Constraints from the Old Quasar APM 08279+5255 on Two Classes of $\Lambda ($t$)$-Cosmologies

Abstract

The viability of two different classes of $\Lambda (t)$CDM cosmologies is tested by using the APM 08279+5255, an old quasar at redshift $z=3.91$. In the first class of models, the cosmological term scales as $\Lambda (t)\sim R^{-n}$. The particular case $n=0$ describes the standard $\Lambda$CDM model whereas $n=2$ stands for the Chen and Wu model. For an estimated age of 2 Gyr, it is found that the power index has a lower limit $n>0.21$, whereas for 3 Gyr the limit is $n>0.6$. Since $n$ can not be so large as $\sim 0.81$, the $\Lambda$CDM and Chen and Wu models are also ruled out by this analysis. The second class of models is the one recently proposed by Wang and Meng which describes several $\Lambda (t)$CDM cosmologies discussed in the literature. By assuming that the true age is 2 Gyr it is found that the $\epsilon$ parameter satisfies the lower bound $\epsilon >0.11$, while for 3 Gyr, a lower limit of $\epsilon >0.52$ is obtained. Such limits are slightly modified when the baryonic component is included.

arXiv comment7 pages, 3 figures, job presented at the IWARA 2005. Revised paper to match version to be published on GRG

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