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Register a new userConstraints on the generalized tachyon field models from latest observational data
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We consider constraints on generalized tachyon field (GTF) models from latest observational data (including 182 gold SNIa data, the shift parameter, and the acoustic scale). We obtain at 68.3% confidence level $Omega_{rm m}=0.37pm0.01$, $k_0=0.09^{+0.04}_{-0.03}$, $alpha=1.8^{+7.4}_{-0.7}$ (the best-fit values of the parameters) and $z_{q=0}sim 0.47-0.51$ (the transitional redshift) for GTF as dark energy component only; $k_0=0.21^{+0.20}_{-0.18}$, $alpha=0.57pm0.01$ and $z_{q=0}sim 0.49-0.68$ for GTF as unification of dark energy and dark matter. In both cases, GTF evolves like dark matter in the early universe. By applying model-comparison statistics and test with independent $H(z)$ data, we find GTF dark energy scenario is favored over the $Lambda$CDM model, and the $Lambda$CDM model is favored over GTF unified dark matter by the combined data. For GTF as dark energy component, the fluctuations of matter density is consistent with the growth of linear density perturbations. For GTF unified dark matter, the growth of GTF density fluctuations grow more slowly for $ato1$, meaning GTF do not behave as classical $Lambda$CDM scenarios.
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For the constant-roll tachyon inflation, we derive the analytical expressions for the scalar and tensor power spectra, the scalar and tensor spectral tilts and the tensor to scalar ratio up to the first order by using the method of Bessel function approximation. The derived $n_s-r$ results for the constant-roll inflation are also compared with the observations, we find that only one constant-roll inflation is consistent with the observations and observations constrain the constant-roll inflation to be slow-roll inflation. The tachyon potential is also reconstructed for the constant-roll inflation which is consistent with the observations.
We use a dynamical systems approach to study thawing quintessence models, using a multi-parameter extension of the exponential potential which can approximate the form of typical thawing potentials. We impose observational constraints using a compilation of current data, and forecast the tightening of constraints expected from future dark energy surveys, as well as discussing the relation of our results to analytical constraints already in the literature.
Our purpose is to place firm observational constraints on the three most widely used theoretical models for the spatial configuration of the large-scale interstellar magnetic field in the Galactic disk, namely, the ring, the axisymmetric and the bisymmetric field models. We use the rotation measures (RMs) of low-latitude Galactic pulsars and combine them with their dispersion measures and estimated distances to map out the line-of-sight component of the interstellar magnetic field in the near half of the Galactic disk. We then fit our map of the line-of-sight field to the three aforementioned theoretical field models and discuss the acceptability of each fit, in order to determine whether the considered field model is allowed by the pulsar data or not. Strictly speaking, we find that all three field models are ruled out by the pulsar data. Furthermore, none of them appears to perform significantly better than the others. From this we conclude that the large-scale interstellar magnetic field in the Galactic disk has a more complex pattern than just circular, axisymmetric or bisymmetric.
We examine whether tachyon matter is a viable candidate for the cosmological dark matter. First, we demonstrate that in order for the density of tachyon matter to have an acceptable value today, the magnitude of the tachyon potential energy at the onset of rolling must be finely tuned. For a tachyon potential $V(T)sim M_{Pl}^4exp(-T/tau)$, the tachyon must start rolling at $Tsimeq 60tau$ in order for the density of tachyon matter today to satisfy $Omega_{T,0}sim 1$, provided that standard big bang cosmology begins at the same time as the tachyon begins to roll. In this case, the value of $Omega_{T,0}$ is exponentially sensitive to $T/tau$ at the onset of rolling, so smaller $T/tau$ is unacceptable, and larger $T/tau$ implies a tachyon density that is too small to have interesting cosmological effects. If instead the universe undergoes a second inflationary epoch after the tachyon has already rolled considerably, then the tachyon can begin with $T$ near zero, but the increase of the scale factor during inflation must still be finely tuned in order for $Omega_{T,0} sim 1$. Second, we show that tachyon matter, unlike quintessence, can cluster gravitationally on very small scales. If the starting value of $T/tau$ is tuned finely enough that $Omega_{T,0}sim 1$, then tachyon matter clusters more or less identically to pressureless dust. Thus, if the fine-tuning problem can be explained, tachyon matter is a viable candidate for cosmological dark matter.
Most dark energy models have the $Lambda$CDM as their limit, and if future observations constrain our universe to be close to $Lambda$CDM Bayesian arguments about the evidence and the fine-tuning will have to be employed to discriminate between the models. Assuming a baseline $Lambda$CDM model we investigate a number of quintessence and phantom dark energy models, and we study how they would perform when compared to observational data, such as the expansion rate, the angular distance, and the growth rate measurements, from the upcoming Dark Energy Spectroscopic Instrument (DESI) survey. We sample posterior likelihood surfaces of these dark energy models with Monte Carlo Markov Chains while using central values consistent with the Planck $Lambda$CDM universe and covariance matrices estimated with Fisher information matrix techniques. We find that for this setup the Bayes factor provides a substantial evidence in favor of the $Lambda$CDM model over most of the alternatives. We also investigated how well the CPL parametrization approximates various scalar field dark energy models, and identified the location for each dark energy model in the CPL parameter space.
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