National Geographic Daily News
An illustration of the Milky Way galaxy.

An illustration of the Milky Way shows the expected distribution of dark matter as a blue halo.

Illustration courtesy L. Calçada, ESO

Ker Than

for National Geographic News

Published April 19, 2012

Dark matter is mysteriously missing from the sun's neighborhood, according to a new study that could provide ammunition for skeptics who argue that the invisible substance is just an illusion.

"There will be people claiming dark matter doesn't exist because of this result," predicted study leader Christian Moni-Bidin, an astronomer at Chile's University of Concepción.

"These observations alone do not prove that dark matter does not exist. Still, it is not where we expected it and where we needed it."

(See "Dark Matter Is an Illusion, New Antigravity Theory Says.")

Even though dark matter particles can't be detected with current instruments, astronomers think the substance must make up about a quarter of the universe, based on the gravitational effect it has on visible matter such as galaxies and galaxy clusters.

Since its discovery in the 1930s, the material has become crucial for galaxy-formation theories, which say that dark matter functions as a kind of invisible scaffold around which normal matter gravitationally coalesces to form stars and larger objects.

(Also see "Dark Matter Blob Should Not Exist, But There It Is.")

Current models of how galaxies form and rotate suggest that the Milky Way is embedded in a cloud, or halo, of dark matter. Astronomers can't tell precisely what shape this halo takes, but they expected to find significant amounts of dark matter in the region around the sun.

Sun's Motion Reveals No Dark Matter?

In the new study, Moni-Bidin and his colleagues used the European Southern Observatory's La Silla and Las Campanas telescopes in Chile to map the three-dimensional motions of more than 400 red giant stars up to 13,000 light-years from the sun.

Stars in the sky may appear static, but they're constantly in motion as they get minutely pushed and pulled by the gravitational effects of neighboring objects, including other stars, gas clouds, or clumps of dark matter.

(Related picture: "Cluster Smashup Is Dark Matter Proof.")

The team compared their measurements of stellar motion with what they predicted the motions would be if the stars' movements were affected by visible matter alone.

To their surprise, the two sets of measurements matched. In other words, dark matter was not necessary to explain the motions of the sun and its close neighbors.

"These observations point to the fact that in this volume [of space], there is no dark matter," Moni-Bidin said.

Dark-Matter Study Has Achilles Heel

However, the new results are being met with some skepticism, because they rely on ten simplifying assumptions, said Avi Loeb, chair of the astronomy department at Harvard University, who was not involved in the study.

For example, the analysis assumes that the average speed at which the stars in question orbit the center of the Milky Way is the same no matter the stars' distances from the galactic center. Knowing this speed is important for creating a complete picture of the factors that influence a star's motion.

But "this assumption by itself requires dark matter, unless gravity is modified," Loeb said in an email.

One reason scientists think dark matter exists is because stars in the outer parts of galaxies have been seen to orbit the center as fast as stars in the inner parts. The laws of gravity dictate that such fast-moving stars near the edges of galaxies should fly off into space. Instead, the theory goes, the added mass of dark matter holds them in place.

An alternative idea is that gravity itself behaves differently at different distances from the galactic core. However, "if gravity is modified, then it is unclear which equations one should use in the [new study's] modeling," Loeb said.

"Extraordinary claims require extraordinary evidence," he added, "and these ten assumptions are the Achilles heel of the claim that dark matter is absent in the solar neighborhood."

(Related: "Dark-Matter Galaxy Detected—Hidden Dwarf Lurks Nearby?")

For his part, study author Moni-Bidin thinks it's too early for astronomers to give up on dark matter, because the substance is still too important for widely held theories about how the universe works.

"We are far from being able to live without [dark matter]," he said. "At the moment we need it to explain too many observations where alternative theories fail."

The new dark-matter research will be detailed in an upcoming issue of the Astrophysical Journal.

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