Question

Explain the big bang theory.

Short answer

The Big Bang Theory describes the origin of the universe as an expansion from a hot, dense state about 13.8 billion years ago.

Step-by-step solution

Understanding the Foundation

The Big Bang Theory is a scientific explanation about the origin of the universe. According to this theory, the universe began as a singular, incredibly hot and dense point roughly 13.8 billion years ago. From this point, the universe started to expand and cool.

The Initial Expansion

The Big Bang itself was not an explosion in the traditional sense, but rather a rapid expansion. As the universe expanded, fundamental particles began to form, eventually leading to the creation of simple atoms, predominantly hydrogen and helium.

Formation of the Cosmic Microwave Background

Around 380,000 years after the initial expansion, the universe cooled enough for electrons to combine with protons and form neutral hydrogen atoms. This recombination allowed light to travel freely, creating what is known as the Cosmic Microwave Background Radiation, a critical piece of evidence for the theory.

Emergence of Structure

Over the next billions of years, slight fluctuations in the density of matter led to the gravitational collapse of gas clouds, forming stars and galaxies. This structured the universe as we observe it today.

Key concepts

Origin of the Universe

The Big Bang Theory is the most widely accepted explanation for the origin of the universe. It describes how the universe started from an incredibly hot and dense singular point. This event is thought to have occurred around 13.8 billion years ago.

At the beginning, all matter and energy were concentrated in this singularity, a term which refers to a point at which gravitational forces cause matter to have infinite density.
As time progressed, this singularity began to expand—this was not an explosion, but rather an expansion of space itself.

This expansion allowed the universe to cool down significantly, paving the way for different forms of matter to begin forming.

• The initial expansion phase was incredibly rapid, sometimes referred to as "inflation."
• After this initial rapid expansion, the rate of expansion slowed down but has continued to occur to the present day.

Cosmic Microwave Background

The Cosmic Microwave Background (CMB) is a key observational evidence for the Big Bang Theory.
It refers to the afterglow radiation left over from the early stages of the universe.
Around 380,000 years after the Big Bang, the universe had expanded and cooled enough to allow electrons to combine with protons, forming neutral hydrogen.
This event is called "recombination."

Before recombination, the universe was a plasma, opaque to radiation, but afterwards, photons were able to travel freely through space.
This transition allowed light to diffuse across the universe, creating the Cosmic Microwave Background.

The CMB fills the entire universe and is almost uniform in all directions.

• The temperature of the CMB is extremely cold, about 2.73 Kelvin, but it is detectable by modern instruments.
• Observations of the CMB provide critical insights into the early conditions and composition of our universe.

Formation of Atoms

The formation of atoms, known as nucleosynthesis, was a pivotal phase following the Big Bang.
As the universe expanded and cooled, fundamental particles such as protons and neutrons began to collide and bind together.
This process allowed the creation of simple atomic nuclei—primarily hydrogen and helium, which are the simplest and most abundant elements in the universe.

• Hydrogen, the simplest atom with one proton, formed first.
• Helium formed through fusion processes whereby two protons and two neutrons combined.

Approximately three minutes after the Big Bang, the universe had cooled enough for these nuclei to exist.
After the event of recombination, these nuclei captured electrons to form the first stable atoms.

This creation of atoms was crucial as it led to the formation of the first simple molecules, and eventually more complex elements in stars.

Expansion of the Universe

The expansion of the universe is a fundamental aspect of the Big Bang Theory.
Initially, the universe expanded at an almost incomprehensibly fast rate—a period known as cosmic inflation.
Following this, the expansion slowed, but it has continued ever since.

One important piece of evidence supporting the continuous expansion is the redshift observed in light from distant galaxies.
As the universe expands, light waves stretching through it become longer and shift to the red end of the spectrum.

• Hubble's Law quantifies this expansion, stating that galaxies move away from us at speeds proportional to their distances.
• This implies that the universe is still expanding even today, with galaxies moving further apart over time.

Recent observations suggest that not only is the universe expanding, but the rate of this expansion is accelerating, likely due to a mysterious force called dark energy. This continuous expansion shapes the universe's size and structure as we observe it now.