arXiv subjects

Amir H. Shirazi

Publications and source records attributed to Amir H. Shirazi.

3 recordsLinked to original sources

arXiv2020

Ups and Downs in Dark Energy: phase transition in dark sector as a proposal to lessen cosmological tensions

Based on tensions between the early and late time cosmology, we proposed a double valued cosmological constant which could undergo a phase transition in its history. It is named "double-$Λ$ Cold Dark Matter": $ΛΛ$CDM. An occurred phase transition results in (micro-) structures for the dark sector with a proper (local) interaction. In this paper, inspired by the physics of critical phenomena, we study a simplified model such that the cosmological constant has two values before a transition scale factor, $a_t$, and afterwards it becomes single-valued. We consider both the background and perturbation data sets including CMB, BAO distances and R19 data point. $ΛΛ$CDM has its maximum likelihood for $a_t= 0.916^{+0.055}_{-0.0076}$ and $H_0= 72.8\pm 1.6$. This result shows no inconsistency between early and late time measurements of Hubble parameter in $ΛΛ$CDM model. In comparison to $Λ$CDM, our model has better fit to data such that $Δχ^2=-11$ and even if we take care of two additional degrees of freedom we do have better AIC quantity $Δ$AIC$=-7$. We conclude that a phase transition in the behavior of dark energy can address $H_0$ tension successfully and may be responsible for the other cosmological tensions.

astro-ph.CO↗

arXiv2020

Ginzburg-Landau Theory of Dark Energy: A Framework to Study Both Temporal and Spatial Cosmological Tensions Simultaneously

A dark energy model (DE) is proposed based on Ginzburg-Landau theory of phase transition (GLT). This model, GLTofDE, surprisingly provides a framework to study not only temporal tensions in cosmology e.g. $H_0$ tension but also spatial anomalies of CMB e.g. the hemispherical power asymmetry and quadrupole-octopole alignment. In the mean field (or Landau) approximation of GLTofDE, there is a spontaneously symmetry breaking exactly like the Higgs potential. We modeled this transition, phenomenologically, and showed that GLTofDE can resolve both the $H_0$ tension and Lyman-$α$ anomaly in a non-trivial way. According to $χ^2$-analysis the transition happens at $z_t=0.738\pm0.028$ while $H_0=71.89\pm0.93$ km/s/Mpc and $Ω^{like}_{k}=-0.225\pm0.049$ which are consistent with the latest $H(z)$ reconstructions. In addition, the GLTofDE proposes a framework to address the CMB anomalies when it is considered beyond the mean field approximation. In this regime existence of a long wavelength mode is a typical consequence which is named the Goldstone mode in the case of continuous symmetries. This mode, which is an automatic byproduct in GLTofDE, makes cosmological constant, direction dependent. This means one side of the sky should be colder than the other side in agreement with what has been already observed in CMB. In addition between initial stochastic pattern and the final state with one long wavelength mode, we can observe smaller patches or protrusions of the biggest remaining patch in the simulation. Our simulations show these protrusions are few in numbers and will be evolved according to Alan-Cahn mechanism. These protrusions can give an additional effect on CMB which is the existence of aligned quadrupole-octopole mode and its direction should be orthogonal to the dipole direction. We conclude that GLTofDE is a fertile framework both theoretically and phenomenologically.

astro-ph.CO↗

arXiv2019

Mean-field solution of structural balance dynamics in nonzero temperature

In signed networks with simultaneous friendly and hostile interactions, there is a general tendency to a global structural balance, based on the dynamical model of links status. Although the structural balance represents a state of the network with a lack of contentious situations, there are always tensions in real networks. To study such networks, we generalize the balance dynamics in nonzero temperatures. The presented model uses elements from Boltzmann-Gibbs statistical physics to assign an energy to each type of triad, and it introduces the temperature as a measure of tension tolerance of the network. Based on the mean-field solution of the model, we find out that the model undergoes a first-order phase transition from an imbalanced random state to structural balance with a critical temperature $T_{c}$, where in the case of $T > T_{c}$ there is no chance to reach the balanced state. A main feature of the first-order phase transition is the occurrence of a hysteresis loop crossing the balanced and imbalanced regimes.

physics.soc-ph↗