End of the universe
According to recent research, space and time's demise could come from a famous, particle-sized source
Top NASA astrophysicists uncover how and why it's sooner than we initially thought
There are many ways the world could end: a deadly asteroid impact, a global pandemic of some super-contagious bug, a president with an itchy trigger finger. But what about the cosmos as a whole? Is there anything powerful enough to shatter the entire universe?
The ultimate fate of the universe has been a hot topic among astronomers for decades. Initially they contemplated a Big Crunch – effectively a Big Bang in reverse. If the universe contains enough material then, eventually, its collective gravity would overturn the outwards expansion from its creation and it would start to shrink back down. That would ultimately lead to a calamitous collision to end the universe in fire and fury. Thankfully that picture is now more than a little out of favour, thanks to a discovery made 20 years ago.
In 1998, and thanks to data from NASA's fleet of space telescopes, two teams of astronomers independently discovered something remarkable: the expansion of the universe isn't slowing down – it's speeding up. The explanation is that there is some invisible entity lurking out there in the shadows called dark energy. It would act as a sort
of anti-gravity, pushing galaxies apart at an everincreasing pace. The more space between galaxies the more their gravitational attraction wanes, meaning dark energy dominates even more. It's a viscous cycle that could well tear the universe apart. Eventually space will stretch so much that entire solar systems would be ripped to shreds. Gaining even more traction, dark energy will even tear atoms and atomic nuclei apart until there is no longer any biology and chemistry in the cosmos. No stars and no life. This event – known as 'The Big Rip' – will leave the universe a splintered husk of its former self. According to research - including that conducted by teams at NASA - it could happen in as little as 22 billion years from now.
Despite the 2011 awarding of the Physics Nobel prize for its discovery, astronomers still know so little about dark energy and what it might be made of. This has led some researchers to claim it is just an illusion, the result of us applying the laws of physics beyond their usefulness. A figment of our collective imagination. Others have been looking into alternative culprits for the cosmos' ultimate destruction. According to particle physicists there is another contender we should add to the list of potential harbingers of doom: the Higgs boson.
Sometimes known as 'the God particle', it was famously discovered by researchers using the Large Hadron Collider (LHC) at CERN, Geneva, in 2012. It was first predicted to exist in 1964 by scientists including Peter Higgs, who now lends the particle his name. It was the missing piece in a sub-atomic jigsaw puzzle known as the Standard Model.
Think of the Standard Model as a cookbook for the cosmos, containing the ingredients and recipes needed to make everything in the universe we see around us, from stars and galaxies to planets and people. The Higgs was seen as the Standard Model's crowning glory, a resounding success for the ingenuity of particle physicists, but it could also be the end of us all.
The particle, along with the associated Higgs field, is what gives all other particles in the universe mass. Imagine the Higgs field as an all-pervading cosmic ocean. A particle’s mass is the result of how much it gets bogged down in this ocean. Particles capable of zipping through it like agile fish are incredibly light, but those that find the Higgs field like swimming through treacle are much more massive. However, the mass of the Higgs itself is capable of changing. If it ever did it would alter the strength of the Higgs field and fundamentally change everything else in the universe to boot,
“The Higgs was seen as the Standard Model’s crowning glory, but it could also be the end of us all”
including the masses of sub-atomic particles like the electron. “Chemistry relies on these masses being what they are,” says Anders Johan Andreassen from Harvard University. If they change? “Life in that sort of universe wouldn't be sustainable,” he says.
To understand how the Higgs might one day change its mass, it helps to picture two valleys either side of a central mountain. One valley sits at a higher altitude than its neighbour. According to measurements made at the LHC, the Higgs boson currently resides in this upper valley. Thankfully, the mountain – or potential barrier – blocking access to the lower valley is a high one. However, should the Higgs ever make its way down to the valley below it would then exist in a lower energy state than it is now, meaning the new Higgs field would contain a lot more energy. According to Andreassen, at least 100 trillion times more. This would fundamentally alter the way particles are able to move through it and radically change their masses. Such a change would lead to a burst of energy tearing across the universe at the speed of light, like an enormous cosmic tidal wave destroying everything it its path. The universe would be extinguished and we'd never see it coming.
What could make the Higgs jump down to the lower valley? The answer is simple probability. There's a rule in physics called the Heisenberg Uncertainty Principle which says that you cannot ever simultaneously know a particle's
“The Standard Model is very well tested, but these estimates are pushing it to its limit”
Ruth Gregory
exact momentum and position. There's always some uncertainty. It means you can only ever say where a particle is statistically most likely to be. There is a tiny probability that a Higgs boson somewhere in the universe may one day suddenly and spontaneously find itself on the other side of the mountain. The universe is a big place with a dizzying number of bosons that could do just that.
Andreassen and his colleagues have been working to calculate exactly when this might happen. According to their work – the most accurate estimation ever made – it is so unlikely that we might have to wait 10139 years for it to occur. That's one followed by 139 zeroes! Andreassen stresses just how long this is. “If you counted every atom in the universe, at the rate of one every 14 billion years – roughly the current age of the universe – you could count all the atoms in the universe and still that amount of time wouldn't have elapsed,” he says. In short, it's not something to lose sleep over. They are at least 95 per cent sure that the universe will last another 1058 years. Optimistically it could be as long as 10549 years.
It's worth saying that estimates like these are based solely on the Standard Model. Despite all its successes, it's a theory with some serious problems. It can't, for example, explain why the universe contains more matter than antimatter. Nor can it account for why tiny particles called neutrinos have a miniscule mass and can shapeshift between three different varieties. It completely omits gravity. The
weakest of nature's four fundamental forces stubbornly refuses to be brought into the fray. Dark energy is also not included.
“The Standard Model is very well tested, but these estimations are pushing it to its limit,” says Ruth Gregory, a cosmologist at the University of Durham. “There might be new physics beyond the Standard Model that could come in and completely lift the lower valley up,” she says. If that's the case, there'd be no lower valley for the Higgs to transition into and we'd all be safe. “It would remove this danger,” Gregory says. “But at the moment we just don't know.”
Gregory's own research has been looking into how black holes might change the picture. In particular she's been investigating the impact of primordial black holes – those thought to have formed in the early universe shortly after the Big Bang. “They change the calculation radically,” she says. “It makes it a lot more probable.” Stephen Hawking calculated that all black holes have a temperature – they radiate energy into space.
The smaller the black hole, the more energy – or Hawking radiation – they emit. So these tiny primordial black holes can emit quite a lot of energy. According to Gregory's calculations, by the time they weigh just a ton they're producing so much energy that they might be able to give a Higgs boson enough of kick to boost it over the mountain and into the valley below.
“Accelerating expansion means distant parts of the cosmos are being carried ever farther from us”