Our growing knowledge of physical laws has allowed us to rewind the tape on the universe, tracing its evolution back to within a fraction of a second after the Big Bang. Here, however, when the sum total of matter and energy coalesces in a ball of infinite density and temperature, the equations of general relativity break down.

As a theory, “the Big Bang leaves out the bang,” physicist Brian Green writes in The Fabric of the Cosmos. Whatever happened in that instant, let alone before that moment, is anyone’s (well-reasoned) guess — and there is no shortage of guesses of how the universe began.

What Happened Before the Big Bang?

First, a caveat: Many experts argue that the word “before” misses the mark. It assumes there was some pre-existing time separate from the universe, when really, time and space may have emerged out of the universe.

In this view, the question — “what came before the big bang?” — is literally meaningless. Stephen Law, an Oxford philosopher, has suggested in interviews that what we need is not an answer to these kinds of questions, but “a kind of therapy, an explanation that will make us realize why it’s time to stop asking the question.”

The idea pushes human language and intuition to their breaking point, but we can try to make sense of it with a favorite analogy of the late physicist Stephen Hawking: Wondering what happened before the big bang is like wondering what’s south of the South Pole. It’s not even accurate to say there is nothing farther south; the point is that the question itself is nonsensical. We’re trying to pin down something that simply doesn’t exist.

How Did the Universe Begin?

That response may seem intellectually unsatisfying. Surely the universe came from something. How could all this bewildering beauty and complexity have its origin in … nothing?

One solution, dating back to Aristotle, is that there was no origin for motion in the universe — it has always existed. Newton, Einstein and others of their caliber believed the cosmos to be eternal and static, until the astronomer Edwin Hubble discovered in the 1920s that all galaxies are expanding away from each other. That implied a starting point, the famed “singularity.”

So, whence the singularity? Scoffing at Aristotle’s need for a “first cause,” some physicists today respond that our notions of causality are irrelevant in the extreme conditions of the Big Bang. We must take a closer look at the scientific theories of how the universe began.

Read More: Could the Big Bang Be Wrong?

A Quantum Mechanics Solution

Quantum mechanics has shown that even seemingly empty space is filled with fluctuating virtual particles which, through a process known as tunneling, may be able to generate matter. We only see such behavior at ultra-small scale, but back then the universe was the right size.

One prominent advocate of this perspective is Alexander Vilenkin, a cosmologist at Tufts University. In a 1982 paper, written for an audience of professional physicists, he conceded that “the concept of the universe being created from nothing is a crazy one.”

Nevertheless, he argued that the laws of physics alone could have given rise to all we see around us. (Physicists have even considered whether it’s possible to create a universe in a lab.)

As MIT physicist Alan Lightman has described it, “the entire universe could have ‘suddenly’ appeared from wherever things originate in the impossible-to-fathom haze of quantum probabilities.”

Still, you might suspect that this “nothing,” if it was compatible with the creation of reality as we know it, was “something” after all.

A Universe From Nothing

David Albert, a philosopher at Columbia University, has argued exactly that: “If what we formerly took for nothing turns out, on closer examination, to have the makings of protons and neutrons and tables and chairs and planets and solar systems and galaxies and universes in it, then it wasn’t nothing, and it couldn’t have been nothing, in the first place.”

When it comes to understanding the concept of a universe from nothing, there’s an important difference between philosophical nothing and physical nothing. Namely, the latter still includes the laws of nature required for cosmic genesis.

Even granting Albert’s point, though, we’re merely kicking the can down the road. Whatever our universe came from, that too must have come from something else (at least according to the commonsense expectations of feeble human brains).

In other words, it’s “turtles all the way down,” as some academics put it. So, tabling that issue, let’s delve into some theories slightly less mired in philosophy.

The Multiverse and Eternal Inflation

When we try to imagine the Big Bang, the best we can do is to envision an event of extraordinary force and grandeur, the fireworks show to end them all — or, start them all. But what if, from the perspective of an even vaster cosmological landscape, it was just another Tuesday?

For example, we could be the offspring of a larger proto-universe, which is continuously spawning new ones. This concept, known as eternal inflation, was developed in the 1980s, primarily by the physicists Alan Guth, Andrei Lind and Paul Steinhardt.

Under the right conditions, they believe, quantum fluctuations can spark the outrageously fast expansion of  “pocket universes.” That process could continue indefinitely, leading to a potentially infinite multiverse. Despite the theory’s name, however, inflation can only be eternal in the future, not in the past — how it began remains a mystery.

Cyclic Universe Theory

It’s also possible the Big Bang was not the start of our universe, but rather a transition from some earlier state.

It could be that the cosmos cycles infinitely, each phase ending where the next begins, making the interval between the two more of a bounce than a bang. The cyclic universe theory supports the eternal universe idea, with all its comforting logic (that is, they don’t try to get something from nothing), while still accounting for cosmic evolution.

According to one version of this story, the ekpyrotic model, our universe began in a collision between two “branes” — unconfirmed theoretical objects that exist in as many as 10 or 11 dimensions, depending on which version of string theory you subscribe to.

The thinking goes that we live within a three-dimensional brane, which routinely slams into a second parallel brane, the two separated by higher-dimensional space. The energy produced by their encounter causes them to expand, then contract, and eventually come together again in the next clash.

Read More: Did the Big Bang Happen More Than Once?

Conformal Cyclic Cosmology

Another alternative is conformal cyclic cosmology, the controversial brainchild of Nobel Prize-winning physicist Roger Penrose, who was inspired by a striking similarity between the birth and predicted death of the universe.

Over the unimaginable course of a googol years (1 followed by 100 zeros), black holes will swallow every last bit of matter, then boil away in a process known as Hawking radiation, leaving behind a sea of massless photons. Surprisingly, that cold, quiet end is mathematically equivalent to the hot, energetic Big Bang — they are essentially the same, suggesting that one could blend into the other.

In 2020, Penrose even claimed to have detected the imprint of a previous “cosmic eon” on our own, though many physicists are unconvinced.

Until scientists find the much-sought-after unified theory, which would combine Einstein’s gravitational insights with the mind-bending mechanisms of the quantum world, our picture of the big bang will likely remain fuzzy.

Strange as these scenarios are, the truth may be stranger still. In the meantime, lucky for us, physicists love to speculate how the universe began.

Read More: Scientists Attempt to Map the Multiverse