Surajit Chattopadhyay
Analyzing nearby and distant Supernovae Type Ia data, Riess et al. [1] and Perlmutter et al. [2] discovered the universe's late-time accelerated expansion.(Noble Prize in Physics, 2011) Once abandoned by Einstein, the cosmological constant Λ has seen a resurgence of interest after this groundbreaking discovery of modern Cosmology. Alongside Λ, the simplest dark energy (DE) candidate to explain this late-time acceleration, other DE models with the time-varying equation of state (EoS) parameter have been proposed by various authors. In the present study, we have presented a reconstruction scheme for f(T) gravity, an essential candidate for modified gravity theories. A DE model, so-called Veneziano ghost DE (GDE), has been proposed [3,4]. A DE model, so-called Veneziano ghost DE (GDE), has been proposed in [5]. The key ingredient of this new model is that the Veneziano ghost, which is unphysical in the usual Minkowski spacetime quantum field theory (QFT), exhibits important physical effects in dynamical spacetime or spacetime with non-trivial topology. Veneziano ghost is supposed to exist for solving the U(1) problem in the low-energy effective theory of QCD. Although in the flat Minkowski spacetime the QCD ghosts are unphysical and make no contribution, in curved/time-dependent backgrounds, the cancellation of their contribution to the vacuum energy leave a small energy density ρ ∼ Λ3QCD.
References:
[1] A.G. Riess et al., Astron. J. 116, 1009 (1998). [2] S. Perlmutter, Astrophys. J. 517, 565 (1999). [3] F.R. Urban, A.R. Zhitnitsky, Phys. Lett. B 688, 9 (2010). [4] R. Garcia-Salcedo et al., Phys. Rev. D 88, 043008 (2013), [5] F.R. Urban, A.R. Zhitnitsky, Phys. Lett. B 688, 9 (2010).
f(T) gravity based on QCD ghost dark energy. In the modified field
equations we have considered ρ as the ρgde in a flat universe with
power law form of the scale factor. Because of the choice of the scale
factor, the ρgde could be expressed as a function of t. Subsequently,
considering the two field equations, we have reconstructed f(T) in
two forms described as Case I and Case II, respectively.
In both cases f(T) → 0 as T → 0, which indicates a realistic model
in both cases. Since both of the forms appear as functions of t, we
could get their time derivatives and could successfully reconstruct the
density ρT and pressure pT contributions due to torsion T.
Using these reconstructed ρT and pT we could generate effective
equation-of-state parameter weff in both cases and we observed that
in Case I, weff ≪ −1 and, in Case II, weff ≥ −1. Thus Case I and Case
II generates “phantom”- and “quintessence”-like weff , respectively.
One prominent difference was that for Case I we are staying far
below the phantom boundary and in Case II it is getting asymptotic at
the phantom boundary coming from weff > −1.
Based on this difference of the behaviours of weff it may be
interpreted that Case II, represents a more acceptable model at is can
show an approach towards the phantom phase of the universe starting
from quintessence.
Due to non-positivity of the squared sound speed v2s as seen in the
plots, both QCD ghost f(T) models are classically unstable against
perturbations in flat and non-flat Friedmann-Robertson-Walker
backgrounds. This instability problem is consistent with the result
presented for QCD ghost dark energy model by Garcia-Salcedo et al.
(2013).