Modern cosmological theory is based on the Friedmann-Robertson-Walker (FRW) metric. Often written in terms of co-moving coordinates, this well-known solution to Einstein's equations owes its elegant and highly practical formulation to the Cosmological principal and Weyl's postulate, upon which it is founded. But there is physics behind such symmetries, and not all of it has yet been recognized. In this paper, we derive the FRW metric coefficients from the general form of the spherically-symmetric line element, and demonstrate that, because the co-moving frame also happens to be in free fall, the symmetries in FRW are valid only for a medium with zero active mass. In other words, the spacetime of a perfect fluid in cosmology may be correctly written as FRW only when its equation-of-state is $\rho+3p=0$, in terms of the {\it total} pressure $p$ and {\it total} energy density $\rho$. There is now compelling observational support for this conclusion, including the Alcock-Paczy\'nski test, which shows that only an FRW cosmology with zero active mass is consistent with the latest model-independent Baryon Acoustic Oscillation data.

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Master of Science / Department of Physics / Bharat Ratra / Recent observations in cosmology suggest that the universe is undergoing accelerating expansion. Mysterious component responsible for acceleration is called "Dark Energy" contributing to 70% of total energy density of the universe. Simplest DE model is [Lambda]CDM, where Einstein’s cosmological constant plays role of the dark energy. Despite the fact that it is consistent with observational data, it leaves some important theoretical questions unanswered. To overcome these difficulties different Dark energy models are proposed. Two of these models XCDM parametrization and slow rolling scalar field model [phi]CDM, along with "standard" [Lambda]CDM are disscussed here, constraining their parameter set. In this thesis we start with a general theoretical overview of basic ideas and distance measures in cosmology. In the following chapters we use H II starburst galaxy apparent magnitude versus redshift data from Siegel et al.(2005) to constrain DE model parameters. These constraints are generally consistent with those derived using other data sets, but are not as restrictive as the tightest currently available constraints. Also we constrain above mentioned cosmological models in light of 32 age measurements of passively evolving galaxies as a function of redshift and recent estimates of the product of the cosmic microwave background acoustic scale and the baryon acoustic oscillation peak scale.

We begin with an account of the Prime Mover argument. This originated in the "laws" of Plato, where it is argued that a self-moving mover must exist as source of other motions, and that it must be a kind of soul. In Aristotle this Prime Mover is not itself moved, and elaborate proofs of its existence are offered in the "Physics": all motion requires a mover, and the series cannot go on to infinity, but must end in one or more unmoved movers. His proofs, however, were far from watertight, and later Peripatetics like Theophrastus and Strato rejected them. The argument reappeared in Proclus, but only as subsidiary to a First Cause argument; moreover,the Prime Mover is only the second member of the Neoplatonic "trinity". John Philoponus' theory of "impetus" should have undermined the argument, but in fact did not, and it continued to be used by the Arabs (despite criticism by Avicenna) and the Jews (notably Maimonides). It was taken over by the Christian scholastics like Aquinas. But criticism also continued, especially from Algaael in Islam and by Ockham and his followers in Christendom, and a detailed refutation was offered by the Jew Crescas. The arrival of non-Aristotelian physics was fatal to the argument; it is indeed still defended occasionally by neo-Scholastic philosophers, but none of their defences is adequate. Outside Scholasticism it has few supporters, though Samuel Clarke used it for the Platonic purpose of pointing out an analogy between the Prime Mover and mind, Lotze, however, advanced a quite different kind of argument, but based, like the Platonic and Aristotelian ones, on the existence of change; he argued that change ought always to be internal to that which changes, and hence that the universe must be in some sense a unity. The relationship between this unity and individual things would then be analogous to that between a mind and its states. [Continued in text ...]

This thesis examines various properties of galaxies and clusters within hierarchical models of structure formation. A simple model based on analytical scaling relations is applied to X-ray observations of clusters at low and high redshift, in an attempt to constrain cosmological parameters from their evolutionary properties. It is found that the density parameter, Ωo cannot be constrained using the data alone. Two independent constraints on the slope of the linear power spectrum, infer values of Ωo < 0.7 at 95 per cent confidence. The remainder of this thesis concentrates on the method of cosmological simulation, a self- consistent approach to the modelling of structure in the Universe. A parameter-space study is performed for the simplest model of galaxy formation: the radiative cooling of baryons within the cores of dark matter haloes. It is found that the properties of the galaxies in the simulations are insensitive to the range of parameters studied, with the exception of those that affect the cooling rate of the gas. For modest resolution and reasonable choices of physical parameters, the amount of baryons in galaxy material is around a factor of 2 too high. An investigation is then performed for including the effects of star formation and energy from supemovae (feedback) within cosmological simulations, to reduce the amount of gas that cools. The star formation rate is driven by the minimum density for which the stars formation occurs and, for high star formation efficiencies, is limited by the cooling rate of the gas. A successful model for feedback is found to require the prevention of reheated gas from cooling for a short period of time, as an attempt to mimick the properties of a multiphase medium. Finally, preliminary results are presented for simulations of a galaxy cluster, including the effects of radiative cooling, star formation and feedback. The properties of the cluster are found to vary significantly between models with and without feedback, due to the feedback reducing the star formation rate by reheating gas that cools.

Thesis (Ph.D.). / Written for the Dept. of Physics. Title from title page of PDF (viewed 2009/06/09). Includes bibliographical references.

This thesis investigates the evolution of characteristic structures in neutrino or adiabatic baryon-dominated models of galaxy formation. We discuss the collapse of protocluster or protosupercluster clouds in terms of the behaviour of non-rotating, homogeneous triaxial ellipsoids, predicting that galaxies should populate filamentary or quasi-spherical structures rather than the generic flat structures (pancakes). Secondly we have designed a numerical code which allows us to do fast cosmological hydrodynamics. We investigate the effect of explosions on the standard pancake picture for galaxy formation. Blast waves created by the early evolution of galaxies can not produce the anti-biasing effect required to reconcile the rapid evolution of clustering in n-body simulations of neutrino models with observations of galaxies at redshifts greater than one.

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