5 research outputs found
Fermionic vacuum densities in higher-dimensional de Sitter spacetime
Fermionic condensate and the vacuum expectation values of the energy-momentum
tensor are investigated for twisted and untwisted massive spinor fields in
higher-dimensional de Sitter spacetime with toroidally compactified spatial
dimensions. The expectation values are presented in the form of the sum of
corresponding quantities in the uncompactified de Sitter spacetime and the
parts induced by non-trivial topology. The latter are finite and the
renormalization is needed for the first parts only. Closed formulae are derived
for the renormalized fermionic vacuum densities in uncompactified
odd-dimensional de Sitter spacetimes. It is shown that, unlike to the case of
4-dimensional spacetime, for large values of the mass, these densities are
suppressed exponentially. Asymptotic behavior of the topological parts in the
expectation values is investigated in the early and late stages of the
cosmological expansion. When the comoving lengths of compactified dimensions
are much smaller than the de Sitter curvature radius, to the leading order the
topological parts coincide with the corresponding quantities for a massless
fermionic field and are conformally related to the corresponding flat spacetime
results. In this limit the topological parts dominate the uncompactified de
Sitter part and the back-reaction effects should be taken into account. In the
opposite limit, for a massive field the asymptotic behavior of the topological
parts is damping oscillatory.Comment: 19 pages, 4 figures, cosmological applications are adde
Categorizing Different Approaches to the Cosmological Constant Problem
We have found that proposals addressing the old cosmological constant problem
come in various categories. The aim of this paper is to identify as many
different, credible mechanisms as possible and to provide them with a code for
future reference. We find that they all can be classified into five different
schemes of which we indicate the advantages and drawbacks.
Besides, we add a new approach based on a symmetry principle mapping real to
imaginary spacetime.Comment: updated version, accepted for publicatio
Pan-cancer analysis of whole genomes
Cancer is driven by genetic change, and the advent of massively parallel sequencing has enabled systematic documentation of this variation at the whole-genome scale. Here we report the integrative analysis of 2,658 whole-cancer genomes and their matching normal tissues across 38 tumour types from the Pan-Cancer Analysis of Whole Genomes (PCAWG) Consortium of the International Cancer Genome Consortium (ICGC) and The Cancer Genome Atlas (TCGA). We describe the generation of the PCAWG resource, facilitated by international data sharing using compute clouds. On average, cancer genomes contained 4-5 driver mutations when combining coding and non-coding genomic elements; however, in around 5% of cases no drivers were identified, suggesting that cancer driver discovery is not yet complete. Chromothripsis, in which many clustered structural variants arise in a single catastrophic event, is frequently an early event in tumour evolution; in acral melanoma, for example, these events precede most somatic point mutations and affect several cancer-associated genes simultaneously. Cancers with abnormal telomere maintenance often originate from tissues with low replicative activity and show several mechanisms of preventing telomere attrition to critical levels. Common and rare germline variants affect patterns of somatic mutation, including point mutations, structural variants and somatic retrotransposition. A collection of papers from the PCAWG Consortium describes non-coding mutations that drive cancer beyond those in the TERT promoter; identifies new signatures of mutational processes that cause base substitutions, small insertions and deletions and structural variation; analyses timings and patterns of tumour evolution; describes the diverse transcriptional consequences of somatic mutation on splicing, expression levels, fusion genes and promoter activity; and evaluates a range of more-specialized features of cancer genomes