The Avocado Night Thread

Science Sunday Day Thread (December 6, 2020)

Alright nerds, settle down. It’s Science Sunday, and today we’re talking about space; space not just in terms of planets and stars, or even galaxies, but in clusters and superclusters of galaxies, in scales so large that your brain will spontaneously crack open to let the goo spill out if you think about it too much.

Let’s learn about the Shapley Attractor and the Dipole Repeller; which, much like your mom, is an enormous region of space that’s pushing everything away from it.

Since the Big Bang, everything in the universe has been steadily expanding, but not in a uniform fashion. The effect of gravity on cosmic scales has led large enough groups of objects to move closer to each other faster than the growth of the universe itself.

This leads to Supervoids – regions of space in which there is a near total absence of mass in thousands of light years in any direction – and Superclusters, such as the local one we find ourselves in. As we orbit the Sun and our solar system in turn spins within the cluster of stars that make up the Milky Way, our galaxy and the others around us within our cosy little Local Group are moving together through space at over six hundred kilometres per second in a unified direction.

What are we being pulled towards? It’s difficult to parse certain regions of space because they lie behind the Zone of Avoidance: the area of the universe hidden behind our own galaxy. As optical, infrared and x-ray astrophysics have advanced, observations suggested we were being pulled towards a “Great Attractor”, a concentration of galaxies with more than ten billion times the mass of our sun, a hundred and fifty light years away.

More recent observations discovered that it isn’t really all that great after all. Instead, the Milky Way is part of a surge of galaxies moving past the Great Attractor, towards the Shapley Attractor, over five hundred million light years away, an overdense accumulation of dozens of clusters of galaxies. The area was named after Harlow Shapley, the American scientist who surveyed the galaxies the southern skies in the 1930s.

This flowing of mass is all occurring around the hundred thousands of galaxies that make up the Local Cluster, or Laniakea Supercluster – the name taken from the Hawaiian words ‘lani’, which means heaven, and ‘akea’, which means spacious or immeasurable.

The Laniakea Supercluster

The Laniakea Supercluster

But despite the gravitational pull of the Shapley Attractor, cosmologists suspected that the galaxy was not merely being pulled, but was also being pushed, in a way that might be visualized as a Repeller. By 2017 enough data could be gathered by the most sensitive devices to support this idea.

Yehuda Hoffman, a cosmologist at Hebrew University in Jerusalem, worked with scientists in France and Hawaii to build a 3D map of the nearest galaxies. They drew on measurements from an array of observatories, including the Hubble space telescope, to work out how more than 8,000 galaxies were moving in the ever-expanding universe.

The map revealed a steady flow of galaxies towards the Shapley attractor and away from another region of space almost directly behind the Milky Way on the same axis. The scientists called the newly-identified region of space the “Dipole Repeller.”

The Dipole Repeller

The Dipole Repeller

Not enough is yet known about this area, however; it may simply be a void of effective repulsion, or there may yet be forces that actively push us and all the billions of other stars around us, away from it. Or maybe there’s a boatload of other things to discover; clusters become superclusters which turn into mere arms of even larger crazy huge clusters as new, more precise measurements are taken because everything’s just so darn BIG!


Have an awesome Sunday, everyone!