The size of the universe
The observable cosmos is not the whole story when astronomers consider how big the universe really is
The cosmic microwave background (CMB) is the furthest that astronomers are able to see from Earth. This ancient light was released when the universe was approximately 380,000 years old, and has been travelling across the cosmos ever since. The furthest galaxies we can see are almost as far away as the CMB – some 13.8 billion light years away. At least that is where they were when the light originally set off. While their light has been making its way to Earth, the universe has carried that part of space ever further from us. That point is now around 45 billion light years away, in effect making the observable universe around 90 billion light years across.
period of ‘expansion on steroids’ is known as cosmic inflation. According to inflationary theory, the small temperature variations in the CMB are the result of tiny quantum fluctuations during the preinflation phase which were frozen into the universe when it ballooned. As these quantum fluctuations were random and approximately equal in size, so should be the hot and cold spots they caused. That’s why the presence of one particularly large cold spot has cosmologists scratching their heads. “It’s a very significant breaking of the expected uniformity of the CMB,” explains Mersini-Houghton.
Her explanation is radical and is by no means accepted by all cosmologists, but it may help us answer the question of where our universe ends. According to one version of inflationary theory, the process didn’t just happen once. “Many other universes were created, which are similar to ours,” she says. Before each of these universes inflated they would have shared a quantum link between them. “We traced forward that quantum link to see what it would look like at present,” she says. The outcome of her calculations was the prediction of a cold spot in the CMB. Crucially, Mersini-Houghton and her team made this prediction public before the cold spot was identified beyond reasonable doubt. Not all cosmologists are convinced, however. “The overall consensus [within the cosmological community] is that the current data doesn’t support it,” Pontzen says. “It’s one of those cases where an extraordinary claim requires extraordinary evidence.” Nevertheless, Mersini-Houghton has been able to suggest where the edge of our universe might be if she is right. Her answer? “It is at least 1,000-times further from us than the edge of our cosmic horizon,” she says.
She isn’t the only one pushing the boundaries of our cosmological thinking and suggesting a potentially revolutionary interpretation of the universe in which we live. Nikodem Poplawski at the University of New Haven, Connecticut, is another physicist challenging our perceived wisdom. “According to general relativity the Big
“There is no evidence for an edge to the universe, but there is an edge to what we can see”
Andrew Pontzen
Bang started with a singularity,” Poplawski says. A singularity is an infinitely small, infinitely heavy point. This point would have grown to the size of a marble during inflation and then carried on expanding. But the birth of our universe isn’t the only place you encounter singularities. “The matter falling into a black hole also ends up at a singularity,” explains Poplawski. Black holes are gravitational monsters from which nothing can escape if drawn in. He wondered whether that final singularity in a black hole might actually provide an initial singularity for a new universe.
The trouble with singularities is that an infinitely small, infinitely dense point makes no physical sense. How can something have no size or be infinitely heavy? So Poplawski hit upon a mechanism by which the matter falling into a black hole gets very close to forming a singularity – an incredibly small, incredibly dense point – but ‘bounces’ before it gets infinitely small and dense. But where does it bounce? Material can’t bounce out of the black hole because by definition nothing can escape from its clutches. “It has to go somewhere,” he says. “After the bounce it explodes and creates new space – a new universe.” When explaining his idea to his students he uses an analogy based on the famous science-fiction series Doctor Who. “When you enter the TARDIS you realise you’re in a space larger than a police box,” he says. “Just as the TARDIS door is a door to a spacecraft, so a black hole is a spherical door to a new universe.”
If Poplawski is right, our universe was created by a black hole in another universe. What does that mean for the edge of our universe? “It wouldn’t have one,” he says. “It would be like the surface of a sphere.” The Earth’s surface, for example, has no edge. If you were to walk out of your house and travel in a straight line, you would end up back where you started. According to Poplawski’s idea, if you travelled far enough away from Earth you would end up looping right the way back round and returning home. No edge, no boundary.
For now it is hard to know which of these pictures is the right one. It could be that the universe ends somewhere beyond our cosmic horizon. Or it could go on forever, meaning there are definitely copies of you out there in space. Equally, though, we could be nested inside one great multiverse, or the calamitous result of a black hole forming in another cosmos. Only further investigation, more observations and a higher volume of astronomical data will be able to tell us more. What is certain, however, is that there is more to this universe than meets the eye.