| 1. |
- Berg, Marcus, 1973-, et al.
(författare)
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Growth Histories in Bimetric Massive Gravity
- 2012
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Ingår i: Journal of Cosmology and Astroparticle Physics. - Bristol : Institute of Physics (IOP). - 1475-7516. ; :12
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Tidskriftsartikel (refereegranskat)abstract
- We perform cosmological perturbation theory in Hassan-Rosen bimetric gravity for general homogeneous and isotropic backgrounds. In the de Sitter approximation, we obtain decoupled sets of massless and massive scalar gravitational fluctuations. Matter perturbations then evolve like in Einstein gravity. We perturb the future de Sitter regime by the ratio of matter to dark energy, producing quasi-de Sitter space. In this more general setting the massive and massless fluctuations mix. We argue that in the quasi-de Sitter regime, the growth of structure in bimetric gravity differs from that of Einstein gravity.
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| 2. |
- Uggla, Claes, 1957-, et al.
(författare)
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Second order cosmological perturbations : simplified gauge change formulas
- 2019
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Ingår i: Classical and quantum gravity. - : Institute of Physics (IOP). - 0264-9381 .- 1361-6382. ; 36:3, s. 1-19
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we present a new formulation of the change of gauge formulas in second order cosmological perturbation theory which unifies and simplifies known results. Our approach is based on defining new second order scalar perturbation variables by adding a multiple of the square of the corresponding first order variables to each second order variable. A bonus is that these new perturbation variables are of broader significance in that they also simplify the analysis of second order scalar perturbations in the super-horizon regime in a number of ways, and lead to new conserved quantities.
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| 3. |
- Alho, Artur, et al.
(författare)
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Cosmological global dynamical systems analysis
- 2022
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Ingår i: Classical and quantum gravity. - : Institute of Physics (IOP). - 0264-9381 .- 1361-6382. ; 39:14, s. 1-14
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Tidskriftsartikel (refereegranskat)abstract
- We consider a dynamical systems formulation for models with an exponential scalar field and matter with a linear equation of state in a spatially flat and isotropic spacetime. In contrast to earlier work, which only considered linear hyperbolic fixed point analysis, we do a center manifold analysis of the non-hyperbolic fixed points associated with bifurcations. More importantly though, we construct monotonic functions and a Dulac function. Together with the complete local fixed point analysis this leads to proofs that describe the entire global dynamics of these models, thereby complementing previous local results in the literature.
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| 4. |
- Alho, Artur, et al.
(författare)
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Inflationary alpha-attractor cosmology : A global dynamical systems perspective
- 2017
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Ingår i: Physical Review D. - : American Physical Society. - 2470-0010 .- 2470-0029. ; 95:8
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Tidskriftsartikel (refereegranskat)abstract
- We study flat Friedmann-Lemaitre-Robertson-Walker alpha-attractor E- and T-models by introducing a dynamical systems framework that yields regularized unconstrained field equations on two-dimensional compact state spaces. This results in both illustrative figures and a complete description of the entire solution spaces of these models, including asymptotics. In particular, it is shown that observational viability, which requires a sufficient number of e-folds, is associated with a particular solution given by a one-dimensional center manifold of a past asymptotic de Sitter state, where the center manifold structure also explains why nearby solutions are attracted to this "inflationary attractor solution." A center manifold expansion yields a description of the inflationary regime with arbitrary analytic accuracy, where the slow-roll approximation asymptotically describes the tangency condition of the center manifold at the asymptotic de Sitter state.
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| 5. |
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| 6. |
- Sandin, Patrik, 1980-, et al.
(författare)
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Bianchi Type I Models with Two Tilted Fluids
- 2008
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Ingår i: Classical and quantum gravity. - : IOP Publishing. - 0264-9381 .- 1361-6382. ; 25:22, s. 1-23
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Tidskriftsartikel (refereegranskat)
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| 7. |
- Sandin, Patrik
(författare)
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Cosmological Models and Singularities in General Relativity
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This is a thesis on general relativity. It analyzes dynamical properties of Einstein's field equations in cosmology and in the vicinity of spacetime singularities in a number of different situations. Different techniques are used depending on the particular problem under study; dynamical systems methods are applied to cosmological models with spatial homogeneity; Hamiltonian methods are used in connection with dynamical systems to find global monotone quantities determining the asymptotic states; Fuchsian methods are used to quantify the structure of singularities in spacetimes without symmetries. All these separate methods of analysis provide insights about different facets of the structure of the equations, while at the same time they show the relationships between those facets when the different methods are used to analyze overlapping areas.The thesis consists of two parts. Part I reviews the areas of mathematics and cosmology necessary to understand the material in part II, which consists of five papers. The first two of those papers uses dynamical systems methods to analyze the simplest possible homogeneous model with two tilted perfect fluids with a linear equation of state. The third paper investigates the past asymptotic dynamics of barotropic multi-fluid models that approach a `silent and local' space-like singularity to the past. The fourth paper uses Hamiltonian methods to derive new monotone functions for the tilted Bianchi type II model that can be used to completely characterize the future asymptotic states globally. The last paper proves that there exists a full set of solutions to Einstein's field equations coupled to an ultra-stiff perfect fluid that has an initial singularity that is very much like the singularity in Friedman models in a precisely defined way.
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| 8. |
- Sandin, Patrik, et al.
(författare)
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Perfect Fluids and Generic Spacelike Singularities
- 2010
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Ingår i: Classical and quantum gravity. - : IOP Publishing. - 0264-9381 .- 1361-6382. ; 27:2, s. 025013-
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Tidskriftsartikel (refereegranskat)abstract
- We present the 1+3 Hubble-normalized conformal orthonormal frame approach to Einstein field equations, and specialize it to a source that consists of perfect fluids with general barotropic equations of state. We use this framework to give specific mathematical content to conjectures about generic spacelike singularities that were originally introduced by Belinskii, Khalatnikov and Lifshitz. Assuming that the conjectures hold, we derive results about how the properties of fluids and generic spacelike singularities affect each other.
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| 9. |
- Sandin, Patrik, 1980-
(författare)
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Tilted Two-Fluid Bianchi Type I Models
- 2009
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Ingår i: General Relativity and Gravitation. - : Springer Science and Business Media LLC. - 0001-7701 .- 1572-9532. ; 41, s. 2707-2724
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Tidskriftsartikel (refereegranskat)
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| 10. |
- Uggla, Claes, 1957-, et al.
(författare)
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Simple expressions for second order density perturbations in standard cosmology
- 2014
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Ingår i: Classical and quantum gravity. - : Institute of Physics (IOP). - 0264-9381 .- 1361-6382. ; 31:10, s. 105008-
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we present four simple expressions for the relativistic first and second order fractional density perturbations for ΛCDM cosmologies in different gauges: the Poisson, uniform curvature, total matter and synchronous gauges. A distinctive feature of our approach is the use of a canonical set of quadratic differential expressions involving an arbitrary spatial function, the so-called comoving curvature perturbation, to describe the spatial dependence, which enables us to unify, simplify and extend previous seemingly disparate results. The simple structure of the expressions makes the evolution of the density perturbations completely transparent and clearly displays the effect of the cosmological constant on the dynamics, namely that it stabilizes the perturbations. We expect that the results will be useful in applications, for example, studying the effects of primordial non-Gaussianity on the large scale structure of the universe.
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