http://arxiv.org/abs/astro-ph/0504416(adsbygoogle = window.adsbygoogle || []).push({});

Authors: Mustapha Ishak (Princeton University)

Comments: 10 pages, 5 figures

Associated with the cosmic acceleration are the old and new cosmological constant problems, recently put into the more general context of the dark energy problem. In broad terms, the old problem is related to an unexpected order of magnitude of this component while the new problem is related to this magnitude being of the same order of the matter energy density during the present epoch of cosmic evolution. Current plans to measure the equation of state certainly constitute an important approach; however, as we discuss, this approach is faced with serious feasibility challenges and is limited in the type of conclusive answers it could provide. Therefore, is it really too early to seek actively for new tests and approaches to these problems? In view of the difficulty of this endeavor, we argue in this work that a good place to start is by questioning some of the assumptions underlying the formulation of these problems and finding new ways to put this questioning to the test. Motivated by some theorems, we discuss if the full identification of the cosmological constant with vacuum energy is unquestionable. Next, we evaluate how much fine tuning the cosmic coincidence problem represents. We discuss some implications of the simplest solution for the principles of General Relativity. We stress the potential of some cosmological probes such as weak gravitational lensing to identify novel tests to probe dark energy questions and assumptions. Also, we discuss the relevance of experiments at the interface of astrophysics and quantum field theory, focusing on the Casimir effect in gravitational and cosmological contexts. We conclude that challenging some of the assumptions underlying the cosmological constant problems and putting them to the test may prove useful and necessary to make progress on these questions

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# Cosmological constant and Dark energy

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