Leandros Perivolaropoulos | University of Ioannina/Greece (original) (raw)
Papers by Leandros Perivolaropoulos
We demonstrate analytically and numerically the existence of geodesically complete singulari-ties... more We demonstrate analytically and numerically the existence of geodesically complete singulari-ties in quintessence and scalar tensor quintessence models with scalar field potential of the form V (φ) ∼ |φ| n with 0 < n < 1. In the case of quintessence, the singularity which occurs at φ = 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, 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) = as + b(ts − t) + c(ts − t) 2 + d(ts − t) q where as, b, c, d are constants obtained from the dynamical equations and ts is the time of the singularity. In the case of quintessence we find q = n + 2 (i.e. 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, with a constant equation of state w = p ρ , is present. The linear and quadratic terms in (ts − t) 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 sin-gularity, 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 ˙ H = 3 2 Ω0m(1 + zs) 3 − 3H 2. These terms should be taken into account when searching for future or past time such singularities, in cosmological data.
Physical Review D, 2000
We use numerical simulations and semi-analytical methods to investigate the stability and the int... more We use numerical simulations and semi-analytical methods to investigate the stability and the interactions of nontopological stationary qball solutions. In the context of a simple model we map the parameter sectors of stability for a single qball and verify the result using numerical simulations of time evolution. The system of two interacting qballs is also studied in one and two space dimensions. We find that the system generically performs breather type oscillations with frequency equal to the difference of the internal qball frequencies. This result is shown to be consistent with the form of the qball interaction potential. Finally we perform simulations of qball scattering and show that the right angle scattering effect observed in topological soliton scattering in two dimensions, persists also in the case of qballs where no topologically conserved quantities are present. For relativistic collision velocities the qball charge is split into a forward and a right angle scattering component. As the collision velocity increases, the forward component gets amplified at the expense of the right angle component.
Physical Review D, 1997
We demonstrate that field theories involving explicit breaking of continous symmetries, incorpora... more We demonstrate that field theories involving explicit breaking of continous symmetries, incorporate two generic classes of topological defects each of which is stable for a particular range of parameters. The first class includes defects of the usual type where the symmetry gets restored in the core and vacuum energy gets trapped there. We show however that these defect solutions become unstable for certain ranges of parameters and decay not to the vacuum but to another type of stable defect where the symmetry in not restored in the core. In the wall case, initially spherical, bubble-like configurations are simulated numerically and shown to evolve generically towards a planar collapse. In the string case, the decay of the symmetric core vortex resembles the decay of a semilocal string to a skyrmion with the important difference that while the skyrmion is unstable and decays to the vacuum, the resulting non-symmetric vortex is topologically stable.
Physical Review D, 1992
We use arguments based on Derrick's theorem to show that the property of collapse which is the ke... more We use arguments based on Derrick's theorem to show that the property of collapse which is the key feature of global texture appears in several field theory models with broken global O(N) symmetry. Such models do not necessarily have nontrivial third homotopy group of the vacuum manifold but may give rise to collapsing global field configurations with properties similar to textures. It is verified that configurations with planar and cylindrical geometries do not collapse. The existence of critical parameters for collapse of spherically symmetric global field configurations is verified both analytically and numerically.
Physical Review Letters, 2001
We show the existence of new stable ring-like localized scalar field configurations whose stabili... more We show the existence of new stable ring-like localized scalar field configurations whose stability is due to a combination of topological and nontopological charges. In that sense these defects may be called semitopological. These rings are Noether charged and also carry Noether current (they are superconducting). They are local minima of the energy in scalar field theories with an unbroken U(1) global symmetry. We obtain numerical solutions of the field configuration corresponding to large rings and derive virial theorems demonstrating their stability. We also derive the minimum energy field configurations in 3D and simulate the evolution of a finite size Q ring on a three dimensional lattice thus generalizing our demonstration of stability.
Physical Review D, 2002
In the lightcone frame, where the supermembrane theory and the Matrix model are strikingly simila... more In the lightcone frame, where the supermembrane theory and the Matrix model are strikingly similar, the equations of motion admit an elegant complexification in even dimensional spaces. Although the explicit rotational symmetry of the target space is lost, the remaining unitary symmetries apart from providing a simple and unifying description of all known solutions suggest new ones for rotating spherical and toroidal membranes. In this framework the angular momentum is represented by U(1) charges which balance the nonlinear attractive forces of the membrane. We examine in detail a six dimensional rotating toroidal membrane solution which lives in a 3-torus, T3T^3T3 and admits stable radial modes. In Matrix Theory it corresponds to a toroidal N-$D_{0}$ brane bound state. We demonstrate its existence and discuss its radial stability.
Journal of High Energy Physics, 2000
Motivated by recent work we study rotating ellipsoidal membranes in the framework of the light-co... more Motivated by recent work we study rotating ellipsoidal membranes in the framework of the light-cone supermembrane theory. We investigate stability properties of these classical solutions which are important for the quantization of super membranes. We find the stability modes for all sectors of small multipole deformations. We exhibit an isomorphism of the linearized membrane equation with that of the SU(N) matrix model for every value of NNN. The boundaries of the linearized stability region are at a finite distance and they appear for finite size perturbations.
The dynamics of texture-like configurations are briefly reviewed. Emphasis is given to configurat... more 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 tension
Journal of Cosmology and Astroparticle Physics, 2007
If the dark energy equation of state parameter w(z) crosses the phantom divide line w=-1 (or equi... more If the dark energy equation of state parameter w(z) crosses the phantom divide line w=-1 (or equivalently if the expression d(H^2(z))/dz - 3\Omega_m H_0^2 (1+z)^2 changes sign) at recent redshifts, then there are two possible cosmological implications: Either the dark energy consists of multiple components with at least one non-canonical phantom component or general relativity needs to be extended to a more general theory on cosmological scales. The former possibility requires the existence of a phantom component which has been shown to suffer from serious theoretical problems and instabilities. Therefore, the later possibility is the simplest realistic theoretical framework in which such a crossing can be realized. After providing a pedagogical description of various dark energy observational probes, we use a set of such probes (including the Gold SnIa sample, the first year SNLS dataset, the 3-year WMAP CMB shift parameter, the SDSS baryon acoustic oscillations peak (BAO), the X-ray gas mass fraction in clusters and the linear growth rate of perturbations at z=0.15 as obtained from the 2dF galaxy redshift survey) to investigate the priors required for cosmological observations to favor crossing of the phantom divide. We find that a low \Omega_m prior (0.2<\Omega_m <0.25) leads, for most observational probes (except of the SNLS data), to an increased probability (mild trend) for phantom divide crossing. An interesting degeneracy of the ISW effect in the CMB perturbation spectrum is also pointed out.
consists of five distinct subsets defined by the group or instrument that discovered and analyzed... more 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)w(z)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=−1w=-1w=−1 from below (w_0<-1,w_1>0). We also isolate six SnIa that are mostly responsible for this behavior of the HZSST subset.
Physical Review D, 2008
) but does not involve derivatives of the expansion rate H(z)H(z)H(z). This also leads to the fact that... more ) but does not involve derivatives of the expansion rate H(z)H(z)H(z). This also leads to the fact that LCDM provides an excellent fit to the current linear growth data.
Journal of Cosmology and Astroparticle Physics, 2007
The Gold06 SnIa dataset recently released in astro-ph/0611572 consists of five distinct subsets d... more 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 (w0, w1) 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 ) = (w0, w1) 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 (w0, w1) 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 ΛCDM (w0 = −1, w1 = 0) while the HZSST subset is statistically distinct and strongly 'pulls' towards a varying w(z) crossing the line w = −1 from below (w0 < −1, w1 > 0). We also isolate six SnIa that are mostly responsible for this behavior of the HZSST subset. PACS numbers: 98.80.Es,98.65.Dx,98.62.Sb
Physical Review D, 2004
] (hereafter CKW) that bound structures get dissociated in phantom cosmologies but we demonstrate... more ] (hereafter CKW) that bound structures get dissociated in phantom cosmologies but we demonstrate that this happens earlier than anticipated by CKW. In particular we find that the `rip time' when a bound system gets unbounded is not the time when the repulsive phantom energy gravitational potential due to the average (rho+3p)(\rho + 3p)(rho+3p) balances the attractive gravitational potential of the mass M of the system. Instead, the `rip time' is the time when the minimum of the time dependent effective potential (including the centrifugal term) disappears. For the Milky Way galaxy this happens approximately 180Myrs before the Big Rip singularity instead of approximately 60Myrs indicated by CKW for a phantom cosmology with w=-1.5. A numerical reconstruction of the dissociating bound orbits is presented.
Physical Review D, 2004
We study the expansion history of the universe up to a redshift of z=1.75 using the 194 recently ... more We study the expansion history of the universe up to a redshift of z=1.75 using the 194 recently published SnIa data by Tonry et. al. and Barris et. al. In particular we find the best fit forms of several cosmological models and H(z)H(z)H(z) ansatze, determine the best fit values of their parameters and rank them according to increasing value of chimin2\chi_{min}^2chimin2 (the minimum value of chi2\chi^2chi2 for each H(z)H(z)H(z) ansatz). We use a prior of Omega0m=0.3\Omega_{0m} = 0.3Omega0m=0.3 and assume flat geometry of the universe. No prior assumptions are made about validity of energy conditions. The fitted models are fourteen and include SCDM, LCDM, dark energy with constant equation of state parameter www (quiessence), third order polynomial for H(1+z)H(1+z)H(1+z), Chaplygin gas, Cardassian model, w(z)=w0+w1zw(z)=w_0 + w_1 zw(z)=w0+w1z, w(z)=w0+zw1/(1+z)w(z)=w_0 + z w_1/(1+z)w(z)=w0+zw1/(1+z), an oscillating ansatz for H(z)H(z)H(z) etc. All these models with the exception of SCDM are consistent with the present data. However, the quality of the fit differs significantly among them and so do the predicted forms of w(z)w(z)w(z) and H(z)H(z)H(z) at best fit. The worst fit among the data-consistent models considered corresponds to the simplest model LCDM ($\chi_{min}^2 = 198.7$ for Omega0m=0.34\Omega_{0m} = 0.34Omega0m=0.34) while the best fit is achieved by the three parameter oscillating ansatz ($\chi_{min}^2 = 194.1$). Most of the best fit ansatze have an equation of state parameter w(z)w(z)w(z) that varies between w(z)simeq−1w(z) \simeq -1w(z)simeq−1 for z<0.5z<0.5z<0.5 to w(z)>0w(z) > 0w(z)>0 for z>1z>1z>1. This implies that the sign of the pressure of the dark energy may be alternating as the redshift increases. The goodness of fit of the oscillating H(z)H(z)H(z) ansatz lends further support to this possibility.
Physical Review D, 2007
We use a low redshift expansion of the cosmological equations of extended (scalar-tensor) quintes... more We use a low redshift expansion of the cosmological equations of extended (scalar-tensor) quintessence to divide the observable Hubble history parameter space in four sectors: A forbidden sector I where the scalar field of the theory becomes imaginary (the kinetic term becomes negative), a forbidden sector II where the scalar field rolls up (instead of down) its potential, an allowed `freezing' quintessence sector III where the scalar field is currently decelerating down its potential towards freezing and an allowed `thawing' sector IV where the scalar field is currently accelerating down its potential. The dividing lines between the sectors depend sensitively on the time derivatives of the Newton's constant G over powers of the Hubble parameter. For minimally coupled quintessence which appears as a special case for a constant G our results are consistent with previous studies. Observable parameter \chi^2 contours based on current data (SNLS dataset) are also constructed on top of the sectors, for a prior of \Omega_m=0.24. By demanding that the observed 2\sigma \chi^2 parameter contours do not lie entirely in the forbidden sectors we derive stringent constraints on the current second time derivative of Newton's constant G. In particular we find {\ddot G}/G >-1.91 H_0^2=-2 10^{-20}h^2 yrs^{-2} at the 2\sigma level which is complementary to solar system tests which constrain only the first derivative of G as |{\dot G}/G|<10^{-14}yrs^{-1} at 1\sigma.
Physical Review D, 2005
... This would leave only models based on extended gravity theories [16,17] and combina-tions of ... more ... This would leave only models based on extended gravity theories [16,17] and combina-tions of multiple fields [18,19](quintessence + phantom) as dark energy candidates. ... Indeed, phantom [14] dynamical dark energy mod--1.75 -1.5 -1.25 -1 -0.75 -0.5 -0.25 0 w0 -2 0 2 4 6 8 ...
We demonstrate analytically and numerically the existence of geodesically complete singulari-ties... more We demonstrate analytically and numerically the existence of geodesically complete singulari-ties in quintessence and scalar tensor quintessence models with scalar field potential of the form V (φ) ∼ |φ| n with 0 < n < 1. In the case of quintessence, the singularity which occurs at φ = 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, 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) = as + b(ts − t) + c(ts − t) 2 + d(ts − t) q where as, b, c, d are constants obtained from the dynamical equations and ts is the time of the singularity. In the case of quintessence we find q = n + 2 (i.e. 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, with a constant equation of state w = p ρ , is present. The linear and quadratic terms in (ts − t) 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 sin-gularity, 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 ˙ H = 3 2 Ω0m(1 + zs) 3 − 3H 2. These terms should be taken into account when searching for future or past time such singularities, in cosmological data.
Physical Review D, 2000
We use numerical simulations and semi-analytical methods to investigate the stability and the int... more We use numerical simulations and semi-analytical methods to investigate the stability and the interactions of nontopological stationary qball solutions. In the context of a simple model we map the parameter sectors of stability for a single qball and verify the result using numerical simulations of time evolution. The system of two interacting qballs is also studied in one and two space dimensions. We find that the system generically performs breather type oscillations with frequency equal to the difference of the internal qball frequencies. This result is shown to be consistent with the form of the qball interaction potential. Finally we perform simulations of qball scattering and show that the right angle scattering effect observed in topological soliton scattering in two dimensions, persists also in the case of qballs where no topologically conserved quantities are present. For relativistic collision velocities the qball charge is split into a forward and a right angle scattering component. As the collision velocity increases, the forward component gets amplified at the expense of the right angle component.
Physical Review D, 1997
We demonstrate that field theories involving explicit breaking of continous symmetries, incorpora... more We demonstrate that field theories involving explicit breaking of continous symmetries, incorporate two generic classes of topological defects each of which is stable for a particular range of parameters. The first class includes defects of the usual type where the symmetry gets restored in the core and vacuum energy gets trapped there. We show however that these defect solutions become unstable for certain ranges of parameters and decay not to the vacuum but to another type of stable defect where the symmetry in not restored in the core. In the wall case, initially spherical, bubble-like configurations are simulated numerically and shown to evolve generically towards a planar collapse. In the string case, the decay of the symmetric core vortex resembles the decay of a semilocal string to a skyrmion with the important difference that while the skyrmion is unstable and decays to the vacuum, the resulting non-symmetric vortex is topologically stable.
Physical Review D, 1992
We use arguments based on Derrick's theorem to show that the property of collapse which is the ke... more We use arguments based on Derrick's theorem to show that the property of collapse which is the key feature of global texture appears in several field theory models with broken global O(N) symmetry. Such models do not necessarily have nontrivial third homotopy group of the vacuum manifold but may give rise to collapsing global field configurations with properties similar to textures. It is verified that configurations with planar and cylindrical geometries do not collapse. The existence of critical parameters for collapse of spherically symmetric global field configurations is verified both analytically and numerically.
Physical Review Letters, 2001
We show the existence of new stable ring-like localized scalar field configurations whose stabili... more We show the existence of new stable ring-like localized scalar field configurations whose stability is due to a combination of topological and nontopological charges. In that sense these defects may be called semitopological. These rings are Noether charged and also carry Noether current (they are superconducting). They are local minima of the energy in scalar field theories with an unbroken U(1) global symmetry. We obtain numerical solutions of the field configuration corresponding to large rings and derive virial theorems demonstrating their stability. We also derive the minimum energy field configurations in 3D and simulate the evolution of a finite size Q ring on a three dimensional lattice thus generalizing our demonstration of stability.
Physical Review D, 2002
In the lightcone frame, where the supermembrane theory and the Matrix model are strikingly simila... more In the lightcone frame, where the supermembrane theory and the Matrix model are strikingly similar, the equations of motion admit an elegant complexification in even dimensional spaces. Although the explicit rotational symmetry of the target space is lost, the remaining unitary symmetries apart from providing a simple and unifying description of all known solutions suggest new ones for rotating spherical and toroidal membranes. In this framework the angular momentum is represented by U(1) charges which balance the nonlinear attractive forces of the membrane. We examine in detail a six dimensional rotating toroidal membrane solution which lives in a 3-torus, T3T^3T3 and admits stable radial modes. In Matrix Theory it corresponds to a toroidal N-$D_{0}$ brane bound state. We demonstrate its existence and discuss its radial stability.
Journal of High Energy Physics, 2000
Motivated by recent work we study rotating ellipsoidal membranes in the framework of the light-co... more Motivated by recent work we study rotating ellipsoidal membranes in the framework of the light-cone supermembrane theory. We investigate stability properties of these classical solutions which are important for the quantization of super membranes. We find the stability modes for all sectors of small multipole deformations. We exhibit an isomorphism of the linearized membrane equation with that of the SU(N) matrix model for every value of NNN. The boundaries of the linearized stability region are at a finite distance and they appear for finite size perturbations.
The dynamics of texture-like configurations are briefly reviewed. Emphasis is given to configurat... more 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 tension
Journal of Cosmology and Astroparticle Physics, 2007
If the dark energy equation of state parameter w(z) crosses the phantom divide line w=-1 (or equi... more If the dark energy equation of state parameter w(z) crosses the phantom divide line w=-1 (or equivalently if the expression d(H^2(z))/dz - 3\Omega_m H_0^2 (1+z)^2 changes sign) at recent redshifts, then there are two possible cosmological implications: Either the dark energy consists of multiple components with at least one non-canonical phantom component or general relativity needs to be extended to a more general theory on cosmological scales. The former possibility requires the existence of a phantom component which has been shown to suffer from serious theoretical problems and instabilities. Therefore, the later possibility is the simplest realistic theoretical framework in which such a crossing can be realized. After providing a pedagogical description of various dark energy observational probes, we use a set of such probes (including the Gold SnIa sample, the first year SNLS dataset, the 3-year WMAP CMB shift parameter, the SDSS baryon acoustic oscillations peak (BAO), the X-ray gas mass fraction in clusters and the linear growth rate of perturbations at z=0.15 as obtained from the 2dF galaxy redshift survey) to investigate the priors required for cosmological observations to favor crossing of the phantom divide. We find that a low \Omega_m prior (0.2<\Omega_m <0.25) leads, for most observational probes (except of the SNLS data), to an increased probability (mild trend) for phantom divide crossing. An interesting degeneracy of the ISW effect in the CMB perturbation spectrum is also pointed out.
consists of five distinct subsets defined by the group or instrument that discovered and analyzed... more 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)w(z)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=−1w=-1w=−1 from below (w_0<-1,w_1>0). We also isolate six SnIa that are mostly responsible for this behavior of the HZSST subset.
Physical Review D, 2008
) but does not involve derivatives of the expansion rate H(z)H(z)H(z). This also leads to the fact that... more ) but does not involve derivatives of the expansion rate H(z)H(z)H(z). This also leads to the fact that LCDM provides an excellent fit to the current linear growth data.
Journal of Cosmology and Astroparticle Physics, 2007
The Gold06 SnIa dataset recently released in astro-ph/0611572 consists of five distinct subsets d... more 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 (w0, w1) 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 ) = (w0, w1) 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 (w0, w1) 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 ΛCDM (w0 = −1, w1 = 0) while the HZSST subset is statistically distinct and strongly 'pulls' towards a varying w(z) crossing the line w = −1 from below (w0 < −1, w1 > 0). We also isolate six SnIa that are mostly responsible for this behavior of the HZSST subset. PACS numbers: 98.80.Es,98.65.Dx,98.62.Sb
Physical Review D, 2004
] (hereafter CKW) that bound structures get dissociated in phantom cosmologies but we demonstrate... more ] (hereafter CKW) that bound structures get dissociated in phantom cosmologies but we demonstrate that this happens earlier than anticipated by CKW. In particular we find that the `rip time' when a bound system gets unbounded is not the time when the repulsive phantom energy gravitational potential due to the average (rho+3p)(\rho + 3p)(rho+3p) balances the attractive gravitational potential of the mass M of the system. Instead, the `rip time' is the time when the minimum of the time dependent effective potential (including the centrifugal term) disappears. For the Milky Way galaxy this happens approximately 180Myrs before the Big Rip singularity instead of approximately 60Myrs indicated by CKW for a phantom cosmology with w=-1.5. A numerical reconstruction of the dissociating bound orbits is presented.
Physical Review D, 2004
We study the expansion history of the universe up to a redshift of z=1.75 using the 194 recently ... more We study the expansion history of the universe up to a redshift of z=1.75 using the 194 recently published SnIa data by Tonry et. al. and Barris et. al. In particular we find the best fit forms of several cosmological models and H(z)H(z)H(z) ansatze, determine the best fit values of their parameters and rank them according to increasing value of chimin2\chi_{min}^2chimin2 (the minimum value of chi2\chi^2chi2 for each H(z)H(z)H(z) ansatz). We use a prior of Omega0m=0.3\Omega_{0m} = 0.3Omega0m=0.3 and assume flat geometry of the universe. No prior assumptions are made about validity of energy conditions. The fitted models are fourteen and include SCDM, LCDM, dark energy with constant equation of state parameter www (quiessence), third order polynomial for H(1+z)H(1+z)H(1+z), Chaplygin gas, Cardassian model, w(z)=w0+w1zw(z)=w_0 + w_1 zw(z)=w0+w1z, w(z)=w0+zw1/(1+z)w(z)=w_0 + z w_1/(1+z)w(z)=w0+zw1/(1+z), an oscillating ansatz for H(z)H(z)H(z) etc. All these models with the exception of SCDM are consistent with the present data. However, the quality of the fit differs significantly among them and so do the predicted forms of w(z)w(z)w(z) and H(z)H(z)H(z) at best fit. The worst fit among the data-consistent models considered corresponds to the simplest model LCDM ($\chi_{min}^2 = 198.7$ for Omega0m=0.34\Omega_{0m} = 0.34Omega0m=0.34) while the best fit is achieved by the three parameter oscillating ansatz ($\chi_{min}^2 = 194.1$). Most of the best fit ansatze have an equation of state parameter w(z)w(z)w(z) that varies between w(z)simeq−1w(z) \simeq -1w(z)simeq−1 for z<0.5z<0.5z<0.5 to w(z)>0w(z) > 0w(z)>0 for z>1z>1z>1. This implies that the sign of the pressure of the dark energy may be alternating as the redshift increases. The goodness of fit of the oscillating H(z)H(z)H(z) ansatz lends further support to this possibility.
Physical Review D, 2007
We use a low redshift expansion of the cosmological equations of extended (scalar-tensor) quintes... more We use a low redshift expansion of the cosmological equations of extended (scalar-tensor) quintessence to divide the observable Hubble history parameter space in four sectors: A forbidden sector I where the scalar field of the theory becomes imaginary (the kinetic term becomes negative), a forbidden sector II where the scalar field rolls up (instead of down) its potential, an allowed `freezing' quintessence sector III where the scalar field is currently decelerating down its potential towards freezing and an allowed `thawing' sector IV where the scalar field is currently accelerating down its potential. The dividing lines between the sectors depend sensitively on the time derivatives of the Newton's constant G over powers of the Hubble parameter. For minimally coupled quintessence which appears as a special case for a constant G our results are consistent with previous studies. Observable parameter \chi^2 contours based on current data (SNLS dataset) are also constructed on top of the sectors, for a prior of \Omega_m=0.24. By demanding that the observed 2\sigma \chi^2 parameter contours do not lie entirely in the forbidden sectors we derive stringent constraints on the current second time derivative of Newton's constant G. In particular we find {\ddot G}/G >-1.91 H_0^2=-2 10^{-20}h^2 yrs^{-2} at the 2\sigma level which is complementary to solar system tests which constrain only the first derivative of G as |{\dot G}/G|<10^{-14}yrs^{-1} at 1\sigma.
Physical Review D, 2005
... This would leave only models based on extended gravity theories [16,17] and combina-tions of ... more ... This would leave only models based on extended gravity theories [16,17] and combina-tions of multiple fields [18,19](quintessence + phantom) as dark energy candidates. ... Indeed, phantom [14] dynamical dark energy mod--1.75 -1.5 -1.25 -1 -0.75 -0.5 -0.25 0 w0 -2 0 2 4 6 8 ...