Ever feel like you are stuck in a hole? Newsflash: you are. Astronomers call it the “local hole”, but that is quite the understatement. It is vast, gigantic, enormously huge – although, in truth, adjectives fail us when it comes to this expanse of nothingness. It is the largest cosmic void we know of, spanning 2 billion light years. Our galaxy happens to be near its centre, but the trouble with this hole isn’t that it presents a proximate danger – more that it shouldn’t exist at all.
That is, if one of our most firmly held beliefs about the cosmos is true. That assumption, known as the cosmological principle, says that the universe’s matter should be evenly distributed on the largest scales. It is the cornerstone on which much of modern cosmology is built. If the void is real, then that stone might be crumbling.
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For this reason, few dared to believe the void could be genuine. But evidence has mounted in recent years, and astronomers have moved from doubt to begrudging acceptance. They have also discovered other similarly vast structures. So now the question is being asked with increasing urgency: if we really are living in a void, do we need to drastically modify our models of the cosmos? That might involve rethinking gravity, the nature of dark matter, or both.
The idea that the universe has the same character through and through can be traced back at least as far as Isaac Newton. He argued that the motions of the stars and planets can be explained by a law of universal gravitation that applies in the same way, everywhere. Today, a similar idea applies to the distribution of stuff. The cosmological principle says that the universe should be both isotropic and homogeneous, meaning its larger-scale composition should look the same in every direction, no matter where you observe from.
There is a clear rationale for this. Just after the big bang, matter would have been contained in an exceedingly dense ball, which then ballooned in size during an epoch we call inflation. This very rapid period of expansion would have smoothed out the density of matter, ultimately leading to a universe in which galaxies are evenly distributed.
Standard model of cosmology
Today, these ideas are baked into the standard model of cosmology, our supremely successful explanation of how the cosmos grew and took shape. Along with the cosmological principle, the model also includes the assumption that the universe consists mainly of unidentified dark energy and dark matter, the first exerting a force linked to the continued expansion of the cosmos, and the second an unidentified substance that only interacts gravitationally with other matter. Based on this, the model perfectly explains key features of the cosmos, including the amounts of helium and deuterium made in the first few minutes of the universe and the pattern of light left over from the big bang, known as the cosmic microwave background.
But think of the universe as being like crunchy peanut butter rather than the smooth variety. This is because the cosmological principle is statistical in nature, meaning the universe is smooth on average, but clumps and gaps aren’t completely forbidden as long as they aren’t too numerous. Plus, we would expect gravity to have pulled matter together into large structures, but only up to a point, given the age of the universe. Oddities within this general evenness may crop up, but they should be rare. “You can calculate the probability of these kinds of structures and it may be very low – but it is not zero,” says astrophysicist at Yale University. The bottom line is that we don’t expect to see voids or structures wider than about 1.2 billion light years.
The first hints that our part of the universe was thumbing its nose at this came in 1990, when at the University of Durham, UK, and his colleagues looked at a survey of our cosmic locality, made up of optical photographs, and counted the number of galaxies. They . In 1997, a team of astronomers counted the galaxies in the same region based on infrared images and found the same.
The evidence hardened further in 2013, when three researchers – Ryan Keenan at the Academia Sinica Institute of Astronomy and Astrophysics in Taiwan, Amy Barger at the University of Wisconsin-Madison and Lennox Cowie at the University of Hawaii – looked again using a cutting-edge infrared survey. They found the same low density of galaxies and, crucially, mapped out the structure of the void. “It became very clear that there was a local under-density,” says Cowie. The trio found that we live in a 2-billion-light-year-wide expanse of space with .
This region is now known as either the local hole or the KBC void, following the initials of its discoverers. But there were lingering doubts over whether it was quite as empty as it seemed. After all, there is plenty of stuff out there that doesn’t emit visible or infrared light, and so could have been missed.
However, the void has since been mapped using most of the observational tools at our disposal, including in and also concluded that the void was less dense than the universe on average. “The observational evidence is strong,” says Shanks. “Counting galaxies is a simple, reliable technique.”
That leaves cosmologists with quite the headache. What if the universe isn’t as isotropic and homogeneous as we have always assumed it must be? “Finding that it isn’t would really push cosmology into changing things,” says Cowie.
He doesn’t think the existence of the KBC void on its own is enough to force a panic. But there are other worries. In 2019, Alexia Lopez at the University of Central Lancashire, UK, was trying out a new method to analyse images from the Sloan Digital Sky Survey, which is mapping a vast swath of the sky. Her work used the ability of extremely bright objects called quasars to illuminate fainter matter. In a 2021 paper, she revealed she had discovered an arc of galaxies extending for 3.3 billion light years. This is just one of several astoundingly huge formations discovered over the past few years (see “Super-size me!”, below).
Hubble tension
Oversized voids and structures aren’t the only problem facing the standard model of cosmology. There is also the Hubble tension. We know that the universe is expanding at an ever-increasing pace, given by a figure known as the Hubble constant. The trouble is, various measures of this rate of expansion don’t agree. When we gauge it at large scales, the number comes out about 10 per cent lower than when we measure it based on nearby galaxies. In other words, our local patch of the universe seems to be expanding faster than the rest. Just last week, new results emerged that may help soothe the tension, but it remains a contested topic among astronomers.
On top of that, there is the issue of “bulk flows”, which refers to the way streams of galaxies move. Last year, astronomer Brent Tully at the University of Hawaii and his colleagues observed these flows in the KBC void and found they were . “These measurements couple with the Hubble tension dispute to suggest a real problem with the standard model [of cosmology],” says Tully.
He isn’t the only one who feels that way. , a researcher at the University of St Andrews in the UK, who has studied the KBC void, says the Hubble tension, bulk flow problem and the void are together creating a crisis in cosmology. He says it is “impossible” to fix it while sticking with the standard model – so it is time to look for other solutions.
That solution must realistically involve a mechanism by which structures can form more quickly than we normally expect. There are only really two options for this. Either you propose a tweak to the strength of gravity, or you adjust the properties of dark matter and the way it exerts its gravitational pull.
Banik and his colleagues recently explored the first of these options, tinkering with an old idea called modified Newtonian dynamics (MOND), which posits that gravity is stronger than normal when operating over very large distances. They calculated how MOND would affect our local environment, assuming that we live in a void with 20 per cent less matter than the cosmic average.
The researchers found that it would because matter, including the supernovas and galaxies used to measure the Hubble expansion, would continuously flow out of the region – being gravitationally attracted to the more abundant matter outside the void. So living in a void would ultimately inflate the local measurement of how fast the cosmos is expanding. The model also matched the latest figures for bulk flows. For Lopez, the results are interesting because they could potentially also explain the behemoth structures she found. “It’s good science to follow the data,” she says.
Yet MOND is a deeply controversial hypothesis because it does away with dark matter, an idea that is well-supported by observations. Even Banik stresses that he doesn’t see his work as the solution to this crisis per se. Rather, he says, it illustrates that some tweak to the standard cosmological model could allow for the faster formation of structures. Banik thinks a less-interventionist version of MOND might do the trick, one where the laws of general relativity are tweaked to make gravity ever-so-slightly stronger on scales above a million light years, but not so much that anything else in the standard model is affected, including dark matter. He points out that the strength of gravity hasn’t been tested at those scales yet.
Dark matter
If he is wrong about gravity needing a tweak, then perhaps the void can be explained with an alteration to dark matter. The standard cosmological model assumes that dark matter is “cold”, meaning it is slow-moving and barely interacts with normal matter or light except through gravity. The model also suggests that cosmic structures grow hierarchically, with small objects combining to form larger ones. But some cosmologists argue that dark matter may instead be “hot”, moving at close to light speed. In such a scenario, dark matter would be made of almost massless particles, such as neutrinos, and structures would form the other way around – starting with giant entities that break apart into smaller objects such as galaxies. This ultimately fits better with the existence of megastructures – and the KBC void – but worse with other observations. Or maybe dark matter interacts with other matter through an unknown fifth force of nature. “You could probably cook up some sort of dark matter interaction either with itself or with baryons [the key components of ordinary matter] that would boost structure formation,” says , a cosmologist at the University of Portsmouth, UK.
There are more outlandish ideas too. One wild option – that nonetheless is allowed in the standard cosmological model – is cosmic strings. These hypothetical strands are thinner than a proton but billions of light years long. Lopez floats the idea that such strings could function as an extra way to gravitationally attract matter. Banik says this is unlikely to be the solution because, even if these strings exist, they would be rare.
Working out a solution to the void isn’t going to be easy, then. Natarajan says our existing cosmological model is “exceedingly difficult to falsify”. Even minor adjustments to one aspect may cause unintended problems elsewhere. While changing the nature of dark matter might explain the presence of the void, “I don’t know what speeding up structure formation would do to star formation, galaxy formation or black hole formation,” she says.
For his part, Shanks wonders if voids might actually be more common than we realise. Most of our data on the structure of the universe is based on bright galaxies – naturally enough, because those are what we can see most easily. Yet it is possible that less-visible matter might cluster very differently, perhaps making large-scale structures or gaping voids more common than we realise. If so, the KBC void would be no outlier.
But assuming our void is as rare as we think – that would be quite something. Over the past century, our interrogation of the universe has taught us the same lesson time and again: we are nothing special. Earth is one of many planets orbiting one of billions of stars in one of billions of galaxies – possibly even existing in one of many universes. The discovery of the KBC void points in the opposite direction; it makes us unusual. If we are living in a hole, maybe that thought is more special than it sounds.
Super-size me!
Astronomers can't stop finding structures that are shockingly big. As early as 1989, a team discovered a "" of galaxies that is at least 500 million light years across. And measuring in at around 1.5 billion light years from end to end is the , a vast line of galaxies packed together in a linear shape that was discovered in 2003. A decade later, astronomers found the biggest example yet: the , which is a whopping 10 billion light years in length. Such giants are in conflict with the so-called cosmological principle, which says that structures in the universe shouldn't be bigger than about 1.2 billion light years.
Since then, more cases have cropped up. In 2021, Alexia Lopez at the University of Central Lancashire, UK, reported a huge arc of matter stretching for 3.3 billion light years. And this year she has found that there is a "" 1.2 billion light years across right next door to the arc. According to existing ideas about the makeup of the universe, there should be a low probability of finding any of these structures; to find two so close together is even more unlikely. "We should think about what these structures could be hinting at, not simply dismiss them as statistical anomalies," says Lopez.
It is possible that some of these observations could be mistakes. Commenting on Lopez's findings, astronomer Thomas Shanks at the University of Durham, UK, says the sighting is interesting, but warns that the eye "is very sensitive to picking up shapes like rings".
Assuming they are real, though, these megastructures seem to be in conflict with the standard model of cosmology, which suggests such massive forms shouldn't be possible. If that is the case, maybe our model of the universe needs adapting (see main story).
Miriam Frankel is a freelance journalist based in London
