Superconducting Semilocal Stringy (Hopf) Textures

The dynamics of texture-like configurations are briefly reviewed. Emphasis is given to configurations in 2+1 dimensions which are constructed numerically. Confirming previous semi-analytical studies it is shown that they can be stabilized by partial gauging of the vacuum manifold (semilocality) in a finite range of parameter space. When these configurations are extended to 3+1 dimensions (stringy textures) it is shown that they can support persistent currents if a twist (Hopf charge) is introduced in the scalar field sector. The pressure induced by these persistent currents is also studied in closed loops. In the context of a simple model, twist induced pressure is shown to be insufficient to stabilize the loops against collapse due to tensionComment: Talk presented at the NATO Advanced Study Institute of the ESF Network on 'Topological Defects and the Non-Equilibrium Dynamics of Symmetry Breaking Phase Transitions' at Les Houches, France 16-26/2/1999. 8 pages of two column revtex, 5 figure

LCDM: Triumphs, Puzzles and Remedies

The consistency level of LCDM with geometrical data probes has been increasing with time during the last decade. Despite of these successes, there are some puzzling conflicts between LCDM predictions and dynamical data probes (bulk flows, alignment and magnitude of low CMB multipoles, alignment of quasar optical polarization vectors, cluster halo profiles). Most of these puzzles are related to the existence of preferred anisotropy axes which appear to be unlikely close to each other. A few models that predict the existence of preferred cosmological axes are briefly discussed.Comment: 9 pages, 1 figure. Invited talk at the `New Directions in Modern Cosmology' workshop (Lorentz Center, Leiden Sep. 2010). To appear in the workshop proceeding

Gravitational Interactions of Finite Thickness Global Topological Defects with Black Holes

It is well known that global topological defects induce a repulsive gravitational potential for test particles. 'What is the gravitational potential induced by black holes with a cosmological constant (Schwarzschild-de Sitter (S-dS) metric) on finite thickness global topological defects?'. This is the main question addressed in the present analysis. We also discuss the validity of Derrick's theorem when scalar fields are embedded in non-trivial gravitational backgrounds. In the context of the above question, we consider three global defect configurations: a finite thickness spherical domain wall with a central S-dS black hole, a global string loop with a S-dS black hole in the center and a global monopole near a S-dS black hole. Using an analytical model and numerical simulations of the evolving spherical wall we show that the spherical wall experiences a repelling gravitational potential due to the mass of the central black hole. This potential is further amplified by the presence of a cosmological constant. For initial domain wall radius larger than a critical value, the repulsive potential dominates over the wall tension and the wall expands towards the cosmological horizon of the S-dS metric where it develops ghost instabilities. For smaller initial radius, tension dominates and the wall contracts towards the black hole horizon where it also develops ghost instabilities. We also show, using the same analytical model and energetic arguments that a global monopole is gravitationally attracted by a black hole while a cosmological constant induces a repulsive gravitational potential as in the case of test particles. Finally we show that a global string loop with finite thickness experiences gravitational repulsion due to the cosmological constant which dominates over its tension for a radius larger than a critical radius leading to an expanding rather than contracting loop.Comment: 13 pages, 9 Figures. The Mathematica file used for the numericala analysis and the construction of the Figures of the paper may be downloaded from http://leandros.physics.uoi.gr/defects-gravity

Scalar-Tensor Quintessence with a linear potential: Avoiding the Big Crunch cosmic doomsday

All quintessence potentials that are either monotonic with negative interval or have a minimum at negative values of the potential, generically predict a future collapse of the scale factor to a "doomsday" singularity. We show that this doomsday is generically avoided in models with a proper non-minimal coupling of the quintessence scalar field to the curvature scalar $R$. For simplicity we consider linear quintessence potential $V=-s\phi$ and linear non-minimal coupling $F=1-\lambda \phi$. However our result is generic and is due to the fact that the non-minimal coupling modifies the effective potential that determines the dynamics of the scalar field. Thus for each positive value of the parameter $s$ we find a critical value $\lambda_{crit}(s)$ such that for $\lambda>\lambda_{crit}(s)$ the negative potential energy does not dominate the universe and the cosmic doomsday Big Crunch singularity is avoided because the scalar field eventually rolls up its potential. We find that $\lambda_{crit}(s)$ increases approximately linearly with $s$. For $\lambda>\lambda_{crit}(s)$ the potential energy of the scalar field becomes positive and it eventually dominates while the dark energy equation of state parameter tends to $w=-1$ leading to a deSitter Universe.Comment: 6 pages, 5 figures. Extended version. Accepted in Phys. Rev. D as regular article (to appear

Core Phase Transitions for Embedded Topological Defects

Vortices in superfluid 3He-B have been observed to undergo a core transition. We discuss the analog phenomenon in relativistic field theories which admit embedded global domain walls, vortices and monopoles with a core phase structure. They are present in scalar field theories with approximate global symmetries which are broken both spontaneously and in parts explicitly. For a particular range of parameters their symmetric core exhibits an instability and decays into the nonsymmetric phase.Comment: Talk presented at the "Formation of Topological Defects" ESF Network Meeting, Grenoble, France, September 199

Reconstructing a Model for Gravity at Large Distances from Dark Matter Density Profiles

Using the Navarro-Frenk-White (NFW) dark matter density profile we reconstruct an effective field theory model for gravity at large distances from a central object by demanding that the vacuum solution has the same gravitational properties as the NFW density profile has in the context of General Relativity (GR). The dimensionally reduced reconstructed action for gravity leads to a vacuum metric that includes a modified Rindler acceleration term in addition to the Schwarzschild and cosmological constant terms. The new term is free from infrared curvature singularities and leads to a much better fit of observed galaxy velocity rotation curves than the corresponding simple Rindler term of the Grumiller metric, at the expense of one additional parameter. When the new parameter is set to zero the new metric term reduces to a Rindler constant acceleration term. We use galactic velocity rotation data to find the best fit values of the parameters of the reconstructed geometric potential and discuss possible cosmological implications.Comment: 13 pages, 2 figures. The Mathematica file with the code for the construction of the Figures may be downloaded from http://leandros.physics.uoi.gr/dim-reduction

Tension and Systematics in the Gold06 SnIa Dataset

The Gold06 SnIa dataset recently released in astro-ph/0611572 consists of five distinct subsets defined by the group or instrument that discovered and analyzed the corresponding data. These subsets are: the SNLS subset (47 SnIa), the HST subset (30 SnIa), the HZSST subset (41 SnIa), the SCP subset (26 SnIa) and the Low Redshift (LR) subset (38 SnIa). These subsets sum up to the 182 SnIa of the Gold06 dataset. We use Monte-Carlo simulations to study the statistical consistency of each one of the above subsets with the full Gold06 dataset. In particular, we compare the best fit $w(z)$ parameters (w_0,w_1) obtained by subtracting each one of the above subsets from the Gold06 dataset (subset truncation), with the corresponding best fit parameters (w^r_0,w^r_1) obtained by subtracting the same number of randomly selected SnIa from the same redshift range of the Gold06 dataset (random truncation). We find that the probability for (w^r_0,w^r_1)=(w_0,w_1) is large for the Gold06 minus SCP (Gold06-SCP) truncation but is less than 5% for the Gold06-SNLS, Gold06-HZSST and Gold06-HST truncations. This result implies that the Gold06 dataset is not statistically homogeneous. By comparing the values of the best fit (w_0,w_1) for each subset truncation we find that the tension among subsets is such that the SNLS and HST subsets are statistically consistent with each other and `pull' towards LCDM (w_0=-1,w_1=0) while the HZSST subset is statistically distinct and strongly `pulls' towards a varying w(z) crossing the line $w=-1$ from below (w_00). We also isolate six SnIa that are mostly responsible for this behavior of the HZSST subset.Comment: 10 pages, 6 Figures. References added. The mathematica files with the numerical analysis of the paper may be found at http://leandros.physics.uoi.gr/gold06/gold06.ht

Propagation of gravitational waves in an expanding background in the presence of a point mass

We solve the Laplace equation $\Box h_{ij}=0$ describing the propagation of gravitational waves in an expanding background metric with a power law scale factor in the presence of a point mass in the weak field approximation (Newtonian McVittie background). We use boundary conditions at large distances from the mass corresponding to a standing spherical gravitational wave in an expanding background which is equivalent to a linear combination of an incoming and an outgoing propagating gravitational wave. We compare the solution with the corresponding solution in the absence of the point mass and show that the point mass increases the amplitude of the wave and also decreases its frequency (as observed by an observer at infinity) in accordance with gravitational time delay.Comment: 10 pages, 6 figures. The Mathematica files with the numerical analysis of this study may be downloaded from https://drive.google.com/file/d/0B7rg6X3QljQXck5YSmQ5Rl9HeUU/view . Minor modifications compared to previous version (typos and improved presentation). Accepted in Phys. Rev. D. To appea

Sudden Future Singularities in Quintessence and Scalar-Tensor Quintessence Models

We demonstrate analytically and numerically the existence of geodesically complete singularities in quintessence and scalar tensor quintessence models with scalar field potential of the form $V(\phi)\sim \vert \phi\vert^n$ with $0<n<1$. In the case of quintessence, the singularity which occurs at $\phi=0$, involves divergence of the third time derivative of the scale factor (Generalized Sudden Future Singularity (GSFS)), and of the second derivative of the scalar field. In the case of scalar-tensor quintessence with the same potential and with a linear minimal coupling ($F(\phi)=1-\lambda \phi$), the singularity is stronger and involves divergence of the second derivative of the scale factor (Sudden Future Singularity (SFS)). We show that the scale factor close to the singularity is of the form $a(t)=a_s+b(t_{s}-t) + c(t_{s}-t)^2 +d(t_{s}-t)^q$ where $a_s,b,c,d$ are constants obtained from the dynamical equations and $t_s$ is the time of the singularity. In the case of quintessence we find $q=n+2$ (ie $2<q<3$), while for the case of scalar-tensor quintessence $q=n+1$ ($1<q<2$). We verify these analytical results numerically and extend them to the case where a perfect fluid is present. The linear and quadratic terms in $(t_{s}-t)$ that appear in the expansion of the scale factor around $t_s$ are subdominant for the diverging derivatives close to the singularity, but can play an important role in the estimation of the Hubble parameter. Using the analytically derived relations between these terms, we derive relations involving the Hubble parameter close to the singularity, which may be used as observational signatures of such singularities in this class of models. For quintessence with matter fluid, we find that close to the singularity $\dot H=\frac{3}{2}\Omega_{0m} (1+z_{s})^{3}-3H^{2}$.Comment: 15 pages, 12 Figures. The mathematica file that were used for the construction of the Figures may be downloaded from http://leandros.physics.uoi.gr/quint-singularities/math-quint.zi

Spinning particle orbits around a black hole in an expanding background

We investigate analytically and numerically the orbits of spinning particles around black holes in the post Newtonian limit and in the presence of cosmic expansion. We show that orbits that are circular in the absence of spin, get deformed when the orbiting particle has spin. We show that the origin of this deformation is twofold: a. the background expansion rate which induces an attractive (repulsive) interaction due to the cosmic background fluid when the expansion is decelerating (accelerating) and b. a spin-orbit interaction which can be attractive or repulsive depending on the relative orientation between spin and orbital angular momentum and on the expansion rate.Comment: 12 pages, 4 figures. Accepted in 'Classical and Quantum Gravity'. To appea