Key Ideas

Cosmic Inflation: A brief period of rapid expansion, called inflation, is thought to have occurred immediately after the Big Bang. During a tiny fraction of a second, the universe expanded to a size many times larger than it would have reached through its normal expansion rate.

• Inflation explains why the universe is nearly flat and the 2.725-K microwave background is almost perfectly isotropic.

The Four Forces and Their Unification: Four basic forces—gravity, electromagnetism, the strong force, and the weak force—explain all the interactions observed in the universe.

• The Standard Model accurately describes all the known particles in nature and their observed interactions (except for gravity).
• The weak force and electromagnetic force become unified into a single force called the electroweak force at higher energies than those typically found in today’s universe. This unification has been observed in high-energy particle accelerators.
• Grand unified theories (GUTs) are attempts to explain three of the forces (strong force, weak force, and electromagnetic force) in terms of a single force. This has not been observed, and particle accelerators fall far short of having the energy to directly probe the high energy where this unification is predicted to occur.
• A supergrand unified theory would explain all four forces (including gravity) at extremely high energies as a single force acting similarly on all the particles in nature. String theory attempts to make this unification, and it would describe the quantum nature of gravity. Supergrand unification is hypothesized to occur before the Planck time (t = 10⁻⁴³ seconds after the Big Bang).

Spontaneous Symmetry Breaking: As the universe expands and cools, the unified forces break into separate forces. Starting around the Planck time, gravity became a distinct force through a spontaneous symmetry breaking. During a second spontaneous symmetry breaking, the strong nuclear force became a distinct force. A final spontaneous symmetry breaking separated the electromagnetic force from the weak nuclear force; from that moment on, the universe behaved as it does today.

Particles and Antiparticles: Heisenberg’s uncertainty principle states that the amount of uncertainty in the mass of a subatomic particle increases as it is observed for shorter and shorter time periods.

• Because of the uncertainty principle, particle-antiparticle pairs can spontaneously form and disappear within a fraction of a second. These pairs, whose presence can be detected only indirectly, are called virtual pairs.
• The collision of two high-energy photons can produce a real particle-antiparticle pair. In this process, called pair production, the photons disappear, and their energy is transformed into the masses of the particle-antiparticle pair. In the process of annihilation, a colliding particle-antiparticle pair disappears and two high-energy photons appear.

The Origin of Matter: Just after the inflationary epoch, the universe was filled with particles and antiparticles formed from numerous high-energy photons. The particles also annihilated to produce a state of thermal equilibrium between the particles and the photons.

• As the universe expanded, its temperature decreased. When the temperature fell below the threshold temperature required to produce each kind of particle, annihilation of that kind of particle began to dominate over production.
• Matter is much more prevalent than antimatter in the present-day universe, because particles and antiparticles were not created or maintained in exactly equal numbers. What caused this initial asymmetry is not known.

Nucleosynthesis: Helium could not have been produced until the cosmological redshift eliminated most of the high-energy photons. These photons created a deuterium bottleneck by breaking protons apart from neutrons before they could combine further to form helium.

Density Fluctuations and the Origin of Stars and Galaxies: The large-scale structure of the universe arose from primordial density fluctuations.

• The first stars were much more massive and luminous than stars in the present-day universe. The material that they ejected into space seeded the cosmos for all later generations of stars.
• Galaxies are generally located on the surfaces of roughly spherical voids. Models based on dark energy and cold dark matter give good agreement with details of this large-scale structure.

The Frontier of Knowledge: The search for a theory that unifies gravity with the other fundamental forces suggests that the universe might actually have 11 dimensions (10 of space and 1 of time), 7 of which are folded on themselves so that we cannot see them. The fundamental objects of our universe may be very small strings, rather than pointlike particles.