Question about Expanding Universe

[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.