Todays Irish Times
THE CONVENTIONAL picture presented by science is that the universe began about 14 billion years ago in a massive explosion at a single point (the Big Bang) and it has been expanding outwards ever since from this point of origin, and time and space only arrived with the Big Bang. But new concepts are being developed. To understand the matter fully we need a reliable quantum theory of gravity. One candidate for such a theory predicts that time may have existed before the Big Bang and that the universe that existed then may have shrunk to a point of maximum density and then bounced back (the event we conventionally call the Big Bang) into our familiar expanding universe. This fascinating research is described by Martin Bojowald in this month's Scientific American .
We know that at the finest level matter resolves into the graininess of atoms. There is good evidence that space-time also ultimately resolves into "atoms" of space-time, ie the smallest indivisible units of distance (about 1/1035m). Einstein's general theory of relativity, our current theory of gravity, which predicts that if we rewind the expanding universe back in time, all the galaxies will converge into a single point of infinite temperature and density (a singularity), is blind to the fine-structure of space time. But, Bojowald explains, there is something wrong with the prediction of general relativity because the infinite values of the singularity indicate that general relativity itself breaks down. To understand what really happens we need a quantum theory of gravity.
There are several candidates for a quantum theory of gravity and the one favoured by Martin Bojowald is called the theory of loop quantum gravity. This theory predicts the existence of space-time atoms, producing an ultimately porous mesh-like structure of space-time. This ultimate mesh-like nature goes unnoticed under ordinary conditions when the spacing of the "atoms" is so tight that space-time looks continuous. Over large distances in the expanding universe, the dynamism of the space-time atom mesh is describable in terms of general relativity. But when space-time becomes jam-packed with energy, as it was at the time of the Big Bang, the fine structure of space-time comes into its own and the predictions of loop gravity differ from those of general relativity.
Normally, gravity attracts. In the continuous space-time envisaged by general relativity, you can pack an infinite amount of energy into the tiniest space. But, Bojowold says, "loop gravity suggests that the atomic structure of space-time changes the nature of gravity at very high energy densities, making it repulsive". Just as a sponge can only store so much water before it starts to repel it, so space-time is porous and can only store so much energy before it starts to repel it. In this model, no state of infinite density can arise. It predicts that the early universe had a massively high, but finite, density. At this extreme, gravity acted as a repulsive force, causing the universe to expand, initially at an exponential rate (inflation). As the density of the universe dropped, gravity switched over to being a force of attraction, but inertia has kept the expansion going.
Conventionally, the infinitely dense singularity marked the beginning of time, but if this could never have existed the history of the universe may go back long before cosmology thought possible. One possibility is that a universe existed before the Big Bang; it collapsed under the attractive force of gravity, but, when the density of the contracting universe grew high enough, gravity switched over to being repulsive and the universe started expanding again. In other words, the Big Bang was really the Big Bounce.
But how could the universe be very much older than 14 billion years? Doesn't thermo-dynamics tell us that disorder increases with time and an extremely old universe should at this stage be completely randomised. According to Bojowold, the chaotic quantum conditions of the massively dense early uni- verse allows it to "tidy up" and to "present the young growing universe with a clean state irrespective of all the mess that may have built up before". This is because loop theory allows the number of space atoms to change. So quantum effects during the Big Bounce erased almost all traces of prior history.
But there are still ways of probing this period. Gravitational waves and neutrinos scarcely interact with matter and therefore passed through the primordial plasma with minimum loss. These may soon bring us news of a time before the Big Bang/Bounce.
© 2008 The Irish Times
What I want to know is how much time will it take to make more space in the garage?
THE CONVENTIONAL picture presented by science is that the universe began about 14 billion years ago in a massive explosion at a single point (the Big Bang) and it has been expanding outwards ever since from this point of origin, and time and space only arrived with the Big Bang. But new concepts are being developed. To understand the matter fully we need a reliable quantum theory of gravity. One candidate for such a theory predicts that time may have existed before the Big Bang and that the universe that existed then may have shrunk to a point of maximum density and then bounced back (the event we conventionally call the Big Bang) into our familiar expanding universe. This fascinating research is described by Martin Bojowald in this month's Scientific American .
We know that at the finest level matter resolves into the graininess of atoms. There is good evidence that space-time also ultimately resolves into "atoms" of space-time, ie the smallest indivisible units of distance (about 1/1035m). Einstein's general theory of relativity, our current theory of gravity, which predicts that if we rewind the expanding universe back in time, all the galaxies will converge into a single point of infinite temperature and density (a singularity), is blind to the fine-structure of space time. But, Bojowald explains, there is something wrong with the prediction of general relativity because the infinite values of the singularity indicate that general relativity itself breaks down. To understand what really happens we need a quantum theory of gravity.
There are several candidates for a quantum theory of gravity and the one favoured by Martin Bojowald is called the theory of loop quantum gravity. This theory predicts the existence of space-time atoms, producing an ultimately porous mesh-like structure of space-time. This ultimate mesh-like nature goes unnoticed under ordinary conditions when the spacing of the "atoms" is so tight that space-time looks continuous. Over large distances in the expanding universe, the dynamism of the space-time atom mesh is describable in terms of general relativity. But when space-time becomes jam-packed with energy, as it was at the time of the Big Bang, the fine structure of space-time comes into its own and the predictions of loop gravity differ from those of general relativity.
Normally, gravity attracts. In the continuous space-time envisaged by general relativity, you can pack an infinite amount of energy into the tiniest space. But, Bojowold says, "loop gravity suggests that the atomic structure of space-time changes the nature of gravity at very high energy densities, making it repulsive". Just as a sponge can only store so much water before it starts to repel it, so space-time is porous and can only store so much energy before it starts to repel it. In this model, no state of infinite density can arise. It predicts that the early universe had a massively high, but finite, density. At this extreme, gravity acted as a repulsive force, causing the universe to expand, initially at an exponential rate (inflation). As the density of the universe dropped, gravity switched over to being a force of attraction, but inertia has kept the expansion going.
Conventionally, the infinitely dense singularity marked the beginning of time, but if this could never have existed the history of the universe may go back long before cosmology thought possible. One possibility is that a universe existed before the Big Bang; it collapsed under the attractive force of gravity, but, when the density of the contracting universe grew high enough, gravity switched over to being repulsive and the universe started expanding again. In other words, the Big Bang was really the Big Bounce.
But how could the universe be very much older than 14 billion years? Doesn't thermo-dynamics tell us that disorder increases with time and an extremely old universe should at this stage be completely randomised. According to Bojowold, the chaotic quantum conditions of the massively dense early uni- verse allows it to "tidy up" and to "present the young growing universe with a clean state irrespective of all the mess that may have built up before". This is because loop theory allows the number of space atoms to change. So quantum effects during the Big Bounce erased almost all traces of prior history.
But there are still ways of probing this period. Gravitational waves and neutrinos scarcely interact with matter and therefore passed through the primordial plasma with minimum loss. These may soon bring us news of a time before the Big Bang/Bounce.
© 2008 The Irish Times
What I want to know is how much time will it take to make more space in the garage?
