36 research outputs found
Inflation and the quantum measurement problem
We propose a solution to the quantum measurement problem in inflation. Our model treats Fourier modes of cosmological perturbations as analogous to particles in a weakly interacting Bose gas. We generalize the idea of a macroscopic wave function to cosmological fields, and construct a self-interaction Hamiltonian that focuses that wave function. By appropriately setting the coupling between modes, we obtain the standard adiabatic, scale-invariant power spectrum. Because of central limit theorem, we recover a Gaussian random field, consistent with observations
DSR as an explanation of cosmological structure
Deformed special relativity (DSR) is one of the possible realizations of a
varying speed of light (VSL). It deforms the usual quadratic dispersion
relations so that the speed of light becomes energy dependent, with preferred
frames avoided by postulating a non-linear representation of the Lorentz group.
The theory may be used to induce a varying speed of sound capable of generating
(near) scale-invariant density fluctuations, as discussed in a recent Letter.
We identify the non-linear representation of the Lorentz group that leads to
scale-invariance, finding a universal result. We also examine the higher order
field theory that could be set up to represent it
Chirality of tensor perturbations for complex values of the Immirzi parameter
In this paper we generalise previous work on tensor perturbations in a de
Sitter background in terms of Ashtekar variables to cover all complex values of
the Immirzi parameter gamma (previous work was restricted to imaginary gamma).
Particular attention is paid to the case of real gamma. Following the same
approach as in the imaginary case, we can obtain physical graviton states by
invoking reality and torsion free conditions. The Hamiltonian in terms of
graviton states has the same form whether gamma has a real part or not; however
changes occur for the vacuum energy and fluctuations. Specifically, we observe
a gamma dependent chiral asymmetry in the vacuum fluctuations only if gamma has
an imaginary part. Ordering prescriptions also change this asymmetry. We thus
present a measurable result for CMB polarisation experiments that could shed
light on the workings of quantum gravity.Comment: 6 pages, 1 figur
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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
Rainbow universe
The formalism of rainbow gravity is studied in a cosmological setting. We
consider the very early universe which is radiation dominated. A novel
treatment in our paper is to look for an ``averaged'' cosmological metric
probed by radiation particles themselves. Taking their cosmological evolution
into account, we derive the modified Friedmann-Robertson-Walker(FRW) equations
which is a generalization of the solution presented by Magueijo and Smolin.
Based on this phenomenological cosmological model we argue that the spacetime
curvature has an upper bound such that the cosmological singularity is absent.
These modified equations can be treated as effective equations in the
semi-classical framework of quantum gravity and its analogy with the one
recently proposed in loop quantum cosmology is also discussed.Comment: 5 page
A generalized Hartle-Hawking wave function
The Hartle–Hawking wave function is known to be the Fourier dual of the Chern–Simons or Kodama state reduced to mini-superspace, using an integration contour covering the whole real line. But since the Chern–Simons state is a solution of the Hamiltonian constraint (with a given ordering), its Fourier dual should provide a solution (i.e. beyond mini-superspace) of the Wheeler DeWitt equation representing the Hamiltonian constraint in the metric representation. We write down a formal expression for such a wave function, to be seen as the generalization beyond mini-superspace of the Hartle–Hawking wave function. Its explicit evaluation (or simplification) depends only on the symmetries of the problem, and we illustrate the procedure with anisotropic Bianchi models and with the Kantowski–Sachs model. A significant difference of this approach is that we may leave the torsion inside the wave functions when we set up the ansatz for the connection, rather than setting it to zero before quantization. This allows for quantum fluctuations in the torsion, with far reaching consequences
Modified (A)dS Schwarzschild black holes in Rainbow spacetime
A modified (Anti-)de Sitter Schwarzschild black hole solution is presented in
the framework of rainbow gravity with a cosmological constant. Its
thermodynamical properties are investigated. In general the temperature of
modified black holes is dependent on the energy of probes which take the
measurement. However, a notion of intrinsic temperature can be introduced by
identifying these probes with radiation particles emitted from black holes. It
is interesting to find that the Hawking temperature of this sort of black holes
can be reproduced by employing the extended uncertainty principle and modified
dispersion relations to the ordinary (A)dS Schwarzschild black holes.Comment: 11 pages. The version to appear in CQ
Qualitative Analysis of Universes with Varying Alpha
Assuming a Friedmann universe which evolves with a power-law scale factor,
, we analyse the phase space of the system of equations that describes
a time-varying fine structure 'constant', , in the
Bekenstein-Sandvik-Barrow-Magueijo generalisation of general relativity. We
have classified all the possible behaviours of in ever-expanding
universes with different and find new exact solutions for . We
find the attractors points in the phase space for all . In general, will be a non-decreasing function of time that increases logarithmically in
time during a period when the expansion is dust dominated (), but
becomes constant when . This includes the case of negative-curvature
domination (). also tends rapidly to a constant when the
expansion scale factor increases exponentially. A general set of conditions is
established for to become asymptotically constant at late times in an
expanding universe.Comment: 26 pages, 6 figure
The Polarization of the Cosmic Microwave Background Due to Primordial Gravitational Waves
We review current observational constraints on the polarization of the Cosmic
Microwave Background (CMB), with a particular emphasis on detecting the
signature of primordial gravitational waves. We present an analytic solution to
the Polanarev approximation for CMB polarization produced by primordial
gravitational waves. This simplifies the calculation of the curl, or B-mode
power spectrum associated with gravitational waves during the epoch of
cosmological inflation. We compare our analytic method to existing numerical
methods and also make predictions for the sensitivity of upcoming CMB
polarization observations to the inflationary gravitational wave background. We
show that upcoming experiments should be able either detect the relic
gravitational wave background or completely rule out whole classes of
inflationary models.Comment: 25 pages, 4 figures, review published in IJMP
Gravity waves in parity-violating Copernican universes
In recent work minimal theories allowing the variation of the cosmological constant, Λ , by means of a balancing torsion, have been proposed. It was found that such theories contain parity violating homogeneous and isotropic solutions, due to a torsion structure called the Cartan spiral staircase. Their dynamics are controlled by Euler and Pontryagin quasitopological terms in the action. Here we show that such theories predict a dramatically different picture for gravitational wave fluctuations in the parity violating branch. If the dynamics are ruled solely by the Euler-type term, then linear tensor mode perturbations are entirely undetermined, hinting at a new type of gauge invariance. The Pontryagin term not only permits for phenomenologically sounder background solutions (as found in previous literature), but for realistic propagation of gravitational wave modes. These have the general property that the right and left handed gravitational waves propagate with different speeds. More generally they imply modified dispersion relations for the graviton, with both parity violating and non-violating deformations, including an effective mass for both gravitational wave polarizations. We discuss the observational constraints and predictions of these theories