India

Subhasis Nalui

Subhra Bhattacharya

Addendum to “Herrera complexity and shadows of spherically symmetric compact objects” [Phys. Lett. B 861 (2025) 139261]

In this work we investigate the effect of complexity factor on the formation of photon spheres for spherically symmetric compact objects. The complexity factor obtained from the orthogonal splitting of the Riemann curvature tensor connects the geometric attributes of a compact spherically symmetric gravitating object with its matter inhomogeneity and pressure anisotropy via a scalar term. The novelty of the complexity factor is the inherent simple definition that identifies the evolution of matter tensors inside a given region of space-time. Such identification helps to obtain an equivalence class of gravitating compact objects based on their degree of complexity with zero complexity identified as the simplest system. On the other hand shadows and photon rings have become essential for identifying compact regions of space time characterised by massive gravity. Advanced observational data analysis tools augments the hope for identification of exotic gravitational objects, like the so called “black hole mimickers” and may serve as testing ground for other gravity theories. In this context we explore how complexity of compact objects (a fundamentally theoretical classification) is connected to the photon ring (an astrophysical observable in the universe) and its stability. We consider zero complexity systems and discuss its significance with respect to (wrt) formation of photon rings and hence shadows.

Herrera complexity and shadows of spherically symmetric compact objects

It is investigated the reconstruction during the slow-roll inflation in the most general class of scalar-torsion theories whose Lagrangian density is an arbitrary function  $$f(T,\phi )$$  <math> <mrow> <mi>f</mi> <mo>(</mo> <mi>T</mi> <mo>,</mo> <mi>ϕ</mi> <mo>)</mo> </mrow> </math>   of the torsion scalar T of teleparallel gravity and the inflaton  $$\phi $$  <math> <mi>ϕ</mi> </math>  . For the class of theories with Lagrangian density  $$f(T,\phi )=-M_{pl}^{2} T/2 - G(T) F(\phi ) - V(\phi )$$  <math> <mrow> <mi>f</mi> <mrow> <mo>(</mo> <mi>T</mi> <mo>,</mo> <mi>ϕ</mi> <mo>)</mo> </mrow> <mo>=</mo> <mo>-</mo> <msubsup> <mi>M</mi> <mrow> <mi>pl</mi> </mrow> <mn>2</mn> </msubsup> <mi>T</mi> <mo>/</mo> <mn>2</mn> <mo>-</mo> <mi>G</mi> <mrow> <mo>(</mo> <mi>T</mi> <mo>)</mo> </mrow> <mi>F</mi> <mrow> <mo>(</mo> <mi>ϕ</mi> <mo>)</mo> </mrow> <mo>-</mo> <mi>V</mi> <mrow> <mo>(</mo> <mi>ϕ</mi> <mo>)</mo> </mrow> </mrow> </math>  , with  $$G(T)\sim T^{s+1}$$  <math> <mrow> <mi>G</mi> <mrow> <mo>(</mo> <mi>T</mi> <mo>)</mo> </mrow> <mo>∼</mo> <msup> <mi>T</mi> <mrow> <mi>s</mi> <mo>+</mo> <mn>1</mn> </mrow> </msup> </mrow> </math>   and the power s as constant, we consider a reconstruction scheme for determining both the non-minimal coupling function  $$F(\phi )$$  <math> <mrow> <mi>F</mi> <mo>(</mo> <mi>ϕ</mi> <mo>)</mo> </mrow> </math>   and the scalar potential  $$V(\phi )$$  <math> <mrow> <mi>V</mi> <mo>(</mo> <mi>ϕ</mi> <mo>)</mo> </mrow> </math>   through the parametrization (or attractor) of the scalar spectral index  $$n_{s}(N)$$  <math> <mrow> <msub> <mi>n</mi> <mi>s</mi> </msub> <mrow> <mo>(</mo> <mi>N</mi> <mo>)</mo> </mrow> </mrow> </math>   and the tensor-to-scalar ratio r(N) as functions of the number of  $$e-$$  <math> <mrow> <mi>e</mi> <mo>-</mo> </mrow> </math>  folds N. As specific examples, we analyze the attractors  $$n_{s}-1 \propto 1/N$$  <math> <mrow> <msub> <mi>n</mi> <mi>s</mi> </msub> <mo>-</mo> <mn>1</mn> <mo>∝</mo> <mn>1</mn> <mo>/</mo> <mi>N</mi> </mrow> </math>   and  $$r\propto 1/N$$  <math> <mrow> <mi>r</mi> <mo>∝</mo> <mn>1</mn> <mo>/</mo> <mi>N</mi> </mrow> </math>  , as well as the case  $$r\propto 1/N (N+\gamma )$$  <math> <mrow> <mi>r</mi> <mo>∝</mo> <mn>1</mn> <mo>/</mo> <mi>N</mi> <mo>(</mo> <mi>N</mi> <mo>+</mo> <mi>γ</mi> <mo>)</mo> </mrow> </math>   with  $$\gamma $$  <math> <mi>γ</mi> </math>   a dimensionless constant. In this sense and depending on the attractors considered, we obtain different expressions for the function  $$F(\phi )$$  <math> <mrow> <mi>F</mi> <mo>(</mo> <mi>ϕ</mi> <mo>)</mo> </mrow> </math>   and the potential  $$V(\phi )$$  <math> <mrow> <mi>V</mi> <mo>(</mo> <mi>ϕ</mi> <mo>)</mo> </mrow> </math>  , as also the constraints on the parameters present in our model and its reconstruction.

Chile

Manuel Gonzalez-Espinoza

Ramón Herrera

Giovanni Otalora

Joel Saavedra

Reconstructing inflation in scalar-torsion  $$f(T,\phi )$$  <math> <mrow> <mi>f</mi> <mo>(</mo> <mi>T</mi> <mo>,</mo> <mi>ϕ</mi> <mo>)</mo> </mrow> </math>   gravity

We investigate warm inflationary scenario in which the accelerated expansion of the early Universe is driven by chameleon-like scalar fields. Due to the non-minimal coupling between the scalar field and the matter sector, the energy-momentum tensor of each fluid component is not conserved anymore, and the generalized balance equation is obtained. The new source term in the energy equation can be used to model warm inflation. On the other hand, if the coupling function varies slowly, the model reduces to the standard model used for the description of cold inflation. To test the validity of the warm chameleon inflation model, the results for warm inflationary scenarios are compared with the observational Planck2018 Cosmic Microwave Background data. In this regard, the perturbation parameters such as the amplitude of scalar perturbations, the scalar spectral index and the tensor-to-scalar ratio are derived at the horizon crossing in two approximations, corresponding to the weak and strong dissipative regimes. As a general result it turns out that the theoretical predictions of the chameleon warm inflationary scenario are consistent with the Planck 2018 observations.

Iran

South Africa

Haidar Sheikhahmadi

Italy

Abolhassan Mohammadi

Ali Aghamohammadi

Romania

China

Tiberiu Harko

Christian Corda

Amare Abebe

Khaled Saaidi

Constraining chameleon field driven warm inflation with Planck 2018 data

The intermediate, logamediate and exponential inflationary models in the context of Galileon inflation or G-inflation are studied. By assuming a coupling of the form $$G(\phi ,X)\propto \phi ^{\nu }\,X^{n}$$ <math><mrow><mi>G</mi><mrow><mo>(</mo><mi>ϕ</mi><mo>,</mo><mi>X</mi><mo>)</mo></mrow><mo>∝</mo><msup><mi>ϕ</mi><mi>ν</mi></msup><mspace width="0.166667em"></mspace><msup><mi>X</mi><mi>n</mi></msup></mrow></math>  in the action, we obtain different analytical solutions from the background cosmological perturbations assuming the slow-roll approximation. General conditions required for these models of G-inflation to be realizable are determined and discussed. In general, we analyze the condition of inflation and also we use recent astronomical and cosmological observations for constraining the parameters appearing in these G-inflationary models.

Nelson Videla

Marco Olivares

G-inflation: from the intermediate, logamediate and exponential models

The reconstruction of a warm inflationary universe model from the scalar spectral index nS(N) and the tensor to scalar ratio r ( N ) as a function of the number of e-folds N is studied. Under a general formalism we find the effective potential and the dissipative coefficient in terms of the cosmological parameters nS and r considering the weak and strong dissipative stages under the slow roll approximation. As a specific example, we study the attractors for the index nS given by nS-1∝N-1 and for the ratio r∝N-2 , in order to reconstruct the model of warm inflation. Here, expressions for the effective potential V(ϕ) and the dissipation coefficient Γ(ϕ) are obtained.

Reconstructing warm inflation

An intermediate inflationary Universe model in the context of non-minimal coupling to the scalar curvature is analyzed. We will conduct our analysis in the slow-roll approximation of the inflationary dynamics and the cosmological perturbations considering a coupling of the form F(φ)=κ+ξnφn . Considering the trajectories in the r – ns plane from the Planck data, we find the constraints on the parameter space in our model.

Carlos González

Intermediate inflation in a generalized induced-gravity scenario

Both scalar fields and (generalized) Chaplygin gases have been widely used separately to characterize the dark sector of the universe. Here we investigate the cosmological background dynamics for a mixture of both these components and quantify the fractional abundances that are admitted by observational data from supernovae of type Ia and from the evolution of the Hubble rate. Moreover, we study how the growth rate of (baryonic) matter perturbations is affected by the dark-sector perturbations.

Sergio del Campo

Russia

Brazil

Júlio C. Fabris

Winfried Zimdahl

Is the cosmological dark sector better modeled by a generalized Chaplygin gas or by a scalar field?

We present a new cosmological model that mimics the Lambda Cold Dark Matter by using a stealth field. This kind of field is characterized as not coupling directly to gravity; however, it is connected to the underlying matter content of the universe model. As is well known, stealth fields do not back-react on the space-time; however, their mimicry skills show how this field and its self-interaction potential determines the cosmic evolution. We show the study of the simplest model that can be developed with the stealth field.

Mexico

Cuauhtemoc Campuzano

Víctor H. Cárdenas

Mimicking the LCDM model with stealths

A warm intermediate inflationary model in the context of generalized Chaplygin gas is investigated. We study this model in the weak and strong dissipative regimes, considering a generalized form of the dissipative coefficient Γ=Γ(T,ϕ) , and we describe the inflationary dynamics in the slow-roll approximation. We find constraints on the parameters in our model considering the Planck 2015 data, together with the condition for warm inflation T&gt;H , and the conditions for the weak and strong dissipative regimes.

Warm intermediate inflationary Universe model in the presence of a generalized Chaplygin gas