Existence: Why is there a universe?

Source: http://www.newscientist.com/section/life

AS DOUGLAS ADAMS once wrote: "The universe is big. Really big." And yet if our theory of the big bang is right, the universe was once a lot smaller. Indeed, at one point it was non-existent. Around 13.7 billion years ago time and space spontaneously sprang from the void. How did that happen?

Or to put it another way: why does anything exist at all? It's a big question, perhaps the biggest. The idea that the universe simply appeared out of nothing is difficult enough; trying to conceive of nothingness is perhaps even harder.

It is also a very reasonable question to ask from a scientific perspective. After all, some basic physics suggests that you and the rest of the universe are overwhelmingly unlikely to exist. The second law of thermodynamics, that most existentially resonant of physical laws, says that disorder, or entropy, always tends to increase. Entropy measures the number of ways you can rearrange a system's components without changing its overall appearance. The molecules in a hot gas, for example, can be arranged in many different ways to create the same overall temperature and pressure, making the gas a high-entropy system. In contrast, you can't rearrange the molecules of a living thing very much without turning it into a non-living thing, so you are a low-entropy system.

By the same logic, nothingness is the highest entropy state around - you can shuffle it around all you want and it still looks like nothing.

Given this law, it is hard to see how nothing could ever be turned into something, let alone something as big as a universe. But entropy is only part of the story. The other consideration is symmetry - a quality that appears to exert profound influence on the physical universe wherever it crops up. Nothingness is very symmetrical indeed. "There's no telling one part from another, so it has total symmetry," says physicist Frank Wilczek of the Massachusetts Institute of Technology.

And as physicists have learned over the past few decades, symmetries are made to be broken. Wilczek's own speciality is quantum chromodynamics, the theory that describes how quarks behave deep within atomic nuclei. It tells us that nothingness is a precarious state of affairs. "You can form a state that has no quarks and antiquarks in it, and it's totally unstable," says Wilczek. "It spontaneously starts producing quark-antiquark pairs." The perfect symmetry of nothingness is broken. That leads to an unexpected conclusion, says Victor Stenger, a physicist at the University of Colorado in Boulder: despite entropy, "something is the more natural state than nothing".

"According to quantum theory, there is no state of 'emptiness'," agrees Frank Close of the University of Oxford. Emptiness would have precisely zero energy, far too exacting a requirement for the uncertain quantum world. Instead, a vacuum is actually filled with a roiling broth of particles that pop in and out of existence. In that sense this magazine, you, me, the moon and everything else in our universe are just excitations of the quantum vacuum.

Before the big bang

Might something similar account for the origin of the universe itself? Quite plausibly, says Wilczek. "There is no barrier between nothing and a rich universe full of matter," he says. Perhaps the big bang was just nothingness doing what comes naturally.

This, of course, raises the question of what came before the big bang, and how long it lasted. Unfortunately at this point basic ideas begin to fail us; the concept "before" becomes meaningless. In the words of Stephen Hawking, it's like asking what is north of the north pole.

Even so, there is an even more mind-blowing consequence of the idea that something can come from nothing: perhaps nothingness itself cannot exist.

Here's why. Quantum uncertainty allows a trade-off between time and energy, so something that lasts a long time must have little energy. To explain how our universe has lasted for the billions of years that it has taken galaxies to form, solar systems to coalesce and life to evolve into bipeds who ask how something came from nothing, its total energy must be extraordinarily low.

That fits with the generally accepted view of the universe's early moments, which sees space-time undergoing a brief burst of expansion immediately after the big bang. This heady period, known as inflation, flooded the universe with energy. But according to Einstein's general theory of relativity, more space-time also means more gravity. Gravity's attractive pull represents negative energy that can cancel out inflation's positive energy - essentially constructing a cosmos for nothing. "I like to say that the universe is the ultimate free lunch," says Alan Guth, a cosmologist at MIT who came up with the inflation theory 30 years ago.

Physicists used to worry that creating something from nothing would violate all sorts of physical laws such as the conservation of energy. But if there is zero overall energy to conserve, the problem evaporates - and a universe that simply popped out of nothing becomes not just plausible, but probable. "Maybe a better way of saying it is that something is nothing," says Guth.

None of this really gets us off the hook, however. Our understanding of creation relies on the validity of the laws of physics, particularly quantum uncertainty. But that implies that the laws of physics were somehow encoded into the fabric of our universe before it existed. How can physical laws exist outside of space and time and without a cause of their own? Or, to put it another way, why is there something rather than nothing?