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ACT-compatible Inflation in Scalar Field Coupled Gravity
Shin'ichi Nojiri, S. D. Odintsov, V. K. Oikonomou
In this work, we construct models compatible with ACT constraints on inflation in the framework of gravity, with gravity coupled to a scalar fiel…
Horizon singularity, energy conditions and shadows in time-dependent and spherically symmetric spacetime
Shin'ichi Nojiri, Sergei D. Odintsov, Diego Sáez-Chillón Gómez +1
Some dynamical spacetime solutions that describe ultra-compact objects are analysed. We show that by turning a static spacetime that describes a one-way wormhole into a dynamical t…
ACT-DR6 consistent inflation in generalised entropic cosmology and gravity
Shin'ichi Nojiri, Sergei D. Odintsov
Within the framework of generalised entropic cosmology and its equivalent gravity, we construct inflationary scenarios consistent with recent Atacama Cosmology Telescope (AC…
Observational signatures of negative mass wormholes through their shadows
Shin'ichi Nojiri, Sergei D. Odintsov, Diego Sáez-Chillón Gómez
Systems containing objects with negative mass (NMOs) are considered. Such a system consists of one object with positive mass and one NMO, where a bound state exists even though the…
Ghost-free non-local Gravity Compatible with ACT
Shin'ichi Nojiri, S. D. Odintsov, V. K. Oikonomou
We confront the ghost-free non-local gravity theories with the latest Atacama Cosmology Telescope (ACT) constraints on the spectral index of the scalar perturbations and the…
May Negative Mass Objects exist in the sky?
Shin'ichi Nojiri, S. D. Odintsov
We conjecture the possibility of negative mass objects (NMOs) existing in the sky. It is shown that they may not be so exotic as usually expected. We show that NMOs appear as solut…
Is Phantom Divide Crossing in General Relativity Completely Impossible? Shortcomings in canonical and minimally coupled scalar field and Possible Solutions in -essence Models
Shin'ichi Nojiri, S. D. Odintsov, V. K. Oikonomou
General relativity has its successes at the local astrophysical level, however, it seems to be insufficient in describing the Universe at large scales. In this work we investigate…
Phantom Crossing and Oscillating Dark Energy with Gravity
Shin'ichi Nojiri, S. D. Odintsov, V. K. Oikonomou
In this work, we shall consider how a dynamical oscillating and phantom crossing dark energy era can be realized in the context of gravity. We approach the topic from a theo…
Black holes and their shadows in gravity
Shin'ichi Nojiri, S. D. Odintsov
We investigate the radii of the photon sphere and the black hole shadow in the framework of gravity. For this purpose, we derive the field equation for the corresponding the…
Black holes, photon sphere, and cosmology in ghost-free gravity
Shin'ichi Nojiri, S. D. Odintsov
The improved version of ghost-free gravity introduced in Phys.~Lett.~B \textbf{631} (2005), 1-6 is proposed and investigated. It is demonstrated that improved ghos…
gravity with Gauss-Bonnet corrections: from early-time inflation to late-time acceleration
Shin'ichi Nojiri, S. D. Odintsov
We show that in the gravity with a non-metricity scalar , the curvatures in Einstein's gravity, that is, the Riemann curvature constructed from the standard Levi-Civita c…
Well-defined Gravity, Reconstruction of FLRW Spacetime and Unification of Inflation with Dark Energy Epoch
Shin'ichi Nojiri, S. D. Odintsov
We formulate the convenient and mimetic gravities in terms of the metric and four scalar fields with the being the non-metricity scalar. As a result, it is shown that th…
Gravitational collapse, primordial black hole, and gravitational wave in Einstein--Gauss-Bonnet theory with two scalar fields
Shin'ichi Nojiri, G. G. L. Nashed
In this paper, we investigate the gravitational collapse to form the black hole in the acceleratingly expanding universe in the frame of Einstein--Gauss-Bonnet theory having two sc…
General geometry realized by four-scalar model and application to gravity
G. G. L. Nashed, Shin'ichi Nojiri
In this paper, we propose a model including four scalar fields coupled with general gravity theories, which is a generalization of the two-scalar model proposed in Phys. Rev. D \te…
Wormholes inside stars and black holes
Shin'ichi Nojiri, S. D. Odintsov, Vladimir Folomeev
We construct models of two exotic objects: (i) a wormhole whose throat is hidden by a stellar object like a neutron star; and (ii) a wormhole inside a black hole. We work within Ei…
Propagation of Gravitational Waves in a Dynamical Wormhole Background for Two-scalar Einstein-Gauss-Bonnet Theory
E. Elizalde, Shin'ichi Nojiri, S. D. Odintsov +1
In this work, we propose a model of Einstein--Gauss-Bonnet gravity coupled with two scalar fields. The two scalar fields are considered to be ``frozen'' or they become non-dynamica…
Stable gravastar with large surface redshift in Einstein's gravity with two scalar fields
Shin'ichi Nojiri, G. G. L. Nashed
We propose a class of models, in which stable gravastar with large surface redshift becomes a solution. In recent decades, gravastars have become a plausible substitute for black h…
Modified Gravity Theories on a Nutshell: Inflation, Bounce and Late-time Evolution
S. Nojiri, S. D. Odintsov, V. K. Oikonomou
We systematically review some standard issues and also the latest developments of modified gravity in cosmology, emphasizing on inflation, bouncing cosmology and late-time accelera…
Newton law in covariant unimodular gravity
S. Nojiri, S. D. Odintsov, V. K. Oikonomou
We propose a covariant ghost-free unimodular gravity theory, which contains a three-form field and study its structure using the analogy of the proposed theory with a quantu…
Unimodular-Mimetic Cosmology
S. Nojiri, S. D. Odintsov, V. K. Oikonomou
We combine the unimodular gravity and mimetic gravity theories into a unified theoretical framework, which is proposed to provide a suggestive proposal for a framework that may ass…
The bounce universe history from unimodular gravity
S. Nojiri, S. D. Odintsov, V. K. Oikonomou
In this paper we investigate how to realize various quite well known cosmological bouncing models in the context of the recently developed unimodular gravity. Particularly,…
Singular inflation from generalized equation of state fluids
S. Nojiri, S. D. Odintsov, V. K. Oikonomou
We study models with a generalized inhomogeneous equation of state fluids, in the context of singular inflation, focusing to so-called Type IV singular evolution. In the simplest c…