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New map of dark matter spanning 10 million galaxies Tips for a failure in our physics



An invisible force has an effect on our universe. We cannot see it, and we cannot detect it – but we can observe how it interacts gravitationally with the things we can see and discover, such as light.

Now, an international team of astronomers has used one of the world's most powerful telescopes to analyze that effect over 10 million galaxies associated with Einstein's general relativity. The result? The most comprehensive map of dark matter over the history of the universe to date.

It has not yet completed peer review, but the map has suggested something unexpected – that the structures in dark matter can develop more slowly than previously calculated.

"If additional data shows that we are definitely right, it suggests that something is missing from our current understanding of the standard model and general relativity," said physicist Chiaki Hikage of the Kavli Institute for Physics and Mathematics of the Universe.

dark matter over timeVisualization of the dark matter map (lighter = darker matter) based on the lentils produced by galaxies, indicated by white lines. (HSC project / UTokyo)

We do not know what dark matter is. What we know is that the gravitational effects we see in the universe cannot be accounted for by observable thing alone. For example, the rotation speed of galaxies would be quite different if it was based solely on the gravity of observable mass.

We also know that gravity can bend the path of light, as we see with gravitational lensing. This effect can also be used to map dark matter – when you subtract the gravitational effect of visible matter, what you have left is the gravitational effect of dark matter.

This is a common method for finding dark matter, and that is what Hikage's law is also used for. They utilized the 8.2-meter Subaru telescope's 870-megapixel Hyper Suprime-Cam to reach galaxies billions of light-years away.

Since their light has taken so long to reach us, we see them as they existed several billion years ago, which means that the map covers much of the history of the universe, so astronomers can observe how dark matter has evolved over billions of years.

The resulting 3D map reveals the clumsy layout of the dark matter of the universe, consistent with the results of previous research – except how quickly the structures evolve. According to this new map, it is slower than predicted by previous results.

Not so much, but enough to stand out as weird. As said, the jury is still out of what it means. It may indicate that something is missing from the standard model, which would be quite amazing; or it may indicate a statistical fluctuation in data.

It may take a while before we find out too. The team has been working on this project since 2014, with only the first year's observer value, or 11 percent of the Hyper Suprime-Cams survey, which has not yet been completed. The capture is scheduled to end in 2020 at some point.

So let's not get too excited – there is still a whole maternity burden of work to be done. But it is still an exciting result, and we are waiting for more information with breath.

"With a little more work, if we can get better accuracy, we may find something concrete," Hikage said. "This is a great motivational factor for me."

The team's research has been approved in Publication of the Astronomical Society of Japan, and can be read in its entirety on pre-print server arXiv.


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