|MadSci Network: Physics|
thank you for posting this interesting question. In usual cosmology, it is
assumed that space is expanding, that means the distance between two stars.
The size of an atom or of an atomic nucleus, for example is not expanding.
First of all, all distance are supposed to increase, therefore also the
distance between an atomic nucleus and its electrons. But the binding force
keeps the atom together. The orbits of an electron are not arbitrary, but
they are fixed. So expansion may either leave the atom unaltered or may
push the electron into an excited state. In the latter case, the electron
will eventually drop back into the ground state emitting electromagnetic
radiation. Or the atom may be ionized, that means the electron is driven
away. But the expansion is not a strong effect, so atoms and matter are
The above picture is not valued for the early universe. There, matter existed in a different form. It was far too hot for atoms to be stable, even protons and neutrons did not exist. The constituents of the nucleons (protons and neutrons), the quarks built a plasma, densely packed. Atoms and other constituents were not smaller than they are today, but the environment of the early universe did no allow their formation. If such a state was built, it would be destroyed immidiately afterwards.
For a reference see for example Big Bang Introduction.
I hope I could help you. Greetings,
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