Does the Milky Way Live in a Void?

A sparse galactic neighborhood could clear up certain problems with our understanding of the universe.

Cosmology has a minor problem on its hands. The universe is getting bigger over time, that much we know for sure. But measurements of how fast the universe is currently expanding depend on what you measure.

Hubble constant measurements

This plot shows some representative measurements of the expansion rate of today's universe, known as the Hubble constant. The figure includes three values derived from measurements of the cosmic microwave background (left side of the plot) and four values derived from "nearby" objects (right side of plot). The values presented here are the bare-bones results from each study; any of these values can be made more precise by combining information from multiple datasets.
Ana Aceves / Monica Young

Observations of the cosmic microwave background, the afterglow of the Big Bang, suggest that today’s universe is expanding at a rate between 66.3 and 67.6 kilometers per second per megaparsec. But studies of relatively local objects, like variable Cepheid stars or Type Ia supernovae in nearby galaxies, point to a faster expansion rate: between 71.5 and 75 km/s/Mpc. Granted, one measurement comes from photons released 370,000 years after the Big Bang and the other measurement from photons released billions of years later. Nevertheless, the two methods should be getting the same answer for today’s expansion rate — and they aren’t.

At the 230th meeting of the American Astronomical Society, Benjamin Hoscheit and Amy Barger (both at University of Wisconsin, Madison) presented a possible workaround. What if, they proposed, the Milky Way lives in a cosmic void? That could skew the measurements of local stars and supernovae, but it wouldn’t affect the faraway CMB.

The Relative Emptiness Around Us

Large-scale structure

This frame from a computer simulation shows the foamy large-scale structure of the universe, in which long filaments of galaxy clusters surround immense voids. The image measures 300 million light-years on a side.
Virgo Project

In 2013 Barger and two colleagues, Ryan Keenan (then at the Academia Sinica Institute of Astronomy and Astrophysics, Taiwan) and Lennox Cowie (University of Hawai‘i) counted some 35,000 galaxies from multiple surveys. What they found is that the Milky Way appears to live in a relatively empty area. Per unit volume, there’s half again as much light reaching us from galaxies 1.5 billion light-years away as there is from galaxies right around us.

It’s as if we’re living in the suburbs, and the skyglow we see in our backyard comes more from distant cities than from our neighbors.

If this sparse region that we live in is a true cosmic void, then at 1.5 billion light-years in radius, it’s well above average in size, says Hoscheit. Typical voids have radii between 90 million light-years and 450 million light-years, he says. But this void would be so big, it would encompass the Laniakea Supercluster, which the Milky Way and its Local Group of galaxies call home, as well as the Tully Local Void, which Laniakea borders. “It would be the largest void known to science,” he says.

Such an immense void could bring local measurements of the universe's current rate of expansion in line with measurements of the CMB, Hoscheit says. But measurements in the middle zone, such as lensed quasars (such as the rightmost point in the plot above) wouldn't be affected by any local void and could still pose a problem.

A Void by Any Other Name

However, Radek Wojtak (University of Copenhagen, Denmark and KIPAC, Stanford University) isn’t so sure that the KBC region (for Keenan, Barger, and Cowie) is a void at all.

First, he notes, the galaxy counts that define the potential void aren’t taken from the whole sky — and the surveys the team used look away from the Laniakea Supercluster.

Moreover, Wojtak adds, “A change in density by factor of 1.5 is not enough to call it a void — but this is of course a matter of definition.” Observational cosmologists will sometimes call any big, underdense region a void, he says, but typical cosmic voids have densities 5 times smaller than the universe’s average density.

So, does the Milky Way live in a void? It’s really too soon to say. Measurements from nearby standard-candle supernovae are too scattered to confirm or rule out the existence of the KBC void, Hoscheit points out. What could clear up the situation, Wojtak says, is more data: counting galaxies to great distances and across the whole sky instead of just certain regions could help determine whether the KBC void is real.

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