398 research outputs found

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### Singular inflation

We prove that a homogeneous and isotropic universe containing a scalar field with a power-law potential, V(Ď•)=AĎ•^n, with 00 always develops a finite-time singularity at which the Hubble rate and its first derivative are finite, but its second derivative diverges. These are the first examples of cosmological models with realistic matter sources that possess weak singularities of â€śsuddenâ€ť type. We also show that a large class of models with even weaker singularities exists for noninteger n>1. More precisely, if k<n<k+1 where k is a positive integer then the first divergence of the Hubble rate occurs with its (k+2)th derivative. At early times these models behave like standard large-field inflation models but they encounter a singular end state when inflation ends. We term this singular inflation.A.A.H.G. and J.D.B. are supported by the STFC.This is the author accepted manuscript. The final version is available from APS via http://dx.doi.org/10.1103/PhysRevD.91.08351

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### Cosmological models in energy-momentum-squared gravity

We study the cosmological effects of adding terms of higher-order in the usual energy-momentum tensor to the matter lagrangian of general relativity. This is in contrast to most studies of higher-order gravity which focus on generalising the Einstein-Hilbert curvature contribution to the lagrangian. The resulting cosmological theories give rise to field equations of similar form to several particular theories with different fundamental bases, including bulk viscous cosmology, loop quantum gravity, K-essence, and brane-world cosmologies. We find a range of exact solutions for isotropic universes, discuss their behaviours with reference to the early and late-time evolution, accelerated expansion, and the occurrence or avoidance of singularities. We briefly discuss extensions to anisotropic cosmologies and delineate the situations where the higher-order matter terms will dominate over anisotropies on approach to cosmological singularities

### Hyperbolic inflation in the light of Planck 2015 data

Rubano and Barrow have discussed the emergence of a dark energy, with
late-time cosmic acceleration arising from a self-interacting homogeneous
scalar field with a potential of hyperbolic power type. Here, we study the
evolution of this scalar field potential back in the inflationary era. Using
the hyperbolic power potential in the framework of inflation, we find that the
main slow-roll parameters, like the scalar spectral index, the running of the
spectral index and the tensor-to-scalar fluctuation ratio can be computed
analytically. Finally, in order to test the viability of this hyperbolic scalar
field model at the early stages of the Universe, we compare the predictions of
that model against the latest observational data, namely Planck 2015.S.â€‰B. acknowledges support by the Research Center for Astronomy of the Academy of Athens in the context of the program â€śTracing the Cosmic Acceleration.â€ť J.â€‰D.â€‰B. acknowledges STFC support.This is the author accepted manuscript. The final version is available from APS via http://dx.doi.org/10.1103/PhysRevD.91.10351

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### New singularities in unexpected places

Spacetime singularities have been discovered which are physically much weaker than those predicted by the classical singularity theorems. Geodesics evolve through them and they only display infinities in the derivatives of their curvature invariants. So far, these singularities have appeared to require rather exotic and unphysical matter for their occurrence. Here, we show that a large class of singularities of this form can be found in a simple Friedmann cosmology containing only a scalar-field with a power-law self-interaction potential. Their existence challenges several preconceived ideas about the nature of spacetime singularities and has an impact upon the end of inflation in the early universe. This is the author accepted manuscript. The final version is available from World Scientific via http://dx.doi.org/10.1142/S021827181544012

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### Turning on gravity with the Higgs mechanism

We investigate how a Higgs mechanism could be responsible for the emergence of gravity in extensions of Einstein theory, with a suitable low energy limit. In this scenario, at high energies, symmetry restoration could 'turn off' gravity, with dramatic implications for cosmology and quantum gravity. The sense in which gravity is muted depends on the details of the implementation. In the most extreme case gravity's dynamical degrees of freedom would only be unleashed after the Higgs field acquires a non-trivial vacuum expectation value, with gravity reduced to a topological field theory in the symmetric phase. We might also identify the Higgs and the Brans-Dicke fields in such a way that in the unbroken phase Newton's constant vanishes, decoupling matter and gravity. We discuss the broad implications of these scenarios

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### Stability of the Kasner universe in f (T) gravity

f(T) gravity theory offers an alternative context in which to consider gravitational interactions where torsion, rather than curvature, is the mechanism by which gravitation is communicated. We investigate the stability of the Kasner solution with several forms of the arbitrary lagrangian function examined within the f(T) context. This is a Bianchi type--I vacuum solution with anisotropic expansion factors. In the f(T) gravity setting, the solution must conform to a set of conditions in order to continue to be a vacuum solution of the generalized field equations. With this solution in hand, the perturbed field equations are determined for power-law and exponential forms of the f(T) function. We find that the point which describes the Kasner solution is a saddle point which means that the singular solution is unstable. However, we find the de Sitter universe is a late-time attractor. In general relativity, the cosmological constant drives the the isotropization of the spacetime while in this setting the extra f(T) contributions now provide this impetus

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