“Space” is certainly an apt nickname for our cosmos, since there’s a heckuva lot of it out there.
Between here and the moon, about a quarter-million miles away, there’s virtually nothing — just stray hydrogen, helium and the odd dust particle. On far grander scales, this barrenness becomes unimaginably vast. A desolate, virtually starless, 2.5 million light-year gulf — that’s nearly 15 quintillion miles — separates our home galaxy, the Milky Way, from its nearest sizable neighbor, the Andromeda Galaxy.
Yet compared to cosmic scales, the Milky Way and Andromeda are right next door. Like neighbors awkwardly catching glances of each other through the windows, we can see Andromeda with the naked eye as a glowing smudge in its namesake constellation. The vast majority of the universe’s galaxies similarly huddle together. They gather into the equivalent of neighborhoods, cities and interconnected megalopolises known in astro-jargon as groups, clusters and filaments. Here in our Local Group, for instance, some 50-odd galaxies nestle within a dumbbell-shaped space 10 million light-years long.
In contrast to such typically close-knit galactic communities, enormous zones called voids are the boonies. For example, only several dozen small galaxies dot the Boötes Void, a spherical, bucolic region that spans a whopping 250 million light-years. (A more urban part of space might pack 10,000 galaxies into such a volume.) “Void galaxies are the loneliest galaxies,” says Kathryn Kreckel, a researcher at the Max Planck Institute for Astronomy in Heidelberg, Germany.
We’ve learned in recent decades that these void hinterlands, not galactic metropolises, are actually the cosmic norm. “Voids occupy most of the universe,” says Michael Vogeley, an astrophysicist at Drexel University in Philadelphia. “We find that over 60 percent of the universe is in voids.”
Once thought of as uninteresting backwaters, voids are emerging as the new big thing in several realms of astrophysics. “Voids are a comparatively young field, but people are excited,” says Bhuvnesh Jain, a professor of astrophysics at the University of Pennsylvania. Because of their profound emptiness, voids make unique laboratories for testing why the universe looks and behaves the way it does. Astronomers can study voids to tackle the cosmological bugaboos of dark matter and, in particular, dark energy.
“Voids are the best place to look for the signature of dark energy,” says Vogeley. If those signatures never turn up, voids might instead put the kibosh on dark energy, ushering in new forms of gravity or even a new force of nature. If all that weren’t enough, studying those rare, loner galaxies that call voids home should shed light on how all galaxies evolved over the universe’s eons.
Indeed, as a decade-old astronomy journal paper whimsically put it, void science is — with apologies to Shakespeare — much ado about nothing.
Spinning the Cosmic Web
The first inkling of the gaping holes in the universe’s distribution of galaxies came in the late 1970s, when astronomers began sketching out the three-dimensional structure of the cosmos. They relied on the redshifting of galaxies’ light to estimate their respective distances: The farther away a galaxy is, the more the expanding universe stretches its light toward the red end of the light spectrum. These redshifted galaxies’ locations outlined the shapes of hollow pockets. The mammoth Boötes Void was discovered this way in 1981, and six years later came the boringly named Local Void, skirting our Local Group.
These newfound voids upended the prevailing view of the universe as a smooth, uniform mosaic. The cosmos, we learned, is akin to Swiss cheese or foam, with galaxies clumping by the hundreds of thousands around colossal cavities.
Scientists think this “Cosmic Web,” to use the preferred nomenclature, emerged from fluctuations in the primordial cosmos that arose 13.8 billion years ago in the Big Bang. Dark matter — the mysterious, invisible substance reckoned to comprise 80 percent of the universe’s matter — clumped here and there, gravitationally drawing regular matter toward it. As the universe expanded and matured, these overdense regions of matter gelled into galaxy clusters, leaving underdense voids to grow emptier.For years, it was the light-emitting parts of the Cosmic Web that held cosmologists’ attention as they tried to explain dark matter, gravity and the universe’s unfurling. No one cared much about the voids. “I remember very prominent cosmologists for a long time said, ‘Oh, voids, they’re not important,’ ” says University of Groningen astrophysicist Rien van de Weygaert, a pioneer in the field of void research. “I got a lot of flak in the beginning.”
In the past 20 years, van de Weygaert and his colleagues have demonstrated how voids are not just null, passive places. Voids change over time, actually spurring the universe’s hordes of galaxies into their filamentous structures. To know how the universe got from there to here, van de Weygaert reasoned, we have to grasp it holistically. “You need an understanding of the evolution of the voids to understand the whole development of this weblike network we call the Cosmic Web,” he says.
Filling in the Holes
Real insights into the characteristics of voids - and how they shape the universe - truly only came around with the Sloan Digital Sky Survey, the biggest redshift survey to date, begun in 2000. "People had identified individual voids," says Jain, "but to have a whole population to work with was only possible after Sloan."
Sloan and other new surveys have now bagged thousands upon thousands of voids. Looking at them as a whole, we're gleaning that they're typically oval-shaped and span 50 million to 150 million light-years in the modern, nearby universe. A few billion years ago, though, voids tended to be smaller. That suggests they're growing, joining together in places, squeezing and concentrating dark and luminous matter between them. "Voids evolve in a hierarchical way," says van de Weygaert. "They build up into bigger soap suds, like in your kitchen sink, where you see the suds merging into larger bubbles."
As voids grow, simulations and observations show, they also become ever emptier. Such underdense areas have lower gravitational attraction than the surrounding overdense, galaxy-laced regions, and mass keeps on attracting mass. As the universe expanded, voids have, in effect, acted repulsively, losing matter toward their more massive, galaxy-lined edges. "You see the repulsive nature of these guys," says van de Weygaert. "You see them really pushing matter around."
Look at the Local Group. Along with our galactic neighbors, we Milky Wayers are moving at a clip of 392 miles per second toward the most massive nearby object, named the Great Attractor (a region somewhere in the vicinity of the Norma, Centaurus and Hydra clusters). Brent Tully at the University of Hawaii's Institute of Astronomy has used nearby galaxies' motions to survey the repulsive effect of the Local Void, which he co-discovered. As it continues to balloon and clear house, the void contributes a hefty 161 miles per second (roughly 40 percent) to that total Local Group velocity. "That was a beautiful survey," van de Weygaert says. "You get a very good idea of how important voids are in building structure."
The Forces at Work
Voids’ importance doesn’t end there. In the late 1990s, scientists realized to their shock that the universe’s expansion is accelerating, and apparently has been for more than half its lifetime. No theory of cosmology can readily explain a cosmos seemingly hell-bent on ripping itself apart.
Two schools of thought have cropped up explaining this stunning discovery. One posits that gravity as we know it, as laid out in Albert Einstein’s general theory of relativity, is correct, but that space itself also generates a weird energy that drives the cosmos apart — dark energy.
The other school believes Einsteinian gravity is flawed. It works spectacularly well at describing smaller-scale interactions, like planets’ orbits in the solar system, but on sprawling cosmological scales, gravity might act differently — the idea behind so-called modified gravity theories. This modification, some scientists wager, might even be due to a mystical-sounding “fifth force” of nature, joining gravity, electromagnetism and the strong and weak nuclear forces.
Either way, researchers hope, voids can help astronomers make sense of things. Compared with the galaxy-laced, matter-filled regions where we’ve so far directed most of our study, voids should “feel” the enlarging effects of dark energy or modified gravity more, on account of having less gravity-generating matter acting as a counterbalance.
“Dark energy ‘blows up’ the voids, in the sense that dark energy takes over in voids first, driving the acceleration of the universe,” says Drexel’s Vogeley. And Yan-Chuan Cai of the University of Edinburgh waxes similar about the fifth force he’s hunting: “Cosmic voids are underdense environments where the fifth force is expected to be more active,” he says, so they “are perhaps better laboratories for testing modified gravity.”
Vogeley, Cai and others in their field are keenly interested in gauging voids’ shape, size, distribution and mass (they do have some — they’re only virtually empty), much as we’ve done already for galaxies and clusters. All of these properties depend on the strength of the fundamental forces at work, whether they include gravity, dark energy or a phantasmic fifth force. The more we learn about voids, the more we learn about what’s pushing the universe apart.
To date, most of our knowledge regarding voids had been limited to where and how big they are. Worse, this data is squishy, dependent on the technique used for identifying a void. “We still do not have a universal definition of what a void is,” notes van de Weygaert.
It’s also tricky to judge the amount of matter that voids furtively harbor and evict. To better “weigh” voids, Penn graduate student Joseph Clampitt and his mentor Jain recently adapted a technique called gravitational lensing. Massive, foreground objects like galaxy superclusters warp light rays from background objects, similar to a magnifying glass or fun house mirror. The opposite effect, Clampitt and Jain showed, happens with voids. Instead of focusing light, underdense zones bend light away as it arcs toward higher-mass areas. The extent of this gravitational “demagnification” gives away how little mass a void contains and where it’s located, letting us see how void populations have changed over time as they have emptied and expanded. The Penn researchers intend to use the method to figure out how fast voids are kicking matter out, and then see which gravitational theory comes closest.
Meanwhile, for fifth force advocates, voids are also scientifically appealing. “The fifth force may behave differently in different circumstances,” explains Cai. It might not be measurable on local scales, like here in the solar system, or in any matter-strewn environs, such as galaxy clusters. Why? A theoretical “screening” mechanism, known as a chameleon field, that suppresses the fifth force in the presence of matter. Voids, mostly free of matter, would then be the prime places to see the fifth force without such chameleonic camouflage.
As theories go, dark energy — for all its mystery — is currently the best bet, with chameleon fields and a fifth force being the long shots. But as we determine the repulsive power of voids over cosmic history with better accuracy, the latter may still win out. “The repulsive force in voids drives voids to grow faster and bigger,” says Cai. “How empty a void is, is deeply relative to how active the fifth force may be.”
Islands in the Abyss
Despite their insights into the big-picture questions of cosmic structure and fundamental forces, voids might have the most to say about the growth of galaxies. “Void galaxies are rare and interesting objects,” says Vogeley. “In contrast to the well-studied galaxies in clusters — the ‘cities’ of the universe — we know relatively little about the properties of galaxies in voids.”
The conventional wisdom about how galaxies evolve supposes a hierarchical buildup from small to medium to large, just like for voids. Lilliputian galaxies spawned by the early universe attracted their fellow young galaxies, which glommed together into bigger galaxies. The multiple rounds of galactic mergers that have since taken place have given us hefty galaxies like our Milky Way and nearby Andromeda.
Void galaxies, however, have antisocially skipped out on galactic merging. Splendidly isolated, they have evolved almost entirely unto themselves in sparse environments. “This is one of the reasons to study void galaxies,” says Kreckel. “They may represent less-evolved galaxies.” Accordingly, void galaxies might provide a window into what the first galaxies looked like, telling us about conditions in the primordial universe.
Among the most fruitful efforts to date is the aptly named Void Galaxy Survey. Van de Weygaert, fellow University of Groningen professor Thijs van der Hulst, Kreckel and other members of the “Void Gang,” as they’ve called themselves, have culled 60 of the most isolated void galaxies from the Sloan Digital Sky Survey. Space- and ground-based telescopes further observed these hermit galaxies to tease out their shapes, rates of star formation and other galactic vital signs.
Based on this small sample, void galaxies have thrown researchers for a loop. “The first impression that’s a bit to our surprise is that galaxies in voids are not too much different than those in filaments,” says van der Hulst. Void galaxies are, as expected, tiny. Yet they’re still vibrant. In optical light, void galaxies look bluer on average than galaxies in denser regions, thanks to starlight beaming from a goodly number of hulking, bluish stars. These stars don’t live long, so their presence implies recent bouts of star formation. Fittingly, observations reveal that many void galaxies do possess the ample supplies of gas needed to generate new stars, even more than “normal” galaxies seen elsewhere.
These unexpected similarities to urban galaxies suggest that the conventional hierarchical model, wherein galaxies stack together like Legos into bigger cosmic structures, might not be the complete picture. “Void galaxies are a strong test of our theories of galaxy formation,” says Vogeley. Perhaps, some — or even most — galaxies might simply grow larger over time by sucking up available gas from their environments. If this galactic fuel is really there in voids, and we can find it, the so-called accretion theory for galactic evolution could explain why void galaxies do not look like bumpkins compared to their cosmopolitan cousins.
If current reckonings of dark energy or its still-more enigmatic cousins are right, the universe will keep on expanding at an ever-faster pace. The voids will swell ever larger, eventually taking up almost all the space in space. Distant galaxies will slip out of view, and with them the history of the universe. Even the galaxies in our Local Group will eventually either subsume each other or fly apart as emptiness asserts its reign.
If any vestige of humanity remains many billions of years from now, and the universe’s ciphers remain undecoded, our descendants might have only an all-encompassing abyss to stare into — not just space, but truly, the void.