arXiv subjects

Mairi Sakellariadou

Publications and source records attributed to Mairi Sakellariadou.

At least 163 records · Page 9Linked to original sources

arXiv2008

Cusps on cosmic superstrings with junctions

The existence of cusps on non-periodic strings ending on D-branes is demonstrated and the conditions, for which such cusps are generic, are derived. The dynamics of F-, D-string and FD-string junctions are investigated. It is shown that pairs of FD-string junctions, such as would form after intercommutations of F- and D-strings, generically contain cusps. This new feature of cosmic superstrings opens up the possibility of extra channels of energy loss from a string network. The phenomenology of cusps on such cosmic superstring networks is compared to that of cusps formed on networks of their field theory analogues, the standard cosmic strings.

hep-th↗

arXiv2008

Dynamics of F/D networks: the role of bound states

We study, via numerical experiments, the role of bound states in the evolution of cosmic superstring networks, being composed by p F-strings, q D-strings and (p,q) bound states. We find robust evidence for scaling of all three components of the network, independently of initial conditions. The novelty of our numerical approach consists of having control over the initial abundance of bound states. This indeed allows us to identify the effect of bound states on the evolution of the network. Our studies also clearly show the existence of an additional energy loss mechanism, resulting to a lower overall string network energy, and thus scaling of the network. This new mechanism consists of the formation of bound states with an increasing length.

hep-th↗

arXiv2008

Unique factor ordering in the continuum limit of LQC

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

gr-qc↗

arXiv2008

Numerical techniques for solving the quantum constraint equation of generic lattice-refined models in loop quantum cosmology

To avoid instabilities in the continuum semi-classical limit of loop quantum cosmology models, refinement of the underlying lattice is necessary. The lattice refinement leads to new dynamical difference equations which, in general, do not have a uniform step-size, implying complications in their analysis and solutions. We propose a numerical method based on Taylor expansions, which can give us the necessary information to calculate the wave-function at any given lattice point. The method we propose can be applied in any lattice-refined model, while in addition the accuracy of the method can be estimated. Moreover, we confirm numerically the stability criterion which was earlier found following a von Neumann analysis. Finally, the `motion' of the wave-function due to the underlying discreteness of the space-time is investigated, for both a constant lattice, as well as lattice refinement models.

gr-qc↗

arXiv2008

Cosmic Superstrings

Cosmic superstrings are expected to be formed at the end of brane inflation, within the context of brane-world cosmological models inspired from string theory. By studying properties of cosmic suprestring networks, and comparing their phenomenological consequences against observational data, we aim at pinning down the successful and natural inflationary model and get an insight into the stringy description of our universe.

hep-th↗

arXiv2008

On the possibility of Dark Energy from corrections to the Wheeler-De Witt equation

We present a method for approximating the effective consequence of generic quantum gravity corrections to the Wheeler-DeWitt equation. We show that in many cases these corrections can produce departures from classical physics at large scales and that this behaviour can be interpreted as additional matter components. This opens up the possibility that dark energy (and possible dark matter) could be large scale manifestations of quantum gravity corrections to classical general relativity. As a specific example we examine the first order corrections to the Wheeler-De Witt equation arising from loop quantum cosmology in the absence of lattice refinement and show how the ultimate breakdown in large scale physics occurs.

gr-qc↗

arXiv2007

Numerical experiments with p F- and q D-strings: the formation of (p,q) bound states

We investigate the behaviour of (p,q) string networks, focusing on two aspects: (1) modelling more realistic (p,q) string networks than the Z_N networks used so far and (2) investigating the effect of long-range interactions on the evolution of the network. We model the network with no long-range interactions using two sets of fields, complex scalars coupled to gauge fields, with a potential chosen such that the two types of strings will form bound states. This way we can model junctions of 3 strings with different tension; in Z_N models used so far in simulations all the strings have identical tensions. In order to introduce long-range interactions we also study a network in which one of the scalars forms global strings. We observe that in the absence of long-range interactions the formation of bound states has a significant influence on the evolution of the network. When long-range interactions are turned on the bound states are short-lived and have a minimal effect on the network evolution.

hep-th↗

arXiv2007

The necessity of dark matter in MOND within galactic scales

To further test MOdified Newtonian Dynamics (MOND) on galactic scales -- originally proposed to explain the rotation curves of disk galaxies without dark matter -- we study a sample of six strong gravitational lensing early-type galaxies from the CASTLES database. To determine whether dark matter is present in these galaxies, we compare the total mass (from lensing) with the stellar mass content (from a comparison of photometry and stellar population synthesis). We find 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 this excess 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 MONDian scale of $\sim 10^{-10}$m/s$^2$. This is a serious challenge to MOND unless the proper treatment of lensing is qualitatively different (possibly to be developed within a consistent theory such as TeVeS).

astro-ph↗

arXiv2007

Lattice Refining LQC and the Matter Hamiltonian

In the context of loop quantum cosmology, we parametrise 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.

gr-qc↗

arXiv2007

On the Onset of Inflation in Loop Quantum Cosmology

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.

gr-qc↗

arXiv2007

Relativistic Modified Newtonian Dynamics from String Theory?

We argue that TeVeS-like vector fields appear naturally in certain string theory backgrounds involving D0-branes, as a result of the recoil velocity field, expressing the interaction of neutrino string matter with point-like branes. However, the similarity with TeVeS models is restricted only to the bi-metric properties of space time, namely the difference of the background metric from the one felt by (some) matter fields interacting, in a topologically non-trivial manner, with the D0-brane defects. In our approach, neutrinos appear as dark matter candidates that could be ``captured'' by the D0 branes, as a result of stringy properties, and thus couple with the recoil-vector fields. Moreover, we argue in support of a possibly preferential role of neutrinos in inducing novel non-perturbative contributions to ``vacuum'' (dark) energy, in addition to their ordinary dark matter contribution. In fact, the role of neutrinos as providing substantial contributions to dark matter and dark energy components of the Universe, suggested by our approach, appears also to be necessitated by the need to reproduce the peaks in the Cosmic Microwave Background radiation spectrum, as claimed recently in the literature. Thus, our framework may be viewed as providing a microscopic explanation of such phenomenological conclusions concerning TeVeS-like, Lorentz-violating models.

hep-th↗

arXiv2007

Lattice Refining Loop Quantum Cosmology and Inflation

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 quantun 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 CMB data on large angular scales. For a $m^2ϕ^2/2$ 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.

gr-qc↗

arXiv2007

Cosmological evolution of cosmic string loops

The existence of a scaling evolution for cosmic string loops in an expanding universe is demonstrated for the first time by means of numerical simulations. In contrast with what is usually assumed, this result does not rely on any gravitational back reaction effect and has been observed for loops as small as a few thousandths the size of the horizon. We give the energy and number densities of expected cosmic string loops in both the radiation and matter eras. Moreover, we quantify previous claims on the influence of the network initial conditions and the formation of numerically unresolved loops by showing that they only concern a transient relaxation regime. Some cosmological consequences are discussed.

astro-ph↗

arXiv2007

Cosmic Strings

Cosmic strings, a hot subject in the 1980's and early 1990's, lost its appeal when it was found that it leads to inconsistencies in the power spectrum of the measured cosmic microwave background temperature anisotropies. However, topological defects in general, and cosmic strings in particular, are deeply rooted in the framework of grand unified theories. Indeed, it was shown that cosmic strings are expected to be generically formed within supersymmetric grand unified theories. This theoretical support gave a new boost to the field of cosmic strings, a boost which has been recently enhanced when it was shown that cosmic superstrings (fundamental or one-dimensional Dirichlet branes) can play the role of cosmic strings, in the framework of braneworld cosmologies. To build a cosmological scenario we employ high energy physics; inflation and cosmic strings then naturally appear. Confronting the predictions of the cosmological scenario against current astrophysical/cosmological data we impose constraints on its free parameters, obtaining information about the high energy physics we employed. This is a beautiful example of the rich and fruitful interplay between cosmology and high energy physics.

hep-th↗

arXiv2007

Production of topological defects at the end of inflation

Hybrid inflation within supersymmetric grand unified theories, as well as inflation through brane collisions within braneworld cosmological models, lead to the formation of one-dimensional defects. Observational data, mainly from the cosmic microwave background temperature anisotropies but also from the gravitational wave background, impose constraints on the free parameters of the models. I review these inflationary models and discuss the constraints from the currently available data.

hep-th↗

arXiv2007

The origin of spacetime dimensionality

I address the issue of spacetime dimensionality within Kaluza-Klein theories and theories with large extra dimensions. I review the arguments explaining the dimensionality of the universe, within the framework of string gas cosmology and braneworld cosmology, respectively.

hep-th↗

arXiv2006

D-term inflation in non-minimal supergravity

D-term inflation is one of the most interesting and versatile models of inflation. It is possible to implement naturally D-term inflation within high energy physics, as for example SUSY GUTs, SUGRA, or string theories. D-term inflation avoids the $η$-problem, while in its standard form it always ends with the formation of cosmic strings. Given the recent three-year WMAP data on the cosmic microwave background temperature anisotropies, we examine whether D-term inflation can be successfully implemented in non-minimal supergravity theories. We show that for all our choices of Kähler potential, there exists a parameter space for which the predictions of D-term inflation are in agreement with the measurements. The cosmic string contribution on the measured temperature anisotropies is always dominant, unless the superpotential coupling constant is fine tuned; a result already obtained for D-term inflation within minimal supergravity. In conclusion, cosmic strings and their rôle in the angular power spectrum cannot be easily hidden by just considering a non-flat Kähler geometry.

hep-th↗

arXiv2006

Scattering of cosmic strings by black holes: loop formation

We study the deformation of a long cosmic string by a nearby rotating black hole. We examine whether the deformation of a cosmic string, induced by the gravitational field of a Kerr black hole, may lead to the formation of a loop of cosmic string. The segment of the string which enters the ergosphere of a rotating black hole gets deformed and, if it is sufficiently twisted, it can self-intersect chopping off a loop of cosmic string. We find that the formation of a loop, via this mechanism, is a rare event. It will only arise in a small region of the collision phase space, which depends on the string velocity, the impact parameter and the black hole angular momentum. We conclude that generically, the cosmic string is simply scattered or captured by the rotating black hole.

gr-qc↗