Research News

Big Bang or Big Bounce?

There is a new dark-horse entry in the cosmological sweepstakes.

Cosmologists Alan Guth and Paul Steinhardt
Cosmologists Alan Guth, left, and Paul Steinhardt

In the last 50 years, the Big Bang theory has gradually become the standard scientific model for how the universe began and has been written into the grade school science textbooks. That is because it has done an amazing job of explaining many of the physical properties of the observable universe.

In the last few years, however, a rival theory, which I’ve dubbed the Big Bounce, has arisen that its proponents claim is more elegant and describes what we know about the universe equally well.

Last month members of the Vanderbilt community had the opportunity to listen to the chief proponents of the two theories, which paint dramatically different pictures of the basic nature of the universe. Supporting the standard model was Alan Guth, the Victor F. Weisskopf Professor of Physics at MIT. In the opposite corner was Paul Steinhardt, the Albert Einstein Professor of Science at Princeton, one of the chief proponents of the challenger called cyclic cosmology. In 2007, he and Neil Turok of Cambridge University have written a popular book on their ideas, called Endless Universe.

Big Bang ‘last theory standing’

First, let’s turn the dial back to the 1950’s. Scientists were debating whether the universe started out with a “Big Bang” (a label given it by its opponents in hopes of discrediting it) or whether it existed in an unchanging, steady state.

Then, new astronomical observations showed the universe is expanding and scientists discovered the cosmic background radiation, a weak microwave signal that comes from every direction in the sky. Scientists recognized that it was fossil radiation left over from the early days of the universe and that it implied the universe originated in a hot dense phase. These observations knocked the steady-state model out of contention, leaving the Big Bang the last theory standing.

NASA illustration of inflationary big bang universe
NASA illustration of inflationary big bang universe

In its original form, the Big Bang theory had a number of problems. In particular, it couldn’t explain why astronomers’ measurements found that the universe is exceptionally flat.

This issue was addressed by adding the idea of cosmological inflation – that, in the first second after it was created, the universe expanded so rapidly that a volume the size of an atom grew to the size of the visible universe!!!

This hyper-expansion was attributed to an unstable form of negative gravity that, when it quit, decayed into the energy and matter that make up the universe today.

One of the latest successes of inflationary theory is its prediction that following the inflationary period, space was not perfectly smooth, but contained a very subtle variations in density, a pattern that astronomers have found in cosmic background radiation.

Recently, cosmologists have been forced to add another type of negative gravity, called dark energy, to explain evidence that the rate of expansion of the universe has been increasing for the last five billion years, rather than slowing as cosmologists had expected.

From universe to multiverse

The final twist in the Inflationary Big Bang theory has been the recognition that it is difficult, if not impossible, to totally turn inflation off. As the inflationary force decays, there are always areas that continue to expand. They are expanding so rapidly that they continually outstrip the decay process. The net result is a “multiverse” made up a series of pocket universes, each with different basic physical properties.

This is either an advantage or a disadvantage depending on an individual’s view of the anthropic principle, the philosophical argument that the basic physical properties of the universe appear to be compatible with the existence of conscious life. If inflation is randomly churning out different kinds of pocket universes, then it is inevitable that we find ourselves in one that is compatible with life.

Although Guth is willing to accept this possibility, Steinhardt is not. “Because the process is completely random, it doesn’t have any explanatory power,” he objected, in a discussion following his lecture at Dyer Observatory.

Bouncing branes

Steinhardt and a small cadre of collaborators have come up with a theory that has a cyclic structure, harking back to Heraclitus and Vedic scripture. To do so, they have borrowed some basic concepts from string theory, which is built on the idea that matter is built out of tiny, vibrating loops instead of points or balls.

In the string-theory universe, our three-dimensional world is a subset of the entire universe and exists on the surface of a four-dimensional membrane (nicknamed a brane) floating in multi-dimensional spacetime called the bulk.

The normal matter that we are made from is stuck to the surface of this brane, so we can’t perceive what is going on in the bulk. In fact, gravity is the only force that we can measure that penetrates into the bulk.

The basis of their theory is that our brane world is connected to an invisible twin brane world. The force connecting them acts something like a giant rubber band that brings the two branes together once every trillion years or so. The collision of two brane worlds looks just like the Big Bang, creating large amounts of energy and matter and triggering a modest period of expansion. View an animation.

Our twin brane world has mass and energy but, according to Steinhardt, it is unlikely to have physical properties compatible with life. The gravitational force from stars and galaxies in our universe should effect the matter in the twin brane and vice versa. This can explain why we haven’t found any visible traces of dark matter, invisible matter that scientists have detected by its gravitational influence on the motion of stars circling other galaxies. It could be dark matter is invisible because it exists on the twin brane.

For the first eight billion years, or so, the brane worlds are dominated by matter and energy. But, as they continue expanding, dark energy takes over. Although the expansion caused by dark energy is much, much slower than the hyper-expansion in the Big Bang model, it has billions of years to operate and, as a result, can flatten spacetime in the same fashion. As a result, the model doesn’t need to invoke inflation at the very beginning of the cycle.

According to Steinhardt, the Planck satellite launched last year by the European Space Agency is making observations that will differentiate between the two. The Big Bang model predicts that the cosmic background radiation will show a pattern of polarization caused by intense gravity waves. The colliding brane worlds would not produce similar gravity waves, so there should be no such pattern.

Both theories explain what we currently know about the universe, so pick the one that you prefer. Do you like the straight forward, linear nature of the Big Bang? Or does the cyclic nature of the alternative appeal?