HOLE UNIVERSE
WHAT LURKS WITHIN THE MOST MYSTERIOUS PLACE IN THE COSMOS?
“If the Milky Way had been in the centre of the Boötes Void , we wouldn’t have known there were other galaxies until the 1960s.” This was a quote from American astronomer Greg Aldering in an article for Discover magazine in 1995. He was talking about a region of space that appears to be a cosmological ‘dead zone’, containing very few stars or galaxies.
Galaxies have been observed for hundreds of years with telescopes, while some, such as Andromeda, Triangulum and the Magellanic Clouds, have been visible to the naked eye throughout human existence. But inside a cosmic void such as Boötes, none of that would have been the case until the development of modern instruments. The Milky Way would have been an anomaly inside a dark bubble of ‘nothing’. As it happens, our galaxy exists among others in a cluster, called the Local Group.
The first void was discovered in 1978 by two separate teams: one at the Kitt Peak National Observatory in Arizona, and the other at the Tartu Astrophysical Observatory in Estonia. Looking at the foreground region of Abell 1656 and Abell 1367 – together called the Coma Supercluster – 300 million light years away, they noticed an unusual absence of galaxies.
Then, in 1981, Robert Kirshner and others discovered a large galaxy-free zone centred 700 million light years away in the constellation of Boötes. Looking at the redshift of surrounding galaxies – the degree to which galaxies’ light spectra are reddened due to their recession speeds, and thus distances, a result of the expansion of space – they noticed a gap at around 1,500 kilometres (932 miles) per second. Follow-up work constrained this to 1,200 to 1,900 kilometres (745 to 1,180 miles) per second. There were very few galaxies with those velocities.
This roughly spherical 330 million light year expanse, disguised by surrounding galaxies, had finally shown itself.
The Boötes Void is roughly 3,300 times the size of the Milky Way, and is sometimes called a ‘supervoid’. If the entire observable universe was one metre (3.2 feet) wide, then Boötes would span about 2.7 millimetres (0.1 inches). In cosmological terms, that’s vast. By 1997 a total of 60 galaxies had been discovered within – roughly 33 times fewer than would normally be expected for other parts of space. Boötes isn’t truly barren, but it isn’t lush.
But are voids fundamentally different from other areas of space? “Voids are almost completely empty. You can go millions of light years without encountering anything more than a hydrogen atom,” says cosmologist, science educator and author Paul M. Sutter. “But deep inside we do find some material, like dark matter and small, dim dwarf galaxies. It’s the same stuff that’s in the clusters and filaments, just less of it.”
One person who studies galaxy formation in voids is Professor Rien van de Weygaert of Groningen University in the Netherlands. “The mass available to protogalaxies in voids is far less than in more moderate cosmic environments like filaments,” he says. “Voids are low density, and mass is continuously streaming out. The mass clumps in voids not only start out lower mass, but are also not able to accrete much during their evolution.” This is borne out of simulations Weygaert has conducted with numerous colleagues. “Because of the lower mass available to the void galaxies, the gravitational contraction and collapse of these objects proceeds more slowly.” The team is studying the star formation history of these void galaxies, allowing researchers to have a better understanding of their assembly, which in turn will provide direct information on the cosmological background. But how did voids themselves form?
The best theory involves quantum fluctuations
“Deep inside we do find some material, like dark matter and small, dim dwarf galaxies”
Paul m. Sutter
during the Big Bang growing enormously during a rapid expansion event called inflation. 380,000 years later, the universe switched from being a hot, dense plasma of subatomic particles to a transparent universe of atoms – an event known as recombination. That’s because as the universe expanded it became less dense, allowing the particles to form atoms. This ‘froze’ into place any density fluctuations that had originated during the Big Bang. As the universe continued expanding, areas of high density became sites of galaxy formation, while low-density areas saw minimal formation, becoming voids. Boötes is thought to be composed from smaller voids, otherwise it couldn’t have attained its size in the time that it did. So does this mean there are other voids?
“There are actually many voids; the universe is really empty,” says Dr David Alonso of Oxford University. In fact, voids account for 70 per cent of the observable universe’s volume. Alonso and his colleagues studied a particular phenomenon using data from the Sloan Digital Sky Survey’s (SDSS) Baryon Oscillation Spectroscopic Survey (BOSS) and the European Space Agency’s (ESA) Planck telescope. BOSS mapped the distributions of red galaxies and quasars. The Planck telescope was a space-borne observatory that mapped the relic radiation from the recombination era, which the expansion of space has stretched over 13.769
billion years into the microwave region, hence its name: the cosmic microwave background (CMB).
Alonso’s team used the BOSS data to identify and map 774 cosmic voids. These were all in the Southern Hemisphere, so Boötes wasn’t included. They then stacked them onto Planck’s CMB data. They were studying the Sunyaev-Zel’dovich effect, where photons of the CMB are boosted by high-energy electrons in galaxy clusters. This causes distortions in the CMB that reveal the locations of clusters and voids. They compared the energy of the observed CMB photons from voids to those of modelled values. “We figured out that we can use voids to measure the mean gas pressure as long as they’re sufficiently ‘empty’,” says Alonso.
There are hints that the void gas is slightly warmer than expected. “The most likely causes are either a statistical fluctuation in the data, or small errors in the model used to describe the gas pressure in voids,” says Alonso, but he points out that the data is tentative. Another explanation is that powerful cosmic jets from supermassive black holes could be injecting energy into space, which then shows up as a slight overtemperature in void gas. The mystery could be solved soon. Alonso says that the Atacama Cosmology Telescope or the Simons Observatory should obtain cleaner CMB mapping data that could then be combined with future galaxy surveys, like that of DESI, the Dark Energy Spectroscopic Instrument.
Much of the action in the universe seems to be in galaxy clusters, superclusters and filaments, so why go to voids for answers at all? “Voids are the ultimate time capsule for the universe,” says Sutter. “Unlike clusters and filaments, they’ve barely changed over the course of billions of years. They’ve preserved a memory of the early universe, and by looking at voids we can see what it was like back then.” In effect, voids are pristine archaeological sites for cosmologists wanting to