Where Was Dark Matter in the Early Universe?

Observations of early-forming galaxies shows they’re pretty…normal. And that’s unexpected.

Artist's depiction of rotating disc galaxies in the early Universe (right) and the present day (left). Galaxies in the early Universe were less influenced by dark matter (shown in red). As a result the outer parts of distant galaxies rotate more slowly than comparable regions of galaxies in the local Universe.

In the modern universe, dark matter dominates the motions of stars in the outskirts of the Milky Way and other disk-shaped galaxies. But new research suggests that wasn’t the case 10 billion years ago. Instead, galaxies were dominated by the “normal” matter that makes up gas, dust, and stars—everything we can see and touch.

Dark matter produces no detectable energy, but reveals its presence by exerting a gravitational pull on visible matter. Leading theories say it consists of subatomic particles created in the Big Bang, although efforts to find the particles using detectors in underground laboratories so far have been unsuccessful.

Astronomers discovered dark matter in part by measuring its influence on the motions of stars in spiral galaxies. If a galaxy consists only of the matter that we can see, then stars at the edge of its disk should move more slowly than those in the densely packed center, just as planets farther from the Sun move slower than those closer to the Sun.

Instead, stars on the rims of galaxies in the present-day universe move just as fast as those near the center. That means that some unseen matter is tugging at the stars to make them move faster: dark matter. Astronomers calculate that dark matter outweighs the normal matter in the universe by about five to one.

But a new study has found that when the universe was only about four billion years old, when the rate of galaxy formation reached its peak, galaxies were dominated by normal matter. The study was led by Reinhard Genzel of the University of California, Berkeley, and the Max Planck Institute for Extraterrestrial Physics in Germany and published in the March 16 issue of Nature.

Researchers measured the rotation of six disk-shaped galaxies with the Very Large Telescope in Chile. The galaxies are all of similar mass to the Milky Way. The measurements showed that stars in the inner regions of the galaxies moved much faster than those at the edges, suggesting that the role of dark matter was “modest to negligible,” according to the Nature paper.

Several other recent studies have found similar results. One averaged the observations of about 100 galaxies, while another modeled the motions of 240 galaxies. A third, led by team member Hannah Übler but not yet published, “also shows that the contribution of dark matter to the dynamical mass on the galaxy scale is larger for galaxies that are [closer],” Übler said in an email.

The findings don’t mean that dark matter didn’t exist in that earlier epoch. Normal matter was just more densely packed in the younger, smaller early universe, the researchers say.

“Our early, star-forming galaxies are very gas rich and compact,” Genzel and Übler said in the email. “Gas...can migrate to the centers of galaxies through loss of angular momentum, creating dense galactic cores and disks.” With the normal matter squeezed together more tightly, it exerted a stronger gravitational influence on stars at a galaxy’s edge than it does today.

To more fully understand the role of dark matter in the earlier universe, the team plans to study less massive galaxies, “which are the progenitors of galaxies like our Milky Way,” Genzel and Übler said. “Will they have a different contribution from dark matter to their dynamics, as it seems to be the case for the Milky Way today? This is what we want to find out next.”

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