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How Did the First Atom Form? Where did it come from? | Big Bang Nucleosynthesis

Where did the first atom come from? The short answer is the big bang. In the early universe there was an immense amount of energy, The energy condensed, atoms formed. But there’s a lot more that happened, which will be explained here.

The big bang is often thought of as the theory explaining the beginning. but it’s not. We don’t know when the universe actually started, or whether it did. Our best theory of the early universe is the standard model of cosmology, We can only go back to one Planck time, about 10^-43 seconds. This is the smallest unit of time that can theoretically exist according to quantum mechanics. We don’t know what came before this.

The earliest time we can theorize what happened is around the time of inflation. This is when the universe expanded exponentially from about 10^-36 to 10^-33 seconds after the big bang. The universe expanded faster than the speed of light. This is permissible because there is no theoretical restriction on how fast space can expand. I

The proper way to understand the term “big bang”, is not as some point or object from which the universe started, but as a period in the early universe, when the universe was very hot, very dense, and expanding rapidly.

The theory of the standard model of cosmology is well understood starting at about 10-12 seconds, because the universe at this point had energies that can be replicated in our particle accelerators.

In terms of the forces, gravity separated from the unified force shortly after the Planck Epoch at 10^-43 seconds. The strong force separated at around the time of inflation 10^-32 seconds. But from 10^-32 seconds to 10^-12 seconds, the electromagnetic and weak forces were still united as the electroweak force. The universe probably consisted of quarks and gluons in a quark-gluon plasma. All these fundamental particles were massless, because the Higgs field was massless at this point.

At 10^-11 seconds, the temperature fell to one quadrillion Kelvin. This leads to electroweak symmetry breaking and the beginning of the quark epoch. The electromagnetic and weak forces become separate forces, and the Higgs field gained a non-zero potential, the particles of the Standard Model obtain their rest mass.

The universe is however still too hot for the quarks to combine together to form hadrons like protons and neutrons.
As temperatures cool to1 trillion Kelvin at 10-5 seconds, the quark plasma turns into a hadron gas made of protons, neutrons, and some mesons.

As the universe keeps cooling down, matter antimatter begin annihilating with particles creating lighter particle and antiparticle pairs, eventually ending up as the lightest particles – neutrinos and photons. For some reason more particles were created than antiparticles. If this annihilation were symmetric, then we would have had a universe consisting of nothing except photons and neutrinos.

A few protons, neutrons, and electrons were left over, the building blocks needed for atoms. After the universe was a few minutes old, the temperature dropped below 1 billion Kelvin, big bang nucleosynthesis or BBN happened.

BBN lasts up to the universe at 20 minutes old. The universe had 75% Hydrogen and 25% Helium-4, & trace amount of deuterium, Helium-3 and Lithium-9 nuclei. The universe consisted of 87% protons and 13% neutrons.
Note that at this point in time it’s all ionized nuclei, only the core of the atoms exists – no electrons bound to them. The universe so hot that the electrons could not attach to the nucleons.

The universe was still opaque because the photons that carry light as they interacted with the nucleons and electrons flying around. They were not free to propagate through space. This photon epoch lasts for 380,000 years until the universe cools down to 3000 Kelvin. Electrons can now bind to the nucleons to make neutral and stable atoms. This is called recombination.

Photons were now free to propagate through the universe. This first light of the universe is what we see today as the cosmic microwave background or CMB. This light was released as the first stable neutral atoms were formed.

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