Everything in our universe is constantly moving. Planets orbit stars. Stars dance within galaxies. Galaxies whirl within giant clusters. And generally, on average, these clusters move further apart from each other. This is the expansion of the universe, first discovered by Hubble in the 1920s. And according to our best observations, our remarkably successful theories, and about four centuries’ worth of astronomy, the expansion of the Universe is uniform. So, once you get to large enough scales, galaxies just move out without any other preferred direction.
But at relatively smaller scales, dynamics are not entirely random, if you wish to put it that way. Take our own galaxy, the Milky Way. It is en route to collide with our nearest neighbor, the Andromeda. In about five billion years these two will meet 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.
And of course, there’s more. The Milky Way, Andromeda, the Virgo Cluster, and all the other galaxies in our nearby cosmic 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.
Galilean relativity tells us that there is no preferred frame of reference to define absolute stillness. In other words, the laws of Physics are the same no matter your speed. In Astronomy, however, there is one frame of reference that is, in some ways, “preferred” over others. It’s the Cosmic Microwave Background radiation; the CMB.
The CMB is the leftover glow of the early universe. It is, as its name suggests, microwave radiation; a remnant of the Big Bang that provides an important source of data on the primordial universe. What’s worth noting about the CMB is that in all directions it is observed to read the same temperature, around 2.7 Kelvin. But due to the motion of the Earth, the Sun, and virtually every celestial body, we record the CMB to be Doppler-shifted: some wavelengths are stretched and some are shrunk. This leads to the impression of “cooler” and “warmer” bands of radiation. Invariably, this begs the thought that if the CMB is asymmetric to our motion, there must be some reference frame in which we do not observe any Doppler shifts. In that reference frame, if you added all the peculiar velocities of the galaxies in the universe, you’d expect them to cancel out. So the CMB can be used to define “stillness”; or alternatively, to measure deviations from stillness. But there are caveats here as well.
Enter: the Kinematic Sunyaev-Zeldovich Effect
Actually, before beginning with the kinematic KSZ effect, let’s begin with the regular old thermal KSZ effects.
The most massive clusters in our universe are vast conglomerations of thousands of galaxies that are bathed in a searing hot plasma, with temperatures up to 100 million Kelvin. As the CMB radiation passes through this plasma, some of its photons interact with the electrons in the plasma, causing the photons to gain energy. This results in a small shift in the spectrum of the CMB radiation, known as the thermal SZ effect. Even in superclusters, the effect is not strong (< 8 μK), but with precise enough equipment, measuring this distortion can give a glimpse into large-scale structure formation [1].
The kinematic KSZ effect is much harder to measure than its thermal twin. If clusters have a peculiar velocity in addition to the velocity they have due to the expansion of the universe, then the thermal SZ effect adds an extra Doppler shift to the photons that pass through the cluster. This is the kinematic Sunyaev-Zeldovich effect. Measuring this Doppler shift can then yield the velocities of individual galaxy clusters. There is a caveat here that I should mention.
The Doppler effect can only tell us the peculiar velocities of galaxies that move towards or away from us, i.e., in our line of sight. Any other motion is inconclusive. And given how weak this effect is, the KSZ measurement for just one cluster isn’t very useful. But measuring hundreds of clusters across the universe yields fascinating results.
Scientists led by Sasha Kashlinsky, of the Goddard Space Flight Center in the US, did exactly that. They used the Wilkinson Microwave Anisotropy Probe to study tiny fluctuations in the CMB due to KSZ effects. Their study concluded the non-random motion of celestial bodies after analyzing the WMAP data. They constructed a peculiar velocity map, if you will, containing about 700 galaxy clusters across the sky, spanning billion of light years. After factoring out the Hubble flow, on average, it seems as if a large number of these clusters are moving in the same direction. This is incredibly controversial.
It violates an idea rooted in fundamental cosmology: the Universe is isotropic and homogenous on the largest of scales. Homogenous means that regardless of which part of the Universe you were in, everything would look roughly the same. Isotropic means that the Universe shouldn’t have a preferred direction. It shouldn’t be stretched out in a particular direction and it certainly shouldn’t be moving in any preferred direction either. So, the idea of a universe-wide dark flow still warrants skepticism. Because, if galaxy clusters are moving to a preferential point, they must be pulled from a force beyond the edges of our Universe.
Most researchers have been quick to refute this claim. The strongest argument is offered by the Planck team. The Planck satellite is one of the latest CMB probes that is far more advanced than WMAP. In a catalog of over a thousand clusters, it reported no apparent dark flow. Other scientists, including the team led by Kashlinsky, however, reproduced their method to observe an apparent dark flow even in the Planck data.
But let’s forget the skepticism for a minute and consider the implications of a preferential direction. The data indicated that clusters are moving towards the Centaurus and Hydra constellations. And since 1973, we’ve known that galaxies in the local part of the Universe seem to be drawn to that region which we’ve termed the Great Attractor. We now think that this may be the center of the Laniakea supercluster, which is a vast cluster of several-hundred galaxy clusters. The Great Attractor, however, is not causing the dark flow because a preferred direction can be observed for galaxies within a 2.5 billion light-year radius; a distance far beyond the gravitational influence of the Great Attractor.
It is possible that, if real, dark flow is the relic of a massive gravitational attraction at the beginning of the Universe. Whatever may have been beyond the observable Universe could have caused a large gravitational pull and although today it is far beyond the reach of light, gravity, or any other phenomenon, its effects may remain active.
Whether or not dark flow is real seems to be unresolved. A large number of physicists, do, however, contest that it is not.
Thank you for reading!
