What Caused the Big Bang?

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Virtually all astronomers and cosmologists agree the universe began with a “big bang” — a tremendously powerful creation of space-time that sent matter and energy reeling outward. The evidence is clear — from the underpinnings of Albert Einstein’s general theory of relativity, to the detection of the cosmic microwave background radiation 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 breaks down into several eras and key events. Standard cosmology, the set of ideas that is most reliable in helping to decipher the universe’s history, applies from the present time back to about 1/100 second after the Big Bang. Before then, particle physics and quantum cosmology describe 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 which lasted only until 10–35 second).

Matter and energy were inter­ changeable and in equilibrium during this period, and the weak and strong nuclear forces and electromagnetism were all equivalent. 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 10–33 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 otherwise would 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, condensing out matter — neutrinos, electrons, quarks, and photons, followed by protons and neutrons. Antiparticles were produced in abundance, carrying opposite charge from 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 1 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 made up more of the 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. The relic radiation of the Big Bang decoupled 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 and describe all of what happened up to the formation of galaxies, stars, and black holes to be “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 observational astronomers can gain a clear picture of that process, they need to consider the role of the wild card — dark matter.

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