Section 12.4 Epochs of the Early Universe
Let's now see what was going on in the universe during each epoch, the intervals between the crucial events of Section 12.3, and explain briefly why.
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Bang to \(\mathbf{10^{-43}}\Xunits{s}\text{:}\).
Very speculative. This would be the time of universal symmetry, with quantized gravity incorporated with the other three interactions. Nobody really knows how.
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\(\mathbf{10^{-43}}\) to \(\mathbf{10^{-34}}\Xunits{s}\text{:}\).
Grand unification prevails. The strong, weak and electromagnetic interactions are on equal footings. Quarks and leptons are not yet distinct, since there are plenty of \(X\) bosons to facilitate interconversion.
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\(\mathbf{10^{-34}}\) to \(\mathbf{10^{-10}}\Xunits{s}\text{:}\).
The strong interaction acts separately from the electro-weak. The \(X\) bosons are now too heavy to be produced in random collisions and they decay away quickly leaving distinct quarks, antiquarks, leptons and antileptons. However, due to a very subtle flaw in symmetry (which we won't explain here), the \(X\) boson and its antiparticle (\(\overline X\)) decay slightly differently. This leaves a slight excess (about one part in \(10^9\)) of quarks over antiquarks. The universe now consists of a hot “soup” of quarks, leptons, and gauge bosons (\(W\)'s and \(Z\)'s, gluons, photons) all in exact thermal equilibrium.
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\(\mathbf{10^{-10}}\) to \(\mathbf{10^{-4}}\Xunits{s}\text{:}\).
The weak interaction now acts separately from electromagnetism, because \(W\)'s and \(Z\)'s are no longer produced abundantly. By the end of this epoch, the density of the universe has fallen sufficiently that quarks begin collecting into hadrons.
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\(\mathbf{10^{-4}}\) to 1 s:.
With quarks permanently confined, the “soup” now consists of hadrons and leptons. Energies are now insufficient to create exotic particles or to produce hadronic particle-antiparticle pairs. Thus annihilation of antiparticles occurs rapidly and essentially all baryonic antimatter is wiped out, leaving the small original (since the time \(t=10^{-34}\Xunits{s}\)) excess of matter. We are left with primarily protons, neutrons, electrons, positrons, neutrinos and photons in thermal equilibrium. So much radiation is released from the annihilation of quarks that there are approximately \(10^9\) photons/baryon, the value we observe today.
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1 to 400 s:.
The universe is now diffuse enough ( “only” about 400,000 times as dense as water) that neutrinos don't interact enough to stay in equilibrium with the rest. From 1 second on they evolve separately. This means that inverse beta decay:
\begin{equation} \overline\nu_e + p \to n + e^+\tag{12.5} \end{equation}stops turning protons back into neutrons. But a neutron can spontaneously decay to a proton, with 10 percent decaying every 100 seconds. So the ratio of protons to neutrons starts growing. Also during this epoch the available thermal energy falls below that required to create electron-positron pairs and so positrons pretty much disappear. At the same time, thermal energies start to fall below the binding energies of small nuclei, most notably the deuteron. This means that protons and neutrons can start coalescing to form helium and lithium nuclei without being blasted apart by high energy photons. By the end of this period, essentially all the neutrons are bound into light nuclei. The ratio of protons to neutrons and the relative abundances of primordial elements are fixed at this time.
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400 s to \(10^5\) years:.
The universe consists of a plasma of protons, helium nuclei, electrons and photons, and decoupled neutrinos. Any remaining free neutrons have long since decayed away. The photons interact easily with the free electrons via the Compton effect and thermal equilibrium between radiation and matter is maintained.
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\(10^5\) years to the present:.
At last it is cool enough that electrons and nuclei can combine to form atoms, since the photon energies are too low to easily ionize the atoms. Later the photon energies fall below even that level necessary to excite atomic transitions, so there is no longer any efficient way for photons to transfer energy. The universe becomes transparent to light as the photons decouple from the matter. Those primordial photons are still wandering around the universe.