Big Bang

The Big Bang is a physical theory that explains how the universe expanded from a very hot and dense beginning. This theory helps us understand many things we see in space, like the presence of certain elements, the cosmic microwave background radiation, and how galaxies move away from each other.

Timeline of the expansion of the universe, where space is represented schematically at each time by circular sections. On the left, the dramatic expansion of inflation; at the center, the expansion accelerates (artist's concept; neither time nor size are to scale).

Scientists use the Big Bang theory to describe the early moments of the universe. As the universe grew, it cooled down. Tiny particles formed and came together to create stars and galaxies. We now know that the universe began about 13.8 billion years ago.

The idea of an expanding universe was first shown through work by physicist Edwin Hubble in 1929. Later, important discoveries like the cosmic microwave background gave more support to the Big Bang theory. Scientists are still working to understand some mysteries, like the nature of dark matter and dark energy.

Features of the models

Big Bang cosmology models depend on three main ideas: that physical laws work the same everywhere, that the universe looks mostly the same in all directions on large scales, and that matter can be thought of as a smooth flow. These ideas have been tested and mostly hold true.

The universe's expansion and shape depend on how much mass and energy it contains. Today, we know that only about 5% of the universe is made of stars and planets, with 27% being dark matter and 68% being dark energy. Because the universe has a limited age and light travels at a finite speed, there are limits to how far back and forward in time we can observe or affect the universe.

Timeline

Main article: Chronology of the universe

The Big Bang theory tells us that the universe began very hot and very small. It has been expanding and cooling ever since.

Panoramic view of the entire near-infrared sky reveals the distribution of galaxies beyond the Milky Way. Galaxies are color-coded by redshift.

In the very beginning, the universe was so hot and dense that we cannot fully describe it with our current science. Soon after, the universe expanded very quickly in a process called inflation. This helps explain why the universe looks almost the same in all directions.

As the universe cooled, tiny particles formed. Later, atoms formed, making the universe clear for the first time. This is why we can see the cosmic microwave background radiation today. Over time, gravity pulled matter together to form stars, galaxies, and all the structures we see now.

Finally, a mysterious force called dark energy began to make the universe expand faster again after many billions of years.

Concept history

Main article: History of the Big Bang theory

The term Big Bang became popular in the 1970s. Astronomer Fred Hoyle first used it in 1949 during a BBC Radio talk. He talked about it when comparing it to his own steady-state idea about the universe.

Before this name was used, scientists were studying the universe. In 1912, Vesto Slipher found that many objects called "spiral nebulae" were moving away from Earth. Later, Edwin Hubble showed these were actually faraway galaxies. In 1927, Georges Lemaître suggested the universe was expanding. This meant it started from a tiny point a long time ago. This idea was not always popular, but evidence grew to support it.

Over time, more discoveries helped scientists understand the Big Bang better. In 1964, they found special energy called cosmic background radiation. This matched what the Big Bang theory predicted. New tools and space telescopes have given us more details about how the universe began and how it has changed over billions of years.

Observational evidence

Redshift of absorption lines due to recessional velocity

The Big Bang theory helps explain many things we see in space. It talks about the amounts of light elements, the cosmic microwave background radiation, and how galaxies move. Scientists use these clues to support the idea that the universe started from a very hot, dense state and has been growing ever since.

One important clue is Hubble's law. It shows that galaxies are moving away from us, and how fast they move depends on how far away they are. This helps us understand how the universe is expanding. Another clue is the cosmic microwave background radiation. This is a faint glow found everywhere that matches what we expect from the early universe cooling down after the Big Bang. The amounts of light elements like helium and hydrogen also fit with what the Big Bang model predicts, which gives us more confidence in this theory.

Problems and related issues in physics

The Big Bang theory, like any scientific theory, has some mysteries that scientists are still trying to solve. Some of these mysteries have been partly explained, but others remain puzzling. For example, there are questions about why the universe looks the same in all directions and why it seems to be flat.

One big mystery is why the universe has more matter than antimatter. When the universe was very young and hot, it likely had equal amounts of both. But today, we see almost only matter. Scientists think special conditions allowed a tiny bit more matter to survive, but exactly how this happened is still not fully understood. Another mystery is dark energy, which seems to be making the universe expand faster and faster. We don’t know what dark energy is, but it helps explain why the universe looks the way it does. There’s also dark matter, which we can’t see but which we know must exist because of how galaxies move and how light bends around them. Scientists are working hard to uncover the secrets of dark matter and dark energy.

Misconceptions

The Big Bang theory is often misunderstood. Many people think it explains how the universe began, but it actually describes how the universe grew from a very dense and hot starting point. It does not explain how space, time, and energy first appeared.

Another common mistake is about how fast galaxies move away from us. According to Hubble's law, galaxies beyond a certain distance seem to move away faster than the speed of light. This does not mean they are traveling faster than light; it is a different way to measure their movement in the expanding universe.

Main article: confusion about the nature of cosmic expansion
Main articles: origin of the universe, faster-than-light

Implications

The Big Bang theory helps us understand how the universe began and what might happen far in the future. Scientists can make guesses about the universe's fate, but these are still not certain because we can't see or test conditions from the very beginning.

One big question is what happened before the Big Bang. Some ideas suggest tiny changes in energy might have sparked the universe's beginning. Others imagine the universe existed in a quieter state before expanding suddenly.

In the past, scientists thought the universe might eventually stop expanding and collapse back in on itself, called a Big Crunch. But today, we know the universe's expansion is speeding up, likely due to something called dark energy. This could mean the universe will continue to grow forever, getting colder and darker.