An extragalactic void is shoving our galaxy from behind

What causes some galaxies to speed along faster than others? The answer, it turns out, is nothing. Cathal O'Connell reports.

An artist's impression of the Milky Way galaxy. There's a large region of nothing in our extragalactic neighbourhood that's repelling our Local Group of galaxies.
NASA / JPL-Caltech / R. Hurt (SSC / Caltech)

Astronomers have finally discovered why the Milky Way is barrelling through space faster than the universe’s rate of expansion. It is being pushed from behind by an enormous void dubbed the “dipole repeller”.

The work, published today in Nature Astronomy, fills a gaping hole in our understanding of the local universe.

The Milky Way galaxy is a whirling disc 100,000 light-years across, sparkling with 100 billion stars. But zoom out and our galaxy is but a speck of dust, tossed about by cosmic currents of gravity.

As discovered by Edwin Hubble in the 1930s, galaxies fly apart because the universe itself is expanding. But on top of this general expansion, there is also a local movement of galaxies driven by gravity – like little eddies on the surface of a rushing tide.

By mapping these eddies, known as the “peculiar velocities”, astronomers are gradually identifying the seats of gravitational power in our local universe.

In the 1970s, astronomers discovered that the Milky Way, along with all the other galaxies in our neighbourhood, were hurtling towards the same region of space – as if someone had pulled a plughole in the cosmos.

This mysterious region, 150 million light years away, must have an enormous gravitational pull, and so astronomers dubbed it the Great Attractor.

Later, astronomers realised the Great Attractor itself was in motion, being pulled towards an even stronger source of gravity, the Shapley Supercluster, a further 600 million light-years distant.

But neither of these two massive structures could fully explain the breakneck hurtling of our own Milky Way. Their direction of pull didn’t quite match up.

Now, astronomers led by Yehuda Hoffman at the Hebrew University in Jerusalem have identified the missing piece of the puzzle.

Through mapping the ‘flow’ of galaxies in our pocket of the universe, the team realised out that besides being pulled in one direction by Shapley and the Great Attractor, the Milky Way is also being pushed from behind – most probably by a huge region of space almost completely empty of galaxies.

What’s more, the direction of this push aligns almost exactly with the direction we’re actually travelling.

They dub the void the “dipole repeller” because the flow lines of galaxies (away from the Repeller and towards Shapley) create field lines similar to a dipole magnet.

Like everywhere else in the universe, the void is expanding, but without the gravity of any galaxies to keep its expansion in check. This means the expanding void pushes on nearby galaxies (including our own) a bit like bubbles of air expanding inside a rising cake.

In 2014, study co-authors Brent Tully from the University of Hawaii in the US and Helene Courtois from the University of Lyon in France used the same technique to measure the extent of our home supercluster, Laniakea – a local metropolis of 100,000 galaxies.

The next step is to survey the Repeller itself, and confirm the huge region empty of galaxies they expect. A strange situation where astronomers will look into their telescopes and hope to see … nothing.

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Cathal O'Connell is a science writer based in Melbourne.