Search Results (30 total)

We show that the factor ordering ambiguities associated with the loop quantization of the gravitational part of the cosmological Hamiltonian constraint disappear at the level of the Wheeler-DeWitt equation only for a particular choice of lattice refinement model, which coincides with constraints imposed from phenomenological and consistency arguments.

To test modified Newtonian dynamics (MOND) on galactic scales, we study six strong gravitational lensing early-type galaxies from the CASTLES sample. Comparing the total mass (from lensing) with the stellar mass content (from a comparison of photometry and stellar population synthesis), we conclude that strong gravitational lensing on galactic scales requires a significant amount of dark matter, even within MOND. On such scales a 2 eV neutrino cannot explain the excess of matter in contrast with recent claims to explain the lensing data of the bullet cluster. The presence of dark matter is detected in regions with a higher acceleration than the characteristic MOND scale of ∼10^-10  m/s². This is a serious challenge to MOND unless lensing is qualitatively different [possibly to be developed within a covariant, such as Tensor-Vector-Scalar (TeVeS), theory].

In the context of loop quantum cosmology, we parametrize the lattice refinement by a parameter, A, and the matter Hamiltonian by a parameter, δ. We then solve the Hamiltonian constraint for both a self-adjoint and a non-self-adjoint Hamiltonian operator. Demanding that the solutions for the wave functions obey certain physical restrictions, we impose constraints on the two-dimensional, (A,δ), parameter space, thereby restricting the types of matter content that can be supported by a particular lattice refinement model.

Using a Liouville measure, similar to the one proposed recently by Gibbons and Turok, we investigate the probability that single-field inflation with a polynomial potential can last long enough to solve the shortcomings of the standard hot big bang model, within the semiclassical regime of loop quantum cosmology. We conclude that, for such a class of inflationary models and for natural values of the loop quantum cosmology parameters, a successful inflationary scenario is highly improbable.

We study the importance of lattice refinement in achieving a successful inflationary era. We solve, in the continuum limit, the second order difference equation governing the quantum evolution in loop quantum cosmology, assuming both a fixed and a dynamically varying lattice in a suitable refinement model. We thus impose a constraint on the potential of a scalar field, so that the continuum approximation is not broken. Considering that such a scalar field could play the role of the inflaton, we obtain a second constraint on the inflationary potential so that there is consistency with the cosmic microwave background data on large angular scales. For a m²ϕ²/2 inflationary model, we combine the two constraints on the inflaton potential to impose an upper limit on m, which is severely fine-tuned in the case of a fixed lattice. We thus conclude that lattice refinement is necessary to achieve a natural inflationary model.

Standard D-term inflation is studied in the framework of supergravity. D-term inflation produces cosmic strings; however, it can still be compatible with cosmic microwave background (CMB) measurements without invoking any new physics. The cosmic strings contribution to the CMB data is not constant, nor dominant, contrary to some previous results. Using current CMB measurements, the free parameters (gauge and superpotential couplings, as well as the Fayet-Iliopoulos term) of D-term inflation are constrained.

We study cosmic string formation within supersymmetric grand unified theories (GUTs). We consider gauge groups having a rank between 4 and 8. We examine all possible spontaneous symmetry breaking patterns from the GUT down to the standard model gauge group. Assuming standard hybrid inflation, we select all the models which can solve the GUT monopole problem leading to baryogenesis after inflation, and are consistent with proton lifetime measurements. We conclude that, in all acceptable spontaneous symmetry breaking schemes, cosmic string formation is unavoidable. The strings which form at the end of inflation have a mass which is proportional to the inflationary scale. Sometimes a second network of strings form at a lower scale. Models based on gauge groups which have a rank greater than 6 can lead to more than one inflationary era; they all end by cosmic string formation.

Graviton production due to collapsing extra dimensions is studied. The momenta lying in the extra dimensions are taken into account. A D-dimensional background is matched to an effectively four-dimensional standard radiation-dominated universe. Using observational constraints on the present gravitational wave spectrum, a bound on the maximal temperature at the beginning of the radiation era is derived. This expression depends on the number of extra dimensions, as well as on the D-dimensional Planck mass. Furthermore, it is found that the extra dimensions have to be large.

We study non-Gaussian signatures on the cosmic microwave background (CMB) radiation predicted within inflationary models with non-vacuum initial states for cosmological perturbations. The model incorporates a privileged scale, which implies the existence of a feature in the primordial power spectrum. This broken-scale-invariant model predicts a vanishing three-point correlation function for the CMB temperature anisotropies (or any other odd-numbered-point correlation function) whilst an intrinsic non-Gaussian signature arises for any even-numbered-point correlation function. We thus focus on the first non-vanishing moment, the CMB four-point function at zero lag, namely the kurtosis, and compute its expected value for different locations of the primordial feature in the spectrum, as suggested in the literature to conform with observations of large scale structure. The excess kurtosis is found to be negative and the signal to noise ratio for the dimensionless excess kurtosis parameter is equal to |S/N|≃4×10^-4, almost independently of the free parameters of the model. This signature turns out to be undetectable. We conclude that, subject to current tests, Gaussianity is a generic property of single field inflationary models. The only uncertainty concerning this prediction is that the effect of back reaction has not yet been properly incorporated. The implications for the trans-Planckian problem of inflation are also briefly discussed.

The recently released BOOMERanG data were taken as “contradicting topological defect predictions.” We show that such a statement is partly misleading. Indeed, the presence of a series of acoustic peaks is perfectly compatible with a non-negligible topological defect contribution. In such a mixed perturbation model (inflation and topological defects) for the source of primordial fluctuations, the natural prediction is a slightly lower amplitude for the Doppler peaks, a feature shared by many other purely inflationary models. Thus, for the moment, it seems difficult to rule out these models with the current data.

We compute the energy spectra for massless Kalb-Ramond axions in four-dimensional anisotropic string cosmology models. We show that, when integrated over directions, the four-dimensional anisotropic model leads to infrared divergent spectra similar to the one found in the isotropic case.

In the context of inflation, nonvacuum initial states for cosmological perturbations that possess a built in scale are studied. It is demonstrated that this assumption leads to a falsifiable class of models. The question of whether they lead to conflicts with the available observations is addressed. For this purpose, the power spectrum of the Bardeen potential operator is calculated and compared with the CMBR anisotropies measurements and the redshift surveys of galaxies and clusters of galaxies. Generic predictions of the model are a high first acoustic peak, the presence of a bump in the matter power spectrum and non-Gaussian statistics. The details are controlled by the number of quanta in the nonvacuum initial state. Comparisons with observations show that there exists a window for the free parameters such that good agreement between the data and theoretical predictions is possible. However, in the case where the initial state is a state with a fixed number of quanta, it is shown that this number cannot be greater than a few. On the other hand, if the initial state is a quantum superposition, then a larger class of initial states could account for the observations, even though the state cannot be too different from the vacuum. Planned missions such as the MAP and Planck satellites and the Sloan Survey will demonstrate whether the new class of models proposed here represents a viable alternative to the standard theory.

Pre-big-bang cosmology predicts tiny first-order dilaton and metric perturbations at very large scales. Here we discuss the possibility that other, more copiously generated, perturbations may act, at second order, as scalar seeds of large-scale structure and CMB anisotropies. We study, in particular, the cases of electromagnetic and axionic seeds. We compute the stochastic fluctuations of their energy-momentum tensor and determine the resulting contributions to the multipole expansion of the temperature anisotropy. In the axion case it is possible to obtain a flat or slightly tilted blue spectrum that fits present data consistently, both for massless and for massive (but very light) axions.

We compute cosmic microwave background angular power spectra for scaling seed models of structure formation. A generic parametrization of the energy momentum tensor of the seeds is employed. We concentrate on two regions of parameter space inspired by global topological defects: O(4) texture models and the large- N limit of O(N) models. We use χ² fitting to compare these models to recent flat-band power measurements of the cosmic microwave background. Only scalar perturbations are considered.

We study microwave background anisotropies induced by scaling seed perturbations in a universe dominated by cold dark matter. Using a gauge-invariant linear perturbation analysis, we solve the perturbation equations on super-horizon scales, for CMB anisotropies triggered by generic gravitational seeds. We find that perturbations induced by seeds—under very mild restrictions—are nearly isocurvature. Thus, compensation, which is mainly the consequence of physically sensible initial conditions, is very generic. We then restrict our study to the case of scaling sources, motivated by global scalar fields. We parametrize the energy-momentum tensor of the source by “seed functions” and calculate the Sachs-Wolfe and acoustic contributions to the CMB anisotropies. We discuss the dependence of the anisotropy spectrum on the parameters of the model considered. Even within the restricted class of models investigated in this work, we find a surprising variety of results for the position and height of the first acoustic peak as well as for the overall amplitude. In particular, for certain choices of parameters, the spectrum resembles very much the well known adiabatic inflationary spectrum, whereas for others, the position of the first acoustic peak is significantly shifted towards smaller angular scales.

We examine the scaling properties of an evolving network of strings in Minkowski spacetime and study the evolution of length scales in terms of a three-scale model proposed by Austin, Copeland, and Kibble (ACK). We find good qualitative and some quantitative agreement between the model and our simulations. We also investigate small-scale structure by altering the minimum allowed size for loop production E_c. Certain quantities depend significantly on this parameter: for example, the scaling density can vary by a factor of 2 or more with increasing E_c. Small-scale structure as defined by ACK disappears if no restrictions are placed on loop production, and the fractal dimension of the string changes smoothly from 2 to 1 as the resolution scale is decreased. Loops are nearly all produced at the lattice cutoff. We suggest that the lattice cutoff should be interpreted as corresponding to the string width, and that in a real network loops are actually produced with this size. This leads to a radically different string scenario, with particle production rather than gravitational radiation being the dominant mode of energy dissipation. At the very least, a better understanding of the discretization effects in all simulations of cosmic strings is called for.

We investigate scaling and correlations of the energy and momentum in an evolving network of cosmic strings in Minkowski space. These quantities are of great interest, as they must be understood before accurate predictions for the power spectra of the perturbations in the matter and radiation in the early universe can be made. We argue that Minkowski space provides a reasonable approximation to a Friedmann background for string dynamics and we use our results to construct a simple model of the network, in which it is considered to consist of randomly placed segments moving with random velocities. This model works well in accounting for features of the two-time correlation functions, and even better for the power spectra.

The Doppler peaks (Sacharov peaks) in the angular power spectrum of the cosmic microwave background anisotropies are due mainly to coherent oscillations in the baryon radiation plasma before recombination. Here we present a calculation of the Doppler peaks for perturbations induced by global textures and cold dark matter. We find that the height of the first Doppler peak is smaller than in standard cold dark matter models, and that its position is shifted to ℓ∼350. We believe that our analysis can be easily extended to other types of global topological defects and general global scalar fields.

We show that there are inflationary models for which perturbations in the energy-momentum tensor, which are of second order in the scalar field, cannot be neglected. We first specify the conditions under which the usual first order perturbations are absent. We then analyze classically the growth and decay of our new type of perturbations for one mode of fluctuations δφ_k in the scalar field. We generalize this analysis, considering the contribution from the whole spectrum of δφ to a given wavelength of geometrical perturbations. Finally, we discuss the evolution of the perturbations during the subsequent radiation-dominated era and discuss the resulting spectrum of density fluctuations. In the case of a massless scalar field we find a spectral index n=4. For massive scalar fields we obtain n=0 but the resulting amplitude of fluctuations for inflation around a GUT scale are by far too high. Hence, ‘‘conventional’’ inflationary models must not be influenced by this new type of perturbations, in order to lead acceptable perturbations.

Some of the essential general principles governing cosmic string mechanics in a conformally expanding blackbody radiation background are described. It is shown that the effect of dissipative drag damping may be given a strictly conservative (i.e., variational) representation in which the usual Goto-Nambu action is simply multiplied by an appropriate cosmological temperature-dependent conformal factor. A simplified thermodynamic description is used to investigate approximately stationary equilibrium states such as may occasionally be produced as the long term outcome of large scale damping in the case of a cosmic string loop for which the (thermal or more general) distribution of surviving microscopic wiggles on an isolated cosmic string loop is characterized by a strong preponderance of ‘‘right movers’’ over ‘‘left movers’’ (or vice versa). For nonsuperconducting strings, such states can be represented very simply using the nondispersive ‘‘warm’’ cosmic string model whose dynamics is characterized by a pair of ‘‘left’’- and ‘‘right’’-moving characteristic surface currents that will be independently conserved so long as the effective heat loss to the environment is negligible. It is predicted that one of these currents will still remain conserved in the long run when account is taken of radiative energy loss from the approximately stationary equilibrium state, which will evolve with negative specific heat, monotonically increasing its effective temperature as it contracts.

We derive the effective action for a domain wall with a small thickness in curved spacetime and show that, apart from the Nambu term, it includes a contribution proportional to the induced curvature. We then use this action to study the dynamics of a spherical thick bubble of false vacuum (de Sitter) surrounded by an infinite region of true vacuum (Schwarzschild).

We study the evolution of cosmic strings taking into account the frictional force due to the surrounding radiation. We consider small perturbations on straight strings, oscillation of circular loops, and small perturbations on circular loops. For straight strings, friction exponentially suppresses perturbations whose comoving scale crosses the horizon before the cosmological time t_*∼μ^-2 (in Planck units), where μ is the string tension. Loops with a size much smaller than t_* will be approximately circular at the time when they start the relativistic collapse. We investigate the possibility that such loops will form black holes. We find that the number of black holes which are formed through this process is well below present observational limits, so this does not give any lower or upper bounds on μ. We also consider the case of straight strings attached to walls and circular holes that can spontaneously nucleate on metastable domain walls.

We present a class of exact spherically symmetric solutions to the constraint equations of general relativity coupled to a Klein-Gordon field. We analyze the subsequent evolution of the consistent Cauchy data represented by those solutions, showing that only certain special initial conditions eventually lead to successful inflationary cosmologies. We argue, however, that these initial conditions are precisely the likely outcome of quantum events which occurred before the inflationary era.

We discuss the onset of inflation in an inhomogeneous, asymptotically Friedmann-Robertson-Walker universe coupled to a scalar inflaton field. We consider a three-parameter family of inhomogeneous Cauchy data, for which we can solve analytically the constraint equations. Inflation only occurs if the Cauchy data are homogeneous over several horizon lengths.

Making use of the relationship between the corresponding field configurations, we derive the metric around a straight global cosmic string with traveling waves in terms of the static metric (without traveling waves) in the weak-field limit. We discuss under which conditions the effect of the traveling waves may overcome the repulsive gravitational potential of the static straight global string. We also extend the calculation beyond the weak-field limit combining our result with the recent observation made by Garfinkle and Vachaspati that the exact solution must be of the generalized Kerr-Schild type.