Other
Scientific paper
Oct 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999phdt.........1z&link_type=abstract
Thesis (PhD). UNIVERSITY OF PENNSYLVANIA, Source DAI-B 60/04, p. 1653, Oct 1999, 110 pages.
Other
Quintessence, Dark Matter, Tracker Fields
Scientific paper
Recent observations suggest that a large fraction of the energy density of the universe has negative pressure. One explanation is vacuum energy density; another is quintessence in the form of a scalar field slowly evolving down a potential. In either case, a key problem is to explain why the energy density nearly coincides with the matter density today. The densities decrease at different rates as the universe expands, so coincidence today appears to require that their ratio be set to a specific, infinitesimal value in the early universe. In this thesis, we introduce the notion of a ``tracker field,'' a form of quintessence, and show how it may explain the coincidence. We also address the following questions: What is the general condition to have tracker fields? What is the relation between the matter energy density and the equation-of-state of the universe imposed by tracker solutions? And, can tracker solutions help to explain why quintessence is becoming important today rather than during the early universe? We also apply the tracker idea to the standard cold dark matter component in cases where it is composed of oscillating fields. Combining these ideas, we construct a model in which quintessence, cold dark matter, and ordinary matter all contribute comparable amounts to the total energy density today irrespective of initial conditions.
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