Dark Flow (Script) (Patreon)

There’s controversial evidence that on the largest scales, the universe is flowing. It may be that much of the matter in the cosmos is drifting due to the ancient gravitational pull of something outside our observable universe.

Space is not static. Everything moves. Planets orbit stars, stars orbit within galaxies, galaxies whirl within the gravitational fields of giant clusters. And of course, the universe is expanding; distant galaxies are thrust apart from each other as the space between them grows. But there’s no preferred direction to any of this motion. Motion due to the expansion of the universe – what we call the Hubble flow – is equal in all directions. The random motion of galaxies – what we call their peculiar motion – should also have no preferred direction. On the largest scales of the universe, there should be no preference for up or down or left or right. At least that’s what we thought. Observations of the cosmic microwave background suggest that the galaxy clusters across the cosmos may be moving, ever so slightly, towards the same point beyond the cosmic horizon. We call this dark flow.

Galilean relativity tells us that there is no preferred frame of reference to define absolute “stillness”. The laws of physics work the same no matter your speed. And yet there is one frame of reference that in some ways is a little more preferred, or at least special than all of the others. It’s the frame of reference of the cosmic microwave background radiation – the CMB. The CMB is the left-over heat glow from the hot dense early universe. In all directions it appears to be the same temperature – around 2.7 Kelvin, and hence the same microwave wavelength. Actually, when we look at the CMB from our moving platform of planet Earth we see that in one direction of the sky it’s a little warmer, and in the opposite it’s a little cooler. This is due to the Sun’s …km/s orbit around the Milky Way. That motion causes the CMB to be Doppler shifted – its wavelengths a little stretched out behind us and a little compacted ahead.

But if the CMB is asymmetric due to our motion, then there must be some reference frame – some velocity – in which there’s no observed Doppler shift in any direction. If you were in that reference frame you would be “still” with respect to the CMB. In that reference frame, if you added together the peculiar velocities of all galaxies in the universe, you would expect them to cancel out. After all, those galaxies formed from the hot hydrogen plasma that produced the CMB. So the peculiar motion of galaxies can be defined as motion relative to the cosmic microwave background. The CMB can be used to define this notion of “stillness”, but it can also be used to measure deviations from that stillness. Doing so has seemed to reveal that the motion of galaxies does NOT cancel out. On the largest scales of the cosmos, there’s controversial evidence of a slight drift in one preferred direction.

To explain this I’ll need to tell you about the kinematic Sunyaev-Zeldovich or KSZ effect. Actually, let’s start with the regular old thermal Sunyaev-Zeldovich effect. OK, so the most massive clusters of galaxies in our universe are vast conglomerations of thousands of galaxies and are bathed in a diffuse but searing-hot plasma – hydrogen and helium with temperatures up to 100 million Kelvin. As the photons of the CMB pass through that plasma they steal a little of its energy. As a result, when we look at the cosmic microwave background through one of these clusters, we see that its energy is boosted just slightly. This allows astronomers to find extremely distant galaxy clusters just by studying the CMB. The kinematic Sunyaev-Zeldovich effect is much, much harder to see than the thermal SZ effect. If a galaxy cluster has some extra peculiar velocity in addition to the  Hubble flow – then the SZ effect adds an extra Doppler shift to the CMB photons that pass through that cluster. It sort of resets the apparent velocity - the frame of reference - of that patch of the CMB to the peculiar velocity of the cluster. It gives that part of the CMB a Doppler shift relative to the rest of the CMB, and that shift can tell us the peculiar velocity of the cluster.

This effect is really, really tiny, and it can only tell us the component of the peculiar velocity that’s either towards us or away from us, not side to side. A KSZ measurement from a single cluster isn’t very useful. But what if you measure the effect from hundreds of clusters across the observable universe? That’s exactly what Alexander Kashlinsky and collaborators did. They used the WMAP satellite observations of the cosmic microwave background to build up a peculiar velocity map of around 700 clusters in all directions on the sky and out to billions of light years. Their claim is pretty astonishing. Once you factor out the Hubble flow, on average those clusters seem to be drifting in the same direction! It’s like a lot of the matter in the universe is flowing towards some point beyond the edge of our universe.

This claim is extremely controversial. There’s a pretty entrenched idea in cosmology: the Universe should be homogeneous and isotropic on the largest scales. Homogeneous means that everything looks roughly the same in all locations. That certainly seems to be the case; when you look at scales larger than around a billion light years, you have basically the same amount of matter everywhere. Isotropic means that the universe shouldn’t have a preferred direction. Matter shouldn’t be more stretched out in any particular direction, and it shouldn’t be moving in any one direction more than any other. So the idea of a universe-wide dark flow surprised a lot of people, and made others very skeptical.

Other teams have been quick to try to refute this claim, and the strongest challenge came from the team behind the Planck satellite. Planck is the latest cosmic microwave background probe and has produced a much more detailed map compared to WMAP. Analyzing around 1000 clusters using the new data, the Planck satellite team say that no dark flow is apparent. However Kashlinsky and team, as well as some other researchers, also redid their work with the Planck data and claim that dark flow is still very real.

But let’s talk about the implications of this finding, assuming it’s real. The direction of the claimed dark flow is towards the constellations Centaurus and Hydra. In fact it’s towards something beyond the cosmic horizon in the same direction as these constellations. This is an odd coincidence. We already know that there’s something very massive in that direction. It’s the so-called Great Attractor. Since 1973 we’ve noticed that galaxies in the local part of the universe seem to be drawn in that direction due to an unseen gravitational influence. We now think that this might be the center of the Laniakea supercluster, which is the vast cluster-of-clusters that encompasses hundreds of millions of light years, and several hundred entire galaxy clusters. But the Great Attractor isn’t causing Dark Flow, because that flow seems to affect galaxies across the observable universe – to 2.5 billion light years – far beyond the gravitational reach of Laniakea.

So what causes Dark Flow? IF it’s real, then the leading explanation is that it’s the relic of a gravitational attraction towards something beyond the edge of the observable universe.  Now that part of the greater universe can no longer influence us – it’s moving away from us at faster than the speed of light and is now forever beyond the reach of light or gravity or anything. However that wasn’t always the case. In the earliest instant of the Big Bang the observable universe and much, much more was compressed into a subatomic scale. Things that are now far beyond the cosmic horizon were close enough to affect us gravitationally.

Then, the event known as cosmic inflation caused an exponential expansion that threw these once-neighbors far, far apart. Yet the relics of their influence may remain. What if, somewhere beyond our cosmic horizon there exists a region of much more stuff? A different bubble of “observable universe” with more galaxies, more clusters, more dark matter. Such a region may have given our entire universe such a gravitational tug that, even 13.7 billion years later, we still see a faint drift in that direction.

Whether or not Dark Flow is real is still unresolved. We need to better understand why the different teams get different results. However we may also need a better map of the cosmic microwave background so that we can measure more clusters to greater distances and higher precision. However if Dark Flow does turn out to be real, we may have detected for the first time the influence of a neighboring region of the greater universe, beyond the horizon of observable Space Time.

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