Have we all been here before?
The Universe appeared from a cataclysmic explosion 13.7 billion years ago in which space, time and matter were all created. So is there any truth in the theory that universes and civilisations existed before our own? Robert Matthews investigates
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.
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
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
Robert Matthews is a Visiting Reader in Science at Aston University
Testing the theories
Two ways to investigate the truth about cyclic universes
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.”
Robert is a science writer and visiting professor of science at Aston University.