If the Big Bang theory is correct, what exactly happened?

The Big Bang theory describes a mind-blowing event that took place around 14 billion years ago. The observable universe expanded from the size of a minuscule bacterium to the mammoth dimensions of the Milky Way, all within a fraction of a second.

According to currently accepted theory, this early universe was a hot, dense environment. So, how do we know this massive cosmic event actually happened?

Much of what we know comes from evidence collected in the last century. In the late 1920s, American astronomer Edwin Hubble found that galaxies are moving away from each other, leading to the idea that our universe is always expanding.

If we trace this expansion back, we discover a universe that is as old as the oldest astronomical objects we see.

Believers and skeptics of Big Bang theory

Initially, this radical idea was met with skepticism. Interestingly, it was a skeptic, an Englishman named Fred Hoyle, who facetiously dubbed this hypothesis as the “Big Bang” during a BBC radio interview in 1949.

The tune of skepticism began to change when, in 1964, Arno Penzias and Robert Wilson detected cosmic microwave background (CMB) radiation — an afterglow of the Big Bang that filled all space.

This discovery, which earned them the 1978 Nobel Prize in Physics, opened a window into the hot, dense beginning of our universe.

First moments after the Big Bang

Over the years, experiments at particle accelerators like the Large Hadron Collider have shed light on conditions that existed closer to the time of the Big Bang.

Our understanding of physics at these high energies suggests that the four fundamental forces of physics we recognize now were initially unified as a single force.

What are these four forces? We know them as gravity, electromagnetism, and the strong and weak nuclear forces.

As our universe expanded and cooled down, it underwent dramatic changes, known as phase transitions, splitting these forces.

Experiments suggest that a few billionths of a second after the Big Bang, the breakdown of electroweak unification took place.

This was when electromagnetism and the weak nuclear force separated, leading to all matter in the universe acquiring its mass.

Universe’s primordial soup

Our walk through the early universe reveals a world filled with a peculiar substance called quark-gluon plasma.

As its name suggests, this ‘primordial soup’ consisted of quarks and gluons, sub-atomic particles responsible for the strong nuclear force. This plasma didn’t last long, though.

A few millionths of a second post-Big Bang, as the universe expanded and cooled, quarks and gluons clumped together to form protons and neutrons, the building blocks of atoms. This event is referred to as quark confinement.

Big Bang nucleosynthesis

With fewer high energy photons in the universe, the process called Big Bang nucleosynthesis (BBN) kicked in. This was when the first atomic nuclei — dense lumps of matter made from protons and neutrons — formed.

Once, the higher amount of high energy photons would have quickly destroyed any atomic nuclei, but with fewer of them around, BBN could take place.

The fusion reactions stopped just a few minutes after the Big Bang, but their effects are observable to this day.

Big Bang theory and the singularity

Penetrating further into our cosmic past, we reach an intriguing period where there’s no evidence for what existed before the breakdown of electroweak unification.

The energy density at this point was so high that our mathematical equations fumble and become nonsensical. This period is known as a singularity.

In the 1960s, two British theoretical physicists, Stephen Hawking and Roger Penrose, presented a series of mathematical theorems suggesting that the universe’s spacetime must end at a singularity in the past — the Big Bang singularity.

While Penrose received the Nobel Prize in 2020, Hawking, who passed away in 2018, was not eligible for this posthumous honor.

According to these theories, space and time appear at the Big Bang singularity, suggesting there is no ‘before’ the Big Bang. So, in scientific terms, the Big Bang is the dawn of time.

Quantum quandary of Big Bang theory

Despite its accuracy in a vast majority of cases, the theory of general relativity fails to describe atoms, nuclear fusion, or radioactivity. These phenomena are instead explained by quantum theory.

Quantum theory differs from its classical counterparts, such as relativity, by being probabilistic. While classical theories are deterministic and predict a definite outcome from certain initial conditions, quantum theory assigns a probability to multiple possible outcomes.

Gravity is one such area where the classical description falls short, especially when spacetime curvature is extreme, near a singularity.

During these extreme conditions, the quantum nature of gravity surfaces, making spacetime more like the fibers and threads of a carpet seen up close — gnarly, not smooth.

Close to the Big Bang singularity, the structure of spacetime ceases to be smooth. Mathematical theorems suggest that spacetime is overwhelmed by gnarly features, or spacetime foam.

In spacetime foam, causality does not apply, making the question “why did the Big Bang occur?” meaningless. Events do not need a cause to occur.

Universe born out of chaos

To understand the physics at a singularity like the Big Bang, we need a theory explaining the quantum behavior of gravity.

But efforts to formulate one, such as loop quantum gravity and string theory, are still in their infancy, rendering spacetime foam an enigma.

However, the theory of cosmic inflation, introduced by Alexei Starobinsky and Alan Guth, might provide some answers.

Inflation is a period of accelerated expansion in the early universe. As difficult as it may be to comprehend, it explains why our universe is large, uniform, and spatially flat.

“I usually describe inflation as a theory of the ‘bang’ of the Big Bang: It describes the propulsion mechanism that we call the Big Bang,” explained Alan Guth.

What does all of this mean?

To sum it all up, according to a well-accepted theory called the Big Bang, the story of our universe begins with a cataclysmic explosion roughly 14 billion years ago.

This explosion kickstarted a period of explosive expansion, called cosmic inflation. The events before inflation remain a mystery. Is it a spacetime singularity, or is it spacetime foam? The answer is still largely unknown.

We know, or at least what we think we know, is that our universe was born from chaos and has evolved over billions of years into the organized cosmos we see today.

We still have a lot to learn, but we’ve come a very long way in a relatively short period of time.

Rather than beat ourselves up that we still don’t know the “why” part of the question, let’s just enjoy the ride and never stop seeking the answer to the ultimate question: How did it all begin?

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