Our Milky Way Galaxy exists in void –one of the vast holes of the “Swiss-cheese” structure of the cosmos– with a radius measuring roughly 2 billion light years in diameter –the largest void known to science, shaped like a sphere with a shell of increasing thickness made up of galaxies, stars and other baryonic matter. As with other voids, it is not completely empty but contains our own galaxy, the Milky Way (a few hundred million light-years from the void’s center), the Local Group, and a larger part of the Laniakea Supercluster.
In a 2013 observational study of the large-scale structure of the universe, University of Wisconsin-Madison astronomers Amy Barger and Ryan Keenan showed that our galaxy, in the context of the large-scale structure of the universe, resides in an enormous void—a region of space containing far fewer galaxies, stars and planets than expected. The structure of the cosmos is Swiss cheese-like in the sense that it is composed of “normal matter” in the form of voids and filaments. The filaments are made up of superclusters and clusters of galaxies, which in turn are composed of stars, gas, dust and planets. Dark matter and dark energy, which cannot yet be directly observed, are believed to comprise approximately 95 percent of the contents of the universe.
A subsequent study not only firms up the idea that we exist in one of the holes the cosmos, the KBC Void, but helps ease the apparent disagreement or tension between different measurements of the Hubble Constant, the unit cosmologists use to describe the rate at which the universe is expanding today. The void is named after the Wisconsin astronomers Keenan, Barger, and Lennox Cowie.
The tension arises from the realization that different techniques astrophysicists employ to measure how fast the universe is expanding give different results. “No matter what technique you use, you should get the same value for the expansion rate of the universe today,” explained Ben Hoscheit, currently at Caltech. “Fortunately, living in a void helps resolve this tension.”
The reason for that is that a void—with far more matter outside the void exerting a slightly larger gravitational pull—will affect the Hubble Constant value one measures from a technique that uses relatively nearby supernovae, while it will have no effect on the value derived from a technique that uses the cosmic microwave background (CMB), the leftover light from the Big Bang.
“It is often really hard to find consistent solutions between many different observations,” said Barger, an observational cosmologist.”What Ben has shown is that the density profile is consistent with cosmological observables. One always wants to find consistency, or else there is a problem somewhere that needs to be resolved.”
The bright light from a supernova explosion, where the distance to the galaxy that hosts the supernova is well established, is the “candle” of choice for astronomers measuring the accelerated expansion of the universe. Because those objects are relatively close to the Milky Way and because no matter where they explode in the observable universe, they do so with the same amount of energy, it provides a way to measure the Hubble Constant.
Alternatively, the cosmic microwave background is a way to probe the very early universe. “Photons from the CMB encode a baby picture of the very early universe,” explained Hoscheit. “They show us that at that stage, the universe was surprisingly homogeneous. It was a hot, dense soup of photons, electrons and protons, showing only minute temperature differences across the sky. But, in fact, those tiny temperature differences are exactly what allow us to infer the Hubble Constant through this cosmic technique.”
A direct comparison can thus be made, Hoscheit says, between the ‘cosmic’ determination of the Hubble Constant and the ‘local’ determination derived from observations of light from relatively nearby supernovae.
The new analysis shows that there are no current observational obstacles to the conclusion that the Milky Way resides in a very large void. The presence of the void can also resolve some of the discrepancies between techniques used to clock how fast the universe is expanding.
The Daily Galaxy, Max Golderg, via University of Wisconsin-Madison
Image at the top of the page shows great cosmic voids that exist between the great clusters and filaments of the Universe. Andrew Z Colvin, Wikimedia Commons.