To my understanding, at the very beginning there was the big bang when nothing (or everything?) existed in singularity, and then at some early point hydrogen came to existence. I understand how stars churn with gravity and heat and whatnot those bigger atoms such as iron at later stages of the universe. But how and why did hydrogen happen as the first atom, and why didn’t we have, say, uranium straight from the beginning?
In: Physics
Well, before atoms formed, protons and electrons would have to form. One thing at a time – you can’t make an atom without those.
Once protons and electrons form, the protons are *not* going to want to stick together. They have no reason to. They will latch onto electrons, though, and a proton paired with an electron is just regular hydrogen.
Anything bigger requires neutrons and forcing protons together.
The idea is that there is a time very early, when the universe is so hot that if two protons collide with each other, they are more likely to create some exotic particles than they are to simply fuse. Think of the universe as being a bit like a particle collider. After some time passes, the universe has cooled. Then, protons and neutrons become the favorable particles, and the exotic particles decay away. Once that happens, there is a very brief time that the protons and neutrons can fuse to each other before the universe becomes too cool for that. The time during which the universe is cool enough for protons and neutrons to fuse instead of creating exotic particles, but also hot enough for them to fuse, is too brief for very heavy nuclei to be produced.
We also had also lots of helium at the beginning, and probably a tiny bit of lithium. The initial ultra-hot soup of energy and particles after the big bang cooled (read: still extremely hot, just a bit less so) to the point where stuff flying around was mostly protons, neutrons, and electrons. But the heat was still immense and baked them together into slightly larger atoms sometimes, creating deuterium and helium mostly.
But larger stuff was not able to survive in those conditions. A very strong burst of light can literally rip an atomic nucleus apart, and this is easier for heavy stuff. So that tiny bit that maybe was created, if at all, was getting destroyed right away by a still _very_ hot and _bright_ early universe.
Because uranium is basically a bunch of hydrogen stuck together, can’t have that without hydrogen first.
If you look at the periodic table of elements, basically atoms are set apart by how many protons they carry in their nucleus or center. The most basic element is hydrogen, which is just one proton. Uranium has 92 protons. Atoms formed earliest because they were the simplest — as soon as the universe had cooled enough to start forming protons, and then for those protons to hold electrons around them, you had hydrogen atoms.
To get progressively heavier elements than hydrogen, you have to somehow apply enough energy to get protons to fuse together into a larger nucleus. Protons are positively charged and resist this (think about trying to push the north ends of two magnets together — a different kind of natural force, but the same effect — they repel each other, so you have to push to force them together.
For that reason, to get any meaningful quantities of the heavier elements, you have to wait for there to be enough hydrogen to get pulled together into clouds and ignite stars. The stars provide the heat and pressure necessary to fuse together protons and make heavier elements — mostly helium (two protons), but trace amounts of heavier elements.
The more generations of stars you go through, the more amounts of heavy elements you have, thanks to all the fusion from the previous generation. This planet we’re living on is the debris from old star fusion, which in turn used the hydrogen that formed in the early universe.
In order to build something complicated, you often have to build its simpler components first. Once you have protons and electrons, hydrogen is just about the simplest thing you can build, it’s just one proton with an electron buddy.
All other atoms are just more and more complicated combinations of different numbers of protons with electrons, so in a way, hydrogen is the basic building block everything else is made of. That’s why it had to exist first.
Just after bing bang, energy was so intense that they *condensed* (idk the right term) into particles. Such particles are called Higgs Bosons or God’s particles. They form the foundation blocks of all sub atomic particles. With Gravity they kept clubbing until they formed the basics atom with one electron and one proton. This atom is called Hydrogen.
In nature, we usually do not see atoms spontaneously combine; there are forces that repel subatomic particles, so complex atoms like uranium can only be formed in locations that overcome those repelling forces (like supernovae).
But hydrogen doesn’t need to overcome those forces. It’s just one proton and one electron, which are attracted by electromagnetism. So more complex atoms needed something like a star or supernova to form, while hydrogen did not – thus, all hydrogen at the start.
> at the very beginning there was the big bang when nothing (or everything?) existed in singularity
This is not correct. There is no data which suggest singularity at the beginning, the singularity pops up when people imagine and we don’t have another idea to refute the infinity. We have better ideas now that smart people have had decades to think and work the math on the subject.
A singularity is an infinite thing, but infinities don’t exist in nature. Any time we find one it’s a problem with our model, not actually how things are. Our best model right now has the universe coming from a super hot foam that we think was “everywhere”, but of course that model includes another infinity so it can’t be entirely right either.
The short answer to why uranium didn’t just form is that it needs lots of pressure to make protons and neutrons overcome the forces that keep them apart, and that pressure didn’t exist in the early universe.
Hydrogen was the first atom because it’s the most simple combination of things that the quarks made when they cooled off and slowed down to combine into something. To put it in math terms, it’s 1+1, as straightforward a combination as possible.
Uranium comes after stars spend millions of years squishing stuff together to make heavier elements (up to iron and nickel.) Those stars explode and the pressure wave squeezes some of the heavier than nickel elements into existence and leave behind a core remnant that we call a neutron star. And then – millions or billions of years later – those neutron stars smash into each other and make a different type of explosion that makes the rest of the heavy elements.
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