Humans have wrestled with the nature of the universe since time immemorial, but we’ve had science to guide us in recent generations. Most experts on physics and cosmology accept the inflation model, a straight line from the Big Bang to our infinitely expanding universe. However, some scientists hold onto the possibility of a “Big Bounce” instead of a bang, and they’re still actively searching for evidence that could upend the conventional wisdom.
Throughout the 20th century, scientists learned a great deal about the early universe. Most of what we learned supports the idea of an inflationary universe, one that has enough mass to continue expanding forever after the Big Bang. Several major discoveries about the universe have strengthened support for this idea. For example, the universe is flat and uniform in every direction, which is what you’d expect from a rapid expansion. Measurements of the cosmic microwave background radiation (a remnant of the Big Bang) show some spots in the universe are colder than others, which again, is what the inflation model predicts. Inflation also accurately predicted the mass density of the universe.
The inflation model doesn’t explain everything, though. Most researchers agree it’s still incomplete, but others feel it will never explain what we see as well as the Big Bounce. Neil Turok, director of the Perimeter Institute for Theoretical Physics, is one of those scientists. He’s a proponent of the Bounce, which holds that inflation is only one phase. Eventually, the universe collapses into a singularity and “bounces” to begin growing again.
According to bouncers, inflation is too specialized at a fundamental level. To support inflation as it’s currently understood, the early universe would have needed very specific and unlikely conditions. Inflation also implies the existence of an infinite number of pocket universes. The current model says inflation continues forever, stopping only in some regions of space. Meanwhile, the universe continues expanding in other regions faster than the speed of light. These bubbles would be closed off from each other, possibly with incompatible laws of physics. Turok claims this is unfalsifiable and unscientific.
An all-sky map of the cosmic microwave background radiation, from when the universe was just a few hundred thousand years old. The heat fluctuations eventually turned into galaxies.
The Big Bounce isn’t a silver bullet, though. There are several potential versions of the bounce — some cyclical and others that only bounce once. They all require new physics to explain, but Turok and his colleagues say they’ve 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 call on quantum gravity. In this version, negative energy prevents the universe from becoming a true singularity, allowing it to re-expand in normal space after the bounce. This could even explain some things that have been taken as evidence for the Big Bang, for example, 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 the consensus. Inflation has passed a lot of predictive tests, and the physics backing the Big Bounce are unproven. 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.
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