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e.
1 the worst problem of fine-tuning <http://en.wikipedia.org/wiki/Fine-tuning> in phys!ics: there is no known natural way to derive, even roughly, the infinitesimal cosmo1 logical constant observed in cosmology from particle physics <http://en.wikipedia.org/ I wiki/Particle_physics. Some physicists, including Steven Weinberg <http:// 1 en.wikipedia.org/wiki/Steven_Weinberg>, think the delicate balance of quantum 1 vacuum energy is best explained by the anthropic principle <http://en.wikipedia.org/
wiki/Anthropic_principle>, 1 In spite of its problems, the cosmological constant is in many respects the most i economical solution <http://en.wikipedia.org/wiki/Occam%27s_razor> to the prob1 lem of cosmic acceleration. One number successfully explains a multitude of obser
vations. Thus, the current standard model of cosmology, the Lambda-CDM model, includes the cosmological constant as an essential feature.
3.2.2 Quintessence
Alternatively, dark energy might arise from the particle-like excitations in some type of dynamical <http://en.wikipedia.org/wiki/Dynamical_system> field <http:// 1 en.wikipedia.org/wiki/Scalar_field>, referred to as quintessence <http:// 1 en.wikipedia.org/wiki/Quintessence_%28physics%29> (Lee, Seokcheon <http://
adsabs.harvard.edu/cgi-bin/author_form? author = Lee, + S & fullauthor = Lee,% 20 Seokcheon & charset = ISO-8859-1&db_key=AST>; Olive, Keith A. <http:// adsabs.harvard.edu/cgi-bin/author_form? author = Olive, +K & fullauthor = Olive,%20 Keith%20A.&charset=ISO-8859-1&db_key=AST>; Pospelov, Maxim <http:// adsabs.harvard.edu/cgi-bin/author_form? author = Pospelov, + M & fullauthor = Pospelov, %20Maxim&charset=ISO-8859-1&db_key=AST>, 2004). Quintessence differs from the cosmological constant in that it can vary in space and time. In order 1 for it not to clump and form structure <http://en.wikipedia.org/wiki/Large-1 scale_structure_of_the_cosmos> like matter, it must be very light so that it has a 1 large Compton wavelength <http://en.wikipedia.org/wiki/Compton_wavelength>. I
No evidence of quintessence is yet available, but it cannot be ruled out either. It 1 generally predicts a slightly slower acceleration of the expansion of the universe than the cosmological constant. Some researchers think that the best evidence for 1 quintessence would come from violations of Einstein's equivalence principle <http:// len.wikipedia.org/wiki/Equivalence_principle> and variation of the fundamental constants <http://en.wikipedia.org/wiki/Equivalence_principle> in space or time. Scalar fields <http://en.wikipedia.org/wiki/Scalar_field> are predicted by the standard model <http://en.wikipedia.org/wiki/Standard_model> and string theory <http:// en.wikipedia.org/wiki/String_theory>, but an analogous problem to the cosmological constant problem (or the problem of constructing models of cosmic inflation <http://en.wikipedia.org/wiki/Cosmic_inflation>) occurs: renormalization <http:// en.wikipedia.org/wiki/Renormalization theory predicts that scalar fields should acquire large masses.
The cosmic coincidence <http://en.wikipedia.org/w/index.php?title = Cosmic_coincidence & action =edit> problem asks why the cosmic acceleration begins when it did. If cosmic acceleration began earlier in the universe, structures
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