If all the galaxies in the Universe are moving away from each other why are some galaxies crashing into other galaxies?
It is not true that “all” galaxies are moving away from each other.
It is only at the largest scales (perhaps a billion light years) that the expansion of the universe overwhelms the gravitational interaction between galaxies.
In the same way, when astronomers say that space is isotropic (mostly the same in all directions) and homogeneous (matter and galaxies distributed evenly), that is only true once you look at a large enough swath of the universe.
Galaxies cluster and inside a cluster, they do not move away from each other. And clusters form super-clusters. I don’t know if it is known if there is clustering at all scales. The Milky Way, Andromeda, Magellanic Clouds, the Triangulum and some others form the cluster known as the Local Group.
Certain galaxies are affected by other bigger galaxies and veer into another galaxy. Some are slowed down somewhat and get run over by another galaxy behind it. Other galaxies are constantly on their cell phones and arent not paying attention to where theyre going…
Basically, every galaxy is moving away from the point of the Big Bang. Interactions with gravitational forces causes clustering and eventually galactic fender benders.
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There is no point that all galaxies are moving away from which could be said to be the point of the Big Bang. The point that was the big bang expanded to become all of the points in the universe, so that no point in the universe can be differentiated from any other point, relative to their historical origin.
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So generally, all galaxies are moving away from all other galaxies with speed proportional to the distance between them – the farther apart two galaxies are, the faster they are likely to moving away from each other. Clearly, two galaxies that are very close to each other are not likely to be moving away from each other very fast, in fact it is reasonably likely that they could be moving towards each other.
In fact, it is an absolutely certainly. Our own Milky Way galaxy and the Andromeda galaxy will crash into one another in a couple billion years.
There are also many pictures of colliding galaxies in the astronomical records.
I asked a similar question here.
There is not much consensus on this Hubble expansion (My feeling). If the ‘bodies’ do move proportional to distance from the observer, then at a certain point they have to exceed the speed of light and thus leave the universe.
So the mass of the universe decreases and so does the energy and entropy! How do you explain that ?
andy_fl, the bodies don’t actually move, the space between them just gets bigger. Nothing can leave the universe, remember it has no boundaries (ok you could argue a few exceptions but their irrelevant to Hubble expansion). I believe that something ‘on the edge of the universe’ (that’s from our point of view as in truth all points inthe universe are pretty arbitary) would appear to travel pretty close to the speed of light but not above it.
The universes mass does not decrease, infact it increases as new matter is formed in the expanding gaps between the stars.
Here’s a neat animation showing galaxies interacting to form our own, and ejecting some along the way. This only happens when the galaxies are close enough together in the first place.
This is the Hoyle steady-state model, and it’s been absolutely, totally discredited. No cosmologist (except for Hoyle, if he’s still alive) takes it seriously, as it has failed every test comparing it to the standard Big Bang model.
As for the rest, matter can and does leave the observable universe at the distance where recession is the speed of light, but there’s a difference between the observable universe and the Universe. All of the conservation laws remain intact. There is, in fact, a nearly unanimous consensus (among cosmologists, at least) on the Hubble expansion. There are still some details which are in debate, but the presence of the expansion is udenied.
Fred Hoyle (1915-2001) is resting peacefully in the arms of his heavenly father. Or possibly at another location.
What is the difference between observable universe and the universe ?? As I understand relatavity, if anything exceeds the speed of light, we can no way know anything about it and that means that the mass, energy, everything is gone! What is wrong here ?
Just like you cannot say that the universe existed before the big bang, similarly you cannot say that “objects” exceeding the speed of light exist in this universe. Because, you cannot measure any property of it - it does’nt exist.
In relation to matter leaving or not leaving the universe:
Does light have mass?
What happens to the light emitted from stars at the very very edge of the universe?
Does it emit past the boundaries of the measurable universe? If so then what happens to that mass?
andy_fl, no “thing” exceeds the speed of light. Space itself can grow faster than the speed of light without any violation of relativity.
Imagine that you are on an island watching a ship sail off into the distance. When that ship disappears over the horizon it doesn’t cease to exist. It is simply no longer observable. Similarly, although we can observe only a tiny fraction of the entire universe (assuming inflation), what we call the universe still includes the portions that are not observable.
You are correct though that we have no way to measure anything beyond the observable universe.
Okay rsa, I’m more confused now. I thought in science something exists only if you can measure it or at least have some kind of effect from it. If not it is not science, just like I have my belief that the soul exists after death - but it is not a scientific belief.
So can you please explain as to how this unobservable universe exists ?
And it is happening now. The dwarf galaxy in Sagitttarius is colliding with the Milky Way as we speak.
Actually, I wasn’t referring to the Hoyle’s steady state model. It 's along time since I studied this but I believed the consensus among physicists was that some matter is created in the gaps between the stars (not leading to a steady state though).
And also I was lead to believe that the outer reaches of the universe do not appear to be travelling faster than the speed of light as the expanison is at (roughly) the speed of light.
Here is my I-am-not-an-astrophysicist comment.
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I think the statement that “space” can expand/move at C+ rate (ie, “faster than the speed of light”) is an error. Any measurement that one would interpret as indicating that the space between two “determined points” (usually: two material objects, or rays of whatever) was expanding at C+ is disallowed by general(?) relativity, as it is equivalent to one “marker” departing from the other beyond the speed limit.
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I believe this applies to any and all forms of detection. For example, the metric of space, signified by the path of a ray of light, cannot warp over time in such a manner that the rate of warpage is C+.
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I suppose it would be in keeping with the Heisenbergian spirit to say that space can move as fast as it wants PROVIDED that its C+ rates can never be detected. So if objects of ever-increasing distance appear to be moving faster and faster, showing a smooth graph approaching C, and then vanish AT C–does that kind of logical deduction (as to what happens thereafter) constitute “detection” or not? I would argue in the affirmative.
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But now consider that we’re also looking back in time. Isn’t this supposed “vanishing” occurring very close to the time of the Big Bang? Can the “vanishing” be understood as the fact that a universe of material objects did not exist prior to its first moment?
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Hoyle’s “continuous creation” (and some variants thereupon) are not considered part of the standard cosmological model. The weight of evidence is pretty one-sided. But as to EXACTLY what the Big Bang was, or how it worked, is still very much under discussion. To venture well beyond what I am qualified to say: there is still a question as to whether, or how much, the “banging” was a result of intra-nuclear forces that we still recognize today, acting under conditions of absolutely ultimate density/compression-- OR whether now-extinct (or now-undetectible) kinds of forces played a role-- OR how much of what we are seeing has to do with expansion/warpage of the spacetime metric itself.
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But now let me ask a relevant question. Every time somebody asks “If the universe is expanding, where is the point it’s expanding away from?” we get that old analogy of the dots on the surface of a balloon as its being blown-up. I guess the more formal way to say this is the business above about one point expanding to fill all space, etc. Taking the balloon analogy seriously, does this mean that the direction of primary expansion (equivalent to the radius of the balloon) is in the fourth dimension? The fourth dimension, as in TIME? Or a fourth spatial dimension? Or does the theory deny that there IS a “direction of primary expansion”–that the third-dimensional “no-center” situation prevails all the way up?
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And is it really the case that Hubble’s Constant does not apply to astronomically small spaces? OR is it more that we don’t know, because local gravitational effects drown the relevant data?
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Which leads to: Empty space is not all that empty, and filled space is not all that filled. Solid matter contains plenty of “space” (with various forces in it, just as gravitation fills the spaces between galaxies). Indeed, if “space” is simply the metric itself, “space” is absolutely everywhere–all-pervasive. So are “solid objects” really expanding at some undetectible rate?
Well, for most working astronomers it might as well not exist. But cosmology theorists don’t restrict themselves to mundane matters such as the ability to measure something. 
In the classical model of the universe, it didn’t matter very much because it was thought that the observable universe may have made up as much as 75% of the entire universe. But is was because of problems with the classical theory explaining certain things about the observable universe when the theorists started getting more creative. As a result of these new theories calculations could be made which gave a much larger estimate for the size of the universe. In one version of the chaotic inflation theory, the size of the universe worked out to 10[sup]10[sup]12[/sup][/sup]cm which would be written out as a one followed by a trillion zeros!
We would obviously have no way of directly observing such a unimaginably huge universe, but these various new theories can still be tested against each other by how well they explain what we know about the origin and evolution of the universe. Don’t get me wrong though, none of these theories will ever be proved to be the “correct”.
Since our observable universe might be such a tiny part of the whole, it may seem presumptuous to assume that the rest of the universe is anything like our little piece. But a common assumption in cosmology is that there is no reason to think that we occupy any special place in space or time. So the conclusion is that it is most likely that our observable universe is like the parts that we can’t observe.
If you still think that much of this smacks of speculation and is more akin to “belief”, you have plenty of company. Even one of the leading inflation theorists, Andrei Linde, had this to say:
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Yes, light has relativistic mass (but must have zero rest mass for quantum physics to work)
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what do you mean by ‘measurable’ universe? mass cannot leave the universe as it has no boundaries.