Humans have struggled with the nature of the universe since time immemorial, but over the last few generations we have had the science to guide us. Most physics and cosmology experts accept the inflation model, a straight line from the big bang to our infinitely expanding universe. However, some scientists hold to the possibility of a “Big Bounce” rather than a bang, and they are still actively looking for evidence that could overturn conventional wisdom.
Throughout the 20th century, scientists have learned a lot about the early universe. Most of what we have learned supports the idea of an inflationary universe, a universe that has enough mass to expand forever after the big bang. Several important discoveries about the universe have bolstered support for this idea. For example, the universe is flat and uniform in all directions, which is what you would expect from a rapid expansion. Measurements of the cosmic microwave background radiation (a remnant of the Big Bang) show that some places in the Universe are colder than others, which is again what the inflation model predicts. Inflation also accurately predicted the mass density of the universe.
However, the inflation model does not explain everything. Most researchers agree that it is still incomplete, but others think it will never explain what we see as much as the Big Bounce. Neil Turok, director of the Perimeter Institute for Theoretical Physics, is one of those scientists. He is a supporter of the Bounce, which states that inflation is just one stage. Ultimately the universe collapses into a singularity and “bounces” to grow again.
According to bouncers, inflation is too specialized at a fundamental level. To sustain inflation as it is currently understood, the early universe would have required very specific and unlikely conditions. Inflation also implies the existence of an infinite number of pocket universes. The current model says inflation will last forever and will only end in some areas of the space. Meanwhile, the universe continues to expand faster than the speed of light in other regions. These bubbles would be separated from each other, possibly with incompatible laws of physics. Turok claims this is unfalsifiable and unscientific.
However, the Big Bounce is not a panacea. There are several possible versions of the bounce – some cyclical and others that only bounce once. They all need new physics to explain, but Turok and his colleagues say they built a simple model that uses quantum tunneling to explain how a singularity could collapse and then emerge from the quantum realm as an expanding universe. Meanwhile, Paul Steinhardt and Anna Ijjas of Princeton University have a version of the bounce model that doesn’t rely on quantum gravity. In this version, negative energy prevents the universe from becoming a true singularity, allowing it to re-expand into normal space after bouncing. This could even explain some of the things that have been considered evidence of the Big Bang, such as the uniform flatness of the universe.
We don’t have all the answers yet, but bouncers will have to make a lot of progress to shift consensus. Inflation has passed many predictive tests and the physics supporting the Big Bounce has not been proven. Maybe one day we’ll crack the theory of quantum gravity, and the Bounce will suddenly look like the only plausible solution. For now? Not so much.