120 research outputs found
Gravitational tests of the Generalized Uncertainty Principle
We compute the corrections to the Schwarzschild metric necessary to reproduce
the Hawking temperature derived from a Generalized Uncertainty Principle (GUP),
so that the GUP deformation parameter is directly linked to the deformation of
the metric. Using this modified Schwarzschild metric, we compute corrections to
the standard General Relativistic predictions for the light deflection and
perihelion precession, both for planets in the solar system and for binary
pulsars. This analysis allows us to set bounds for the GUP deformation
parameter from well-known astronomical measurements.Comment: 20 pages, 2 figure
Discovery and Characterization of Methylation of Arginine 42 on Histone H3: A Novel Histone Modification with Positive Transcriptional Effects
Eukaryotic genomic DNA is packaged in the form of chromatin, which contains repeating nucleosomal units consisting of roughly two super-helical turns of DNA wrapped around an octamer of core histone proteins composed of four histone species: one histone H3/H4 tetramer and two histone H2A/H2B dimers. Histones are basic globular proteins rich in lysine and arginine residues, with unstructured N-terminal “tail” regions protruding outside the nucleosome structure, and structured “core” domains in the DNA-associated portion. Several core residues, and in particular arginines in H3 and H4, mediate key interactions between the histone octamer and DNA in forming the nucleosomal particle. Histone post-translational modifications (PTMs) lead to downstream effects indirectly by allowing or preventing docking of effector molecules, or directly by changing the intrinsic biophysical properties of local chromatin. To date, little has been done to study PTMs that lie outside of the unstructured tail domains of histones. I describe here the identification by mass spectrometry of a novel methylation site on histone H3, the asymmetric dimethylation of arginine 42 (H3R42me2a). H3R42 is conserved through evolution and is at the DNA entry/ exit position within the nucleosome core, with likely interactions with the DNA backbone. I show that methyltransferases CARM1 and PRMT6 methylate this residue in vitro and in vivo. Using chemically-defined “designer” histones I also show that methylation of H3R42 stimulates transcription in vitro from chromatinized templates. Using peptide pull down experiments combined with enzymatic assays I demonstrate that H3R42me2a prevents the stimulation of the histone deacetylase activity of the N-CoR co-repressive complex by impeding its binding to H3. Thus, H3R42 is a new histone methylation site with stimulating effects on transcription. I propose that methylation of basic histone residues at the DNA interface may be a general mechanism to disrupt histone:DNA interactions, with effects on downstream processes, including transcription
Gravity coupling from micro-black holes
Recently much work has been done in lowering the Planck threshold of quantum
gravitational effects (sub-millimeter dimension(s), Horava-Witten fifth
dimension, strings or branes low energy effects, etc.). Working in the
framework of 4-dim gravity, with semi-classical considerations based on Hawking
evaporation of planckian micro-black holes, I shall show here as quantum
gravity effects could occur also near GUT energies.Comment: LaTex file, 5 pages, no figure
Minimum length (scale) in Quantum Field Theory, Generalized Uncertainty Principle and the non-renormalisability of gravity
The notions of minimum geometrical length and minimum length scale are
discussed with reference to correlation functions obtained from in-in and
in-out amplitudes in quantum field theory. Whereas the in-in propagator for
metric perturbations does not admit the former, the in-out Feynman propagator
shows the emergence of the latter. A connection between the Feynman propagator
of quantum field theories of gravity and the deformation parameter
of the generalised uncertainty principle (GUP) is then exhibited, which allows
to determine an exact expression for in terms of the residues of the
causal propagator. A correspondence between the non-renormalisability of (some)
theories (of gravity) and the existence of a minimum length scale is then
conjectured to support the idea that non-renormalisable theories are
self-complete and finite. The role played by the sign of the deformation
parameter is further discussed by considering an implementation of the GUP on
the lattice.Comment: LaTeX, 12 pages, no figures, final version to appear in PL
Higher Order Slow-Roll Predictions for Inflation
We study the WKB approximation beyond leading order for cosmological
perturbations during inflation. To first order in the slow-roll parameters, we
show that an improved WKB approximation leads to analytical results agreeing to
within 0.1% with the standard slow-roll results. Moreover, the leading WKB
approximation to second order in the slow-roll parameters leads to analytical
predictions in qualitative agreement with those obtained by the Green's
function method.Comment: Replaced to match published versio
Adiabatic Invariant Treatment of a Collapsing Sphere of Quantized Dust
The semiclassical collapse of a sphere of quantized dust is studied. A
Born-Oppenheimer decomposition is performed for the wave function of the system
and the semiclassical limit is considered for the gravitational part. The
method of adiabatic invariants for time dependent Hamiltonians is then employed
to find (approximate) solutions to the quantum dust equations of motions. This
allows us to obtain corrections to the adiabatic approximation of the dust
states associated with the time evolution of the metric. The diverse
non-adiabatic corrections are generally associated with particle (dust)
creation and related fluctuations. The back-reaction due to the dominant
contribution to particle creation is estimated and seen to slow-down the
collapse.Comment: LaTeX, 16 pages, no figures, final version to appear in Class. and
Quantum Gravit
Point-like sources and the scale of quantum gravity
We review the General Relativistic model of a (quasi) point-like particle
represented by a massive shell of neutral matter which has vanishing total
energy in the small-volume limit. We then show that, by assuming a Generalised
Uncertainty Principle, which implies the existence of a minimum length of the
order of the Planck scale, the total energy instead remains finite and equal to
the shell's proper mass both for very heavy and very light particles. This
suggests that the quantum structure of space-time might be related to the
classical Equivalence Principle and possible implications for the late stage of
evaporating black holes are briefly mentioned.Comment: 5 pages, 3 figures, revtex4 styl
Improved WKB analysis of Slow-Roll Inflation
We extend the WKB method for the computation of cosmological perturbations
during inflation beyond leading order and provide the power spectra of scalar
and tensor perturbations to second order in the slow-roll parameters. Our
method does not require that the slow-roll parameters be constant. Although
leading and next-to-leading results in the slow-roll parameters depend on the
approximation technique used in the computation, we find that the inflationary
theoretical predictions obtained may reach the accuracy required by planned
observations. In two technical appendices, we compare our techniques and
results with previous findings.Comment: REVTeX 4, 13 pages, no figures, final version to appear in Phys. Rev.
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