How did scientists reach the conclusion that the big bang was the beginning of our universe?
In: Physics
We can see that the galaxies in the sky are red-shifted, which means that they are drifting away from us, and from each other, which obviously means that they were closer together in the past. And if you turned the clock back by around 14 billion years, all the matter in the universe would have been in roughly the same place.
So after this discovery around 90 years ago, someone came up with this idea, probably just some abstract calculation model at first, that everything started out from a single point. I’m sure that most physicists did not believe that there was anything real to this single-point-thing back then, but over time more and more observations were made that supported this big bang: For example, if the big bang was real, then the universe would not only get smaller the further you go back in time, but also hotter. Before a certain point in time, calculated around 400.000 years after the big bang, everything would have been glowing hot and also the universe would have been more or less opaque, since everything must have been just a gas cloud with no actual empty space in between.
The thing is, we then discovered the background radiation: If you point your instruments at any empty spot in the sky, there is some microwave radiation, at frequencies that correspond to the heat radiation of a very cold object, just 3°C above absolute zero. But since light waves get stretched together with the expansion of space itself, and since this light, coming from further away than even the furthest stars, was also older than these stars, this meant that whatever sent out this light must have been hotter back then.
Let me rephrase that, since its pretty significant: If you look at any seemingly empty point in the sky with a microwave antenna, you can see light from something very hot very far away in the very distant past, and you can see absolutely nothing from beyond that!
And it just so happens that if you crunch the numbers, this fits very well with the big-bang-model, in which the universe became transperent about 400.000 years after the bang when everything was about 3000°C hot. Thats when people realized that this single-point thing might be more than just a model.
Of course, that was only the first step. Knowing that the universe was something in line with a model 400.000 years after the big bang is still far away from knowing that the universe had been concentrated in a single point. However, pretty much every time we figured something new out about the physics about this theoretical very early universe – for example with particle collider experiments – we only ever got results that were in line with the big bang model.
We can now say with confidence that the history of the universe corresponds to the big bang model up to a much earlier time than the age of 400.000 years. We can NOT confidently extrapolate this to the single point of the big bang. Our knowledge only permits us to extrapolate back to a certain point in time, and this point so far has moved further and further back to the hypothetical big bang itself.
Maybe in the future we will figure something out about high-energy-physics that contradicts the idea of a single-point-big-bang. Maybe the universe did not actually have a beginning. But whatever happened, it certainly was very very small, very very hot, very very dense 14 billion years ago.
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