11.7 Evolution of the early universe

In the previous section, we discussed the structure and dynamics of universe. In particular, the universe appears to be expanding and even accelerating. But what was the universe like at the beginning of time? In this section, we discuss what evidence scientists have been able to gather about the early universe and its evolution to present time.

The early universe

Before the short period of cosmic inflation, cosmologists believe that all matter in the universe was squeezed into a space much smaller than an atom. Cosmologists further believe that the universe was extremely dense and hot, and interactions between particles were governed by a single force. In other words, the four fundamental forces (strong nuclear, electromagnetic, weak nuclear, and gravitational) merge into one at these energies ( [link] ). How and why this “unity” breaks down at lower energies is an important unsolved problem in physics.

Scientific models of the early universe are highly speculative. [link] shows a sketch of one possible timeline of events.

1. Big Bang $\left(t<10^{\text{−}43}\text{s}\right):$ The current laws of physics break down. At the end of the initial Big Bang event, the temperature of the universe is approximately $T=10^{32}\text{K}.$
2. Inflationary phase $\left(t=10^{\text{−}43}\text{to 1}{0}^{\text{−}35}\text{s}\right):$ The universe expands exponentially, and gravity separates from the other forces. The universe cools to approximately $T=10^{27}\text{K}.$
3. Age of leptons $\left(t=10^{\text{−}35}\text{to 1}{0}^{\text{−}6}\text{s}\right):$ As the universe continues to expand, the strong nuclear force separates from the electromagnetic and weak nuclear forces (or electroweak force). Soon after, the weak nuclear force separates from the electromagnetic force. The universe is a hot soup of quarks, leptons, photons, and other particles.
4. Age of nucleons $\left(t=10^{\text{−}6}\text{to 225 s}\right):$ The universe consists of leptons and hadrons (such as protons, neutrons, and mesons) in thermal equilibrium. Pair production and pair annihilation occurs with equal ease, so photons remain in thermal equilibrium:
   
   $\begin{array}{}\\ \\ \gamma +\gamma \leftrightarrow {\text{e}}^{\text{−}}+{\text{e}}^{+}\hfill \\ \gamma +\gamma \leftrightarrow \text{p}+\stackrel{\text{−}}{\text{p}}\hfill \\ \gamma +\gamma \leftrightarrow \text{n}+\stackrel{\text{−}}{\text{n}}.\hfill \end{array}$
   
   The number of protons is approximately equal to the number of neutrons through interactions with neutrinos:
   
   $\begin{array}{}\\ \\ {\nu }_e+\text{n}\leftrightarrow {\text{e}}^{\text{−}}+\text{p}\hfill \\ {\stackrel{\text{−}}{\nu }}_e+\text{p}\leftrightarrow {\text{e}}^{+}+\text{n.}\hfill \end{array}$
   
   The temperature of the universe settles to approximately $10^{11}\text{K}$ —much too cool for the continued production of nucleon-antinucleon pairs. The numbers of protons and neutrons begin to dominate over their anti-particles, so proton-antiproton $\left(\text{p}\stackrel{\text{−}}{\text{p}}\right)$ and neutron-antineutron ( $\text{n}\stackrel{\text{−}}{\text{n}}$ ) annihilations decline. Deuterons (proton-neutron pairs) begin to form.
5. Age of nucleosynthesis ( $t={225}^{}\text{s}$ to 1000 years): As the universe continues to expand, deuterons react with protons and neutrons to form larger nuclei; these larger nuclei react with protons and neutrons to form still larger nuclei. At the end of this period, about 1/4 of the mass of the universe is helium. (This explains the current amount of helium in the universe.) Photons lack the energy to continue electron-positron production, so electrons and positrons annihilate each other to photons only.
6. Age of ions ( $t=1000$ to 3000 years): The universe is hot enough to ionize any atoms formed. The universe consists of electrons, positrons, protons, light nuclei, and photons.
7. Age of atoms ( $t=3000$ to 300,000 years): The universe cools below ${10}^5\text{K}$ and atoms form. Photons do not interact strongly with neutral atoms, so they “decouple” (separate) from atoms. These photons constitute the cosmic microwave background radiation to be discussed later.
8. Age of stars and galaxies ( $t=\mathrm{300,000}$ years to present): The atoms and particles are pulled together by gravity and form large lumps. The atoms and particles in stars undergo nuclear fusion reaction.