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  • Open access
  • 126 Reads
Reconstruction of models with variable cosmological parameter in f(R,T) theory
Published: 01 March 2021 by MDPI in 1st Electronic Conference on Universe session Dark Matter and Dark Energy

The standard ΛCDM model is reasonably successful in describing the universe, and is the most widely acceptable model in cosmology. However, there are several theoretical issues, such as the initial singularity, the cosmological constant problem, the particle nature of dark matter, the existence of anomalies in the cosmic microwave background radiation and on small scales, the predictions and tests of the inflationary scenario, and whether general relativity is valid on the largest possible scales. Hence there is growing interest in looking at modified theories. In this presentation, a reconstruction is made of the Friedmann-Lemaitre-Robertson-Walker models with a dynamic cosmological parameter in f(R,T) modified gravity. This theory has a number of pleasing features, such as the avoidance of the initial big-bang singularity and a variable cosmological parameter. A dynamic cosmological parameter, which arises naturally in this theory, can solve the cosmological constant problem, and is also a candidate for dark energy. In addition, a variable cosmological parameter fits observations better than the standard ΛCDM model. The model exhibits a transition from deceleration to acceleration. The time evolution of the physical parameters such as energy density, pressure and equation of state are analyzed. Also the energy conditions and an Om(z) diagnostic analysis are discussed.

  • Open access
  • 61 Reads
Dynamics of Anisotropic Cylindrical Collapse in Energy-Momentum Squared Gravity

This paper deals with the dynamics of cylindrical collapse with anisotropic matter configuration in the context of energy-momentum squared gravity. This covariant generalization of general relativity allows the presence of T_abT^ab in the action of functional theory. Consequently, the relevant field equations are different from general relativity only in the presence of matter sources. In this theory, there is a maximum energy density and a minimum scale factor at the early Universe. This means that there is a bounce at early times which avoids the presence of an early-time singularity. Moreover, this theory possesses a true sequence of cosmological eras. Although cosmological constant does not play an important role in the early times and becomes important only after the matter-dominated era. In this theory, the “repulsive” nature of the cosmological constant plays a crucial role at early times in resolving the singularity. We formulate the corresponding field equations as well as junction conditions. We construct dynamical equations through Misner-Sharp technique and examine the impact of energy-momentum squared gravity on the collapse rate. We develop a relation among fluid parameters, correction terms and Weyl scalar and examine the effects of anisotropy, effective matter variables as well as correction terms on the collapsing phenomenon. Due to the presence of anisotropic pressure, spacetime is no longer considered to be conformally flat. To obtain conformally flat spacetime, we neglect the impact of anisotropy and assume the isotropic matter distribution which yields homogeneity of the energy density and conformally flat spacetime. The hydrodynamical force determines the stability of the system and prevents the collapsing as well as expanding process for the constant energy-momentum squared gravity model. We conclude that positive correction terms and anisotropy provide the anti-gravitational behavior leading to the stability of self-gravitating objects and hence prevent the collapsing process.

  • Open access
  • 67 Reads
The Cosmological Model Based on the Uncertainty-Mediated Dark Energy
Published: 19 March 2021 by MDPI in 1st Electronic Conference on Universe session Dark Matter and Dark Energy

Existence of the effective Lambda-term is a commonly-accepted paradigm of the modern cosmology, but physical essence of this quantity remains absolutely unknown, and its numerical values are drastically different in the early and modern Universe. In fact, the Lambda-term is usually introduced in literature either by postulating the arbitrary additional terms in the Lagrangians or by employing the empirical equations of state. In the series of our recent papers [Yu.V. Dumin. Grav. & Cosmol., v.25, p.169 (2019); v.26, p.259 (2020); v.27, in press (2021)], we tried to provide a more rigorous physical basis for the effective Lambda-term, starting from the time-energy uncertainty relation in the Mandelstam-Tamm form, which is appropriate for the long-term evolution of quantum systems. This results in the time-dependent Lambda-term, decaying as 1/t. The uncertainty-mediated cosmological model possesses a number of specific features, some of which look rather appealing: (1) While the standard cosmology involves a few very different stages (governed by the Lambda-term, radiation, dust-like matter, and again Lambda-term), our model provides a universal description of the entire evolution of the Universe by the same "quasi-exponential" function. (2) As follows from the analysis of causal structure, the present-day cosmological horizon comprises a single domain developing from the Bing Bang. Therefore, the problems of homogeneity and isotropy of the matter, the absence of topological defects, etc. should be naturally resolved. (3) Besides, our model naturally explains the observed approximately flat 3D space, i.e., solution with zero curvature is formed "dynamically", starting from the arbitrary initial conditions. (4) The age of the Universe turns out to be much greater than in the standard cosmology; but this should not be a crucial drawback, because the most of problems are associated with insufficient rather than excessive age of the Universe.

  • Open access
  • 52 Reads
Can extra dimension pull space-time?

Over the years, efforts to unify gravity with other fundamental forces in nature is an active field of research. Looking for the common origin of fundamental interactions, one may arrive at Kaluza-Klein type theories. Generalized Kaluza-Klein models offer an attractive possibility of unifying gravity with the other fundamental forces aiming at the extension of space-time from 4D to higher “mathematical” dimensions. In this paper, a generalization of the standard class of exact solutions in Kaluza-Klein (4+1) gravity are obtained for a homogeneous cosmological model filled with vacuum energy. In the algebraic and physical sense, these solutions generalize the previously found solutions in the literature. A unified and systematic treatment by solving the field equations in a straight forward manner is more appealing. The deceleration parameter shows that the model exhibits a transition from a decelerated to an accelerated universe. Recent observations have generated strong theoretical and observational evidence that the present expansion of the universe is in an accelerated phase. There is also observational evidence that beyond a certain value of redshift, the universe has been undergoing decelerated expansion. The models which describe transition from a decelerated to an accelerated phase are in the line of observational outcomes and of physical interest. The standard 3-space expands indefinitely. Extra dimensions exhibit contraction as well as expansion with suitable values of the parameters. The model rejects the hypothesis of manifesting matter from extra dimensions. However, extra dimensions generate some attractive force similar to gravity during the early evolution. Consequently, extra dimensions can be responsible for the past deceleration of the universe. The model seems to suggest an alternative mechanism pointing to a smooth transition from a decelerated phase to accelerated phase where the extra dimensions cause the transition.

  • Open access
  • 55 Reads
Second Order Glauber Correlation of Gravitational Waves using the LIGO observatories as Hanbury Brown and Twiss detectors

In this paper the second order Glauber correlation of a simplified gravitational wave is investigated, using parameters from the first signal detected by LIGO. This simplified model spans the inspiral, merger, and ringdown phases of a black hole merger and was created to have a continuous amplitude, so there is no discontinuity between the phases. This allows for a trivial extraction of the intensity, which is necessary to determine the correlation between detectors. The two LIGO observatories can be used as detectors in a Hanbury Brown and Twiss interferometer for gravitational waves, these observatories measure the amplitude of the wave, so these measurements were used as the basis of the simplified model. The signal detected by the observatories is transient and is not consistent with chaotic or steady electromagnetic waves and thus the second order Glauber correlation function was calculated to produce physically meaningful results. A correlation was also calculated using a sine-Gaussian model in hopes to describe the waveform in an even simpler model. To find correlations that are consistent with applications to electromagnetic waves weighting functions for both models were studied in the integral equations for the Glauber correlation functions. The relationship between the transient and chaotic signals of both waveforms and their respective correlation functions was also examined. The second order Glauber correlation functions are a measure of intensity interference between independent detectors and has proven to be useful in both optics and particle physics. It has also been used in theoretical studies of primordial gravitational waves. The correlations can be used to define the degrees of coherence of a field, characterize multi-particle processes, and assist in image enhancement.

  • Open access
  • 41 Reads
Gravity Variation Effects on the Growth of Maize Shoots

Gravity-variation effects on plants give definite changes. Normal-earth-gravity (1G) and microgravity (µg) are possible variations for experimental purposes. On-board spaceflight microgravity-experiments are rare and expensive, as microgravity-environment is an outstanding platform for research, application and education. Clinostat was used for ground-based experiment to investigate the shoot-morphology of maize plants at the Space-Agency of Nigeria – National Space Research and Development Agency (NASRDA). A Clinostat device uses rotation to negate gravitational-pull effects on plant growth and development. Maize was selected for this experiment because of its nutritional and economic importance; and its usability on the Clinostat. Plant shoot-morphology is important for gravi-responses. Shoot curvature and shoot growth-rate analyzes were done on the shoots of provitamin variety of maize. The seeds were planted into 3 petri-dishes (in-parallels) in a wet-chamber using plant substrate – agar-agar. The experimental conditions were subject to relative-humidity, temperature and light-conditions. After 3 days of germination under 1G, two of the petri-dishes were left under 1G serving as controls for shoot curvature and shoot growth-rate analyzes. The clinorotated-sample was mounted on Clinostat under: fast rotation-speed of 80 rpm, horizontal rotation position and clockwise rotation-direction. The images of the samples were taken 30 minutes interval for 4 hours. After observations, the shoot morphology of the seedlings were studied using ImageJ-software. The grand-average shoot-angles and shoot-lengths of all the seedlings were calculated following the experimental period to give the shoot curvatures and shoot growth-rates respectively. The results showed that the clinorotated-sample had reduced response to gravity with 12.69°/hr for the shoot-curvature while the 1G-sample had 13.87°/hr. The shoot growth-rate for the 1G-sample was 0.31mm/hr while the clinorotated was 0.32mm/hr. The clinorotated had increased growth-rate per hour than the counterpart 1G. These analytical results serve as a preparation for future real-space experiments on maize and could be beneficial to agriculture-sector.