Cosmology challenges some of the most fundamental questions we have about our existence – how did this all begin? How will it end?
For decades, astronomers have been exploring the Universe around us, using new technology and insights to better understand the time at the very start of our Universe. By observing galaxies so distant that their light has taken billions of years to reach Earth, they’re effectively looking back to a time when the Universe was in its infancy.
Alas, there is only so far back in time we can look. Stars only started shining a few hundred thousand years into the Universe’s lifespan. The further back we go, the more the signs of what came before are overwritten by the exploding stars and colliding galaxies that came after them.
Go back far enough and even those traces vanish, erased in the Big Bang – the very explosion that created our Universe.
So rather than looking back in time, perhaps the answers might instead come from looking forward. By understanding the end of the Universe, might we also learn about its beginning?
Professor Henry Tye from Cornell University is one of the theoretical physicists trying to predict the final fate of our Universe. Using the latest observations of the Universe, Tye and his team have been able to build detailed simulations of the cosmos. They then run the clock forward billions of years to find out what might be in store for us.
These simulations start with the Big Bang, which kicked off the Universe 13.8 billion years ago. According to our current understanding, space itself has been expanding ever since, carrying galaxies ever further away from each other. Initially, cosmologists thought this expansion would slowly decline over time as it ran out of steam, leading to one of two main outcomes.
The first scenario is that, eventually, the expansion of our Universe will dwindle to the point it can no longer overpower the pull of gravity trying to crunch everything together again. The expansion would turn on its head, and the cosmos would start to collapse until it all came together again in a Big Crunch.
The alternate scenario is that expansion could drive the Universe far enough apart that it never quite gets overtaken by gravity. Matter and energy would continue to separate until it was evenly spread across space, unable to clump together to form stars or galaxies, ending our reality in an eternal Big Freeze.
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Then, in the late 90s, astronomers discovered that rather than slowing down, the expansion of our Universe is in fact accelerating. Something was driving the Universe ever further apart. Uncertain what that something might be, astronomers came up with the concept of dark energy.
“About 70 per cent of the content in our Universe today is dark energy,” Tye told BBC Science Focus.
Cosmologists are unsure exactly what dark energy is, but the leading theory is that it’s related to some fundamental quality of empty space. They incorporate dark energy into their equations about the Universe using ‘the cosmological constant’ – a concept related to how much energy space has, even when it seems completely empty.
But growing evidence hints there might be something else at work. As the name suggests, astronomers thought the cosmological constant was, well, constant. If that is true, then the effects of dark energy should be the same across all of cosmic history.
But new observations suggest this is, in fact, not the case. Two projects – the Dark Energy Survey (DES) and Dark Energy Spectroscopic Instrument (DESI) – have spent years measuring the positions of millions of galaxies to create detailed maps of the 3D Universe. Astronomers can then use these to search the cosmos for signatures left behind by dark energy.
Both these surveys seem to show that dark energy has been evolving and changing over time. That means either the cosmological constant isn’t as constant as we thought, or there’s something else at work.
Turning cosmology on its head
To get to the bottom of dark energy, Tye and his colleagues have used the DES and DESI results to come up with their own theory – that a particle known as an axion is also responsible for the Universe’s expansion.
“It’s simplest to assume that dark energy comes entirely from the cosmological constant,“ says Tye, but that doesn’t appear to be what’s happening.
Axions are still theoretical as they only interact with normal matter via gravity, making them very difficult to detect in a lab.
Their theory goes that the cosmological constant is indeed a constant, but that axions are also at work, driving the Universe apart.
In the early eras of the Universe, axions were the dominant factor. But as space has expanded, these axions have been spread out, lessening their impact, which is why dark energy appears to evolve over time.
Meanwhile, the cosmological constant has been flipped on its head. In most current theories, the constant is positive, meaning it drives the Universe apart. Tye’s team suggest it’s actually negative, and is trying to draw everything together. Eventually, when the axions are so spread out they no longer dominate, the cosmological constant takes over and starts to draw everything back in.
“The Universe’s expansion is still accelerating now in our theory,” says Dr Hoang Nhan from Donostia International Physics Centre, who also took part in the study. “The Universe’s expansion only slows down when the so-called ‘crunching’ phase starts, which may happen in the next 11 billion years, as a rough estimation, followed by a Big Crunch around eight billion years afterwards.
“Unfortunately, our model can't give much prediction after the Universe collapses.”
The problem is that as everything squeezes back in together, the strange world of quantum mechanics begins to become just as important as the laws of gravity and relativity. Quantum can explain what happens on very small scales quite happily, while gravity takes care of the large-scale Universe. Under most conditions, the two rarely bother each other.
The problem is those rare occasions when the laws of gravity and quantum both apply – physicists have never quite been able to work out how to make the two work together. And without that understanding, they can’t work out exactly what will happen next.
What we do know is that the last time everything was all together in one place, the Big Bang happened. Some cosmologists have put forward the idea that a Big Crunch could lead to a second Big Bang – a Big Bounce. A cyclical Universe that starts with an explosion, expands and then contracts, only to explode over and over again.
The Big Bounce is a tempting theory, as it explains not only how our Universe might end, but how it could have begun. But the truth is, we’ll probably never know for sure exactly what our Universe’s fate will be.
“Here, we predict the end of our Universe,” says Tye, “but precisely how it ends remains an open question. Just as one can predict a person’s life expectancy without predicting the circumstances of the death.”
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