Galaxies exist in the first place because the early universe was not completely uniform. Some regions were slightly denser than others. These density variations were initially at the level of one part in 10-100 thousand (10^(-5) to 10^(-4)), but gravity amplified them over time. Denser regions tended to pull in surrounding matter, becoming still denser. Eventually, the densest patches formed galaxies.

However, initial variations in the density of the universe also existed at scales much larger than galaxies. Due to this large-scale structure, galaxies are now moving toward regions of higher density and away from regions of lower density.

Here's an example movie. "z" in the corner is the redshift, essentially inverse time (smaller is later). The key point is that galaxies form (the yellowish color) but continue moving as they coalesce into still larger systems.

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Great video. Thank you👍

I'm confused about the concept of the universe. When I was a kid we were told the universe was "everything" now is not quite everything but currently expanding size. However logically even nothing has dimensions. Seriously, could we go to the edge of the universe then stick a meter stick a little bit further.

(not serious) or is the edge of the universe an ice wall guarded by some galactic guardians?

> the universe was "everything"

Correct. It's expanding, but that's an independent statement.

> Seriously, could we go to the edge of the universe then stick a meter stick a little bit further.

There is no edge and no center either. On a large scale, the universe is the same in every place and every direction.

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I've heard the analogy used that you can think of 3D space as flattened onto the surface of a balloon which is expanding. No matter which direction you take, you always end up back where you started (after a relativity breaking amount of time.

Edit: as the balloon expands, the actual material expands similar to how spacetime is expanding. Interestingly, at some point eons into the future, this expansion will make the milky way an island where everything else is so far away, any future civilization will never receive its light and will think our galaxy is all that exists in the universe.

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This is a trick question. Our current understanding of the universe is this: The portion of the universe that is visible to us is about 93-ish billion light years in diameter or about 28.5 gigaparsecs. Space is expanding at a rate of about 45mi (73km) for every megaparsec. Consequently, beyond the observable universe there are things racing away from earth (frame of reference) faster than light speed thus are part of our particle universe but forever disconnected from our reality. We will never know because any information emitted will never reach us.

I'm hopeful that one day we will devise the necessary physics to dwell into the edges of the universe much like the edges between us and the point of singularity of the black holes.

An interesting thought experiment, as I said earlier, there are things racing away from us at faster than light speed. From their frame of reference, we are receding from them at equal rate. So technically we're both right. We are going through the universe on a roller coaster traveling faster than light. "Make it so"

A note on the “size” of the universe - you will see articles referencing the size of the “observable” universe. This is just the bit of the universe that we can see. As /u/mfb- says, there is currently no known “edge” to the universe.

However, the current “observable” universe is likely the most of the universe we will ever see. This is because the universe is expanding. After a certain distance, things are moving away from us faster than the speed of light. Those objects are unreachable to us now (and probably forever). You can think of it like a moving walkway that is moving faster than a person can run. Even if you run full tilt against the moving walkway, you will not reach the other side.

So there is no “edge” in the sense of a wall or something, but there is a limit to what we can see and (as far as current models predict) a limit to what we will ever see.

Thank you. I'm a little puzzled, however in that wouldn't the attractive forces weaken with distance? That is to say, shouldn't, say, Andromeda and the Milky Way have collided a long time ago?

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That's the cool thing about infinity. There is no end. You can always add 1 one more.

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Gravitational forces do indeed weaken with the square of the distance, but they strengthen between more massive objects. You also need to factor in the relative velocity between the objects.

Just for a complete picture, the Andromeda galaxy is about 2.5 million light years away from the milky way (~0.9 Megaparcecs), and moving towards us at about 110 km/s. The expansion rate of the universe is about 73km/s per megaparcec.

While this is probably a massive oversimplification, we can think of the Andromeda galaxy receding from us at about 73 * 0.9 = 65km/s, but is moving towards us faster than it's receding, at 175km/s, but the expansion rate of the universe brings its net speed down to 110km/s. This is just a snapshot in time, since as the distance gets smaller, the force of gravity gets stronger, and there's less expansion of the universe to deal with. In other words, Andromeda should be accelerating towards us over time, assuming just gravitational force and the expansion rate of the universe.

Why didn't Andromeda and the Milky way already merge? Basically, both galaxies were originally two separate denser regions in space that were far enough apart to form two independent galaxies, but close enough to not recede over time thanks to the expansion of the universe.

> After a certain distance, things are moving away from us faster than the speed of light. Those objects are unreachable to us now

That's a common misconception. We see things where the distance to them always increased faster than the speed of light. The matter that emitted the CMB we see today is an example. The number of things we can see is still increasing as the universe gets older.

In the distant future, in a universe completely dominated by dark energy, your statement will be right.

It's not exactly like raisins in a loaf, though that's a good way to explain it to layperson. Unlike a loaf of bread or surface of a balloon, there isn't a firmament in which galaxies are suspended, and only the firmament expands such that they never touch. It's more like an infinite stew that keeps getting infinitely watered down - slowly. Sometimes the bits in the stew will still hit each other as everything cooks and swirls. They'll just do it less and less often as more water gets added.

The observable universe is not infinite. And it keeps expanding such that more and more of it is not visible. But that's just because we're limited by time and the speed of light with respect to what we can see. If you went to the edge of the earth's observable universe you'd probably just find more universe. You'd be at the centre of a new observable bubble with the earth and a portion of the earth's observable universe bubble in one corner. As you move anywhere the "observable universe" moves with you. You take a step left and now the observable universe is one step larger to the left (though it's expanding faster than that so you wouldn't see anything new). Make sense?

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There's amble evidence that the Milkyway has collided with another galaxy and currently is in the process of incorporating another galaxy into its structure. The other galaxies are drawf galaxies so we maintained out spiral structure. With Andromeda we will not be a spiral any more.

Andromeda is slightly larger and it and 29 other galaxies (including the Milky Way) are part of the Local Group of galaxies. There may have been many more galaxes but they have since been incorporated through glactice mergers. Andromeda and the Milky Way's orbits around our center of gravity will bring about a merger in the distant future. By that I mean one of these galaxies are not in the stable orbit (on galactic time scale). Even after the merger the combined mass and velocity of the the merged galaxies will put us into a different orbit around our center of gravity.

You know what they say: If Andromeda doesn't come to us, we will come to Andromeda.

Yup. Except that there is no expansion of space between our two galaxies. As miniscule as it is, the gravitational "force" is much stronger than the expansionary forces so that rate is 0.

We are in the Laniakea Supercluster and the space between our Supercluster and the next one, Perseus–Pisces Supercluster, are expanding at 73km/s. Within Laniakea, the gravitational "force" keeps us together in the same orbit. Everything is orbiting something.

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Edit: 73km/s/megaparsec

So you're saying that because our mass isn't fixed, every time it changes we more or less move to a new orbit relative to our common center of mass? Is that basically correct?

Gravity.

For short distances, the pull of the expansion of the universe is weak. The closer the 2 objects are, the weaker the expansion. I like to use a rubberband as an example because even if you can't visualize it, you can easily do it to see. Take a rubber band and cut it so it's not a loop. Place 3 dots on it, one "main" dot, one that's close to the main dot, and one that's far away. Now stretch the rubber band. You'll notice that the near dot doesn't move away from the main dot nearly as much as the far dot. The expansion of the universe works in the exact same way.

Inversely, gravity gets stronger the closer 2 objects are. Like magnets.

Andromeda, for example, is close enough to where the gravitational pull of our 2 galaxies is significantly greater than the expansion of the universe so our 2 galaxies will eventually collide in the distant future. However, anything outside our local group of galaxies is far enough away where the expansion of the universe is significantly stronger so we will NEVER collide with anything out there.

I thought galaxies existed because of supermassive blackholes and the nebulae we think of as "galaxies" are their accretion disks?

Supermassive black holes form because of galaxies, not the reverse.

It has been suggested that supermassive black holes might form from "seed" primordial black holes, which would have existed before galaxies. But even then, it's the galaxy-scale initial density variations that allow galaxies to form around these seeds and grow them to supermassive scales.

Well, now my curiosity is piqued! I think I understand how ordinary black holes form as a result of stellar evolution, but can you point me to some decent resources on supermassive black hole formation? I'd prefer something closer to a book than a web page.

Yes

Don't forget the relative speed as well.

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There's a very nice recent review article about the origins of massive black holes.

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Expansion-wise, sure. But we haven't concretely proven the 'shape' of the universe. Would we end up at the other side? Or would we keep going infinitely far?

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So, are you saying that past a certain magnitude threshold, the force of gravity effectively causes the expansion of space in that region to stop?

I just watched a video about the concept of black hole stars and the theory is honestly fascinating.

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The 3D universe can be thought of as the surface of an expanding hypersphere. If the universe weren’t expanding, you could go all the way around and come back to where you started.

But since it’s expanding, you’ll never be able to move fast enough to come all the way back around again.

A “finite but unbounded universe”

Yes, but in the case that the universe is just the 3D surface of a hypersphere, it would also be expanding, expanding faster than you could move to come back all the way around again.

This is what Carl Sagan meant by “finite but unbounded”

Awesome thanks! What do the spheres represent at the full scale volume?

Furthermore, from the perspective of somebody 93 billion light years away the universe extends equally in all directions, including the direction opposite to that pointing at us. Therefore it must be infinite.

It's not that gravity causes the expansion of space to stop, it's just that on relatively "small" scales we can safely ignore the expansion of space. Gravity still dominates even at the scale of galactic superclusters (for now). But increase the scale by comparing entities billions of light years away, and it's the expansion of space that dominates over gravity.

It's not that gravity causes the expansion of space to stop, it's just that on relatively "small" scales we can safely ignore the expansion of space. Gravity still dominates even at the scale of galactic superclusters (for now). But increase the scale by comparing entities billions of light years away, and it's the expansion of space that dominates over gravity.

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What do you think about the idea that supermassive black holes at the center of galaxies were one the first generation of stars? Additionally, what do you think about black hole stars?

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Supermassive blackholes aren't nearly massive enough to hold together galaxies. If Sagittarius A* disappeared tomorrow, the Milky Way would be pretty much unaffected.

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Just to add some numbers, Sagittarius A* is 10⁶ solar masses while the Milky Way is 10¹², a million times more massive; comparable to the difference between the Earth and the Sun.

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Forces have an effective range. At the sub-atomic range, the Strong and Weak forces act on particles like quarks. Above that is is the electromagnetic force which works at the atomic level to the macroscopic level (normal everyday experiences). After that is the gravitational "force" which works at the normal everyday objects like apples, cars, rockets, and people to galaxies, local clusters, and superclusters. All of the above forces are orders of magnitude stronger than the dark force that caused the expansion of the universe.

So, the space between superclusters is vast and gravity no longer hold sways over the space fabric so it stretches.

One posit is that gravity is not a force but rather a time gradient around mass. The closer you are to the central mass the greater the time curvature so the differential time difference causes you to spiral downward through space instead of an actual field that interacts with a particle (like photon to the electromagnetic field or the Higgs boson with the Higgs field to give us mass).

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It’s not proven to have positive curvature like a balloon, at the largest scales we could measure the universe has no base curvature. It expands in every direction all at once like dots on a balloon but isn’t shaped like it as far as we know

the argument I’d give in return is that it only appears locally flat (local as in the entire observable universe) because the total thing is much larger than 93 billion light years across. Like if your entire observable universe was Kansas, but you didn’t know Kansas was part of a globe.

The margin of error for positive curvature is 0.4% so… within the limits of things that are known and possible.

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Yeah that’s why I said as far as we know we can think up and theorize many a thing but the evidence we have suggests it’s flat

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There’s probably an infinite number of areas of space like what we call “the universe” for trillions of light years in every direction, some expanding, some colliding, some contracting. This is one idea some people have.

In addition to what others have said, "nothing" doesn't have dimension.

I'm not sure actually! They look like they could be indicating the "virial radius" of each dark matter halo, which is a common way of approximating the system's size. As context, the virial radius of the Milky Way's halo is something like 700000 light years in radius, over ten times larger than its galactic disk. So these spheres would be much larger than galaxies, but they would generally contain galaxies at their centers.

The precise definition of the virial radius varies, but a typical definition is that it's the radius inside which the average density is 200 times the cosmological mean. That would mean that each sphere is exactly 200 times denser than the cosmological mean.

The basic idea of the virial radius is that the material inside this radius should be orbiting stably. There's a theoretical reason for the factor of 200 (technically the theory suggests 178, but it's approximate enough that people usually round it), and its derivation uses the idea that stably orbiting material should obey the virial theorem. That's where the name comes from.

This reinforces my belief that there is life beyond Earth. And also dismally displays how small we are to the universe.

The speed of light is fixed. So when we look at light that is emitted from something that is moving away from us very fast it gets red shifted. When it’s moving towards us it gets blue shifted. Same idea if we’re the ones moving and seeing.

Also because light moves at fixed speed the light we see that’s far away was emitted a very long time ago. So we see things as they were when the light was emitted.

We can’t see infra red, but our cameras can. And we can make sensors that can see much lower frequencies than that. However some stuff is so far/long ago away that it gets shifted beyond any ability to measure and becomes cosmic background radiation that essentially acts as noise.

Some things will simply never reach us as long as the universe is expanding, and somethings are so shifted that they cannot be observed meaningfully.

We can infer a lot about the parts we cannot observe because there is a general spherical symmetry to the Big Bang.

But the edge is just the farthest anything from the Big Bang has gone that we can observe . We haven’t seen/understood evidence that there are things that weren’t from the Big Bang yet.

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Superclusters are still expanding, they are just overdense regions that would eventually collapse if there were no dark energy. Assuming dark energy persists, only our Local Group will remain nearby.

Thank you.

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Perhaps I'm looking at it wrong. And these concepts are a big stretch to my ability to comprehend.

Yes thank you. The concept is much clearer now.

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Please say more about dark matter- in its universe, why would that be true?

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Is there anything to suggest why the density wasn't uniform? For example if we consider having particle-antiparticle pairs constantly popping into and out of existence could something like that have been enough perturbation to start things moving?

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Given the sheer SIZE of supermassive black holes and how quickly the formed post the big bang it seems there are great odds of SMBHs coming from some MEGA big stars that collapsed very quickly. Problem is to my knowledge we've never observed a star big enough to collapse INTO a supermassive black hole.

https://phys.org/news/2021-03-massive-stars-early-universe-progenitors.html#:~:text=The%20leading%20theory%20suggests%20the,into%20supermassive%20black%20holes%20today.

We would have to look back far enough to a now VERY distant galaxy to observe such a massive stars collapsing and merging. Hopefully JWST can confirm them!!

Regarding black hole stars: https://www.youtube.com/watch?v=aeWyp2vXxqA&t=609s

I think black hole stars could well be the origin of some of the supermassive black holes if a whole bunch could smash together!

A bit late but I’ve pondered this question before and finally there’s a relevant thread to ask and maybe have an answer.

Does this mean that over time the denser regions become denser (attracting matter around them constantly) to the point where the entirety of the universe becomes a dense point which condenses into an infinitely massive black hole? Which then collapses and causes a big bang event.

What I’m saying for a while Ive been thinking that the big bang isn’t the creation event. There is no specific “creation” event. The universe expands and then shrinks recursively, forever over trillenia. When it shrinks all life is erased and then life restarts once big bang occurs and galaxies reform.

This is a very interesting relevant article:

https://www.scirp.org/journal/paperinformation.aspx?paperid=80777

Do we know that the early universe wasn’t uniformly dense because of, like, mathematical laws, or is it an inference we make from the fact that it is currently not uniformly dense and therefore must not have started that way?

In short, no, this has been proposed before.

The current rate of expansion of the universe would mean that the universe keeps expanding forever. There is not enough gravity/mass in the observable universe to make it collapse into itself.

Also, dark energy is increasing the expansion rate. The most likely scenario is called Heat Death.

What you described is normally referred as Big Crunch.

We don't know, but the most popular hypothesis is that the density variations originated as quantum fluctuations during inflation (the hypothesized early period of accelerated expansion). They would begin around the Planck scale but rapidly expand due to inflation. This process creates fluctuations over a huge range of scales, as fluctuations created earlier grow larger than later ones, and that matches what we observe.

That's a yes an no answer mostly because the space between the local clusters are too large. There's a region around the supercluster where objects are gravitationally bound to the center of gravity and outside of that radius the local clusters will eventually escape. Much like the orbital mechanic of our solar system. So from that region outward the dark force appears to dominate and expand the space.

Edit: With our current understanding of the universe, within the Supercluster the dark force responsible for the expansion of the universe is too weak to overcome the gravitational "force" within the bounded section of the supercluster.

We can see the initial density fluctuations as temperature fluctuations in the cosmic microwave background (CMB). Almost all of the CMB was causally disconnected at its emission time, as the horizon scale at the time is around 1 degree on the sky. We see temperature variations larger than that, and since they are not causally connected, we know that they must have been frozen in time since whatever process created them in the much earlier universe. (Likely inflation, as I noted in another comment.)

Also, gravity can only amplify already existing density variations. Thus the smaller-scale (causally connected) CMB temperature variations, and the density variations in the universe today (responsible for galaxies and larger-scale structure), must have originated from similar initial density variations. In fact we understand quite well (mathematically) how density variations gravitationally amplify over time, and a wide range of observations generally all point to initial density variations having essentially the same average amplitude at every scale (the one part in 10-100 thousand that I mentioned).

If a cluster of galaxies is virialized (its constituents are orbiting stably), we call it a cluster, not a supercluster. Superclusters are expanding with the Hubble flow by definition. A supercluster could certainly have a virialized cluster at its center though!

Dark energy (which I discussed) and dark matter (which I didn't) are completely different things.

A universe with only dark energy (or where everything else is negligible) expands exponentially, i.e. if you follow the distance between two objects over time then this distance increases exponentially. It has a constant expansion rate. If you emit light at a distance where the distance increases at the speed of light then the light will always keep that distance - the expansion perfectly matches the speed of the light, and the expansion rate doesn't change so the light will never come closer.

In our universe, where matter still plays a role (~3/4 dark energy, 1/4 matter today), the expansion rate is decreasing a bit. Light emitted at the same distance of "light speed distance increase" doesn't get closer to us today, but it will start getting closer "tomorrow" (will take hundreds of millions of years before this is significant, of course).

>the entirety of the universe becomes a dense point which condenses into an infinitely massive black hole? Which then collapses and causes a big bang event.

While theoretically possible, we don't see enough observable evidence to support this.

Additionally, think of this. At what specific point of size/mass would a black hole actually "explode" into another Big Bang? As far as we know, each black hole that currently exists at any size or mass is already infinitely dense. Even if you combined all the matter in the Milky Way Galaxy into one black hole, it would still be an infinitely dense black hole, it can't get any more dense than it already is. It *would* become more massive and have a larger event horizon.

But there's nothing fundamentally different about a solar-mass size black hole and a galactic core black hole, aside from just being much much more massive. Again, as far as I'm aware.

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>now is not quite everything but currently expanding size.

Well, the space between stuff is what is expanding/being created constantly,the galaxies are stationary (kinda, not really.)

>Seriously, could we go to the edge of the universe then stick a meter stick a little bit further.

No.

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Forget about accretion disks, that's a local phenomenon.You're right in the sense that galaxies exist because the Black hole at the center does not become massive enough to have all the matter. In the same sense that earth exists because of the sun because it the sun is not massive enough to have subsumed the earth. Also, nebulae are gas features within galaxies, but entire galaxies we're once called nebula before we realized they're just far away galaxies.

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Fantastic, thank you!

The ant on a rubberband example does not work for the universe. Even if the Hubble constant was constant with time, the universe expands exponentially, not linearly (so the “universe” rubberband length goes 1,2,4,8; not 1,2,3,4). An ant on an exponentially growing rubberband cannot reach everywhere.

For the past 10 billion years a linear expansion was a pretty decent approximation. The early universe slowed its expansion, which makes the relative reach of the ant even larger (or, equivalently, the early recession speeds were larger).

The Hubble rate is still decreasing. It's expected to approach a constant in the future. I covered that in the second paragraph:

> In the distant future, in a universe completely dominated by dark energy, your statement will be right.

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