Collaborative Research: Cosmology with the Lyman Alpha Forest

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ABSTRACT AST 0098437HuiThis is a Collaborative Research Project with Matias Zaldarriaga (0098506) of New York University.The light from a quasar (also called a qso) often has multiple dips in intensity, or absorption lines, at positions corresponding to the redshifted Lyman alpha line (at rest wavelength 1216 angstroms) from neutral hydrogen. These often occur as a series of absorption lines, called the Lyman alpha forest. The redshift is caused by the expansion of the universe. If we can calculatehow the universe is expanding (based on a cosmological model), we can tell where the photons were absorbed in relation to us. Thus, we can use the absorption map to plot the positions ofclouds of intervening hydrogen between the quasar and us. The clouds in the Lyman alpha systems are not very massive compared to objects like galaxies. Because of this, reliable computer simulations (numerical experiments) of their gravitational collapse (formation) from primordial fluctuations are possible. These Lyman alpha forest simulations produce structure similar to what we see in the nearby universe (filaments, walls, and voids ) by starting with small fluctuations in matter density and then letting gravity and other known forces act. These detailed calculations of the structure and distribution of matter are at the frontiers of current research and these recent advances in computer modeling have made it possible to use observations of the Lyman-alpha forest at redshifts between 2 and 4 as powerful cosmological tools. The forest has offered, and will continue to offer, interesting constraints on many quantities such as the amplitude and shape of the primordial mass power spectrum, the neutrino mass, the matter density, the cosmological constant, and the reionization epoch. These constraints, in conjunction with those from other observations, provide important checks on our fundamental understanding of cosmology.This project will primarily focus on the "transmission power spectrum", a statistic used in cosmological models which is directly observable. The program has two goals:1) Examination of fundamental assumptions: The project will test several methods to address questions about the influence of continuum-fitting in determining the measured fluctuations in the forest , the interpretation of these fluctuations in terms of mass fluctuations resulting from gravitational instability, and the effect of fluctuations in the ionizing background and the effective equation of state of the intergalactic medium on the interpretation. 2) Maximizing the scientific return from the data: To this end the program will address redshift-distortions, (which affect both the program of mass power spectrum recovery, as well asthe ability to measure the cosmological constant through its geometrical distortion of thecorrelation function), degeneracies among parameters (by performing a full likelihood analysiswhich will reveal the degeneracies in the 4 to 6 free parameters in the current model of the forest), and obtaining a better understanding of the thermal history of the universe, (by using the drop in temperature with time after reionization to infer or at least limit the epoch of reionization from measurements of the temperature using the small-scale cut-off of the transmission powerspectrum at redshifts between 2 and 4.)Funding for this project was provided by the NSF program for Extragalactic Astronomy & Cosmology (AST/EXC).***