From the July 2019 issue

How Did the Big Bang Happen?

Virtually all astronomers and cosmologists agree the universe began with a “big bang” — a tremendously powerful genesis of space-time that sent matter and energy reeling outward.
By | Published: July 1, 2019 | Last updated on May 18, 2023
BIG-TIME THEORY. The ­discovery of the cosmic microwave background (CMB) confirmed the Big Bang theory. The CMB’s clumpiness gives astronomers evidence for theories ranging from what our universe’s contents are to how modern structure formed. 
Astronomy: Roen Kelly
The evidence is clear, ranging from the underpinnings of Albert Einstein’s general theory of relativity, to the detection of the cosmic microwave background by Arno Penzias and Robert Wilson in the 1960s, to the confirmation of ripples in the fabric of ancient space-time from the Cosmic Background Explorer (COBE) satellite in 1992. But the devil is in the details, and that’s where figuring out how Big Bang cosmology really works gets interesting.

The Big Bang model is typically broken down into a few key eras and events. Standard cosmology, the set of ideas that are most reliable in helping decipher the universe’s history, applies from the present time back to about a hundredth of second after the Big Bang. But before then, particle physics and quantum cosmology ruled the universe.

When the Big Bang occurred, matter, energy, space, and time were all formed, and the universe was infinitely dense and incredibly hot. The often-asked question “What came before the Big Bang?” is outside the realm of science because it can’t be answered by scientific means. In fact, science says little about the way the universe behaved until some 10–43 second after the Big Bang, when the Grand Unification Epoch began (and lasted only until about 10–35 second). Matter and energy were interchangeable and in equilibrium during this period, and the weak and strong nuclear forces and electromagnetism were all equivalent.

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HIGH-RES ECHO. Evidence for the Big Bang comes from detailed data from the Cosmic Background Explorer (COBE) and Wilkinson Microwave Anisotropy Probe (WMAP) satellites. In 1992, COBE produced the first good CMB map (top); nine years later, WMAP followed with a far-more detailed version. 
NASA/WMAP Science Tea
The universe cooled rapidly as it blew outward, however, and by 10–35 second after the Big Bang, the epoch of inflation occurred, enlarging the universe by a factor of 1050 in only 10–34 second. During this wild period, cosmic strings, monopoles, and other exotic species likely came to be. As sensational as inflation sounds, it explains several observations that would otherwise be difficult to reconcile. After inflating, the universe slowed down its expansion rate but continued to grow, as it does still. It also cooled significantly, allowing for the formation of matter — first neutrinos, electrons, quarks, and photons, followed by protons and neutrons. Likewise, antiparticles were produced in abundance, carrying the opposite charge of their corresponding particles (positrons along with electrons, for example).

As time went on and particles’ rest-mass energy was greater than the thermal energy of the universe, many were annihilated with their partners, producing gamma rays in the process. As more time crept by, these annihilations left an excess of ordinary matter over antimatter. 

Chemistry has its roots deep in the history of the universe. At a key moment about one second after the Big Bang, nucleosynthesis took place and created deuterium along with the light elements helium and lithium. After some 10,000 years, the temperature of the universe cooled to the point where massive particles contributed more to the universe’s overall energy density than light and other radiation, which had dominated until then. This turned on gravity as a key player, and the little irregularities in the density of matter were magnified into structures as the universe expanded.
COSMIC DISCOVERY. Robert Wilson (left) and Arno Penzias unexpectedly discovered the cosmic microwave background radiation with this horn-shaped antenna.
Astronomical Society of the Pacific

The relic radiation of the Big Bang decoupled (picture heavy traffic suddenly clearing) nearly 400,000 years later, creating the resonant echo of radiation observed by Penzias and Wilson with their radio telescope. This decoupling moment witnessed the universe changing from opaque to transparent. Matter and radiation were finally separate. 

Observational astronomers consider much of the history of the early universe the province of particle physicists, describing what happened up to the formation of galaxies, stars, and black holes as “a lot of messy physics.”  They are more interested in how the first astronomical objects, the large-scale inhabitants of the universe, came to be about 1 billion years after the Big Bang. But before these astronomers can gain a clear picture of that process, they need to consider the role of the wild card — dark matter.