arXiv subjects

Shinji Mukohyama

Publications and source records attributed to Shinji Mukohyama.

At least 253 records · Page 14Linked to original sources

arXiv2008

Reheating a multi-throat universe by brane motion

We propose a mechanism of reheating after inflation in multi-throat scenarios of warped extra dimensions. Validity of an effective field theory on the standard model (SM) brane requires that the position of the SM brane during inflation be different from the position after inflation. The latter is supposed to be near the tip of the SM throat but the former is not. After inflation, when the Hubble expansion rate becomes sufficiently low, the SM brane starts moving towards the tip and eventually oscillates. The SM fields are excited by the brane motion and the universe is reheated. Since interaction between the brane position modulus and the SM fields is suppressed only by the local string scale, the modulus effectively decays into the SM fields.

hep-th↗

arXiv2008

Nonlinear superhorizon perturbations of non-canonical scalar field

We develop a theory of non-linear cosmological perturbations at superhorizon scales for a scalar field with a Lagrangian of the form $P(X,ϕ)$, where $X=-\partial^μϕ\partial_μϕ$ and $ϕ$ is the scalar field. We employ the ADM formalism and the spatial gradient expansion approach to obtain general solutions valid up to the second order in the gradient expansion. This formulation can be applied to, for example, DBI inflation models to investigate superhorizon evolution of non-Gaussianities. With slight modification, we also obtain general solutions valid up to the same order for a perfect fluid with a general equation of state $P=P(ρ)$.

gr-qc↗

arXiv2008

Decaying D-branes and Moving Mirrors

We present an exact time-dependent solution to the effective D-brane world-volume theory which describes an inhomogeneous decay of a brane-antibrane system. We compute the quantum energy flux induced by the particle creation in this inhomogeneous and time-dependent background. We find that this calculation is essentially equivalent to that of the moving mirror system. In the initial stage, the energy flux turns out to be thermal with the temperature given by the inverse of the distance between the brane and the antibrane. Later it changes its sign and becomes a negative energy flux. Our result may be relevant for the (p)reheating process or/and the evolution of cosmic string network after stringy brane inflation.

hep-th↗

arXiv2008

Constraints on Wrapped DBI Inflation in a Warped Throat

We derive constraints on the tensor to scalar ratio and on the background charge of the warped throat for DBI inflation driven by D5- and D7-branes wrapped over cycles of the throat. It is shown that the background charge well beyond the known maximal value is required in most cases for DBI inflation to generate cosmological observables compatible with the WMAP3 data. Most of the results derived in this paper are insensitive to the details of the inflaton potential, and could be applied to generic warped throats.

hep-th↗

arXiv2008

Rapid roll Inflation with Conformal Coupling

Usual inflation is realized with a slow rolling scalar field minimally coupled to gravity. In contrast, we consider dynamics of a scalar with a flat effective potential, conformally coupled to gravity. Surprisingly, it contains an attractor inflationary solution with the rapidly rolling inflaton field. We discuss models with the conformal inflaton with a flat potential (including hybrid inflation). There is no generation of cosmological fluctuations from the conformally coupled inflaton. We consider realizations of modulated (inhomogeneous reheating) or curvaton cosmological fluctuations in these models. We also implement these unusual features for the popular string-theoretic warped inflationary scenario, based on the interacting D3-anti D3 branes. The original warped brane inflation suffers a large inflaton mass due to conformal coupling to 4-dimensional gravity. Instead of considering this as a problem and trying to cure it with extra engineering, we show that warped inflation with the conformally coupled, rapidly rolling inflaton is yet possible with N=37 efoldings, which requires low energy scales 1-100 TeV of inflation. Coincidentally, the same warping numerology can be responsible for the hierarchy. It is shown that the scalars associated with angular isometries of the warped geometry of compact manifold (e.g. S^3 of KS geometry) have solutions identical to conformally coupled modes and also cannot be responsible for cosmological fluctuations. We discuss other possibilities.

hep-th↗

arXiv2007

Instability of de Sitter brane and horizon entropy in a 6D braneworld

We investigate thermodynamic and dynamical stability of a family of six-dimensional braneworld solutions with de Sitter branes. First, we investigate thermodynamic stability in terms of de Sitter entropy. We see that the family of solutions is divided into two distinct branches: the high-entropy branch and the low-entropy branch. By analogy with ordinary thermodynamics, the high-entropy branch is expected to be stable and the low-entropy branch to be unstable. Next, we investigate dynamical stability by analyzing linear perturbations around the solutions. Perturbations are decomposed into scalar, vector and tensor sectors according to the representation of the 4D de Sitter symmetry, and each sector is analyzed separately. It is found that when the Hubble expansion rates on the branes are too large, there appears a tachyonic mode in the scalar sector and the background solution becomes dynamically unstable. We show analytically that the onset of the thermodynamic instability and that of the dynamical instability exactly coincide. Therefore, the braneworld model provides a new example illustrating close relations between thermodynamic and dynamical instability.

hep-th↗

arXiv2007

Towards a Higgs phase of gravity in string theory

We consider a braneworld scenario with a black brane parallel to our brane, aiming towards a Higgs phase of gravity in string theory. The existence of the black brane spontaneously breaks the Lorentz symmetry on our brane but preserves the rotational and translational invariance. If all moduli are stabilized then this should lead to a Higgs phase of gravity. We investigate moduli stabilization by using a KKLT-type moduli potential in the context of type IIB warped flux compactification.

hep-th↗

arXiv2006

A doubly covariant formula of deficit angle and its application to six-dimensional braneworld

We reformulate boundary conditions for axisymmetric codimension-2 braneworlds in a way which is applicable to linear perturbation with various gauge conditions. Our interest is in the thin brane limit and thus this scheme assumes that the perturbations are also axisymmetric and that the surface energy-momentum tensor of the brane is proportional to its induced metric. An advantage of our scheme is that it allows much more freedom for convenient coordinate choices than the other methods. This is because in our scheme, the coordinate system in the bulk and that on the brane are completely disentangled. Therefore, the latter does not need to be a subset of the former and the brane does not need to stay at a fixed bulk coordinate position. The boundary condition is manifestly doubly covariant: it is invariant under gauge transformations in the bulk and at the same time covariant under those on the brane. We take advantage of the double covariance when we analyze the linear perturbation of a particular model of six-dimensional braneworld with warped flux compactification.

hep-th↗

arXiv2006

Accelerating Universe and Cosmological Perturbation in the Ghost Condensate

In the simplest Higgs phase of gravity called ghost condensation, an accelerating universe with a phantom era (w<-1) can be realized without ghost or any other instabilities. In this paper we show how to reconstruct the potential in the Higgs sector Lagrangian from a given cosmological history (H(t), ρ(t)). This in principle allows us to constrain the potential by geometrical information of the universe such as supernova distance-redshift relation. We also derive the evolution equation for cosmological perturbations in the Higgs phase of gravity by employing a systematic low energy expansion. This formalism is expected to be useful to test the theory by dynamical information of large scale structure in the universe such as cosmic microwave background anisotropy, weak gravitational lensing and galaxy clustering.

hep-th↗

arXiv2006

Dynamics of Gravity in a Higgs Phase

We investigate the universal low-energy dynamics of the simplest Higgs phase for gravity, `ghost condensation.' We show that the nonlinear dynamics of the `ghostone' field dominate for all interesting gravitational sources. Away from caustic singularities, the dynamics is equivalent to the irrotational flow of a perfect fluid with equation of state p \propto ρ^2, where the fluid particles can have negative mass. We argue that this theory is free from catastrophic instabilities due to growing modes, even though the null energy condition is violated. Numerical simulations show that solutions generally have singularities in which negative energy regions shrink to zero size. We exhibit partial UV completions of the theory in which these singularities are smoothly resolved, so this does not signal any inconsistency in the effective theory. We also consider the bounds on the symmetry breaking scale M in this theory. We argue that the nonlinear dynamics cuts off the Jeans instability of the linear theory, and allows M \lsim 100GeV.

hep-ph↗

arXiv2006

Spontaneous Lorentz Breaking at High Energies

Theories that spontaneously break Lorentz invariance also violate diffeomorphism symmetries, implying the existence of extra degrees of freedom and modifications of gravity. In the minimal model (``ghost condensation'') with only a single extra degree of freedom at low energies, the scale of Lorentz violation cannot be larger than about M ~ 100GeV due to an infrared instability in the gravity sector. We show that Lorentz symmetry can be broken at much higher scales in a non-minimal theory with additional degrees of freedom, in particular if Lorentz symmetry is broken by the vacuum expectation value of a vector field. This theory can be constructed by gauging ghost condensation, giving a systematic effective field theory description that allows us to estimate the size of all physical effects. We show that nonlinear effects become important for gravitational fields with strength \sqrtΦ > g, where g is the gauge coupling, and we argue that the nonlinear dynamics is free from singularities. We then analyze the phenomenology of the model, including nonlinear dynamics and velocity-dependent effects. The strongest bounds on the gravitational sector come from either black hole accretion or direction-dependent gravitational forces, and imply that the scale of spontaneous Lorentz breaking is M < Min(10^{12}GeV, g^2 10^{15}GeV). If the Lorentz breaking sector couples directly to matter, there is a spin-dependent inverse-square law force, which has a different angular dependence from the force mediated by the ghost condensate, providing a distinctive signature for this class of models.

hep-th↗

arXiv2006

Dynamical Stability of Six-dimensional Warped Flux Compactification

We show the dynamical stability of a six-dimensional braneworld solution with warped flux compactification recently found by the authors. We consider linear perturbations around this background spacetime, assuming the axisymmetry in the extra dimensions. The perturbations are expanded by scalar-, vector- and tensor-type harmonics of the four-dimensional Minkoswki spacetime and we analyze each type separately. It is found that there is no unstable mode in each sector and that there are zero modes only in the tensor sector, corresponding to the four-dimensional gravitons. We also obtain the first few Kaluza-Klein modes in each sector.

hep-th↗

arXiv2005

Anti-D-brane as Dark Matter in Warped String Compactification

It is pointed out that in the warped string compactification, motion of anti-D-branes near the bottom of a throat behaves like dark matter. Several scenarios for production of the dark matter are suggested, including one based on the D/anti-D interaction at the late stage of D/anti-D inflation.

hep-th↗

arXiv2005

Warped Flux Compactification and Brane Gravity

We find a simple exact solution of 6-dimensional braneworld which captures some essential features of warped flux compactification, including a warped geometry, compactification, a magnetic flux, and one or two 3-brane(s). In this setup we analyze how the Hubble expansion rate on each brane changes when the brane tension changes. It is shown that effective Newton's constant resulting from this analysis agrees with that inferred by simply integrating extra dimensions out. Based on the result, a general formula for effective Newton's constant is conjectured and its application to cosmology with type IIB warped string compactification is discussed.

hep-th↗

arXiv2005

Kaluza-Klein bubble like structure and celestial sphere in inflationary universe

We consider five dimensional deSitter spacetimes with a deficit angle due to the presence of a closed 2-brane and identify one dimension as an extra dimension. From the four dimensional viewpoint we can see that the spacetime has a structure similar to a Kaluza-Klein bubble of nothing, that is, four dimensional spacetime ends at the 2-brane. Since a spatial section of the full deSitter spacetime has the topology of a sphere, the boundary surface surrounds the remaining four dimensional spacetime, and can be considered as the celestial sphere. After the spacetime is created from nothing via an instanton which we describe, some four dimensional observers in it see the celestial sphere falling down, and will be in contact with a 2-brane attached on it.

hep-th↗

arXiv2005

Ghost Condensation and Gravity in Higgs Phase

A tachyon is considered to be sick in the context of particle mechanics, but in field theory just indicates instability of a background. We consider a similar possibility that a ghost in field theory might be just an indication of instability of a background and that it can condense to form a different background around which there is no ghost. We construct a low energy effective field theory based on the derivative expansion around the stable background. Possible applications are discussed, including dark energy, dark matter, inflation and black hole.

hep-th↗

arXiv2005

Black Holes in the Ghost Condensate

We investigate how the ghost condensate reacts to black holes immersed in it. A ghost condensate defines a hypersurface-orthogonal congruence of timelike curves, each of which has the tangent vector u^μ=-g^{μν}\partial_νϕ. It is argued that the ghost condensate in this picture approximately corresponds to a congruence of geodesics. In other words, the ghost condensate accretes into a black hole just like a pressure-less dust. Correspondingly, if the energy density of the ghost condensate at large distance is set to an extremely small value by cosmic expansion then the late-time accretion rate of the ghost condensate should be negligible. The accretion rate remains very small even if effects of higher derivative terms are taken into account, provided that the black hole is sufficiently large. It is also discussed how to reconcile the black hole accretion with the possibility that the ghost condensate might behave like dark matter.

hep-th↗

arXiv2004

Gravity in the dynamical approach to the cosmological constant

One of the most disturbing difficulties in thinking about the cosmological constant is that it is not stable under radiative corrections. The feedback mechanism proposed in [hep-th/0306108] is a dynamical way to protect a zero or small cosmological constant against radiative corrections. Hence, while this by itself does not solve the cosmological constant problem, it can help solving the problem. In the present paper we investigate stability and gravity in this approach and show that the feedback mechanism is both classically and quantum mechanically stable and has self-consistent, stable dynamics.

hep-th↗