- The Big Bang Theory
- Fundamental Forces
- The Big Bang Timeline


[SLIDE 1]

The Big Bang Theory states that everything started as an unimaginably hot and dense collection of matter and radiation, which has cooled over time as the universe has expanded.

At the very beginning, the universe was so hot that photon energy could transform freely into particles of matter and anti-matter, which could then collide and turn back into energy.


[SLIDE 2]

To understand how things were different in the earliest stages of the universe, we first need to understand the roles of forces. There are four forces that govern the universe today: gravity, electromagnetism, the strong force, and the weak force.

We know about gravity, which holds together planets, stars, and galaxies. We are also familiar with electromagnetism, which depends on the electrical charges of particles. As we have learned previously, the strong force refers to the force that binds protons and neutrons together in atomic nuclei. The last of the four forces -- the "weak" force -- is a different force that plays a critical role in nuclear reactions such as fission and fusion.

As we will see, right after the Big Bang, these four forces may not have been as distinct as they are today.


[SLIDE 3]

The Big Bang theory charts a timeline of periods after the Big Bang occurred. The first four eras were over in the blink of an eye, lasting a total of about one millisecond.

The first era was the Planck Era, in which the four fundamental forces may have been unified as one superforce. Space and time would have been randomly warped due to massive fluctuations in energy and mass, which were interchangeable.

The second era was the GUT (narrator: spell out the G-U-T) Era, which stands for the Grand Unified Theories Era. During this time, gravity became distinct, while the strong, weak, and electromagnetic forces remained unified as a single GUT force.


[SLIDE 4]

At the end of the GUT Era, the GUT force split up into the strong force and the electroweak force, marking the beginning of the Electroweak Era. When this split occurred, a massive amount of energy was released, causing a dramatic expansion of the universe known as inflation. In a tiny fraction of a second, a piece of the universe the size of an atomic nucleus would have expanded into the size of our solar system.

The Electroweak Era ended when the electroweak force split into the electromagnetic force and the weak force, meaning the four fundamental forces were now distinct.

During the Particle Era, the spontaneous exchange of matter and energy continued, with photons turning into a variety of exotic subatomic particles and then changing back into energy. 


[SLIDE 5]

During the Planck Era, the GUT Era, the Electroweak Era, and the Particle Era, temperatures were high enough for matter and energy to change back and forth spontaneously. When the temperature fell to about 1 trillion Kelvin, it was no longer hot enough for this to continue -- pure energy could not spontaneously turn into matter and antimatter.

This marked the beginning of the Era of Nucleosynthesis, which lasted about five minutes. During this period, protons and neutrons began to fuse into heavier nuclei. By the time temperatures had dropped to about a billion Kelvins and the universe was no longer dense enough to maintain fusion, the chemical content of the universe had become about 75% hydrogen and 25% helium, with trace amounts of deuterium and lithium.

By the time fusion ceased, the universe was still nothing like we see it today. During the next phase, called the Era of Nuclei, the universe was a hot plasma of hydrogen nuclei, helium nuclei, and free electrons. The fully ionized nuclei moved independently from the electrons -- they were not bound together in neutral atoms. Photons bounced around rapidly, unable to travel very far before being redirected in another collision, similar to what happens in the radiation zone of the Sun today.


[SLIDE 6]

The Era of Nuclei lasted about 380,000 years until the temperature of the universe dropped to about 3,000 Kelvin. At this point, hydrogen and helium nuclei were finally able to capture electrons and settle into stable, neutral atoms. With the electrons no longer trapping the photons and keeping them from moving freely, it was as if a thick fog had lifted and the universe became transparent. Photons now streamed freely across the universe, causing what is known today as the cosmic microwave background.

With the end of the Era of Nuclei began the era of atoms, when atoms and plasma began to draw together in higher-density regions, assembling into protogalactic clouds. By the time the universe was about 1 billion years old, the first galaxies began to form, and the era of galaxies began, which continues to this day.