A  busy patch of space has been captured in this image from the NASA/ESA  Hubble Space Telescope. Scattered with many nearby stars, the field also  has numerous galaxies in the background. Located  on the border of Triangulum Australe (The Southern Triangle) and Norma  (The Carpenter’s Square), this field covers part of the Norma Cluster  (Abell 3627) as well as a dense area of our own galaxy, the Milky Way. The  Norma Cluster is the closest massive galaxy cluster to the Milky Way,  and lies about 220 million light-years away. The enormous mass  concentrated here, and the consequent gravitational attraction, mean  that this region of space is known to astronomers as the Great  Attractor, and it dominates our region of the Universe. The  largest galaxy visible in this image is ESO 137-002, a spiral galaxy  seen edge on. In this image from Hubble, we see large regions of dust  across the galaxy’s bulge. What we do not see here is the tail of glowing X-rays that has been observed extending out of the galaxy — but which is invisible to an optical telescope like Hubble. Observing  the Great Attractor is difficult at optical wavelengths. The plane of  the Milky Way — responsible for the numerous bright stars in this image —  both outshines (with stars) and obscures (with dust) many of the  objects behind it. There are some tricks for seeing through this —  infrared or radio observations, for instance — but the region behind the  centre of the Milky Way, where the dust is thickest, remains an almost  complete mystery to astronomers. This image consists of exposures in blue and infrared light taken by Hubble’s Advanced Camera for Surveys.

A busy patch of the Great Attractor

A busy patch of space has been captured in this image from the NASA/ESA Hubble Space Telescope. Scattered with many nearby stars, the field also has numerous galaxies in the background. Located on the border of Triangulum Australe (The Southern Triangle) and Norma (The Carpenter’s Square), this field covers part of the Norma Cluster (Abell 3627) as well as a dense area of our own galaxy, the Milky Way. The Norma Cluster is the closest massive galaxy cluster to the Milky Way, and lies about 220 million light-years away. The enormous mass concentrated here, and the consequent gravitational attraction, mean that this region of space is known to astronomers as the Great Attractor, and it dominates our region of the Universe. The largest galaxy visible in this image is ESO 137-002, a spiral galaxy seen edge on. In this image from Hubble, we see large regions of dust across the galaxy’s bulge. What we do not see here is the tail of glowing X-rays that has been observed extending out of the galaxy — but which is invisible to an optical telescope like Hubble. Observing the Great Attractor is difficult at optical wavelengths. The plane of the Milky Way — responsible for the numerous bright stars in this image — both outshines (with stars) and obscures (with dust) many of the objects behind it. There are some tricks for seeing through this — infrared or radio observations, for instance — but the region behind the centre of the Milky Way, where the dust is thickest, remains an almost complete mystery to astronomers. This image consists of exposures in blue and infrared light taken by Hubble’s Advanced Camera for Surveys.

Photo by: ESA/Hubble & NASA

ESA/Hubble & NASA

What is “Dark Flow”?

It sounds super-scary: something from outside the universe, a force so unimaginable, is pulling every single galaxy towards it. What monstrosity of cosmic physics could it be?

November 05, 2021

It’s the dark flow.

At least the good news is that we don’t know if it exists, and even if it did it wouldn’t be that big of a deal.

Everything in our universe is constantly moving. Planets orbit stars. Stars dance within galaxies. And generally, on average, galaxies are getting further apart from each other. This is the expansion of the universe, first discovered by astronomer Edwin Hubble in the 1920s. According to our best observations, and our understanding of the evolution of the universe, this expansion is uniform. It’s the same in all directions. Once you get to big enough scales, galaxies just move out without any other preferred direction.

Dr. Edwin P. Hubble, one of America's foremost astronomers, runs the 48-inch Schmidt Photographic Telescope through its final series of rehearsals for the National Geographic Society-Palomar Observatory sky survey. The project will prove the first ever definitive photo atlas of the heavens.

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Dr. Edwin P. Hubble, who first discovered "dark flow."

Photo by: Bettmann

Bettmann

Dr. Edwin P. Hubble, who first discovered "dark flow."

But at smaller scales, there can be some very interesting dynamics. For example, our own galaxy, the Milky Way, is on a collision course with our nearest neighbor, the Andromeda Galaxy. In about five billion years our galaxies will collide in a titanic collision, and if our descendants are around to see the show it will be quite spectacular.

On top of that motion, both the Milky Way and Andromeda are heading towards the Virgo Cluster, a dense cosmic city home to about a thousand galaxies.

Beautiful and Bright Milky Way

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The Milky Way is moving.

Photo by: Natapong Supalertsophon

Natapong Supalertsophon

The Milky Way is moving.

And there’s more. The Milky Way, Andromeda, the Virgo Cluster itself, and all the other galaxies in our nearby patch of space are together moving in the direction of an even bigger cluster, the Norma Cluster, located in a region of space known as the Great Attractor.

Here’s where things start to get tricky. The further out we go in our observations, the harder it gets to pinpoint the precise movements of galaxies. Compounding that, astronomers are really good at measuring the movement of galaxies toward or away from us (based on the shifting of light from the galaxies), but calculating movements in other directions is much harder. So we have to rely on computer modeling to make an educated guess.

While most observations of the larger universe have held up the “expanding uniformly in all directions” point of view, some inconsistencies have emerged. Some observations have seen a tiny, subtle, blink-and-you-miss-it movement. It appears that many galaxies might be moving in a preferred direction, instead of just uniformly outwards.

Not to be confused with our neighbouring Andromeda Galaxy, the Andromeda constellation is one of the 88 modern constellations. More importantly for this image, it is home to the pictured NGC 7640. Many different classifications are used to identify galaxies by shape and structure — NGC 7640 is a barred spiral type. These are recognisable by their spiral arms, which fan out not from a circular core, but from an elongated bar cutting through the galaxy’s centre. Our home galaxy, the Milky Way, is also a barred spiral galaxy. NGC 7640 might not look much like a spiral in this image, but this is due to the orientation of the galaxy with respect to Earth — or to Hubble, which acted as photographer in this case! We often do not see galaxies face on, which can make features such as spiral arms less obvious.  There is evidence that NGC 7640 has experienced some kind of interaction in its past. Galaxies contain vast amounts of mass, and therefore affect one another via gravity. Sometimes these interactions can be mild, and sometimes hugely dramatic, with two or more colliding and merging into a new, bigger galaxy. Understanding the history of a galaxy, and what interactions it has experienced, helps astronomers to improve their understanding of how galaxies — and the stars within them — form.

A spiral in Andromeda

Andromeda is moving toward the Norma Cluster.

Photo by: ESA/Hubble & NASA

ESA/Hubble & NASA

Andromeda is moving toward the Norma Cluster.

This is the “dark flow” because it’s an unexplained motion observed in the behavior of galaxies. Like a flock of birds veering to one side, you have to ask: what’s making them go in that direction?

It could be caused by some catastrophe in the early universe, setting up a motion that took billions of years to unfold. It could be something frighteningly massive lying just outside the edge of the visible universe, tugging at all of its with its almost-imperceptible gravity. It could also be just a mistake or bias in the data, an artifact of our inadequate observations.

Whatever it is, the dark flow is nothing to worry about. If it does exist, it’s just a tiny effect that isn’t going to tear apart the universe or jump out from under the bed.

Sleep tight.

Journey Through the Cosmos in an All-New Season of How the Universe Works

The new season premieres March 24 on Science Channel and streams on discovery+.

Paul M. Sutter

Paul M. Sutter is an astrophysicist at Stony Brook University and the Flatiron Institute, host of Ask a Spaceman and Space Radio, and author of How to Die in Space.

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