The Physics Police

The Physics Police

Tuesday, July 23, 2013

Local Dark Matter

I share with you this splendid example of terrible journalism:
The Incredible Dark Matter Mystery: Why Astronomers Say it is Missing in Action
This article attempts to invent a controversy where none, in fact, exists:
So astronomers are left scratching their heads. On the one hand, they say dark matter must hold our galaxy together with a vice-like gravitational grip. On the other, its gravitational effect on the Solar System is negligible. Something has to give.
This implies some contradiction between theory and experiment. There is no such contradiction!

Look no further than the physics paper cited in the news article, Constraints on Dark Matter in the Solar System, which concludes:
Our results show that the mass of the dark matter, if present, and its density are much lower than the present-day errors in these parameters.
The author of the news article (actually, a blog post, really) seems to have misunderstood the idea of a limit. In scientific parlance, a "limit" is the maximum (or minimum) amount of some measurable quantity that may exist. We get limits by doing experiments.

In this case, previous work has set maximum limits on dark matter density in the solar system. This paper sets some new limits based on several planetary orbits. Cool, new limits, yeah!

So the question remains, do these new limits contradict theories of dark matter?

The paper finds an upper limit of 1.1 * 10-20 g/cm3, whereas it's generally accepted that the dark matter density near Earth is about 0.3 GeV/cm3, or 20,000 times smaller!

M / V = (E / c2) / V = ((0.3 GeV) / c2) / (1 cm3) = 5.35 * 10-25 g/cm3

This expected dark matter density is infinitesimal compared to the new upper-limit set by the paper. That means, while the paper has gotten us one step closer to testing theory, we aren't there just yet. Experimental error is still too large to detect local dark matter.
Now astronomers say they can find no evidence of dark matter’s gravitational influence on the planets. What gives?
Yeah, what gives? I'll tell you what gives!

MIT Technology Review just got policed!

No comments:

Post a Comment