Physical basis for dark energy--I think Bojo found it After reading his new paper over the course of a couple of weeks, I'm seeing strong indications that Bojowald is on the right track explaining the dark energy effect as a quantum geometrical correction. the math is straightforward, most of it quite easy to follow. the reasoning is robust---doesn't depend on a particular version of loop cosmology but is based on general features shared by a broad class of models. it explains the "coincidence" that the acceleration of expansion seems to have taken off recently we are talking an intrinsic "vacuum curvature" which arises as a quantum correction when the Gen Rel spacetime geometry is quantized and acquires aspects of discreteness. Anyone at all interested in these issues (cosmological constant, dark energy, observed expansive acceleration, testing different theories of dark energy) should read the paper. It has an excellent explanation of Loop quantum cosmology for newcomers. Bojowald's physical basis for dark energy is, as he says, eminently TESTABLE by measuring acceleration in deeper redshift with more precision. It is readily FALSIFIABLE, just like General Relativity itself was in 1915 when it first came out (all Eddington needed to do was observe the 1919 eclipse and he could have shot down Gen Rel). No negative pressure "inflaton" field needed here. No unicorns put in by hand. Comes right out of QG as a quantum correction to the inverse volume operator as it affects the kinetic term of the Hamiltonian. Good work Bojowald! It's really beautiful. http://arxiv.org/abs/0705.4398 The Dark Side of a Patchwork Universe Martin Bojowald 24 pages, 2 figures, Contribution to the special issue on Dark Energy by General Relativity and Gravitation "While observational cosmology has recently progressed fast, it revealed a serious dilemma called dark energy: an unknown source of exotic energy with negative pressure driving a current accelerating phase of the universe. All attempts so far to find a convincing theoretical explanation have failed, so that one of the last hopes is the yet to be developed quantum theory of gravity. In this article, loop quantum gravity is considered as a candidate, with an emphasis on properties which might play a role for the dark energy problem. Its basic feature is the discrete structure of space, often associated with quantum theories of gravity on general grounds. This gives rise to well-defined matter Hamiltonian operators and thus sheds light on conceptual questions related to the cosmological constant problem. It also implies typical quantum geometry effects which, from a more phenomenological point of view, may result in dark energy. In particular the latter scenario allows several non-trivial tests which can be made more precise by detailed observations in combination with a quantitative study of numerical quantum gravity. If the speculative possibility of a loop quantum gravitational origin of dark energy turns out to be realized, a program as outlined here will help to hammer out our ideas for a quantum theory of gravity, and at the same time allow predictions for the distant future of our universe."