Dark Matter Filaments
January 25, 2016
Filaments are everywhere in our modern world. Electricity comes to your home on metal wires and the Internet on an optical fiber. Before the legislated death of the incandescent light bulb, tungsten filaments in such bulbs gave us light. Your clothing is built from a weave of many threads. About 160 billion pounds of textile fiber is produced annually
The most famous filament is the one that held the sword above the head of Damocles. As the story goes, the courtier, Damocles, wished to enjoy the pleasures of kingship. His king granted his wish, allowing Damocles to sit in his throne to enjoy the perquisites of kingship.
After a time, Damocles noticed that the king had hung a sword over the throne by a single hair from a horse's tail. Damocles realized that kingship involved not just luxury, but also great responsibility and danger. Our modern idiom, "hanging by a thread," derives from this story. The sword of Damocles fails as a parable of executive compensation, since the sword in that case is missing, possibly sold to pay the CEO's salary.
The longest objects called filaments are galaxy filaments, composed of galaxies that form the boundaries of voids in the universe. I wrote about these voids in a previous article (Our Lumpy Universe, June 6, 2014). It's conjectured that matter in the universe is aligned with a superstructure of dark matter that gravitationally attracts the normal matter, and that the voids might also contain faint filaments of galaxies.[1-2]
The idea that filaments of dark matter pervade our normal matter spaces, including our own Solar System, is explored in a recent paper in The Astrophysical Journal by Gary Prézeau, an astrophysicist at NASA's Jet Propulsion Laboratory (Pasadena, California). He proposes the existence of strands of concentrated dark matter filaments, or dark matter "hairs," that are concentrated at the planets, including Earth itself.[3-4] Calculations show that the dark matter concentration at Jupiter's core could be a trillion times denser than average.[3-4]
Dark matter is important to astrophysics, since there's more than five times as much dark matter in the universe as ordinary matter. Dark energy, which comprises the rest of the universe, is responsible for the universal expansion that we see. The "dark" parts of the universe are conjectured, since neither dark energy nor dark matter has been directly detected. Although it doesn't emit or absorb light, dark matter makes its existence known through its gravitational effects.
Since there's so much dark matter, it's thought that the galaxies of normal matter that we see formed around density fluctuations of dark matter. Computer simulations of galaxy formation over the past two decades indicate that dark matter is organized into fine-grained streams of particles that travel along with normal matter. Says Prézeau,
"A stream can be much larger than the solar system itself, and there are many different streams crisscrossing our galactic neighborhood... When gravity interacts with the cold dark matter gas during galaxy formation, all particles within a stream continue traveling at the same velocity."
What happens when such a dark matter stream comes near a planet? Prézeau did a computer simulation using an algorithm called the Fast Accurate Integrand Renormalization. He found that when dark matter streams move through a compact body such as a planet, their density becomes greatly magnified along the stream velocity axis passing through the center of the body. While a stream of ordinary matter could not pass through the Earth, the Earth is no obstacle to dark matter. These dark matter streams are focused into an ultra-dense filament, and there should be many such hairs of dark matter sprouting from the Earth.
Such dark matter hairs will have a root, where the dark matter concentration is highest, and a tip, where the concentration ends. In the case of the Earth, dark matter streams would form a root that's a billion times concentrated at a distance from the Earth of about a million kilometers (600,000 miles). This distance is slightly more than twice the distance to the Moon. The tip, formed by dark matter streams that just graze the Earth, will be at twice the distance of the root.
Massive Jupiter is another story altogether. For Jupiter, the root will have a density enhancement of 1011, and the root is located at about 105 kilometers, which is inside the planet, whose radius is about 7 x 104 kilometers. Since dark matter is so difficult to detect, such an enhancement leads to an intriguing experimental possibility. Says Prézeau, "If we could pinpoint the location of the root of these hairs, we could potentially send a probe there and get a bonanza of data about dark matter."
When dark matter detection becomes routine, the effect can be used to map the inner structures of the planets. For Earth, the density changes from the inner core, through the outer core and mantle, to the crust, would be noted as "kinks" in the hairs.[3-4] This existence of such hairs is just conjectured at this point, so much future work on this topic is required.
- E. Tempel, R. S. Stoica, V. J. Martínez, L. J. Liivamägi, G. Castellan and E. Saar, "Detecting filamentary pattern in the cosmic web: a catalogue of filaments for the SDSS," MNRAS, vol. 438, no. 4 (March 11, 2014), pp. 3465-3482.
- These aren't the voids you're looking for, International Centre for Radio Astronomy Research Press Release, March 10, 2014.
- G. Prézeau, "Dense Dark Matter Hairs Spreading Out From Earth, Jupiter, and Other Compact Bodies," The Astrophysical Journal, vol. 814, no. 2 (November 25, 2015), DOI: 10.1088/0004-637X/814/2/122.
- Elizabeth Landau, "The solar system might be a lot hairier than we thought," NASA Jet Propulsion Laboratory Press Release, November 23, 2015.
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