

No.

They look like they are moving fast. So let's say they are.

They look like they are being flung out. Maybe they are.

In other words, maybe they are unstable. Which is what I said before is a logical possibility based on the observation of a single galaxy, without any other context.

But again, that's just not plausible, because there is context, and it's not just one galaxy. Why is basically EVERY galaxy we observe coincidentally unstable at the moment we are observing it, but we don't actually see the aftermath of this instability in any other galaxy we observe? Where are all the galaxies that flew apart? We don't see any. And how did they coalesce in the first place, if they don't have the gravitational attraction to stay bound together? Even getting a single galaxy into such a configuration requires an amazing coincidence. It's not gravitationally bound, but it has to come together while looking like it is. The probabilities of such cosmic coincidence happening by chance so that it LOOKS like all these galaxies are stable when they really just came together at this moment and will now explode is just so absurdly unlikely that we don't have to seriously consider it.

ETA: it's the cosmological equivalent of saying that God placed dinosaur bones in the earth to make it look like earth is older than it actually is.

 

[Mike Helland]

Why is basically EVERY galaxy we observe coincidentally unstable at the moment we are observing it, but we don't actually see the aftermath of this instability in any other galaxy we observe?

The aftermath being what? Stars flung out?

They'd definitely be harder to see against the more dense galaxy from which they were flung.

And that's probably why there are dwarf galaxies in several directions.

 

[Ziggurat]

The aftermath being what? Stars flung out?

No. The aftermath would be the majority of a galaxy being ripped apart. Those stars wouldn't become invisible, we would still see the aftermath.

And again, there's no plausible way to get those stars into such a configuration to begin with. Stars don't spontaneously coalesce around points that they aren't gravitationally bound to, and they REALLY don't do that all together in a coordinated fashion. And that coordination wouldn't just have to happen within one galaxy (which there's still no mechanism for), it would have to be coordinated between all these different galaxies at different places and at different times, in order for all of them to be appearing to do it when humans on earth today see it, but any civilization in a different galaxy or any civilization in our galaxy at a different time wouldn't.

The universe isn't conspiring to fool us.

And that's probably why there are dwarf galaxies in several directions.

No, it really isn't.

 

[jonesdave116]

And that's probably why there are dwarf galaxies in several directions.

Errrr, yeah, right. If they were flung out due to centrifugal forces at the edge of spirals, they would all be in the same plane of the galaxy, yes? Are they? They would all be receding, yes? Are they?

 

[Ziggurat]

Errrr, yeah, right. If they were flung out due to centrifugal forces at the edge of spirals, they would all be in the same plane of the galaxy, yes? Are they? They would all be receding, yes? Are they?

I don't think he's saying that dwarf galaxies were flung out from the edge of regular galaxies, I think he's saying that dwarf galaxies are the result of regular galaxies flinging off a bunch of stars.

Still not plausible, but for different reasons.

 

[jonesdave116]

I don't think he's saying that dwarf galaxies were flung out from the edge of regular galaxies, I think he's saying that dwarf galaxies are the result of regular galaxies flinging off a bunch of stars.

Still not plausible, but for different reasons.

OK. I'm glad you know what he is talking about