No one knows what the universe was like before the Big Bang, what caused it or exactly how the process took place.

We do know that the standard model is consistent with the astronomical evidence that the universe is expanding, and the expansion projects backward to a point of origin around 13.8 billion years ago.

The standard model has not yet been able to explain dark matter and dark energy, but theoreticians are hard at work to incorporate them meaningfully.

As the universe expanded after the blast, the energy spread out through a larger volume. This reduced its energy density the same way that blowing a bubble reduces the thickness of its bubble-gum skin.

The universe is not expanding into something outside itself. It is space itself that expands. We believe that it does not violate what we consider universal laws such as the law of gravity or the laws of thermodynamics.

Temperature is energy. The more energy there is, the higher the temperature. The higher the temperature, the more energy. When the energy falls below the mass equivalent of a given particle, then that particle can exist within it. The mass equivalent is expressed by Einstein’s famous equation, E=MC2. This means that energy and matter are different forms of the same thing.

That thing is a field, and that is a story for another column.

As the universe expanded, temperatures successively fell below different mass equivalent levels, allowing particles to condense out of the energy. This is analogous to water condensing from steam or ice forming in water. You might say the energy field became chunky soup.

A more complex analogy would be the order of crystallization of minerals in a cooling magma.

Now we are ready to understand the origin of matter as the universe expands and cools after the Big Bang.

At first there was only energy and a temperature of trillions of trillions of degrees. As the temperature decreased, the four fundamental forces separated.

The temperatures are inconceivably high and the times are inconceivably short. Gravity separated within 10-43 seconds, before the temperature had cooled to 1032 degrees. The strong force separated at around 10-34 seconds, leaving only the electroweak force. The weak force separated from the electromagnetic force at 10-12 seconds, resulting in the four forces we know of today.

At this time, quarks began to form. By 10-6 seconds (1 microsecond), quark energy had decreased enough that the strong force had captured them and confined them in hadrons. At around 1 second, neutrinos ceased to interact and other lepton/anti-lepton pairs had been annihilated, leaving electrons as the dominant lepton.

At 3 minutes, nucleosynthesis began creating helium nuclei. By 70,000 years, the universe had cooled enough that matter dominated over energy. By 377,000 years after the Big Bang, hydrogen and helium nuclei could capture electrons to form stable atoms. Photons no longer had the energy to ionize most atoms. This allowed the photons to travel greater distances. The universe became transparent, releasing the photons that formed the cosmic microwave background.

After that, hydrogen and helium clumped together and eventually formed stars, galaxies and planets like ours.

Richard Brill is a professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.