02:02 - Source: CNN
Brian Greene sheds light on dark matter

Editor’s Note: Don Lincoln is a senior scientist at Fermi National Accelerator Laboratory. He is the author of “The Large Hadron Collider: The Extraordinary Story of the Higgs Boson and Other Stuff That Will Blow Your Mind” and produces a series of science education videos. Follow him on Facebook. The opinions expressed in this commentary are his.

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Albert Einstein is universally regarded to be a pretty smart guy. His theory of special relativity taught us about counterintuitive truths of nature, such as how stationary and moving clocks tick at different speeds and the inability to travel faster than light. And his theory of general relativity was even more mind-blowing, revealing that gravity is really the bending of space and time.

General relativity has been stringently tested and is accepted by the world’s scientific community. However, while this theory is used to describe the entire universe, it turns out that it has only been tested at size scales no larger than our solar system. Well, that is, until now.

Astronomers have just announced an exciting new measurement that has validated Einstein’s theory of general relativity at a galactic scale. Using a quirk of the theory, where one galaxy can act as a lens for a more distant galaxy, researchers have extended Einstein’s century-long run of accurate predictions. His predictions are now confirmed to be correct even for things as big as an entire galaxy.

Not only is this an unprecedented accomplishment, it has significant consequences in the ongoing scientific debate into not only the nature, but even the existence, of dark matter, a substance which has been hypothesized but never observed.

General relativity makes many surprising predictions, one of them being that gravity can bend the passage of light. This is surprising because gravity is a force that affects objects with mass, and light has no mass. But if gravity can bend space, and light travels in straight lines in that bent space, it looks as if light will travel in a curved path.

Astronomers can use this phenomenon to study very distant galaxies. Suppose you are looking at a relatively close galaxy and there is a much more distant galaxy directly along that line of sight. The path of light from the more distant galaxy will be curved in its passage around the closer galaxy and directed toward Earth. This is exactly analogous to how an ordinary lens, you might find in a pair of binoculars works. Accordingly, physicists call this phenomenon “gravitational lensing.”

If the alignment of the near and distant galaxy is nearly perfect, the result is that we see the distant galaxy in the form of a ring around the nearer one. This phenomenon was predicted by Einstein in 1936, two decades after he invented his theory of general relativity and astronomers call them “Einstein rings.”

The recent observation that astronomers announced is of a nearly perfect Einstein ring.

Many Einstein rings have been observed by modern astronomers, so it’s not the simple observation that is exciting. But there are several very interesting features of this case.

The first is that the closer galaxy, called ESO 325-G004 and what astronomers call the lensing galaxy, is relatively nearby – about 450 million light years away. This proximity makes it possible for astronomers to accurately measure the distribution of mass inside the galaxy. This was accomplished using an instrument called MUSE, located on the European Southern Observatory Very Large Telescope.

Using the Hubble Space Telescope, astronomers were able to precisely measure the size of the Einstein ring. This ring was much smaller than most, with a radius about 6,500 light years, meaning that the ring is actually observed within the lensing galaxy.

Einstein’s theory of general relativity makes predictions about the relationship between the size of an Einstein ring and the mass of the lensing galaxy. When the prediction is compared to this recent measurement, the agreement is spectacular, with an uncertainty of less than 10%.

This measurement is a very important addition to the scientific conversation about dark matter. You see, if you take the simplest form of Einstein’s theory of general relativity and apply it to a variety of astronomical observations, they don’t agree and the disagreement is severe.

This led astronomers to postulate the existence of dark matter, which is a substance that feels the force of gravity, but is otherwise undetectable. Dark matter is thought to be five times more prevalent than ordinary matter. Despite never having been observed, its existence is the most popular explanation for the disagreement between observations and Einstein’s predictions.

Another possible explanation for the discrepancy between measurement and predictions is simply that Einstein’s theory is wrong. This has been a plausible conjecture because, until now, validation of his theory has been limited to the solar system.

Given this lack of verification, physicists proposed alternative theories of gravity; ones which made predictions identical to general relativity for distances up to the size of the solar system, but made different predictions for bigger objects like galaxies and even clusters of galaxies. These alternate theories are lumped under the umbrella term MOND (Modifications of Newtonian Dynamics).

The recent observation validates that Einstein’s theory applies to a large fraction of the galaxies in the universe – those with a diameter of up to 13,000 light years. Our Milky Way galaxy is larger than most, with a diameter of 100,000 light years, meaning that the theory of general relativity might not apply to the entirety of our galaxy. It should, of course, and astronomers certainly assume that it does. But to be strictly accurate, their observation only confirms that general relativity governs these somewhat smaller galaxies.

The fact that Einstein’s theory of general relativity has been validated at galactic size scales makes it much harder for those trying to devise MOND theories. By extension, it also supports the idea that dark matter exists.

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    As a scientist, I do urge caution. This is a single measurement and confirmation is always crucial in scientific endeavors. However, there is no question that this observation is an exciting and valuable contribution to our understanding of the laws of nature. We can, however, throw caution to the wind, and make one important conclusion.

    That Einstein was a pretty smart guy.