Monday, April 27, 2009

What banged?

Until recently scientists and priests seemed to be in awkward agreement. Genesis started with a bang! It happened 13.7 billion years ago; and questions like “what caused it?” or “what was there before?” were considered a scientific no-man’s land where no decent, career-minding, physicist dared to venture. After all science is about things that can be measured. How could one measure something before it happened?

This was precisely the subject of the public lecture given by Cambridge physicist Neil Turok on April 17th at the Athens Concert Hall. The daring title was “what banged?” and it aimed to introduce a radical new cosmological theory. Turok, together with Paul Steinhardt of Princeton, have named their theory “the ekpyrotic universe” and explain it in their trade book “Endless Universe” (published in Greece by Avgo Books, http://www.avgobooks.com/). According to the pair of authors, there have been countless Big Bangs, followed by long terms of space-time expansion, an endless cycle of universal birth and re-birth. “Only by positing an endless universe can we explain the mysteries of the cosmos,” said Turok like a modern-day Brahman.

Cosmic PuzzlesAnd there are mysteries aplenty to keep cosmologists on their toes. Following the observational confirmation of the Big Bang theory in 1964 by Penzias and Wilson, scientists had to explain how the universe was so uniform (same average density of matter and energy wherever you chose to turn your telescope). They thus hypothesized something that happened during the first critical moments of the Big Bang, a force field that guaranteed thermal equilibrium across the universe as well as uniformity of matter and energy (think of an electric heater trying to heat up uniformly a room that keeps expanding, and you will understand the difficulty cosmologists were facing). The force field was called "inflationary field".

In summary, the traditional viewpoint holds that a tiny fraction of time after the Big Bang, the Universe expanded with an amazing rate, doubling itself in size every billionth of a billionth of a second. The cause of this incredible expansion - named “inflation” - was first suggested by Alan Guth, now at MIT. Inflation worked on the early universe only for a very short while and then died out, allowing for a much slower expansion afterwards. It was an elegant, albeit ad hoc, hypothesis that seemed to satisfy observation data - until two new observational puzzles arrived to upset it.

The first puzzle is the so-called “dark matter” that accounts for 25% of the cosmos (“normal” matter, the stuff of stars, galaxies, you and me, accounts for only 5%). No one knows what dark matter is, but we do know it is out there, in the same way that we know there is water in a glass even if transparent (light bends when travelling through it). The second weirdness is “dark energy”; it accounts for 70% of the universe, and a few billion years ago started to accelerate the universe’s rate of expansion. The inflationary hypothesis had to be urgently overhauled in order to account for both.

Getting rid of InflationTurok and Steinhardt had both worked as theorists on the inflationary model of the Big Bang, but became increasingly disillusioned with its ad hoc character; until they decided to apply the mathematics of string theory to the early universe and see what happens. When they did so all puzzles and weirdness disappeared! No need for an invented inflationary field! Dark matter and dark energy were acounted for and made perfect sense! It was an incredible eureka moment! All you had to hypothesize was that we live in a universe of normal matter that hovers parallel to another universe of dark matter. Between the two parallel universes flows dark energy, pulling and pushing them apart. When the two universes collide a Big Bang occurs; all matter and energy becomes light and a new pair of twin universes is born; the “normal matter universe” expands and cools by pulling away from its “dark matter” twin sister and then, when expansion arrives at its maximum, dark energy pulls the two universes back for another collision - and the whole cycle starts anew.

Ekpyrotic StringsTo understand better the ekpyrotic universe hypothesis you have to understand strings. Since the early 20th century physicists have two wonderful theories to explain everything: the relativity theory of Einstein that deals with gravity and explains natural phenomena from a multi-molecular scale upwards to planets and galaxies and clusters of galaxies – and quantum theory, which explains what happens at a submolecular and subatomic scale. The problem is that scientists cannot reconcile – or “unify” - the two theories together. This is extremely annoying because it suggests that nature is operating different laws at macroscopic and microscopic levels, which is absurd. String theory, rich in exotic mathematics and developed during the past twenty years, comes to the rescue! It suggests that nature is built by tiny, dimensionless strings, objects that look like rubber bands. Depending on the oscillations and twists of those tiny strings, the cosmos behaves in a quantum or relativistic way.

According to string theory our universe can be seen as a three dimensional membrane where space-time is stretched and pulled by dark energy. Think of space-time as the arena and planets and galaxies as the objects inside it. String theory suggests that the sun and Earth spring into existence from within the fundamental geometry of space-time. In other words, everything is the physical realization of mathematical entities. The world is a shadow show of maths-at-work in a multi-dimensional background! Plato, had he been among the audience of Athens Concert Hall that night, would have felt vindicated.

Complexity begets complexityWhat I find exceptionally thrilling with string cosmologies such as the Ekpyrotic Universe is that instead of assuming material nothingness as the primal cause of the Universe they presuppose mathematical complexity. Indeed, the mathematical complexity of the vacuum assumed by the ekpyrotic theory - and its parent M theory – is (at least for the time being) enormous, to an extent as yet unimaginable. But we do not need to see the whole mathematical picture in order to appreciate that, the very idea whereby a complex universe - such as the one we inhabit - manifests out of complex mathematical-geometrical entities offers a new and deep insight. Could these complex mathematical-geometrical entities be the natural laws? Could the laws of nature resemble abstract, immaterial casts into which planets, starts and creatures and minds are molded? If that is the true nature of reality, I somehow feel that it makes far more sense than silly ideas such as the Copenhagen Interpenetration, or the Multiple Worlds interpretation of quantum physics.

LISA will testTurok and Steinhardt applied string mathematics to the early universe and found that the theory works perfectly. But how can they be sure? One must always judge the efficacy of a beautiful scientific idea by the means that can be tested. Can we do so in this case? Surely the only way to tell if the ekpyrotic theory is correct is to peer into the moment of Big Bang itself, to go behind the primal afterglow as sensed by W-MAP. But can we measure anything before radiation was born? According to Turok we can! We can measure gravitational waves, the ripples of cosmic turbulence that travel across space-time, like waves on the surface of a pond when a stone has been tossed. And that is exactly what is going to happen in 2018 when spacecraft LISA (Laser Interferometer Space Antenna), a joint venture of NASA and the European Space Agency, will aim to detect and confirm the existence of gravitational waves. If Turok is right there should be no gravitational waves from the Big Bang.

He certainly believes in his theory, so much that he made a wager with a very famous wager-man who also happens to be Neil’s friend as well as his colleague-down-the-corridor at Cambridge University. “I betted Stephen Hawking I’m right”, he said smiling, to the general applause of the Athenian audience.