Tag Archive: Dark Matter

Astronomers mapped the motions of hundreds of stars in the Milky Way in order to deduce the amount of dark matter that must be tugging on them from the vicinity of our sun. Their surprising conclusion? There’s no dark matter around here.

As the researchers write in a forthcoming paper in the Astrophysical Journal, the stellar motion implies that the stars, all within 13,000 light-years of Earth, are gravitationally attracted by the visible material in our solar system — the sun, planets and surrounding gas and dust — and not by any unseen matter.

“Our calculations show that (dark matter) should have shown up very clearly in our measurements. But it was just not there!” said lead study author Christian Moni-Bidin, an astronomer at the University of Concepcion in Chile.

If the analysis of the data from Chile’s European Southern Observatory (ESO) is correct — a big “if,” several physicists say — it overturns the decades-old theory that dark matter permeates space in our region of the Milky Way. Dark matter is an invisible material thought to make up 80 percent of all matter in the universe. Although it doesn’t interact with light and so cannot be seen, its presence is invoked to explain why the outskirts of galaxies, including the Milky Way, rotate much more quickly than would be expected based on the gravitational pull of visible matter alone. Commonly accepted as fact, dark matter plays an essential role in models of galaxy formation and evolution, and several experiments are under way to detect dark matter particles on Earth.

But if dark matter isn’t here in the solar system, it may not be anywhere, because its distribution through the galaxy would have to be extremely peculiar to avoid this region in space. “Modern theories have serious troubles to explain the formation of a (dark matter) halo so curiously shaped,” Moni-Bidin told Life’s Little Mysteries.

Scott Tremaine, professor of physics at Princeton University’s Institute for Advanced Study, said, “If the authors’ conclusions are correct, this is indeed a serious blow to dark matter.”

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Dark matter, to re-interpret Obi Wan Kenobi, surrounds us and penetrates us. It binds the galaxy together.

At least that’s what a new scientific study seems to show. Dark matter appears to stretch well beyond the visible limits of galaxies, flowing through and filling even the vast, previously-thought empty space between galaxies. The researchers, led by Shogo Masaki of Nogoya University, used computer simulations to model how dark matter behaves over time as it helps form galaxies, and found that while it’s concentrated in and around galaxies, it doesn’t fade away into nothing with distance. It does get thinner, but still exists to a measurable degree well outside of galaxies. The model structure they found is actually quite lovely.

Remember, this is a model, and not an actual map. It does show concentrations of dark matter along galaxies and clusters of galaxies, but also shows how even “empty” space well outside of galaxies has pervasive dark matter in it.

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Dark Matter Mystery Deepens

Like all galaxies, our Milky Way is home to a strange substance called dark matter. Dark matter is invisible, betraying its presence only through its gravitational pull. Without dark matter holding them together, our galaxy’s speedy stars would fly off in all directions. The nature of dark matter is a mystery — a mystery that a new study has only deepened.

“After completing this study, we know less about dark matter than we did before,” said lead author Matt Walker, a Hubble Fellow at the Harvard-Smithsonian Center for Astrophysics.

The standard cosmological model describes a universe dominated by dark energy and dark matter. Most astronomers assume that dark matter consists of “cold” (i.e. slow-moving) exotic particles that clump together gravitationally. Over time these dark matter clumps grow and attract normal matter, forming the galaxies we see today.

Cosmologists use powerful computers to simulate this process. Their simulations show that dark matter should be densely packed in the centers of galaxies. Instead, new measurements of two dwarf galaxies show that they contain a smooth distribution of dark matter. This suggests that the standard cosmological model may be wrong.

“Our measurements contradict a basic prediction about the structure of cold dark matter in dwarf galaxies. Unless or until theorists can modify that prediction, cold dark matter is inconsistent with our observational data,” Walker stated.

Dwarf galaxies are composed of up to 99 percent dark matter and only one percent normal matter like stars. This disparity makes dwarf galaxies ideal targets for astronomers seeking to understand dark matter.

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