Dark Energy and Gravity

I review the problem of dark energy focusing on the cosmological constant as the candidate and discuss its implications for the nature of gravity. Part 1 briefly overviews the currently popular `concordance cosmology' and summarises the evidence for dark energy. It also provides the observational and theoretical arguments in favour of the cosmological constant as the candidate and emphasises why no other approach really solves the conceptual problems usually attributed to the cosmological constant. Part 2 describes some of the approaches to understand the nature of the cosmological constant and attempts to extract the key ingredients which must be present in any viable solution. I argue that (i)the cosmological constant problem cannot be satisfactorily solved until gravitational action is made invariant under the shift of the matter lagrangian by a constant and (ii) this cannot happen if the metric is the dynamical variable. Hence the cosmological constant problem essentially has to do with our (mis)understanding of the nature of gravity. Part 3 discusses an alternative perspective on gravity in which the action is explicitly invariant under the above transformation. Extremizing this action leads to an equation determining the background geometry which gives Einstein's theory at the lowest order with Lanczos-Lovelock type corrections. (Condensed abstract).Comment: Invited Review for a special Gen.Rel.Grav. issue on Dark Energy, edited by G.F.R.Ellis, R.Maartens and H.Nicolai; revtex; 22 pages; 2 figure

Late-time Entropy Production from Scalar Decay and Relic Neutrino Temperature

Entropy production from scalar decay in the era of low temperatures after neutrino decoupling will change the ratio of the relic neutrino temperature to the CMB temperature, and, hence, the value of N_eff, the effective number of neutrino species. Such scalar decay is relevant to reheating after thermal inflation, proposed to dilute massive particles, like the moduli and the gravitino, featuring in supersymmetric and string theories. The effect of such entropy production on the relic neutrino temperature ratio is calculated in a semi-analytic manner, and a recent lower bound on this ratio, obtained from the WMAP satellite and 2dF galaxy data, is used to set a lower bound of ~ 1.5 x 10^-23 Gev on the scalar decay constant, corresponding to a reheating temperature of about 3.3 Mev.Comment: 13 pages, to appear in PR

A retrograde monazite-forming reaction in bearthide-bearing high-pressure rocks

Bearthite,Ca2Al[PO4](2)(OH),an aluminium phosphate that may contain up to similar to 10 wt% of light rare-earth elements (LREE) + Th, shows various breakdown textures in high-pressure metamorphic rocks. Two such reactions are described: (1) a pyrope-bearing kyanite-phengite-quartz (coesite) schist from the high-pressure Dora-Maira terrane, Italy, in which bearthite displays a symplectitic rim that consists of apatite + monazite; (2) a garnet-bearing muscovite-biotite gneiss of the Monte Rosa nappe, Italy, in which a symplectitic pseudomorph consisting of apatite + corundum + monazite occurs as an inclusion in allanite. In rocks with a high Al/Ca ratio, we suggest that bearthite is more stable than monazite as the LREE-bearing phase under high P/low T conditions (that is, a subduction zone environment). It breaks down to an assemblage of symplectitic monazite + apatite + corundum (or Al-silicate with free SiO2) during decompression. Bearthite that coexists with HREE-dominated minerals like xenotime does not show increased HREE contents. Because of its considerable Th-content, bearthite could be used to constrain the time of high-pressure metamorphism

Monazite analysis; from sample preparation to microprobe age dating and REE quantification.

Despite the recognized importance of monazite in geochronology and petrology, a range of fundamental analytical and preparational problems remains. For example, chemical Th-U-Pb dating of monazite requires special lead-free sample preparation. This is achieved efficiently and at high quality with specially developed grooved ND-PE polyethylene polishing disks. Techniques useful in locating and characterizing monazite are evaluated. Back scattered electron imaging is an effective way to determine zonation patterns, particularly with respect to thorium. Quantitative analysis of monazite by EMP is delicate and time consuming. A whole series of X-ray peak interferences has been ignored in published work. For example, for monazite containing 12% Th, the commonly disregarded interference of Th hit on Pb hla causes an overestimation of 11% (relative) in Pb. This propagates to an age overestimation of similar to 50 Ma for a sample of 400 to 500 Ma in age. A judicious choice of X-ray peaks used in quantitative EMP analysis avoids or minimises peak overlap for all elements, including REE. Only for U a correction factor is required: U wt%(corrected) = U wt%(measured) - (0.0052 * T wt%(measured)) based on the analytical lines U Mb and Th Ma

Comparative genomics in the triticeae

International audienceThe genomes of grasses are very different in terms of size, ploidy level and chromosome number. Among them, the Triticeae species (wheat, barley, rye) have some of the largest and complex genomes. Comparative mapping studies between rice, maize, sorghum, barley and wheat have pioneered the field of plant comparative genomics a decade ago. They showed that the linear order (colinearity) of genetic markers and genes is very well conserved opening the way to accelerated map-based cloning and defining rice as a model for grasses. More recently, the availability of BAC libraries and large sets of genomic sequences including the completion of the rice genome have permitted micro-colinearity studies that revealed rearrangements between the grass genomes and provided some insights into mechanisms that have shaped their genome during evolution. This review summarizes a decade of comparative genomics Studies In grasses with a special emphasis on the wheat and barley genomes