The short simple version is that if galaxies happen to be close enough to each other they fall toward each other faster than the space between them expands. That's what "gravitationally bound" means.
this post was submitted on 18 May 2024
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There are other galaxy clusters. Gravitational binding is not unique to the local cluster. From Wikipedia,
Notable galaxy clusters in the relatively nearby Universe include the Virgo Cluster, Fornax Cluster, Hercules Cluster, and the Coma Cluster.
The expansion of the universe is very tricky to explain. Oversimplifying can lead to an explanation that seems to be contradictory.
Gravitational pull in local clusters of galaxies can exist and even contradict larger clusters or the universe as a whole, just in a smaller scale. The passage of time is also localised due to gravitational intensity, too. We know the universe is not expanding at the same rate in different places, as well.
This can happen in sets of complex systems of systems. In your query it somewhat analogous to if you were throwing a ball northbound at 100km/h in a train moving southbound at 50km/h. On a planet circling its star at 107,800 km/h in a total different direction. While the star drags the earth along with other planets in its own orbit in a different direction.
FYI might want to rethink your choice of punctuation for your edit: https://en.m.wikipedia.org/wiki/Triple_parentheses
Dont we see other galaxy groups though? Im no astronomer, but I do recall the universe having some degree of structure above the scale of individual galaxies, with groups and clusters of them forming larger groups or filaments surrounding voids of space with fewer galaxies in them.
Edit: quick search in wikipedia brings up a list of a few groups and clusters known, of which the local group is merely one: https://en.wikipedia.org/wiki/List_of_galaxy_groups_and_clusters
You are so correct, I'm sorry for not checking myself. I'm not surprised I had errors, just that it was so quick and large.
So I guess the expanding universe is galaxy CLUSTERS moving away from each other? I don't feel like it's explained that way.
I imagine any explanation of the expansion of the universe for people that are not themselves studying astronomy is going to be simplified in a way that gives the average person the basic idea but not the complete picture to avoid confusion when explaining the concept. Ive not studied astronomy, but I did get most of the way through a physics degree, and know that at least there, a lot ideas are explained in that sort of way to people without much knowledge of the subject, especially the more confusing concepts. I wouldnt be surprised if thats the case for most fields of science. For a different example as an analogy, its common knowledge that you cant move faster than light (ignoring the whole expanding spacetime stuff), but it isnt always explained why this is the case, leading to questions from some people like "what happens if I fly a spaceship to the speed of light, and then turn on the rockets to try to go faster?" which have easy answers or just dont make sense as a question if one has had the behavior of objects at high speed explained, but which seem reasonable enough questions to ask if all youve been told is that the speed of light is just some cosmic speed limit. People cant reasonably blame you for finding an incomplete explanation you've been given, well, incomplete, and then asking questions that come to mind as a result.
Well now I want to know what does happen if I go the speed of light and then turn on the rockets to go faster! ππΈπ³οΈβοΈπ π₯
If you're actually curious, or someone else reading this is, you never can get a rocket, or anything with mass, to the speed of light either, not just faster than it, but you can get arbitrarily close. However, you never notice anything stopping you going faster than your current speed, there's no point where your rockets stop working or anything, rather, time and space stretch and squeeze such that neither you nor anybody else see you going faster than light. If you have a magic rocket that somehow has infinite fuel and can fire forever, you can actually get anywhere as fast as you want, from your perspective.
Alpha centauri is famously about 4 light years away, but you can get there in 2 seconds, from your perspective, if you go fast enough. But, everyone on earth will see slightly over that roughly 4 years go by in the time that for you is just 2 seconds. (You'll see them move slowly too at first, since they're moving relative to you just as fast as your ship is moving to them, but when you slow down, you'll see them seem to speed up until you'll have seen them do 4 years worth of stuff by the time you stop). Meanwhile on your ship, you don't see yourself crossing that 4 light year distance in less than the allowed time either, because space itself is squished kinda, so that the distance to alpha centauri is shortened to the point that if you're getting there in 2 seconds, it's now less than 2 light seconds away, from your perspective, and you're not moving faster than light to cross that distance in that time. People outside will also see your ship compressed like this too.
This isn't just a regular optical illusion either, space and time really are different for the people on and off the ship (and indeed very slightly different for everyone anywhere). Nobody has the "correct" view of the universe, because everyone's perspective is equally valid.
Ah yes, I seem to remember a few things now, which helped me to understand this before:
- You have a fixed energy output (not sure what to call it, but let's go with that, "energy output"). Either you output all of your energy in the physical dimensions, or in the time dimension (I can't think of anything that does this though), or some in both. Most things output some energy in both, time and the physical, like us for example!
- Light is basically all of its energy in the physical dimension and none in the time dimension. Thus a photon doesn't really experience time at all. To it, it feels like absolutely zero time has passed at all between traveling from one end of the universe until it is absorbed by an object at the other end.
- To attain the speed of light takes infinite energy, as it takes more and more energy to accelerate mass the faster you get to c, in a way that you never reach c.
These concepts are really helpful in my opinion, to understanding the fact that you can't reach the speed of light, as well as the whole time dilation concept.
I hope it helps someone else.
(Do note I'm not an astrophysicist, so this may be a bit wrong, but I think the main part of it is right.) Not exactly. Everything in the universe is constantly drifting away from everything else. The reason it is pretty much only visible at the scale of galactic clusters is that literally every force in the universe overpowers this expansion, unless the distances between the objects are truly absurd, in the range of millions or billions of light years.
This! A nice additional detail is that the expansion is accellerated, so there will be some interesting things happening when the relative strength of the fundamental forces start competing with the expansion.
Here, found a Kurzgesagt video for you
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