I stumbled across a paper the other morning on arXiv which, at first glance, looked so bizarre I nearly forgot about my coffee. I'll freely admit, I do rather like audacious concepts, and this one is about as audacious as they come! Given the immense fuel requirements for interstellar travel, collecting your fuel en route would be economically very favourable. So what about dark matter...?

Resting a little tenuously upon the ideas in supersymmetry (that if dark matter is composed of supersymmetric particles, they could annihilate and release energy) the concept is fairly simple. What if we could harness that annihilation and use it as propulsion? The immediate idea is something not unlike the Bussard ramjets popularised by Carl Sagan and Larry Niven. In a way directly analogous to the atmospheric ramjets utilised by some aircraft, a "dark matter engine" would take in dark matter, pressurise it to cause annihilation and use the products as propulsion.

Of course, the immediate problem (acknowledged in the paper's conclusion) is that it may be difficult to construct such an engine, at least using baryonic matter. As any self-respecting astrophysicist will tell you, dark matter interacts only weakly with regular matter, making its presence felt through gravity and gravity alone. It's nice, but fanciful to suggest building this engine out of material from the "dark sector".

But just suppose, for a moment, that such a thing was indeed possible. Suppose that you could create some kind of container which dark matter couldn't readily escape from. Assuming, for simplicity, a static distribution of dark matter, the stuff is everywhere. Theoretically, there should be some in the same room as you while you're reading this. The mass density of dark matter in the universe is around five times that of regular matter. If some kind of dark matter ramjet of this ilk could be constructed, it could hypothetically reach relativistic speeds in as short a distance as 20.6 AU (a little over the orbital radius of Uranus). Increased dark matter density, or increased surface area of the engine would thus increase the acceleration rate. Acceleration itself would be proportional to velocity, theoretically causing an exponential increase in overall speed.

To cut a long story short (and skimp somewhat on the calculus), with the estimated dark matter density in a subhalo, it would still be impossible to reach relativistic speeds -- although you could still theoretically achieve up to 300 kilometres per second, comparable to the average velocity of the solar wind. Using the vastly increased density around a black hole, however, it would theoretically be possible to accelerate to around 90% the speed of light!

I must admit, I'm not sure about this (although I'm by no means an expert on dark matter). In truth, I suspect this concept may be destined to remain, simply, a concept. We still don't really know enough about dark matter, if it even exists, to be able to test such an incredible hypothesis. If it's even remotely possible, however, it's certainly a fascinating idea.

Mind you, this paper does forget to account for one thing. If you could accelerate to 90% of the speed of light, there's no suggestion as to how precisely you might stop!


Source -- arXiv:0908.1429

So apparently, everything we can see in the whole universe is just the tip of the proverbial iceberg. Like the back jelly beans in this picture (which I've stolen from the Chandra Blog), most of the universe is completely invisible and undetectable to us. Regular matter makes up about 4% (0.4% stars and 3.6% intergalactic gas) while 23% is dark matter, and a whopping 73% is the mysterious dark energy.

Frankly, I'm not buying it.
The trouble with both of these concepts is that they rest on some pretty hefty suppositions. Are we to believe that we can see all the baryonic matter in the universe? If so, are we to believe that we actually know what it is we're looking at? As an astrochemist, I can say quite readily that we do not. Phenomena such as the (often overlooked) Diffuse Interstellar Bands, for instance tell us that almost everywhere we look, we see things that cannot be explained. Technically, the diffuse bands come from what could be called "dark matter". We know it's there, but we don't know what it is. How can anyone realistically say how much of the universe is made up of these mysterious chemicals? You might notice that the percentages I gave in the first paragraph make no reference to interstellar gas. Or dust. While it may certainly be true that out of the solar system's known mass, the Sun accounts for around 99.86% of it. This however, cannot account for objects like the Oort cloud, or other regions of the Solar system that aren't fully understood.

The actual concept of Dark Matter, I fully agree with. We can't see everything, pure and simple. It seems logical that a substantial part of the universe is composed of things that we might not even know how to look for yet, but -- are people really looking for it in the right way. Lots of people talk about Weakly Interacting Massive Particles (WIMPs) as a possible contender for dark matter. So far, no one's actually found any, but some people seem to speak about them as if they were fact. We know there are hypothetical cousins of our familiar matter with quixotic names like "Strange Matter" and "Charmed Matter", but no one's really seen them in the cosmos either. Should astronomers really keep looking for things that they can't see, or should they start to reconsider whether or not they fully understand what they can see?

So that's dark matter, but what about this dark energy stuff? Personally, I don't believe in dark energy. I'm sorry, I just don't. Observations were made that implied it's existence. Implied. Not proved. Too many people seem to take the existence of dark energy to be a given. Many alternative hypotheses exist, from a failure of general relativity over the largest scales, to a simple optical illusion created by the propagation of radiation over comological scales. I don't think there's any way to be sure of the apparent acceleration of the universe's expansion until we understand what it's made of - in other words, until we get what dark matter is, perhaps we shouldn't be considering dark energy, if indeed it does exist.

Occam's Razor is a principle traditionally used in formulating scientific theories. It states that "the explanation of any phenomenon should make as few assumptions as possible, eliminating those that make no difference in the observable predictions of the explanatory hypothesis or theory". In other words, of two competing theories, if they are the same in all other respects, the one that makes the fewest assumtions and makes the fewest references to things that may or may not exist is the better one. To that end, perhaps Modified Newtonian Dynamics (MOND) is a better explanation than dark matter.

Then again, perhaps this is all resting on the assumption that we understand the fabric of reality and how spacetime works at a fundamental level. In my humble opinion, I don't believe that to be true. Perhaps we will one day. For the meantime, we shall have to wait and see!