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New observational constraints on the cosmic matter density $\Omega_m$ and aneffectively redshift-independent equation of state parameter $w_x$ of the darkenergy are obtained while simultaneously testing the strong and null energyconditions of general relativity on macroscopic scales. The combination ofREFLEX X-ray cluster and type-Ia supernova data shows that for a flat Universethe strong energy condition might presently be violated whereas the null energycondition seems to be fulfilled. This provides another observational argumentfor the present accelerated cosmic expansion and the absence of exotic physicalphenomena related to a broken null energy condition. The marginalization of thelikelihood distributions is performed in a manner to include a large fractionof the recently discussed possible systematic errors involved in theapplication of X-ray clusters as cosmological probes. This yields for a flatUniverse, $\Omega_m=0.29^{+0.08}_{-0.12}$ and $w_x=-0.95^{+0.30}_{-0.35}$($1\sigma$ errors without cosmic variance). The scatter in the differentanalyses indicates a quite robust result around $w_x=-1$, leaving little roomfor the introduction of new energy components described by quintessence-likemodels or phantom energy. The most natural interpretation of the data is apositive cosmological constant with $w_x=-1 or something like it.Comment: 11 pages, 5 figures, Astron. Astrophys. (in press

astronomy and astrophysics

Observational constraints on general relativistic energy conditions, cosmic matter density and dark energy from X-ray clusters of galaxies and type-Ia supernovae