Narrow Results

In this work, we propose and investigate an inflationary model with a kinetic coupling term $\kappa G^{\mu \nu }{\partial }_{\mu }\varphi {\partial }_{\nu }\varphi$ and a combined scalar potential $V(\varphi )=\frac{\alpha {\varphi }^2}{V_0}+\frac{arctan(\beta \varphi )}{V_0}$. Exploiting the dynamical system analysis, we identify the stationary points and examine their stability showing certain restrictions on the involved parameters $\alpha$, $\beta$ and $p=\frac{\kappa }{V_0}$. Then, we compute and study the relevant observable quantities such as the spectral index $n_s$ and the tensor-to-scalar ratio $r$ from slow-roll approximations. For certain values of the restricted parameters, we find that $n_s$ and $r$ are in a good agreement with the current observation data. To support such findings, we consider the limit corresponding to the vanishing kinetic coupling contribution. We find that this limit provides a boomerang-like geometry intersecting almost all Planck contours associated with the $(n_s-r)$ curves.

This work analyzes a power law solution under $f(R,T)$ gravity for an isotropic and homogeneous universe. To construct $f(R,T)$ gravity model, we consider the functional form of $f(R,T)$ as the sum of two independent functions of the Ricci scalar R and the trace of the energy–momentum tensor T, i.e. $f(R,T)=R+\xi RT$, with $\xi$ being a positive constant where we study the cosmological model under the following two cases: (i) $f_1(R)=f_2(R)=R$ and (ii) $f_3(T)=\xi T$. In the framework of $f(R,T)$ gravity with homogeneous and isotropic spacetime, the constructed model yields several features on application of the scale factor $a=\alpha t^{\beta }$. We employed the Markov Chain Monte Carlo (MCMC) approach to get model parameters $\alpha$, $\beta$ and $H_0$ over a redshift range of $0\le z\le 1.965$. The model parameter’s restricted values are listed below: $H_0=67.098_{-1.792}^{+2.148}$ km ${\text{s}}^{-1}$Mpc${}^{-1}$, $H_0=67.588_{-2.170}^{+2.229}$km${\text{s}}^{-1}$ Mpc${}^{-1}$, $H_0=66.270_{-2.181}^{+2.215}$kms${}^{-1}$ Mpc${}^{-1}$, $H_0=65.960_{-1.834}^{+2.380}$kms${}^{-1}$Mpc${}^{-1}$, $H_0=66.274_{-1.864}^{+2.015}$kms${}^{-1}$Mpc${}^{-1}$. The model was constrained using the Hubble parameter ($H(z)$) dataset, Baryon Acoustic Oscillations (BAO) dataset, Pantheon dataset, joint $H(z)$ + Pantheon dataset and collective $H(z)$ + BAO + Pantheon dataset. The results from the Planck collaboration group are consistent with these calculated Ho observed values. In order to study and analyze the model, we first look at how the energy circumstances affected our power law assumption. The validity of the model has also been evaluated using the $Om$ diagnostic and the jerk parameter, which are state finding diagnostic tools. We find that the model under investigation agrees with the observed fingerprints within a certain range of constraints.

We investigate inflationary models in $f(R,T)$ modified gravity with a kinetic coupling term ${\omega }^2{G}^{\mu \nu }{\partial }_{\mu }\varphi {\partial }_{\nu }\varphi$ having a positive factor needed to remove the ghosts. Taking $f(R,T)=R+2\beta T$, we calculate and analyze the relevant observable quantities including the spectral index $n_s$ and the tensor-to-scalar ratio $r$ using the slow-roll approximations. Concretely, we consider two scenarios described by the decoupling and the coupling behaviors between the scalar potential and the $f(R,T)$ gravity via the moduli space by dealing with two potentials being the quartic one $V(\varphi )=\lambda {\varphi }^4$ and the small field inflation $V(\varphi )=V_0(1-{(\frac{\varphi }{\mu })}^{\alpha })$. For the quartic inflation model, we consider a decoupling behavior. For the small field inflation, however, we present the parameter decoupling and coupling scenarios. For both scenarios, we compute and inspect $n_s$ and $r$ showing interesting results. For three different values of the number of e-folds $N=60,65$ and 70, we find that the coupling between $f(R,T)$ and the scalar potential via the moduli space provides an excellent agreement with the observational findings. In the last part of this work, we provide a possible discussion on the amplitude of the scalar power spectrum needed to provide a viability of the proposed theory. Considering the second potential form in the parameter coupling scenario, we find acceptable values in certain points of the moduli space.

We have studied constant-roll inflation using the q-de Sitter scale factor. Both the cold and warm inflation have been considered, and the scalar potential and inflaton field have been determined. Also tensor to scalar ratio and the spectral index are obtained. In the cold inflation case, the perturbations have been investigated using Mukhanov–Sasaki equation, and the results determined for the spectral index and tensor to scalar ration are consistent with observations. In the warm inflation two cases of constant damping ($\mathrm{\Gamma }={\mathrm{\Gamma }}_0$) and the field dependent damping ($\mathrm{\Gamma }={\mathrm{\Gamma }}_{1}V(\varphi )$) have been studied. In both cases the power spectrum and the tensor to scalar ratio are determined as a function of the inflaton field. It has been shown that in both cases the spectral index is independent of the inflaton field. In the case of constant damping the obtained value for the spectral index is inconsistent with observations but for the case of field dependent damping the parameter $q$ can be chosen such that the value obtained for all the physical quantities including the spectral index be consistent with present cosmological observations.

A warm inflation model, drawn upon the vector cosmology, was applied in the present work to calculate cosmological fluctuations. Moreover, parameters of the model were constrained and compared using the latest observational data such as Planck and WMAP9.

In this work, we focus on the gravitationally influenced adiabatic particle creation process, a mechanism that does not need any dark energy or modified gravity models to explain the current accelerating phase of the universe. Introducing some particle creation models that generalize some previous models in the literature, we constrain the cosmological scenarios using the latest compilation of the Type Ia Supernovae data only, the first indicator of the accelerating universe. Aside from the observational constraints on the models, we examine the models using two model independent diagnoses, namely the cosmography and $Om$. Further, we establish the general conditions to test the thermodynamic viabilities of any particle creation model. Our analysis shows that at late-time, the models have close resemblance to that of the $\mathrm{\Lambda }$CDM cosmology, and the models always satisfy the generalized second law of thermodynamics under certain conditions.

This study set out to investigate the effect of anisotropy on the $\mathrm{\Lambda }$CDM model in the framework of Brans−Dicke theory. To this end, astrophysical constraints on this model using current available data including type Ia supernovae (SNIa), the Baryon Acoustic Oscillation (BAO), and the Hubble parameter $H(z)$ data were deployed. Here, we present combined results from these probes, deriving constraints on ${\mathrm{\Omega }}_{{\sigma }_0}$ of $\mathrm{\Lambda }$CDM model and its anisotropy energy density in an anisotropic universe. It is found that ${\mathrm{\Omega }}_{{\sigma }_0}$ can be constrained by the $\mathrm{\text{SNIa}}+H(z)+\mathrm{\text{BAO}}$ data, with the best fitting value ${\mathrm{\Omega }}_{{\sigma }_0}=(-6.52\pm 4.21)\times 10^{-4}$ for the Brans–Dicke cosmology. We extend our study to the case of $\mathrm{\Lambda }$CDM model in an anisotropic universe and Brans–Dicke framework and find out that the equation of state parameter (${\omega }_{\mathrm{\Lambda }}$) cannot cross the phantom line and eventually the universe approaches a quintessence era.

The impact of anisotropy on the Ricci dark energy cosmologies is investigated where it is assumed that the geometry of the universe is described by Bianchi type I (BI) metric. The main goal is to determine the astrophysical constraints on the model by using the current available data as type Ia supernovae (SNIa), the Baryon Acoustic Oscillation (BAO), and the Hubble parameter $H(z)$ data. In this regard, a maximum likelihood method is applied to constrain the cosmological parameters. Combining the data, it is found out that the allowed range for the density parameter of the model stands in $-3.6\times 10^{-3}\lesssim {\mathrm{\Omega }}_{{\sigma }_0}\lesssim -2.6\times 10^{-3}$. With the help of the Supernova Legacy Survey (SNLS) sample, we estimate the possible dipole anisotropy of the Ricci dark energy model. Then, by using a standard ${\chi }^2$ minimization method, it is realized that the transition epoch from early decelerated to current accelerated expansion occurs faster in Ricci dark energy model than $\mathrm{\Lambda }$CDM model. The results indicate that the BI model for the Ricci dark energy is consistent with the observational data.

In this paper, we propose a specialized parameterization for the Hubble parameter, inspired by $\mathrm{\Lambda }$CDM cosmology, to investigate the cosmic expansion history of the Universe. This parameterization is employed to analyze the universe’s late-time behavior within the context of $Q^n$ gravity, where $Q$ represents non-metricity. By using data from 57 Hubble data points, 1048 supernova (SNe) data points, and six Baryon Acoustic Oscillation (BAO) data points, we determine the optimal values for the model parameters. Additionally, we explore three distinct cosmological models based on the parameter $n$, specifically when it takes on the values of $0.55$, $1.5$ and $2.0$. The results of our analysis indicate that our proposed parameterization, along with the associated models for different values of $n$, predicts an accelerated cosmic expansion phase.

In this paper, we employ parametrization techniques within the framework of $f(T)$ gravity to investigate the deceleration parameter (DP), a key quantity characterizing the universe’s expansion dynamics. By analyzing the DP, we gain valuable insights into the nature of cosmic constituents and their impact on the universe’s evolution. We utilize a combination of observational data, including 31 Cosmic Chronometers (CC) measurements, 1048 Type Ia Supernovae (SNIa), 17 Baryon Acoustic Oscillation (BAO) measurements, Cosmic Microwave Background (CMB), 162 Gamma Ray Bursts (GRB), and 24 observations of compact radio quasars (Q). We employ the Markov chain Monte Carlo (MCMC) sampling technique to estimate the best-fit range of model parameters. Cosmological and cosmographic parameters are investigated, and their implications in the context of cosmology have been discussed. We also analyze the statefinder and Om diagnostic to gain deeper insights into the universe’s behavior. Furthermore, we conduct statistical analyses to compare our model with the standard $\mathrm{\Lambda }$CDM model. Our investigation also includes the study of physical parameters, providing comprehensive insights into the cosmological behavior within the $f(T)$ gravity framework. The results and comparisons presented in this work contribute to a deeper understanding of the universe’s dynamics and provide valuable implications for the cosmological model under consideration.

Digital health technologies such as wearable devices have transformed health data analytics, providing continuous, high-resolution functional data on various health metrics, thereby opening new avenues for innovative research. In this work, we introduce a new approach for generating causal hypotheses for a pair of a continuous functional variable (e.g., physical activities recorded over time) and a binary scalar variable (e.g., mobility condition indicator). Our method goes beyond traditional association-focused approaches and has the potential to reveal the underlying causal mechanism. We theoretically show that the proposed scalar-function causal model is identifiable with observational data alone. Our identifiability theory justifies the use of a simple yet principled algorithm to discern the causal relationship by comparing the likelihood functions of competing causal hypotheses. The robustness and applicability of our method are demonstrated through simulation studies and a real-world application using wearable device data from the National Health and Nutrition Examination Survey.