how do we know the universe is expanding and will do this till infinity?
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The red light shift caused by the Doppler effect. And it will do this until infinity only in theory. Eventually everything will be so far apart and moving so fast away from everything else that light will not be able to bridge the gap and nothing will be visible in the sky.
In short, because everywhere we look, everything is moving away from us.
Thanks to the Doppler effect, objects moving towards us get more blue, and those moving away become more red.
As it turns out, everything outside our local area (that is, our galaxy and neighbouring galaxies) is moving away from us. Not only that but the rate at which they’re moving away is proportional to how far away they are.
Both of these would be explained if its space itself that is expanding. It would mean that you would see this no matter where you are in the universe, so it avoids implying that we’re somehow in a special place in the universe
Imagine a bouncing buoy on a still pond. It makes ripples in circles. If you push this buoy away from yourself, those ripples become further apart by the time they hit you, because each time it bounces, it covered a slight bit of distance away from you before making the next bounce. If it was moving towards you, the ripples would be closer together for the opposite reason.
Stars make ripples and those ripples are light. If they are moving closer they will have compressed ripples (higher frequency). If they are moving further, we see less frequent ripples than we should see if it was stationary. We can confirm how much we ‘should’ see through a whole bunch of different ways.
Whether this will happen forever or not in reality is not yet known. Some theory says yes it will continue forever. From our perspective, it would seem as if were speeding up away from the buoy until we were going to fast the ripples couldn’t ever reach us because we’re moving faster than the ripples themselves.
There’s a kind of exploding star that is so bright that we can see it billions of light-years away. That means these explosions, called supernovae, happened billions of years ago, and their light is only now reaching us.
These supernovae turn out to have a very consistent brightness. Because of this property, when we see one, we can infer how distant it was by how bright it appears to us. If the supernova appears dimmer, that means it was more distant.
Since we know the distance, we can infer precisely how long the light took to reach us and therefore how long ago the supernova happened.
Meanwhile, the light from these supernovae was emitted when the universe was smaller. As the universe expanded, the light’s wavelength got spread out by the expansion. We can measure how much the light we receive got spread out in this way. Therefore, we know precisely how much smaller the universe was when each supernova happened.
Putting these ingredients together, each supernova event gives us a record for how much smaller the universe was at the time of that supernova. With many supernovae, we can reconstruct the whole expansion history of the universe.
With these data, we find that not only is the universe expanding, but *the universe’s expansion is speeding up*. This is a major reason why we don’t think the expansion will stop (although it’s not impossible).
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