7,577 research outputs found
Running Newton Constant, Improved Gravitational Actions, and Galaxy Rotation Curves
A renormalization group (RG) improvement of the Einstein-Hilbert action is
performed which promotes Newton's constant and the cosmological constant to
scalar functions on spacetime. They arise from solutions of an exact RG
equation by means of a ``cutoff identification'' which associates RG scales to
the points of spacetime. The resulting modified Einstein equations for
spherically symmetric, static spacetimes are derived and analyzed in detail.
The modifications of the Newtonian limit due to the RG evolution are obtained
for the general case. As an application, the viability of a scenario is
investigated where strong quantum effects in the infrared cause Newton's
constant to grow at large (astrophysical) distances. For two specific RG
trajectories exact vacuum spacetimes modifying the Schwarzschild metric are
obtained by means of a solution-generating Weyl transformation. Their possible
relevance to the problem of the observed approximately flat galaxy rotation
curves is discussed. It is found that a power law running of Newton's constant
with a small exponent of the order would account for their
non-Keplerian behavior without having to postulate the presence of any dark
matter in the galactic halo.Comment: 72pp, to appear in Phys. Rev.
Electron-hole pairs during the adsorption dynamics of O2 on Pd(100) - Exciting or not?
During the exothermic adsorption of molecules at solid surfaces dissipation
of the released energy occurs via the excitation of electronic and phononic
degrees of freedom. For metallic substrates the role of the nonadiabatic
electronic excitation channel has been controversially discussed, as the
absence of a band gap could favour an easy coupling to a manifold of
electronhole pairs of arbitrarily low energies. We analyse this situation for
the highly exothermic showcase system of molecular oxygen dissociating at
Pd(100), using time-dependent perturbation theory applied to first-principles
electronic-structure calculations. For a range of different trajectories of
impinging O2 molecules we compute largely varying electron-hole pair spectra,
which underlines the necessity to consider the high-dimensionality of the
surface dynamical process when assessing the total energy loss into this
dissipation channel. Despite the high Pd density of states at the Fermi level,
the concomitant non-adiabatic energy losses nevertheless never exceed about 5%
of the available chemisorption energy. While this supports an electronically
adiabatic description of the predominant heat dissipation into the phononic
system, we critically discuss the non-adiabatic excitations in the context of
the O2 spin transition during the dissociation process.Comment: 20 pages including 7 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.html [added two references, changed
V_{fsa} to V_{6D}, modified a few formulations in interpretation of spin
asymmetry of eh-spectra, added missing equals sign in Eg.(2.10)
A General Effective Theory for Dense Quark Matter
A general effective action for quark matter at nonzero temperature and/or
nonzero density is derived. Irrelevant quark modes are distinguished from
relevant quark modes, and hard from soft gluon modes, by introducing two
separate cut-offs in momentum space, one for quarks, , and one for
gluons, . Irrelevant quark modes and hard gluon modes are then
exactly integrated out in the functional integral representation of the QCD
partition function. Depending on the specific choice for and
, the resulting effective action contains well-known effective
actions for hot and/or dense quark matter, for instance the ``Hard Thermal
Loop'' (HTL) or the ``Hard Dense Loop'' (HDL) action, as well as the
high-density effective theory proposed by Hong and others.Comment: 10 pages, 6 figures, contribution to proceedings of SEWM 200
Implementing antiretroviral therapy in rural communities: the Lusikisiki model of decentralized HIV/AIDS care.
Health worker shortages are a major bottleneck to scaling up antiretroviral therapy (ART), particularly in rural areas. In Lusikisiki, a rural area of South Africa with a population of 150,000 serviced by 1 hospital and 12 clinics, Médecins Sans Frontières has been supporting a program to deliver human immunodeficiency virus (HIV) services through decentralization to primary health care clinics, task shifting (including nurse-initiated as opposed to physician-initiated treatment), and community support. This approach has allowed for a rapid scale-up of treatment with satisfactory outcomes. Although the general approach in South Africa is to provide ART through hospitals-which seriously limits access for many people, if not the majority of people-1-year outcomes in Lusikisiki are comparable in the clinics and hospital. The greater proximity and acceptability of services at the clinic level has led to a faster enrollment of people into treatment and better retention of patients in treatment (2% vs. 19% lost to follow-up). In all, 2200 people were receiving ART in Lusikisiki in 2006, which represents 95% coverage. Maintaining quality and coverage will require increased resource input from the public sector and full acceptance of creative approaches to implementation, including task shifting and community involvement
Blue-fluorescence of NADPH as an indicator of marine primary production
Nicotinamide Adenine Dinucleotide Phosphate (NADPH) is the primary product of photosynthesisand can therefore serve as an indicator of biomass and photosynthetic activity. Pure NADPH whichis the reduced form of NADP shows an absorption maximum at 340 nm and a maximum of emissionat 460 nm. NADPH concentrations in terrestrial vegetation have already been studied since1957 in great detail with optical methods. However, its potential as a biomass parameter of oceanicphytoplankton which can be assessed in situ and remotely with fluorescence spectroscopy has notyet been investigated.In this paper, we report on laboratory investigations of the blue-fluorescence spectrum in algalsuspensions of Chlorella and Thalassiosira when excited with UV-A light. It is shown that cell densitiesof about 106 per litre as they are typically found under natural conditions are too low for precisedetection of NADPH fluorescence, while concentrated samples with 108-1010 cells per litre exhibitsignificant blue-fluorescence which can be related to NADPH. Inhibition of photosynthetic activityby addition of DCMU decreases the strength of blue-fluorescence remarkably. Since NADPHis an end product of photosynthesis, changes of PAR illumination levels should directly affect itsconcentration and hence the intensity of blue-fluorescence. However, no effect of illumination onblue-fluorescence could be observed in our study. Possible reasons of these observations are discussed,and perspectives for practical applications of the method used are proposed
Renormalization Group Flow of Quantum Gravity in the Einstein-Hilbert Truncation
The exact renormalization group equation for pure quantum gravity is used to
derive the non-perturbative \Fbeta-functions for the dimensionless Newton
constant and cosmological constant on the theory space spanned by the
Einstein-Hilbert truncation. The resulting coupled differential equations are
evaluated for a sharp cutoff function. The features of these flow equations are
compared to those found when using a smooth cutoff. The system of equations
with sharp cutoff is then solved numerically, deriving the complete
renormalization group flow of the Einstein-Hilbert truncation in . The
resulting renormalization group trajectories are classified and their physical
relevance is discussed. The non-trivial fixed point which, if present in the
exact theory, might render Quantum Einstein Gravity nonperturbatively
renormalizable is investigated for various spacetime dimensionalities.Comment: 58 pages, latex, 24 figure
Running Gauge Coupling in Asymptotically Safe Quantum Gravity
We investigate the non-perturbative renormalization group behavior of the
gauge coupling constant using a truncated form of the functional flow equation
for the effective average action of the Yang-Mills-gravity system. We find a
non-zero quantum gravity correction to the standard Yang-Mills beta function
which has the same sign as the gauge boson contribution. Our results fit into
the picture according to which Quantum Einstein Gravity (QEG) is asymptotically
safe, with a vanishing gauge coupling constant at the non-trivial fixed point.Comment: 27 page
- …