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CosmosUp | October 15, 2019

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There's Half as much Dark Matter in the Milky Way as We Thought

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There’s Half as much Dark Matter in the Milky Way as We Thought

Using a century-old technique to probe the borders of our galaxy, a team of Australian astronomers has estimated the amount of dark matter in the Milky Way. Intriguingly, according to their new calculations, there could be half as much of this poorly understood substance in our galaxy as once thought.

Scientists have long known that everything we see only makes up a tiny part of the Universe – the majority of it is hidden to us.

“Stars, dust, you and me, all the things that we see, only make up about 4 percent of the entire Universe,” said lead researcher Prajwal Kafle from the University of Western Australia node of the International Center. “About 25 percent is dark matter and the rest is dark energy,” he explained.

But because dark matter is impossible to see, it’s also hard to measure. Now using a technique developed almost 100 years ago in 1915 by British astronomer James Jeans, Kafle has managed to more accurately calculate how much dark matter is in our galaxy, and it’s helped solve a decades-old astronomical mystery.

This is an artist's impression of the Milky Way and its dark matter halo

This is an artist’s impression of the Milky Way and its dark matter halo

Australian astronomers probed the edge of the Milky Way, looking closely at the fringes of the galaxy about three trillion miles (5 trillion kilometres) from Earth. The tehnique involves measuring the speed of stars moving through the galaxy.

Dr Kafle, who is originally from Nepal, was able to measure the mass of the dark matter in the Milky Way by studying the speed of stars throughout the galaxy, including the edges, which had never been studied to this detail before. Dr Kafle’s measurement also helps to solve a mystery that has been haunting theorists for almost two decades.

‘The current idea of galaxy formation and evolution, called the Lambda Cold Dark Matter theory, predicts that there should be a handful of big satellite galaxies around the Milky Way that are visible with the naked eye, but we don’t see that,’ Dr Kafle said.

‘When you use our measurement of the mass of the dark matter the theory predicts that there should only be three satellite galaxies out there, which is exactly what we see; the Large Magellanic Cloud, the Small Magellanic Cloud and the Sagittarius Dwarf Galaxy.’

University of Sydney astrophysicist Professor Geraint Lewis, who was also involved in the research, said the missing satellite problem had been “a thorn in the cosmological side for almost 15 years.”
“Dr Kafle’s work has shown that it might not be as bad as everyone thought, although there are still problems to overcome,” he said.

The most detailed computer simulation of the history of the universe called Illustris, revealing dark matter density in the universe

The most detailed computer simulation of the history of the universe called Illustris, revealing dark matter density in the universe

The study also looked at the Milky Way in a holistic light, which led them to make several other conclusions, such as the speed necessary to escape the gravity of the Milky Way.

“Be prepared to hit 550 kilometers per second (340 miles per second) if you want to escape the gravitational clutches of our galaxy,” Kafle said. “A rocket launched from Earth needs just 11 kilometers per second (5.8 miles per second) to leave its surface, which is already about 300 times faster than the maximum Australian speed limit in a car!” .

Mass measurements conducted by a team of international scientists found that the Milky Way is about half the weight of our neighbor, the Andromeda galaxy.

The study team speculated that Andromeda has more dark matter than our own galaxy, causing it to weigh more.

Source: http://goo.gl/NDQKDa



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