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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
worldwide.
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.
Sword of Damocles, an 1812 oil painting on canvas by Richard Westall (1765-1836).
This painting is in the Ackland Museum, Chapel Hill, North Carolina.
(Via Wikimedia Commons.)
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.[4]
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.[4] 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."[4]
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.[3] While a stream of ordinary matter could not pass through the Earth, the Earth is no obstacle to dark matter.[4] 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.[4]
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.[4]
Massive Jupiter is another story altogether. For Jupiter, the root will have a density enhancement of 10
11, and the root is located at about 10
5 kilometers, which is inside the planet, whose
radius is about 7 x 10
4 kilometers.[3] 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."[4]
The supposed concentration of dark matter filaments inside Jupiter.
This looks like a cover idea for a science fiction novel.
(Modified NASA image.)
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.[4]
References:
- 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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