View source for Cosmological constant

===Dark energy or cosmological constant===

These unresolved issues have motivated the current observational effort to test whether the cosmological constant is a valid cause of the acceleration of the universe.  Other theories, such as fledgling theories of quantum gravity (e.g. brane-motivated cosmologies), naturally produce dark energy candidates with properties different from the standard cosmological constant ([[#padmanabhan03|Padmanabhan, 2003]]).  Phenomenological theories such as quintessence have also been proposed, which have a time-varying value of dark energy.  Although these models are designed partially to negate the coincidence problem by having dark energy solutions that can track the matter density, these then suffer a new fine-tuning problem as they introduce additional parameters whose values need to be fine-tuned to produce the evolution needed ([[#weinberg89|Weinberg 1989, 2000]]).