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Asked by anon-217191 to Philippe on 14 Jun 2019.
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Philippe Gambron answered on 14 Jun 2019:
Looking at stars is nice. Of course, what is interesting is not just to look at beautiful things but understand how they work and, for that, you need more than a small telescope. But there are many things that we can learn simply looking at the sky. For example, the sky is very homogeneous. There are roughly the same number of galaxies everywhere.
Of course, there are clumps of matter but, on large distances, the density is more the same. You would expect that of the ingredients of a dough if it has been mixed well enough. But the different parts of the Universe are too far away to have been able to mix up. So we suppose that they have been able to mix up during the very first moments and then it has expanded very quickly. This theory matches what we see in the cosmic microwave background.
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Comments
Harrison commented on :
Yes, it does. One of the most interesting developments over the past half century is the close relationship between the particle experiments we conduct on Earth and the very early Universe. The particle physics experiments have provided overwhelming evidence in favour of the current theory of particles, which has the uninspired name of the Standard Model (SM). At early times, the temperature of the Universe was much higher than it is today. The higher the temperature, the faster the particles in the early Universe. When the speed of a particle is close to the speed of light, we say that the particle is “relativistic”. At a given temperature, a lighter particle will move faster than a heavier particle. It turns out that the density of energy, that is, the amount of energy per cubic meter in space, depends mostly on the fastest particles and on the temperature. What is remarkable is that the Standard Model, which was built from what we learned in experiments on Earth, together with some brilliant inspired guesswork, tells us exactly how many relativistic particles there are at a given temperature. So, we can relate the energy density of the early Universe to the number of such particles and the temperature. We can thereby describe in precise detail what was happening 14 billion years ago. This is amazing. You might object and say how can we possible know how matter behaved 14 billion years ago; after all, no one was there to observe the Universe! Well, the remarkable thing is that every time we collide two protons together at the Large Hadron Collider, we reproduce for a tiny fraction of a second the conditions of the Universe when it was a billionth of a second old! Therefore, by investigating these collisions, we are in effect investigating the early Universe.