how do we know the universe is expanding and will do this till infinity?
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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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