Has the cosmos existed forever, or did something bring it into existence? Time to grapple with the universe's greatest mystery
AS BIG questions go, it's hard to beat. Has the universe existed forever? Over the years, some of the greatest minds in physics have argued that no matter how far back in time you go, the universe has always been here. Others have argued that the opposite must be true - something must have happened to bring the cosmos into existence. With both sides claiming that observations support their view, until recently an answer seemed as distant as ever.
However, earlier this year, cosmologists Alex Vilenkin and Audrey Mithani claimed to have settled the debate. They have uncovered reasons why the universe cannot have existed forever. Yet what nature grudgingly gives with one hand, it takes back with the other - even though the universe has a beginning, its origins may be lost in the mists of time.
Modern cosmology began in 1916 when Einstein applied his newly formulated theory of gravity, general relativity, to the biggest gravitating mass he could think of: the entire universe. Like Newton, Einstein favoured an unchanging universe - a universe that had existed forever and therefore had no beginning. To achieve this, Einstein realised that the gravity pulling together all the matter in the universe had to be countered by a weird cosmic repulsion of empty space.
Einstein's static universe was unfortunately unstable. As the English physicist Arthur Eddington pointed out, such a universe was balanced on a knife-edge between runaway expansion and runaway contraction. A further blow came in 1929 when American astronomer Edwin Hubble observed that galaxies were flying apart from each other like pieces of cosmic shrapnel. The conclusion was that the universe was expanding.
Yet if the universe was expanding, an unavoidable consequence must be that it had been smaller in the past. Imagine rewinding that expansion back to a time when everything was compressed into the tiniest of volumes. This was the big bang.
The big bang theory that has subsequently developed describes the evolution of the universe from a hot, dense state, but it does not say anything about what brought the universe into existence. That still l eaves crucial questions unanswered - what happened before the big bang and was there really a beginning?
Little wonder, then, that the appeal of the eternal universe became popular, not least because such awkward questions need never be asked. In 1948, Fred Hoyle, Hermann Bondi and Tommy Gold proposed that, as the universe expanded, new matter fountained into existence in the gaps between galaxies and coalesced into new galaxies. According to this steady state picture, the universe looks the same today as it has always done and always will. It has no beginning, it has simply existed forever.
However the steady state theory was scuppered by two observations. The first was the discovery in the 1960s that the distant, and therefore, early universe does not look the same as today's universe. The second was the discovery in 1964 of the cosmic microwave background, the hot afterglow of the big bang fireball. More recently, NASA's WMAP satellite has made detailed measurements of this cosmic background and shown that the big bang happened 13.7 billion years ago.Flaws in forever
A further blow to the eternal universe came from theory. In the 1960s, Roger Penrose and Stephen Hawking were two young theorists at the University of Cambridge. Their work showed that if you reversed the expansion of the universe, it is impossible to avoid reaching a point known as a singularity, where physical parameters such as density and temperature skyrocket to infinity. Crucially, physics breaks down at a singularity making it impossible to predict what lies on the other side. According to Penrose and Hawking, the big bang must truly be the beginning.
So, story over? Well, no. It turns out that there is a loophole in the singularity theorems of Penrose and Hawking. According to Newton's laws, the gravitational pull of an object depends only on its mass. Einstein's insight showed that the strength of gravity also depends on an object's energy density and, crucially, its pressure. In deriving their powerful theorems, Penrose and Hawking had assumed that the pressure of space is always small and positive. But what if they were wrong? "It is just this possibility that has opened the way to modern cosmological theories in which the big bang is not a beginning at all," says Vilenkin. "Chief among them is inflation."
Inflation, a theory that Vilenkin helped to create, starts with a vacuum in an unusually high energy state and with a negative pressure. Together these give the vacuum repulsive gravity that pushes things apart rather than draws them together. This inflates the vacuum, making it more repulsive, which causes it to inflate even faster.
But the inflationary vacuum is quantum in nature, which makes it unstable. All over it, and at random, bits decay into a normal, everyday vacuum. Imagine the vacuum as a vast ocean of boiling water, with bubbles forming and expanding across its length and breadth. The energy of the inflationary vacuum has to go somewhere and it goes into creating matter and heating it to a ferocious temperature inside each bubble. It goes into creating big bangs. Our universe is inside one such bubble that appeared in a big bang 13.7 billion years ago.
One of the striking features of inflation is that it is eternal. New high-energy vacuum is created far faster than it is eaten away by its decay into ordinary vacuum, which means that once inflation starts, it never stops and universes bubble off forever in the future. But because eternal inflation avoids the dreaded singularity, it opens up the possibility that this has always been the case with universes bubbling off forever in the past too.
Inflation is compatible with all our observations and Vilenkin is fairly certain it is fundamentally correct. Yet there is a problem with eternal inflation, which Vilenkin first discovered in 2003 when he teamed up with Arvind Borde of Southampton College in New York and inflation pioneer Alan Guth of the Massachusetts Institute of Technology.
They calculated what would happen in a growing universe and made no assumptions about energy or gravity. Their theorem simply assumed that on average the universe expands. "To our amazement, it showed that space-time does not continue forever in most past directions," says Vilenkin. "Inflation must have a beginning."
However inflation is not the only game in town. So could the alternative scenarios have a beginning? Earlier this year, Vilenkin teamed up with Audrey Mithani, his colleague at Tufts University in Medford, Massachusetts, to examine two of the leading alternative cosmological scenarios.
The first is the "cyclic universe" developed within string theory by Neil Turok of Canada's Perimeter Institute for Theoretical Physics in Waterloo, Ontario, and Paul Steinhardt of Princeton University. In this scenario, our universe is a four-dimensional island, or "brane", in a higher dimensional space. It collides repeatedly with a second brane (see diagram). Think of the two branes as two parallel slices of bread, coming together along a fifth dimension, passing through each other, pulling apart again, then coming together again. Each time the branes touch, their tremendous energy of motion along the fifth dimension creates matter on each brane and heats it to tremendous temperature. To observers on the brane, it looks exactly like a big bang and would lead to the same patterns in the cosmic microwave background and distributions of galaxies. Yet it is a big bang without a beginning, say Turok and Steinhardt, because the cycles have been repeating for eternity.
However, Vilenkin and Mithani have now shown that the cyclic universe cannot continue indefinitely towards the future and the past. According to the theory, matter on the branes expands more with each cycle and this means that the Borde-Guth-Vilenkin theorem of there being a beginning to the universe still applies. "If you run it backwards like a movie in reverse, the cyclic universe encounters either a singularity or some kind of beginning like inflation," he says.
Another cosmological scenario considered by Vilenkin and Mithani is even weirder than the cyclic universe and inflation. This is the "emergent universe" imagined by George Ellis of the University of Cape Town in South Africa and Roy Maartens of the University of Portsmouth, UK. It begins as a small static universe, which exists in this state for an infinite amount of time before suddenly being triggered to inflate. Such scenarios do arise in string theory, so the idea isn't totally out of the blue. "It's a somewhat desperate scenario," says Vilenkin.
To model an eternally slumbering emergent universe is not straightforward. In the same way that Einstein's static universe was unstable and needed the extra ingredient of cosmic repulsion, Ellis and Maartens can only stabilise theirs with two weird ingredients: a vacuum with negative energy, and fault-lines in space-time known as domain walls that are a feature of some models of particle physics. Domain walls should leave an imprint on the temperature of the cosmic microwave background radiation, which has not been seen, but this might be explained if they were diluted away by inflation.Spontaneous collapse
Vilenkin and Mithani are critical of Ellis and Maartens's approach. "At first sight it appears they have concocted a stable universe," says Vilenkin. "However, we find that it's only stable if you ignore the effects of quantum theory."
According to quantum theory, the universe cannot stay at its minimum size forever - there is a chance it would spontaneously collapse. "Although the probability may be very small, since an infinite amount of time is available, it is inevitable," says Vilenkin. "Consequently, if we live in an emergent universe, it cannot have existed forever." According to Maartens, this quantum instability result "is not unreasonable". However, he is not sure this inevitably implies a beginning to the universe. "It strikes me that this is a much deeper question, requiring at least a mature quantum gravity theory," he says. "Unfortunately, we do not have that."
In the context of known physics, however, Vilenkin and Mithani conclude that, whatever way you look at it, the universe cannot have existed forever so must have had a beginning. But how did it begin? According to Vilenkin, quantum theory has a solution because it permits something to pop out of nothing - with that something being a small universe that starts to inflate, cycle or hang for an extremely long time before inflating.
Can we really be sure now that the universe had a beginning? Or are we in for an infinite cycle of belief and disbelief over the matter? "For the first time in history, we have the tools to address the origin question scientifically," says Vilenkin. "So I have a feeling we are getting near to the truth."
Any hope of us observing the ultimate origin is fading, however. Soon after Vilenkin and Mithani published their argument, physicist Leonard Susskind of Stanford University in California responded with two papers. In them, he says that a beginning, if it did indeed occur, is likely to have been so far in the past that for all practical purposes the universe has been around forever.
He argues that because space inflates exponentially, the volume of the vacuum at later times is overwhelmingly greater than at earlier times. With many more bubble universes in existence, chances are that the patch of vacuum we call home formed later on too. The true beginning is likely to have been an awfully long time ago - so far away, that no imprint on the universe has survived. "I find it a paradoxical situation to say that there must have been a beginning, but it is with certainty before any nameable time," says Susskind.
Vilenkin acknowledges this. "It's ironic," he says. "The universe may have a beginning but we may never be able to know exactly what the beginning was like."
Still, cosmologists have plenty of other big questions to keep them busy. If the universe owes its origins to quantum theory, then quantum theory must have existed before the universe. So the next question is surely: where did the laws of quantum theory come from? "We do not know," admits Vilenkin. "I consider that an entirely different question." When it comes to the beginning of the universe, in many ways we're still at the beginning.
Marcus Chown is the author of Tweeting the Universe (Faber & Faber)