10 incredible facts about the Big Bang Theory

If you ask a scientist where the Universe got its start, “the Big Bang” is the answer you’re most likely to get, as for over 60 years now, the scientific evidence that’s come in favoring that theory has overwhelmed all alternatives. Our Universe may be full of stars, galaxies, and a cosmic web of large-scale structure, all separated by the vastness of empty space between them, but it wasn’t born that way. It’s a profound realization that our cosmos hasn’t existed in its current form forever. Instead, the Universe came to be this way because it expanded and cooled from a hot, dense, uniform, matter-and-radiation-filled state with no galaxies, stars, or even simple atoms present at the outset.

Everything, as it exists in its current form, wasn’t the way it is today back some 13.8 billion years ago: at the start of the hot Big Bang. Moreover, all that we know today about our place in cosmic history was figured out only during the past 100-ish years (beginning in 1923). However, even with all of this, there are a whole slew of facts most people — even many scientists — don’t quite get right about the Big Bang.

So, without further ado, here are our top 10 incredible facts about the Big Bang theory!

The results of the 1919 Eddington eclipse expedition showed, conclusively, that the general theory of relativity described the bending of starlight around massive objects, overthrowing the Newtonian picture. This was the first observational confirmation of Einstein’s theory of gravity.

1.) Einstein first dismissed it outright when it was presented to him as a possibility.

Einstein’s general theory of relativity was a revolutionary theory of gravity, proposed in 1915, as a successor to Newton’s theory. It predicted the orbital motion of Mercury to an accuracy Newton’s theory couldn’t, it predicted the bending of starlight by mass confirmed in 1919, and it predicted the existence of gravitational waves, just confirmed a few months ago. But it also predicted that a Universe that was filled uniformly with matter, if it needed to be static, or unchanging over time, would actually be unstable in a gravitational sense.

When the Belgian priest and scientist Georges Lemaître, in 1927, put forth the idea that the spacetime fabric of the Universe could be very large and expanding, having emerged from a smaller, denser, more uniform state in the past, Einstein wrote back to him, “Vos calculs sont corrects, mais votre physique est abominable,” which means “Your calculations are correct, but your physics is abominable!” This Einsteinian dismissal would later be reversed, but only after much more evidence poured in, and after many others came up, independently, with the same ideas that Lemaître put forth in his letter to Einstein.

Edwin Hubble’s original plot of galaxy distances versus redshift (left), establishing the expanding universe, versus a more modern counterpart from approximately 70 years later (right). In agreement with both observation and theory, the universe is expanding.

2.) Hubble’s discovery of the expanding Universe turned it into a serious idea.

Although many scientists considered that the spiral nebulae seen in the night sky actually were distant galaxies all on their own even before Einstein, it was Edwin Hubble’s work in the 1920s that robustly demonstrated that this was true. Moreover, Hubble put together a superior set of observations that further revealed that the more distant a galaxy was from us, the faster it was observed to be receding away from us as well.

This fact — which came to be known as Hubble’s Law, describing the expansion of the Universe — led to a very straightforward interpretation consistent with the Big Bang idea: if the Universe is expanding today, then it stands to reason that it must have been smaller and denser in the past!

This image shows Catholic priest and theoretical cosmologist Georges Lemaître at the Catholic University of Leuven, ca. 1933. Lemaître was among the first to conceptualize the Big Bang as the origin of our Universe within the framework of general relativity, even though he didn’t use that name himself, instead calling it the “primeval atom.”

3.) The idea of the expanding Universe, which the Big Bang is built upon, had been around since 1922, but was widely dismissed for decades.

Soviet Physicist Alexander Friedmann came up with the theory for cosmic expansion in 1922, although this notion, too, was criticized by Einstein. Lemaître’s 1927 work was also dismissed by Einstein, and even after Hubble’s work in 1929, the idea that the Universe was smaller, denser, and more uniform in the past remained only a fringe idea. Hubble provided the observations that would provide the strongest evidence for the expanding Universe, but Hubble himself did not publicly support the theory until much later. (Nor did Einstein, for that matter.)

Critically, however, Lemaître added in to Friedmann’s work the idea that the redshift of galaxies could be explained by this expansion of space, accounting for Hubble’s observations. Lemaître also went a step further than either Friedmann or Hubble and extrapolated that there must have been an initial “moment of creation” at the beginning, assuming that you can extrapolate the expansion so far back that all of matter and even space itself would have come forth from a single point. While Einstein had put forth the concept of a singularity, this initial moment of creation became known as either the “primeval atom” or the “cosmic egg” for decades, long before the term “Big Bang” ever appeared.

There is a large suite of scientific evidence that supports the expanding Universe and the Big Bang. At every moment throughout our cosmic history for the first several billion years, the expansion rate and the total energy density balanced precisely, enabling our Universe to persist and form complex structures. Today, dark energy dominates the Universe, while early on, prior to the onset of the hot Big Bang, a phase of cosmological inflation occurred, preceding it and setting it up.

4.) The Big Bang theory only rose to true prominence in the 1940s, when it was recognized that such an idea would make a startling set of predictions.

George Gamow, an American scientist (who was actually Alexander Friedmann’s PhD student!) swiftly became enamored of Lemaître’s ideas, and expanded upon them. Gamow realized that if the Universe was expanding today, then the wavelength of the light within the Universe must be increasing over time due to cosmic expansion, and therefore the Universe was cooling. But conversely, if the Universe is cooling today, then it must have been not only smaller and denser long ago, but also hotter in the distant past.

Extrapolating backward, he recognized that there once was a time period when it was too hot for neutral atoms to form, and then a period before that when it was too hot for even atomic nuclei to form. Therefore, as the Universe expanded and cooled, it must have formed the light elements and then neutral atoms for the first time, resulting in the existence of a “primeval fireball,” or a cosmic background of cold radiation just a few degrees above absolute zero: what’s known as the cosmic microwave background (or CMB) today.

Fred Hoyle was a regular on BBC radio programs in the 1940s and 1950s, and one of the most influential figures in the field of stellar nucleosynthesis. His role as the Big Bang’s most vocal detractor, even after the critical evidence supporting it had been discovered, is one of his longest-enduring legacies.

5.) The name “Big Bang” came about from the theory’s most fervent detractor, Fred Hoyle.

A theory making a different set of predictions — the Steady-State Theory of the Universe — was actually the leading theory of the Universe in the 1940s, 1950s, and even into the 1960s, favored by most over its main rival: the Big Bang. Whereas Gamow had a scheme that the elements of the periodic table were forged in an early, hot, nucleosynthetic state shortly after the onset of the hot Big Bang, Hoyle claimed, correctly mind you, that the nuclear physics simply wouldn’t work out.

Instead, Hoyle claimed that the vast majority of atoms came from stars that died, and not from this early, hot, dense state. Calculations, and later (in the 1950s), experiments, borne out the fact that nuclear physics supported Hoyle’s position, not Gamow’s. Hoyle, speaking to the BBC, was the one who coined the term “Big Bang” in a 1949 radio interview, saying,

“One [idea] was that the Universe started its life a finite time ago in a single huge explosion, and that the present expansion is a relic of the violence of this explosion. This ‘big bang’ idea seemed to me to be unsatisfactory even before detailed examination showed that it leads to serious difficulties.”

This image shows Arno Penzias and Robert Wilson, co-discoverers of the cosmic microwave background (CMB), with the Holmdel Horn Antenna used to discover it. Although many sources can produce low-energy radiation backgrounds, the properties of the CMB, including its perfectly blackbody nature and uniform temperature in all directions, confirm its cosmic origin. As time goes on and the leftover glow from the Big Bang continues to redshift, larger telescopes sensitive to longer wavelengths and smaller number densities of photons will be required to continue to detect it.

6.) The 1964 discovery of the leftover glow from the Big Bang was initially thought to be from bird poop.

In 1964, scientists Arno Penzias and Bob Wilson, working at the Holmdel Horn Antenna at Bell Labs, discovered a uniform radio signal coming from everywhere in the sky at once. Not realizing it was the Big Bang’s leftover glow, they thought it was a problem with the antenna, and tried to calibrate this “noise” away.

When that didn’t work, they went into the interior of this horn-shaped antenna itself and, to their dismay, discovered nests of pigeons living in there!

Could that be the cause of this suspected noise? They went in with cleaning equipment and physically removed the nests (and cleaned up the droppings) from the pigeons out of the antenna, and only then did they realize they might have found something important, as the signal nevertheless remained. The realization came slowly, after input from other physicists, that what they were seeing was the discovery of Gamow’s long-ago prediction. This vindicated the Big Bang model, entrenching it as the scientific origin of our Universe. It also makes Penzias and Wilson the only Nobel-winning scientists in physics to need to clean up animal excrement as part of their Nobel-worthy research.

The modern cosmic picture of our universe’s history begins not with a singularity that we identify with the Big Bang, but rather with a period of cosmic inflation that stretches the universe to enormous scales, with uniform properties and spatial flatness. The end of inflation signifies the onset of the hot Big Bang, and our Universe has expanded and evolved ever since.

7.) The confirmation of the Big Bang gives us an explicit history and timetable for the formation of stars, galaxies, and rocky planets in the Universe.

If the Universe started off hot, dense, expanding, and importantly, also uniform, then not only would our cosmos cool and lead to the formation of first atomic nuclei and then neutral atoms, but it would take much grander periods of time for gravitation to pull objects together into gravitationally collapsed structures.

The first stars, as of our current understanding would take 30-to-100 million years to form; the first galaxies wouldn’t form until a total of 150-250 million years had elapsed since the Big Bang; Milky Way-sized galaxies might take many hundreds of millions or even a few billion years, and the first rocky planets wouldn’t form until multiple generations of stars lived, burned through their fuel, and died in catastrophic supernovae explosions.

It may not be a coincidence that we’re observing the Universe now, 13.8 billion years after the Big Bang; it might be that the Universe took this long, or close to it, to “ripen” enough to give intelligent life a good enough chance to come into existence!

There is a tremendous scientific story about the Universe that humanity has revealed, from small, subatomic scales up to large, cosmic ones. We can understand this by evaluating the full suite of evidence in light of all we know, but it’s up to us to be honest and scrupulous with ourselves about our own ignorance and limitations.

8.) The fluctuations in the cosmic microwave background tell us how close-to-perfectly uniform the Universe was at the start of the Big Bang.

The leftover glow from the epoch of the hot Big Bang, now known as the cosmic microwave background, has cooled so significantly that it’s just a mere 2.725 K today. However, the fluctuations shown above, which correspond to the imperfections in that leftover glow, are only around ~100 microkelvin in magnitude, or around 0.0001 K.

The fact that the leftover glow from the Big Bang has slight non-uniformities of a particular magnitude at that early time tells us that the Universe was born in an incredibly uniform state: uniform to an impressive sensitivity of just 1-part-in-30,000. However, these fluctuations are incredibly important, from a cosmic perspective. It’s because of these initial imperfections, with cold spots corresponding to initial overdensities and hot spots corresponding to initial underdensities, that all the structure we see in the Universe today — stars, galaxies, etc. — was able to form from these minuscule seeds.

The stars and galaxies we see today didn’t always exist, and the farther back we go, the closer to an apparent singularity the Universe gets, as we go to hotter, denser, and more uniform states. However, there is a limit to that extrapolation, as going all the way back to a singularity creates puzzles we cannot answer.

9.) The Big Bang itself doesn’t correspond to the very beginning of our cosmos, at least, not anymore.

It’s tempting to extrapolate this hot, dense expanding state all the way back to a singularity, as Lemaître did nearly 100 years ago. Unfortunately, that turns out to lead to a slate of problems. There’s a suite of observations — led by the fluctuations in the primeval fireball — that teach us there was a different state prior to the early moments of the hot Big Bang. That state wasn’t a singularity, but rather was one where all the energy in the Universe was inherent to space itself, leading that space to expand in an exponential fashion: doubling in size again and again, relentlessly, with each moment that elapsed.

This period was known as cosmic inflation, and although we’re still researching the details on that, we’re confident that this inflationary state predated and set up the conditions for the hot Big Bang. Science has progressed our understanding further and further back in cosmic history, but so far, there’s no end in sight to where the story actually, truly began.

Possible fates of the expanding Universe. Notice the differences between models in the past; only a Universe with dark energy matches our observations, and the dark energy-dominated solution came from de Sitter all the way back in 1917. By observing the expansion rate today and measuring the components present in the Universe, we can determine both its future and past histories.

10.) Most importantly, knowing the way the Universe began doesn’t reveal the answer as to how the Universe will end.

Finally, the Big Bang tells us there was a race between gravity, trying to recollapse the expanding Universe, and the initial expansion, trying to drive everything apart. But the Big Bang on its own doesn’t tell us what the fate will be; that takes knowing what the entire Universe is made out of in terms of all the different components that make it up, as well as knowing how those individual components evolve.

With the existence of dark energy, discovered only in 1998, we’ve learned that not only will the expansion win, but that the most distant galaxies will continue to speed up in their recession from us. Although many have argued that dark energy may be weakening in its evolution, we lack sufficient data to draw a robust conclusion. Our cold, lonely, empty fate is what we get in a dark energy Universe, but if the Universe were born with just a tiny bit more matter or radiation than what we have today, our fate could’ve been very different!

This article was first published in July of 2022. It was updated in September of 2025.