Dark Flow, The phenomenon needs Evidence

The internet is bombarded with articles about Dark Matter because scientists are very close to confirming its existence. While browsing about it, I went to Wikipedia to read the history of Dark Matter research. On top of that page, Wikipedia showed a list of the unsolved problems in Physics. Yes, Dark Matter is one of them. Among those unsolved problems, after Dark Matter and Dark Energy, I found Dark Flow. That was the first time I learned about Dark Flow, so I became curious and started reading about it.

In 2008, an article was released by Alexander Kashlinsky and his colleagues. It was the first-ever paper published about this cosmic phenomenon. Since then, lots of research has been going on, and many imaginative stories have been wrapped around this cosmic mystery.

So, what exactly is Dark Flow?

After we discovered Dark Energy, scientists began constantly studying the dynamics of massive cosmic bodies like galaxies and galaxy clusters. They noticed that a few large, distant galaxy clusters were moving toward a particular point in space at a speed estimated between 600–1000 km/s. But what is that point? That’s where the mystery begins — the point they calculated lies beyond our cosmic horizon, meaning it is not in our observable universe.

Some articles and videos claim that it’s a pull from another universe, while others say it’s a “hole” in our current balloon-like universe and that the universe is leaking out. These are mostly clickbait meant to attract more audiences.

The team of scientists compiled an all-sky sample of galaxy clusters selected in X-rays and matched them with Cosmic Microwave Background (CMB) data, combining it with WMAP (Wilkinson Microwave Anisotropy Probe) observations to identify the flow of large galaxy clusters. They measured the dipole of the CMB temperature field using the Kinematic Sunyaev–Zeldovich (KSZ) effect. If clusters move in the same direction, a temperature dipole appears in that region of the CMB temperature field. Through precise calculations—considering redshift and minimizing noise—they concluded that there is a dark flow of galaxy clusters. They also found that more X-ray luminous clusters have a greater dipole, which matches the expected result of the KSZ effect.

I read Alexander Kashlinsky’s research paper on this. The authors mentioned that their results represent a 3–4σ detection of coherent large-scale bulk motion of galaxy clusters. That means these results provide strong evidence for Dark Flow, but not a definitive proof. For a definitive proof, the results should reach 5σ. However, the last five years of data from WMAP show the same coherent results, strengthening the evidence for Dark Flow.

They found that the flow of large clusters is directed toward a 20-degree patch between the constellations of Centaurus and Vela. The results were published in the October 20, 2008 issue of Astrophysical Journal Letters.

In 2013, data from the Planck Space Telescope showed no evidence of Dark Flow on such a scale, discounting claims that Dark Flow results from gravitational effects of another universe or regions beyond the cosmic horizon. However, in 2015, Atrio-Barandela and his colleagues published a paper with a detailed analysis of the Planck and WMAP data, providing further support for the existence of Dark Flow.

There has been a lot of criticism surrounding Dark Flow. Some researchers argue that the methodology is flawed, while others say that the team did not consider the anisotropies of the CMB to be as significant as the isotropies. Some scientists suggest that it might be the effect of a “sibling universe” or some massive structure beyond our cosmic horizon. Kashlinsky responded that a few criticisms are invalid, and at this point, we don’t have enough information to determine what it truly is or to constrain it.

Although the ESA Planck Satellite (2013) data suggested there isn’t enough evidence for the existence of Dark Flow, Fernando Atrio-Barandela argued that the data are consistent with earlier findings from WMAP.

There are many criticisms and a lot of imaginative stories about Dark Flow, but we still don’t have enough data to confirm it as a definitive discovery. Let’s see whether it turns out to be the Vulcan or the Neptune of our universe.

References:

1. Dark flow - Wikipedia

2. Alexander Kashlinsky's Article - https://arxiv.org/abs/1012.3214