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

 

[Mike Helland]

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.

We can determine the velocity of a star in a galaxy by its blue or red shift.

How do we tell direction?

The Milky Way has 50 some dwarf galaxies:

https://www.esa.int/ESA_Multimedia/Images/2021/11/Dwarf_galaxies_around_the_Milky_Way

We will probably discover more with better telescopes.

 

[slyjoe]

We can determine the velocity of a star in a galaxy by its blue or red shift.

How do we tell direction?

The Milky Way has 50 some dwarf galaxies:

https://www.esa.int/ESA_Multimedia/Images/2021/11/Dwarf_galaxies_around_the_Milky_Way

We will probably discover more with better telescopes.

Going from memory, blue or red shift is velocity along a line to us. Measuring tangential velocity (sometimes over years or decades) is at right angles to this. Combine those two velocity vectors (which are at right angles to each other) and you get the proper velocity (speed and direction).

ETA: I'm sure the guys that remember this better will correct me.

 

[Mike Helland]

Going from memory, blue or red shift is velocity along a line to us. Measuring tangential velocity (sometimes over years or decades) is at right angles to this. Combine those two velocity vectors (which are at right angles to each other) and you get the proper velocity (speed and direction).

ETA: I'm sure the guys that remember this better will correct me.

Right.

From a short period of time we get the radial velocity.

The tangential velocity is pretty fast, but even at a million miles an hour, we're talking about the outer radius of a galaxy. Do the math on how long it would actually take to clock of change in distance.

Many orders of magnitudes past how long we have been observing them.

 

[slyjoe]

Right.

From a short period of time we get the radial velocity.

The tangential velocity is pretty fast, but even at a million miles an hour, we're talking about the outer radius of a galaxy. Do the math on how long it would actually take to clock of change in distance.

Many orders of magnitudes past how long we have been observing them.

From 2017:
Astronomers measure the motions of stars in a nearby galaxy

https://astronomy.com/news/2017/11/astronomers-measure-the-motions-of-stars-in-a-nearby-galaxy

 

[Ziggurat]

Going from memory, blue or red shift is velocity along a line to us. Measuring tangential velocity (sometimes over years or decades) is at right angles to this. Combine those two velocity vectors (which are at right angles to each other) and you get the proper velocity (speed and direction).

ETA: I'm sure the guys that remember this better will correct me.

No, this is incorrect. You don't need tangential velocity (relative to our line of sight) at all. You measure the velocity towards or away from you on one side of the galaxy, then on the other, and the difference between them is twice the orbital velocity (accounting for the tilt of the galaxy as well).

Right.

From a short period of time we get the radial velocity.

No, that's not how it works. We never have to watch a star change position to figure this out.

 

[Ziggurat]

We can determine the velocity of a star in a galaxy by its blue or red shift.

How do we tell direction?

Blue is coming towards us, red is going away from us. It's blue shifted on one side of the galaxy, red shifted on the other, which tells us which way the galaxy is rotating. We assume that the velocity is tangential, because anything far from tangential would mean an unstable galaxy, and galaxies ARE stable unless they're very close to other galaxies. So, could there be some errors from a not perfectly tangential velocity? Yes. Can these errors be large? Not for every single galaxy we measure the rotation curve of.

The Milky Way has 50 some dwarf galaxies:

What's your point?

 

[Delvo]

The rate of expansion was given above (70 kilometers per megaparsec per second), but the units are a bit awkward, with two of the units being distance. What it means, if we keep both of those distance units, is that it takes a second for a distance of a megaparsec to grow 70 kilometers longer. But there's a simpler way to look at it: any formula with one unit divided or multiplied by another unit for the same physical parameter (distance in this case) can be expressed using the same unit for both, and thus treated as just a ratio.

They just don't normally do that in this case because the difference in scale of the difference between the distance units is so vast that it would make the numbers start looking silly. A megaparsec is about
30,856,775,814,913,673,000 km, so the universal expansion rate would bring it all the way up to
30,856,775,814,913,673,070 in one second. The ratio between those is about
1.0000000000000000022685455026111, which gives us a growth rate of about
0.00000000000000022685455026111% per second. At that rate, for any distance to grow by 1% would take about
4,408,110,830,701,867 seconds, which is over 139,684,603 years.

That's why it's not pushing/pulling people, planets, or galaxies apart (yet): because it's practically nothing (for now). Pretty much everything else in the universe that ever pushes or pulls any two things closer together or farther apart does it many many many times faster than that. If I take a single step in a second, I'm already about 0.000004735% of the way around to the opposite side of Earth, which means I just reduced that distance by almost 21 billion times more than cosmic expansion would have increased it in the same second. The effect simply gets overwhelmed, by a wide margin, compared to practically anything else that isn't literally nothing at all.

It is increasing, but, when scientists postulate a future in which it destroys galaxies/planets/atoms, they're talking about a time when the number of years in the universe's age will be dozens of digits long.

 

[smartcooky]

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.

..and an interesting point occurs to me

Back in the early part of the 20th century, "The Great Debate" raged between two prominent astronomers, Harlow Shapley and Heber Curtis. The debate topic was "are the spiral nebulae we observe inside or outside of the Milky Way, and is the Milky Way the whole Universe or just a small part of it?"

Harlow Shapley believed that these nebulae were relatively small and lay within the outskirts of the Milky Way galaxy, and that our galaxy was the entire Universe.

Heber Curtis believed they were in fact other galaxies, that they were large and very distant, and this implied that the Universe was a lot bigger than just our galaxy.

In the end, the discovery by Edwin Hubble of the red shift in the spectra of spiral nebulae swung the debate in favour of Heber Curtis, but it is interesting to think that if we wind the clock far enough ahead, there will come a time millions of years in the future when the Universe has expanded so much, that no other galaxies will be visible from any planet, orbiting any star in our galaxy. Any future civilisation growing up in that time, will have never seen a galaxy, and so they would likely believe that the Milky Way is the Universe, and would hold the views of Harlow Shapley to be correct.

 

[HansMustermann]

Hell, there'll be a time (very briefly) where there's nothing outside of our solar system.

 

[Chanakya]

The rate of expansion was given above (70 kilometers per megaparsec per second), but the units are a bit awkward, with two of the units being distance. What it means, if we keep both of those distance units, is that it takes a second for a distance of a megaparsec to grow 70 kilometers longer. But there's a simpler way to look at it: any formula with one unit divided or multiplied by another unit for the same physical parameter (distance in this case) can be expressed using the same unit for both, and thus treated as just a ratio.

They just don't normally do that in this case because the difference in scale of the difference between the distance units is so vast that it would make the numbers start looking silly. A megaparsec is about
30,856,775,814,913,673,000 km, so the universal expansion rate would bring it all the way up to
30,856,775,814,913,673,070 in one second. The ratio between those is about
1.0000000000000000022685455026111, which gives us a growth rate of about
0.00000000000000022685455026111% per second. At that rate, for any distance to grow by 1% would take about
4,408,110,830,701,867 seconds, which is over 139,684,603 years.

That's why it's not pushing/pulling people, planets, or galaxies apart (yet): because it's practically nothing (for now). Pretty much everything else in the universe that ever pushes or pulls any two things closer together or farther apart does it many many many times faster than that. If I take a single step in a second, I'm already about 0.000004735% of the way around to the opposite side of Earth, which means I just reduced that distance by almost 21 billion times more than cosmic expansion would have increased it in the same second. The effect simply gets overwhelmed, by a wide margin, compared to practically anything else that isn't literally nothing at all.

It is increasing, but, when scientists postulate a future in which it destroys galaxies/planets/atoms, they're talking about a time when the number of years in the universe's age will be dozens of digits long.

Great post, thanks Delvo. Clearly explains just why it is that the "binding" --- whether atomic, or molecular, or galactic, or of galactic clusters --- overrides the expansion of the universe, by very graphically discussing how totally totally 'stronger' is the former when compared to the latter. And also how utterly inconceivably vast, even at cosmic scales, are the time frames when the expansion rates might catch up and override these "bindings" (I suppose this is the "Big Rip" that Ziggurat mentioned upthread).


---


About this increasing rate of expansion, though:

Why is that a thing, do we know? That is, it is my (vague) understanding that right after the (alleged) Big Bang, there was a (very brief) period of super-massive expansion. But apparently that super-massive expansion rate then suddenly slowed down to a very feeble rate of expansion ("feeble", in terms of, for instance, what you quantify in your comment). And apparently this "feeble" rate of expansion is increasing, slowly but surely, so that one day, in the far far distant future, it will (or might) become 'strong' enough to override the other gravitational etc forces.

I was wondering, do we know why/how this rate of expansion, that was so super-massively-rapid to begin with, slowed down to this far more sedate rate; and/or why/how that rate now keeps on increasing?

 

[Chanakya]

..and an interesting point occurs to me

Back in the early part of the 20th century, "The Great Debate" raged between two prominent astronomers, Harlow Shapley and Heber Curtis. The debate topic was "are the spiral nebulae we observe inside or outside of the Milky Way, and is the Milky Way the whole Universe or just a small part of it?"

Harlow Shapley believed that these nebulae were relatively small and lay within the outskirts of the Milky Way galaxy, and that our galaxy was the entire Universe.

Heber Curtis believed they were in fact other galaxies, that they were large and very distant, and this implied that the Universe was a lot bigger than just our galaxy.

In the end, the discovery by Edwin Hubble of the red shift in the spectra of spiral nebulae swung the debate in favour of Heber Curtis, but it is interesting to think that if we wind the clock far enough ahead, there will come a time millions of years in the future when the Universe has expanded so much, that no other galaxies will be visible from any planet, orbiting any star in our galaxy. Any future civilisation growing up in that time, will have never seen a galaxy, and so they would likely believe that the Milky Way is the Universe, and would hold the views of Harlow Shapley to be correct.

Haha, how quaint that sounds, the idea that the Milky Way might be the whole universe. And yet these were apparently, as you say, prominent astronomers debating this, both of them, and not a couple of cranks. And this isn't even all that much of a long time ago!

Shows how much of what we know today we've only discovered so very recently, relatively speaking!

 

[Delvo]

About this increasing rate of expansion, though:

Why is that a thing, do we know?... do we know why/how this rate of expansion, that was so super-massively-rapid to begin with, slowed down to this far more sedate rate; and/or why/how that rate now keeps on increasing?

No, and it's one of the top few items on every professional physicist's list of remaining unsolved mysteries of physics.

To explain the increase, just explaining why the expansion happens at all is probably a prerequisite, and even that problem alone is already the source of literally the worst prediction in the history of science: two numbers you get by different routes, which according to the only explanatory hypothesis available should be the same number, are actually so far off from each other that dividing the big one by the little one yields an answer 120 digits long.

 

[sphenisc]

Great post, thanks Delvo. Clearly explains just why it is that the "binding" --- whether atomic, or molecular, or galactic, or of galactic clusters --- overrides the expansion of the universe, by very graphically discussing how totally totally 'stronger' is the former when compared to the latter. And also how utterly inconceivably vast, even at cosmic scales, are the time frames when the expansion rates might catch up and override these "bindings" (I suppose this is the "Big Rip" that Ziggurat mentioned upthread).


---


About this increasing rate of expansion, though:

Why is that a thing, do we know? That is, it is my (vague) understanding that right after the (alleged) Big Bang, there was a (very brief) period of super-massive expansion. But apparently that super-massive expansion rate then suddenly slowed down to a very feeble rate of expansion ("feeble", in terms of, for instance, what you quantify in your comment). And apparently this "feeble" rate of expansion is increasing, slowly but surely, so that one day, in the far far distant future, it will (or might) become 'strong' enough to override the other gravitational etc forces.

I was wondering, do we know why/how this rate of expansion, that was so super-massively-rapid to begin with, slowed down to this far more sedate rate; and/or why/how that rate now keeps on increasing?

The detailed particle physics mechanism responsible for inflation ^WP is unknown. The basic inflationary paradigm is accepted by most physicists, as a number of inflation model predictions have been confirmed by observation; however, a substantial minority of scientists dissent from this position. The hypothetical field thought to be responsible for inflation is called the inflaton.

https://en.m.wikipedia.org/wiki/Inflaton

 

[HansMustermann]

About this increasing rate of expansion, though:

Why is that a thing, do we know? That is, it is my (vague) understanding that right after the (alleged) Big Bang, there was a (very brief) period of super-massive expansion. But apparently that super-massive expansion rate then suddenly slowed down to a very feeble rate of expansion ("feeble", in terms of, for instance, what you quantify in your comment). And apparently this "feeble" rate of expansion is increasing, slowly but surely, so that one day, in the far far distant future, it will (or might) become 'strong' enough to override the other gravitational etc forces.

I was wondering, do we know why/how this rate of expansion, that was so super-massively-rapid to begin with, slowed down to this far more sedate rate; and/or why/how that rate now keeps on increasing?

Just to add though, we really don't know how far it will go. All calculations seem to put us somewhere around the border between "nah, bound stuff will stay bound" and "Big Rip". Afaik we don't yet have enough accuracy to tell if we're juuust this side of the border or juuust over it.

As for a possible explanation, well, some speculate that this is actually what we'd see if we're actually falling into a giant black hole, and t switches places with r in our chart. The big rip being basically us hitting the singularity. Essentially what we perceive as space expansion is really the fact that a light cone centered on us starts very wide, and becomes narrower and narrower as we approach the singularity. Right before hitting it, essentially your light cone is a line, and you really can't see anything to your left or right. Even though in another chart it might be 1 micron away, light from it cannot reach you and viceversa, so basically for you it might as well be infinite distance. That has become all your observable universe.

I've touched briefly on it before, but I'll freely admit that my expertise is BY FAR not enough for me to get into much of an argument over it. So take it with a lot of salt.

 

[Chanakya]

No, and it's one of the top few items on every professional physicist's list of remaining unsolved mysteries of physics.

To explain the increase, just explaining why the expansion happens at all is probably a prerequisite, and even that problem alone is already the source of literally the worst prediction in the history of science: two numbers you get by different routes, which according to the only explanatory hypothesis available should be the same number, are actually so far off from each other that dividing the big one by the little one yields an answer 120 digits long.

That's interesting. Not that it's likely to mean much/anything to me, but still, out of curiosity: what is this number, then, that's expected to have just one value but ends up having two such widely differing ones?

 

[Chanakya]

The detailed particle physics mechanism responsible for inflation ^WP is unknown. The basic inflationary paradigm is accepted by most physicists, as a number of inflation model predictions have been confirmed by observation; however, a substantial minority of scientists dissent from this position. The hypothetical field thought to be responsible for inflation is called the inflaton.

https://en.m.wikipedia.org/wiki/Inflaton

Dissent from the position of an expanding universe? Didn't know that! It was my impression that everyone's kind of agreed, at this time, about an expanding universe.

 

[Chanakya]

Just to add though, we really don't know how far it will go. All calculations seem to put us somewhere around the border between "nah, bound stuff will stay bound" and "Big Rip". Afaik we don't yet have enough accuracy to tell if we're juuust this side of the border or juuust over it.

As for a possible explanation, well, some speculate that this is actually what we'd see if we're actually falling into a giant black hole, and t switches places with r in our chart. The big rip being basically us hitting the singularity. Essentially what we perceive as space expansion is really the fact that a light cone centered on us starts very wide, and becomes narrower and narrower as we approach the singularity. Right before hitting it, essentially your light cone is a line, and you really can't see anything to your left or right. Even though in another chart it might be 1 micron away, light from it cannot reach you and viceversa, so basically for you it might as well be infinite distance. That has become all your observable universe.

I've touched briefly on it before, but I'll freely admit that my expertise is BY FAR not enough for me to get into much of an argument over it. So take it with a lot of salt.

Wow, that's a new one! The black hole thing, I mean. Sounds totally sci-fi-ish! Cool.

So, "we", as in all of our universe? But how would that work? (Yeah, I know, not your area of expertise, as you qualify; but any idea, even if vaguely/approximately?) I mean, how on earth would our entire universe be consumed by a black hole --- which itself, the black hole that is, would presumably be part of this universe itself, right? Or, what, are we speaking of metaverses here, and of our universe falling into a black hole of some other much bigger universe, or something exotic like that?

 

[HansMustermann]

No. In that setup, the whole universe is inside the black hole. We're about 14 billion years deep into it in fact. (Again, t and r switch roles in that scenario.) In fact, the universe IS the black hole. Everything in the universe is, well, matter falling towards the singularity.

 

[Chanakya]

No. In that setup, the whole universe is inside the black hole. We're about 14 billion years deep into it in fact. (Again, t and r switch roles in that scenario.) In fact, the universe IS the black hole. Everything in the universe is, well, matter falling towards the singularity.

That's ...mind-bending, that thought, that idea.

Of course, any such hypothesis raises many more questions than it answers! But then I guess that's true of most hypotheses, including our usual (as in no-giant-black-hole) Big Bang theory.



eta: Here's a weird thing. If we're actually in a black hole, our whole universe, then what you have is a number of black holes within a giant black hole!

 

[HansMustermann]

Well, it doesn't contradict Big Bang. It's just that -- IF the maths checks out, which is a bit beyond my pay grade -- the Big Bang is just what the formation of that black hole looks like from the inside.