Starts With A Bang

Ask Ethan: Will The ‘Great Attractor’ Defeat Dark Energy? (Synopsis)

The Laniakea supercluster, with the Milky Way's position shown in red. Image credit: Tully, R. B., Courtois, H., Hoffman, Y & Pomarède, D. Nature 513, 71–73 (2014).

“We detect motion along this axis, but right now our data cannot state as strongly as we’d like whether the clusters are coming or going.” -Alexander Kashlinsky

When dark energy was discovered, and the expansion of the Universe was shown to be accelerating, there was concurrently another puzzle that received much less attention: the problem of the Great Attractor. Galaxies appear to move due to both the Hubble expansion and the local gravitational field, but the gravity from the galaxies we saw didn’t account for all the motion.

The “flows” of galaxies mapped out with the mass field nearby. Image credit: Helene M. Courtois, Daniel Pomarede, R. Brent Tully, Yehuda Hoffman, Denis Courtois, from “Cosmography of the Local Universe” (2013).

There must have been an additional set of masses, revealed only in the 2010s with the identification of the supercluster Laniakea. All the galaxies in our local neighborhood are headed towards it, but are we moving fast enough to overcome the expansive pull of dark energy?

The various galaxies of the Virgo Supercluster, grouped and clustered together. Each individual group/cluster is unbound from all the others. Image credit: Andrew Z. Colvin, via Wikimedia Commons.

The answer looks to be no; come find out why on this week’s Ask Ethan!