Browsing by Subject "GRAVITY"

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  • Bonora, L.; Bytsenko, A. A.; Chaichian, M.; Goncalves, A. E. (2019)
    In this paper, we examine the Ruelle-type spectral functions R(s), which define an overall description of the content of the work. We investigate the Gopakumar-Vafa reformulation of the string partition functions, describe the N = 2 Landau-Ginzburg model in terms of Ruelle-type spectral functions. Furthermore, we discuss the basic properties satisfied by elliptic genera in N = 2 theories, construct the functional equations for R(s), and analyze the modular transformation laws for the elliptic genus of the Landau-Ginzburg model and study their properties in details.
  • Rivera-Ingraham, A.; Ristorcelli, I.; Juvela, M.; Montillaud, J.; Men'shchikov, A.; Malinen, J.; Pelkonen, V. -M.; Marston, A.; Martin, P. G.; Pagani, L.; Paladini, R.; Paradis, D.; Ysard, N.; Ward-Thompson, D.; Bernard, J. -P.; Marshall, D. J.; Montier, L.; Toth, L. V. (2017)
    Context. The onset of star formation is intimately linked with the presence of massive unstable filamentary structures. These filaments are therefore key for theoretical models that aim to reproduce the observed characteristics of the star formation process in the Galaxy. Aims. As part of the filament study carried out by the Herschel Galactic Cold Cores Key Programme, here we study and discuss the filament properties presented in GCC VII (Paper I) in context with theoretical models of filament formation and evolution. Methods. A conservatively selected sample of filaments located at a distance D <500 pc was extracted from the GCC fields with the getfilaments algorithm. The physical structure of the filaments was quantified according to two main components: the central (Gaussian) region of the filament (core component), and the power-law-like region dominating the filament column density profile at larger radii (wing component). The properties and behaviour of these components relative to the total linear mass density of the filament and the column density of its environment were compared with the predictions from theoretical models describing the evolution of filaments under gravity-dominated conditions. Results. The feasibility of a transition from a subcritical to supercritical state by accretion at any given time is dependent on the combined effect of filament intrinsic properties and environmental conditions. Reasonably self-gravitating (high M-line,M-core) filaments in dense environments (Av greater than or similar to 3 mag) can become supercritical on timescales of t similar to 1 Myr by accreting mass at constant or decreasing width. The trend of increasing M-line,M-tot (M-line,M-core and M-line,M-wing) and ridge A(v) with background for the filament population also indicates that the precursors of star-forming filaments evolve coevally with their environment. The simultaneous increase of environment and filament Av explains the observed association between dense environments and high Mlille,co values, and it argues against filaments remaining in constant single-pressure equilibrium states. The simultaneous growth of filament and background in locations with efficient mass assembly, predicted in numerical models of filaments in collapsing clouds, presents a suitable scenario for the fulfillment of the combined filament mass-environment criterium that is in quantitative agreement with Herschel observations.
  • Enckell, Vera-Maria; Enqvist, Kari; Räsänen, Syksy; Wahlman, Lumi-Pyry (2019)
    We study scalar field inflation in F(R) gravity in the Palatini formulation of general relativity. Unlike in the metric formulation, in the Palatini formulation F(R) gravity does not introduce new degrees of freedom. However, it changes the relations between existing degrees of freedom, including the inflaton and spacetime curvature. Considering the case F(R) = R + alpha R-2, we find that the R-2 term decreases the height of the effective inflaton potential. By adjusting the value of a, this mechanism can be used to suppress the tensor-to-scalar ratio r without limit in any scalar field model of inflation without affecting the spectrum of scalar perturbations.
  • Räsänen, Syksy; Tomberg, Eemeli (2019)
    We study the production of primordial black hole (PBH) dark matter in the case when the Standard Model Higgs coupled non-minimally to gravity is the inflaton. PBHs can be produced if the Higgs potential has a near-critical point due to quantum corrections. In this case the slow-roll approximation may be broken, so we calculate the power spectrum numerically. We consider both the metric and the Palatini formulation of general relativity. Combining observational constraints on PBHs and on the CMB spectrum we find that PBHs can constitute all of the dark matter only if they evaporate early and leave behind Planck mass relics. This requires the potential to have a shallow local minimum, not just a critical point. The initial PBH mass is then below 10(6) g, and predictions for the CMB observables are the same as in tree-level Higgs inflation, n(s) = 0.96 and r = 5 x 10(-3) (metric) or r = 4 x 10(-8) ... 2 x 10(-7) (Palatini).
  • Rasa, Kimmo; Vihera-Aarnio, Anneli; Rytkonen, Peetu; Hyvaluoma, Jari; Kaseva, Janne; Suhonen, Heikki; Jyske, Tuula (2021)
    Novel bioeconomic approaches call for increasingly faster production of lignocellulosic biomass and its bettertailored use for higher added value. The high-yield capacity and structural properties of willows (Salix spp.) suggest their excellent potential for the production of designed biochar for use in agronomic, electronic and technical applications. All these applications rely on the internal pore structure of biochar. However, we lack an in-depth quantitative understanding of the interlinkages between the feedstock properties and the physical quality of the biochar produced. We studied quantitatively how the clonal and within-plant properties of five different willow clones (hybrids of Salix schwerinii E.L. Wolf) affected the micrometre-scale pore properties of the produced biochars (pyrolyzed at + 462 ?C). The porosity and pore size distribution were analysed before and after slow pyrolysis by X-ray microtomography and image analysis. We also studied the potential of conventional low-cost fibre analysis techniques to be used to predict biochar pore properties directly from fresh feedstock. The total porosity (0.55?0.62) and the pore size distribution of willow wood and derived biochars varied between clones. Approximately two-thirds of the biochar total porosity was associated with pores formed by wood fibres. Pyrolysis levelled off the structural variation detected between and within the clones. Pyrolysis-induced shrinkage reduced the pore sizes and narrowed the pore size distribution. The results suggest that conventional fibre analysis techniques could be utilized to predict biochar homogeneity. Short rotation coppice willows are suitable feedstock to produce homogenous biochar precursor for production of bio-based carbon materials to be used in high value-added technical applications. The structural homogeneity of the feedstock and produced biochar can be enhanced by selecting proper harvesting strategy and clones used in plantations. From the industrial perspective, comprehensive understanding of feedstock properties helps to control quality of the produced biochar.
  • David, Francois; Kupiainen, Antti; Rhodes, Remi; Vargas, Vincent (2017)
    Liouville Quantum Field Theory (LQFT) can be seen as a probabilistic theory of 2d Riemannian metrics e(phi(z)) |dz|(2), conjecturally describing scaling limits of discrete 2d-random surfaces. The law of the random field phi in LQFT depends on weights alpha is an element of R that in classical Riemannian geometry parametrize power law singularities in the metric. A rigorous construction of LQFT has been carried out in [3] in the case when the weights are below the so called Seiberg bound: alpha <Q where Q parametrizes the random surface model in question. These correspond to studying uniformized surfaces with conical singularities in the classical geometrical setup. An interesting limiting case in classical geometry are the cusp singularities. In the random setup this corresponds to the case when the Seiberg bound is saturated. In this paper, we construct LQFT in the case when the Seiberg bound is saturated which can be seen as the probabilistic version of Riemann surfaces with cusp singularities. The construction involves methods from Gaussian Multiplicative Chaos theory at criticality.
  • Iosifidis, Damianos; Koivisto, Tomi (2019)
    This article presents an exhaustive classification of metric-affine theories according to their scale symmetries. First it is clarified that there are three relevant definitions of a scale transformation. These correspond to a projective transformation of the connection, a rescaling of the orthonormal frame, and a combination of the two. The most general second order quadratic metric-affine action, including the parity-violating terms, is constructed in each of the three cases. The results can be straightforwardly generalised by including higher derivatives, and implemented in the general metric-affine, teleparallel, and symmetric teleparallel geometries.
  • Casas, Santiago; Karananas, Georgios K.; Pauly, Martin; Rubio, Javier (2019)
    We discuss the cosmological phenomenology of biscalar-tensor models displaying a maximally symmetric Einstein-frame kinetic sector and constructed on the basis of scale symmetry and volumepreserving diffeomorphisms. These theories contain a single dimensionful parameter Lambda(0) -associated with the invariance under the aforementioned restricted coordinate transformations-and a massless dilaton field. At large field values these scenarios lead to inflation with no generation of isocurvature perturbations. The corresponding predictions depend only on two dimensionless parameters, which characterize the curvature of the field manifold and the leading-order behavior of the inflationary potential. For Lambda(0) = 0 the scale symmetry is unbroken and the dilaton admits only derivative couplings to matter, evading all fifthforce constraints. For Lambda(0)( )not equal 0 the field acquires a runaway potential that can support a dark-energy - dominated era at late times. We confront a minimalistic realization of this appealing framework with observations using a Markov chain Monte Carlo approach, with likelihoods from present baryon acoustic oscillation, type la supernova, and cosmic microwave background data. A Bayesian model comparison indicates a preference for the considered model over ACDM, under certain assumptions for the priors. The impact of possible consistency relations among the early and late Universe dynamics that can appear within this setting is discussed with the use of correlation matrices. The results indicate that a precise determination of the inflationary observables and the dark energy equation of state could significantly constrain the model parameters.
  • Koivisto, Tomi S.; Nyrhinen, Hannu J. (2017)
    The no-hair theorem postulates that the only externally observable properties of a black hole are its mass, its electric charge, and its angular momentum. In scalar-tensor theories of gravity, a matter distribution around a black hole can lead to the so called 'spontaneous scalarisation' instability that triggers the development of scalar hair. In the Brans-Dicke type theories, this effect can be understood as a result of tachyonic effective mass of the scalar field. Here we consider the instability in the generalised class of scalar-theories that feature non-conformal, i.e. 'disformal', couplings to matter. Such theories have gained considerable interest in the recent years and have been studied in a wide variety of systems, both cosmological and astrophysical. In view of the prospects of gravitational wave astronomy, it is relevant to explore the implications of the theories in the strong-gravity regime. In this article, we concentrate on the spontaneous scalarisation of matter configurations around Schwarzschild and Kerr black holes. We find that in the more generic scalar-tensor theories, the instability of the Brans-Dicke theory can be enhanced, suggesting violations of the no-hair theorem. On the other hand, we find that, especially if the coupling is very strong, or if the gradients in the matter distribution are negligible, the disformal coupling tends to stabilise the system.
  • Bernal, Nicolas; Rubio, Javier; Veermäe, Hardi (2020)
    In the Starobinsky model of inflation, the observed dark matter abundance can be produced from the direct decay of the inflaton field only in a very narrow spectrum of closeto-conformal scalar fields and spinors of mass similar to 10(7) GeV. This spectrum can be, however, significantly broadened in the presence of effective non-renormalizable interactions between the dark and the visible sectors. In particular, we show that UV freeze-in can efficiently generate the right dark matter abundance for a large range of masses spanning from the keV to the PeV scale and arbitrary spin, without significantly altering the heating dynamics. We also consider the contribution of effective interactions to the inflaton decay into dark matter.