A GIANT hole in the web of galaxies that fills the cosmos. A colossal string of quasars billions of light years across. A ring made out of hugely energetic bursts of radiation that spans 6 per cent of the visible universe. As our observations of the cosmos come into ever sharper focus, astronomers are beginning to identify structures bigger than any seen before. There’s only one problem: none of them should be there.
Ever since Copernicus proposed his revolutionary idea that Earth’s place among the stars is nothing special, astronomers have regarded it as fundamental. The cosmological principle it has evolved into goes a step further, stating that nowhere in the universe is special. You’re allowed to have patches of individuality on the level of solar systems, galaxies and galaxy clusters, of course, but zoom out far enough and the universe should exhibit a drab homogeneity. No vast galactic walls or bald spots, and no huge structures.
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Small wonder that the spate of recent findings has got cosmologists hot under the collar. But the solution could prove equally controversial. One researcher claims these massive structures are illusions projected from another dimension, the first tantalising evidence of realities beyond our own. If he is right, and these behemoths don’t exist as physical objects within our universe, then the cosmological principle might still be safe.
The concept of favoured regions in the universe is anathema to modern cosmology. “All our thinking since the Renaissance has been working against that idea,” says Seshadri Nadathur, a cosmologist at the University of Portsmouth in the UK. It also makes using Einstein’s general theory of relativity to understand gravity’s role in the evolution of our universe an even more fiendish task than it already is. “Einstein’s equations are much easier to solve if you assume a universe that’s almost homogeneous,” says Nadathur. But, at the moment, the cosmological principle is just that – an assumption. There is no concrete evidence that it is true, and the evidence we do have seems increasingly against it.
Take that giant hole in the universe – a void almost 2 billion light years wide, according to its co-discoverer, András Kovács of the Institute for High Energy Physics in Barcelona, Spain. “There are 10,000 fewer galaxies in that part of the sky compared with the universal average,” says Kovács. Based on the latest data, astronomers believe that the cosmological principle must apply on scales of roughly a billion light years, with the average amount of material in any given volume more or less the same. A big empty patch almost double the size of the cut-off stands out like a sore thumb. Kovács and his team call this vast expanse a supervoid, and believe it might explain away the giant cold spot in the cosmic microwave background, an observation that has been puzzling astronomers for over a decade (see “CMB cold spot“).
And the supervoid isn’t the half of it. As far back as 2012, a team led by Roger Clowes at the University of Central Lancashire, UK, claimed to have found an enormous structure strung out over 4 billion light years – more than twice the size of the supervoid. “We thought ‘what is that!?’ It was obviously something very unusual,” says Clowes. Yet this time it wasn’t an empty patch of space, but a particularly crowded one. Known as the Huge Large Quasar Group, it contains 73 quasars – the bright, active central regions of very distant galaxies. Astronomers have known since the early 1980s that quasars tend to huddle together, but never before had a grouping been found on such a large scale.
Then earlier this year a team of Hungarian astronomers uncovered a colossal group of gamma-ray bursts (GRBs) – highly energetic, short-lived flashes of energy erupting from distant galaxies. The galaxies emitting these GRBs appear to form a ring a whopping 5.6 billion light years across – 6 per cent of the size of the entire visible universe. “We really didn’t expect to find something this big,” says Lajos Balázs from the Konkoly Observatory in Budapest, Hungary, who led the study. Its size makes it five times larger than the typical scale at which the cosmological principle tells us that homogeneity should kick in.
So fundamental is the cosmological principle to our understanding of the universe that such apparent violations make astronomers and cosmologists deeply uncomfortable, even those who discovered them in the first place. When it comes to the intense flashes of light that make up the GRB ring, for instance, there’s a possibility they might be surrounded by other galaxies, currently shining less brightly because of an absence of GRBs. It’s like being in a darkened room in which light bulbs are evenly distributed: if only a few are illuminated when you look into the room, you’re likely to draw the wrong conclusions about how they are arranged. “It doesn’t necessarily contradict the cosmological principle,” says Balázs.
Rise of the giant-killers
The huge large quasar group is also the subject of intense debate. “I don’t think it’s really a structure at all,” says Nadathur. In 2013, he published a paper studying the algorithm Clowes and his team used to analyse their data, calculating the probability that a random distribution of quasars would also yield an apparent structure. “The chances of seeing a pattern like the one they see, even if there is nothing there, is quite high,” he says. But the giant might not be dead just yet. Clowes’s PhD student, Gabriel Marinello, is working on a paper countering Nadathur’s claims, which he describes as “conservative and unrealistic”. He argues that instead of modelling a random distribution, Nadathur should have included the fact that quasars – just like other galaxies – are known to huddle together on scales of around 300 million light years.
As well as the quasar group, Nadathur thinks the supervoid could also be reconciled with the cosmological principle. “The principle is not saying that any one place cannot fluctuate from the norm, just that on average the large-scale universe must be homogeneous,” says Nadathur. In short, the probability of finding objects like the supervoid is not zero. There just can’t be too many of them.
But Rainer Dick, a theoretical physicist at the University of Saskatchewan, Canada, believes such attempts to brush these cosmic megastructures aside are misguided. In fact, he says they should be embraced as our best bet of keeping the cosmological principle alive. All we have to do is accept that they don’t actually exist. Instead, they represent the first evidence of other dimensions intruding into our own, leaving dirty footprints behind on our otherwise smooth and homogeneous cosmic background.
It seems a breathtakingly audacious proposal – but it builds on a solid foundation of theoretical work. For one thing, conjuring up other dimensions beyond our own is nothing new. For decades, many theorists have regarded the existence of extra dimensions as our best hope of reconciling Einstein’s general relativity with that other bastion of 20th century physics: quantum theory. A marriage between these two seemingly disparate concepts, one dealing with the very large and the other with the very small, would yield what is often called a theory of everything, a one-size-fits-all framework capable of describing the universe in its entirety.
“That really would be compelling evidence that our universe is not alone”
One popular candidate is M-theory, an extension of string theory that famously suggests we live in an 11 dimensional universe, with the other seven dimensions curled up so tightly as to drop out of sight. It’s an elegant and mathematically appealing framework with a number of influential supporters. But it has one major failing: the lack of solid predictions offering opportunities to verify it. Dick’s work on a generalisation of string theory known as brane theory might provide just such a prediction, and resolve the cosmological principle dilemma to boot.
At the heart of brane theory is the idea that what we perceive as our universe is a single four dimensional membrane floating in a sea of similar branes spanning multiple extra dimensions. Such an idea is not inconsistent with our established theory of gravity, says Dick, as “you can add infinitely large extra dimensions and still get back general relativity”.
Although the other branes occupy extra dimensions, and so would be impossible to observe directly, the theory suggests we might just be able to spot the effects of a neighbouring brane overlapping with ours.
So how does this help with the problem of the cosmological principle? Well, in order to measure our distance to far-off objects, astronomers exploit an effect known as redshift. They break down the light from the object using a spectrometer – a fancy version of a prism – to reveal bands known as spectral lines. Any object moving away from us because of the universe’s ongoing expansion will have its light stretched out to longer, redder wavelengths and the lines will appear shifted towards the red end of the spectrum. The further away the object, the faster it will appear to recede and the more the lines will shift. If astronomers see many objects all exhibiting the same redshift, they will interpret that as some form of structure, just like the GRB ring or the huge quasar group.
Except, looking into a region where another brane is overlapping with our own might skew our redshift measurements. Under these conditions, photons in one brane would exert a force on charged particles in another – a phenomenon Dick calls brane crosstalk. “This would change the distance between the energy levels within hydrogen atoms in the overlap region,” he says. Electrons moving between these energy levels either emit or absorb photons, producing the spectral lines we rely on for working out their distance from Earth.
But if brane crosstalk were to narrow the energy-level gap, this would produce photons of a slightly longer wavelength – a redshift that has nothing to do with the expansion of the universe. If you fail to take this into account, and assume the overall redshift you measure is solely the result of distance, then you will systematically overestimate how far away an object in the overlap region actually is, with large swathes of empty space visible in its true location (see diagram).
If such a model held true, areas of brane overlap would produce an apparent pile-up of objects at one redshift and a distinct lack of objects at another – an optical illusion that would make a homogeneous universe appear to contain massive structures and enormous voids. In a stroke, this would explain the origins of the quasar group and the GRB ring as well as the supervoid, says Dick. “These structures match the potential signal of brane crosstalk.”
Of course, it’s hardly an open-and-shut case. “There are many assumptions that one must accept in order for this to happen, and some of them may just be taking things a bit too far,” says Moataz Emam from the State University of New York College at Cortland. Emam also warns that some of the assumptions about gravity that Dick’s theory relies on have been severely criticised in the past, not least by string theorists who have had difficulty reconciling them with their calculations. “But his model is certainly testable,” he says.
Emam suggests that the necessary evidence could be found by observing parts of the sky where high density regions coexist next to apparent barren patches. Provided the discrepancy in redshift measurements is identical in all cases, it might well suggest that our brane is overlapping with another.
With the help of the Sloan Digital Sky Survey (SDSS) – the most detailed three-dimensional map of the universe ever made – Dick is now planning to scour the databases for redshift data that could support his theory. “That really would be compelling evidence that our universe is not alone,” he says. Such a discovery would not only explain away some of the most perplexing observations in astronomy, but give the abstract field of string theory a tantalising experimental foundation.
But his quest to cut the universe’s largest objects down to size might lead to new monsters arising in their place. The discovery of branes beyond our own, for instance, would pose a serious challenge to humanity’s fragile sense of its place in the cosmos, and make a nonsense of our concept of cosmic homogeneity. In a vast multiverse of interacting membranes, the cosmological principle might not be worth saving after all.
Images from top: Doaly, ESO/M. Kornmesser
CMB cold spot
The cosmic microwave background (CMB), radiation left over after the big bang, bears an impression from when the universe was only around 400,000 years old. Its distinctive maps are littered with red and blue speckles representing the slightly hotter and cooler regions of the infant universe. Our understanding of the physics governing this period predicts that these variations should be small, and for the most part they are. However, in 2004, scientists using the WMAP satellite claimed to have found a cold spot significantly larger than the others. They thought it might be an error in their measurements. Then the European Space Agency’s Planck satellite observed it too. An alternative model was badly needed.
Among the most promising remains the supervoid theory, a thorn in the side of those who defend the idea of a uniform universe (see main story). This proposes that a large patch of empty space sits slap bang in the direction of the cold spot. In order to reach us, CMB photons originating from beyond the supervoid would have had to pass right through it. Thanks to the accelerating expansion of the universe, the photons emerging from this barren area would find that matter was less densely packed than when they went in, leading to a drop in the gravitational potential they experienced, and consequently their energy. As photon energies are used to calculate a source’s temperature, this would in turn lead us to incorrectly interpret their home region as colder than any other point on the sky.
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