Big Bang theory’s rival keeps bouncing back

Most students will have heard of the Big Bang and be able to say that it was the beginning of the Universe. Some might even tell you that it happened about 14 billion years ago. What they are unlikely to know is that Big Bang theory has some big problems.

Big Bang theory tells us that 13.8 billion years ago – before Earth, the Solar System, the Milky Way or even particles existed – the Universe spontaneously materialised from nowhere and nothing, and in less than a blink of any eye, inflated like a balloon. In the subsequent period of more sedate expansion, matter formed and mixed together, leaving a largely blank cosmic canvas with a few flaws here and there that went on to become the life-giving galaxies and stars we see today

But in recent years, questions have grown louder about this view of the Universe’s beginning, and indeed about our current understanding of how the cosmos works. Much of this doubt stems from differences between what theory predicts and what is observed. For example, there should be

Gravitational waves we can detect from quantum fluctuations during the very early Universe’s rapid and brief inflation phase. They don’t exist. Also, the relic radiation left over from the Big Bang that permeates all space, known as the Cosmic Microwave Background, should be roughly uniform. It’s not. Most importantly from a theoretical perspective, at the moment of the Big Bang, everything is squeezed into a space with zero volume and infinite density, known as a singularity. If this happens, physics completely breaks down. These and many more anomalies and problems point to something missing from our cosmic understanding

Though most researchers believe the faults in Big Bang theory will be ironed out in time, an alternative idea gaining traction is that the Universe did not originate in a Bang at all, but in a Bounce. In this scenario, the Universe expands up to a certain point, stops and then contracts back down to a tiny size, only to then start expanding again, over and over. If true, we could be living in the first or any one of an infinite number of Universes made by successive Bounces.

“Dark energy” plays a key role for some Big Bounce supporters. A mysterious property of the fabric of space itself, dark energy is thought to make up around 70% of the Universe. It is believed to power the Universe’s accelerated expansion that we are witnessing right now. But in the far future, it could become less concentrated, leading to decelerating expansion, and the eventual contraction of the Universe – ready for the next Bounce and a fresh new Universe.

The Big Bounce has faced the same criticism as the Big Bang. At the point between contracting and expanding, the Universe forms a nonsensical singularity. Despite sharing this fundamental problem, Big Bang theory’s success in many other regards saw Big Bounce ideas fade from view during the 1980s. However, recently two groups consisting of Paul Steinhardt and Anna Ijjas, and Neil Turok and Steffen Gielen have rekindled the Big Bounce. In very different ways, they have both demonstrated that it is possible the Universe would never actually become a singularity. What’s more, Ijjas may soon identify unique features of a bouncing universe for astronomers to hunt down. If astronomers spot any of these telltale signs, the Big Bang’s rival will have truly bounced back into contention to explain the origin of the Universe.

Dr Benjamin Skuse

Freelance science writer

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