Sagittarius Files: What is Laniakea? Brent Tully, leader of the discovery team, explains in this PWFM interview

Laniakea, our galactic supercluster. The red dot to the right is the location of the Milky Way, or home galaxy. To the right, the next stop is Perseus Pisces, a similar structure 300 million light years away. Amazingly, that is still considered part of the nearby universe.

Please join me for this extraordinary conversation with Brent Tully, the leader of the team that discovered Laniakea — the galactic supercluster of which the Milky Way is a tiny little part. You can see our home galaxy as a little dot to the far right of the image above.

Prof. Brent Tully

What used to be called the Great Attractor is now known as Laniakea — Hawaiian for immense heaven. It is the structure of which the Milky Way galaxy, and all those that are part of the “local group” of galaxies, and the Virgo cluster, are part. It’s a creature consisting of 100,000 other nearby galaxies in our region of space.

The Great Attractor, Lainakea’s focal point, is located at 14 degrees of Sagittarius and is a defining feature of that sign. The core of our galaxy, called Sagittarius A*, is much closer to us, though also located in the sign Sagittarius.

It was defined in September 2014, when a group of astronomers including R. Brent Tully of the University of Hawaiʻi, Hélène Courtois of the University of Lyon, Yehuda Hoffman of the Hebrew University of Jerusalem, and Daniel Pomarède of CEA Université Paris-Saclay published a new way of defining superclusters according to the relative velocities of galaxies.

The new definition of the local supercluster subsumes the prior defined local supercluster, the Virgo Supercluster, as an appendage.

Is the Universe Expanding or Contracting?

In March 2021, I wrote to Prof. Tully with a question:

I have been watching physics courses on Amazon, including one on gravity with Schumacher. In all of these, Hubble’s Constant is mentioned as is the constantly expanding universe.

But nobody talks about the Great Attractor or the many similar features that exist. If that is so, how are the galaxies all moving apart from one another? I keep trying to reconcile this. If you have a moment, or some good references I can follow, I would be most grateful.

Prof. Tully replied:

The matter that you raise probably confuses a lot of people not closely connected with the problem. I need to be clear about what is going on.

There are two processes in competition. One that is well advertised is the overall expansion of the universe which came out of the Big Bang. The other is the gravitational interactions between masses. The first is global while the second is local. For examples in the extreme, we are held to the Earth, the Earth to the Sun, and the Sun to the Milky Way galaxy because on these local scales gravity easily wins over cosmic expansion. It is on scales of about 15-20 million light years where cosmic expansion starts to win over local gravity.

In our maps of the growth of structure in the universe, we plot things in co-moving coordinates. This is a coordinate system where cosmic expansion is cancelled out, leaving the residuals caused by local gravity to be evident. We do this because if we plotted in physical coordinates then cosmic expansion would be so dominant that the local effects would be obscured. Colleagues in the field understand what we do, but others may not, and perhaps we aren’t clear about this fundamental point.

So to your question, it is only very locally, like near the Virgo Cluster or other regions of great mass concentration that matter is trapped and facing back on the central concentration. On the 300 million lightyear scale of the Great Attractor, although in co-moving coordinates galaxies are falling together, in physical coordinates galaxies are flying apart. The motions caused by gravity are only perturbations on cosmic expansion. Still, these perturbations are interesting because they are telling us how much and where there are concentrations of matter.


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