Is the Universe Expanding?

[budcamp]

If the universe quit expanding and started to contract, how whould we know about it?

[timeshifter]

We wouldn't. At least, not until it was a few years from crushing us.

[PhysBrain]

We know the universe is expanding because in every direction we look, the light from distant galaxies is red-shifted, indicating that they are moving away from us. The rate at which each galaxy is moving away increases the further away it is. From this motion, it is concluded that just about every galaxy is moving away from every other galaxy in a manner that is consistent with the space between the galaxies expanding uniformly at all points in the universe.

So, one way we would know if the universe slowed its expansion would be if astronomers began to notice a decrease in the red-shift of light from distant galaxies. This would indicate that they are no longer moving away from each other as rapidly as before. However, since this effect is only noticable accross galactic scale distances, we wouldn't actually observe it in this way until several million years after it had begun.

Alternatively, if the universe is truly expanding at every point in space, then a very sensitive measurement of the speed of light could possibly indicate whether or not the rate of the expansion of the universe is changing a significant amount. That is to say if the amount that space expands in the time it takes light to go from an emitter to a detector changes, then the length that the light has to travel will also change, and hence the speed of light will increase if the expansion of the universe begins to decrease.

Please note that this last experiment is one that I just made up and I am not certain if such a phenomenon would be observed or even observable on length scales that we are able to measure.

[Skulled]

That is a very interesting question but unfortunately the answer is not as simple.

The universe is expanding as we know from the doppler shift and on a more mathematical level, by the cosmological constant.

So we can either figure out if the universe started to contract from these two quantities.

And as for the experiment offered by PhysBrain would not work i believe. As if you take the argument of the speed of light increasing when the amount of space the light has to travel decreases, then the opposite effect should happen now. Since the amount of space is expanding the speed of light must decrease, but this would completely violate relativity, which requires that the speed of light be a constant from any frame of reference.

So either way the experiment would violate relativity, and this would require someone to come up with a theory as widely accepted and accurate as the theory of relaitvity.

Just a few thoughts i had on this subject.

[timeshifter]

As very well put by PhysBrain, the universe does seem to be expanding due to the dopplar shift. However, the dopplar effect we see (red shift) is a result of what we see based on how far away the stars we are seeing actually are. If they are millions of light years away, which they probably are, then we are seeing the red shift they left millions of years ago.

Now, if that shift started yesterday and the universe started to collapse, then we would not see the blue shift, indicative of an object moving towards you, until the universe was nearly on top of us. So, from my point of view, the answer remains, no, we do not know if the universe is still expanding.

All we know is that the universe was expanding at the time we are seeing those stars and galaxies.

[timeshifter]

As very well put by PhysBrain, the universe does seem to be expanding due to the dopplar shift. However, the dopplar effect we see (red shift) is a result of what we see based on how far away the stars we are seeing actually are. If they are millions of light years away, which they probably are, then we are seeing the red shift they left millions of years ago.

Now, if that shift started yesterday and the universe started to collapse, then we would not see the blue shift, indicative of an object moving towards you, until the universe was nearly on top of us. So, from my point of view, the answer remains, no, we do not know if the universe is still expanding.

All we know is that the universe was expanding at the time we are seeing those stars and galaxies.