For decades, the root cause of our Milky Way galaxys bulk flow in the direction of the Centaurus and Hydra constellations has retained an air of mystery. Our own stars motion through the Milky Way is relatively well understood. But a full understanding of the exact forces driving our Local Group of galaxies peculiar velocities at rates of 631 kilometers-per-second remains elusive.

Quite apart from our universes long-documented inflationary expansion (known as the Hubble Expansion), the local cosmos which surrounds us over millions of light years has its own peculiar trajectory and velocity.

What is known is that we are moving in bulk towards the Great Attractor (a region of half a dozen galaxy clusters some 150 million light years away), and the Shapley Concentration (a supercluster of galaxies some three times farther distant).

Hubble Space Telescope view towards the Great Attractor. Credit: ESA/Hubble & NASA

Is this just the result of gravity towards more massive and much more distant galaxy superclusters, or

A paper just submitted to the journal Monthly Notices of the Royal Astronomical Society (MNRAS) details cosmological distance measurements from some 8000 objects, most of which were from within our own Laniakea supercluster. As reported last September by Brent Tully, a cosmologist at the University of Hawaii in Manoa and colleagues in the journal Nature, the Milky Way is actually an outlier of this newly-named supercluster whose estimated 100,000 galaxies span some 500 million light years.

Yehuda Hoffmann, an astronomer at Israels Hebrew University, and colleagues used the Cosmicflows-2 catalogue (CF-2), the largest and most accurate ever catalog of galaxy peculiar velocities to reconstruct a large segment of our local cosmos large scale structure.

We [used] the CF-2 database to uncover the distribution of matter out to distances of hundreds of million of light years, Hoffmann, the papers lead author, told Forbes. Our main result is that the bulk velocity estimated from the CF-2 data is fully consistent with the standard model of cosmology. As Tully told Forbes, from our perspective, both downtown Laniakea and the Shapley supercluster lie in the same direction. Its the combination of these two things lined up like a spring tide that is pulling us. but we dont yet have a full accounting for whats causing our motion, said Tully. Until we can actually add up all the vectors and [still] come up with this number of 600 kilometers per second, theres still an incomplete story.

How do researchers actually use such data?

The team used 8000 separate distance measurements for their MNRAS paper; including 300 distances derived from Type 1a supernovae.

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Chasing The Mystery Of Spacetime's Local 'Bulk Flow'