1988-1992 Πτυχίο Φυσικής από το Πανεπιστήμιο Αθηνών, Τμήμα Φυσικής. Βαθμός πτυχίου 8.9/10 (Άριστα).

1993-1995 Μεταπτυχιακό δίπλωμα (M.Sc.) στη Θεωρητική Φυσική από το Πανεπιστήμιο τηςAlberta,Τμήμα Φυσικής, Edmonton, Canada. Τίτλος διατριβής: "Το σύμπαν τύπου Bianchi-IX στην Ευκλείδια ΚβαντικήΚοσμολογία". Επιβλέπων Καθηγητής D.N. Page.

1996-1999 Διδακτορικό δίπλωμα από το Πανεπιστήμιο Αθηνών, Τμήμα Φυσικής. Τίτλος διατριβής: "Σχετικιστική Κλασσική και Κβαντική Κοσμολογία". Επιβλέπων Επίκ. Καθ. Θ. Χριστοδουλάκης.

Κλασσική βαρύτητα, Εναλλακτικές θεωρίες βαρύτητας, Βαρύτητα σνώτερων διαστάσεων, Μελανές οπές, Βαρυτική ακτινοβολία.

Κοσμολογία, Κοσμολογία μεμβρανών-χορδών.

Κανονική προσέγγιση κβαντικής βαρύτητας και κβαντικής κοσμολογίας.

Φυσική στη Βιολογία.

Φυσική I (μηχανική), Φυσική ΙΙ (ηλεκτρομαγνητισμός), Γραμμική Άλγεβρα, Μαθηματικός Λογισμός, Διαφορικές Εξισώσεις με Μερικές Παραγώγους, Κλασσική Μηχανική (μεταπτυχιακό), Κοσμολογία, Γενική Σχετικότητα, Στατιστική Φυσική, Κβαντική Θεωρία πεδίου (μεταπτυχιακό).

Ώρες γραφείου: Πέμπτη 12:00-14:00 , Παρασκευή 12:00-14:00

A. Bonanno, **G. Kofinas**, V. Zarikas, Effective field equations and scale-dependent couplings in gravity, Phys. Rev. D, Vol. 103, No. 104025, pp. 10, 2021, https://arxiv.org/abs/2012.05338

** Abstract **

A new set of field equations for a space-time dependent Newton's constant G(x) and cosmological constant Λ(x) in the presence of matter is presented. We prove that it represents the most general mathematically consistent, physically plausible, set of evolution equations assuming at most second derivatives in the dynamical variables. In the new Einstein's equations, only Λ-kinetic terms arise, while in the modified conservation equation, derivative terms of G also appear. As an application, this formalism is applied in the context of the Asymptotic Safety scenario to the early universe, assuming a perfect fluid with a radiation equation of state. Cosmological solutions are obtained for all types of spatial curvature, displaying a variety of interesting cosmic evolutions. As an indication of such behaviours, bouncing solutions, recollapsing solutions or non-singular expanding solutions with a transient acceleration era are discussed in details.

** Abstract **

Christoffel connection did not enter gravity as an axiom of minimal length for the free fall of particles (where anyway length action is not defined for massless particles), nor out of economy, but from the weak equivalence principle (gravitational force is equivalent to acceleration according to Einstein) together with the identification of the local inertial frame with the local Lorentz one. This identification implies that the orbits of all particles are given by the geodesics of the Christoffel connection. Here, we show that in the presence of only massless particles (absence of massive particles) the above identification is inconsistent and does not lead to any connection. The proof is based on the existence of projectively equivalent connections and the absence of proper time for null particles. If a connection derived by some kinematical principles for the particles is to be applied in the world, it is better these principles to be valid in all relevant spacetime rather than different principles to give different connections in different spacetime regions. Therefore, our result stated above may imply a conceptual insufficiency of the use of the Christoffel connection in the early universe where only massless particles are expected to be present (whenever at least some notions, like orbits, are meaningful), and thus of the total use of this connection. If in the early universe the notion of a massive particle, which appears latter in time, cannot be used, in an analogous way in a causally disconnected high-energy region (maybe deep interior of astrophysical objects or black holes) the same conclusions could be extracted if only massless particles are present.

F.K. Anagnostopoulos, S. Basilakos, **G. Kofinas**, V. Zarikas, Constraining the Asymptotically Safe Cosmology: cosmic acceleration without dark energy , JCAP, Vol. 02, pp. 053, 2019, https://arxiv.org/abs/1806.10580

** Abstract **

A recently proposed Asymptotically Safe cosmology provides an elegant mechanism towards understanding the nature of dark energy and its associated cosmic coincidence problem. The underlying idea is that the accelerated expansion of the universe can occur due to infrared quantum gravity modifications at intermediate astrophysical scales (galaxies or galaxy clusters) which produce local anti-gravity sources. In this cosmological model no extra unproven energy scales or fine-tuning are used. In this study the Asymptotically Safe model is confronted with the most recent observational data from low-redshift probes, namely measurements of the Hubble parameter, standard candles (Pantheon SnIa, Quasi-stellar objects), Baryonic Acoustic Oscillations (BAOs) and high redshift probes (CMB shift parameters). Performing an overall likelihood analysis we constrain the free parameters of the model and we test its performance against the concordance model (flat Λ CDM) utilizing a large family of information criteria. We find that the Asymptotically Safe model is statistically equivalent with Λ CDM, hence it can be seen as a viable and very efficient cosmological alternative.

F.K. Anagnostopoulos, **G. Kofinas**, V. Zarikas, IR quantum gravity solves naturally cosmic acceleration and its coincidence problem, International Journal of Modern Physics D, Vol. 28, pp. 14, 2019, World Scientific, https://www.worldscientific.com/doi/abs/...

** Abstract **

The novel idea is that the undergoing accelerated expansion of the universe happens due to infrared quantum gravity modifications at intermediate astrophysical scales of galaxies or galaxy clusters, within the framework of Asymptotically Safe gravity. The reason is that structures of matter are associated with a scale-dependent positive cosmological constant of quantum origin. In this context, no extra unproven energy scales or fine-tuning are used. Furthermore, this model was confronted with the most recent observational data from a variety of probes, and with the aid of Bayesian analysis, the most probable values of the free parameters were extracted. Finally, the model proved to be statistically equivalent with Λ \\r\\nΛ\\r\\nCDM, and thus being able to resolve naturally the concept of dark energy and its associated cosmic coincidence problem.

** Abstract **

A novel idea is proposed for a natural solution of the dark energy and its cosmic coincidence problem. The existence of local antigravity sources, associated with astrophysical matter configurations distributed throughout the universe, can lead to a recent cosmic acceleration effect. Various physical theories can be compatible with this idea, but here, in order to test our proposal, we focus on quantum originated spherically symmetric metrics matched with the cosmological evolution through the simplest Swiss cheese model. In the context of asymptotically safe gravity, we have explained the observed amount of dark energy using Newton's constant, the galaxy or cluster length scales, and dimensionless order one parameters predicted by the theory, without fine-tuning or extra unproven energy scales. The interior modified Schwarzschild-de Sitter metric allows us to approximately interpret this result as that the standard cosmological constant is a composite quantity made of the above parameters, instead of a fundamental one.

[6]

** Abstract **

The most general completion of Brans-Dicke gravity is found when energy is exchanged in a uniquely\\r\\ndefined way between the scalar field and ordinary matter. The theory contains a new parameter\\r\\n(integration constant from the integration procedure) and when this is switched off, Brans-Dicke\\r\\ntheory emerges. As usually, the vacuum theory can be defined from the complete Brans-Dicke\\r\\ntheory when the matter energy-momentum tensor vanishes. However, additionally, the complete family\\r\\nof vacuum theories is found, consistent with the free wave equation for the scalar field.\\r\\nThe subclass of this family with identically covariantly conserved energy-momentum tensor is\\r\\nidentified and, thus, can be supplemented by any equation of motion for the scalar field.

** Abstract **

Recently the most general completion of Brans-Dicke theory was appeared with energy exchanged in
a uniquely defined way between the scalar field and ordinary matter, given that the equation
of motion for the scalar field keeps the simple wave form of Brans-Dicke. Here it is first found
the action of the new vacuum theory which arises as the limit of the matter theory.
A symmetry transformation of this vacuum action in the Jordan frame is found which consists of a
conformal transformation of the metric together with a redefinition of the scalar field.
Since the general family of vacuum theories is not exhausted by the above vacuum theory,
the action of this family is also found which is parametrized by an arbitrary function of the scalar
field. As for the full theory with matter it is only found the action of the system when the
matter Lagrangian vanishes on-shell, as for example for pressureless dust.
Due to the interaction, the matter Lagrangian is non-minimally coupled either in the Jordan or the
Einstein frame.

** Abstract **

We discuss the cosmological implications of an extended
Brans-Dicke theory presented recently, in which there is an energy exchange between
the scalar field and ordinary matter, determined by the theory. A new mass scale is
generated in the theory which modifies the Friedmann equations
with field-dependent corrected kinetic terms. In a radiation
universe the general solutions are found and there are branches
with complete removal of the initial singularity, while at the
same time a transient accelerating period can occur within
deceleration. Entropy production is also possible in the early universe.
In the dust era, late-times acceleration has been
found numerically in agreement with the correct behaviour of the
density parameters and the dark energy equation of state, while
the gravitational constant has only a slight variation over a
large redshift interval in agreement with observational bounds.

** Abstract **

General non-singular accelerating cosmological solutions for an initial cosmic period of pure vacuum
birth era are derived. This vacuum era is described by a varying cosmological ``constant''
suggested by the Renormalisation Group flow of Asymptotic Safety scenario near the ultraviolet fixed
point. In this scenario, natural exit from inflation to the standard decelerating cosmology occurs
when the energy scale lowers and the cosmological ``constant'' becomes insignificant.
In the following period where matter is also present, cosmological solutions with characteristics
similar to the vacuum case are generated. Remarkably the set of equations allow for particle production
and entropy generation. Alternatively, in the case of non-zero bulk viscosity, entropy production
and reheating is found. As for the equations of motion, they modify Einstein equations by adding
covariant kinetic terms of the cosmological ``constant'' which respect the Bianchi identities.
An advance of the proposed framework is that it ensures a consistent description of both a quantum
vacuum birth of the universe and a subsequent cosmic era in the presence of matter.

G. Aliferis, **G. Kofinas**, V. Zarikas, Efficient electroweak baryogenesis by black holes, Phys. Rev. D, Vol. 91, No. 4, pp. 045002, 2015, http://arxiv.org/abs/1406.6215

** Abstract **

A novel cosmological scenario, capable to generate the observed baryon number at the electroweak scale for very small CP violating angles, is presented. The proposed mechanism can be applied in conventional FRW cosmology, but becomes extremely efficient due to accretion in the context of early cosmic expansion with high energy modifications. Assuming that our universe is a Randall-Sundrum brane, baryon asymmetry can easily be produced by Hawking radiation of very small primordial black holes. The Hawking radiation reheats a spherical region around every black hole to a high temperature and the electroweak symmetry is restored there. A domain wall is formed separating the region with the symmetric vacuum from the asymmetric region where electroweak baryogenesis takes place. First order phase transition is not needed. The black holes's lifetime is prolonged due to accretion, resulting to strong efficiency of the baryon producing mechanism. The allowed by the mechanism black hole mass range includes masses that are energetically favoured to be produced from interactions around the higher dimensional Planck scale.

** Abstract **

We studied spherically symmetric solutions in scalar-torsion gravity theories in which a scalar field is coupled to torsion with a derivative coupling. We obtained the general field equations from which we extracted a decoupled master equation, the solution of which leads to the specification of all other unknown functions. We first obtained an exact solution which represents a new wormhole-like solution dressed with a regular scalar field. Then, we found large distance linearized spherically symmetric solutions in which the space asymptotically is AdS.

D. Jatkar, **G. Kofinas**, O. Miskovic, R. Olea, Conformal Mass in Einstein-Gauss-Bonnet AdS gravity, Phys. Rev. D, Vol. 91, No. 10, pp. 105030, 2015, http://arxiv.org/abs/1501.06861

** Abstract **

In this paper, we show that the physical information given by conserved charges for asymptotically AdS spacetimes in Einstein-Gauss-Bonnet AdS gravity is encoded in the electric part of the Weyl tensor. This result generalizes the conformal mass definition by Ashtekar-Magnon-Das (AMD) to a gravity theory with a Gauss-Bonnet term. This proof makes use of the Noether charges obtained from an action renormalized by the addition of counterterms which depend on the extrinsic curvature (Kounterterms). If the asymptotic fall-off behaviour of the Weyl tensor is same as the one considered in the AMD method, then the Kounterterm charges and the AMD charges agree in any dimension.

** Abstract **

New general spherically symmetric solutions have been derived with a cosmological \"constant\" \\Lambda as a source. This \\Lambda field is not constant but it satisfies the properties of the asymptotically safe gravity at the ultraviolet fixed point. The importance of these solutions comes from the fact that they describe the near to the centre region of black hole spacetimes as this is modified by the Renormalization Group scaling behaviour of the fields. The consistent set of field equations which respect the Bianchi identities is derived and solved. One of the solutions (with conventional sign of temporal-radial metric components) is timelike geodesically complete, and although there is still a curvature divergent origin, this is never approachable by an infalling massive particle which is reflected at a finite distance due to the repulsive origin. Another family of solutions (of both signatures) range from a finite radius outwards, they cannot be extended to the centre of spherical symmetry, and the curvature invariants are finite at the minimum radius.

** Abstract **

We study a gravity theory where a scalar field with potential, beyond its minimal coupling, is also coupled through a non-minimal derivative coupling with the torsion scalar which is the teleparallel equivalent of Einstein gravity. This theory provides second order equations of motion and we find large-distance non-perturbative static spherically symmetric four-dimensional solutions. Among them a general class of black hole solutions is found for some range of the parameters/integration constants with asymptotics of the form of hyperscaling violating Lifshitz spacetime with spherical horizon topology. Although the scalar field diverges at the horizon, its energy density and pressures are finite there. From the astrophysical point of view, this solution provides extra deflection of light compared to the Newtonian deflection.

** Abstract **

We continue the investigation of a recent proposal on alternative\\r\\nmatching conditions for self-gravitating defects which generalize\\r\\nthe standard matching conditions. The reasoning for this study is\\r\\nthe need for consistency of the various codimension defects and the\\r\\nexistence of a meaningful equation of motion at the probe limit,\\r\\nthings that seem to lack from the standard approach. These matching\\r\\nconditions arise by varying the brane-bulk action with respect to\\r\\nthe brane embedding fields (and not with respect to the bulk metric\\r\\nat the brane position) in a way that takes into account the\\r\\ngravitational back-reaction of the brane to the bulk. They always\\r\\npossess a Nambu-Goto probe limit and any codimension defect is\\r\\nseemingly consistent for any second order bulk gravity theory. Here,\\r\\nwe consider in detail the case of a codimension-1 brane in\\r\\nfive-dimensional Einstein gravity, derive the generic alternative\\r\\njunction conditions and find the $Z_{2}-$symmetric braneworld\\r\\ncosmology, as well as its bulk extension. Compared to the standard\\r\\nbraneworld cosmology, the new one has an extra integration constant\\r\\nwhich accounts for the today matter and dark energy contents,\\r\\ntherefore, there is more freedom for accommodating the observed\\r\\ncosmic features. One branch of the solution possesses the asymptotic\\r\\nlinearized LFRW regime. We have constrained the parameters so that\\r\\nto have a recent passage from a long deceleration era to a small\\r\\ntoday acceleration epoch and we have computed the age of the\\r\\nuniverse, consistent with current data, and the time-varying dark\\r\\nenergy equation of state. For a range of the parameters it is\\r\\npossible for the presented cosmology to provide a large acceleration\\r\\nin the high energy regime.

** Abstract **

We investigate the cosmological implications of the recently
constructed 5-dimensional brane-world cosmology with gravitating
Nambu-Goto matching conditions. Inserting both matter and radiation
sectors, we first extract the analytical cosmological solutions.
Additionally, we use observational data from Type Ia Supernovae
(SNIa) and Baryon Acoustic Oscillations (BAO), along with
requirements of Big Bang Nucleosynthesis (BBN), in order to impose
constraints on the parameters of the model. We find that the
scenario at hand is in very good agreement with observations, and
thus a small departure from the standard Randall-Sundrum scenario is
allowed.

D.P. Jatkar, **G. Kofinas**, O. Miskovic, R. Olea, Conformal Mass in AdS gravity, Phys. Rev. D, Vol. 89, No. 12, pp. 124010, 2014, http://arxiv.org/abs/arXiv:1404.1411

** Abstract **

We show that the Ashtekar-Magnon-Das (AMD) mass and other conserved
quantities are equivalent to the Kounterterm charges in the
asymptotically AdS spacetimes that satisfy the Einstein equations,
if we assume the same asymptotic fall-off behavior of the Weyl
tensor as considered by AMD. This therefore implies that, in all
dimensions, the conformal mass can be directly derived from the bulk
action and the boundary terms, which are written in terms of the
extrinsic curvature.

** Abstract **

Inspired by the teleparallel formulation of General Relativity,
whose Lagrangian is the torsion invariant $T$, we construct the
teleparallel equivalent of Gauss-Bonnet gravity in arbitrary
dimensions. Without imposing the Weitzenbock connection, we extract
the torsion invariant $T_G$, equivalent (up to boundary terms) to
the Gauss-Bonnet term $G$. $T_G$ is constructed by the vierbein and
the connection, it contains quartic powers of the torsion tensor, it
is diffeomorphism and Lorentz invariant, and in four dimensions it
reduces to a topological invariant as expected. Imposing the
Weitzenbock connection, $T_G$ depends only on the vierbein, and this
allows us to consider a novel class of modified gravity theories
based on $F(T,T_G)$, which is not spanned by the class of $F(T)$
theories, nor by the $F(R,G)$ class of curvature modified gravity.
The cosmological application of this new theory proves to be very
interesting, providing a unified description of the cosmological
history from early-times inflation to late-times self-acceleration,
without the inclusion of a cosmological constant. Moreover, the dark
energy equation-of-state parameter can be quintessence or
phantom-like, or experience the phantom-divide crossing, depending
on the parameters of the model.

** Abstract **

The f(T, TG) class of gravitational modification, based on the quadratic torsion
scalar T, as well as on the new quartic torsion scalar TG which is the teleparallel equivalent
of the Gauss-Bonnet term, is a novel theory, different from both f(T) and f(R,G) ones. We
perform a detailed dynamical analysis of a spatially flat universe governed by the simplest
non-trivial model of f(T, TG) gravity which does not introduce a new mass scale. We find
that the universe can result in dark-energy dominated, quintessence-like, cosmologicalconstant-
like or phantom-like solutions, according to the parameter choices. Additionally,
it may result to a dark energy - dark matter scaling solution, and thus it can alleviate
the coincidence problem. Finally, the analysis “at infinity” reveals that the universe may
exhibit future, past, or intermediate singularities depending on the parameters.

** Abstract **

We investigate the cosmological applications of F(T,TG) gravity, which is a novel modified gravitational theory based on the torsion invariant T and the teleparallel equivalent of the Gauss-Bonnet term TG. F(T,TG) gravity differs from both F(T) theories as well as from F(R,G) class of curvature modified gravity, and thus its corresponding cosmology proves to be very interesting. In particular, it provides a unified description of the cosmological history from early-times inflation to late-times self-acceleration, without the inclusion of a cosmological constant. Moreover, the dark energy equation-of-state parameter can be quintessence or phantom-like, or experience the phantom-divide crossing, depending on the parameters of the model.

** Abstract **

We raise on theoretical grounds the question of the physical
relevance of Israel matching conditions and their generalizations to
higher codimensions, the standard cornerstone of the braneworld and
other membrane scenarios. Our reasoning is twofold: First, the
incapability of the conventional matching conditions to accept the
Nambu-Goto probe limit (even the geodesic limit of the Israel
matching conditions is not acceptable since being the geodesic
equation a kinematical fact it should be preserved for all
gravitational theories or defects, which is not the case for these
conditions). Second, in our $D$-dimensional spacetime (maybe $D$=4),
classical defects of any possible codimension should be compatible.
These matching conditions fail to accept codimension-2 and 3 defects
for $D$=4 (which represents effectively the spacetime at certain
length and energy scales) and most probably fail to accept high
enough codimensional defects for any $D$ since there is no high
enough Lovelock density to support them. Here, we propose
alternative matching conditions which seem to satisfy the previous
criteria. Instead of varying the brane-bulk action with respect to
the bulk metric at the brane position, we vary with respect to the
brane embedding fields so that the gravitational back-reaction is
included. For a codimension-2 brane in 6-dim EGB gravity we prove
its consistency for an axially symmetric cosmological configuration.
The cosmologies found have the LFRW behaviour and extra correction
terms. For a radiation brane one solution avoids a cosmological
singularity and undergoes accelerated expansion near the minimum
scale factor. In the presence of an induced gravity term, there
naturally appears in the theory the effective cosmological constant
scale $\lambda/(M_6^4 r_c^2)$, which for a brane tension
$\lambda\sim M_6^4$ (e.g. $TeV^4$) and $r_c \sim H_0^{-1}$ gives the
observed value of the cosmological constant.

** Abstract **

A mechanism capable to provide a natural solution to two major cosmological problems, i.e. the cosmic acceleration and the coincidence problem, is proposed. A specific brane-bulk energy exchange mechanism produces a total dark pressure, arising when adding all normal to the brane negative pressures in the interior of galactic core black holes. This astrophysically produced negative dark pressure explains cosmic acceleration and why the dark energy today is of the same order to the matter density for a wide range of the involved parameters. An exciting result of the analysis is that the recent rise of the galactic core black hole mass density causes the recent passage from cosmic deceleration to acceleration. Finally, it is worth mentioning that this work corrects a wide spread fallacy among brane cosmologists, i.e. that escaping gravitons result to positive dark pressure.

C. Charmousis, **G. Kofinas**, A. Papazoglou, The Consistency of codimension-2 braneworlds and their cosmology, JCAP, Vol. 1001, pp. 022, 2010, IOP, http://arxiv.org/abs/arXiv:0907.1640

** Abstract **

We study axially symmetric codimension-2 cosmology for a distributional braneworld fueled by a localised 4D perfect fluid, in a 6D Lovelock theory. We argue that only the matching conditions (dubbed topological) where the extrinsic curvature on the brane has no jump describe a pure codimension-2 brane. If there is discontinuity in the extrinsic curvature on the brane, this induces inevitably codimension-1 distributional terms. We study these topological matching conditions, together with constraints from the bulk equations evaluated at the brane position, for two cases of regularisation of the codimension-2 defect. First, for an arbitrary smooth regularisation of the defect and second for a ring regularisation which has a cusp in the angular part of the metric. For a cosmological ansatz, we see that in the first case the coupled system is not closed and requires input from the bulk equations away from the brane. The relevant bulk function, which is a time-dependent angular deficit, describes the energy exchange between the brane and the 6D bulk. On the other hand, for the ring regularisation case, the system is closed and there is no leakage of energy in the bulk. We demonstrate that the full set of matching conditions and field equations evaluated at the brane position are consistent, correcting some previous claim in the literature which used rather restrictive assumptions for the form of geometrical quantities close to the codimension-2 brane. We analyse the modified Friedmann equation and we see that there are certain corrections coming from the non-zero extrinsic curvature on the brane. We establish the presence of geometric self-acceleration and a possible curvature domination wedged in between the period of matter and self-acceleration eras as signatures of codimension-2 cosmology.

D.V. Gal'tsov, **G. Kofinas**, P. Spirin, T.N. Tomaras, Transplanckian bremsstrahlung and black hole production, Phys. Lett. B, Vol. 683, pp. 331-334, 2010, Elsevier, http://arxiv.org/abs/arXiv:0908.0675

** Abstract **

Classical gravitational bremsstrahlung in particle collisions at transplanckian energies is studied in ${\mathcal M}_4\times {\mathcal T}^d$. The radiation efficiency $\epsilon\equiv E_{\rm rad}/E_{\rm initial}$ is computed in terms of the Schwarzschild radius $r_S(\sqrt{s})$, the impact parameter $b$ and the Lorentz factor $\gamma_{\rm cm}$ and found to be $\epsilon=C_d (r_S/b)^{3d+3} \gamma_{\rm cm}^{2d+1}$, larger than previous estimates by many powers of $\gamma_{\rm cm}\gg 1$. The result is reliable for impact parameters in the overlap of $r_S**\lambda_C$, with $b_c$ marking (for $d\neq 0$) the loss of the notion of classical trajectories and $\lambda_C\equiv \hbar/mc$ the Compton length of the scattered particles. The condition on $s$ and $m$ for extreme radiation damping and (presumably) no black hole production is also derived.
**

E. Kiritsis, **G. Kofinas**, On Horava-Lifshitz 'Black Holes', JHEP, Vol. 1001, pp. 122, 2010, Springer, http://arxiv.org/abs/arXiv:0910.5487

** Abstract **

The most general spherically symmetric solution with zero shift is found in the non-projectable Horava-Lifshitz class of theories with general coupling constants. It contains as special cases, spherically symmetric solutions found by other authors earlier. It is found that the generic solution has conventional (AdS, dS or flat) asymptotics with a universal 1/r tail. There are several special cases where the asymptotics differ, including the detailed balance choice of couplings. The conventional thermodynamics of this general class of solutions is established by calculating the energy, temperature and entropy. Although several of the solutions have conventional horizons, for particles with ultra-luminal dispersion relations such solutions appear to be horizonless.

D.V. Gal'tsov, **G. Kofinas**, P. Spirin, T.N. Tomaras, Classical ultrarelativistic bremsstrahlung in extra dimensions, JHEP, Vol. 1005, pp. 055, 2010, Springer, http://arxiv.org/abs/arXiv:1003.2982

** Abstract **

The emitted energy and the cross-section of classical scalar bremsstrahlung in massive particle collisions in D=4+d dimensional Minkowski space M_D as well as in the brane world M_4 \times T^d is computed to leading ultra-relativistic order. The particles are taken to interact in the first case via the exchange of a bulk massless scalar field \Phi and in the second with an additional massless scalar \phi confined together with the particles on the brane. Energy is emitted as \Phi radiation in the bulk and/or \phi radiation on the brane. In contrast to the quantum Born approximation, the classical result is unambiguous and valid in a kinematical region which is also specified. For D=4 the results are in agreement with corresponding expressions in classical electrodynamics.

D.V. Gal'tsov, **G. Kofinas**, P. Spirin, T.N. Tomaras, Classical ultra-relativistic scattering in ADD, JHEP, Vol. 0905, pp. 074, 2009, IOP, http://arxiv.org/abs/arXiv:0903.3019

** Abstract **

The classical differential cross-section is calculated for high-energy small-angle gravitational scattering in the factorizable model with toroidal extra dimensions. The three main features of the classical computation are: (a) It involves summation over the infinite Kaluza-Klein towers but, contrary to the Born amplitude, it is finite with no need of an ultraviolet cutoff. (b) It is shown to correspond to the non-perturbative saddle-point approximation of the eikonal amplitude, obtained by the summation of an infinite number of ladder graphs of the quantum theory. (c) In the absence of extra dimensions it reproduces all previously known results.

E. Kiritsis, **G. Kofinas**, Horava-Lifshitz Cosmology, Nucl. Phys. B, Vol. 821, pp. 467-480, 2009, Elsevier, http://arxiv.org/abs/arXiv:0904.1334

** Abstract **

The cosmological equations suggested by the non-relativistic renormalizable gravitational theory proposed by Ho\v{r}ava are considered. It is pointed out that the early universe cosmology has features that may give an alternative to inflation and the theory may be able to escape singularities.

** Abstract **

Thin gravitating defects with conical singularities in higher codimensions and with generalized Israel matching conditions are known to be inconsistent for generic energy-momentum. A way to remove this inconsistency is proposed and is realized for an axially symmetric gravitating codimension-two defect in six dimensional Einstein gravity. By varying with respect to the brane embedding fields, alternative matching conditions are derived, which are generalizations of the Nambu-Goto equations of motion of the defect, consistent with bulk gravity. For a maximally symmetric defect the standard picture is recovered. The four-dimensional perfect fluid cosmology coincides with conventional FRW in the case of radiation, but for dust it has rho^{4/3} instead of rho. A four-dimensional black hole solution is presented having the Schwarzschild form with a short-distance correction r^{-2}.

** Abstract **

A definite form for the boundary term that produces the finiteness of both the conserved quantities and Euclidean action for any Lovelock gravity with AdS asymptotics is presented. This prescription merely tells even from odd bulk dimensions, regardless the particular theory considered, what is valid even for Einstein-Hilbert and Einstein-Gauss-Bonnet AdS gravity. The boundary term is a given polynomial of the boundary extrinsic and intrinsic curvatures (also referred to as Kounterterms series). Only the coupling constant of the boundary term changes accordingly, such that it always preserves a well-posed variational principle for boundary conditions suitable for asymptotically AdS spaces. The background-independent conserved charges associated to asymptotic symmetries are found. In odd bulk dimensions, this regularization produces a generalized formula for the vacuum energy in Lovelock AdS gravity. The standard entropy for asymptotically AdS black holes is recovered directly from the regularization of the Euclidean action, and not only from the first law of thermodynamics associated to the conserved quantities.

** Abstract **

We consider (thin) braneworlds with conical singularities in six-dimensional Einstein-Gauss-Bonnet gravity with a bulk cosmological constant. The Gauss-Bonnet term is necessary in six dimensions for including non-trivial brane matter. We show that this model for axially symmetric bulks does not possess isotropic braneworld cosmological solutions.

** Abstract **

The role of brane-bulk energy exchange and of an induced gravity term on a single braneworld of negative tension and vanishing effective cosmological constant is studied. It is shown that for the physically interesting cases of dust and radiation a unique global attractor which can realize our present universe (accelerating and 0

** Abstract **

A finite action principle for Einstein-Gauss-Bonnet AdS gravity is presented. The boundary term, which is different for even and odd dimensions, is a functional of the boundary metric, intrinsic curvature and extrinsic curvature. For even dimensions, the boundary term corresponds to the maximal Chern form of the spacetime, and the asymptotic AdS condition for the curvature suffices for the well-posedness of this action. For odd dimensions, the action is stationary under a boundary condition on the variation of the extrinsic curvature. The background-independent Noether charges associated to asymptotic symmetries are found and the Euclidean continuation of the action correctly describes the black hole thermodynamics in the canonical ensemble. In particular, this procedure leads to a covariant formula for the vacuum energy in odd-dimensional asymptotically AdS spacetimes.

** Abstract **

We study braneworlds in six-dimensional Einstein-Gauss-Bonnet gravity. The Gauss-Bonnet term is crucial for the equations to be well-posed in six dimensions when non-trivial matter on the brane is included (the also involved induced gravity term is not significant for their structure), and the matching conditions of the braneworld are known. We show that the energy-momentum of the brane is always conserved, independently of any regular bulk energy-momentum tensor, contrary to the situation of the five-dimensional case.

** Abstract **

We study the collapse of a homogeneous braneworld dust cloud in the context of the various curvature correction scenarios, namely, the induced-gravity, the Gauss-Bonnet, and the combined induced-gravity and Gauss-Bonnet. In accordance to the Randall-Sundrum model, and contrary to four-dimensional general relativity, we show in all cases that the exterior spacetime on the brane is non-static.

A. Kehagias, **G. Kofinas**, Cosmology with exponential potentials , Class.Quant.Grav. , Vol. 21, pp. 3871-3886, 2004, IOP, http://arxiv.org/abs/gr-qc/0402059

** Abstract **

We examine in the context of general relativity the dynamics of a spatially flat Robertson-Walker universe filled with a classical minimally coupled scalar field \phi of exponential potential ~ e^{-\mu\phi} plus pressureless baryonic matter. This system is reduced to a first-order ordinary differential equation, providing direct evidence on the acceleration/deceleration properties of the system. As a consequence, for positive potentials, passage into acceleration not at late times is generically a feature of the system, even when the late-times attractors are decelerating. Furthermore, the structure formation bound, together with the constraints on the present values of \Omega_{m}, w_{\phi} provide, independently of initial conditions and other parameters, necessary conditions on \mu. Special solutions are found to possess intervals of acceleration. For the almost cosmological constant case w_{\phi} ~ -1, as well as, for the generic late-times evolution, the general relation \Omega_{\phi}(w_{\phi}) is obtained

E. Kiritsis, **G. Kofinas**, N. Tetradis, T.N. Tomaras, V. Zarikas, Cosmological evolution with brane bulk energy exchange, JHEP, Vol. 0302, pp. 035, 2003, IOP, http://arxiv.org/abs/hep-th/0207060

** Abstract **

The consequences for the brane cosmological evolution of energy exchange between the brane and the bulk are analysed in detail, in the context of a non-factorizable background geometry with vanishing effective cosmological constant on the brane. A rich variety of brane cosmologies is obtained, depending on the precise mechanism of energy transfer, the equation of state of brane-matter and the spatial topology. An accelerating era is generically a feature of our solutions. In the case of low-density flat universe more dark matter than in the conventional FRW picture is predicted. Spatially compact solutions are found to delay their recollapse.

** Abstract **

We study the cosmology of the Randall-Sundrum brane-world where the Einstein-Hilbert action is modified by curvature correction terms: a four-dimensional scalar curvature from induced gravity on the brane, and a five-dimensional Gauss-Bonnet curvature term. The combined effect of these curvature corrections to the action removes the infinite-density big bang singularity, although the curvature can still diverge for some parameter values. A radiation brane undergoes accelerated expansion near the minimal scale factor, for a range of parameters. This acceleration is driven by the geometric effects, without an inflaton field or negative pressures. At late times, conventional cosmology is recovered.

** Abstract **

An analysis of a spherically symmetric braneworld configuration is performed when the intrinsic curvature scalar is included in the bulk action. In the case when the electric part of the Weyl tensor is zero, all the exterior solutions are found; one of them is of the Schwarzschild-(A)dS 4 form, which is matched to a modified Oppenheimer-Volkoff interior solution. In the case when the electric part of the Weyl tensor is non zero, the exterior Schwarzschild-(A)dS 4 black hole solution is modified receiving corrections from the non-local bulk effects. A non-universal gravitational constant arises, depending on the density of the considered object and the Newton's law is modified for small and large distances; however, the conventional limits are easily obtained.

** Abstract **

A new moving domain wall solution is obtained for a flat 3-universe. This consists of a bulk metric depending on both time and the extra coordinate, plus a dynamically interacting domain wall, admitted by the metric and inhabited by the three-universe. The matter contents are cosmological constants on the domain wall and the bulk. The bulk space is shown to be (A)dS_(5). A remarkable fact concerning the three-universe is that its scale factor never vanishes, even though the corresponding scale factor of the bulk metric vanishes. The inclusion of a bulk scalar field is discussed, neglecting back-reaction. Its normalizability and the existence of a positive frequency or adiabatic bulk vacuum are shown.

** Abstract **

An analysis of a spherically symmetric braneworld configuration is performed when the intrinsic curvature scalar is included in the bulk action; the vanishing of the electric part of the Weyl tensor is used as the boundary condition for the embedding of the brane in the bulk. All the solutions outside a static localized matter distribution are found; some of them are of the Schwarzschild-(A)dS_{4} form. Two modified Oppenheimer-Volkoff interior solutions are also found; one is matched to a Schwarzschild-(A)dS_{4} exterior, while the other does not. A non-universal gravitational constant arises, depending on the density of the considered object; however, the conventional limits of the Newton's constant are recovered. An upper bound of the order of TeV for the energy string scale is extracted from the known solar system measurements (experiments). On the contrary, in usual brane dynamics, this string scale is calculated to be larger than TeV.

** Abstract **

We extent our previous study on spherically symmetric braneworld solutions with induced gravity, including non-local bulk effects. We find the most general static four-dimensional black hole solutions with g_{tt}=-g_{rr}^{-1}. They satisfy a closed system of equations on the brane and represent the strong-gravity corrections to the Schwarzschild-(A)dS_4 spacetime. These new solutions have extra terms which give extra attraction relative to the Newtonian-(A)dS_4 force; however, the conventional limits are easily obtained. These terms, when defined asymptotically, behave like AdS_4 in this regime, while when defined at infinitely short distances predict either an additional attractive Newtonian potential or an attractive potential which scales approximately as sqrt(r). One of the solutions found gives extra deflection of light compared to Newtonian deflection.

T. Christodoulakis, **G. Kofinas**, E. Korfiatis, G.O. Papadopoulos, A. Paschos, Time dependent automorphism inducing diffeomorphisms and the complete closed form solutions of Bianchi types II and V vacuum cosmologies , J. Math. Phys., Vol. 42, pp. 3580-3608, 2001, AIP, http://arxiv.org/abs/gr-qc/0008050

** Abstract **

We investigate the set of spacetime general coordinate transformations (G.C.T.) which leave the line element of a generic Bianchi Type Geometry, quasi-form invariant; i.e. preserve manifest spatial Homogeneity. We find that these G.C.T.'s, induce special time-dependent automorphic changes, on the spatial scale factor matrix $\gamma_{\alpha\beta}(t)$ -along with corresponding changes on the lapse function $N(t)$ and the shift vector $N^{\alpha}(t)$. These changes, which are Bianchi Type dependent, form a group and are, in general, different from those induced by the group SAut(G) -advocated in earlier investigations as the relevant symmetry group-, they are used to simplify the form of the line element -and thus simplify Einstein's equations as well-, without losing generality.
As far as this simplification procedure is concerned, the transformations found, are proved to be essentialy unique. For the case of Bianchi Types II and V, where the most general solutions are known -Taub's and Joseph's, respectively-, it is explicitly verified that our transformations and only those, suffice to reduce the generic line element, to the previously known forms. It becomes thus possible, -for these Types- to give in closed form, the most general solution, containing all the necessary ``gauge'' freedom.

T. Christodoulakis, **G. Kofinas**, G.O. Papadopoulos, Conditional symmetries and phase space reduction towards G.C.T. invariant wave functions, for the class A Bianchi type VI and VII vacuum cosmologies , Phys. Lett. B, Vol. 514, pp. 149-154, 2001, Elsevier, http://arxiv.org/abs/gr-qc/0101103

** Abstract **

The quantization of Class A Bianchi Type VI and VII geometries -with all six scale factors, as well as the lapse function and the shift vector present- is considered. A first reduction of the initial 6-dimensional configuration space is achieved by the usage of the information furnished by the quantum form of the linear constraints. Further reduction of the space in which the wave function -obeying the Wheeler-DeWitt equation- lives, is accomplished by revealing a classical integral of motion, tantamount to an extra symmetry of the corresponding classical Hamiltonian. This symmetry generator -member of a larger group- is linear in momenta and corresponds to G.C.T.s through the action of the automorphism group -especially through the action of the outer automorphism subgroup. Thus, a G.C.T. invariant wave function is found, which depends on one combination of the two curvature invariants --which uniquely and irreducibly characterizes the hypersurfaces t=const.

** Abstract **

A general analysis of the induced brane dynamics is performed when the intrinsic curvature term is included in the action. Such a term is known to cause dramatic changes and is generically induced by quantum corrections coming from the bulk gravity and its coupling with matter living on the brane. The induced brane dynamics is shown to be the usual Einstein dynamics coupled to a well defined modified energy-momentum tensor. In cosmology, conventional general relativity revives for an initial era whose duration depends on the value of the five-dimensional Planck mass. Violations of energy conditions may be possible, as well as matter inhomogeneities on the brane in (A)dS_{5} or Minkowski backgrounds. A new anisotropic cosmological solution is given in the above context. This solution, for a fine-tuned five-dimensional cosmological constant, exhibits an intermediate accelerating phase which is followed by an era corresponding to a 4D perfect fluid solution with no future horizons.

T. Christodoulakis, **G. Kofinas**, V. Zarikas, The General untilted diffuse matter Bianchi V universe, Phys. Lett. A, Vol. 275, pp. 182-192, 2000, Elsevier, http://arxiv.org/abs/gr-qc/9907104

** Abstract **

A diffuse matter filled type V Universe is studied. The anisotropic behaviour, the distortion caused to the CMBR and the parameter region allowed by present cosmological bounds are examined. It is shown how the overall sky pattern of temperature anisotropies changes under a non-infinitesimal spatial coordinate transformation that preserves the type V manifest homogeneity.

T. Christodoulakis, **G. Kofinas**, E. Korfiatis, A. Paschos, Isolation of the true degree of freeedom and normalizable wave functions for the general type V cosmology , Phys. Lett. B, Vol. 419, pp. 30-36, 1998, Elsevier, http://www.sciencedirect.com/science/art...

** Abstract **

The quantization of the most general Type V geometry (with all six scale factors as well as the shift vector present) is considered. The information carried by the linear constraints is used to reduce the Wheeler–DeWitt equation (arising from a valid Hamiltonian found earlier), which initially included six variables, to a final PDE in three variables, getting rid of three redundant variables (gauge degrees of freedom). The full space of solutions to this equation is presented. In trying to interpret these wave functions, we are led through further consideration of the action of the automorphism group on the configuration space, to a final reduction to the one and only true degree of freedom, i.e. the only independent curvature invariant of the slice t=constant. Thus, a normalizable wave function in terms of the true degree of freedom is obtained.

T. Christodoulakis, **G. Kofinas**, E. Korfiatis, A. Paschos, Wave functions for the general type II cosmology , Phys. Lett. B, Vol. 390, pp. 55-58, 1997, Elsevier, http://www.sciencedirect.com/science/art...

** Abstract **

The quantization of the most general Type II geometry (having all six scale factors as well as the shift vector) is considered. The information carried by the linear constraints is used to reduce the initial Wheeler-De Witt equation (in six variables) to a final PDE in four variables. The full space of solutions to this equation is exhibited.

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V. Zarikas, **G. Kofinas**, Singularities and Phenomenological aspects of Asymptotic Safe Gravity, 20th International Meeting: Physical Interpretations of Relativity Theory (PIRT 2017), July 2017, Moscow, Russia , J. Phys. Conf. Series 1051, no. 1, pp. 012028, Dec, 2018, https://iopscience.iop.org/article/10.10...

** Abstract **

Asymptotic Safety (AS) Program for quantum gravity keeps the same fields and symmetries with General Relativity and studies the associated gravitational action as a fundamental part of the complete theory at the nonperturbative level with the help of functional renormalization group (RG) techniques. An important phenomenological task that can test the new point of view of AS approach is the discovery of RG improved cosmologies and black holes. In this work, we analyze the properties of recently found non-singular spherically symmetric and non-singular cosmological solutions. Furthermore, we derive a novel consistent set of modified Einstein field equations, in the spirit of AS, which respects the Bianchi identities. This new set of equations completes previously published modified Einstein equations which arise by adding appropriate covariant kinetic terms to the action.

** Abstract **

We show the universal form of the boundary term (Kounterterm series) which regularizes the Euclidean action and background-independent definition of conserved quantities for any Lovelock gravity theory with AdS asymptotics (including Einstein-Hilbert and Einstein-Gauss-Bonnet). We discuss on the connection of this procedure to the existence of topological invariants and Chern-Simons forms in the corresponding dimensions.

**Copyright Notice:** This material is presented to ensure timely dissemination of scholarly and technical work.
Copyright and all rights therein are retained by authors or by other copyright holders.
All persons copying this information are expected to adhere to the terms and constraints invoked
by each author's copyright. In most cases, these works may not be reposted or mass reproduced
without the explicit permission of the copyright holder.

**Copyright Notice:** This material is presented to ensure timely dissemination of scholarly and technical work.
Copyright and all rights therein are retained by authors or by other copyright holders.
All persons copying this information are expected to adhere to the terms and constraints invoked
by each author's copyright. In most cases, these works may not be reposted or mass reproduced
without the explicit permission of the copyright holder.

**Copyright Notice:** This material is presented to ensure timely dissemination of scholarly and technical work.
Copyright and all rights therein are retained by authors or by other copyright holders.
All persons copying this information are expected to adhere to the terms and constraints invoked
by each author's copyright. In most cases, these works may not be reposted or mass reproduced
without the explicit permission of the copyright holder.