Big Bang Theory

In this section:

The Classical Big Bang Theory
Problems with Classic Big Bang Theory
Inflation Theory
Dark Matter
Dark Energy
What is the Universe made of
The Problem with Matter and Antimatter
Cosmic Inflation Theory

The Classical Big Bang Theory:

This is called the Classical Big Bang Theory because this is the original theory before additional discoveries were made. There have since been "extensions" added to the Big Bang Theory and I will cover those below.

Astronomer Georges Lemaître came up with the expanding universe theory in 1927. Two years later astronomer Edwin Hubble observed that other galaxies were moving away from us. This is what Lemaître predicted
The Big Bang Theory states the universe started as an infinitely dense and hot single point. Then it expanded and grew to the size it is today.
The Big Bang is not a theory on how the infinitely dense hot single point came into existence or what was before the big bang. It’s only focused on the big bang itself (when the universe started expanding) and after that. It makes no predictions about how or why the universe was formed to begin with, only what happened after it started expanding.

Problems with the Classical Big Bang Theory:

Originally it was believed the universe expanded gradually throughout its history. As more discoveries were made, this could not be true so the theory had to be updated. Here are some of the major problems that had to be solved.

The Flatness Problem - Scientists have determined the universe is nearly flat. It is flat in the same way as the earth appears to be flat when you are standing on it. The universe is so big that the part we can see appears to be flat. The Big Bang predicted curvature would grow with time so it should not be flat. For it to be as flat as it is today, there would have been extreme fine-tuning of conditions in the past, which most scientists say is not really plausible. If the curvature of the universe had been off by a few percent within a few seconds after the Big Bang, then it would have either re-collapsed or expanded so much that it would appear to be devoid of matter.
The Horizon Problem - With standard Big Bang expansion, there are regions of space in opposite directions of each other that could not have been in contact with each other. This is because moving at the speed of light (the universe’s speed limit), it would take longer to go from one side to the other than the age of the universe. But the universe, and particularly the microwave background, look the same everywhere. This means these regions have been in contact with each other in the past.
The Monopole Problem - Magnets come with two poles, a north and south. The Big Bang predicts that a lot of magnetic monopoles would have been created in the universe. A magnetic monopole only has a north or south pole, not both. These have not been observed in the universe so if they do exist, they are much more rare than the Big Bang predicts.

Inflation Theory:

To solve these problems, the Big Bang extension of the Inflation Theory was created in the late 70’s and early 80’s. It says that the universe expanded very rapidly (faster than the speed of light) for a brief moment shortly after the Big Bang. Essentially the universe (or the visible universe) expanded by a factor of 10 to the 26 (from smaller than a proton to about the size of a grapefruit) in way less than a trillionth of a second. Scientists are still trying to work out how/why inflation started and ended.

How it solves the flatness problem - The universe expands like a balloon being blown up. Without inflation, the balloon would be smaller and scientists could see the curve. But with inflation, the universe is so big that the part we can see appears to be flat. This is the same way that the earth is round but appears to us to be flat.
How it solves the horizon problem - It allows the regions of space that could not have been in contact with standard expansion to have been in contact since the universe expanded much more rapidly during inflation. So it solves the time problem for the opposite sides of the universe not being able to be in contact.
How it solves the magnetic monopoles problem - During inflation, the density or number of monopoles drops drastically which allows them to be at an undetectable level.

Problems with Inflation Theory:

If inflation happened in the early universe, it would create primordial gravitational waves. These gravitational waves would create an imprint in the light of the cosmic microwave background (CMB). These patterns, or polarization are called B-Modes. Currently all attempts by scientists to discover these have failed. So inflation remains an unproven theory.
The other problem with inflation is there is currently no known reason for it to start or end. But here are a few of the theories of how/why it started.

The Eternal Inflation Theory says that inflation never stops. Space stopped inflating in our universe but then space started inflating somewhere else. This leads to the multiverse theory. One universe bubbles up, or inflation causes it to expand, then inflation moves to another part of space and causes another universe to bubble up. This process never ends so there are an infinite number of universes.
At extremely high energies, a special state of matter would turn gravity into a repulsive force rather than an attractive one we see today. This rapid expansion only lasted a small fraction of a second (way less than a trillionth of a second) because the repulsive material quickly decayed. This decay left behind the matter and energy we see today. (There is no evidence for this “special matter”).
Inflation is a quantum field that permeates all of space and time. Quantum fields contain a certain amount of energy even in a vacuum. If the vacuum energy of the quantum field is large, then it can cause the universe to expand.

Dark Matter Theory:

Another Big Bang extension is dark matter. Dark matter was added to the big Bang Theory to solve the following problems.

Starting back in the 1930’s, scientists observed that stars and galaxies did not rotate as expected. As more research was done through the years, calculations showed that many galaxies would fly apart, would never have formed, or would not move the way they do with the current understanding of gravity.
The stars closer to the center of a galaxy should move faster than stars closer to the edge. But they move close to the same speed.
With only gravity and “ordinary” matter, there was not enough time for galaxies and larger structures to form and be distributed through the universe as they are.
Scientists theorize that dark matter exists and solves these problems.

What is dark matter?

That is a question that science has not been able to answer yet. Despite the different theories on what dark matter might be and the numerous experiments to detect it, currently all have failed and dark matter has not been detected.
It is called dark matter because it cannot be seen or dected like ordinary matter.
Some scientists believe that dark matter does not exist and there are other possible explanations but they are in the minority. Most of these theories say we don't fully understand gravity and with a better understanding of gravity we could solve these problems without the dark matter theory.

Dark Energy Theory:

The last Big Bang extension I'm going to cover is dark energy. Dark energy was added to the Big Bang Theory to solve the following problem.

In 1998, it was discovered that the expansion of the universe was accelerating. This did not fit with the Big Bang Theory. Expansion should be slowing down due to gravity.

What is dark energy and how does it solve the problem?

Dark energy is a theoretical repulsive force that counteracts gravity and causes the universe to expand at an accelerating rate.

Just like dark matter, dark energy has not been detected even though scientists have been trying to detect it. It is another theoretical substance added to the Big Bang Theory to try and solve problems with it. Some scientists believe that it does not exist and there are other possible explanations but they are in the minority.

What is the universe made of?

For the Big Bang Theory to work, the universe is made up of the following amounts of ordinary matter, dark matter and dark energy. These numbers vary a little depending on who is providing them but they are all close to these amounts

Dark energy - 70%
Dark matter - 25%
Ordinary matter - 4% to 5%

So in order for the Big Bang Theory as we know it to be true, less than 5% of the universe is matter that we can detect and know for sure it exists. The remaining 95% is all theoretical at this point.

The problem with Matter and Antimatter

The Big Bang should have created the same amount of matter and antimatter. Matter and antimatter particles are always created as a pair.

When matter and antimatter come into contact, they destroy each other leaving only pure energy. The universe should contain only energy and not any matter.
Also when matter and antimatter come in contact, they produce a gamma ray burst of very specific frequencies. These gamma rays are not found when observing the universe so there cannot be much antimatter in the universe.
Our universe is made up of over 99.999% matter. Somehow a tiny portion of matter created in the Big Bang survived and is responsible for everything we see in the universe today. Only one out of every billion particles of matter survived. There are still tiny amounts of antimatter in the universe, mainly around black holes.
Every interaction between particles ever observed has always either created or destroyed both the same number of matter and antimatter particles. Never more matter than antimatter.
Scientists have a few theories on how more matter could exist, but nothing has been proven at this point.

Cosmic Inflation Theory (Cosmic Reheating):

This is a newer twist on the theory that the Big Bang happened and then inflation started. This says inflation happened first and created the Big Bang (the more gradual expansion of space).

The universe began super small, way smaller than a proton, and was full of ultra-high energy matter.
This energy had so much pressure that it generated a repulsive gravitational force which is what caused inflation to start. This is sometimes referred to as a “false vacuum”. This false vacuum generated the repulsive force that causes inflation.
It grew nearly an octillion times larger (1 followed by 27 zeros) in less than a trillionth of a second.
The rapid inflation causes all the matter to spread out, leaving the universe cold and empty.
“Cosmic Reheating” is the key to this theory. During the reheating period, the energy propelling inflation is believed to decay into particles (matter, antimatter and radiation). All these particles start bumping into each other and this is what reheates the universe. This creates the initial conditions for the Big Bang.
The Big Bang is the more gradual expansion of space we see today. And this is the period of time when all the planets, stars, galaxies, etc formed in the universe.
This theory attempts to provide an explanation for why inflation started but it still has the same issues of the other Big Bang theory. Inflation has not been proven and it’s still just a theory.