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.