Busting the top 5 myths about the Big Bang

The Universe we know today, filled with stars and galaxies across the great cosmic abyss, hasn’t been around forever. Despite the fact that there are several trillions of galaxies visible to us, spanning distances of tens of billions of light-years, there’s a limit to how far away we can look. Even here in the JWST era, the most distant galaxy we’ve ever seen is located an impressive 34 billion light-years away, with its light corresponding to a time where the Universe was a mere 280 million years old: just 2% of its current age. Why can’t we see farther than that? The reason isn’t because the Universe is finite — in fact, it may well be infinite after all — but rather because it had a beginning that occurred a finite amount of time ago: the Big Bang.

The fact that we can:

• look at our Universe today,
• see it expanding and cooling,
• and infer our cosmic origins from what we observe,

is one of the most profound scientific achievements of the 20th century. The Universe began from a hot, dense, matter-and-radiation filled state some 13.8 billion years ago, and has been expanding, cooling, and gravitating ever since. But the Big Bang itself doesn’t work the way most people think. Here are the top 5 myths that people believe about the Big Bang, along with the science behind what’s actually true instead.

This rare color photo shows the first nuclear explosion at the Trinity Site in New Mexico on July 16, 1945. This nuclear detonation ushered humanity into the Atomic Age, with the fireball rising 200 meters into the air a mere 16 milliseconds after detonation. If it weren’t for the presence of the ground or the heat-induced airflow of the mushroom cloud, the explosion would appear as a nearly perfectly-symmetric sphere.

1.) The Big Bang is the explosion that began our Universe. Every time we look out at a distant galaxy in the Universe and try to measure what its light is doing, we see the same pattern emerge: the farther away the galaxy is, the more significantly its light is systematically shifted to longer and longer wavelengths. This redshift that we observe for these objects follows a predictable pattern, with double the distance meaning that the light’s wavelength will be shifted by twice as great an amount. The farther away we look, the greater an object’s redshift, indicating that it appears to be receding from us at a much faster speed.

This is what’s encoded in Hubble’s Law (or the Hubble-Lemaître Law): a relationship between the inferred recession speed of a distant object and its measured distance from us, with the Hubble constant indicating the rate of recession as a function of distance. Just as a police car speeding away from you will sound lower-pitched the faster it moves away from you, the greater we measure an object’s distance to be from us, the greater the measured redshift of its light will be. It makes a lot of sense, then, to think that the more distant objects are moving away from us at faster speeds, and that we could trace every galaxy we see today back to a single point in the past: an enormous explosion.

The ‘raisin bread’ model of the expanding Universe, where relative distances increase as the space (dough) expands. The farther away any two raisins are from one another, the greater the observed redshift will be by the time the light is received. The redshift-distance relation predicted by the expanding Universe is borne out in observations, but different methods of measuring the cosmic expansion yield different, incompatible results.

But this is a total misconception about what the Big Bang actually is. It isn’t that these galaxies are moving through the Universe itself, but rather that the fabric of space that makes up the Universe itself is expanding. Just as raisins appear to recede in proportion to their distance in a leavening ball of dough, the galaxies appear to recede from one another as the Universe expands. The raisins aren’t in motion relative to the dough; the action of the expanding dough itself simply appears to drive them apart. In our Universe, the galaxies (or galaxy groups/clusters) caught up in the expansion of the Universe behave as the raisins do, while the dough itself, if it were transparent and invisible, would correspond to our expanding fabric of space.

In other words, the idea of an “initial explosion” has nothing to do with the Big Bang at all. It wasn’t an initial explosion that caused galaxies to move away from one another, but rather the physics of the expanding Universe as governed by Einstein’s general relativity. It’s the fact that we have a Universe that obeys general relativity as our law of gravity, and that our Universe is uniformly filled with energy (at least, on the large-scale average), that causes space (with galaxies contained within it) to expand. There was no explosion, just a rapid expansion that has been evolving based on the cumulative gravitational effects of everything contained within our Universe.

Artist’s logarithmic scale conception of the observable universe. Note that we’re limited in how far we can see back by the amount of time that’s occurred since the hot Big Bang: 13.8 billion years, or (including the expansion of the Universe) 46 billion light years. Anyone living in our Universe, at any location, would see almost exactly the same thing from their vantage point.

2.) There is a point in space that we can trace the Big Bang ‘event’ back to. Similarly, there’s no “center point” to the event of the Big Bang. You might initially think that if everything appears to be expanding away from everything else, then we can extrapolate everything back to when they all originated at the same location. Just as a grenade has a central location from where all the shrapnel must have originated, it makes sense to think the Universe must have had a similar point of origin. If we trace the motions that we see of all the galaxies back to a “center point,” you might wonder where that is, and whether we’re at (or close to) that point itself.

But even asking this question relies on a misconception: a misconception that the Big Bang was like an explosion. As we’ve just covered, the Universe didn’t explode; it simply expanded. In an expanding Universe, every location in space looks the same, at least, whenever you consider a large-enough volume of it. On the large-scale average, the Universe appears to have the same density, the same temperature, and the same number of galaxies everywhere. And if you extrapolate it back in time, it will appear hotter and denser, but that’s because space itself is evolving and expanding, too.

The observable Universe might extend for 46 billion light-years in all directions from our point of view, but that’s not unique to our vantage point; all observers at all locations would experience the same thing. There’s certainly more, unobservable Universe just like ours beyond the limits of what we can see. It’s unfair to associate any particular point with the center, as what we perceive is determined by the amount of time that’s passed since the light observed today was emitted, rather than the geometry of the Universe.

When we extrapolate the Universe backward in time, we can calculate that it must have been smaller and denser in the past, and by how much. But that doesn’t merely apply to our location in space and the part of the Universe we can observe; it applies to all of space, for all observers, and at all times, including for observers that live at or even beyond the edge of the portion of the Universe that we, ourselves, can observe. Every single observer at every point has equal claim to being at the center, just as every region of space has the same large-scale properties as every other similarly sized region of space.

The Big Bang didn’t happen at one single point, but rather occurred everywhere at once, and did so a finite amount of time ago. When we look back at the more distant regions in the Universe, we are looking back in time, and so is every other observer from every other perspective the Universe offers. The fact that the Universe has no repeating structures, shows no identifiable edge, and has no preferred direction all offer evidence that there is no specific origin point for the Big Bang: it happened everywhere at once, with no preferred central location at all. In fact, to the best of our knowledge, the Universe is indistinguishable from an infinite Universe; our limits are purely based on the information we’re capable of seeing.

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.

3.) All of the matter and energy in our Universe was compressed into an infinitely hot, dense state at the Big Bang. If the Universe is expanding and cooling today, then it must have been smaller, denser, and hotter in the past. This aspect of cosmology is true; your intuition hasn’t led you astray. You can imagine, in fact, going all the way back, as far as your imagination can take you, until you’ve achieved a size that gets infinitesimally small, leading to arbitrarily high densities and infinite temperatures. It makes you think about the notion of a singularity: where all the matter and energy of the cosmos was compressed into a single point. Perhaps, you’d think, that was the “instant” of the Big Bang: an infinitely hot, dense state.

That’s not completely wrong, but it’s an old, outdated notion of the Big Bang. Importantly, it’s a hypothesis, and just one idea about how our Universe could have ultimately originated. Although it might not seem like it, we have a few ways to test that hypothesis out!

First off, the temperature fluctuations that we see today, left over in the cosmic microwave background, would have fluctuations that were as large as the maximum temperature compared to the Planck energy scale. Those fluctuations would appear only up to the scale of the cosmic horizon (and smaller). And there ought to be even left-over relics that only appear at high energies, like magnetic monopoles, filling our Universe. All three of these are would-be predictions of the Big Bang originating with a singularity in this fashion.

COBE, the first CMB satellite, measured fluctuations to scales of 7º only. WMAP was able to measure resolutions down to 0.3° in five different frequency bands, with Planck measuring all the way down to just 5 arcminutes (0.07°) in nine different frequency bands in total. All of these space-based observatories detected the cosmic microwave background, confirming it was not an atmospheric phenomenon, and that it had a cosmic origin. The magnitude of the fluctuations within it are 1-part-in-30,000 only; not greater.

The way to test this is to look at the leftover glow from the Big Bang, today’s cosmic microwave background, and to see what critical information gets encoded into its patterns of fluctuations. In the 1990s, 2000s, and 2010s, respectively, humanity received our major results from the COBE, WMAP, and Planck missions. They probed the fluctuations in the leftover glow from the Big Bang: the cosmic microwave background, and helped look for these exact signatures. What they found, along with other experiments (like direct searches for magnetic monopoles), demonstrated that the Universe never reached temperatures that were greater than ~0.03% of the Planck energy scale.

The temperature fluctuations are only 1-part-in-30,000: thousands of times smaller than an infinitely hot state predicts. Fluctuations appear on scales larger than the cosmic horizon (so-called super-horizon fluctuations), robustly measured by both WMAP and Planck, and appearing especially strong in their polarization data. And the constraints on magnetic monopoles and other ultra-high-energy relics strongly disfavor an ultra-high-energy past to our Universe. The conclusion? The Universe had a temperature cutoff in its past, never rising above a critical threshold during even the hottest stages of the hot Big Bang.

As a balloon inflates, any coins glued to its surface will appear to recede away from one another, with “more distant” coins receding more rapidly than the less distant ones. Any light will redshift, as its wavelength ‘stretches’ to longer values as the balloon’s fabric expands. This visualization solidly explains cosmological redshift within the context of the expanding Universe. If the Universe is expanding today, that implies a past where it was smaller, hotter, denser, and more uniform: leading to the picture of the hot Big Bang. If you extrapolate it as far as possible, you wind up with infinite temperatures and densities a finite amount of time ago: the conditions needed for a singularity.

4.) The Big Bang makes it inevitable that our Universe began from a singularity. Even if the Universe reached a maximum temperature in the early stages of the hot Big Bang, there still needed to be a phase that preceded and set up that hot phase. In order to be consistent with what we observe, it must have:

• stretched the Universe so that it would be indistinguishable from flat,
• created quantum fluctuations that get stretched across the Universe, including to super-horizon scales,
• where the fluctuations were also low in magnitude: that 1-part-in-30,000 we mentioned earlier,
• where the fluctuations had a constant entropy (i.e., were adiabatic),
• and then created a hot, dense state full of particles and antiparticles that equates to our hot Big Bang.

The theory that sets up all of these initial conditions for the Big Bang is known as cosmic inflation, and has been verified by multiple lines of evidence. The Big Bang might describe where our Universe comes from, at least, as we know it, but it doesn’t describe the first scientific thing we can say about our Universe at all.

Blue and red lines represent a “traditional” Big Bang scenario, where everything starts at time t=0, including spacetime itself. But in an inflationary scenario (yellow), we never reach a singularity, where space goes to a singular state; instead, it can only get arbitrarily small in the past, while time continues to go backward forever. Only the last minuscule fraction of a second, from the end of inflation, imprints itself on our observable Universe today. The size of the now-observable Universe could’ve been no smaller than about 1 cubic meter in volume at the start of the hot Big Bang.

However, the existence of a singularity, once you accept a period of cosmic inflation that set up and preceded the Big Bang, is no longer inevitable. In fact, accepting cosmic inflation necessarily brings along this also-spectacular realization: if inflation precedes the Big Bang, then it won’t lead to a Universe that reaches an infinitesimal size at a finite point in the past. The Universe expands exponentially during inflation, which means that it will double in size on a certain timescale if you run the clock forward, but will only halve and halve in size on that same timescale if you go backward. No matter how many “halves” you take, you never reach zero.

It’s still possible, and perhaps even likely, that there was a separate phase that existed before cosmic inflation took place. If that were the case, then we have to admit the possibility that perhaps the Universe did begin from a singularity after all. Unfortunately, we have no way to observe, measure, or test that hypothesis with the observable Universe that we’re limited to. We can only state, based on the observational evidence we have, that inflation lasted at least some tiny fraction of a second, didn’t lead to a singularity itself or at the start of the hot Big Bang, and that we do not know what came before inflation began.

The far distant fates of the Universe offer a number of possibilities, but if dark energy is truly a constant, as the data best indicates, it will continue to follow the red curve, leading to the long-term scenario frequently described on Starts With A Bang: of the eventual heat death of the Universe. If dark energy can strengthen, weaken, or reverse sign over time, however, all bets are off, and alternative possibilities, like a big crunch or a big rip, suddenly abound.

5.) Space, time, and the laws of physics did not exist prior to the Big Bang. If you had reached a true singularity, or a place where you reached infinite densities and temperatures, the laws of physics would break down. In general relativity, singularities are where spacetime can either enter or exit existence, and without spacetime, there are not even necessarily rules that govern the physical Universe that could exist within it.

But we don’t reach a singularity when we speak about even the earliest stages of the hot Big Bang, nor do we reach a singularity when we speak about the period of cosmic inflation that preceded it. In fact, we remain well below the Planck scale during all of those phases, and hence the laws of physics do not break down. That means that the laws that we know must certainly have existed during the inflationary phase that set up the Big Bang itself, and that both space and time must have existed as well. With the knowledge we have of inflation, and the observational confirmation of its predictions, however, new questions arise. These include:

1. Was the inflationary state a constant one, or did its properties change while it was ongoing?
2. Did inflation last for an infinite amount of time, eternal to the past, or only a finite amount of time, and if so, for how long?
3. Is inflation connected to the dark energy that dominates the Universe today, as both cause the Universe to expand at an exponential rate?

This diagram shows, to scale, how spacetime evolves/expands in equal time increments if your Universe is dominated by matter, radiation, or the energy inherent to space itself (i.e., during inflation or dark energy dominance). The bottom-most scenario corresponds to exponential expansion via both dark energy (today) and inflation (at early times). Note that visualizing the expansion as either ‘the existing space stretching’ or ‘the creation of new space’ won’t suffice in all instances.

The truth of the matter is that we don’t know the answer to any of these questions for certain. After all, it’s only the final fraction-of-a-second of inflation that leaves even in-principle observable imprints on our Universe. Anything that occurred before that moment, or those final ~10^-32 seconds of inflation, has had its observable signatures literally inflated away. Even theoretical attempts to argue about the complete/incomplete nature of inflationary spacetimes aren’t concrete; it’s possible that inflation didn’t last forever, and had a singular beginning, but it’s also possible that it either endured eternally or even had a cyclic character, with space and time looping back on itself eventually.

Thousands of years ago, philosophers considered three main possibilities for how time began:

• time has always existed,
• time began at an event that occurred a finite duration ago in the past,
• or time itself is cyclical in nature.

Even with all we’ve learned about the Big Bang and what preceded it and set it up, it’s impossible to draw a robust conclusion that answers the question of those ancient philosophers. We don’t have sufficient information in our observable Universe to know whether time is finite or infinite; whether it’s cyclical or linear. But even before the Big Bang, we can be certain that space, time, and the laws of physics themselves definitely existed, even if we can’t extrapolate those same answers back to the beginning of inflation or whatever came before it.

Even without those answers, however, you’ve now conquered what are perhaps the five most common myths and misconceptions about the Big Bang. Now you know the best scientific truths that modern cosmology has to offer.

This article was first published in February of 2020. It was updated in 2025.