What Is the Big Bounce?

The Big Bounce is the name of a scientific theory about the origins of the universe. The theory is intended to replace an earlier theory known as the Big Bang. The Big Bang is not consistent with some aspects of general relativity, Einstein’s famous theory that explains how matter and energy interact. According to the Big Bang theory, the universe initially expanded from a small, static point for reasons that are not yet understood. According to the Big Bounce theory, the current universe resulted from the collapse of a previous universe.

Since prehistory, humans have pondered the origins of the Earth and its surrounding cosmos. Religion, philosophy, and early science each posited theories about how the universe began, with increasing levels of sophistication. Astronomical observations in the 20th century confirmed that the universe has been expanding for billions of years. Working backward, physicists proposed that the universe began as a small, dense area that contained all matter and energy. An unexplained event 14 billion years ago caused this matter and energy to expand explosively, an event called the Big Bang.

The problem with this theory is that it does not explain what existed before the Big Bang or what triggered the expansion. These matters have been described as singularities, areas of the universe in which the laws of physics do not apply. This was the only way to reconcile the Big Bang with Albert Einstein’s theory of general relativity, the best model of how the universe operates. Scientists have also considered alternative explanations of how the universe began. In the first decade of the 21st century, studies in quantum physics resulted in the theory of the Big Bounce.

According to the Big Bounce theory, when the universe expands beyond a certain point, gravity will cause it to collapse in on itself again, the so-called Big Crunch. Once the collapsing universe reaches a certain density, a principle called loop quantum gravity will cause the matter and energy to “bounce,” or begin expanding again, creating a new universe. The Big Bounce theory does not reveal if this has happened once or many times; it could repeat infinitely. Another intriguing possibility is that these infinitely repeating universes are identical, effectively creating a universal time loop.

The Big Bounce theory is appealing to many physicists because it does not involve singularities and because it includes general relativity and quantum mechanics, two principles of physics that are notoriously difficult to reconcile with each other. Further study is required to determine which theory actually describes the origins of the universe. Much depends on quantifying dark matter and dark energy, mysterious substances that may affect the expansion rate of the universe. Meanwhile, the theory has already provided material for writers of science fiction. In a 2011 episode of the animated comedy Futurama, the main characters observed the Big Bounce while traveling through time into the far future.