1,721,008 research outputs found
Symmetries of primordial perturbations
No full textCreminelli, Paolo. (2011). Symmetries of primordial perturbations. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/118941
Inequivalence of coset constructions for spacetime symmetries: Coupling with gravity
No full textWe study how the coupling with gravity of theories with non-linearly realized space-time symmetries is modified when one changes the parametrization of the coset. As an example, we focus on the so-called Galileon duality: a reparametrization which maps a Galilean invariant action into another one which enjoys the same symmetry. Starting with a standard coupling with gravity, with a parametric separation between the Planck scale and the typical scale of the coset, one ends up with a theory without such a separation. In particular an infinite set of higher-dimension operators are relevant when the superluminality of the Galileon is measurable in the effective theory. This addresses an apparent paradox since superluminality arises in the dual theory even when absent in the original one
The effective theory of quintessence: The w<-1 side unveiled
No full textWe study generic single-field dark energy models, by a parametrization of the most general theory of their perturbations around a given background, including higher derivative terms. In appropriate limits this approach reproduces standard quintessence, k-essence and ghost condensation. We find no general pathology associated to an equation of state w Q<-1 or in crossing the phantom divide w Q = -1. Stability requires that the w Q<-1 side of dark energy behaves, on cosmological scales, as a k-essence fluid with a virtually zero speed of sound. This implies that one should set the speed of sound to zero when comparing with data models with w Q<-1 or crossing the phantom divide. We summarize the theoretical and stability constraints on the quintessential plane (1+w Q) vs. speed of sound squared. © 2009 IOP Publishing Ltd and SISSA
Inequivalence of coset constructions for spacetime symmetries
Get PDFNon-linear realizations of spacetime symmetries can be obtained by a
generalization of the coset construction valid for internal ones. The physical
equivalence of different representations for spacetime symmetries is not
obvious, since their relation involves not only a redefinition of the fields
but also a field-dependent change of coordinates. A simple and relevant
spacetime symmetry is obtained by the contraction of the 4D conformal group
that leads to the Galileon group. We analyze two non-linear realizations of
this group, focusing in particular on the propagation of signals around
non-trivial backgrounds. The aperture of the lightcone is in general different
in the two representations and in particular a free (luminal) massless scalar
is mapped in a Galileon theory which admits superluminal propagation. We show
that in this theory, if we consider backgrounds that vanish at infinity, there
is no asymptotic effect: the displacement of the trajectory integrates to zero,
as can be expected since the S-matrix is trivial. Regarding local measurements,
we show that the puzzle is solved taking into account that a local coupling
with fixed sources in one theory is mapped into a non-local coupling and we
show that this effect compensates the different lightcone. Therefore the two
theories have a different notion of locality. The same applies to the different
non-linear realizations of the conformal group and we study the particular case
of a cosmologically interesting background: the Galilean Genesis scenarios
Stability of geodesically complete cosmologies
Get PDFWe study the stability of spatially flat FRW solutions which are geodesically
complete, i.e. for which one can follow null (graviton) geodesics both in the
past and in the future without ever encountering singularities. This is the
case of NEC-violating cosmologies such as smooth bounces or solutions which
approach Minkowski in the past. We study the EFT of linear perturbations around
a solution of this kind, including the possibility of multiple fields and
fluids. One generally faces a gradient instability which can be avoided only if
the operator is present and its coefficient changes sign
along the evolution. This operator (typical of beyond-Horndeski theories) does
not lead to extra degrees of freedom, but cannot arise starting from any theory
with second-order equations of motion. The change of sign of this operator
prevents to set it to zero with a generalised disformal transformation
Spherical collapse in quintessence models with zero speed of sound
No full textWe study the spherical collapse model in the presence of quintessence with zero speed of sound. This case is particularly motivated for w<-1 as it is required by stability. As pressure gradients are negligible, quintessence follows dark matter during the collapse. The spherical overdensity behaves as a separate closed FLRW universe, so that its evolution can be studied exactly. We derive the critical overdensity for collapse and we use the extended Press-Schechter theory to study how the clustering of quintessence affects the dark matter mass function. The effect is dominated by the modification of the linear dark matter growth function. A larger effect occurs on the total mass function, which includes the quintessence overdensities. Indeed, here quintessence constitutes a third component of virialized objects, together with baryons and dark matter, and contributes to the total halo mass by a fraction ~ (1+w) Omega_Q / Omega_m. This gives a distinctive modification of the total mass function at low redshift
Holographic evolution of gauge couplings
No full textWe study the gauge coupling evolution of a unified theory in the compact Randall-Sundrum model with gauge bosons propagating in the bulk. One-loop corrections in AdS are interpreted in the 4d dual theory as the sum of two contributions: CFT insertions subleading in a 1/N expansion and loops of the additional particles coupled to the CFT. We have calculated the scalar loop correction to the low energy gauge couplings both in scenarios where the GUT symmetry is broken by boundary conditions and with the Higgs mechanism. In each case our results are what expected from the holographic dual theory
Sachs-Wolfe at second order: the CMB bispectrum on large angular scales
No full textWe calculate the Cosmic Microwave Background anisotropy bispectrum on large angular scales in the absence of primordial non-Gaussianities, assuming exact matter dominance and extending at second order the classic Sachs-Wolfe result delta T/T = Phi/3. The calculation is done in Poisson gauge. Besides intrinsic contributions calculated at last scattering, one must consider integrated effects. These are associated to lensing, and to the time dependence of the potentials (Rees-Sciama) and of the vector and tensor components of the metric generated at second order. The bispectrum is explicitly computed in the flat-sky approximation. It scales as l(-4) in the scale invariant limit and the shape dependence of its various contributions is represented in 3d plots. Although all the contributions to the bispectrum are parametrically of the same order, the full bispectrum is dominated by lensing. In the squeezed limit it corresponds to f(NL)(local) = -1/6 - cos(2 theta), where theta is the angle between the short and the long modes; the angle dependent contribution comes from lensing. In the equilateral limit it corresponds to f(NL)(equil) similar or equal to 3.13.It is a pleasure to thank Nicola Bartolo, Francis Bernardeau, Eiichiro Komatsu, Roy Maartens,Sabino Matarrese, Toni Riotto, Misao Sasaki, Uros Seljak, Leonardo Senatore and Matias Zaldarriaga for useful discussions. G.D’A. and P.C. thank the Institut de Physique Th´eorique at Saclay and
F.V. thanks the ICTP for hospitality while working on this project. G.D’A., P.C., and F.V. thank the Galileo Galilei Institute, where part of this work was carried out during the workshop “New Horizons for Modern Cosmology”. Furthermore, F.V. thanks the Yukawa Institute for Theoretical Physics at Kyoto University, where part of this work was carried out during the GCOE/YITP workshop
YITP-W-09-01 on “Non-linear cosmological perturbations” and the EU Marie Curie Research and Training network ”UniverseNet” (MRTN-CT-2006-035863) for support..Peer reviewe
Relaxing the cosmological constant: a proof of concept
Get PDFWe propose a technically natural scenario whereby an initially large cosmological constant (c.c.) is relaxed down to the observed value due to the dynamics of a scalar evolving on a very shallow potential. The model crucially relies on a sector that violates the null energy condition (NEC) and gets activated only when the Hubble rate becomes sufficiently small — of the order of the present one. As a result of NEC violation, this low-energy universe evolves into inflation, followed by reheating and the standard Big Bang cosmology. The symmetries of the theory force the c.c. to be the same before and after the NEC-violating phase, so that a late-time observer sees an effective c.c. of the correct magnitude. Importantly, our model allows neither for eternal inflation nor for a set of possible values of dark energy, the latter fixed by the parameters of the theory
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