Why Is There a Mysterious Supervoid in the Middle of the Universe?
A big ‘ol sphere of nothingness 1.8 billion light years across might explain a longstanding mystery about the formation of the universe—but its existence is a mystery in itself.
llustration: Che Saitta-Zelterman
Last April, a team of cosmologists at University of Hawaii found strong evidence in support of the existence of the largest structure ever observed in the universe, which turns out to be nothing at all.
The results of the cosmological survey, published in Monthly Notices of the Royal Astronomical Society, point to the existence of the Eridanus supervoid, a span of nothingness 1.8 billion light years across located about 3 billion light years from the Milky Way.
If the team is right, the discovery might explain a longstanding mystery in physical cosmology that could ultimately help us better understand the evolution of the early universe.
But let's back up a bit. The Cosmic Microwave Background, or CMB, is a map of the radiation left over from the Big Bang. Since this radiation was emitted only a few hundred thousand years after the Big Bang, it provides us with the earliest glimpse of our fledgling universe and has become a critical component in studying its origin and development at cosmic timescales.
In 2004, cosmologists discovered the CMB Cold Spot, a region near the Eridanus constellation, that was much larger and cooler than it should have been according to the standard cosmological model.
A number of theories were advanced to explain this anomaly in the CMB, ranging from the totally mundane (instrumental error) to the fantastical (a parallel universe rubbing up against our own).
Next to chalking it up to a random (and relatively large) fluctuation in cosmic radiation, the next most likely explanation was the presence of a supervoid in the region. If this theoretical object existed, it would distort our perception of the CMB because light would have to travel through this vast expanse of nothingness before it reached us. Since the universe is expanding, this light doesn't regain the energy that is lost while it travels (in other words, the light's speed remains constant, but the wavelength grows as it crosses the void, a phenomenon known as the Sachs-Wolfe effect), thus creating what looks like a cold spot in the CMB.
It would be a pretty huge coincidence if this massive void in the same region of the sky was entirely unrelated
Although cosmologists had mapped out a number of voids prior to this discovery, the existence of the supervoid accounting for the cold spot had remained largely theoretical until April. While some evidence suggested that a void existed in the region, many cosmologists didn't think it was large enough to account for the cold spot.
In terms of the mass distribution of the universe, voids are relatively exotic. They are delineated by "walls," regions of space comprised of galaxy clusters and filaments where the density of matter is approximately the cosmic mean.
To call these structures voids is slightly misleading, however. Voids are not entirely empty; rather, they are just incredibly under-populated, usually having about 10 percent of the matter that you'd expect to find in an average region of the universe. This was what ultimately allowed the team of researchers to "see" the supervoid in the data they pulled from satellites.
Although the existence of the Eridanus supervoid doesn't totally explain the mysterious cold spot (for example, it would only account for about 10 percent of the temperature drop observed in the CMB), it would be a pretty huge coincidence if this massive void in the same region of the sky was entirely unrelated.
"In our paper we compare two hypotheses: one of them is that the super void and the Cold Spot happen to overlap by chance, the other is that the supervoid causing the CS," said Istvan Szapudi of the University of Hawaii's Institute for Astronomy. "The latter is 20,000 times more likely under fairly conservative assumptions."
There's also another glaring problem: if the supervoid does in fact exist, scientists aren't quite sure how it formed. Voids in general are thought to have formed in the immediate aftermath of the Big Bang due to quantum fluctuations, but some astronomers think the Eridanus void is too large for this explanation to make sense. According to Szapudi, there's also a lack of any plausible alternatives.
It looks like for now the Eridanus supervoid is just another mystery in space.