11.7 Evolution of the Early Universe

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 (Figure 11.21). How and why this “unity” breaks down at lower energies is an important unsolved problem in physics.

Figure 11.21 The separation of the four fundamental forces in the early universe.

Scientific models of the early universe are highly speculative. Figure 11.22 shows a sketch of one possible timeline of events.

Figure 11.22 An approximate timeline for the evolution of the universe from the Big Bang to the present.

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 quarks $\left(t=10^{\text{−}35}\text{to 1}{0}^{\text{−}5}\text{s}\right):$ 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 hadrons and leptons $\left(t=10^{\text{−}5}\text{to 10 s}\right):$ The universe consists of leptons (electrons, muons, tauons, and neutrinos) 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 until about 1s, through interactions with neutrinos:
   $$\begin{array}{}\\ \\ {\nu }_e+\text{n}\leftrightarrow {\text{e}}^{\text{−}}+\text{p}\hfill \\ {\stackrel{\text{−}}{\nu }}_e+\text{p}\leftrightarrow {\text{e}}^{+}+\text{n.}\hfill \end{array}$$
   Neutrinos decouple from baryons at around 1s, forming the cosmic neutrino background.
   The temperature of the universe settles to approximately $10^9\text{K}$ —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.
5. Age of Big Bang nucleosynthesis (10s to 20 minutes): Deuterons (proton-neutron pairs) begin to form. 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 stable nuclei, up to ${}^7\text{Li}$. 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 (20 minutes to 370,000 years): The universe is hot enough to ionize any atoms formed. The universe consists of electrons, positrons, protons, the light nuclei produced during the Big Bang nucleosynthesis, and photons.
7. Age of atoms (370,000 years to about ${10}^8$ years): The universe cools below 4000 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. This is also sometimes called the "dark ages" since the universe becomes transparent and the only source of new light is from hydrogen emitting radio waves.
8. Age of stars and galaxies (${10}^8$ years to present): When the universe cools to about 60 K, atoms and particles are pulled together by gravity and form large lumps. The atoms and particles undergo nuclear fusion reaction, creating stars and elements up to iron. Eventually, supernovae explode, creating elements past iron and spreading carbon higher throughout the universe.

To describe the conditions of the early universe quantitatively, recall the relationship between the average thermal energy of particle (E) in a system of interacting particles and equilibrium temperature (T) of that system:

where $k_{\text{B}}$ is Boltzmann’s constant. In the hot conditions of the early universe, particle energies were unimaginably large.

Nucleons form at energies approximately equal to the rest mass of a proton, or 1000 MeV. The temperature corresponding to this energy is therefore

Temperatures of this value or higher existed within the first second of the early universe. A similar analysis can be done for atoms. Atoms form at an energy equal to the ionization energy of ground-state hydrogen (13 eV). The effective temperature for atom formation is therefore

This occurs well after the four fundamental forces have separated, including forces necessary to bind the protons and neutrons in the nucleus (strong nuclear force), and bind electrons to the nucleus (electromagnetic force).

Nucleosynthesis of Light Elements

The relative abundances of the light elements hydrogen, helium, lithium, and beryllium in the universe provide key evidence for the Big Bang. The data suggest that much of the helium in the universe is primordial. For instance, it turns out that that 25% of the matter in the universe is helium, which is too high an abundance and cannot be explained based on the production of helium in stars.

How much of the elements in the universe were created in the Big Bang? If you run the clock backward, the universe becomes more and more compressed, and hotter and hotter. Eventually, temperatures are reached that permit nucleosynthesis, the period of formation of nuclei, similar to what occurs at the core of the Sun. Big Bang nucleosynthesis is believed to have occurred within a few hundred seconds of the Big Bang.

How did Big Bang nucleosynthesis occur? At first, protons and neutrons combined to form deuterons, ${}^2\text{H}$. The deuteron captured a neutron to form triton, ${}^3\text{H}$ —the nucleus of the radioactive hydrogen called tritium. Deuterons also captured protons to make helium ${}^3\text{He}$. When ${}^3\text{H}$ captures a proton or ${}^3\text{He}$ captures a neutron, helium ${}^4\text{He}$ results. At this stage in the Big Bang, the ratio of protons to neutrons was about 7:1. Thus, the process of conversion to ${}^4\text{He}$ used up almost all neutrons. The process lasted about 3 minutes and almost $25\text{\%}$ of all the matter turned into ${}^4\text{He}$, along with small percentages of ${}^2\text{H}$, ${}^3\text{H}$, and ${}^3\text{He}$. Tiny amounts of ${}^7\text{Li}$ and ${}^7\text{Be}$ were also formed. The expansion during this time cooled the universe enough that the nuclear reactions stopped. The abundances of the light nuclei ${}^2\text{H}$, ${}^4\text{He}$, and ${}^7\text{Li}$ created after the Big Bang are very dependent on the matter density.

The predicted abundances of the elements in the universe provide a stringent test of the Big Bang and the Big Bang nucleosynthesis. Recent experimental estimates of the matter density from the Wilkinson Microwave Anisotropy Probe (WMAP) agree with model predictions. This agreement provides convincing evidence of the Big Bang model.

Cosmic Microwave Background Radiation

According to cosmological models, the Big Bang event should have left behind thermal radiation called the cosmic microwave background radiation (CMBR). The intensity of this radiation should follow the blackbody radiation curve (Photons and Matter Waves). Wien’s law states that the wavelength of the radiation at peak intensity is

where T is temperature in kelvins. Scientists expected the expansion of the universe to “stretch the light,” and the temperature to be very low, so cosmic background radiation should be long-wavelength and low energy.

In the 1960s, Arno Penzias and Robert Wilson of Bell Laboratories noticed that no matter what they did, they could not get rid of a faint background noise in their satellite communication system. The noise was due to radiation with wavelengths in the centimeter range (the microwave region). Later, this noise was associated with the cosmic background radiation. An intensity map of the cosmic background radiation appears in Figure 11.23. The thermal spectrum is modeled well by a blackbody curve that corresponds to a temperature $T=2.7\text{K}$ (Figure 11.24).

Figure 11.23 This map of the sky uses color to show fluctuations, or wrinkles, in the cosmic microwave background observed with the WMAP spacecraft. The Milky Way has been removed for clarity. Red represents higher temperature and higher density, whereas blue indicates lower temperature and density. This map does not contradict the earlier claim of smoothness because the largest fluctuations are only one part in one million. (credit: NASA/WMAP Science Team)

Figure 11.24 Intensity distribution of cosmic microwave background radiation. The model predictions (the line) agree extremely well with the experimental results (the dots). Frequency and brightness values are shown on a log axis. (credit: modification of work by George Smoot/NASA COBE Project)

The formation of atoms in the early universe makes these atoms less likely to interact with light. Therefore, photons that belong to the cosmic background radiation must have separated from matter at a temperature T associated with 1 eV (the approximate ionization energy of an atom) . The temperature of the universe at this point was

According to cosmological models, the time when photons last scattered off charged particles was approximately 380,000 years after the Big Bang. Before that time, matter in the universe was in the plasma form and the photons were “thermalized.”

Antimatter and Matter

We know from direct observation that antimatter is rare. Earth and the solar system are nearly pure matter, and most of the universe also seems dominated by matter. This is proven by the lack of annihilation radiation coming to us from space, particularly the relative absence of 0.511-MeV $\gamma$ rays created by the mutual annihilation of electrons and positrons. (Antimatter in nature is created in particle collisions and in ${\beta }^{+}$ decays, but only in small amounts that quickly annihilate, leaving almost pure matter surviving.)

Despite the observed dominance of matter over antimatter in the universe, the Standard Model of particle interactions and experimental measurement suggests only small differences in the ways that matter and antimatter interact. For example, neutral kaon decays produce only slightly more matter than antimatter. Yet, if through such decay, slightly more matter than antimatter was produced in the early universe, the rest could annihilate pair by pair, leaving mostly ordinary matter to form the stars and galaxies. In this way, the vast number of stars we observe may be only a tiny remnant of the original matter created in the Big Bang.

Dark Matter and Dark Energy

In the 1970s, accomplished astronomer Vera Rubin began investigating the movement of spiral galaxies. She observed that their outermost reaches were rotating as quickly as their centers, which was not the predicted phenomenon. She calculated that the rotational velocity of galaxies should have been enough to cause them to fly apart, unless there was a significant discrepancy between their observable matter and their actual matter. This became known as the galaxy rotation problem. Rubin proposed that massive amounts of unseen, or dark, matter must be present in and around the galaxies for the rotation speeds to occur.

In the last two decades, new and more powerful techniques have revealed that the universe is filled with dark matter. This type of matter is interesting and important because, currently, scientists do not know what it is. However, we can infer its existence by the deflection of distant starlight. For example, if light from a distant galaxy is bent by the gravitational field of a clump of dark matter between us and the galaxy, it is possible that two images of the same galaxy can be produced (Figure 11.25). The bending of light by the gravitational field of matter is called gravitational lensing. In some cases, the starlight travels to an observer by multiple paths around the galaxy, producing a ring (Figure 11.26).

Based on current research, scientist know only that dark matter is cold, slow moving, and interacts weakly with ordinary matter. Dark matter candidates include neutralinos (partners of Z bosons, photons, and Higgs bosons in “supersymmetry theory”) and particles that circulate in tiny rings set up by extra spatial dimensions.

Figure 11.25 Light from a distant star is bent around a galaxy. Under the right conditions, two duplicate images of the same star can be seen.

Figure 11.26 Light from a distant star is bent around a galaxy. Under the right conditions, we can see a ring of light instead of a single star. (credit: modification of work by ESA/Hubble & NASA)

Increasingly precise astronomical measurements of the expanding universe also reveal the presence of a new form of energy called dark energy. This energy is thought to explain larger-than-expected values for the observed galactic redshifts for distant galaxies. These redshifts suggest that the universe is not only expanding, but expanding at an increasing rate. Virtually nothing is known about the nature and properties of dark energy. Together, dark energy and dark matter represent two of the most interesting and unsolved puzzles of modern physics. Scientists attribute $68.3\text{\%}$ of the energy of the universe to dark energy, $26.8\text{\%}$ to dark matter, and just $4.9\text{\%}$ to the mass-energy of ordinary particles (Figure 11.27). Given the current great mystery over the nature of dark matter and dark energy, Isaac Newton’s humble words are as true now as they were centuries ago:

“I do not know what I may appear to the world, but to myself I seem to have been only like a boy playing on the sea-shore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me.”

Figure 11.27 Estimated distribution of matter and energy in the universe. (credit: NASA/WMAP Science Team)