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The Observable Universe and Beyond


Dark Matter

Earth-Moon System It seems that the Big Bang Theory has been validated conclusively with all these supporting evidences. However, recent observations in the last few years reveal that there is something amiss. It is noticed that even though there is not enough mass to hold the stars, galaxies and galaxy clusters in place, they are still moving around and would not disperse. It looks as if there is some kind of invisible force (gravity from the dark matter) to hold them together. The situation is similar to a puppet show, where the audience can safely assume that someone behind is manipulating the movements. It is suggested that the mass of dark matter within the lunar orbit can be computed by subtracting the total mass (Earth + Dark Matter) within the lunar orbit from the mass of the Earth measured by a gravity-sensing satellite (Figure 02-10aa). It turns out to be no more than 1.5x1015 kg or about one billion times lower than the mass of the Earth. It means that the

Figure 02-10aa Earth-Moon System [view large image]

difference is too small to be measured by the 2008 technology. All that can be found is the upper bound, which is just another way of saying that there is no difference up to the current level of accuracy.

 Dark Matter, Deficiency of A 19 April 2012 report in Nature News indicates that a survey found only about 1/10 of the dark matter around the Solar system. The researchers measured the velocity of more 400 stars within 13000 light years of the Sun (in a 15-degree cone) below the disk of the Milky Way, and then extrapolate the result to the other side of the disk above the plane. It is found that only about 1/10 the amount of dark matter predicted by models shown in Figure 02-10ab as a blue haze around the spiral Milky Way. Since the modeling involves many assumptions, further observations are required to arrive at a definite conclusion.

Figure 02-10ab Dark Matter, Deficiency of

Dark Matter Distribution Figure 02-10b shows the large-scale distribution of dark matter mapped by the Hubble's Cosmic Evolution Survey in early 2007. Since light will follow the deformed path created by massive object, the quantity and location of the dark matter can be estimated by the amount of the bending. However, it should be cautioned that such image represents only a small facet of the whole picture. Just like representing the distortion of space-time as a piece of stretched rubber sheet, or the probability density of an electron (in the atom) by some foggy orbital, the dark matter distribution map does not include the other interesting properties such as its lack of interaction with other matter except via gravity.

Figure 02-10b Dark Matter Distribution [large image]

Note the increasing clumpiness from distant past to more recent epoch in the picture.


Dark Matter The map of dark matter forms a filamentous 'skeleton' upon which visible matter congregates, eventually producing stars and galaxies. Baryonic structures are expected to form only inside the dark-matter scaffold. But as shown in Figure 02-10ca, the concentrations of dark matter (mapped in contours) usually - but not always - match up with normal matter (coloured). The discrepancies could be a simple error resulting from the way the observations were made. Alternatively it is suggested that dark matter, if the clump is small enough, could have any accumulating visible matter blown out of it by a high-energy phenomenon such as a quasar or a supernova, for example. The

Figure 02-10ca Dark Matter and Baryonic Matter [view large image]

collision of two galaxies could also blow an amount of visible matter out as a faint satellite galaxy that has no associated dark matter.

Some General Properties of Dark Matter from observations and simulations (Figure 02-10cb) :

Dark Matter Propeties
  1. It must be cold (meaning moving with speed much slower than that for light), otherwise it could not clumped together to form structure such as the halo of galaxy.
  2. As it does not absorb or emit electromagnetic radiation, it must be electrically neutral.
  3. It also does not interact with strong interaction as high energy cosmic rays fail to produce anything from the dark matter.
  4. Since there is no known mechanism to replenish dark matter, it must be stable on cosmic timescales. Theorists propose that dark matter possesses a conserved quantity, e.g., dark-parity = +1, while all the non-dark matters have a value of -1. Thus, it is forbidden to decay to non-dark matter. However, both kinds can still interact provided the total dark-parity is unchanged after the interaction.

Figure 02-10cb Dark Matter Properties

Thus, dark matter could interact with non-dark matter gravitationally and probably via weak interaction as well.
Figure 02-10gd below is a 2014 update on the various experiments/observations to detect dark matter :

Figure 02-10gd Dark Matter Update, 2014

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