Implications of a viscosity bound on black hole accretion

Motivated by the viscosity bound in gauge/gravity duality, we consider the ratio of shear viscosity (eta) to entropy density (s) in black hole accretion flows. We use both an ideal gas equation of state and the QCD equation of state obtained from lattice for the fluid accreting onto a Kerr black hole. The QCD equation of state is considered since the temperature of accreting matter is expected to approach 10(12) K in certain hot flows. We find that in both the cases eta/s is small only for primordial black holes and several orders of magnitude larger than any known fluid for stellar and supermassive black holes. We show that a lower bound on the mass of primordial black holes leads to a lower bound on eta/s and vice versa. Finally we speculate that the Shakura-Sunyaev viscosity parameter should decrease with increasing density and/or temperatures. (C) 2012 Elsevier B.V. All rights reserved.

Composition of Extrafloral Nectar Influences Interactions between the Myrmecophyte Humboldtia brunonis and its Ant Associates

Ant-plant interactions often are mediated by extrafloral nectar (EFN) composition that may influence plant visitation by ants. Over a 300 km range in the Indian Western Ghats, we investigated the correlation between the EFN composition of the myrmecophytic ant-plant Humboldtia brunonis (Fabaceae) and the number and species of ants visiting EFN. EFN composition varied among H. brunonis populations and between plant organs (floral bud vs. young leaf EFN). In general, EFN was rich in sugars with small quantities of amino acids, especially essential amino acids, and had moderate invertase activity. In experiments at the study sites with sugar and amino acid solutions and with leaf or floral bud EFN mimics, dominant EFN-feeding ants differentiated between solutions as well as between mimics. The castration parasite Crematogaster dohrni (northern study site) was the least selective and did not exhibit any clear feeding preferences, while the largely trophobiont-tending non-protective Myrmicaria brunnea (middle study site) preferred higher sucrose concentrations and certain essential/non-essential amino acid mixtures. The mutualistic Technomyrmex albipes (southern study site) preferred sucrose over glucose or fructose solutions and consumed the leaf EFN mimic to a greater extent than the floral bud EFN mimic. This young leaf EFN mimic had low sugar concentrations, the lowest viscosity and sugar: amino acid ratio, was rich in essential amino acids, and appeared ideally suited to the digestive physiology of T. albipes. This preference for young leaf EFN may explain the greater protection afforded to young leaves than to floral buds by T. albipes, and may also help to resolve ant-pollinator conflicts. The differential response of dominant ants to sugar, amino acids, or solution viscosity suggests that plants can fine-tune their interactions with local ants via EFN composition. Thus, EFN can mediate local partner-choice mechanisms in ant-plant interactions.

Dependence of climate forcing and response on the altitude of black carbon aerosols

Black carbon aerosols absorb solar radiation and decrease planetary albedo, and thus can contribute to climate warming. In this paper, the dependence of equilibrium climate response on the altitude of black carbon is explored using an atmospheric general circulation model coupled to a mixed layer ocean model. The simulations model aerosol direct and semi-direct effects, but not indirect effects. Aerosol concentrations are prescribed and not interactive. It is shown that climate response of black carbon is highly dependent on the altitude of the aerosol. As the altitude of black carbon increases, surface temperatures decrease; black carbon near the surface causes surface warming, whereas black carbon near the tropopause and in the stratosphere causes surface cooling. This cooling occurs despite increasing planetary absorption of sunlight (i.e. decreasing planetary albedo). We find that the trend in surface air temperature response versus the altitude of black carbon is consistent with our calculations of radiative forcing after the troposphere, stratosphere, and land surface have undergone rapid adjustment, calculated as ``regressed'' radiative forcing. The variation in climate response from black carbon at different altitudes occurs largely from different fast climate responses; temperature dependent feedbacks are not statistically distinguishable. Impacts of black carbon at various altitudes on the hydrological cycle are also discussed; black carbon in the lowest atmospheric layer increases precipitation despite reductions in solar radiation reaching the surface, whereas black carbon at higher altitudes decreases precipitation.

Higher spin quasinormal modes and one-loop determinants in the BTZ black hole

We solve the wave equations of arbitrary integer spin fields in the BTZ black hole background and obtain exact expressions for their quasinormal modes. We show that these quasinormal modes precisely agree with the location of the poles of the corresponding two point function in the dual conformal field theory as predicted by the AdS/CFT correspondence. We then use these quasinormal modes to construct the one-loop determinant of the higher spin field in the thermal BTZ background. This is shown to agree with that obtained from the corresponding heat kernel constructed recently by group theoretic methods.

Higher spin fermions in the BTZ black hole

Recently it has been shown that the wave equations of bosonic higher spin fields in the BTZ background can be solved exactly. In this work we extend this analysis to fermionic higher spin fields. We solve the wave equations for arbitrary half-integer spin fields in the BTZ black hole background and obtain exact expressions for their quasinormal modes. These quasinormal modes are shown to agree precisely with the poles of the corresponding two point function in the dual conformal field theory as predicted by the AdS/CFT correspondence. We also obtain an expression for the 1-loop determinant for the Euclidean non-rotating BTZ black hole in terms of the quasinormal modes which agrees with that obtained by integrating the heat kernel found by group theoretic methods.

Stability of Adult Emergence and Activity/Rest Rhythms in Fruit Flies Drosophila melanogaster under Semi-Natural Condition

Here we report the results of a study aimed at examining stability of adult emergence and activity/rest rhythms under seminatural conditions (henceforth SN), in four large outbred fruit fly Drosophila melanogaster populations, selected for emergence in a narrow window of time under laboratory (henceforth LAB) light/dark (LD) cycles. When assessed under LAB, selected flies display enhanced stability in terms of higher amplitude, synchrony and accuracy in emergence and activity rhythms compared to controls. The present study was conducted to assess whether such differences in stability between selected and control populations, persist under SN where several gradually changing time-cues are present in their strongest form. The study revealed that under SN, emergence waveform of selected flies was modified, with even more enhanced peak and narrower gate-width compared to those observed in the LAB and compared to control populations in SN. Furthermore, flies from selected populations continued to exhibit enhanced synchrony and accuracy in their emergence and activity rhythms under SN compared to controls. Further analysis of zeitgeber effects revealed that enhanced stability in the rhythmicity of selected flies under SN was primarily due to increased sensitivity to light because emergence and activity rhythms of selected flies were as stable as controls under temperature cycles. These results thus suggest that stability of circadian rhythms in fruit flies D. melanogaster, which evolved as a consequence of selection for emergence in a narrow window of time under weak zeitgeber condition of LAB, persists robustly in the face of day-to-day variations in cycling environmental factors of nature.

Thermodynamics of higher spin black holes in 3D

We examine the thermodynamic properties of recently constructed black hole solutions in SL(3, R) x SL(3, R) Chern-Simons theory in the presence of a chemical potential for spin-3 charge, which acts as an irrelevant deformation of the dual CFT with W-3 X W-3 symmetry. The smoothness or holonomy conditions admit four branches of solutions describing a flow between two AdS(3) backgrounds corresponding to two different CFTs. The dominant branch at low temperatures, connected to the BTZ black hole, merges smoothly with a thermodynamically unstable branch and disappears at higher temperatures. We confirm that the UV region of the flow satisfies the Ward identities of a CFT with W-3((2)) x W-3((2)) symmetry deformed by a spin-3/2 current. This allows to identify the precise map between UV and HI thermodynamic variables. We find that the high temperature regime is dominated by a black hole branch whose thermodynamics can only be consistently inferred with reference to this W-3((2)) x W-3((2)) CFT.

Can the viscosity in astrophysical black hole accretion disks be close to its string theory bound?

String theory and gauge/gravity duality suggest the lower bound of shear viscosity (eta) to entropy density (s) for any matter to be mu h/4 pi k(B), when h and k(B) are reduced Planck and Boltzmann constants respectively and mu <= 1. Motivated by this, we explore eta/s in black hole accretion flows, in order to understand if such exotic flows could be a natural site for the lowest eta/s. Accretion flow plays an important role in black hole physics in identifying the existence of the underlying black hole. This is a rotating shear flow with insignificant molecular viscosity, which could however have a significant turbulent viscosity, generating transport, heat and hence entropy in the flow. However, in presence of strong magnetic field, magnetic stresses can help in transporting matter independent of viscosity, via celebrated Blandford-Payne mechanism. In such cases, energy and then entropy produces via Ohmic dissipation. In,addition, certain optically thin, hot, accretion flows, of temperature greater than or similar to 10(9) K, may be favourable for nuclear burning which could generate/absorb huge energy, much higher than that in a star. We find that eta/s in accretion flows appears to be close to the lower bound suggested by theory, if they are embedded by strong magnetic field or producing nuclear energy, when the source of energy is not viscous effects. A lower bound on eta/s also leads to an upper bound on the Reynolds number of the flow.

Black hole formation at the correspondence point

We study the process of bound state formation in a D-brane collision. We consider two mechanisms for bound state formation. The first, operative at weak coupling in the worldvolume gauge theory, is pair creation of W-bosons. The second, operative at strong coupling, corresponds to formation of a large black hole in the dual supergravity. These two processes agree qualitatively at intermediate coupling, in accord with the correspondence principle of Horowitz and Polchinski. We show that the size of the bound state and time scale for formation of a bound state agree at the correspondence point. The time scale involves matching a parametric resonance in the gauge theory to a quasinormal mode in supergravity.

Black hole formation in fuzzy sphere collapse

We study the collapse of a fuzzy sphere, that is a spherical membrane built out of D0-branes, in the Banks-Fischler-Shenker-Susskind model. At weak coupling, as the sphere shrinks, open strings are produced. If the initial radius is large then open string production is not important and the sphere behaves classically. At intermediate initial radius the backreaction from open string production is important but the fuzzy sphere retains its identity. At small initial radius the sphere collapses to form a black hole. The crossover between the later two regimes is smooth and occurs at the correspondence point of Horowitz and Polchinski.

Establishing a Relation between the Mass and the Spin of Stellar-Mass Black Holes

Stellar mass black holes (SMBHs), forming by the core collapse of very massive, rapidly rotating stars, are expected to exhibit a high density accretion disk around them developed from the spinning mantle of the collapsing star. A wide class of such disks, due to their high density and temperature, are effective emitters of neutrinos and hence called neutrino cooled disks. Tracking the physics relating the observed (neutrino) luminosity to the mass, spin of black holes (BHs) and the accretion rate ((M) over dot) of such disks, here we establish a correlation between the spin and mass of SMBHs at their formation stage. Our work shows that spinning BHs are more massive than nonspinning BHs for a given (M) over dot. However, slowly spinning BHs can turn out to be more massive than spinning BHs if (M) over dot at their formation stage was higher compared to faster spinning BHs.

Black holes in higher spin supergravity

We study black hole solutions in Chern-Simons higher spin supergravity based on the superalgebra sl(3 vertical bar 2). These black hole solutions have a U(1) gauge field and a spin 2 hair in addition to the spin 3 hair. These additional fields correspond to the R-symmetry charges of the supergroup sl(3 vertical bar 2). Using the relation between the bulk field equations and the Ward identities of a CFT with N = 2 super-W-3 symmetry, we identify the bulk charges and chemical potentials with those of the boundary CFT. From these identifications we see that a suitable set of variables to study this black hole is in terms of the charges present in three decoupled bosonic sub-algebras of the N = 2 super-W-3 algebra. The entropy and the partition function of these R-charged black holes are then evaluated in terms of the charges of the bulk theory as well as in terms of its chemical potentials. We then compute the partition function in the dual CFT and find exact agreement with the bulk partition function.

Radiatively inefficient accretion flow simulations with cooling: implications for black hole transients

We study the effects of optically thin radiative cooling on the structure of radiatively inefficient accretion flows (RIAFs). The flow structure is geometrically thick, and independent of the gas density and cooling, if the cooling time is longer than the viscous time-scale (i.e. t(cool) greater than or similar to t(visc)). For higher densities, the gas can cool before it can accrete and forms the standard geometrically thin, optically thick Shakura-Sunyaev disc. For usual cooling processes (such as bremsstrahlung), we expect an inner hot flow and an outer thin disc. For a short cooling time the accretion flow separates into two phases: a radiatively inefficient hot coronal phase and a cold thin disc. We argue that there is an upper limit on the density of the hot corona corresponding to a critical value of t(cool)/t(ff)( similar to 10-100), the ratio of the cooling time and the free-fall time. Based on our simulations, we have developed a model for transients observed in black hole X-ray binaries (XRBs). An XRB in a quiescent hot RIAF state can transition to a cold blackbody-dominated state because of an increase in the mass accretion rate. The transition from a thin disc to a RIAF happens because of mass exhaustion due to accretion; the transition happens when the cooling time becomes longer than the viscous time at inner radii. Since the viscous time-scale for a geometrically thin disc is quite long, the high-soft state is expected to be long-lived. The different time-scales in black hole transients correspond to different physical processes such as viscous evolution, cooling and free fall. Our model captures the overall features of observed state transitions in XRBs.

Rhythmic egg-laying behaviour in virgin females of fruit flies Drosophila melanogaster

Fruit fly Drosophila melanogaster females display rhythmic egg-laying under 12: 12 h light/dark (LD) cycles which persists with near 24 h periodicity under constant darkness (DD). We have shown previously that persistence of this rhythm does not require the neurons expressing pigment dispersing factor (PDF), thought to be the canonical circadian pacemakers, and proposed that it could be controlled by peripheral clocks or regulated/triggered by the act of mating. We assayed egg-laying behaviour of wild-type Canton S (CS) females under LD, DD and constant light (LL) conditions in three different physiological states; as virgins, as females allowed to mate with males for 1 day and as females allowed to mate for the entire duration of the assay. Here, we report the presence of a circadian rhythm in egg-laying in virgin D. melanogaster females. We also found that egg-laying behaviour of 70 and 90% females from all the three male presence/absence protocols follows circadian rhythmicity under DD and LL, with periods ranging between 18 and 30 h. The egg-laying rhythm of all virgin females synchronized to LD cycles with a peak occurring soon after lights-off. The rhythm in virgins was remarkably robust with maximum number of eggs deposited immediately after lights-off in contrast to mated females which show higher egg-laying during the day. These results suggest that the egg-laying rhythm of D. melanogaster is endogenously driven and is neither regulated nor triggered by the act of mating; instead, the presence of males results in reduction in entrainment to LD cycles.

Skeletal Muscle Degeneration and Regeneration in Mice and Flies

Many aspects of skeletal muscle biology are remarkably similar between mammals and tiny insects, and experimental models of mice and flies (Drosophila) provide powerful tools to understand factors controlling the growth, maintenance, degeneration (atrophy and necrosis), and regeneration of normal and diseased muscles, with potential applications to the human condition. This review compares the limb muscles of mice and the indirect flight muscles of flies, with respect to the mechanisms of adult myofiber formation, homeostasis, atrophy, hypertrophy, and the response to muscle degeneration, with some comment on myogenic precursor cells and common gene regulatory pathways. There is a striking similarity between the species for events related to muscle atrophy and hypertrophy, without contribution of any myoblast fusion. Since the flight muscles of adult flies lack a population of reserve myogenic cells (equivalent to satellite cells), this indicates that such cells are not required for maintenance of normal muscle function. However, since satellite cells are essential in postnatal mammals for myogenesis and regeneration in response to myofiber necrosis, the extent to which such regeneration might be possible in flight muscles of adult flies remains unclear. Common cellular and molecular pathways for both species are outlined related to neuromuscular disorders and to age-related loss of skeletal muscle mass and function (sarcopenia). The commonality of events related to skeletal muscles in these disparate species (with vast differences in size, growth duration, longevity, and muscle activities) emphasizes the combined value and power of these experimental animal models.