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elnglish 101. a on that siht. don't critique me, not the field I study and obviously written in a short time. got my hippy on with being creative though.
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WIMPs vs MACHOS; Why nonbaryonic matter is superior to baryonic matter as a candidate solution to the dark matter problem
Astrophysics is an observational science. The bulk of our astrophysical knowledge has been obtained through observations of distant bodies, as opposed to the controlled experiments which characterize most of the other natural sciences. The standard laws of physics have been used to describe, and are sometimes derived from, the dynamics of astronomical systems. In some cases however, what we observe through telescopes is inconsistent with what these laws predict. One of the most prominent examples of this is known as the Dark Matter problem.
The dark matter problem describes an inconsistency between the dynamics of large scale astronomical systems and the amount of mass we observe. An example of this is the anomalous rotation curves of galaxies. Newtonian dynamics predicts that the orbital velocity of a star in a spiral galaxy should depend only on the mass enclosed by, and the radius, of its orbit (Fowles & Cassiday, 2005, p243). The density of stars and hot gasses in a galaxy can be measured by looking at how bright it is in different regions. In most regular galaxies, we find that the density generally increases towards its centre. This means that the orbital velocity of stars should decrease towards the edge of the galactic disc. What we observe however, is that these velocities remain relatively constant (Fowles & Cassiday, 2005, p244). These observations indicate that either our laws of physics do not apply on galactic scales, or that the universe contains an abundance of matter that we cannot observe; dark matter.
Most scientists believe that dark matter is responsible for the anomalous motions of galaxies, however the composition of this matter is still unknown. Many different explanations have been hypothesized, and the most likely is probably due to Cold Dark Matter; massive, low temperature objects that do not emit enough light to be directly observed through telescopes (Blumenthal, Faber, Primack, & Rees, 1984). Two suggested forms of this Cold Dark Matter are Weakly Interacting Massive Particles (WIMPs) and
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Massive Compact Halo Objects (MACHOs).
MACHOs are composed of baryonic matter (Primack & Carr, 1990). This is the same “ordinary” matter that makes up stars and molecules and nearly everything else we know. The properties of baryonic matter is well-described by the standard model of particle physics. A MACHO must be smaller than ordinary stars, because it must not be massive enough to sustain nuclear fusion, which would produce light (Blumenthal et al., 1984).WIMPs on the other hand are not baryonic matter (Primack & Carr, 1990). They interact only via gravitational and weak nuclear forces, and therefore do not emit light. Currently, the standard model does not describe this type of particle. The MACHO hypothesis may appear to be an appealing solution to the dark matter problem, because it does not require any new physics. However there is a growing amount of evidence suggesting that WIMPs, not MACHOs, are the best dark matter candidates. A lack of observational evidence for MACHOs, inconsistencies with the Big Bang Nucleosynthesis, and proposed extensions to the Standard Model, suggest that WIMPs are a better candidate solution to the dark matter problem than MACHOs.
Although MACHOs can not be directly observed, if they are abundant in the universe, we should be able to detect them indirectly using gravitational microlensing (Primack & Carr, 1990). Gravitational lensing is a result of the theory of general relativity, which describes gravity as a distortion in spacetime due to massive objects. To an outside observer, light traveling through these regions of space will appear bent, or “lensed”. Very large objects, such as galaxies, are able to distort light enough that we can resolve multiple images of the same source. In the case of MACHOs however, the effect is too small to explicitly resolve, but should still detectable as fluctuations in the brightness of the source image (Lewis, Ibata, & Wyithe, 2000).
Searches for these microlensing events have identified MACHO-sized objects in galactic halos (Alcock et al., 1997)(Lewis et al., 2000). However if the MACHO hypothesis is to explain the dark matter problem, we should be able to detect them with high enough
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probability to suggest they are abundant enough to account for the missing mass (Tisserand et al., 2007). Alcock et al. placed a lower limit on the mass of these potential objects by observing how long the microlensing events took. They also found evidence to suggest that there may be larger MACHOs in the galactic halo. Further studies however, have effectively ruled out the possibility that there is a large abundance of these objects between this lower limit, and the upper limit which is imposed by the requirement that they do not fuse hydrogen. This lack of evidence strongly suggests that MACHOs are not the best candidate, however it does not rule them out completely. For example, the lack of observations might be because MACHOs are clumped, and not spread out evenly in the galactic halo, and we have simply just not observed these clumps (Tisserand et al., 2007)
In addition to the lack of observational evidence for MACHOs, there are some theoretical inconsistencies with the hypothesis. Probably the largest inconsistency has to do with the standard theory of the early universe; the Hot Big Bang Theory.
One of these incompatibilities is a result of the baryon density predicted by Big Bang Nucleosynthesis (BBN). BBN occurred when the universe reached a low enough temperature for light elements to form. The majority of the first elements formed were hydrogen and helium. Helium was produced through a series of intermediate steps that included deuterium. Essentially all of the deuterium ultimately formed into helium, but a small fraction of it remains in the universe today. The ratio of deuterium to helium can indicate the baryon density of the early universe; more deuterium would be a result of less interactions between particles, corresponding to a lower overall baryon density (Cyburt, 2004). Measurements of this ratio indicate that the baryon density of the universe is too low to suggest a baryon-dominated universe (Fields, Freese, & Graff, 2000). The MACHO hypothesis is therefore incompatible with BBN.
Another incompatibility with the big bang theory has to do with the formation of large-scale structures, such as galaxies and galaxy structures. In order for these structures to form, the theory requires an inhomogenous universe to seed their formation. This
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inhomogeneity must arise due to small density perturbations in the early universe. However before photon decoupling, which occurred around the same time as BBN, it is theoretically impossible for these perturbations to arise in baryonic matter (Einasto, 2013). WIMPs on the other hand, which do not interact through the electromagnetic force, would not have been subject to these same constraints, and would be free to form these inhomogeneities (Einasto, 2013). In addition to this, if the universe were dominated by baryonic matter, and the inhomogeneities were produced by some unknown mechanism, we can still determine just how large the density fluctuations must be. Observations of the cosmic microwave background radiation, which is remnant light from the big bang, have determined that these fluctuations are too small to suggest a baryon-dominated universe, and are consistent with non-baryonic one (Challinor, 2004).
If the universe is primarily nonbaryonic, we are forced to look beyond the standard model. Over the past couple decades, there has been considerable theoretical progress in developing extensions to the standard model. Perhaps the most important of these has been supersymmetry (Bertone, 2010). Supersymmetry is a theory that could potentially resolve many inconsistencies with what the standard model predicts, and what we observe through experiments. One of the aspects of supersymmetry is that it predicts WIMP-like particles (Bertone, 2010). The theories are strong enough, that large-scale experiments, such as the Large Hadron Collider at CERN, have included projects to directly search for these particles. If they are found, it will probably remove most doubt about what makes up the dark matter in our universe.
The debate over what constitutes dark matter is important because it is what makes up most of the universe we live in. The lack of evidence for MACHOs and new evidence for WIMPs puts us at perhaps one of the most exciting times in scientific history. Our hunt for WIMPs has already taken us from the largest scales imaginable; the universe as a whole, down to the smallest constituents of matter as described by quantum field theory. The same particles that we hunt for in cutting edge research facilities are being used to
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describe processes that occurred during the very birth of our universe. If the WIMPs win this one, and it seems like they might, there is no telling where they will take physics next. (1371 words)
