Many textbooks and science educators have attempted to describe the Big Bang as the birth of the universe — an explosive start that happened at a specific point creating matter and flinging it into the void like shrapnel from a grenade.
But the Big Bang is not really the moment of creation — more like its aftermath. The Big Bang didn't emerge from a particular location in space, and it wasn't an explosion — at least not in the traditional sense.
Popular culture — and cosmologists, begrudgingly — made the unfortunate mistake of adopting a name for the theory that even evokes the sound of a gunpowder blast. So… bazinga?
"It’s often said that the whole universe we can now observe was once compressed into a volume the size of a golf ball," wrote John Mather, a Nobel Prize-winning astrophysicist and senior project scientist for NASA's James Webb Space Telescope, in an essay for Theedge.org. "But we should imagine that the golf ball is only a tiny piece of a universe that was infinite even then."
The Big Bang Theory describes an event when existing space — much hotter, denser, and smaller at the time — suddenly and rapidly started stretching out. The primitive universe was a scalding goulash of tiny particles, light, and energy, but as it expanded, space cooled enough to allow important processes to occur, such as forming atoms and elements. The expansion continues today.
That's it. It doesn't suggest what the conditions were before expansion. It doesn't suppose what the universe is expanding into. It doesn't even explain what caused the expansion in the first place. And there are reasons why trying to imagine the event as an explosion can lead to some misinformed conclusions.
"No reputable scientist will claim that we understand in detail what happened at the exact moment when the universe began. We just don't," said Don Lincoln, senior scientist at Fermilab in Illinois, in a video. "In spite of the fact that we don't know everything about how the universe began, I'm constantly staggered by the fact that we know so much."
The Big Bang pertains to the visible universe
To understand the Big Bang — and Mather's previous comment — it's first important to clarify that this theory applies to the visible universe, not the universe as a whole. The visible universe is a bubble of the cosmos centered on our perspective from Earth, with a radius determined by the speed of light. The entire bubble is about 92 billion light-years wide.
The bubble's size is not determined by the range of telescopes, but the literal limitation of light. There is a maximum distance from which photons could have traveled to an observer in the age of the universe. This boundary is known as the cosmic light horizon: Any potential signals beyond it haven't had time to reach us — and they never will, not even billions of years into the future. That's because at a certain extreme distance, far-flung objects recede faster than the speed of light.
So what's beyond this bubble? No one knows because it's unseeable, but scientists could speculate there's more universe. After all, with the expansion of space, scientists are aware that, every second, thousands of stars are escaping our view, beyond that horizon.
Where exactly did the Big Bang happen?
The Big Bang should be thought of as a "point" in time but not happening at a particular place. Astronomers will often say that the Big Bang happened everywhere, which is a confounding idea if you've been thinking of the Big Bang like a detonating bomb.
Imagine instead a hypothetical scenario where space was condensed within a speck, like a pinhead-sized balloon. Then imagine that this tiny balloon somehow inflated into the size of an orange. In this analogy, you can begin to understand why there is no "origin point" for the Big Bang: Nothing left the pinhead where it began; the pinhead point got exponentially bigger.
This is one of the reasons why many astrophysicists say everywhere in the knowable universe could be considered part of the Big Bang's center. There was no particular site from which bits were blown away, according to the theory.
The Big Bang wasn't really an explosion
Scientific observations support the idea of rapid universal expansion versus an explosion. If there had been a firecracker-type blast that scattered matter outward, for example, the laws of physics would dictate that debris farther from that place where it exploded would be moving faster than the stuff closer to that starting point.
"That's because objects far away from the firecracker have to be moving faster. That's how they got far away," Lincoln said.
But that is not what astronomers see. In the cosmos, the space between galaxies is increasing, in all directions — not just relative to a central spot. Astronomer Edwin Hubble, for whom the Hubble Space Telescope was later named, discovered this in 1929.
Using the 100-inch Hooker Telescope in California, Hubble noted that the farther a galaxy was from the Milky Way, Earth's home galaxy, the faster it seemed to be receding. He figured this out by plotting 24 nearby galaxies' velocities and distances. The plot showed that everything was drifting uniformly, at speeds proportional to distance, in all directions.
The rate of expansion has been dubbed the Hubble Constant. Two years after Hubble's observations, a Belgian astronomer and priest, Georges Lemaître, used this premise to publish the first Big Bang-like theory to explain the beginnings of the universe.
How astronomers know the universe is expanding
With Hubble's finding that space itself is expanding, scientists have been able to estimate the age of the universe. The formula for velocity — which you might have learned in high school — is distance divided by time. Scientists already know the speeds of galaxies and their distances, so they can figure out the duration by dividing distance by speed.
If scientists rewind the clock from the present day to the time that everything in the knowable universe crumples back into that small deflated balloon, it occurred about 13.8 billion years ago.
So, if the universe is 13.8 billion years old, one might incorrectly assume that the visible bubble of the universe has a radius of 13.8 billion light-years, with an overall width of 27.6 billion light-years. But the universe isn't standing still, and the distance between objects isn't fixed. The expansion of space explains the discrepancy between 27.6 billion light-years and 92 billion light-years, the diameter of the visible universe.
Have scientists disproved the Big Bang?
Scientists have not disproved the Big Bang Theory, but they have discovered disagreements in the rate of expansion — the Hubble Constant — from different research teams' measurements. The disagreement is known as the Hubble tension.
In short, speed measurements based on telescope observations of the present universe are somewhat higher than projections based on known conditions of the universe during its infancy. For the past few years, astronomers have considered that something is causing the expansion rate to speed up. Studies using the Webb telescope have found that the small-but-significant divergence in the expansion rate is probably not the result of miscalculations but an aspect of the universe that is not yet understood.
As scientists work to solve this mystery, the Big Bang might need some tweaking, but so far this disparity has not upended the bottom line, which is that space was once smaller and hotter, then it suddenly stretched out, and it's still expanding.
The expansion rate of the early universe
Researchers have calculated the expansion rate of the baby universe using data from the so-called Cosmic Microwave Background. U.S. physicists Arno Penzias and Robert Wilson accidentally discovered this phenomenon, a faint afterglow from 380,000 years after the Big Bang, using a radio telescope in 1965.
Around the same time, a separate team at Princeton University had predicted that such waves should exist. If astronomers were archaeologists, this discovery would be akin to finding the earliest fossil of light. It is the oldest thing in the universe anyone has seen.
This heat signature, radiating from atoms that are now more than 46 billion light-years away and stretched into microwaves, fills the sky. The European Space Agency’s Planck mission mapped the microwaves to measure teensy fluctuations in temperature. These slight variations allow scientists to infer the expansion rate at the time.
How 'cosmic inflation' theory fits into the Big Bang
Cosmic inflation tries to describe one brief but crucial phase in the Big Bang narrative that launched the universe onto its expansion timeline.
Alan Guth, a theoretical physicist at MIT, put forward the idea in 1980. It suggests that some repulsive form of gravity, something like dark energy, drove the universe's rapid expansion for an early instant. This phase would have lasted for a fraction of a trillionth of a second. Then, the energy that propelled inflation turned off.
"I usually describe inflation as a theory of the 'bang' of the Big Bang," Guth said in a 2014 Q&A by the university. "In its original form, the Big Bang theory never was a theory of the bang. It said nothing about what banged, why it banged, or what happened before it banged."
During the inflation phase, the tiny universe would have expanded at a rate faster than light. And get this: It wouldn't have broken any laws of physics.
"It's true that nothing can move through space faster than light, but there are no restrictions on how fast space can expand," Lincoln said.
How the 'Big Bang' got its name
Fred Hoyle, an astronomer and well-known science communicator in the United Kingdom, is largely credited with coining the "big bang" in 1949. He was in many ways the Neil deGrasse Tyson of his time. But today many astrophysicists and cosmologists lament that the misnomer stuck.
During a BBC broadcast, Hoyle described theories based on the idea that "all the matter in the universe was created in one big bang at a particular time in the remote past," according to a transcript published in a BBC magazine. He later mentioned the phrase again in his 1950 book "The Nature of the Universe."
Hoyle balked at the idea of a sudden origin of the universe, but he didn't use the words "big bang" disparagingly, according to a recent essay about it in the journal Nature. Instead, he meant to convey the hypothesis with descriptive metaphors to help get the point across over radio.
Bazinga, indeed.
