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! 🙂
In: 5
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.
[removed]
We can do this by looking at the light of distant stars and the characteristics of that light. Scientists can measure the red shift caused by universal expansion and the spectra of the light from given stars and calculate the speed with which the appear to be moving away from us. We know the characteristics of different stars based on other observation, hydrogen and helium emit in different ranges of EM. When redshifted they will appear at different wavelengths than they would if you measured them closer to the source so we can calculate the shift. Other related measurements help give us more information.
But it’s not just the observations that neuter, we also have models based on physics that tell us what we should expect to see. When the observations match those we can be more confident the models are right and those models also show various phases of expansion.
So we predict behavior based on models, confirm or refine the models based on observation, wash rinse and repeat, getting more and more accurate understanding of cosmology over time.
We plot stars, their type and luminosity to develop a cosmic yard stick which we can use to measure things like galaxies and how far other galaxies are. We plot those and notice the emission spectrum red shifts in these same stars in other galaxies the further away they are. Also the shape, type and luminosity of galaxy changes the further in time you go. We can measure the expansion of the universe also by measuring the total radiation emitted in space of a certain radio frequency or compare the emissions of distance stars like I mentioned early. Amalgamating this helps us form a model to make logical inferences such as certain formations of subatomic particles several years of the Big Bang which alters the expansion of the Universe.
Latest Answers