Hunt for the multiverse
Could there be another you reading this article somewhere out there in space?
Revealed: the latest findings in our search for another universe
Cosmonaut Alexei Leonov descends to the foot of the ladder before placing his boots into the grey dust. His words are beamed back to Earth and ring out of television and radio sets across the planet. “For my country, my people and the Marxist-Leninist way of life,” he says, as the hammer-and-sickle flag stands proudly behind him. For the first time in history human beings have finally set foot on the Moon.
If you're scratching your head knowing full well that the Soviets didn't land people on the lunar surface, this is the historical twist on which the new Apple TV drama For All Mankind rests: the Americans were beaten to the Moon, and the storyline follows the resulting fallout. Yet there's a chance that these events really did take place – just not in our universe. Many cosmologists increasingly suspect that ‘the’ universe should really be replaced with ‘our’ universe, one of many in a complex landscape of cosmoses called the multiverse.
If there are enough of them, eventually you'll end up in a universe where events repeat. In others history will be eerily similar, but with small variations such as the Soviets getting to the Moon first, you becoming US president or having three arms. It all boils down to the fact that there are only a finite number of ways to arrange things. Imagine a room containing ten objects in a line. There are 3,628,800 unique ways to arrange those ten objects. If you have four million rooms, it is guaranteed that in at least two of them the objects will be in the same configuration. Equally, there are only a limited number of ways you can arrange atoms in a universe. If there are a large number of other universes, eventually you will get a copy of this one. That means an exact replica of you reading an identical copy of this article.
Strictly speaking you don't need the multiverse for this unsettling reality to hold true. Even if our universe is the only one, if it's big enough, travel far enough and it becomes a statistical certainty that events repeat. Just like if you visited every home on Earth to see how each one's objects are arranged, you wouldn't need another Earth to ensure that you found a repeat. How far do you have to go into this universe to find another you? It's much further than the boundary of the observable universe, so there's no prospect of running into your doppelgängers. Cosmologists call this a Level I parallel universe, and it's the simplest model of a multiverse.
A Level II parallel universe is a separate cosmos created by the same process that spawned ours, and for that we need to look back to the Big Bang, and a period called inflation in particular. “If our universe was created by inflation then billions of others were too,” says Tom Shanks from the University of Durham. “We have to test that theory.” Inflation is the idea that our infant universe underwent a short burst of souped-up expansion. In a trillionth of a trillionth of a trillionth of a second it ballooned from smaller than an atom to the size of a grapefruit.
That may not sound like much, but if you increased the size of a red blood cell by the same amount then it would be larger than the entire observable universe. Physicists have spent the last 40 years examining the mechanisms that could have caused the universe to blow up in this way. In most theories the process of inflation is eternal – once started it can never be fully stopped. If true, that means it is still creating universes to this day,
“If our universe was created by inflation then billions of others were too. We have to test that theory”
Professor Tom Shanks
forever adding new members to an already crowded multiverse. According to string theory, the number of other universes could run to as many as one with 500 zeroes following it.
It may sound a little farfetched, and frankly intimidating, but there are ways in which it may be possible to test the idea of a multiverse. Many have turned to the cosmic microwave background (CMB) – the afterglow of the Big Bang – for help. A map of this radiation is speckled with hot and cold spots, and almost all of them are of a similar size. Yet one stands out like a sore thumb – a huge cold spot in the southern sky towards the constellation of Eridanus. If our eyes could see the CMB, this cold spot would appear ten-times wider than the full Moon. Not everyone agrees that it’s real. Some have suggested it is a ghost in the data, the result of the statistical analysis carried out by researchers.
If it’s really there, the simplest explanation of the cold spot is that there is an unusually huge patch of empty space between us and the origin of the CMB. As the light from the early universe passed through this barren zone it would have been robbed of some of its energy, leading us to see an abnormally large cold spot in the CMB. That's exactly what Shanks and his team set out to test.
In 2017 they released details of an extensive survey of galaxies in that direction. They found it to be no more underpopulated than anywhere else in the universe. So what is causing the cold spot? “The next most standard explanation is a bubble collision,” says Shanks. That's a collision between this universe and a neighbouring one created by the process of inflation at a very similar time to ours. The multiverse could quite literally have left
its mark on the sky. It's not an easy pill to swallow. “The idea left me uncomfortable, but we have to test it,” says Shanks.
Unfortunately that's not an easy thing to do.
The bubble collision theory should also have affected the orientation of the microwaves that make up the CMB. If we can measure this so-called ‘polarisation’ and it matches the predictions of inflation theory, it could be strong evidence in favour of the multiverse. Shanks put one of his students on the case, combing through data from the European Space Agency's Planck satellite, but that search was ultimately in vain. “Unfortunately Planck doesn't have the signal-to-noise ratio,” says Shanks. The polarisation signal – if it's there at all
– is so faint that Planck cannot distinguish it from the background noise of the cosmos and the probe's own instrumentation. It's a bit like trying to hear someone whisper when there is a pneumatic drill pounding away outside. One instrument that may be capable of picking it up is the six-metre (19.7-foot)
Atacama Cosmology Telescope in Chile, but it is yet to find that elusive signal.
Our reasonably new ability to pick up gravitational waves may do the trick. They are ripples in the very fabric of space itself, and the ones we've detected so far have come from colliding black holes and neutron star mergers. Yet the theory of inflation also predicts that events in the early universe should have generated so-called primordial gravitational waves. They would be tiny now nearly 14 billion years later, but future gravitational-wave detectors may be sensitive enough to spot them.
Equally, primordial gravitational waves passing through space when the CMB was released are predicted to have left their mark. Just like a bubble collision, they should have changed the orientation of some of the CMB microwaves. Back in March 2014 a result from the BICEP2 experiment in Antarctica made headlines around the world and generated much talk of an impending Nobel Prize. The team claimed to have found evidence of inflation – and a significant hint at other universes – in the CMB. Yet they'd jumped the gun. Further analysis showed that the same effect could have come from dust in our Milky Way galaxy.
Let's imagine for a moment that we do one day find irrefutable evidence that the cold spot in the CMB really is a bruise from a parallel universe and that the multiverse is real. How common would life be across this vast cosmic landscape? Not that rare according to work done last year by a collaboration of researchers from British and Australian universities. It all boils down to dark energy – the mysterious and invisible entity thought to be accelerating the expansion of our universe. The amount and strength of dark energy will be slightly different in each member of the multiverse.
If it's too strong then that universe will expand too quickly to ever allow gravity to gather material into stars and planets. Traditionally we've thought of our universe existing in a sweet spot where the amount and strength of dark energy is just right. “The multiverse theory could be thought of as a lottery. We have a lucky ticket and live in the universe that forms beautiful galaxies which permit life as we know it,” says Jaime Salcido Negrete from Liverpool John Moores University.
“We have a lucky ticket and live in the universe that forms beautiful galaxies which permit life” Jaime Salcido Negrete
“Like the other forms of multiverse, the many-worlds theory is notoriously hard to prove”
Yet stars and planets could still exist even if our universe had hundreds of times more dark energy or substantially less. At least that's according to state-of-the-art supercomputer simulations published in 2018 by Negrete and his team. “Adding dark energy up to a few hundred times the amount observed in our universe would actually have a modest impact upon star and planet formation,” he says. The prospect of other universes with life in has had some big-name backers. Stephen Hawking's final scientific paper – published ten days before his death in 2018 – supported the notion that the laws of physics would be consistent across the multiverse. So what has happened in our universe could well have happened in others too.
That leaves us with a Level III multiverse, and for that we turn away from astronomy and towards quantum physics. The quantum world is famously weird. Unlike objects we encounter in our everyday lives, subatomic particles can exist in multiple places at once. Only once we measure the position of the particle does it ‘decide’ where it is. This is famously illustrated by the Schrödinger's cat thought experiment. The proverbial cat is in a sealed box with a vial of poison rigged to a hammer and a device to measure the position of a particle. If it is in position A then nothing happens. Yet if it's in position B the hammer is deployed, breaking the vial and killing the cat. The thing is, until the measurement is made the particle is simultaneously in position A and B. Meaning – at least in principle – the vial is broken and unbroken and the cat is both dead and alive at the same time.
What leaves a lot of physicists uncomfortable is why the act of measuring forces the particle to ‘choose’. An alternative explanation is that it doesn't choose at all. Whenever a quantum measurement is made the universe fractures into two realities – one with the particle in position A and another where it is in position B. This is known as the many-worlds interpretation. Like the pages of a book, these multiverses sit close together, but are separate from one another. There's no journeying between worlds. Like the other forms of multiverse, the many-worlds theory is notoriously hard to prove. It used to be a fringe theory on the sidelines of serious academic research, yet today it has quite a following.
There are those that rail against the multiverse, claiming it is little more than pseudo-science. Paul Steinhardt, one of the pioneers of the theory of inflation, has had a change of heart and is now an outspoken critic of those pursuing the existence other universes. But the draw is just too great, it fascinating to contemplate the notion that we are merely one of many copies of ourselves living lives both identical and different to our own. And, one day, if the theory of inflation is substantiated, it may just tell us how our own universe flashed into existence all those years ago.