Olga Razina

Бұл жұмыста F(T) = T − 2Λ*[1 − exp(−Ts2/T2)] функция арқылы сипатталатын модификацияланған телепараллельді гравитацияның экрандалған моделі зерттеледі. мұндағы T — бұралу скаляры, Ts өтудің сипаттамалық масштабын анықтайды, ал Λ* кеш ғаламдық (космологиялық) үдеудің амплитудасын сипаттайды. Модельдің негізгі ерекшелігі — |T| үлкен мәндерінде модификациялаушы бөліктің автоматты түрде басылуы. Жоғары бұралу режимінде түзету мүшесі T−2 заңы бойынша кемиді, нәтижесінде теория асимптоталық түрде жалпы салыстырмалылық теориясының телепараллельді эквивалентіне жақындайды. Ал |T| кіші мәндерінде модификациялаушы бөлік тұрақты −2Λ* мәніне ұмтылып, оң космологиялық тұрақтының әсерін қайта жаңғыртады. Кеңістіктік жазық FLRW (Фридман–Леметр–Робертсон–Уокер) геометриясы үшін модификацияланған Фридман теңдеуі қорытылып шығарылды, геометриялық сектордың тиімді энергия тығыздығының өрнегі алынды және E(0) = 1 шарты бойынша нормалау жүргізілді, мұндағы E(z) = H(z)/H0; онымен қатар, E′(z), тиімді күй теңдеуінің параметрі w(z), f(T) функциясының туындылары және тиімді гравитациялық тұрақты Geff үшін аналитикалық өрнектер анықталды. Монотонды космологиялық тармақта wX = −1 фантомдық шекарасының қиылысуы u = 5/4 шартымен анықталатыны көрсетілді. Сандық талдау τ = 0.30, 0.70 және 1.20 мәндері үшін жүргізілді. Кеңею функциясы, H(z) шамасының ΛCDM моделінен салыстырмалы ауытқуы, wX(z) параметрінің эволюциясы, Geff/G қатынасы және fσ8(z) комбинациясы зерттелді. Зерттеу нәтижелері модельдің фондық динамикасының ΛCDM моделіне жақын екенін және 0 ≤ z ≤ 5 аралығында FT > 0 шартын қанағаттандыратынын көрсетті. Алайда қазіргі кездегі Geff/G ≃ 1.29–1.38 мәні гравитациялық өзара әрекеттесудің едәуір күшейетінін көрсетеді. Демек, модельдің минималды нұсқасы жоғары бұралу режиміндегі экрандалуды сәтті жүзеге асырғанымен, ірі ауқымды құрылымдардың өсуін бақылау деректері тарапынан қатаң шектеулерге ұшырауы ықтимал.

A cosmological model of an expanding Universe with scalar–fermion interactions is considered.Our model is based on the interaction of scalar and fermion fields via a Yukawa-type coupling, which influences the development of cosmological expansion in its late stages. The generalized scale factor is used to study the evolution and dynamics of the system. This approach allows us to explore various expansion regimes and analyze the contribution of fields, namely, the scalar and fermion components, to the total energy density and pressure. The viability of the cosmological evolution model is studied using the dimensionless Hubble parameter and the Om(z) diagnostic.The behavior obtained differs from the standard case, where the parameter remains constant. Our results show that the effective component of dark energy dynamically evolves as the Universe expands. Our analysis demonstrates that the interaction of scalar and fermion fields can significantly influence the history and evolution of the expansion and lead to stable cosmological behavior. The proposed model can be used for further studies of dark energy interaction scenarios.

In this work, we shall consider Higgs inflationary Einstein–Gauss–Bonnet theories that are compatible with the GW170817 event, which dictates that the gravitational wave speed of primordial tensor perturbations must satisfy the constraint [ ]. We consider Einstein–Gauss–Bonnet theories that yield directly [ ] for which theories, the scalar potential and the non-minimal Gauss–Bonnet scalar coupling function are related. We assume that the non-minimal Gauss–Bonnet scalar coupling function is identical with the Higgs potential, and we confront the resulting theory with the ACT data, to determine whether the theory is viable. As we show, the resulting theoretical framework is viable and compatible with the ACT data and the updated Planck constraints on the tensor-to-scalar ratio.

By applying Noether symmetry methods, analytic solutions are obtained for a generalized Einstein–scalar–Gauss–Bonnet model with a [ ] component. Variation with respect to the metric, supplemented by small perturbations, produces the equations of motion and the terms that determine the propagation speed of tensor perturbations. The resulting Hubble parameter incorporates contributions from stiff matter and dark energy, the last originating from a scalar field non-minimally coupled to the Gauss–Bonnet invariant. The viability of the model is assessed by using Cosmic Chronometers, Baryon Acoustic Oscillations, and type Ia supernovae data. Best model selection based on information criteria indicates a slight preference for this new framework over the [ ] Cold Dark Matter model. Stability of the model follows from the positive speed of sound and absence of “Ostrogradsky ghosts”. The total equation of state parameter hints toward the presence of a transition from decelerated to accelerated expansion at [ ], corresponding to the transition from matter to dark energy dominance. Early Universe dynamics, derived from the slow-roll parameters, spectral indices, and the tensor-to-scalar ratio, are found to be perfectly consistent with observations from Planck 2018 and the Atacama Cosmology Telescope.

In this study, we explore the recently proposed f ( R , A ) gravity, a novel extension of modified gravity theories, where R represents the Ricci scalar and A denotes the anticurvature scalar. The addition of anticurvature term might play a role in softening or avoiding singularities, offering smoother transitions in regions of high curvature, such as near black holes or during cosmic inflation (Amendol et al 2020 Phys. Lett . B 811 135923). We also incorporate well-established cosmological bouncing scenarios, including the symmetric bounce, oscillatory bounce, matter bounce, little rip, and super bounce, to examine their implications within the framework of underlying gravity theory. In this context, we investigate some cosmic and thermodynamic aspects of flat FRW universe. We analyze the effective equation of state parameter ω _eff , which exhibits a transition from quintessence to phantom regimes across different bouncing cosmologies. The stability of the models is examined through the squared speed of sound parameter v s 2 , confirming stability in certain bouncing scenarios. The validity of the generalized second law of thermodynamics is verified by ensuring the positivity of total entropy production S ̇ tot . Additionally, the ω eff − ω eff ′ plane exhibits the thawing as well as the freezing regions. Our findings demonstrate that the chosen bouncing models provide viable cosmic and thermodynamic behavior, supporting their relevance in modified gravity theories.

In this work, we extend the framework analyzed in some previous work and study in depth the parameter space of the model, obtaining detailed outcomes not revealed in the previous work. We assume that some of the dark matter of the Universe is composed by low-scale mirror dark matter, which is allowed to interact only gravitationally with the Standard Model world. By varying the Yukawa couplings in the low-scale mirror dark matter world, the mirror world can experience dark cosmological phase transitions, which can be first order or second order. We study the observable effects of the dark phase transitions in our Standard Model world, and as we evince, the second-order phase transitions can lead to a deformation of the total background equation of state for temperatures of the order [ ]GeV, but have no other effect on our world. On the contrary, first-order phase transitions can generate a stochastic gravitational wave background that could be detected by future gravitational wave experiments. In addition, we examine when the mirror world has a large abundance and also whether it contains atoms among the dark matter particles. The latter two features depend mainly on the values of the Yukawa couplings and the mirror Higgs self-coupling.

In the framework of loop quantum cosmology, we investigate the dynamics of pre-inflation with a family of [ ]-attractor potentials. At the quantum bounce, either the kinetic energy or the potential energy of the inflation field dominates the background evolution. We determine the physically feasible initial conditions in both classes that yield the slow-roll inflation and enough number of [ ]-folds to be consistent with observations. The background evolution before reheating is consistently divided into three distinct phases: bouncing, transition, and slow-roll inflation in the case of kinetic energy dominated evolution at the bounce, while the first two phases disappear in the case of potential energy dominated evolution, though slow-roll inflation is still possible. We also explore the phase space trajectories for physically feasible initial conditions of the inflaton field and study the phase space analysis for a class of [ ]-attractor potentials.

We study the inflationary phenomenology of a rescaled Einstein–Gauss–Bonnet (EGB) gravity. In this framework, the gravitational constant of the Einstein–Hilbert term is rescaled due to effective terms active in the high curvature era. Basically, the total theory is an F ( R , G , ϕ ) theory with the Gauss–Bonnet part contributing only a non-minimal coupling to the scalar field, so it is a theory with string theory origins and with a non-trivial F(R) gravity part. The F(R) gravity part in the high curvature regime contributes only a rescaled Einstein–Hilbert term and thus the resulting theory is effectively a rescaled version of a standard EGB theory. We develop the formalism of rescaled EGB gravity, taking in account the GW170817 constraints on the gravitational wave speed. We show explicitly how the rescaled theory affects directly the primordial scalar and tensor perturbations, and how the slow-roll and observational indices of inflation are affected by the rescaling of the theory. We perform a thorough phenomenological analysis of several models of interest and we show that is it possible to obtain viable inflationary theories compatible with the latest Planck data. Also among the studied models there are cases that yield a relatively large blue tilted tensor spectral index and we demonstrate that these models can lead to detectable primordial gravitational waves in the future gravitational wave experiments. Some of the scenarios examined, for specific values of the reheating temperature may be detectable by SKA, LISA, BBO, DECIGO and the Einstein Telescope.