How do we know the universe will continuously expand and never contract?
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We don’t really know that, it’s one of a few theories based on what we’ve observed in the universe. Some people think the universe will reach a certain point and then shrink back in on itself, others think the expansion will just run out of steam and the universe will settle into whatever state it is at that time. There’s a lot of data and a lot of theories but we still don’t know the exact answer to a question like that.
Well, we don’t know that- for certain. What we do have is some theories that fit the currently available data, which right now look like the most likely outcome is that the universe will keep expanding- but there’s also a whole lot we don’t know, so maybe that’ll change.
The first step is that you can measure the spectrum of distant galaxies, a shift twards the red end (redshift) shows they are all getting further apart as expansion happens. On top of that you can measure how the redshift varies with time as light from more distant galaxies was emitted when the universe was younger. If the universe was going to contract eventually, expansion should at least be slowing down. It turns out according to the latest data that the opposite is true, expansion appears instead to be speeding up. Current theories suggest that this is due to the mysterious “dark energy” although it’s not yet clear what this, and its counterpart “dark matter”, actually are.
We’re only reasonably sure.
There were and still are some scientist looking into the idea that the universe’s expansion would slow, stop, then reverse, eventually collapsing back into a point in an event called “The Big Crunch”.
There are a lot less of these scientists then there were 20-30 years ago.
The reason as we got better telescopes we were able to see farther.
When you farther, because light takes time to move, you see further back in time.
The further back in time we see, the slower the expansion of the universe seems to be.
Meaning expansion seems to be speeding up.
There are enough error bars on the whole thing that some rational scientist aren’t convinced though.
We don’t really “know” that, we can’t look into the future after all, but the universe is not just expanding, it’s also expanding faster and faster instead of slowing down, so it’s obvious to assume that it won’t stop.
Scientists used to assume that gravitational pull would slow the expansion down over time and ultimately grind it to a halt and reverse it at some point, but further observations showed that it wasn’t slowing down at all and was instead speeding up.
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