Has the universe been around forever? If so, perhaps it's been bouncing back and forth in a never-ending cycle of big bangs in which all matter bubbles out of a singularity, followed by big crunches, in which everything gets swallowed up again to form that dense point from which the universe is born again. And the cycle continues over and over and over.
The math of those theories, however, has never really worked out in a way that could tell us whether our universe is cyclic or has one beginning and one end. But recently, a team of theorists has invoked the powers of so-called string theory to solve some fundamental riddles of the early universe. The result could give us the theoretical push needed to build a universe from scratch, and hence lend support to a repeating universe.
Related: From Big Bang to present: snapshots of our universe through time
Painting the picture
If you want to build your own private theoretical model of the universe, be my guest. Nobody will ever stop you from making your own cosmology. But if you want to play the game of the universe, you have to play by its rules. That means that no matter what your model of the cosmos contains, you have to confront some cold, hard observational evidence.
For instance, we know that we live in an expanding universe, in which galaxies and stars are flying away from us at an ever-increasing speed. Scientists can tell that by using different types of techniques to calculate how fast galaxies at different distances from us are moving away. We also have pictures of the baby universe, when it was just 380,000 years old (and I really do mean "baby," as the universe is currently 13.8 billion years old).
Within that baby picture, we see interesting patterns — tiny splotches and blotches that reveal the existence of slight temperature and pressure differences in that young universe.
We are able to explain all these observations (and more) with what's called Big Bang cosmology, plus an additional idea known as inflation, which is a process that we think happened when the universe was less than a second old. During that process (which itself lasted for the teensiest sliver of a second), the universe became much, much larger, taking quantum differences and making them bigger in the process. Those differences eventually grew, as slightly denser patches had slightly stronger gravity, making them bigger. Over time, those differences became large enough to imprint themselves as splotches in the baby picture of the universe (and billions of years later, things like stars and galaxies, but that's a separate story).
King of the early universe
Tired of the Big Bang Theory and want your own version of cosmology? That's fine, but you'll have to explain things like the expansion of the universe and the splotches in the baby picture of cosmos. In other words, you have to do a better job at explaining the universe than inflation does.
This seems easy, but it isn't. The pressure, density and temperature differences in the universe's early years has bedeviled many alternative cosmologies, including one of the most popular let's-go-bigger-than-the-big-bang ideas, known as (are you ready for this), Ekpyrotic universe. The word ekpyrotic comes from the Greek for word for "conflagration," which refers to an ancient philosophical idea of a constantly repeating universe.
In the Ekpyrotic scenario, the universe … constantly repeats. Under that perspective, we are currently in a "bang" phase, which will eventually (somehow) slow down, stop, reverse, and crunch back down to incredibly high temperatures and pressures. Then, the universe will (somehow) bounce back and re-ignite in a new big bang phase.
The trouble is, it's hard to replicate the blotches and splotches in the baby picture of the universe in an Ekpyrotic universe. When we attempt to put together some vague physics to explain the crunch-bounce-bang cycle (and I do emphasize "vague" here, because these processes involve energies and scales that we aren't even coming close to understanding with known physics), everything just comes out too … smooth. No bumps. No wiggles. No splotches. No differences in temperature, pressure or density.
And that doesn't just mean the theories don't match observations of the early universe. It means that these cosmologies don't lead to a universe filled with galaxies, stars or even people.
So that's kind of a bummer.
Related: How will the universe end?
The S-brane saves the day
The name of the game in the past few years of Ekpyrotic theories is to try to match the same observations that inflation does. In the latest attempt to overcome this hurdle and make Ekpyrotic cosmologies at least somewhat respectable, a team of researchers invoke none other than the S-brane.
Right. S-branes. So you've heard of string theory, right? That's the universe of fundamental physics where every particle is really a tiny, vibrating string. But a few years ago, theorists realized that the strings don't have to be one-dimensional. And the name they give to a multidimensional string? A brane.
As for the "S" part? Well most branes in string theory can roam around freely through both space and time, but the hypothetical S-brane can exist only in one instant in time, under very special conditions.
In this new Ekpyrotic scenario, when the universe was at its smallest and densest configuration possible, an S-brane appeared, triggering the re-expansion of a cosmos filled with matter and radiation (a big bang) and with small variations in temperature and pressure (giving rise to the well-known splotches in the baby pictures of the universe). That's what three physicists propose in a new paper published online in July to the preprint server arXiv, meaning the paper has yet to be peer-reviewed.
Is this idea correct? Who knows. String theory is on thin theoretical ice recently, as experiments like those at the Large Hadron Collider have failed to find any hints of a theory known as supersymmetry, which is a critical underpinning of String theory . And the concept of S-branes is itself a controversial idea within the String Theory community, as it's not exactly known if branes would be allowed to exist only in one moment in time.
There's also the fact that not only is the universe as we know it expanding, but it's accelerating in its expansion, with no sign whatsoever of it slowing down (let alone collapsing) anytime soon. Figuring out what could make it hit the brakes and reverse course, then, is tricky.
Still, Ekpyrotic (and other) ideas are worth exploring, because the earliest moments of the universe provide some of the most puzzling and challenging questions to modern physics.
Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of Ask a Spaceman and Space Radio, and author of Your Place in the Universe.
Originally published in Live Science.