Friday, 19 April 2013

Dark Matter- A Standard Model of Cosmology

We lie off-centre in the disk that is our Milky Way. A crowd of a hundred thousand million stars, accompanied by globular clusters embedded in a galactic halo kilo-parsecs across. We are part of the local group, a community of galaxies with our closest neighbor, the Canis Major Dwarf and our twin, Andromeda. And while a GPS won't get you far in an increasingly homogeneous and isotropic universe, we are only beginning to peel back the layers of the cosmic onion. We see it's expansion via the receding of galaxies, the meandering of supernovae through standard candles and their speeds by their red-shift. But an anomaly remains on how to account for the accelerated expansion in conjunction with attractive gravity. Dark matter is the 'stuff' of the universe that pulls things together via its attractive gravity and boasts thirty percent of it. It's dark, bleak and invisible but when you consider the cluster dynamics of galaxies for instance, there seems to be more mass determined by velocity rather than what can be accounted by with observation. Gravitational lensing also hints it's existence where the curvature of space-time near a mass deflects light and distorts images of background galaxy clusters. We can rest assured that non-baryonic WIMPS, weakly interacting massive particles, are the ideal candidates for exotic dark matter species as opposed to baryonic MACHOS or massive compact halo objects. There may even be no dark matter but rather modified Newtonian mechanics on an intergalactic scale, rogue planets, black holes or even mass challenged dwarf stars. On the flip side, dark energy pushes things apart, exhibits a repulsive gravity and brags seventy percent of the cosmos. An all pervading fluid as envisioned by the Friedman-Robertson-Walker models will be a source of positive pressure at every single point and would drive deceleration of expansion, however dark energy exerts negative pressure to accelerate expansion as inferred from CMB anisotropies. Think of the Casimir effect, when two closely partitioned metallic plates in a vacuum attract each other, arising from the influence that the plates exhibit on the space between them and the effective negative pressure produced. Indeed Einstein's 'greatest blunder' may be a contender for dark energy; matter and energy under the cosmological constant will not be impaired by expansion but would remain uniform and release negative pressure. Moreover, the energy of the quantum vacuum may be another contender; given that vacuum energy is intrinsic to the vacuum, it won't be diluted by expansion and would exert the required negative pressure. Finally, cosmology is topped off with inflation; the hot big bang model presents two unique limitations: the flatness and horizon problems. Stated simply, the universe is essentially flat with a curvature parameter k of 0, the CMB is the same temperature in all directions and Grand Unified Theories predict an abundance of relic magnetic monopoles. Inflationary cosmology tells the tale 10^-36 seconds after the bang where the scale factor of the universe inflated exponentially, solving the problems by smoothing out any rough edges and separating regions with the same temperature across large distances and diluting any relic particles. The source of cosmic energy for such an initial singularity may be resolved via t=0, given that the total energy of the universe is 0 as the gravitational field has a negative quantity; thus cancelling out. And the universe is largely a 'nothing' for 'nothing' phenomenon, we are merely a speck in her finite yet unbound being.

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