Tonight's Sky
Sun
Sun
Moon
Moon
Mercury
Mercury
Venus
Venus
Mars
Mars
Jupiter
Jupiter
Saturn
Saturn

Tonight's Sky — Change location

OR

Searching...

Tonight's Sky — Select location

Tonight's Sky — Enter coordinates

° '
° '

Dark matter mystery deepens

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.
Dwarf galaxy
This artist's conception shows a dwarf galaxy seen from the surface of a hypothetical exoplanet. A new study finds that the dark matter in dwarf galaxies is distributed smoothly rather than being clumped at their centers. This contradicts simulations using the standard cosmological model known as lambda-CDM. Credit: David A. Aguilar (CfA)
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 Matt Walker from the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.

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 said.

Dwarf galaxies are composed of up to 99 percent dark matter and only 1 percent normal matter like stars. This disparity makes dwarf galaxies ideal targets for astronomers seeking to understand dark matter.

Walker and Jorge Penarrubia from the University of Cambridge, United Kingdom, analyzed the dark matter distribution in two Milky Way neighbors — the Fornax and Sculptor dwarf galaxies. These galaxies hold one million to 10 million stars, compared to about 400 billion in our galaxy. The team measured the locations, speeds, and basic chemical compositions of 1,500 to 2,500 stars.

“Stars in a dwarf galaxy swarm like bees in a beehive instead of moving in nice, circular orbits like a spiral galaxy,” said Penarrubia. “That makes it much more challenging to determine the distribution of dark matter.”

Their data showed that in both cases, the dark matter is distributed uniformly over a relatively large region, several hundred light-years across. This contradicts the prediction that the density of dark matter should increase sharply toward the centers of these galaxies.

“If a dwarf galaxy were a peach, the standard cosmological model says we should find a dark matter “pit” at the center. Instead, the first two dwarf galaxies we studied are like pitless peaches,” said Penarrubia.

Some have suggested that interactions between normal and dark matter could spread out the dark matter, but current simulations don’t indicate that this happens in dwarf galaxies. The new measurements imply that either normal matter affects dark matter more than expected, or dark matter isn’t “cold.” The team hopes to determine which is true by studying more dwarf galaxies, particularly galaxies with an even higher percentage of dark matter.

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 Matt Walker from the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.

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 said.

Dwarf galaxies are composed of up to 99 percent dark matter and only 1 percent normal matter like stars. This disparity makes dwarf galaxies ideal targets for astronomers seeking to understand dark matter.

Walker and Jorge Penarrubia from the University of Cambridge, United Kingdom, analyzed the dark matter distribution in two Milky Way neighbors — the Fornax and Sculptor dwarf galaxies. These galaxies hold one million to 10 million stars, compared to about 400 billion in our galaxy. The team measured the locations, speeds, and basic chemical compositions of 1,500 to 2,500 stars.

“Stars in a dwarf galaxy swarm like bees in a beehive instead of moving in nice, circular orbits like a spiral galaxy,” said Penarrubia. “That makes it much more challenging to determine the distribution of dark matter.”

Their data showed that in both cases, the dark matter is distributed uniformly over a relatively large region, several hundred light-years across. This contradicts the prediction that the density of dark matter should increase sharply toward the centers of these galaxies.

“If a dwarf galaxy were a peach, the standard cosmological model says we should find a dark matter “pit” at the center. Instead, the first two dwarf galaxies we studied are like pitless peaches,” said Penarrubia.

Some have suggested that interactions between normal and dark matter could spread out the dark matter, but current simulations don’t indicate that this happens in dwarf galaxies. The new measurements imply that either normal matter affects dark matter more than expected, or dark matter isn’t “cold.” The team hopes to determine which is true by studying more dwarf galaxies, particularly galaxies with an even higher percentage of dark matter.

0

JOIN THE DISCUSSION

Read and share your comments on this article
Comment on this article
Want to leave a comment?
Only registered members of Astronomy.com are allowed to comment on this article. Registration is FREE and only takes a couple minutes.

Login or Register now.
0 comments
ADVERTISEMENT

FREE EMAIL NEWSLETTER

Receive news, sky-event information, observing tips, and more from Astronomy's weekly email newsletter.

ADVERTISEMENT
ADVERTISEMENT
BoxProductcovernov

Click here to receive a FREE e-Guide exclusively from Astronomy magazine.

Find us on Facebook

Loading...