Dark-Matter Asteroids  – “Trillions of Trillions May Exist in the Milky Way”

Dark Matter Asteroids

 

“Dark matter holds the key to understanding the universe,” says astrophysicist Paul Davies. The mysterious dark matter particles compose 80% of the matter in the Universe, and they have been described as ranging from planet-sized particles to dark-matter life.

“After the big bang that created the universe 13.7 billion years ago,” Davies explains, “matter was spread smoothly through space, but aided by the gravitating power of the dark component, ordinary matter was drawn into clumps, which later evolved into galaxies that spawned stars, planets and, in one case at least that we know of life.”

The Large Hadron Collider’s discovery of the Higgs boson in 2012 prompted a burst of optimism that dark matter particles would soon be discovered, but so far none has been seen and previously promising classes of particles now seem to be long-shots.

“The nature of dark matter is one of the biggest mysteries in science and we need to use any related new data to tackle it,” says Harvard’s Avi Loeb of the CfA.

Common on a Galactic Scale

Dark matter emits no light and interacts with visible matter only via gravity. But dark matter might be only the tip of an invisible universe of unknown forces, including newly proposed macroscopic objects researchers have named dark-matter asteroids.

“If a significant portion of dark matter is in the form of dark asteroids, then they would be extremely common on the galactic scale, with trillions of trillions of them wandering the Milky Way,” Stanford University physicist Kevin Zhou told The Daily Galaxy when asked about the ubiquity of such dark-matter asteroids. Hence the hypothetical dark-matter asteroids zooming through space could potentially outnumber the stars in our Milky Way Galaxy by more than a trillion to one. 

Is Dark Matter Only the Tip of an Invisible Universe of Unknown Forces?

Zhou and researchers at SLAC National Accelerator Laboratory and Université Paris Saclay have proposed that dark-matter objects traveling through stars could produce potentially detectable shock waves. They have recently carried out a study that could introduce a new way of searching for dark matter, reflecting a growing trend within the astrophysics community to use astronomical objects as enormous dark matter detectors. Their paper, “Stellar Shocks From Dark Matter Asteroid Impacts”, shows that when spherical macroscopic dark matter objects travel through a star, they do not disintegrate. Instead, they could produce shock waves that might reach the star’s surface, emitting a distinctive and transient optical, UV and X-ray signal that could be detected by sophisticated telescopes, such as the upcoming Vera Rubin Observatory.

Impacting the Solar System?

But Zhou has a caveat: “Since space is so vast and empty,” he explained in his email, “they would still be very rare on the solar system scale. For the typical dark asteroid masses we consider, an impact on our Sun has likely not happened since the beginning of recorded history, while impacts on Earth have likely never occurred since its formation. That’s why, to have a shot at seeing impact events, powerful telescopes are needed to monitor hundreds of thousands of stars at once. While the Vera Rubin Observatory is not directly designed for such a search, its impressive power makes it one of the most promising avenues to detect dark asteroid collisions.’  

Relics of the Big Bang –Dark Matter is Composed of Primordial Black Holes

Within the standard model of physics, the closest analogue to a dark asteroid impact would be a comet impact. However, comets are expected to be rare outside of planetary systems, with the interstellar comet density bounded orders of magnitude below the dark-matter density. Comets are also “rubble piles” that fall apart before even reaching the stellar surface, leading to a qualitatively different signature. By contrast, in simple dark sector models, the binding energy of a dark asteroid may easily exceed its kinetic energy, and so dark-matter asteroids are more likely to penetrate the surface of a star. 

Dark Matter Density at Milky Way’s Center

The high dark-matter density at the Milky Way’s galactic center would make it ideal for a focused search. Globular clusters could be promising, especially if new nearby clusters are found, or confirmed to contain an intermediate-mass black hole. Milky Way satellite galaxies are more distant but are known to host a high dark-matter density, and could likely be used to probe higher masses of dark matter objects. If dark-matter asteroids have typical masses smaller than expected, then impacts on the Sun may occur annually and would still be energetic enough to be detected by solar observatories. It would be interesting, the researchers noted, to see if the resolution of these instruments permits such impacts to be distinguished from solar flares. In many of these cases, it may be possible to find impact events in a reanalysis of archival data.

Image credit: Shutterstock License

Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via Kevin Zhou and  Physical Review Letters

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