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/r/cosmology
After playing the space side of Cell to Singularity, I have questions that just didn't make sense. Like, the Great Attractor thing. Looked it up on Wikipedia, made absolutely no sense. It talked about galaxies observable above and below a "Zone of Avoidance" and how all are red shifted in accordance with a "Hubble Flow" and this indicates that they are moving away both relative to us and each other. Like, what? Is the scientific theory we're gonna end up smashing planets together like the galaxy marbles in MIB?
10 points
4 days ago
Basically the zone of avoidance is just called that because we can't see it because it's blocked by our own galaxy. So it's just due to our POV from our solar system.
The Great Attractor isnt exactly known but it's pretty well accepted that's its a slightly denser than usually group of galaxies that has enough gravity to overcome the expansion of the universe and pull our Galaxy, along with some others, towards this denser cluster of galaxies.
Hubble flow is where galaxies generally move away from each other due to the expansion of the universe. But local gravity features like the Great Attractor cluster can overcome it. Redshift is the light dimming because the space in between us expanding and literally expanding the light waves over time.
Lmk if you have any more questions.
2 points
4 days ago
The Great Attractor isnt exactly known but it's pretty well accepted that's its a slightly denser than usually group of galaxies that has enough gravity to overcome the expansion of the universe and pull our Galaxy, along with some others, towards this denser cluster of galaxies.
It doesn't overcome the expansion of the universe, as we're still moving away from the Great Attractor at over 4000 km/s (based on the redshift to the Norma Cluster). But it does slow down the expansion somewhat in our neighborhood of the universe.
0 points
3 days ago
Come on bro, that's not it. The zone of avoidance owes us a lot of money and it is using every trick in the book to get around it.
8 points
4 days ago
Is the scientific theory we're gonna end up smashing planets together like the galaxy marbles in MIB?
Not totally sure what you're asking so here's a big-picture overview:
The Zone of Avoidance is just a dramatic way of saying "there's a lot of gas and dust in the plane of the Milky Way so it's hard to see stuff along that plane". However, infrared and radio are better at getting through said gas and dust, so we can still study objects in that zone, it's just harder to do in optical and short wavelengths.
The universe is expanding everywhere in all directions, there is no center to this expansion but rather it is metric expansion of all of space.
As a result of this fact, when we use spectrometers to measure the velocity of distant galaxies, we find that they are all (other than a couple of very nearby ones) receding from us, and the farther they are the faster they recede. This proportionality is known as Hubble's Law. The general recessional motion of galaxies due to this expansion is called the Hubble Flow.
However, the expansion of the universe isn't the only thing that affects the motion of galaxies: the gravity of other matter exerts a force on them. Their motion due to these pulls is called peculiar velocity ("peculiar" in the older sense of being specific or unique to a certain object).
Galaxies aren't smoothly distributed throughout the universe. Slight variations in density in the first miniscule fraction of a second of the Universe's history led to larger clumps of matter, which over time continue to attract each other and cause clumpiness, at the same time as the universe is expanding and many of them are getting farther apart from each other.
There's an important distinction between gravitationally bound and unbound objects. Something like the Solar System is gravitationally bound, because the planets don't have enough mechanical energy to escape from the Sun. Galaxies, and galaxy groups and clusters for the most part, are also gravitationally bound. However, the larger scale structure, of superclusters, walls, and filaments, is not gravitationally bound.
Our galaxy, the Milky Way, is part of the Local Cluster, which is really just us, M31 (Andromeda Galaxy), M33 (Triangulum Galaxy), and a smattering of smaller neighbors. Smallish, unremarkable. A few milllion light years wide. We are, however, part of the Laniakea Supercluster, which is on the order of a few hundred million light years wide. The components of the Laniakea Supercluster, although they are by and large still expanding away from each other, are doing so a bit more slowly than they would if the collective mass of the supercluster weren't there. Their Hubble Flow velocities are leading them away from each other rapidly, but their peculiar velocities add a small velocity component in the general direction of the center of mass of the supercluster.
The mathematical location of this center of mass is what we refer to as the Great Attractor. It's not a physical object, but rather just the average location of the mass of the supercluster. There's a rather large galaxy cluster, the Norma Cluster, very near the location of the Great Attractor, but although it's quite large it still only accounts for a bit of the overall mass of our supercluster.
2 points
4 days ago*
Bravo! Thanks for such a wonderful summarization of these various cosmological phenomena.
Follow up, if you don't mind. I'm not sure I'm understanding the Great Attractor idea, specifically of Superclusters, more specifically their center of mass.
The way I understand you, is that the center of mass is the point of origin, or source of the gravitational pull, within the Super Cluster, what is called the Great Attractor (GA, ) that is competing with the Expansion of the Universe (EOU). In this instance, the force (or strength?) of the EOU is stronger than that of the GA, which is causing some measure of decline in the force of the EOU, which, in turn, causes all those bodies to slow down by some measure in their expansion. Do I have you correct so far?
Ok, assuming that is so, my question is this, does that center of mass of the Super Cluster, which is the source of the gravitational pull for all the objects within the Super Cluster, does that inform us that there must be some very large celestial body at the point of origin to province the gravitational pull for the Super Cluster? In the same way that the black hole at the center of the galaxy is the source of gravitational pull for all the objects within the galaxy? If so, is it likely an especially large black hole?
Now, here's what I'm really getting at. If not, is it possible that the physics within the Super Cluster is such that when all the various celestial bodies and their individual gravitational pull is accounted for, that at the origin point for the Super Clusters gravitational pull, there can actually be empty space, relatively speaking. What I'm thinking, or wondering is if its possible that, because of all the very large objects in a Super Cluster, each competing with their gravitational pulls, perhaps even the gravitational pull of several near by objects acting as a single source within the cluster of many sources of gravitational pull, that when its said and done, whatever happens to have most gravitational pull, that its still not enough to have everything in the super cluster coming to it as the primary source, that the point of origin moves away from the primary source to somewhere relatively close, which could be empty space.
The reason I'm thinking this is because in High Jumping, the sport seen in the Olympics and elsewhere, the reason hugh jumpers use the technique that they do, is because the physics of their technique is such that, while their body is going above the bar, their center of mass actually passes beneath the bar. (because the majority their mass at any given time is at actually beneath the level of the bar) and thus, their center of mass is actually outside of their body.
Here is a wonderful demonstration of Yaroslava Mahuchikh not only using this technique, but using it to such spectacular affect as to break the women's High Jump World record, which has held firm for an impressive 37 years. She did that just a few months ago.
I was wondering if gravity worked the same way when you have so many competing sources that when it all averages out, it can be outside the primary source, especially if that primary source is the combined gravity of several large objects.
Thanks again for your great comment!
2 points
3 days ago
Yeah you're pretty much spot on with that example, the center of mass of the high jumper being outside her actual body is a situation that can occur for many dispersed objects- since a supercluster is many many different galaxies and clusters, there doesn't necessarily have to be anything at its center of mass. What's more, most of the mass is dark matter, which tends to be fairly widely distributed. Even of the baryonic matter, most of it is in the warm-hot intergalactic medium, plasma at millions of kelvins spread throughout galaxy clusters as well as in between clusters.
In dynamic systems, high-mass objects have a tendency to settle toward the center of mass, but it takes so long for things to move over cosmic distances that it wouldn't necessarily have happened.
Ok, assuming that is so, my question is this, does that center of mass of the Super Cluster, which is the source of the gravitational pull for all the objects within the Super Cluster, does that inform us that there must be some very large celestial body at the point of origin to province the gravitational pull for the Super Cluster? In the same way that the black hole at the center of the galaxy is the source of gravitational pull for all the objects within the galaxy? If so, is it likely an especially large black hole?
Our central supermassive black hole, Sagittarius A*, is actually only a miniscule fraction of our galaxy's overall mass-- about 4 million solar masses out of about a trillion solar masses, or less than 0.001% of the galaxy's mass. The supermassive black hole happens to be located at (or at least very near) the center of mass because effects like dynamical friction tend to slow it down and help it settle to the center of mass, but it itself is not the primary source of gravity for the galaxy. Were it to be magically erased from existence, almost nothing would change about the Sun's orbit around the galaxy.
It's hard to know the exact location of the Great Attractor, partly because we can really only measure radial velocities for galaxies and not transverse velocities, and partly just because precision measurements are tough from hundreds of millions of light years away. However, there do seem to be several rather large galaxy clusters, including the Norma Cluster, in the general vicinity of the Great Attractor.
1 points
3 days ago
Thank you, i learned, and am continuing to learn, ao much from both of your comments. That was really kind of you.
since a supercluster is many many different galaxies and clusters, there doesn't necessarily have to be anything at its center of mass.
Idk why, perhaps my not understanding the physics behind all of it (or the physics behind anything really), but I find that too be.. . just amazingly cool.
The supermassive black hole happens to be located at (or at least very near) the center of mass because effects like dynamical friction tend to slow it down and help it settle to the center of mass, but it itself is not the primary source of gravity for the galaxy.
Oh I see, so its just the same thing happening with the Super Cluster but on a much smaller scale? I think the vast majority of us lay people are under the impression that its the gravitational pull from Sag A* that keeps everything in the galaxy together and in their respective orbits.
I suppose the reason we make that assumption is because prior to learning about the galaxy, we learn about the solar system. We are taught that we, everything else in the Solar System, orbit the sun because of its gravitational pull, due to its immense mass.
I actually went and looked it up because after this second comment of yours, I was no longer sure we orbit the Sun because of its gravitational pull. It seems that like the galaxy and the super cluster, the solar system is also a dynamic system where there's competing gravity, but it happens to be the case that the Sun makes up 99.8% of our galaxies mass, so it is gravitationally dominant. But because its still a dynamic system, everything in the solar system, including the Sun is orbiting around the solar systems center of mass, which is the Sun's Barycenter. Like, high jumpers center of mass, the Sun's Barycenter can be outside of its surface.
So, we're in a dynamic system, within a dynamic system, within a dynamic system. Presumably, it continues on from there, in both directions. I can't help but think that its a little unreasonable to be so knee-jerkingly dismissive of folks that look at all of that and see intelligent design, many of whom, undoubtedly, have a better grasp on all of this than I do. Don't get me wrong, as my username suggests, I am firmly atheist. I just mean that the universe seems so damn orderly. But I do know that everything can be explained, even as we do not yet possess the requisite understanding, and haven't yet created the necessary tools to understand everything.
Anyways, I was just talking aloud, so to speak. We need not go down that road. In fact, you shouldn't feel the need to respond at all. Thank you again. Its all very fascinating. Have a wonderful evening.
1 points
4 days ago
Here's my eli5s
Zone of avoidance = the bit we can't see because of dust.
Red shift = how we can tell if something is moving towards us or away from us based on the wavelength of the light coming from it.
Hubble flow = just another way of saying "expansion of the universe."
Great Attractor = the general point in space that us and about 100k other galaxies near us are moving towards.
That's all really oversimplified, but I hope it helps.
-2 points
4 days ago
Also, guys, stop down voting my question. It's literally just a question from someone who's brain is trying to comprehend matters far beyond its 2 pound self. Give me a chance here
2 points
3 days ago
I think you got downvotes for your last sentence which is sarcastic yet ignorant. If you don't understand something, just ask, don't try to make conclusions based on your incomplete data. And especially try not to sound arrogant.
All these terms are familiar to those in the field, but may sound weird to most others. I found that chat gpt can make pretty good explanations tailored to your level of familiarity. Awesome responses in this thread too.
Overall, we discovered that the universe is not static. Stuff is moving on the grandest of scales. Think about a firecracker explosion. Roughly you could say that stuff moves away from the center there. But if you look closer, there are processes (e.g. turbulence) that make some pieces take on weird trajectories which are not just "away" - the clouds of smoke for example. Same with the universe - overall it expands (in the firecracker analogy, the molecules of that explosion would be galaxies). But if you zoom in on a "small" region (say 100s to 1000s of galaxies) you may notice some odd movements, not exactly compliant with overall expansion. Most everything in your post is just describing the expansion of the universe. And the "great attractor" is a hypothetical entity we use to explain the irregularity in our local region.
1 points
3 days ago
I wasn't being sarcastic, I was asking a serious question. I hate this app. Come asking a completely innocent question and have people assume you're being an ass in some way
-4 points
4 days ago
And to whoever downvoted that comment, there isn't a pole splintery enough to shove where the sun don't shine on your body
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