Integrable cosmological models with an additional scalar field

We consider modified gravity cosmological models that can be transformed into two-field chiral cosmological models by the conformal metric transformation. For the $$R^2$$ R 2 gravity model with an additional scalar field and the corresponding two-field model with the cosmological constant and nonstandard kinetic part of the action, the general solutions have been obtained in the spatially flat FLRW metric. We analyze the correspondence of the cosmic time solutions obtained and different possible evolutions of the Hubble parameters in the Einstein and Jordan frames.

Free electron Compton scattering produces the illusion of the universe expansion

Because it is consistent with many astronomical phenomena and successfully predicted the cosmic microwave background radiation (CMBR) and cosmic abundance, the theory of universe expansion has been widely recognized by the scientific community. Hubble's law is the foundation of universe expansion theory, but 100 years of observations have shown that Hubble parameters are not constants, and with the improvement of Hubble parameter measurement accuracy, the problem of inconsistent Hubble parameters obtained by different star types and different methods has become more and more difficult to solve. So the cosmological redshift may not only be related to distance but also to other factors, and the universe may not be really expanding. The Compton effect of free electrons and low energy photons has been observed in the laboratory. Photons interact with a large number of free electrons on their way to us from a distant source (free electron Compton scattering FEC). FEC causes photons (plane electromagnetic waves) to redshift, and the photon beam to expand along the propagation direction, these produce the illusion of cosmic expansion, showing the same astronomical phenomena as the expansion of the universe (FEC model).

Model-independent constraints on cosmic curvature: implication from the future space gravitational-wave antenna DECIGO

In order to estimate cosmic curvature from cosmological probes like standard candles, one has to measure the luminosity distance $$D_L(z)$$ D L ( z ) , its derivative with respect to redshift $$D'_L(z)$$ D L ′ ( z ) and the expansion rate H(z) at the same redshift. In this paper, we study how such idea could be implemented with future generation of space-based DECi-hertz Interferometer Gravitational-wave Observatory (DECIGO), in combination with cosmic chronometers providing cosmology-independent H(z) data. Our results show that for the Hubble diagram of simulated DECIGO data acting as a new type of standard siren, it would be able to constrain cosmic curvature with the precision of $$\varDelta \varOmega _k= 0.09$$ Δ Ω k = 0.09 with the currently available sample of 31 measurements of Hubble parameters. In the framework of the third generation ground-based gravitational wave detectors, the spatial curvature is constrained to be $$\varDelta \varOmega _k= 0.13$$ Δ Ω k = 0.13 for Einstein Telescope (ET). More interestingly, compared to other approaches aiming for model-independent estimations of spatial curvature, our analysis also achieve the reconstruction of the evolution of $$\varOmega _k(z)$$ Ω k ( z ) , in the framework of a model-independent method of Gaussian processes (GP) without assuming a specific form. Therefore, one can expect that the newly emerged gravitational wave astronomy can become useful in local measurements of cosmic curvature using distant sources.

Evidence for cosmic acceleration with next-generation surveys: a model-independent approach

ABSTRACT We quantify the evidence for cosmic acceleration using simulations of H(z) measurements from SKA- and Euclid-like surveys. We perform a non-parametric reconstruction of the Hubble parameters and its derivative to obtain the deceleration parameter q(z) using the Gaussian Processes method. This is a completely model-independent approach, so we can determine whether the Universe is undergoing accelerated expansion regardless of any assumption of a dark energy model. We find that Euclid-like and SKA-like band 1 surveys can probe cosmic acceleration at over 3 and 5σ confidence level, respectively. By combining them with an SKA-like band 2 survey, which reaches lower redshift ranges, the evidence for a current accelerated phase increases to over 7σ. This is a significant improvement from current H(z) measurements from cosmic chronometers and galaxy redshift surveys, showing that these surveys can underpin cosmic acceleration in a model-independent way.

Anisotropic cosmological dynamics in f(T) gravity in the presence of a perfect fluid

We consider the cosmological evolution of a flat anisotropic Universe in f(T) gravity in the presence of a perfect fluid. It is shown that the matter content of the Universe has a significant impact of the nature of a cosmological singularity in the model studied. Depending on the parameters of the f(T) function and the equation of state of the perfect fluid in question the well-known Kasner regime of general relativity can be replaced by a new anisotropic solution, or by an isotropic regime, or the cosmological singularity changes its nature to a non-standard one with a finite values of Hubble parameters. Six possible scenarios of the cosmological evolution for the model studied have been found numerically.

FRW cosmology in f(R,T) gravity

In the present paper, the work of Moreas et al. [P. H. R. S. Moraes, G. Ribeiro and R. A. C. Correa, A transition from a decelerated to an accelerated phase of the universe expansion from the simplest non-trivial polynomial function of T in the [Formula: see text] formalism, Astrophys. Space Sci. 361 (2016) 227–231] is extended to study the FRW model in [Formula: see text] gravity. The expressions for deceleration and Hubble parameters are determined in terms of redshift. The age of the universe is established using [Formula: see text] high-redshift type Ia supernovae data from the Supernova cosmology project and 15 low-redshift type Ia supernovae data from the Calán/Tolono Supernova survey [S. Permutter et al., Measurements of Omega and Lambda from 42 High-Redshift Supernovae, Astrophys. J. 517 (1999) 565–585]. For these redshifts, the data of observed apparent magnitude and luminosity distance are used for the comparison with the obtained theoretical values.

On the Eternal Role of Planck Scale in Machian Cosmology

With reference to Planck scale, Mach's relation and by introducing two new parameters Gamma and Beta, right from the beginning of Planck scale, we make an attempt to estimate ordinary matter density ratio, dark matter density ratio, mass, radius, temperature, age and expansion velocity (from and about the Planck mass in all directions). In analogy with currently believed cosmic acceleration, with a decreasing trend of total matter density ratio, cosmic expansion velocity can be shown to be increasing. By considering H 0 ≅ 70 km/sec/Mpc, estimated current cosmic mass, radius, total matter density, expansion velocity, temperature and age are: 4.3352 × 10 53   kg , 3.207 × 10 26   m , 3.138 × 10 − 27   kg . m - 3 , 2.43 c , 2.721   K and 19.78   Billion   years respectively. Point to be noted is that, with reference to Planck scale, cosmic temperature seems to be redshifted by a factor ( Z T ) t ≅ 1 + ln ( H p l / H t ) − 1 where ( H p l , H t ) represent Planck scale and time dependent Hubble parameters respectively. As a peculiar case, considering the equality of current Hubble parameter and current angular velocity, current cosmic rotational kinetic energy can be estimated to be 0.667 times the current critical energy. It needs further study.

On the Eternal Role of Planck Scale in Machian Cosmology

Considering Planck mass as a representation of baby universe and evolving universe as a growing Planck ball, a heuristic model of quantum cosmology can be developed. By considering the famous relation GM = c2R, a Machian model of evolving quantum cosmology can also be developed with increasing matter content and decreasing matter density. In all directions, ‘from and about’ the baby universe, cosmic expansion velocity can be shown to be increasing with decreasing matter density. Estimated current cosmic mass, radius, matter density, expansion velocity and age are: 4.3352×1053 kg, 3.207×1026 m, 3.318×10-27 kg.m-3, 2.43c and 19.78 Billion respectively. As a peculiar case, considering the equality of current Hubble parameter and current angular velocity, current cosmic rotational kinetic energy can be shown to be 0.667 times the current critical energy! Proceeding further, with reference to Planck scale, cosmic temperature seems to be redshifted by a factor( ( Z T ) t + 1 ) ≅ 1 + ln ( H p l / H t ) ; where (Hpl, Ht) represent Planck scale and time dependent Hubble parameters respectively.

On the Eternal Role of Planck Scale in Machian Cosmology

Considering Planck mass as a baby universe and evolving universe as a growing Planck ball, a hypothetical spherical model of cosmology can be developed. In addition, by considering the famous relation GM = c2R, a Machian model of quantum cosmology can also be developed. Proceeding further, in all directions, cosmic expansion velocity can be referred to ‘from and about’ the baby universe. This hypothetical scenario can be made versatile by adding a heuristic idea: apart from normal expansion, ‘cosmic temperature seems to be redshifted by a factor ( ( Z T ) t + 1 ) ≅ 1 + ln ( H p l / H t ) ; where (Hpl, Ht) represent Planck scale and time dependent Hubble parameters respectively.