Don’t panic. Cosmic voids are actually all around us.
Imagine an especially hole-y block of Swiss cheese, and you have a pretty good visual for the leading theory for the structure of the universe. Voids, vast expanses of nearly empty space, account for about 80 percent of the observable universe. The other stuff, like dust and stars and galaxies like the Milky Way, exists in thread-like filaments between these voids. As the universe expanded, gravity drew matter into clumps, leaving behind cavernous spheres. These empty regions, which can measure hundreds of millions of light years across, do contain some galaxies, but they’re dark caverns compared to the dense, bright bands of millions of galaxies ringing their edges.
According to researchers at the University of Wisconsin-Madison, our very own Milky Way galaxy may float near the center of one of these voids.
Using data from large-scale telescope surveys that count galaxies, the researchers concluded that the Milky Way exists near the center of a region that has fewer galaxies than other parts of the universe. They estimated the size of this void to have a radius of about 1 billion light-years. If they’re right, humans are living in the middle of the largest known void in the observable universe.
The researchers first advanced this idea in 2013, but they took it a step further this week, in findings presented at a meeting of the American Astronomical Society in Austin. The Milky Way’s place inside a void, they said, would help explain a question in the way astronomers measure how fast the universe is expanding.
The universe has been expanding ever since the Big Bang, more than 13 billion years ago, and evidence suggests its expansion rate is accelerating. There is, however, dispute about the precise rate of expansion. Some astronomers observe bright objects like Cepheid stars or supernovae in the nearby cosmic neighborhood, studying their light to determine how fast they’re moving away from Earth. Others peer deeper into the universe’s history and study the cosmic microwave background, the radiation leftover from the Big Bang that fills the universe to this day. Different measurements yield different results, and the measurements from the local universe turn out to be higher than those gleaned from the early universe. Astronomers don’t know whether the discrepancies are a result of statistical fluctuations or hints of new physics we don’t yet understand.
If the Milky Way were in a void, the difference in results would make sense, according to Ben Hoscheit, one of the University of Wisconsin-Madison researchers, who graduated from the school this spring.
“If you’re living inside this void, you’re going to see things being pulled away from you, towards the more dense regions of the universe,” he said. So if you’re sitting in this void, your surroundings expand faster than the rest of the universe. From this vantage point, observers would calculate a higher rate of expansion compared to what they find in the distant, early universe—like they do now.
“We should take into account our place in this very large universe, and we should be aware of how our place could potentially influence the measurements we make on Earth,” Hoscheit said.
Previous research has suggested the Milky Way may exist in a region less dense than others, said Peter Melchior, an astrophysicist at Princeton University who studies the distribution of matter in the universe. The idea that the Milky Way might exist in a void is not unreasonable, given the abundance of voids out there. Astronomers believe the Milky Way and its neighboring galaxies reside near the Local Void, a region 150 million light-years across and so empty that it’s pushing galaxies like ours away. But there’s no way to zoom out far enough to pinpoint the galaxy’s spot in the wider universe.
The size of the void Hoscheit and his team have proposed, though, is remarkable, Melchior said. The void is far larger than any previously observed by telescope observations, like the Sloan Digital Sky Survey, which in 2000 allowed astronomers to start investigating the large, Swiss-cheese scale of the universe for the first time. Most voids measure between 90 million and 450 million light-years in radius.
“They’re kind of like bubbles—they get bigger and bigger as the universe not only expands, but as more galaxies get pulled out over time,” said Greg Aldering, an astrophysicist at the Lawrence Berkeley National Laboratory who studies cosmological measurements and dark energy. “But it gets harder and harder to make a really, really big void.”
The suggestion that we might be in the center of that is a little uncomfortable, too, Melchior said. “It’s mathematically unlikely,” he explained over email. “But even more so, being in the center of the largest void in the observable universe would put us at a very special place, and since the times when we learned that the Earth (or the sun) isn’t at the center of the universe, astronomers try to avoid theories that put us at special places.”
Eventually, astronomers hope to use such voids’ shapes and distributions to explore the nature of dark energy, the unknown force responsible for accelerating the cosmos’ expansion.
Last year, astronomers used data from the Sloan Digital Sky Survey to compile a catalog of cosmic voids, characterizing their sizes and densities. From observations of a quarter of the night sky, they found hundreds of voids. Scientists hope these patches of darkness can help them investigate dark energy, the invisible force accelerating the expansion of the cosmos. The effects of dark energy may be easier to detect in the darkest territories of the universe, away from the glare of stars and galaxies.
The study of cosmic voids, whether we live in them or not, serves as a reminder of how young scientists’ understanding remains of humanity’s place in the cosmos.
“I don’t know how many people realize just how filamentary and Swiss cheese-y the universe is,” Aldering said.
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