Will we ever… know what happened before the Big Bang?

If you think theories about the universe are mind-bending, rest assured that many scientists feel the same way. But the question isn’t a philosophical one: it has potentially real, testable aspects.
In many ways, it’s strange to us humans that the Universe should be the age it is. The Universe – by definition, everything that physically exists – should either be infinite in age, or somehow tied to the lifespan of the human species, as it does in many mythologies. However, thanks to studies on the rate the Universe is expanding, and applying this knowledge in reverse, we know its age. Roughly 13.8 billion years ago, all we can observe on Earth, in our solar system, other galaxies and everything in between expanded out rapidly from an initial point much smaller than an atom, which we call the Big Bang.
The Big Bang model is our best explanation for why the cosmos appears as it does. Nevertheless, it’s not able to answer some of the more challenging questions, including what – if anything – came before it? Despite how it might sound, this question isn’t a philosophical one: it has potentially real, testable aspects.
For many reasons, cosmologists think the early Universe underwent inflation: an incredibly rapid expansion right after the Big Bang. As the Universe expanded, it also cooled, so in the distant past, it was hotter, more dense, and opaque to all forms of light. When the cosmos became transparent, about 380,000 years after the Big Bang, it left behind a bath of photons, detectable today as the Cosmic Microwave Background (CMB). As space observatories like COBE, WMAP and more recently Planck have shown, the CMB is remarkably smooth, but not quite uniform. Within this were tiny ripples that were stretched to enormous sizes during inflation, and in turn these became the seeds for large-scale objects like galaxies and galactic clusters we see today.
Pocket universes
However, none of this really tells us what, if anything, came before the Big Bang. In many models for inflation, as in some of the older Big Bang theories, this is the only Universe that exists – or at the very least, the only Universe we can observe.
A partial exception to this is a model known as eternal inflation. In this scheme, the observable Universe is part of a “pocket universe”, a bubble in a larger froth of inflation that is ongoing. In our particular bubble, inflation began and ended, but in other pocket universes – unconnected (“parallel”) and thereby unobservable to our pocket universe – inflation might have had different properties. Eternal inflation effectively emptied the regions outside of bubbles of all matter; these would have no stars, galaxies, or other familiar hallmarks. If eternal inflation is correct, then the Big Bang is the origin of our pocket universe, but not the beginning of the whole Universe, which may have begun much earlier.
Trillion-year cycle
Many cosmologists regard inflation as being the worst model we have, except for all the alternatives. Inflation’s generic properties are pretty nice, thanks to its usefulness in solving difficult problems in cosmology, but the specifics are slippery.
There is one possible alternative to inflation, which bypasses these questions and, along the way, resolves what came before the Big Bang. In Paul Steinhardt and Neil Turok’s cyclic universe model, the observable Universe resides in a higher-dimensional void. Coupled to our universe is a parallel shadow universe that we can’t directly observe, but is connected via gravity. The Big Bang was not the beginning, but a moment when the two “branes” (short for “membrane”) collided. The Universe in the cyclic model goes between periods when the branes are moving apart, accelerated expansion, and new Big Bangs when the branes re-collide. While each cycle would take about a trillion years to complete, the whole cosmos could be infinitely old, bypassing the philosophical problems with inflationary models.
The cyclic universe is not a popular model among working cosmologists, but at least it could be ruled out by experimental observations: if the gravitational-wave signature of inflation is found, then the cyclic model is dead. The cyclic model isn’t complete: it doesn’t explain how much dark energy there is in the Universe any more than standard cosmology does, for example. In other words, the cyclic model is not complete, so at present there’s no physical evidence to distinguish it from inflationary models.
If you think all these options are fairly mind-bending, rest assured that professional scientists feel the same way. Since the observable Universe is currently accelerating with no sign of re-collapse even in the far future, why should there be a cosmos with a beginning but no similar ending? In another decade or century, the questions and the methods we use to answer these questions will most likely have evolved.