Viewing a single comment thread. View all comments

Aseyhe t1_j2miy9u wrote

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.

345

Obvious-Display-6139 t1_j2p960y wrote

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

12

Aseyhe t1_j2q1xbv wrote

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.

22

BrobdingnagLilliput t1_j2oqs4d wrote

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

3

Aseyhe t1_j2ot4uq wrote

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.

49

BrobdingnagLilliput t1_j2ovrhh wrote

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.

12

StandardSudden1283 t1_j2pbegv wrote

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?

4

deja_entend_u t1_j2qkfok wrote

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!

4

choicemeats t1_j2p4srj wrote

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

2

charlesfire t1_j2pcutp wrote

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

11

purpleoctopuppy t1_j2ppxg6 wrote

Just to add some numbers, Sagittarius A* is 10ā¶ solar masses while the Milky Way is 10Ā¹Ā², a million times more massive; comparable to the difference between the Earth and the Sun.

11

Nayir1 t1_j2w3wrm wrote

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.

1

redpandaeater t1_j2qj2gk wrote

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?

3

Aseyhe t1_j2r0e5t wrote

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.

6

greenwavelengths t1_j2qp83c wrote

Do we know that the early universe wasnā€™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?

3

Aseyhe t1_j2r1bdv wrote

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).

5

arncore t1_j2ql8ob wrote

A bit late but Iā€™ve pondered this question before and finally thereā€™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 Iā€™m saying for a while Ive been thinking that the big bang isnā€™t the creation event. There is no specific ā€œcreationā€ 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

2

iwaslegit t1_j2qq5q3 wrote

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.

7

enderjaca t1_j2rqeg2 wrote

>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.

2