Just been on a space video kick lately and am pretty ignorant about most of it.
One thing I see a lot of these videos saying is that the expansion of the universe seemed to be slowing down until around 5 billion years ago.
How can we tell that it was slowing down 5 billion years ago if it started speeding up afterwords? Like how do they map out the universe and say “yeah it was slowing down until this time.”
Is it just a theory or is this something commonly accepted as fact? Appreciate any info and thanks! 🙂
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So a lot of what we know and understand about the universe is due to one thing that we almost never think about in our daily lives, light speed is actually pretty slow.
Or rather, space is big. In the words of douglas adams, “You just won’t believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space”
This means that it takes a lot of time for light to travel from one place in the universe to another. Using this fact, and a few other pieces of knowledge, means that when we look at distant things (stars, galaxies, nebula) we are not only looking at something far away, but we are actually looking back in time.
If a star is 5 billion light years away, we are not looking at that star and how it is NOW, rather, we see the star as it was 5 billion years ago. We can see how fast it was moving 5 billion years ago.
If we look at a star that is 3 billion light years away, we can see how fast it was moving 3 billion years ago…and so on.
A key assumption that we make here when we talk about the formation of the universe is that we assume that it is pretty much the same everywhere. Earth isnt special, and neither are any of the other stars or galaxies that we can see. This is generally a pretty safe assumption as if you look at a whole bunch of stars that are 5 billion light years away, they all seem to be moving away from us at approximately the same speed.
Being able to see into the “past” like this also is how we know the approximate age of the universe. The furthest light we can see is from objects that are around 13 billion light years away. If the universe was older, we should be able to see things further away than that.
So we can study how the universe has changed simply by looking at things that are further or closer away from us.
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