What is the Big Bang? Simply put, it’s our modern understanding of the history and evolution of the universe. It is not, however, a theory of the creation of our cosmos, because we do not understand that event—yet.
That said, the Big Bang tells us that the entire observable universe, including every atom, every star, and every galaxy in a span over 90 billion light years across, was once compressed into a volume no bigger than a peach.
A very hot peach.
In the Beginning
For centuries, philosophers and scientists believed that the universe was static. Sure, planets and even stars may move around and occasionally blow up, but at the very largest scales, the universe is and always will be.
This view was so entrenched that it even fooled Einstein. In the early 1900s, he applied his new formulation of gravity, called general relativity, to the evolution of the universe as a whole. He found that his theory naturally predicted a dynamic, evolving cosmos, one that was either expanding or contracting—but definitely not static. To fix this, he added a fudge factor to his equations, known as the “cosmological constant.”
Several years later, the astronomer Edwin Hubble would announce a stunning one-two punch of cosmological proportions. First, he discovered that galaxies exist and are very far away from us (our nearest neighbor, the Andromeda galaxy, sits over 2.5 million light years away). Then, he found that, on average, all galaxies are moving away from us.
Astronomers put forth many potential explanations for this apparent motion—that light itself would get “tired” as it traveled, or that Hubble had miscalculated the distances to the galaxies. But further evidence would put those ideas to rest. Not only were all galaxies receding away from us, but they were receding away from each other. The spaces between galaxies were growing; we live in an expanding universe.
In other words, our universe was evolving exactly as Einstein’s equations had predicted—if only he had trusted them.
Putting on a Show
A Belgian Catholic priest and astronomer, Georges Lemaître, first proposed what we now call the “Big Bang theory” even before Hubble’s observations. Lemaître argued that the entire cosmos was once compressed into a “primaeval atom” (his words) that then exploded and expanded, resulting in our modern-day universe.
Suspicious that Catholic teachings might be leaking into hard physics, scientists initially dismissed the idea. But with Hubble’s observations and Einstein’s math, the idea gained momentum. Still, the Big Bang theory wouldn’t reign supreme until the 1950s.
The fundamental idea behind the theory is that our universe is evolving and changing; it was different in the past, and it will be different in the future. This was a radical departure from every cosmological model up until that point, but at least it made testing the idea relatively easy.
If our universe was smaller in the past, then it should have also had higher density and higher temperatures. At some point long ago, all the material in the universe would have been crammed into a small enough volume that its temperature and density would turn it into a plasma, a state of matter in which electrons separate from atoms. But as the universe expanded and cooled, it would then convert from that state into a neutral gas, releasing a flood of radiation that would persist to the present day, soaking the sky.
In 1964, two radio engineers, Arno Penzias and Robert Wilson, were testing a new microwave receiver for Bell Labs. They spent over a year trying to remove a stubborn background hiss in their signal. It turns out that they had accidentally discovered the radiation left over from when our cosmos cooled down from its plasma state: the cosmic microwave background.
A Modern Universe
Half a century later, the Big Bang model remains the only theory capable of explaining our wealth of cosmological data. Cosmologists (a new branch of science developed in tandem with Hubble’s discoveries) have found that the Big Bang can explain the expansion of the universe, the appearance of the cosmic microwave background, the abundance of light elements, the formation of structures like galaxies, and much more.
In our modern picture, the universe is approximately 13.77 billion years old, and the observable portion of that universe is approximately 90 billion light years across. We do not have the necessary knowledge of physics to understand the extreme conditions of the earliest moments of the cosmos, especially when it comes to figuring out how the universe came into existence in the first place. But after that, we have a relatively decent handle on the timeline:
- Within the first fraction of a second, our cosmos underwent a period of extremely rapid expansion known as inflation. This event would lay down the gravitational seeds that would later turn into clumps of matter like stars and galaxies.
- Within the first one or two dozen minutes, all the hydrogen and helium in the cosmos coalesced into a soup of fundamental particles.
- At a ripe old age of 380,000 years, the plasma cooled and released the cosmic microwave background.
- Stars and galaxies first ignited in the “cosmic dawn” a few hundred million years later.
- Galaxies grouped together to form clusters, filaments, and walls in a pattern known as the cosmic web, which we see in the present day.
Left in the Dark
The modern formulation of the Big Bang picture is known as the ΛCDM model (pronounced lambda CDM), and that acronym essentially stands for “we have a lot more to learn.”
Even though we know that the general Big Bang picture is correct, based on the wealth of evidence, modern-day cosmologists are busy trying to fill in many details. For example, “CDM” stands for “cold dark matter,” which is some form of matter that accounts for 80 percent of the mass of every galaxy; yet, it does not interact with light. We do not yet understand what the dark matter is made of, but we know it’s out there, based on its gravitational influence.
The Greek letter Λ (lambda) goes all the way back to Einstein’s attempts to stabilize the universe. In the late 1990s, two teams of astronomers were trying to measure the deceleration of the expansion of the universe, which would be caused by the gravitational attraction of all the matter in it. Instead, they found that the expansion is accelerating—our universe is getting bigger and bigger, faster and faster every day. The easiest way to model this acceleration is with Einstein’s mistake, the cosmological constant, which is denoted by Λ. Today, we call it dark energy.
Current measurements estimate that all matter (both normal and dark) makes up only 32 percent of all the contents of the universe, with the rest being dark energy. That mysterious force switched on about five billion years ago, and it’s currently in the processes of ripping apart the cosmic web.
There’s still plenty of work to be done: understanding dark matter and dark energy, figuring out the process of structure formation, peering into the earliest moments of the Big Bang, and more. But a century of work on the Big Bang model has provided a rich, compelling, evidence-based story of the history of our universe, one in which our cosmos was smaller and hotter in the past, and continues to expand into the future.
Paul M. Sutter is a science educator and a theoretical cosmologist at the Institute for Advanced Computational Science at Stony Brook University and the author of How to Die in Space: A Journey Through Dangerous Astrophysical Phenomena and Your Place in the Universe: Understanding Our Big, Messy Existence. Sutter is also the host of various science programs, and he’s on social media. Check out his Ask a Spaceman podcast and his YouTube page.
