Dark Matter Wasn't as Important to the Early Universe as We Thought

Dark matter in the galaxy
Flickr / European Southern Observatory

Today, we know dark matter as a mysterious, elusive presence in our universe, exerting a gravitational pull on our Milky Way and other galaxies. But in an unexpected twist, scientists are now learning dark matter was far less of a powerful presence in the early days of the universe than we once thought.

In a study to be published Thursday in Nature, as well as three accompanying papers in the Astrophysical Journal, an international team of researchers found that 10 billion years ago, the galaxy was dominated by baryonic or “normal” matter, which constitutes what makes up the gas, dust, stars, and planets in our universe today.

Today, dark matter, which does not emit, absorb, or reflect light, has a greater influence on our universe. It interacts with normal matter only through gravity, and it’s responsible for the rotation speeds in spiral galaxies.

“If you take a textbook case of a nearby spiral disk, there are large amounts of dark matter present that causes the rotational speeds to be high, even far from the galaxy center,” coauthor Stijn Wuyts of the University of Bath tells Inverse.

The team used telescopes to measure the rotation curves of six massive galaxies. Since these galaxies were so distant, it was like looking back in time 10 billion years ago, when stars were still forming.

This chart shows observations of six distant galaxies. The left column shows the distribution of the total surface brightness. The right column shows the velocity map. The galaxies show a rotation pattern, with blue areas moving towards the observer and red parts moving away.

Max Planck Institute for Extraterrestrial Physics

But in early galaxies, researchers observed that the outer regions of the galaxies were spinning slower than central regions — meaning less dark matter than expected was present, and dark matter played a smaller role. These early galaxies were also more turbulent than spiral galaxies today. Two further studies of 240 star-forming galaxies supported these findings.

“From the peak epoch of galaxy formation 10 billion years ago to the present day, the reconstruction of this evolutionary path will be something we will be able to do more accurately,” Wuyts says.

Three to four billion years after the Big Bang, the gas in galaxies condensed into flat, rotating disks, while large dark matter halos surrounded them. Before, these halos were more spread out, but after billions of years, the dark matter has condensed and now has a larger effect on the rotation of many modern galaxies, like our Milky Way.

These findings have three possible implications. One is that because normal matter was more influential in the early universe, galaxies today have central regions that are rich in normal matter.

“It’s driven by the fact that galaxies in the young universe have so much gas in them, the gas that later in their lives will be used to form stars,” Wuyts says.

In addition, these findings imply that the winds driving out from the center of the galaxies may have altered the distribution of dark matter.

There’s also a speculative third possibility that the current hypothesis of dark matter may be incorrect. Perhaps the dark matter particle is lighter or not a particle at all, or perhaps it’s similar to black holes.

“We are learning that the situation is more complicated than people had assumed as far as dark matter is concerned, so we need to consider why that is,” lead author Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics in Germany tells Inverse. “If this might lead to a recognition that the dark matter paradigm needs to be rethought, that would be pretty sensational, but we need to first be conservative.”

The team saw the first hints of dropping rotation curves eight years ago, showing that dark matter was not as influential in the early universe. It continued to gather more evidence using data from other galaxies. In the next phase of their research, scientists plan to study galaxies with lower masses to help them better understand our universe’s origins.

“What I would predict is people will be very suspicious,” Genzel says. “That’s the nature of science. This is not what we would’ve expected. Other people will have to check it. In the meantime, the theorists hopefully will start thinking about the various options we’ve discussed and give us ideas on what we’ll do next.”

Abstract: In the cold dark matter cosmology, the baryonic components of galaxies - stars and gas - are thought to be mixed with and embedded in non-baryonic and non-relativistic dark matter, which dominates the total mass of the galaxy and its dark matter halo. In the local Universe, the mass of dark matter within a galactic disk increases with disk radius, becoming appreciable and then dominant in the outer, baryonic regions of the disks of star-forming galaxies. This results in rotation velocities of the visible matter within the disk that are constant or increasing with disk radius – a hallmark of the dark matter model. Comparison between the dynamical mass and the sum of stellar and cold-gas mass at the peak epoch of galaxy formation ten billion years ago, inferred from ancillary data, suggest high baryon factions in the inner, star-forming regions of the disks. Although this implied baryon fraction may be larger than in the local Universe, the systematic uncertainties (stellar initial mass function, calibration of gas masses) render such comparisons inconclusive in terms of the mass of dark matter. Here we report rotation curves for the outer disks of six massive star-forming galaxies, and find that the rotation velocities are not constant, but decrease with radius. We propose that this trend arises because of a combination of two main factors: first, a large fraction of the massive, high-redshift galaxy population was strongly baryon dominated, with dark matter playing a smaller part than in the local Universe; and second, the large velocity dispersion in high-redshift disks introduces a substantial pressure term that leads to a decrease in rotation velocity with increasing radius. The effect of both factors appears to increase with redshift. Qualitatively, the observations suggest that baryons in the early Universe efficiently condensed at the centres of dark matter halos when gas fractions were high, and dark matter was less concentrated.
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