THE MOST DOMINANT FORCE IN THE UNIVERSE IS ALSO ITS MOST MYSTERIOUS AND MOST UNANTICIPATED

Reported by Robert Lea

You have to feel for physicists at the end of the 19th century. After massive advances in physics in the previous 100 years, many Y thought they had a good handle on the laws of the universe and all that remained for them to do was make more precise measurements. Quickly, the 20th century began to deliver insights that turned this confidence on its head. Arguably, the most shocking of those discoveries came at the end of the century, when it was found that the universe is expanding and that expansion is accelerating. The force behind this was labelled ‘dark energy’, but that’s just a placeholder. Troublingly, dark energy isn’t just an aspect of the cosmos, it’s the dominating aspect. And it’s something that can’t be ignored, as the nature of dark energy could decide what happens at the end of the universe.

Fittingly, scientists are in the dark about what dark energy actually is. Luz Ángela García Peñaloza is a cosmologist at Universidad ECCI in Colombia who has spent the last decade trying to understand it. “It’s like looking for a needle in a haystack,” she tells All About Space. “Except with a needle, you know what it looks like, and you know what it will feel like when it pricks your finger. With dark energy, we don’t know what we are looking for. It’s a bit hilarious in the sense that we are studying something we just aren’t seeing. But Understanding dark energy will help us understand so many fundamental questions.”

Even before dark energy was found in the cosmic recipe of the universe, the 1900s had been delivering findings denting scientists’ confidence. Quantum physics emerged, showing the rules of the subatomic were not just different from those of the macroscopic everyday world, but were counterintuitive and disturbing, challenging the notion of what’s real. Einstein revealed space and time could unite as a single four-dimensional entity called ‘space-time’, set the speed limit of the cosmos to the speed of light and then showed in 1915 that gravity wasn’t as Newton imagined, but emerges from the very curvature of space-time.

Hints at the expansion of the universe began when using this latter theory, general relativity, and its equations to formulate an equation to describe the state of the cosmos. Einstein found that it predicted that the universe wouldn’t ‘hold still’. The consensus at the time was that the universe was static, neither expanding nor contracting, and Einstein agreed with this. But general relativity predicted a dynamic universe. To combat this, Einstein introduced a ‘fudge factor’ to his work to counteract gravity. This was known as the cosmological constant, and it was represented by the Greek letter lambda. The cosmological constant would be a headache for physicists for decades to come, albeit in a different form, but we’ll get to that.

Einstein would rue the introduction of the cosmological constant when Edwin Hubble made observations of distant galaxies that showed they were receding away from each other, and from us. This showed that the universe isn’t in a steady state, but is expanding. In light of this evidence, Einstein dropped the cosmological constant, later describing it as his greatest blunder. But the cosmological constant wouldn’t stay in the theoretical dustbin for long, thanks to a massive revelation that came at the end of the 20th century.