Martino, Matthew C.
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Publication ENVIRONMENTAL EFFECTS IN MODIFIED GRAVITY MODELS(2009-12-22) Martino, Matthew CRecent observations indicate that the expansion rate of the universe is accelerating. For this to be the case, either the universe needs to be dominated by an unusual substance dubbed "dark energy", or our model of gravity needs to be changed. There are many consequences that result from modifying gravity, and these need to be considered when we consider a modified gravity model. In particular, this thesis will examine environmental effects in standard gravity with an interest in understanding how these effects might be changed in a modified gravity model, and will also consider the formation of clusters and voids in a particular modified gravity model with the goal of calculating mass functions that can be used in halo model calculations.Publication Spherical Collapse and Cluster Counts in Modified Gravity Models(2009-04-08) Martino, Matthew C.; Sheth, Ravi K.; Stabenau, Hans F.Modifications to the gravitational potential affect the nonlinear gravitational evolution of large scale structures in the Universe. To illustrate some generic features of such changes, we study the evolution of spherically symmetric perturbations when the modification is of Yukawa type; this is nontrivial, because we should not and do not assume that Birkhoff’s theorem applies. We then show how to estimate the abundance of virialized objects in such models. Comparison with numerical simulations shows reasonable agreement: When normalized to have the same fluctuations at early times, weaker large scale gravity produces fewer massive halos. However, the opposite can be true for models that are normalized to have the same linear theory power spectrum today, so the abundance of rich clusters potentially places interesting constraints on such models. Our analysis also indicates that the formation histories and abundances of sufficiently low mass objects are unchanged from standard gravity. This explains why simulations have found that the nonlinear power spectrum at large k is unaffected by such modifications to the gravitational potential. In addition, the most massive objects in models with normalized cosmic microwave background and weaker gravity are expected to be similar to the high-redshift progenitors of the most massive objects in models with stronger gravity. Thus, the difference between the cluster and field galaxy populations is expected to be larger in models with stronger large scale gravity.