As most of us are aware our universe is 13.8 billion years old. The apparent result of a big bang that expanded our universe into what it is today. But what was there anything before the Big Bang? Well, the truth is, we don't know. But that doesn't mean that there was nothing, because there was definitely something. First, it's important to understand exactly what the Big Bang is. "The Big Bang is a moment in time, not a point in space," says Sean Carroll, a theoretical physicist at the California Institute of Technology and author of "The Big Picture: On the Origins of Life, Meaning and the Universe Itself"
Forget the image of a tiny fragment of dense matter suddenly exploding outward into a void. "For one thing, the universe at the Big Bang may not have been particularly small," Carroll said. "Sure, everything in the observable universe today-a sphere with a diameter of about 93 billion light-years containing at least 2 trillion galaxies-was crammed into a space less than a centimeter across. But there could be plenty outside of the observable universe that Earthlings can't see because it's physically impossible for the light to have traveled that far in 13.8 billion years."
It's possible that before the Big Bang, the universe was an immeasurable tract of an ultra hot, dense material, continuing in a steady state until the Big Bang occurred. This extra-dense universe may have been managed by quantum mechanics, the physics of the miniscule scale, Carroll said. "The Big Bang, then, would have represented the moment that classical physics took over as the major driver of the universe's evolution."
For Stephen Hawking, this moment was all that mattered: Before the Big Bang, he said, events are unmeasurable, and thus undefined. Hawking called this the no-boundary proposal: Time and space, he said, are finite, but they don't have any boundaries or starting or ending points, the same way that the planet Earth is finite but has no edge.
Or perhaps there was something else before the Big Bang that's worth pondering. One idea is that the Big Bang isn't the beginning of time, but rather that it was a moment of symmetry. In this idea, prior to the Big Bang, there was another universe, identical to this one but with entropy increasing toward the past instead of toward the future. Carroll and his colleague Jennifer Chen have their own pre-Big Bang vision. In 2004, the physicists suggested that perhaps the universe as we know it is the offspring of a parent universe from which a bit of space-time has ripped off.
It's like a radioactive nucleus decaying, Carroll said: "When a nucleus decays, it spits out an alpha or beta particle. The parent universe could do the same thing, except instead of particles, it spits out baby universes, perhaps infinitely. It's just a quantum fluctuation that lets it happen," Carroll said. "These baby universes are literally parallel universes," Carroll said, "and don't interact with or influence the other."
Theoretical physicists like Carroll still have a lot of work to do. Like making more-precise predictions about how quantum forces like quantum gravity might work.
"We don't even know what we're looking for," Carroll said, "until we have a theory."
