Finding our galaxy’s exact place in the vast structure of the Universe is surprisingly hard. We know the Milky Way is part of the Local Group of galaxies (along with Andromeda and a few dozen smaller galaxies), but beyond that it gets complicated. That’s because on one side, we have a huge conglomeration of galaxy clusters — Virgo, Centaurus, and other large collections of galaxies — while on the other, we have the Local Void, which is nearly empty of galaxies. Does the Milky Way belong to the big city or the wilderness, cosmically speaking?
It’s not an idle question. If we’re in a deep void, we can be fooled into thinking the expansion of the Universe is accelerating when it’s not. We would see galaxies rushing away from us faster than the cosmic average simply because we’re in a region that’s emptying out as the Universe expands. On the other hand, if we’re part of a supercluster — a large association of galaxies that isn’t bound tightly by gravity, but still is tenuously connected — it could explain the “Great Attractor”, the apparent motion of the Milky Way and other galaxies toward a specific region in the sky. We might not be in either of those situations, either, but it’s hard to tell.
The large-scale structure of the Universe is the result of a conflict between two tendencies. Gravity pulls things together, but the entire cosmos is expanding, pushing galaxies apart. That conflict creates the network of galaxies we observe, and the dark voids between them. While everything on average is moving farther apart, the motion of an individual galaxy may be toward a cluster or other concentration of matter. [Read more…]
As I described in my most recent column for The Daily Beast, some astronomers measured the distances and speeds to more than 8,000 galaxies in our cosmic neighborhood. They separated out cosmic expansion from local motion, and from that determined we’re part of a huge supercluster they named Laniakea, meaning “spacious heavens” in Hawaiian.
The way these astronomers defined a “supercluster” is somewhat non-standard, but I’m not concerned with that. More interesting to me is that they used their own value of the Hubble parameter, calculated from their distance and velocity measurements. The Hubble parameter is the number describing the expansion rate of the Universe, by connecting the apparent speed of a galaxy to its distance from us. The number obtained from the Planck cosmic microwave background data is 67.3 km/s/Mpc, which means a galaxy one megaparsec (or Mpc, or 3.3 million light-years) away appears to be moving 67.3 kilometers per second away from us due to cosmic expansion. The number used in the Laniakea paper is 75.2 km/s/Mpc, which is a lot faster. The difference in Hubble parameter values makes a huge difference in Laniakea’s size … and even whether it exists at all.
This team is led by University of Hawaii astronomer R. Brent Tully, who is well known in astronomy circles for the Tully-Fisher relation for determining distances to galaxies. Since he knows what he’s doing, it’s hard to just write off this project with its anomalously large Hubble parameter value. However, I suspect we’re far from done talking about Laniakea and the Milky Way’s position in the spacious heavens.