Is expansion actually slowing?

  • Thread starter Megalepton
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In summary: The rate of expansion is slowing faster for more distant galaxies, since there is more time for the expansion to slow down.New member here, thanks for your patience. In summary, the farthest galaxies are the most redshifted, and the old light from them does give us information about the rate of expansion closer to the beginning of the Universe. However, the current acceleration of the universe is not solely based on the light from those galaxies, but also on the changing rate of expansion as we look at galaxies at different distances and times. The expansion of the universe is currently accelerating, but it is still slowing down, just at a slower rate. This is more apparent in nearby galaxies compared to distant ones.
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Megalepton
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New member here, thanks for your patience. The farthest galaxies are the most redshifted, but doesn't that old light tell us more about the rate of expansion closer to the beginning of the Universe?
 
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Megalepton said:
The farthest galaxies are the most redshifted, but doesn't that old light tell us more about the rate of expansion closer to the beginning of the Universe?

Yes, it does. But we don't think the expansion of the universe is currently accelerating just because of light from those old galaxies. We think the expansion of the universe is currently accelerating because the expansion rate we infer from looking at light from galaxies at various distances (and therefore from various times) changes in a way that makes it clear that the expansion was slowing down until a few billion years ago, but since then has been speeding up.
 
  • #3
Megalepton said:
New member here, thanks for your patience. The farthest galaxies are the most redshifted, but doesn't that old light tell us more about the rate of expansion closer to the beginning of the Universe?
Yes and no.

First, a constant rate of expansion is an accelerated expansion. This is because expansion is speed per distance, so that an object twice as far away is receding twice as rapidly. This means that if the rate of expansion is a constant, then as objects move further away they speed up.

The current rate of expansion isn't quite a constant. It is still slowing down. But the rate of slowing is low enough that objects are still speeding up as they get further. Note that this is mostly only true for relatively nearby galaxies (as PeterDonis mentioned).
 

1. Is the expansion of the universe actually slowing down?

Yes, according to current scientific understanding, the expansion of the universe is indeed slowing down. This is due to the influence of dark matter and dark energy, which have been observed to make up the majority of the universe's mass and energy. While the expansion of the universe is still ongoing, it is decelerating over time.

2. How do we know that the universe's expansion is slowing down?

Scientists have been able to observe and measure the rate of expansion of the universe using various methods, such as the redshift of light from distant galaxies and the cosmic microwave background radiation. These measurements have consistently shown that the expansion is slowing down, providing evidence for the presence of dark matter and dark energy.

3. Will the expansion of the universe eventually stop or reverse?

It is currently believed that the expansion of the universe will continue indefinitely, but at a slower and slower rate. This means that the universe will continue to expand, but the rate of expansion will approach zero over time. There is no evidence to suggest that the expansion will stop or reverse.

4. Are there any theories about why the expansion is slowing down?

One popular theory is the "Big Rip," which suggests that the expansion of the universe will eventually accelerate to the point where it tears apart all matter and energy. However, this theory is not supported by current observations. Another theory is that the expansion is simply due to the repulsive force of dark energy, which is thought to be responsible for the acceleration of the expansion.

5. Could the expansion of the universe speed up again in the future?

While it is possible that the expansion of the universe could accelerate again in the future, this is not currently predicted by any scientific theories or observations. The most widely accepted scenario is that the expansion will continue to slow down, leading to a "heat death" of the universe in which all matter and energy become evenly distributed and the universe reaches a state of maximum entropy.

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