Dark flow

Groups of stars move in space.

Most move in random ways. But some groups move together. They all flow in one direction. This is a big mystery.

Some think something far away pulls them. This pull might come from outside our view. Scientists use big tools to look. They want to see what is there. They are still looking for answers.

Most galaxy clusters move in random ways. But some scientists found something odd. They saw many clusters moving in one direction. This is called dark flow.

Alexander Kashlinsky led a team to study this. They used data from a tool called WMAP. They found clusters flowing at high speeds. This speed was 600 to 1000 km/s. They were moving toward a patch of sky. This patch is between two star groups. These are the Centaurus and Vela constellations.

Why does this happen? Some think something far away pulls them. This pull might come from outside our visible universe. Our telescopes cannot see past a certain point. This point is about 46 billion light years away. The thing causing the pull might be beyond that. Some think it could be another universe.

Not all scientists agree. Data from the Planck space telescope showed no dark flow. Other experts say the math might be wrong. But in 2015, some researchers found more support for it. They used both WMAP and Planck data. The mystery of dark flow is still being studied.

Scientists study how galaxy clusters move through space. Most clusters move in random directions. This is what standard models of the universe expect to see. However, some researchers found a strange pattern called dark flow. This idea suggests that clusters move together in one direction. This movement is called bulk flow. It is a big mystery in astrophysics.

To find this, astronomers look at peculiar velocity. This is the extra speed a cluster has beyond what is expected. The total speed is the sum of Hubble's law and this extra flow. Alexander Kashlinsky led a team to study this using WMAP data. They used a method called the kinematic Sunyaev–Zeldovich effect. They found clusters flowing at 600 to 1000 km/s. This flow moves toward a patch of sky between Centaurus and Vela.

This discovery happened around October 2008. The results were shared in the Astrophysical Journal Letters. In 2010, Kashlinsky expanded his work. He used five years of WMAP data instead of three. He also looked at twice as many clusters. He studied 1,400 clusters instead of 700. He even sorted them into four different distance slices. He wanted to see if the flow stayed the same.

Some scientists think the cause is very far away. Our telescopes cannot see past the particle horizon. This is a distance of about 46 billion light years. The thing pulling the clusters might be outside our visible universe. It could be a remnant from before inflation. It might even be a sibling universe or a multiverse. However, the Planck space telescope data in 2013 showed no evidence of this flow.

Not all experts agree on these findings. Ned Wright argued that the methods used were flawed. Ryan Keisler said the math might not account for certain cosmic patterns. In 2015, Atrio-Barandela claimed to find support for the flow. They used data from both WMAP and Planck. The dark flow might be related to the Great Attractor. That is a huge mystery discovered in 1973. It is located near the Norma Cluster.

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.