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How Was the Universe Born?

The universe in an hourglass

Image: iStockphoto

By Geraint Lewis

Modern cosmology tells us that the universe as we know it arose 13.7 billion years ago in the fiery birth of the Big Bang, but our understanding of the laws of physics is incomplete and we are currently unable to answer the questions of where the universe actually came from. Cosmologists have many ideas, ranging from the reasonably strange to the extremely outlandish.

For much of human history we were confident about the purpose of the universe, with the Bible telling us that it was born only a few thousand years ago and that it exists as a stage for humans to play out their physical existence before moving onto a more ethereal realm.

While this view is comforting for the soul, it is in stark contrast with our present view of the universe. Over the past 400 years our position has been displaced from being the central, and most important, object in the universe to being nothing but the residents of a small planet orbiting an unremarkable star in one of more than a billion galaxies that inhabit the observable universe.

Since the early 1900s our understanding of the underlying nature of the universe has also radically changed. Written within the framework of Einstein’s theory of general relativity, we now understand the universe as being a dynamic place, expanding and evolving over time, with generations of stars being born, living and dying, and eventually enriching the cosmos with a richer mix of chemical elements with each generation.

The dynamic universe was observationally confirmed with the work of Edwin Hubble, showing that other galaxies are rushing away from the Milky Way with the more distant galaxies rushing away faster than those nearby. This is in precise agreement with our mathematical expectations, but does have a surprising outcome that has lead to a wealth of ongoing discussion on the very origin of the universe.

Simply put, running a film of the universe backwards reveals that, in the past, galaxies were closer together, and temperatures were hotter as gases were squeezed in the smaller universe. Pushing time further back, the galaxies get closer still until 13.7 billion years ago, when the distance between galaxies goes to zero, the density of material goes to infinity and we have reached the Big Bang, the point generally considered the birth of the universe.

This picture of the universe is quite simple. The universe was born in an extremely hot, dense state and has been expanding and cooling ever since. Out of this cooling and condensing universe, stars and planets form, eventually leading to the rise of people on our own little planet.

While simple, this picture makes many people uncomfortable, leaving us to ponder the question: “Just where did the universe come from?” Those of a religious flavour liken the Big Bang to the biblical story of creation, with a creator kick-starting the cosmos. In fact, this argument has formed the basis of numerous religious arguments for the existence of God. But what does science actually say?

Some famous work by Roger Penrose and Stephen Hawking in the 1970s revealed that if general relativity is universally applicable then the Big Bang represents a special state where the density of the universe was infinite. This infinite state is known as a singularity, and it has achieved a rather mythical status within the public conscience.

What Penrose and Hawking showed was that if we trace the history of a particle back over cosmic history we can continue this path back into the earliest epochs – back before the first stars and into the hot plasma of the initial stages of the universe. However, if we try to follow the path back into the Big Bang we hit a snag; the path just stops. We reach a point 13.7 billion years ago where we cannot follow the path back any further.

There is a rather surprising conclusion from this – that the Big Bang appears to be the true “birth” of the universe. Before this epoch there was no universe into which we can follow a particle, there was no time, and there was no space. We have reached the ultimate start of the universe.

Or have we? Considering only Einstein’s theory of relativity, we are left with no wriggle room, and the singular state represents the birth of the universe. But in considering only relativity we are ignoring the other edifice on which modern physics is built – namely quantum mechanics.

Generally, these two theories rule supreme in quite different regimes, with quantum mechanics used to describe the microscopic world of electrons and quarks whereas relativity allows us to understand the dynamics of the universe at large.

However, in extreme environment, with immense densities and super-strong gravity we need to use both relativity and quantum mechanics to fully understand physical behaviour. Unfortunately, both theories treat the nature of time and space, and forces and particles, in fundamentally different ways, and hence they refuse to work together.

But the early stages of the universe were such an extreme state, with all matter squeezed to densities much higher than within an atomic nucleus. Without a unified theory where quantum mechanics and gravity work together, we simply cannot determine what was really happening during the initial phases of our universe.

It is thought that, when properly included, quantum mechanics will smooth out the Big Bang singularity and allow us to trace the history of particles to before the birth of our present universe. But what could have come before?

Not being able to peer back through the Big Bang clearly places this within the realm of speculation, but ideas abound. Potentially the picture is not too radical, with our universe arising as a “quantum fluctuation” in a true nothingness of no time and space. But such a picture is not very appealing, leaving us with more questions than it answers.

Others speculate that our universe is part of an endlessly repeating sequence of universes that are born, live and eventually die, only to be reborn again. There is, however, little sign that our current cosmos will ever re-collapse and be reborn.

Things then get really strange, with the idea that our universe was born during the death of a star in a previously existing, but quite separate, universe, with the violent shaking of space–time that leads to the production of a black hole also causing space–time to bud off and form the universe in which we live. This means that as giant stars die in our universe, forming black holes, they may be giving birth to many other daughter universes.

The most modern of physical theories, superstrings, is trying to unify the actions of quantum forces and gravity, presenting us with a slightly different picture of the multiverse in which our universe is a sheet, or brane, that floats in an 11-dimensional uber-universe. We share this space with a myriad of other universes, some similar to our own and some very different, all interacting and coming and going. In this ekpyrotic scenario, the fiery birth of our universe was the result of a collision between branes, and it will ultimately be recycled into a new universe in a future collision. This bubbling sea of universes has always been there, and always will, with our universe nothing but a temporary inhabitant.

It may seem that we have strayed from the world of science to that of science fiction, but cosmologists are pushing the bounds of our knowledge to try and truly understand what came before the universe. Ultimately we will find the true link between the quantum and gravity, and we will peer behind the curtain of the Big Bang and understand where our universe really came from.

Geraint Lewis is Professor of Astrophysics at The University of Sydney.