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How do we know if the universe will keep expanding forever?

I have been reading about the universe and I wanted to know a few things that seem unclear to me. I am not sure if you can answer these two questions but I am hoping you will!! First, how do we know if the universe if going to expand forever? And if it does expand, will the expansion stop or will it just keep expanding forever? Thank you for your help. I have found your web pages very helpful and interesting when I have questions.

There is really only one way for the universe to stop expanding: that is if there is enough mass in the universe for the gravity to overcome the expansion. The density of mass (amount of mass per volume of space) that is required to halt the expansion is often called the "critical density." If the universe is more dense than critical, the gravity of all the stuff in the universe will be able to overcome the expansion, causing it to stop, and eventually re-collapse. If the density in the universe is smaller than the critical density, then the expansion will continue forever.

It is very difficult to determine what the density of the universe is, because most of the matter in the universe doesn't give off light that we can see in our telescopes. But we can go out and measure how many galaxies are out there, and how fast they orbit each other (The more massive galaxies are, the faster they will orbit). This gives us a fairly good idea of the density. We can also try to directly measure how quickly the expansion is decelerating by measuring the expansion speeds of distant galaxies. This is also a very difficult experiment.

Update: Astronomers' understanding of this problem has changed dramatically since we first answered this question in January 1999. There's now convincing evidence that the expansion of the universe is actually accelerating, not decelerating. This cannot be due to gravity. It must be due to "dark energy", a repulsive force of empty space that counteracts gravity on large scales. (You can read more about this here.)

Assuming astronomers' interpretations of the data are correct, and the universe is indeed accelerating at an ever-faster rate, we still don't know what the ultimate fate of the universe is going to be because we don't know if the dark energy will always be repulsive. There are some models that allow for it to become attractive in the future, which would cause the universe to ultimately recontract. But if dark energy is real, then the density of matter will no longer have much impact on the final fate of the universe, because dark energy tends to win out over gravity in the long run.

January 1999, Christopher Springob (more by Christopher Springob) (Like this Answer), Dave Kornreich (more by Dave Kornreich) (Like this Answer)

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