The Big Bounce: A Variation on the Big Bang

The Big Bang model is the current consensus theory on the origin of the universe. Yet it still has problems which physicists are working to iron out, particularly with regard to that initial condition right before the universe began to expand.

At that moment, the Big Bang model stipulates that all matter was confined to a single geometric point of infinite density. Such a condition begs fundamental questions about the nature of both spacetime and gravity. A quantum theory of gravity called loop quantum gravity (or simply loop gravity) attempts to answer some of these fundamental questions. In the process, it proposes a modification of the Big Bang model often called the "Big Bounce."

In an ancient thought exercise called the Dichotomy Paradox, a finite distance is divided in half an infinite number of times. Space has long been thought to be infinitely divisible in this way.

Once it was thought that matter, too, is infinitely divisible, but the concept of atoms (and later, of the quarks that make up subatomic particles) denies that idea. Matter is now known to come in infinitesimally small, indivisible packages. And loop quantum gravity propose that spacetime is the same way. According to loop gravity, spacetime has a fine structure consisting of "atoms" of spacetime which cannot be subdivided and which can contain only a finite amount of matter-energy.

The Big Bang theory proposes that just before the "explosion" in which the universe began expanding, all matter was compressed into an infinitely small point. If space has a fine structure composed of indivisible, finite quanta of length and volume, this infinite compression would not be possible.

Under the conditions familiar to humans, gravity is an attractive force whose strength increases exponentially as the inverse of distance. As matter grows denser and denser, gravity becomes stronger and stronger. This principle has been understood to be the nature of gravity since Newton.

The mathematics of the loop gravity hypothesis suggest an interesting twist. According to loop gravity equations, at extremely high densities, gravity reverses and becomes a repulsive force.

Loop gravity results in an oscillatory model in which repeated Big Bangs are each followed by a Big Crunch. In other words, the universe existed before the Big Bang. As the universe collapses to a very high (but not infinite) density, gravity switches from an attractive force to a repulsive force, resulting in a new expansion.

The mathematics of loop gravity indicate that specific information about the pre-Big Bang universe is lost during the high-density period. We cannot use observations of our universe to turn the clock back and learn details of this earlier universe.

Bojowald, Martin. "Big Bang or Big Bounce?: New Theory on the Universe's Birth." Scientific American (October 2008).