|Can you spot the dark stuff?|
Time for another very sensible post. There will be no reference to Japanese snipers or ferrets (go Shagger!) in the following pontifications.
The title of this post, ‘Dark Stuff’, refers to ‘stuff’ more commonly referred to as ‘Dark Matter’. I’m constrained/restrained from using the word, ‘matter’ in the title as we really have no idea of what this mysterious thing is composed of. In fact, we infer the presence of ‘Dark Matter’ mainly by the properties it doesn’t seem to have.
A little background: The Swiss astronomer, Fritz Zwicky, observed an interesting galactic phenomenon back in the 1930s. By measuring the light emitted from a remote galaxy cluster he was able to measure its total mass. The velocities of the galaxies in the cluster are governed by gravity and the extent of gravitational attraction is dependant on the overall mass present. He noted, that given the measured mass, the respective velocities of the galaxies in the cluster appeared too high. There seemed too little mass present to account for this aberrant cosmic choreography. Zwicky concluded that there must be an appreciable amount of invisible mass present which he called, ‘Dark Matter’. This observation remained an isolated anomaly until the 1970s when the cosmologist Vera Rubin Cooper noticed a similar mass deficit whilst measuring star velocities in a neighbouring galaxy. Rubin Cooper found that stars in the outer arms of the galaxy were moving faster than that predicted by their visible mass. Again to account for this discrepancy the presence of a large amount of none-visible matter was postulated. Subsequently, these observations have been confirmed using measurements on a number of celestial bodies.
It appears that normal, or baryonic matter, comprises only 5% of the matter/energy in the universe, while a further 25% is composed of exotic dark matter. The remaining 65% seems to belong to an equally mysterious and intractable force now known as ‘Dark Energy’. The nature (or the un-nature) of dark energy will have to await another post.
The only positive attribute we can ascribe to dark matter is that it exhibits gravity. Gravity is an intrinsic property of matter and is best described as an artefact of mass warping space-time. I’ve pontificated upon the topic of gravity previously. My erudite musings can be accessed/expedited here and here. Intriguingly, dark matter emits no electromagnetic radiation, as far as we are aware, and therefore we are unable to detect its presence by conventional light or radio-telescopes. Dark matter does not appear to interact with normal matter in any way, or at least in any way we are able to discern. Furthermore, not only does dark matter have no truck with 'known matter' it also has no ambition to interact with itself: curiouser and curiouser.
Scientists are at a complete loss as to the nature of this exotic ‘substance’. There exists a particle in nature which appears to share the property of none interaction, the neutrino. Neutrinos are particles that do not exhibit an electromagnetic charge and thus are not repelled by the atomic forces present in the nucleus. It is this repulsive power of atoms which gives the illusion of solidity. As I write, millions of neutrinos are passing through my body before continuing their journey through the Earth. Most of what we call, normal matter, is empty space. Without the repulsive charge of atoms, our hands would simply pass through a solid object such as a chair. As neutrinos are without a charge they pass through objects without being impeded. It is only on the very rare occasion when a neutrino physically strikes a nucleus that this rather odd particle is liable to detection. These rare interactions with atoms result in the emittance of detectable (perhaps delectable?) photons. This option for detection seems denied to dark matter.
The neutrino model represents an analogy which may bear no affinity to dark matter. Are there any other scenarios? Could it be that there is no extra ‘matter’ as such, but a form of extra ‘energy’ (cf Dark Energy). How could this energy manifest? Clearly, it is not a form of conventional electromagnetic wave energy, otherwise, we would be able to detect it using our current technology. Thus, the postulation of dark matter is not the only theory put forth to explain the problem of discrepant gravity. It could be argued that perhaps there are other mechanisms at work. Indeed, the theory of dark matter could be no more than a ‘fudge factor’ or the addition of a constant for which there is no evidence. Remember Einstein in the early 1920s assumed a static eternal universe and thus felt the need to add a ‘cosmological constant’ to the equations of general relativity. When it became known that the universe was expanding this constant became no longer necessary. Einstein acknowledged that the addition of his cosmological constant was his greatest blunder in an illustrious career. Perhaps scientists are looking in the wrong area/arena. Perhaps, therefore, there is no extra mass at all. Perhaps the problem lies with our interpretation of gravity. Could Newton and Einstein have been in error when it comes to understanding the fundamental nature of gravity? There have been several theories advanced that do not rely on the presence of invisible matter. These theories are lumped under the umbrella of Modified Newtonian Dynamics (MOND). They tend to be quite complex for the none specialist and rely on a thorough understanding of string theory (another post, perhaps?). These theories although able to explain some of the observations are not able to account for all the observed phenomenon. Strangely, the postulate of invisible matter appears to be the most accurate description of our universe- makes ya think, dun it?.
Tis ironic that our best model of the universe is actually no model at all. Our understanding of reality at the scale of the very large and the extremely small (quantum physics) is a mess of contradictions and confusion. Tis enough to send a sane man quite maaad: or a mad man sane? I'll leave my gentle readers to be the judge.