The big bang theory

The big bang theory is the leading cosmological model explaining how the universe began. The theory states the universe was an infinitely hot and dense single point 13.8 billion years ago that inflated and stretched to produce the still-expanding universe we observe today. Cosmologists believe the big bang not only created the majority of matter, but also the physical laws that govern the universe. Much of the big bang theory is based on models, observations of the universe expanding, and studies of cosmic microwave background radiation.

The big bang theory was first proposed by Belgian astronomer Georges Lemaître in the 1920s and boosted by Hubble's observations of the expanding universe and the discovery of cosmic microwave radiation in the 1960s by Arno Penzias and Robert Wilson.

Based on theory and observation, scientists make three assumptions about the universe:

• The laws of physics hold across the universe irrespective of time or location in space.
• The universe is homogeneous, or roughly the same in every direction or isotropic (though not necessarily for all of time).
• Humans do not observe the universe from a privileged location (such as at its very center).

Applying these assumptions to Einstein's theory of relativity indicates the universe has the following properties:

• The universe expands (astronomers measure light from the universe's distant regions redshifted by the expansion of the space between).
• The universe emerged from a hot, dense state at some finite time in the past.
• The lightest elements were created in the first moments of time.
• A background of microwave radiation fills the universe due to the transition that occurred when the hot, early universe cooled enough for atoms to form.

Given the current observations of the expanding universe in all directions, scientists theorize it must have started at a single point of infinite density a finite period of time in the past. After initial rapid expansion, the big bang theory maintains that the universe cooled enough to allow the formation of subatomic particles and later simple atoms. Giant clouds of these early elements later coalesced through gravity to form stars and galaxies.

Scientists cannot observe the big bang directly; therefore, the timeline and circumstances of the early universe are the subjects of much speculation and competing ideas. However, the big bang theory can be broken down into a series of stages:

• Singularity
• Inflation Epoch
• Cooling Epoch
• Structure Epoch

The big bang theory states that the universe was condensed in an infinitesimally small singularity (also known as the Planck Epoch) of infinite denseness and heat. This period extends from point 0 to approximately 10^-43 seconds. From 10^-43 to 10^-36 the universe temperature transitioned such that the fundamental forces of the universe began to separate.

The singularity began expanding faster than the speed of light (known as cosmic inflation). This period was incredibly brief, lasting roughly 10^-32 seconds according to Alan Guth's theory. Most cosmological models suggest the universe was filled homogeneously with a high-energy density that rapidly expanded and cooled. Baryogenesis occurred with particles moving at relativistic speeds creating particle-antiparticle pairs continuously being created and destroyed.

As the universe further cooled, it decreased in density and temperature particles reached energies that particle physics experiments can obtain. Since temperatures were no longer high enough to produce new matter-antimatter pairs, mass annihilation immediately followed, leaving just one in 10¹⁰ of the original protons and neutrons and none of their antiparticles.

A few minutes into the expansion, Big Bang nucleosynthesis began. With temperatures dropping to 1 billion kelvin and energy densities dropping to about the equivalent of air, neutrons and protons began to combine to form the universe's first deuterium (a stable isotope of hydrogen) and helium atoms. However, most of the universe's protons remained uncombined as hydrogen nuclei.

After roughly 379,000 years, electrons combined with nuclei to form atoms, primarily hydrogen, while the radiation decoupled from matter and continued to expand through space, largely unimpeded. This radiation is now known to be what constitutes the Cosmic Microwave Background (CMB).

Over several billion years, small differences in density in the almost uniformly distributed matter of the universe became gravitationally attracted to each other—producing gas clouds, stars, galaxies, and the other astronomical structures that we regularly observe today.

There are two major scientific discoveries supporting the big bang theory:

1. Hubble's discovery of the relationship between a galaxy’s distance from earth and its speed
2. The discovery of cosmic microwave background radiation.

The phrase "big bang" was first introduced by English astronomer Fred Hoyle in 1949. However, the term did not become popular until the 1960s. Hoyle was one of the founders of the steady-state theory of the universe and opposed cosmological theories with a beginning in time. On March 28th, 1949, Hoyle gave a talk on "continual creation" theory to the BBC's Third Programme, during which he contrasted the steady-state theory with early explosion theories that would later be referred to as the big bang. He stated:

the hypothesis that all matter of the universe was created in one big bang at a particular time in the remote past.

The name is widely considered a misnomer as it implies suggests the image of an explosion.