eli5: What evidence indicate towards the phenomenon of the Big Bang? How did they come to the conclusion of it happening?
In: Physics
TLDR: Because we can see it, or more specifically we can see the shockwave from it’s explosion.
Edwin Hubble (form whom the Hubble Space Telescope is named) catalogued a large number of stars and galaxies and in the process made a phenomenal discovery.
Due to the Doppler effect the light of far away things tends to get shifted into a different part of the spectrum. As things move towards us they appear more blue, while as they move away they appear more Red.
Hubble discovered that almost everything in the universe is red shifted, meaning that the entire universe is moving away from us. The only plausible explanation was that the universe was expanding, and not just that but accelerating as well.
With this new found information scientist concluded that if the universe is expanding, at one point it must have been tiny. If you reverse the clock back then the universe would have been a single point, the Big Bang.
Scientists predicted that such an explosion would have left a shock wave like the light from a distant explosion, a kind of cosmic background radiation.
Then they found it, radio detectors are able to see this cosmic background radiation and it’s coming from every direction because the universe is expanding. They can also use it to determine how old the universe is, about 13 billion years old.
You can see the Cosmic Background Radiation yourself, just turn on an old TV. The fuzz and static on an old TV is primarily caused by your TV picking up this background radiation on it’s antenna.
I’m sure some more astrophysics-minded people can elaborate but for one, nearly all the matter in the universe is moving apart, with all sorts of galaxies and stars all moving away from each other. If you think about it, it kinda makes sense that they all came from the same place and that some sort of large bang sent them all flying apart.
Of course, the galaxies and stars formed after the big bang but their motion suggests that the matter all originated in one location and was dispersed from there
There are two big pieces of evidence we have that the big bang occurred:
1. Astronomers noticed that everything in the universe is moving further apart from each other. This means if you were to “rewind” the universe, everything would be closer together, the further back you go. That leads to the conclusion that if you go far enough back, everything must be in one single location before it “exploded” outwards.
2. The big bang must have been extremely hot. As the universe expanded, it cooled down. Like all hot things, when the universe was very young and hot, it would have actually glowed with light. Since that light can’t escape out of the universe, it must still be around, but very dim. Scientists have been able to observe the very dim echos of that bright light in the form of “microwave background radiation”. This very faint radiation is the same no matter what direction you look, it is spread through the entire universe. This matches up exactly with the calculations that have been made of how the faint afterglow of the big bang should look.
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