The Big Bang is usually regarded as being the start of everything – even space and time. But new theories of space and time point to a radically different picture, in which the Big Bang was really a Big Bounce from an earlier universe. Some theorists even claim it may be possible to probe back before the Big Bang, to find out about the universe before our own. These claims are about to be tested with the launch of Planck, an orbiting observatory able to detect signs that the Universe is just the latest in an endless series of cycles.
What happened before the Big Bang? It’s a question so perplexing even the likes of Stephen Hawking have dismissed it as being beyond answer. But breakthroughs in our understanding of the basic nature of space and time are prompting theorists to suggest it may now be possible to peer into the past and look at the start of the Universe – and even before that.
There is now a growing belief that the Big Bang wasn’t the moment a universe was born for the first time – it was just the latest in an endless cycle of cosmic birth and deaths. And if universes existed before ours, this surely means that life, in some form, could have existed as well.
The theory of the ‘cyclic universe’ is about to be put to the test, with the launch of Planck, an orbiting observatory that will study the aftermath of the Big Bang in unprecedented detail, looking for clues to the creation of today’s Universe some 13.7 billion years ago.
Many Eastern religions have long included the idea of a universe in their accounts of creation. Only now is it being taken seriously by scientists, even though it was shown to be a possibility almost a century ago. When Albert Einstein applied his theory of gravity – called General Relativity (GR) – to the Universe, he expected it to predict that space was infinite, static and everlasting. But his equations revealed a host of possible universes – including ones that go through endless cycles.
Such universes offered a solution to the mystery of what happened before the Big Bang. But the idea was dealt an apparently fatal blow in the 1930s by the US physicist Richard Tolman. He claimed the amount of radiation within the Universe would grow with every cycle, making each last longer. But if our Universe is just the latest in an infinite number, it should by now contain an infinite amount of energy – which it clearly doesn’t.
Tolman’s argument, however, contains a crucial flaw. It assumes Einstein’s equations can be trusted at the moment when one universe emerges from another – and they can’t. Instead, they go haywire at the precise moment of the Big Bang, predicting a state of infinitely high temperatures in zero volume called a ‘singularity’ – conditions which are extremely improbable. No-one can therefore be certain what happened to the radiation between each cosmic cycle.
Two rival ideas could explain cyclic universesM-theory
Attempts to create a single, unified explanation for all forces and particles in the cosmos have led to the possibility that our four-dimensional Universe is part of an infinite, multi-dimensional ‘membrane’ (hence the ‘M’) – or ‘brane’ for short. According to this, the Big Bang is no longer the moment when the entire cosmos is crammed into a single point, known as a ‘singularity’ when conditions achieve infinite values, and standard theories break down. Instead, this moment is envisaged as being the result of a collision between two branes, releasing the colossal amount of energy we call the Big Bang. As well as avoiding the problematic singularity, M-theory implies that the question of what happened before the Big Bang amounts to asking what happened before the collision of the branes. And one possibility is that there were an infinite number of previous collisions, each one triggering a fresh Big Bang.
Loop quantum gravity (LQG)
Another result of trying to unify Einstein’s conception of space, time and gravity with quantum theory, Loop Quantum Gravity leads to the view of space-time as being a kind of fabric made up of subatomic loops acting together to create what we call the ‘force’ of gravity. As with M-theory, when applied to the Big Bang, LQG no longer has the troublesome singularity which causes conventional views of gravity to break down. In contrast to M-theory, however, LQG appears to give a much clearer account of what happened before the Big Bang, with little need for extra speculation to bring about the rebirth of a previous universe. According to LQG, the Big Bang was actually just one half of a ‘Big Bounce’, in which a previous universe collapsed down on itself before re-expanding to form a new universe. Instead of collapsing down to a point of infinite density and temperature – as in a singularity – it just reaches a small but finite size, and a high but finite temperature and density.
A tale of two theories
But now theorists think they have solved the singularity problem, and breathed new life into the cyclic universe idea. It’s all due to breakthroughs in research aimed at fixing the problems with Einstein’s theory by combining it with quantum theory – the laws of the sub-atomic world. Devising such a ‘unified theory’ has proved to be far from simple, but to date, two candidates have emerged – M-theory and Loop Quantum Gravity (see ‘The theories’, left).
The resurrection of the cyclic model began in 1999, when theorists Professor Paul Steinhardt of Princeton University and Professor Neil Turok, then at the University of Cambridge, suggested that the Big Bang is just one of an endless series of collisions between multi-dimensional objects predicted by M-theory called ‘branes’. Steinhardt and Turok calculated that each ‘Big Bounce’ avoids the singularity state and also the build-up of radiation which wrecked previous theories of a cyclic universe.
They dubbed their theory the Ekpyrotic Universe – from the Greek for ‘born from fire’. Yet despite its apparent attractions, theorists pointed out that the Ekpyrotic Universe theory’s reliance on M-theory, which is itself still in its infancy, makes it highly speculative. “There are some open questions that need to be addressed, such as a better understanding of the conditions when the branes collide,” says Dr Parampreet Singh of Canada’s Perimeter Institute.
Singh is one of the leaders in probing the mysteries of the Big Bang with the principal rival to M-theory, known as Loop Quantum Gravity (LQG). Developed over the last 20 years, LQG is more mathematically rigorous than M-theory, but it produces a broadly similar view of the Big Bang. Calculations by Singh and his collaborators have revealed that LQG also produces a universe that goes through a singularity-free ‘Big Bounce’. So it seems that the idea of a cyclic universe is not just some bizarre quirk of M-theory.
But the greater power of LQG allows it to do more, opening up the possibility to glimpse what happened before the Big Bounce. Not surprisingly, the results so far remain very controversial.
Initially, theorists hoped they could probe conditions in the previous universe with some confidence, but subatomic or ‘quantum’ effects are notoriously difficult to pin down, and would have been important when the Big Bang took place – making predictions of exactly what happened around this time almost impossible. “Violent quantum effects near the Big Bang are important, and it’s much more difficult to decide how the pre-Big Bang universe may have behaved,” says LQG theorist Martin Bojowald of Penn State University in the US.
In the last few months, new calculations by Singh and colleagues have given theorists renewed hope, by showing that these quantum effects would only be important if our Universe – and the one before it – were incredibly small. And clearly our Universe is not small. “This is easy to understand from common lab physics,” says Singh. “Though quantum effects are present, their relevance is insignificant.”
So what was the previous universe like? If the calculations by Singh and his colleagues are right, then it may have been quite similar to our own, with galaxies, planets and perhaps even life. But Bojowald is far from convinced that the theory of LQG can give reliable answers, and believes that the previous universe could have been quite unlike ours.
For now, theorists are focusing on building confidence in LQG by showing that – like GR itself – it can produce a universe resembling our own. And they’ve been encouraged by the discovery that the theory is compatible with what most cosmologists believe is one of the most crucial features of our own Universe: cosmic inflation.
Inflation is widely held to be the driving force of the Big Bang. A sub-atomic force field with powerful anti-gravitational effects, it is thought to have led to the expansion of the early Universe after the giant explosion. Its existence explains many phenomena we can see in today’s Universe.
Cosmologists are deeply suspicious of theories that don’t include inflation – hence the delight of Singh and his colleagues that LQG does. And that could also prove to be a decisive advantage over the ekpyrotic concept of Steinhardt and Turok, as their theory doesn’t. Instead of inflation, their theory envisages a relatively leisurely expansion following the collision between the two branes. The end result is a universe that looks almost identical to that predicted by inflation – almost, but not quite.
Rise and fall Cyclic universes: a theory that’s had cycles of popularity
Hindu accounts of the Universe describe an endless cycle of creation, destruction and rebirth triggered by the ‘play of the gods’1922
Russian mathematician Alexander Friedman reveals the possibility of a cyclic universe exists in the equations of General Relativity, Albert Einstein’s theory of gravity
American physicist Richard Tolman suggests the laws of thermodynamics prevent endlessly cyclic universes as they end up being filled with an infinite amount of radiation
Theorists Paul Steinhardt and Neil Turok come up with the idea of colliding multi-dimensional ‘branes’ as a means of allowing an endless cycle of universes
Researchers at Penn State University show that so-called Loop Quantum Gravity theory leads to a ‘Big Bounce’ rather than a Big Bang
The Planck orbiting observatory is scheduled to look for clues to conditions before the Big Bang in the CMB – the heat left over by the explosion
It’s those differences that are about to become the focus of intense study, as the Planck mission begins looking for telltale signs of inflation in the radiation left over from the Big Bang (see ‘Testing the theories’, p58). Known as the cosmic microwave background (CMB), the radiation is the fading heat of that primordial explosion, stretched by billions of years of cosmic expansion into microwaves.
Inflation leaves evidence in the form of a pattern of hot and cold spots in the CMB we can observe today. The trouble is that the inflation process makes that pattern very similar to what we’d expect from the ekpyrotic universe – and the difference between the two is so subtle that it will be hard to get a definitive answer to the question of which theory is right.
That said, Planck should be able to get impressive evidence tending to support one over the other. While Planck’s instruments are capable of analysing the hot and cold spots, they can do much more besides. The radiation they detect also carries the scars of the upheaval in the very fabric of space-time triggered by the Big Bang. Known as gravitational waves, they remain intact over billions of years, and give direct evidence of what happened during the Big Bang.
Inflation is expected to create gravitational waves quite unlike those triggered by the brane collisions of the ekpyrotic theory, making these ripples in space-time crucial in deciding which of the two theories holds the most water. The only slight drawback is that, to date, no evidence for any kind of cosmic gravitational waves has been found.
That could be about to change thanks to Planck, whose instruments can detect such waves through their effect on the CMB. It’s a long shot – and many theorists agree that the definitive test of the rival theories needs direct detection of gravitational waves using future missions like the Big Bang Observer. Even so, the fact that both of the two rival theories about the early Universe both point to a Big Bounce rather than a Big Bang suggests the concept of a cyclic universe must now be taken seriously, having previously been largely denied. And that demands a radical rethink of our cosmic past and future.
Until now, cosmology textbooks have claimed the cosmos somehow appeared out of nowhere 13.7 billion years ago, inflated, and then continued to expand forever. They also paint a picture where, trillions of years in the future, the expansion will leave our galaxy forever isolated from every other, and left to undergo a slow, lingering fade-out.
But if the cyclic universe theory is correct, the textbooks will have to be re-written. Today’s Universe will then be just the latest in an endless series, ending in another Big Bounce trillions of years from now, and a fresh start. So perhaps the future isn’t so dull after all. Who knows – perhaps next time we’ll learn to harness the forces of the cosmos and mould a truly everlasting universe to suit ourselves.
Robert Matthews is a Visiting Reader in Science at Aston University
Testing the theories
Two ways to investigate the truth about cyclic universes
Due for launch onboard an Ariane 5 rocket in October, the European Space Agency’s Planck satellite is designed to observe the background heat left over from the Big Bang in unprecedented detail. Some theories of cyclic universes predict the appearance of patterns in this background heat caused by so-called quantum gravity effects triggered at and before the Big Bang. Hints of these effects have already been seen with ground-based observatories, but Planck is needed to confirm them.
The radiation released in the Big Bang is ‘stretched’ by billions of years of cosmic expansion until it turns into microwaves. Parked at a gravitationally stable point 1.6 million kilometres from the Earth, Planck will study these microwaves, which are predicted to interact with gravitational waves – ripples in the fabric of space-time – causing them to vibrate in certain directions, an effect called polarisation. Analysis of the polarisation signal gathered over the duration of Planck’s 21-month mission will allow theorists to check competing theories of what happened at and before the Big Bang.
Big Bang Observer
Predicted by Einstein’s theory of gravity but never directly observed, gravitational waves are undulations in the fabric of space and time with characteristics capable of giving insights into events at and before the Big Bang. Ground-based gravitational wave detectors have already been built, but theory suggests that the waves created by the Big Bang will only be detectable using vast space-based observatories such as the Big Bang Observer (BBO) which NASA hopes to build some time in the “coming decades”.
The BBO will consist of three sets of three satellites arranged in equilateral triangles. The sides will stretch 50,000km long, each set forming a triangle in solar orbit. Gravitational waves generated by cosmic events pass through the Solar System, altering space-time – and thus altering the distance between the satellites. The laser beams passing between the sets of satellites detect gravitational waves through the effects of interference, which shifts the light waves relative to one another. Data collected during the mission is analysed to see which explanation of the Big Bang gives the best fit.
For and Against
Should we take the cyclic universe theory seriously?
Professor Martin Bojowald , Pennsylvania State University, USA
“The problem with the standard Big Bang scenario is that it seems to predict a ‘beginning’ some time in the finite past. That’s an incorrect interpretation of Einstein’s theory, which actually just breaks down at the Big Bang in a so-called singularity, and doesn’t tell us anything about what happened. What we really need is a theory which is free of this singularity, and the most straightforward alternative is a cyclic universe, which simply reverses the direction at the Big Bang – so a universe that was shrinking in size bounces and begins expanding. “There are quantum gravity effects which can bring about this turnaround and also get rid of the singularity problem. But they’re a double-edged sword, because we also end up with counterintuitive quantum effects, such as a loss of certainty about what happened before the Big Bang. Understanding the universe before the Big Bang involves daringly long extrapolations. And while theory may tell us that there was a universe before the Big Bang, the most important questions concerning its behaviour remain to be addressed.”
No Professor Andreas Albrecht, University of California, USA
“The principal problem with both the cyclic universe and models based on Loop Quantum Gravity (LQG) is the lack of knowledge about the fundamental equations we should be using to understand what happened before the Big Bang. Not many people are convinced that LQG offers a compelling theoretical framework for addressing these questions. “Nor am I enthusiastic about Steinhardt and Turok’s original ekpyrotic concept, because it does not allow different possible starting conditions for the colliding branes to produce a universe like the one we actually observe. They have tried to remedy this in their cyclic model, but they still put essential parts of their explanation into the era before the Big Bang – and no-one knows how to reliably calculate what happens when the universe passes through that event. “But trying things like this is how we learn, and the hope of new theoretical insights and good observational tests certainly keeps me excited about this field.”
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