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Articles de revues sur le sujet « Spherical collapse model »

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Auteur : Grafiati
Publié le 11 mars 2023

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1

DEL POPOLO, ANTONINO. « IMPROVEMENTS TO THE SPHERICAL COLLAPSE MODEL ». International Journal of Modern Physics D 15, no 07 (juillet 2006) : 1067–88. http://dx.doi.org/10.1142/s0218271806008553.

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We study the joint effect of dynamical friction, tidal torques and cosmological constant on clusters of galaxy formation. We show that within high-density environments, such as rich clusters of galaxies, both dynamical friction and tidal torques slow down the collapse of low-ν peaks producing an observable variation in the time of collapse of the perturbation and, as a consequence, a reduction in the mass bound to the collapsed perturbation. Moreover, the delay of the collapse produces a tendency for less dense regions to accrete less mass, with respect to a classical spherical model, inducing a biasing of over-dense regions toward higher mass. We show how the threshold of collapse is modified if dynamical friction, tidal torques and a non-zero cosmological constant are taken into account and we use the Extended Press–Schecter (EPS) approach to calculate the effects on the mass function. Then, we compare the numerical mass function given in D. Reed, Mon. Not. R. Astron. Soc.346, 565 (2003) with the theoretical mass function obtained in the present paper. We show that the barrier obtained in the present paper gives rise to a better description of the mass function evolution with respect to other previous models, R. K. Sheth and G. Tormen, Mon. Not. R. Astron. Soc.308, 119 (1999) and R. K. Sheth and G. Tormen, Mon. Not. R. Astron. Soc.329, 61 (2002).
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Taddei, Laura. « Spherical Collapse in the Symmetron Model ». Journal of Physics : Conference Series 470 (6 décembre 2013) : 012006. http://dx.doi.org/10.1088/1742-6596/470/1/012006.

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3

GOVENDER, M., K. S. GOVINDER, S. D. MAHARAJ, R. SHARMA, S. MUKHERJEE et T. K. DEY. « RADIATING SPHERICAL COLLAPSE WITH HEAT FLOW ». International Journal of Modern Physics D 12, no 04 (avril 2003) : 667–76. http://dx.doi.org/10.1142/s0218271803003086.

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We present here a simple model of radiative gravitational collapse with radial heat flux which describes qualitatively the stages close to the formation of a superdense cold star. Starting with a static general solution for a cold star, the model can generate solutions for the earlier evolutionary stages. The temporal evolution of the model is specified by solving the junction conditions appropriate for radiating gravitational collapse. The results will be useful in constructing models for the evolution of X-ray pulsars, like Her X-1.
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4

Del Popolo, Antonino, et Morgan Le Delliou. « Splashback Radius in a Spherical Collapse Model ». Universe 8, no 9 (6 septembre 2022) : 462. http://dx.doi.org/10.3390/universe8090462.

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It was shown several years ago that dark matter halo outskirts are characterized by very steep density profiles in a very small radial range. This feature has been interpreted as a pile-up of different particle orbits at a similar location, namely, splashback material at half an orbit after collapse. Adhikari et al. (2014) obtained the location of the splashback radius through a very simple model by calculating a dark matter shell trajectory in the secondary infall model while it crosses a growing NFW profile-shaped dark matter halo. Because they imposed a halo profile instead of calculating it from the trajectories of the shells of dark matter, they were not able to find the dark matter profile around the splashback radius. In the present paper, we use an improved spherical infall model taking into account shell crossing as well as several physical effects such as ordered and random angular momentum, dynamical friction, adiabatic contraction, etc. This allows us to determine the density profile from the inner to the outer region and to study the behavior of the outer density profile. We compare the density profiles and their logarithmic slope of with the simulation results of Diemer and Kravtsov (2014), finding a good agreement between the prediction of the model and the simulations.
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5

Del Popolo, A. « Some improvements to the spherical collapse model ». Astronomy & ; Astrophysics 454, no 1 (juillet 2006) : 17–26. http://dx.doi.org/10.1051/0004-6361:20054441.

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6

Cupani, Guido, Marino Mezzetti et Fabio Mardirossian. « Angular momentum in cluster Spherical Collapse Model ». Monthly Notices of the Royal Astronomical Society 417, no 4 (6 octobre 2011) : 2554–61. http://dx.doi.org/10.1111/j.1365-2966.2011.19419.x.

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7

Lee, Seokcheon. « Spherical collapse model with and without curvature ». Physics Letters B 685, no 2-3 (mars 2010) : 110–14. http://dx.doi.org/10.1016/j.physletb.2010.01.058.

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8

Sanchez-Conde, M. A., J. Betancort-Rijo et F. Prada. « The spherical collapse model with shell-crossing ». Monthly Notices of the Royal Astronomical Society 378, no 1 (11 juin 2007) : 339–52. http://dx.doi.org/10.1111/j.1365-2966.2007.11798.x.

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9

DEL POPOLO, A., F. PACE et J. A. S. LIMA. « EXTENDED SPHERICAL COLLAPSE AND THE ACCELERATING UNIVERSE ». International Journal of Modern Physics D 22, no 08 (21 juin 2013) : 1350038. http://dx.doi.org/10.1142/s0218271813500387.

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The influence of the shear stress and angular momentum on the nonlinear spherical collapse model is discussed in the framework of the Einstein–de Sitter and ΛCDM models. By assuming that the vacuum component is not clustering within the homogeneous nonspherical overdensities, we show how the local rotation and shear affect the linear density threshold for collapse of the nonrelativistic component (δc) and its virial overdensity (ΔV). It is also found that the net effect of shear and rotation in galactic scale is responsible for higher values of the linear overdensity parameter as compared with the standard spherical collapse model (no shear and rotation).
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10

Mohanty, Sujata, et Rajesh Gopal. « Analysis of cosmological bias within spherical collapse model ». EUREKA : Physics and Engineering, no 5 (30 septembre 2022) : 3–11. http://dx.doi.org/10.21303/2461-4262.2022.002429.

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The goal of our research work is to analyze cosmological bias parameter. Parametric equations of spherical collapse model are used to calculate the values of spherical collapse over density and mass variance, which is further used in bias formulae to find the values of cosmological bias. Spherical collapse over density has been calculated in the range of redshift 0 to 1. Also, it is compared with the value according to the spherical collapse model. Bias is one of the parameters which are utilized to infer cosmological parameters. Extracting the cosmological parameters is very much useful to know and understand about the birth and evolution of our universe. As there is no direct probe to get the idea about the existence of dark matter. Bias factor helps to analyze about dark matter. The bias coefficient of higher order terms in Taylor series expansion are found to be in ascending order. Increasing values of bias indicate the large-scale structure formation at current epoch is more and more clustered. Values of bias are discussed in result. Also, bias values have been analyzed for redshift in the range 2 to 0. The graph has been plotted bias versus redshift. Let’s found bias decreases with decrease of redshift. That means bias evolves with redshift. Bias value less than one and negative value of bias implies that structure formation is in linear region and higher values of bias indicates the structure formation occurs in nonlinear region. Negative value of bias is also called as antibias. That means the structure formation has not started yet. It is still in linear region. The bias value nearly equal to one indicates that the structure formation has been transformed from linear region to nonlinear region. So, the result showing bias values greater than one indicates that evolution of structure formation occurs in nonlinear region.
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11

Lee, Seokcheon, et Kin-Wang Ng. « Spherical collapse model with non-clustering dark energy ». Journal of Cosmology and Astroparticle Physics 2010, no 10 (25 octobre 2010) : 028. http://dx.doi.org/10.1088/1475-7516/2010/10/028.

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12

Grammenos, Th, et Ch Kolassis. « An analytic model of radiating spherical gravitational collapse ». Physics Letters A 169, no 1-2 (septembre 1992) : 5–11. http://dx.doi.org/10.1016/0375-9601(92)90796-o.

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13

Zhang, Han, Tobias Weinzierl, Holger Schulz et Baojiu Li. « Spherical accretion of collisional gas in modified gravity I : self-similar solutions and a new cosmological hydrodynamical code ». Monthly Notices of the Royal Astronomical Society 515, no 2 (22 juillet 2022) : 2464–82. http://dx.doi.org/10.1093/mnras/stac1991.

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ABSTRACT The spherical collapse scenario has great importance in cosmology since it captures several crucial aspects of structure formation. The presence of self-similar solutions in the Einstein-de Sitter (EdS) model greatly simplifies its analysis, making it a powerful tool to gain valuable insights into the real and more complicated physical processes involved in galaxy formation. While there has been a large body of research to incorporate various additional physical processes into spherical collapse, the effect of modified gravity (MG) models, which are popular alternatives to the Λ cold dark matter paradigm to explain the cosmic acceleration, is still not well understood in this scenario. In this paper, we study the spherical accretion of collisional gas in a particular MG model, which is a rare case that also admits self-similar solutions. The model displays interesting behaviours caused by the enhanced gravity and a screening mechanism. Despite the strong effects of MG, we find that its self-similar solution agrees well with that of the EdS model. These results are used to assess a new cosmological hydrodynamical code for spherical collapse simulations introduced here, which is based on the hyperbolic partial differential equation engine ExaHyPE 2. Its good agreement with the theoretical predictions confirms the reliability of this code in modelling astrophysical processes in spherical collapse. We will use this code to study the evolution of gas in more realistic MG models in future work.
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14

Mota, D. F., et C. van de Bruck. « On the spherical collapse model in dark energy cosmologies ». Astronomy & ; Astrophysics 421, no 1 (11 juin 2004) : 71–81. http://dx.doi.org/10.1051/0004-6361:20041090.

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15

Li, Wei, et Lixin Xu. « Spherical collapse for a viscous generalized Chaplygin GaS model ». Journal of Experimental and Theoretical Physics 120, no 4 (avril 2015) : 613–17. http://dx.doi.org/10.1134/s1063776115020156.

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16

Grammenos, Th. « Thermodynamics of a model of nonadiabatic spherical gravitational collapse ». Astrophysics and Space Science 211, no 1 (1994) : 31–40. http://dx.doi.org/10.1007/bf00658039.

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17

Malomed, Boris A., et Eugene M. Maslov. « Collapse of a spherical kink in the ø4 model ». Physics Letters A 160, no 3 (novembre 1991) : 233–36. http://dx.doi.org/10.1016/0375-9601(91)90768-4.

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18

BANERJEE, A., S. CHATTERJEE et N. DADHICH. « SPHERICAL COLLAPSE WITH HEAT FLOW AND WITHOUT HORIZON ». Modern Physics Letters A 17, no 35 (20 novembre 2002) : 2335–39. http://dx.doi.org/10.1142/s0217732302008320.

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We present a class of solutions for a heat conducting fluid sphere, which radiates energy during collapse without the appearance of horizon at the boundary at any stage of the collapse. A simple model shows that there is no accumulation of energy due to collapse since it radiates out at the same rate as it is being generated.
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19

Karbasi, S., et H. Razmi. « Spherical "Top-Hat" collapse in a modified Chaplygin gas dominated universe ». International Journal of Modern Physics D 24, no 07 (27 mai 2015) : 1550050. http://dx.doi.org/10.1142/s0218271815500509.

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Considering perturbation growth in spherical Top-Hat (STH) model of structure formation in a generalized Chaplygin gas (GCG) dominated universe, we want to study this scenario with modified Chaplygin gas (MCG) obeying an equation of state p = A - B/ρα model. Different parameters of this scenario for positive and negative values of A are computed. The evolution of background and collapsed region parameters are found for different cases. The stability of the model and the collapse time rate are considered in different cases. The turn-around redshifts for different values of α are computed; the results are in relatively good agreement with current observational data.
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20

Nakamura, Takashi, et Masataka Fukugita. « A Rotating Stellar Collapse Model for Supernova 1987a ». International Astronomical Union Colloquium 108 (1988) : 432–33. http://dx.doi.org/10.1017/s0252921100094288.

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It is shown that the bunch structure of the Kamiokande neutrino events associated with SN1987a can be naturally understood, if one assumes that the core of the progenitor star was rotating moderately with q(≡ Jc/GM2) ≈ 3 with J the total angular momentum and M the gravitational mass of the core.We assume that the presence of the observed gap, at least that between the second and the third bunch, is real and consider its implications in the dynamics of the core collapse. Let us define a nondimensional angular momentum q(≡ Jc/GM2) with J the total angular momentum and M the gravitational mass of the core. We assume that the value of q for the core of SN1987a was about 3. Then we expect that the effect of the angular momentum plays an important role when the size of the core r shrinks to < 107cm. In the spherically symmetric collapse model, the size of the unshocked homologous core is of the order of 107cm. Hence we expect that the core at the bounce is essentially governed by the spherically symmetric dynamics. To verify this point we may refer to the simulation by Symbalisty1. Our model almost corresponds to Model ROT2 in which one obtains a neutrinosphere with a roughly spherical radius of approximately 50km. We then expect that the infalling nonhomologous matter onto the core will liberate a gravitational energy of the order of 1052ergs.
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21

Pace, Francesco, Sven Meyer et Matthias Bartelmann. « On the implementation of the spherical collapse model for dark energy models ». Journal of Cosmology and Astroparticle Physics 2017, no 10 (25 octobre 2017) : 040. http://dx.doi.org/10.1088/1475-7516/2017/10/040.

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22

GOVENDER, M. « NONADIABATIC SPHERICAL COLLAPSE WITH A TWO-FLUID ATMOSPHERE ». International Journal of Modern Physics D 22, no 07 (juin 2013) : 1350049. http://dx.doi.org/10.1142/s0218271813500491.

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In this paper, we present an exact model of a spherically symmetric star undergoing dissipative collapse in the form of a radial heat flux. The interior of the star is matched smoothly to the generalized Vaidya line element representing a two-fluid atmosphere comprising null radiation and a string fluid. The influence of the string density on the thermal behavior of the model is investigated by employing a causal heat transport equation of Maxwell–Cattaneo form.
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23

Shaw, Douglas J., et David F. Mota. « An Improved Semianalytical Spherical Collapse Model for Nonlinear Density Evolution ». Astrophysical Journal Supplement Series 174, no 2 (février 2008) : 277–81. http://dx.doi.org/10.1086/522339.

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24

Engineer, S., N. Kanekar et T. Padmanabhan. « Non-linear density evolution from an improved spherical collapse model ». Monthly Notices of the Royal Astronomical Society 314, no 2 (11 mai 2000) : 279–89. http://dx.doi.org/10.1046/j.1365-8711.2000.03275.x.

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25

Korkidis, Giorgos, Vasiliki Pavlidou, Konstantinos Tassis, Evangelia Ntormousi, Theodore N. Tomaras et Konstantinos Kovlakas. « Turnaround radius of galaxy clusters in N-body simulations ». Astronomy & ; Astrophysics 639 (juillet 2020) : A122. http://dx.doi.org/10.1051/0004-6361/201937337.

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Aims. We use N-body simulations to examine whether a characteristic turnaround radius, as predicted from the spherical collapse model in a ΛCDM Universe, can be meaningfully identified for galaxy clusters in the presence of full three-dimensional effects. Methods. We use The Dark Sky Simulations and Illustris-TNG dark-matter-only cosmological runs to calculate radial velocity profiles around collapsed structures, extending out to many times the virial radius R200. There, the turnaround radius can be unambiguously identified as the largest nonexpanding scale around a center of gravity. Results. We find that: (a) a single turnaround scale can meaningfully describe strongly nonspherical structures. (b) For halos of masses M200 > 1013 M⊙, the turnaround radius Rta scales with the enclosed mass Mta as Mta1/3, as predicted by the spherical collapse model. (c) The deviation of Rta in simulated halos from the spherical collapse model prediction is relatively insensitive to halo asphericity. Rather, it is sensitive to the tidal forces due to massive neighbors when these are present. (d) Halos exhibit a characteristic average density within the turnaround scale. This characteristic density is dependent on cosmology and redshift. For the present cosmic epoch and for concordance cosmological parameters (Ωm ∼ 0.3; ΩΛ ∼ 0.7) turnaround structures exhibit a density contrast with the matter density of the background Universe of δ ∼ 11. Thus, Rta is equivalent to R11 – in a way that is analogous to defining the “virial” radius as R200 – with the advantage that R11 is shown in this work to correspond to a kinematically relevant scale in N-body simulations.
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26

Sharif, M., et M. Zeeshan Gul. « Dynamics of spherical collapse in energy–momentum squared gravity ». International Journal of Modern Physics A 36, no 01 (10 janvier 2021) : 2150004. http://dx.doi.org/10.1142/s0217751x21500044.

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This paper investigates the dynamics of spherical collapse in the framework of energy–momentum squared gravity. This theory overcomes the big-bang singularity and provides viable cosmological consequences in the early time universe. We proceed our work by considering the nonstatic spherically symmetric space–time in the interior and static spherically symmetric metric in the exterior regions of the star. The Darmois junction conditions between interior and exterior geometries are derived. We construct dynamical equations through the Misner–Sharp technique to analyze the impact of matter variables and dark source terms on the collapsing phenomenon. A correlation among dark source terms, Weyl scalar and matter variables is also established. Due to the presence of multivariate function and its derivatives, space–time is no longer considered to be conformally flat. To obtain conformally flat space–time, we have considered a particular model of this gravity which yields conformally flat space–time and homogeneity of the energy density through the entire system. We conclude that positive dark source terms as well as negative pressure gradient provide the anti-gravitational behavior leading to the stability of self-gravitating objects and hence prevent the collapsing process.
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27

Aganin, A. A., Т. S. Guseva et Т. F. Khalitova. « Modeling the evolution of small distortions of the sphericity of a collapsing bubble ». Proceedings of the Mavlyutov Institute of Mechanics 5 (2007) : 60–65. http://dx.doi.org/10.21662/uim2007.1.002.

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Results of calculating the evolution of small axisymmetric distortions of the spherical shape of a bubble during its collapse are presented. The full model based on two-dimensional gas dynamics equations (gas and liquid are considered inviscid non-heat-conducting) and a number of simplified models are used. The latter are obtained from the full model by splitting the gas and liquid motion into a spherical component and its small non-spherical perturbation. The differences of the simplified models are defined by the assumptions used in the realization of the splitting.
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28

Del Popolo, A., et Xiguo Lee. « Deviations from Spherical Symmetry, Typical Parameters of the Spherical Collapse Model, and Dark Energy Cosmologies ». Astronomy Reports 62, no 8 (31 juillet 2018) : 475–82. http://dx.doi.org/10.1134/s1063772918080012.

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29

Giani, Leonardo, Oliver F. Piattella et Alexander Yu Kamenshchik. « Bianchi IX gravitational collapse of matter inhomogeneities ». Journal of Cosmology and Astroparticle Physics 2022, no 03 (1 mars 2022) : 028. http://dx.doi.org/10.1088/1475-7516/2022/03/028.

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Abstract We investigate a model of gravitational collapse of matter inhomogeneities where the latter are modelled as Bianchi type IX (BIX) spacetimes. We found that this model contains, as limiting cases, both the standard spherical collapse model and the Zeldovich solution. We study how these models are affected by small anisotropies within the BIX potential. For the spherical collapse case, we found that the model is equivalent to a closed FLRW Universe filled with matter and two perfect fluids representing the anisotropic contributions. From the linear evolution up to the turnaround, the anisotropies effectively shift the value of the FLRW spatial curvature, because the fluids have effective Equation of State (EoS) parameters w ≈ -1/3. Then we estimate the impact of such anisotropies on the number density of haloes using the Press-Schechter formalism. If a fluid description of the anisotropies is still valid after virialization, the averaged over time EoS parameters are w ≈ 1/3. Using this and demanding hydrostatic equilibrium, we find a relation between the mass M, the average radius R and the pressure p of the virialized final structure. When we consider within the BIX ansatz small deviations from the Zeldovich solution, our qualitative analysis suggests that the so called pancakes exhibit oscillatory behavior, as would be expected in the case of a vacuum BIX spacetime.
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30

Batista, R. C., H. P. de Oliveira et L. R. W. Abramo. « Spherical collapse of non-top-hat profiles in the presence of dark energy with arbitrary sound speed ». Journal of Cosmology and Astroparticle Physics 2023, no 02 (1 février 2023) : 037. http://dx.doi.org/10.1088/1475-7516/2023/02/037.

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Abstract We study the spherical collapse of non-top-hat matter fluctuations in the presence of dark energy with arbitrary sound speed. The model is described by a system of partial differential equations solved using a pseudo-spectral method with collocation points. This method can reproduce the known analytical solutions in the linear regime with an accuracy better than 10-6% and better than 10-2% for the virialization threshold given by the usual spherical collapse model. We show the impact of nonlinear dark energy fluctuations on matter profiles, matter peculiar velocity and gravitational potential. We also show that phantom dark energy models with low sound speed can develop a pathological behaviour around matter halos, namely negative energy density. The dependence of the virialization threshold density for collapse on the dark energy sound speed is also computed, confirming and extending previous results in the limit for homogeneous and clustering dark energy.
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Fahimi, K., K. Karami, S. Asadzadeh et K. Rezazadeh. « Structure formation in clustering DBI dark energy model with constant sound speed ». Monthly Notices of the Royal Astronomical Society 481, no 2 (5 septembre 2018) : 2393–406. http://dx.doi.org/10.1093/mnras/sty2416.

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ABSTRACT Within the framework of DBI non-canonical scalar field model of dark energy, we study the growth of dark matter perturbations in both the linear and non-linear regimes. In our DBI model, we consider the anti-de Sitter warp factor $f(\phi)=f_0\, \phi ^{-4}$ with constant f0 > 0 and assume the DBI dark energy to be clustered and its sound speed cs to be constant. In the linear regime, we use the pseudo-Newtonian formalism to obtain the growth factor of dark matter perturbations and conclude that for smaller cs (or $\tilde{f_0} \equiv f_0 H_0^2/M_P^2$), the growth factor of dark matter is smaller for clustering DBI model compared to the homogeneous one. In the non-linear regime based on the spherical collapse model, we obtain the linear overdensity δc($z$c), the virial overdensity Δvir($z$c), overdensity at the turn around ζ($z$c), and the rate of expansion of collapsed region hta($z$). We point out that for the smaller cs (or $\tilde{f_0}$), the values of δc($z$c), Δvir($z$c), ζ($z$c), and hta($z$) in non-clustering DBI models deviate more than the ΛCDM compared to the clustering DBI models. Finally, with the help of spherical collapse parameters we calculate the relative number density of halo objects above a given mass and conclude that the differences between clustering and homogeneous DBI models are more pronounced for the higher mass haloes at high redshift.
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32

Aganin, A. A., A. I. Davletshin et T. F. Khalitova. « Numerical simulation of bubble dynamics in central region of streamer ». Multiphase Systems 13, no 3 (29 juin 2018) : 11–22. http://dx.doi.org/10.21662/mfs2018.3.002.

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A mathematical model and a numerical technique for studying strong expansion and collapse of cavitation bubbles located in the central region of a streamer where the bubbles are almost motionless are developed. They are essentially efficient combinations of the models and techniques previously created by the authors for calculating the dynamics of interacting weakly-non-spherical bubbles in a streamer and the dynamics of a single axisymmetric bubble. The first model and technique are applied at the low-speed stage of expansion and compression of bubbles where their hydrodynamic interaction is significant. The second ones are used at the final high-speed stage of their collapse where the interaction is inessential. The simplest case of the streamer comprising three bubbles is considered as an example to illustrate the features of the developed model and numerical technique. It is shown that under the strong expansion and collapse of an initially spherical cavitation bubble, the presence of neighboring bubbles can substantially deflect the bubble cavity vapor dynamics from what is realized inside a similar but single bubble.
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33

BIZOŃ, PIOTR, et BERND G. SCHMIDT. « HOW TO BYPASS BIRKHOFF THROUGH EXTRA DIMENSIONS : A SIMPLE FRAMEWORK FOR INVESTIGATING THE GRAVITATIONAL COLLAPSE IN VACUUM ». International Journal of Modern Physics D 15, no 12 (décembre 2006) : 2217–22. http://dx.doi.org/10.1142/s0218271806009649.

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It is fair to say that our current mathematical understanding of the dynamics of gravitational collapse to a black hole is limited to the spherically symmetric situation and, in fact, even in this case much remains to be learned. The reason is that Einstein's equations become tractable only if they are reduced to a (1 + 1)-dimensional system of partial differential equations. Owing to this technical obstacle, very little is known about the collapse of pure gravitational waves because by Birkhoff's theorem there is no spherical collapse in vacuum. In this essay, we describe a new cohomogeneity-two symmetry reduction of the vacuum Einstein equations in five and higher odd dimensions which evades Birkhoff's theorem and admits time-dependent asymptotically flat solutions. We argue that this model provides an attractive (1 + 1)-dimensional geometric setting for investigating the dynamics of gravitational collapse in vacuum.
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34

Batista, Ronaldo C. « A Short Review on Clustering Dark Energy ». Universe 8, no 1 (30 décembre 2021) : 22. http://dx.doi.org/10.3390/universe8010022.

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We review dark energy models that can present non-negligible fluctuations on scales smaller than Hubble radius. Both linear and nonlinear evolutions of dark energy fluctuations are discussed. The linear evolution has a well-established framework, based on linear perturbation theory in General Relativity, and is well studied and implemented in numerical codes. We highlight the main results from linear theory to explain how dark energy perturbations become important on the scales of interest for structure formation. Next, we review some attempts to understand the impact of clustering dark energy models in the nonlinear regime, usually based on generalizations of the Spherical Collapse Model. We critically discuss the proposed generalizations of the Spherical Collapse Model that can treat clustering dark energy models and their shortcomings. Proposed implementations of clustering dark energy models in halo mass functions are reviewed. We also discuss some recent numerical simulations capable of treating dark energy fluctuations. Finally, we summarize the observational predictions based on these models.
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35

LEE, SEOKCHEON. « THE INFLUENCE OF DARK ENERGY ON THE LARGE SCALE STRUCTURE FORMATION ». Modern Physics Letters A 25, no 11n12 (20 avril 2010) : 874–84. http://dx.doi.org/10.1142/s0217732310000034.

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Inhomogeneity at small scales (Mpcs) of our universe is manifest from observations. In regions smaller than 8h-1 Mpc galaxies tend to cluster instead of showing a Poisson distribution. The properties of the spherical collapse model which approximately depicts this nonlinearity of the density perturbation are investigated when there exits dark energy. The evolution of a spherical perturbation depends only on the initial critical overdensity threshold for the collapse, δc and its value approaches to 1.58 instead of the conventionally known 1.69 independent of the value of the equation of state of dark energy.
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36

SHINKAI, HISA-AKI, et YUTA YAMADA. « NUMERICAL INVESTIGATION OF FIVE-DIMENSIONAL GRAVITATIONAL COLLAPSES ». International Journal of Modern Physics : Conference Series 07 (janvier 2012) : 148–57. http://dx.doi.org/10.1142/s2010194512004217.

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We introduce our numerical studies of gravitational collapses in five-dimensional (5D) space-time, with a purpose of studying the cosmic censorship hypothesis and the hoop conjecture. The first model is the collapse of spindle matter which was performed by Shapiro and Teukolsky (1991) who announced an appearance of a naked singularity in 4D. Comparing with 4D cases, we found that 5D collapses proceed more rapidly, the final configurations tend to be spherical, and apparent horizon (AH) forms in wider parameter ranges. We also observed positive evidence for formation of a naked singularity in highly spindle cases as well. The second model is the formation of black-ring in 5D. Our code does not include angular momentum, but the model would be helpful for basic understandings. We constructed an initial data sequence with ring-shaped matter, and observed the topology of AHs, if formed. We found a critical ring radius for ring-shaped AH, and it suggests a dynamical transition of AH topology from ring-shaped to spherical. We demonstrate such an example in time evolution.
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37

Malekjani, M., S. Basilakos et N. Heidari. « Spherical collapse model and cluster number counts in power-lawf(T) gravity ». Monthly Notices of the Royal Astronomical Society 466, no 3 (24 décembre 2016) : 3488–96. http://dx.doi.org/10.1093/mnras/stw3367.

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38

Fosalba, Pablo, et Enrique Gaztañaga. « Cosmological Perturbation Theory and the Spherical Collapse model — I. Gaussian initial conditions ». Monthly Notices of the Royal Astronomical Society 301, no 2 (décembre 1998) : 503–23. http://dx.doi.org/10.1046/j.1365-8711.1998.02033.x.

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39

Del Popolo, A., F. Pace et J. A. S. Lima. « Spherical collapse model with shear and angular momentum in dark energy cosmologies ». Monthly Notices of the Royal Astronomical Society 430, no 1 (22 janvier 2013) : 628–37. http://dx.doi.org/10.1093/mnras/sts669.

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40

HERRERA, L., G. LE DENMAT, N. O. SANTOS et A. WANG. « SHEAR-FREE RADIATING COLLAPSE AND CONFORMAL FLATNESS ». International Journal of Modern Physics D 13, no 04 (avril 2004) : 583–92. http://dx.doi.org/10.1142/s0218271804004840.

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Here we study some general properties of spherical shear-free collapse. Its general solution when imposing conformal flatness is re-obtained (Refs. 1 and 2) and matched to the outgoing Vaidya spacetime. We propose a simple model satisfying these conditions and study its physical consequences. Special attention deserve, the role played by relaxational processes and the conspicuous link betweeen dissipation and density inhomogeneity.
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41

Davletshin, A. I., et T. F. Khalitova. « Numerical simulation of single vapor bubble dynamics in a liquid in an intense acoustic field ». Multiphase Systems 13, no 4 (24 décembre 2018) : 127–35. http://dx.doi.org/10.21662/mfs2018.4.018.

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The possibility of increasing the calculation efficiency by the joint use of two models of the dynamics of a single weakly-nonspherical vapor bubble under its strong collapse in liquid is studied. In both models the motion of liquid and vapor is split into a spherical component and its small nonspherical perturbation. The models differ in the description of the spherical component. In the first (simplified) model, it is described by a system of ODE together with partial differential equations in temperature, derived under the assumption of weak compressibility of liquid and bubble homobaricity. In the second model, one-dimensional gas dynamics equations are applied. The advantage of the simplified model consists in determining a numerical solution with much-less computer time costs in comparison with what is required for the numerical integration of gas dynamics equations. The assumptions used in the simplified model in the final stage of collapse become incorrect, and, as a result, the numerical solution errors increase. Therefore, the simplified model is applied at the beginning of bubble collapse, whereas the gas dynamics equations are used at its end. Within this approach, the numerical solution in the final stage of collapse is dependent on the moment of transition to the gas dynamics equations. It is shown that satisfactory description of evolution of bubble sphericity distortion is achieved when the transition is made under the condition that the Mach number M of vapor in the vicinity of the bubble surface is less than 0.4. Satisfactory resolution of the shock wave in the bubble is attained when the transition is performed at M<0.2.
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42

Setare, M. R., F. Felegary et F. Darabi. « Evolution of spherical overdensities in new agegraphic dark energy model ». International Journal of Modern Physics D 26, no 09 (24 mars 2017) : 1750101. http://dx.doi.org/10.1142/s0218271817501012.

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We study the structure formation by investigating the spherical collapse model in the context of new agegraphic dark energy (NADE) model in flat FRW cosmology. We compute the perturbational quantities [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text] for the NADE model and compare the results with those of EdS and [Formula: see text] models. We find that there is a dark energy-dominated universe at low redshifts and a matter-dominated universe at high redshifts in agreement with the observations. Also, the size of structures, the overdense spherical region, and the halo size in the NADE model are found to be smaller, denser, and larger than those of EdS and [Formula: see text] models. We compare our results with the results of tachyon scalar field and holographic dark energy models.
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43

Jia, Fei, Ousmane Kodio, S. Jon Chapman et Alain Goriely. « On the figure of elastic planets I : gravitational collapse and infinitely many equilibria ». Proceedings of the Royal Society A : Mathematical, Physical and Engineering Sciences 475, no 2224 (avril 2019) : 20180815. http://dx.doi.org/10.1098/rspa.2018.0815.

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A classic problem of elasticity is to determine the possible equilibria of an elastic planet modelled as a homogeneous compressible spherical elastic body subject to its own gravitational field. In the absence of gravity, the initial radius is given and the density is constant. With gravity and for small planets, the elastic deformations are small enough so that the spherical equilibria can be readily obtained by using the theory of linear elasticity. For larger or denser planets, large deformations occur and the general theory of nonlinear elasticity is required to obtain the solution. Depending on the elastic model, we show that there may be parameter regimes where there exist no equilibrium or arbitrarily many equilibria. Yet, at most two of them are dynamically stable with respect to radial disturbances. In some of these models, there is a critical initial radius at which spherical solutions cease to exist. For planets with larger initial radii, there is no spherical solution as the elastic forces are not sufficient to balance the gravitational force. Therefore, the system undergoes gravitational collapse, an unexpected phenomenon within the framework of classical continuum mechanics.
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44

Andréasson, Håkan. « On gravitational collapse and cosmic censorship for collisionless matter ». International Journal of Geometric Methods in Modern Physics 11, no 02 (février 2014) : 1460002. http://dx.doi.org/10.1142/s0219887814600020.

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The weak cosmic censorship conjecture is a central open problem in classical general relativity. Under the assumption of spherical symmetry, Christodoulou has investigated the conjecture for two different matter models; a scalar field and dust. He has shown that the conjecture holds true for a scalar field but that it is violated in the case of dust. The outcome of the conjecture is thus sensitive to which model is chosen to describe matter. Neither a scalar field nor dust are realistic matter models. Collisionless matter, or Vlasov matter, is a simple matter model but can be considered to be realistic in the sense that it is used by astrophysicists. The present status on the weak cosmic censorship conjecture for the Einstein–Vlasov system is reviewed here.
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45

Mandal, Ankush, et Sharvari Nadkarni-Ghosh. « One-point probability distribution function from spherical collapse : early dark energy versus ΛCDM ». Monthly Notices of the Royal Astronomical Society 498, no 1 (20 juillet 2020) : 355–72. http://dx.doi.org/10.1093/mnras/staa2073.

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ABSTRACT We compute the one-point probability distribution function (PDF) of an initially Gaussian dark matter density field using spherical collapse (SC). We compare the results to other forms available in the literature and also compare the PDFs in the Λ-cold dark matter model with an early dark energy (EDE) model. We find that the skewed lognormal distribution provides the best fit to the non-linear PDF from SC for both cosmologies, from a = 0.1 to 1 and for scales characterized by the comoving width of the Gaussian: σG = 0.5, 1, and 2. To elucidate the effect of cosmology, we examine the linear and non-linear growth rates through test cases. For overdensities, when the two models have the same initial density contrast, the differences due to cosmology are amplified in the non-linear regime, whereas, if the two models have the same linear density contrast today, then the differences in cosmology are damped in the non-linear regime. This behaviour is in contrast with voids, where the non-linear growth becomes ‘self-regulatory’ and is less sensitive to cosmology and initial conditions. To compare the PDFs, we examine the difference of the PDFs and evolution of the width of the PDF. The trends with scale and redshift are as expected. A tertiary aim of this paper was to check if the fitting form for the non-linear density–velocity divergence relation, derived for constant equation of state (w) models by Nadkarni-Ghosh holds for the EDE model. We find that it does with an accuracy of 4 per cent, thus increasing its range of validity.
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46

Nap, Rikkert J., Sung Hyun Park et Igal Szleifer. « Competitive calcium ion binding to end-tethered weak polyelectrolytes ». Soft Matter 14, no 12 (2018) : 2365–78. http://dx.doi.org/10.1039/c7sm02434g.

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We have developed a molecular model to describe the structural changes and potential collapse of weak polyelectrolyte layers end-tethered to planar surfaces and spherical nanoparticles as a function of pH and divalent ion concentration.
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47

Gaztañaga, Enrique, et Pablo Fosalba. « Cosmological perturbation theory and the spherical collapse model — II. Non-Gaussian initial conditions ». Monthly Notices of the Royal Astronomical Society 301, no 2 (décembre 1998) : 524–34. http://dx.doi.org/10.1046/j.1365-8711.1998.02034.x.

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48

Shibusawa, Y., K. Ichiki et K. Kadota. « The influence of primordial magnetic fields on the spherical collapse model in cosmology ». Journal of Cosmology and Astroparticle Physics 2014, no 08 (7 août 2014) : 017. http://dx.doi.org/10.1088/1475-7516/2014/08/017.

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49

Müller, Bernhard. « A critical assessment of turbulence models for 1D core-collapse supernova simulations ». Monthly Notices of the Royal Astronomical Society 487, no 4 (10 juin 2019) : 5304–23. http://dx.doi.org/10.1093/mnras/stz1594.

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Abstract It has recently been proposed that global or local turbulence models can be used to simulate core-collapse supernova explosions in spherical symmetry (1D) more consistently than with traditional approaches for parametrized 1D models. However, a closer analysis of the proposed schemes reveals important consistency problems. Most notably, they systematically violate energy conservation as they do not balance buoyant energy generation with terms that reduce potential energy, thus failing to account for the physical source of energy that buoyant convection feeds on. We also point out other non-trivial consistency requirements for viable turbulence models. The Kuhfuss model from the 1980s proves more consistent than the newly proposed approaches for supernovae, but still cannot account naturally for all the relevant physics for predicting explosion properties. We perform numerical simulations for a $20 \, \mathrm{M}_\odot$ progenitor to further illustrate problems of 1D turbulence models. If the buoyant driving term is formulated in a conservative manner, the explosion energy of ${\sim }2\times 10^{51}\, \mathrm{erg}$ for the corresponding non-conservative turbulence model is reduced to $\lt 10^{48} \, \mathrm{erg}$ even though the shock expands continuously. This demonstrates that the conservation problem cannot be ignored. Although plausible energies can be reached using an energy-conserving model when turbulent viscosity is included, it is doubtful whether the energy budget of the explosion is regulated by the same mechanism as in multidimensional models. We conclude that 1D turbulence models based on a spherical Reynolds decomposition cannot provide a more consistent approach to supernova explosion and remnant properties than other phenomenological approaches before some fundamental problems are addressed.
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50

CALVISI, M. L., J. I. ILORETA et A. J. SZERI. « Dynamics of bubbles near a rigid surface subjected to a lithotripter shock wave. Part 2. Reflected shock intensifies non-spherical cavitation collapse ». Journal of Fluid Mechanics 616 (10 décembre 2008) : 63–97. http://dx.doi.org/10.1017/s0022112008003054.

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In this paper we use the boundary integral method to model the non-spherical collapse of bubbles excited by lithotripter shock waves near a rigid boundary. The waves we consider are representative of those developed by shock wave lithotripsy or shock wave therapy devices, and the rigid boundaries we consider are representative of kidney stones and reflective bony tissue. This study differs from previous studies in that we account for the reflection of the incident wave and also the asymmetry of the collapse caused by the presence of the rigid surface. The presence of the boundary causes interference between reflected and incident waves. Quantities such as kinetic energy, Kelvin impulse and centroid translation are calculated in order to illuminate the physics of the collapse process. The main finding is that the dynamics of the bubble collapse depend strongly on the distance of the bubble relative to the wall when reflection is taken into account, but much less so when reflection is omitted from the model. The reflection enhances the expansion and subsequent collapse of bubbles located near the boundary owing to constructive interference between incident and reflected waves; however, further from the boundary, the dynamics of collapse are suppressed owing to destructive interference of these two waves. This result holds regardless of the initial radius of the bubble or its initial state at the time of impact with the lithotripter shock wave. Also, the work done by the lithotripter shock wave on the bubble is shown to predict strongly the maximum bubble volume regardless of the standoff distance and the presence or absence of reflection; furthermore, allowing for non-sphericity, these predictions match almost exactly those of a previously developed spherical model.
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