Evidence that the Universe is Expanding?

  • Thread starter zketrouble
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  • #1
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Hi all,

First let me say that I'm not an astrophysicist but that I have some understanding of classical physics. So it has been said that the universe is expanding, but what evidence is there of this other than the fact that things seem to be drifting apart? Since much of the universe is unobservable to us due to the rate which light from foreign objects travel to us, how can it be so asserted that the universe is expanding and that it is finite rather than infinite? If something is getting bigger it must be finite. Couldn't things just be drifting away from each other in empty space? Sure gravity would probably cause the opposite, but only if two bodies were large/close enough. Seems like just as good of an explanation as saying that empty space, nothingness, is being created from nothing.

Of course the second law of thermodynamics states that entropy of the universe is always increasing. In this case, what would happen in so many billion years when the universe reaches maximum entropy? Well if the universe and matter were infinite, maximum entropy would never be achieved. In a finite universe, more space would increase the number of states in which matter could exist.

How theoretical is the statement that the universe is getting expanding? Just curious.
 

Answers and Replies

  • #2
Nabeshin
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So it has been said that the universe is expanding, but what evidence is there of this other than the fact that things seem to be drifting apart?
Well, there's a lot of evidence for this. For one, everything is moving uniformly in one direction: away from us! Now, under your point of view, you could simply assert that we are, in some sense, a privileged point in the universe and everything is moving away from us because we smell bad. But we have something called the cosmological principle that is an extension of the Copernican principle. Basically it says that we are in no special place in the universe.

Furthermore, it's not just that everything is moving away, but everything is moving away with speed linearly proportional to its distance. This is very difficult to explain if everything is simply "moving away", but yet falls right out of the mathematics of an expanding universe.

Since much of the universe is unobservable to us due to the rate which light from foreign objects travel to us, how can it be so asserted that the universe is expanding and that it is finite rather than infinite?
Saying the universe is expanding is not linked to the question of whether or not the universe is finite.

If something is getting bigger it must be finite.
This is a very common misconception. When we think of a balloon expanding, or some other such analogy, we see the balloon as a finite object expanding into 3-dimensional space. Unfortunately for our human minds, this need not be the only case. Imagine the cartesian plane, (x,y). Now, this is clearly an infinite plane as you can choose arbitrarily large values for x,y, or both. Now imagine that there are little dots on the plane at every integer intersection, i.e (1,1), (2,2), (-3,2), (-1,-1), etc, so that they form a grid. Now double the distance between any two adjacent dots. Someone sitting on one of the dots would certainly claim that his universe has expanded! His nearest neighbors are now 2 units away, and those which used to be 2 units away are now 4 units away! But yet, we started with an infinite object and we end with an infinite object.

It comes down to a misunderstanding of limits and infinity. A good calculus course should usually clear this up.

How theoretical is the statement that the universe is getting expanding? Just curious.

No more theoretical than Newton's law of gravity. It is very, very well established.
 
  • #3
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Thanks!
 
  • #4
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Am I wrong, or is the only evidence of an expanding universe the red shift---????
r rosenthal
 
  • #5
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Am I wrong, or is the only evidence of an expanding universe the red shift---????
r rosenthal

Dark Energy though not an evidence,yet.
 
  • #6
Chronos
Science Advisor
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One independent confirmation of redshift distance is supernovae Ia luminosity. This was also used to detect expansion of the universe.
 

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