Earth’s Dark Matter Sabers

hubble_lightsaber_image2.jpgOrion B Molecular Cloud Complex, a stellar nursery. Credits: NASA/ESA

This week NASA released a celestial light saber photo. It’s a stunning visualization of a ‘light saber’ apparition in the Orion B Molecular Cloud Complex. This is the universe of ordinary matter.

But in the world of dark matter which cohabits with ordinary matter in our universe, there are saber-like objects as well. In an earlier blog we talked about dark matter filaments and walls on the very large scale, at the scale of superclusters of galaxies and above. On the small scale, dark matter particles also align into structures as well.

Dark matter is a ‘collisionless’ fluid. That is, it interacts so rarely with either ordinary matter or itself, that it doesn’t thermalize and dissipate energy via radiation as does ordinary matter. It interacts through gravity alone, both with itself and with ordinary matter, but that can lead to structure as well, even at the very small scale. Regions of very enhanced density (gravitational clumping) are known as caustics.

If there were to be caustics in our solar system – regions of enhanced dark matter density – then it could ease the direct detection search for dark matter. There are many direct detection experiments underway on Earth, mostly with negative results, although for a number of years the DAMA/LIBRA experimenters have claimed direct detection; these results are highly disputed.

Our best understanding of dark matter is that it forms “fine-grained streams” or clumps that move together at the same velocity. These streams can be quite large and many streams should be found in our galactic neighborhood.

Now Gary Prézeau of NASA-JPL has noted that compact bodies, such as the Sun and planets, should act as lenses of sorts, focusing dark matter streams into strands of higher density. He refers to these as ‘dark matter hairs’ and calculates where the roots should lie relative to the center of the Sun or the center of the Earth or other planets.

In the case of the Earth the root of the ‘hair’ would be around 1 million kilometers behind the Earth – behind in this case meaning relative to our orbital motion around the galaxy’s center. The density enhancement might be as large as 1 billion times the normal density. In the solar neighborhood that average normal density is estimated to be 1/3 of a proton mass per cubic centimeter, so it’s not surprising that direct detection of dark matter is so difficult. But at hundreds of millions of proton masses per cc, or some tens of millions of dark matter particles per cc (depending on the dark matter mass) it could be much more feasible.

PIA20176_ipCredit: NASA/JPL – CalTech

I prefer to think of these as dark matter sabers emanating from the Earth.

It turns out that the James Webb Space Telescope, the successor to Hubble, will be placed at one of the Lagrange points (L2) in the Sun-Earth gravitational system, about 1.5 million kilometers out. Perhaps this would be a good place to put a dark matter detection experiment. The earthbound ones are getting to be quite massive, but with the potentially much greater sensitivity, a small scale detector could be quite effective. At the L2 point it would be aligned with the Sun’s orbit around our galaxy each year around the first of June. That could be the best time for detection, but there could be other hairs as well with density enhancements in the millions, and detectable at other times of the year.

References: – Ethan Siegel article – NASA release on possible ‘dark matter hairs’ around Earth – Gary Prezeau, 2015. “Dense Dark Matter Hairs Spreading out from Earth,  Jupiter, and other Compact Bodies” – dark matter structure at the very large scale


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