Dark flow

In the field of astrophysics, dark flow is a controversial hypothesis. It attempts to explain unusual measurements in the movement of galaxy clusters. Usually, these clusters move in random directions. This randomness is what standard cosmological models expect to see. However, some researchers suggest a large-scale correlated flow exists. This movement is known as bulk flow. It suggests that many clusters are moving together toward a single direction.

To understand this, we must look at peculiar velocity. This is the extra speed a cluster has beyond what is expected. The total measured velocity is the sum of Hubble's law plus this possible small velocity. Hubble's law describes the expected expansion of the universe. The dark flow would be an additional movement flowing in a common direction. Astronomers use the kinematic Sunyaev–Zeldovich effect to measure these movements. This effect helps them analyze data from the cosmic microwave background.

In 2008, a team led by Alexander Kashlinsky found surprising evidence. They analyzed three years of data from the Wilkinson Microwave Anisotropy Probe, or WMAP. They found a coherent flow of 600 to 1000 km/s. This motion is directed toward a 20-degree patch of sky. This patch lies between the constellations of Centaurus and Vela. Kashlinsky later extended this research in 2010. He used five years of WMAP data instead of three. He also doubled his sample size from 700 to 1,400 galaxy clusters.

Kashlinsky’s team organized the clusters into four distinct distance slices. They wanted to see if the flow direction stayed consistent across different depths. The results showed that while the exact position varied slightly, the trends agreed. The team cataloged this effect as far out as 2.5 billion light-years. They hoped to eventually expand this catalog to twice that distance. This study was published in the October 20, 2008, issue of Astrophysical Journal Letters. The researchers noted they could not yet say if clusters were coming or going.

Where could this mysterious pull come from? The source might be outside our visible universe. Telescopes cannot see events earlier than 380,000 years after the Big Bang. This time marks when the universe became transparent. This limit is called the particle horizon. It is a distance of about 46 billion light-years. The matter causing this motion might be outside that range. However, it would still be in our past light cone. It could be a remnant of regions from before inflation.

Not all scientists agree that dark flow is real. Astrophysicist Ned Wright argued that the original methods were flawed. Ryan Keisler also claimed the possibility of dark flow was ruled out. He argued that the researchers did not treat primary anisotropies of the cosmic microwave background as importantly as necessary. In 2013, data from the Planck space telescope showed no evidence of dark flow. This data discounted claims of gravitational effects reaching beyond the visible universe. It also cast doubt on the idea of a multiverse.

Despite these criticisms, the debate continues. In 2015, Atrio-Barandela claimed to find support for the flow. This team used data from both the Planck and WMAP missions. They suggested the data were consistent with the earlier findings. Some scientists, like Laura Mersini-Houghton, suggest the flow could involve a sibling universe. Others suggest it relates to the Great Attractor. This was a gravitational mystery discovered in 1973. It is located near the Norma Cluster, about 250 million light-years away.