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The lightcone gauge is a set of what are called the observational coordinates adapted to our past lightcone. We develop this gauge by producing a perturbed spacetime metric that describes the geometry of our past lightcone where observations are usually obtained. A verification is made of the observables in the perturbed lightcone gauge with those in the standard gauge. The advantage of the method developed is that the observable relations are simpler than in the standard formalism. We use the perturbed lightcone gauge in galaxy surveys and galaxy number density contrast. The significance of the new gauge is that by considering the null-like light propagations, the calculations are much simpler since angular deviations are not considered. We also explore classes of irrotational-fluid cosmological models in the context of f(R) gravity in an attempt to put some theoretical and mathematical restrictions on the form of the f(R) gravitational Lagrangian. Furthermore, we explore exact f(R) gravity solutions that mimic Chaplygin-gas inspired CDM cosmology. Starting with the original, generalized and modified Chaplygin gas equations of state, we reconstruct the forms of f(R) Lagrangians.
About the author
Maye Y. A. Elmardi, Bsc in physics from University of Khartoum-Sudan. MSc in Mathematics and Applied Mathematics: investigated the magneto-genesis in Cosmology from University of Cape Town (UCT). Currently pursuing a PhD in Cosmology, working on Dark Energy and Inhomogeneous Universe at UCT.
