17211178
N-flation: Non-Gaussianity in the horizon-crossing approximation
We analyze the cosmic nongaussianity produced in inflation models with multiple uncoupled fields with monomial potentials, such as Nflation. Using the horizon-crossing approximation to compute the nongaussianity, we show that when each field has the same form of potential, the prediction is independent the number of fields, their initial conditions, and the spectrum of masses/couplings. It depends only on the number of $e$-foldings after the horizon crossing of observable perturbations. We also provide a further generalization to the case where the fields can have monomial potentials with different powers. Unless the horizon-crossing approximation is substantially violated, the predicted nongaussianity is too small to ever be observed.
17174387
N-flation: Observable predictions from the random matrix mass spectrum
We carry out numerical investigations of the perturbations in N-flation models where the mass spectrum is generated by random matrix theory. The tensor-to-scalar ratio and non-Gaussianity are already known to take the single-field values, and so the density perturbation spectral index is the main parameter of interest. We study several types of random field initial conditions and compute the spectral index as a function of mass spectrum parameters. Comparison with microwave anisotropy data from the Wilkinson Microwave Anisotropy Probe shows that the model is currently viable in the majority of its parameter space.
17082200
N-flation: Multifield inflationary dynamics and perturbations
We carry out numerical investigations of the dynamics and perturbations in the Nflation model of Dimopoulos et al. (2005). This model features large numbers of scalar fields with different masses, which can cooperate to drive inflation according to the assisted inflation mechanism. We extend previous work to include random initial conditions for the scalar fields, and explore the predictions for density perturbations and the tensor-to-scalar ratio. The tensor-to-scalar ratio depends only on the number of e-foldings and is independent of the number of fields, their masses, and their initial conditions. It therefore always has the same value as for a single massive field. By contrast, the scalar spectral index has significant dependence on model parameters. While normally multi-field inflation models make predictions for observable quantities which depend also on the unknown field initial conditions, we find evidence of a ‘thermodynamic’ regime whereby the predicted spectral index becomes independent of initial conditions if there are enough fields. Only in parts of parameter space where the mass spectrum of the fields is extremely densely packed is the model capable of satisfying the tight observational constraints from WMAP3 observations.
14036761
Dynamics of assisted quintessence
We explore the dynamics of assisted quintessence, where more than one scalar field is present with the same potential. For potentials with tracking solutions, the fields naturally approach the same values — in the context of inflation this leads to the assisted inflation phenomenon where several fields can cooperate to drive a period of inflation though none is able to individually. For exponential potentials, we study the fixed points and their stability confirming results already in the literature, and then carry out a numerical analysis to show how assisted quintessence is realized. For inverse power-law potentials, we find by contrast that there is no assisted behaviour — indeed those are the unique (monotonic) potentials where several fields together behave just as a single field in the same potential. More generally, we provide an algorithm for generating a single-field potential giving equivalent dynamics to multi-field assisted quintessence.
119489565
Core-Collapse Times of Two-Component Clusters
We examine the corecollapse times of isolated, two-mass-component star clusters using Fokker-Planck models. With initial condition of Plummer models, we find that the corecollapse times of clusters with M_1/M_2 >> 1 are well correlated with (N_1/N_2)^{1/2}(m_1/m_2)^{2} t_{rh}, where M_1/M_2 and m_1/m_2 are the light to heavy component total and individual mass ratios, respectively, N_1/N_2 is the number ratio, and t_{rh} is the initial half-mass relaxation time scale. We also find two-component cluster parameters that best match multi-component (thus more realistic) clusters with power-law mass functions.
118384654
EUNHA: a new cosmological hydro simulation code
We have developed a parallel cosmological hydrodynamic simulation code designed for the study of formation and evolution of cosmological structures. The gravitational force is calculated using the TreePM method and the hydrodynamics is implemented based on the smoothed particle hydrodynamics. The initial displacement and velocity of simulation particles are calculated according to second-order linear perturbation theory using the power spectra of dark matter and baryonic matter. The initial background temperature is given by Recfast and the temperature uctuations at the initial particle position are determined by the adiabatic model. We use a time-limiter scheme over the individual time steps to capture shock-fronts and to ease the time-step tension between the shock and preshock particles. We also include the astrophysical gas processes of radiative heating/cooling, star formation, metal enrichment, and supernova feedback. We have tested the code in several standard cases such as one-dimensional Riemann problems, Kelvin-Helmholtz, and Sedov blast wave instability. Star formation on the galactic disk is investigated to check whether the Schmidt-Kennicutt relation is properly recovered. We also study global star formation histories at different simulation resolutions and compare them with observations.
118561263
Extremal solutions of the S3 model and nilpotent orbits of G2(2)
We study extremal black hole solutions of the S3 model (obtained by setting S=T=U in the STU model) using group theoretical methods. Upon dimensional reduction over time, the S3 model exhibits the pseudo-Riemannian coset structure $ {{G} \left/ {{\tilde{K}}} \right.} $ with G = G2(2) and $ \tilde{K} = {\text{S}}{{\text{O}}_0}\left( {2,2} \right) $. We study nilpotent $ \tilde{K} $-orbits of G2(2) corresponding to non-rotating single-center extremal solutions. We find six such distinct $ \tilde{K} $-orbits. Three of these orbits are supersymmetric, one is non-supersymmetric, and two are unphysical. We write general solutions and discuss examples in all four physical orbits. We show that all solutions in supersymmetric orbits when uplifted to five-dimensional minimal supergravity have single-center Gibbons-Hawking space as their four-dimensional Euclidean hyper-Kähler base space. We construct hitherto unknown extremal (supersymmetric as well as non-supersymmetric) pressureless black strings of minimal five-dimensional supergravity and briefly discuss their relation to black rings.
122749310
RENO: Reactor experiment for neutrino oscillation at Yonggwang
The RENO experiment is under construction to measure the value of the smallest and unknown neutrino mixing angle θ13. The experiment will compare the measured fluxes of electron antineutrinos at two detectors located at 290 m and 1.4 km distances from the center of the Yonggwang nuclear reactors in Korea. It is planned to start data-taking in early 2010. An estimated systematic uncertainty associated with the measurement is less than 0.6%. With three years of data, the experiment will search for the mixing angle values of sin22θ13 down to 0.02.
14066902
Classification of the BPS states in Bagger-Lambert Theory
We classify, in a group theoretical manner, the BPS configura tions in the multiple M2-brane theory recently proposed by Bagger and Lambert. We present three types of BPS equations preserving various fractions of supersymmetries: in the first type we have const ant fields and the interactions are purely algebraic in nature; in the second type the equations are inv ariant under spatial rotation SO(2), and the fields can be time-dependent; in the third class the equat ions are invariant under boost SO(1;1) and provide the eleven-dimensional generalizations of the Nahm equations. The BPS equations for different number of supersymmetries exhibit the division a lgebra structures: octonion, quarternion or complex.