Lesson 57: The Big Bang Theory
In this lesson, we will learn about the Big Bang Theory. We will start by
learning about the four forces of nature that govern the universe today. We
will then look at a timeline of eight different eras in the universe"s history,
starting from the Planck Era right after the Big Bang, all the way to the Era
of Galaxies, which we are in today.
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.
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.
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.
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.
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.
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.