Moduli stabilization with the string Higgs effect

A string cosmological models with magnetic field along X and Z-axis are investigated in the context of Ruban’s space time. To obtain a determinant solution, equation of state for string model and relation between metric potential is considered. The physical and geometrical aspects are also discussed.

String cosmology models predict a relic background of gravitational wave produced during the dilaton-driven inflation. It's spectrum is most likely to be detected by ground gravitational wave laser interferometers (IFOs), like LIGO, Virgo, GEO, as the energy density grows rapidly with frequency. We show the certain ranges of the parameters that underlying string cosmology model using two approaches, associated with 5% false alarm and 95% detection rate. The result presents that the approach of combining multiple pairs of IFOs is better than the approach of directly combining the outputs of multiple IFOs for LIGOH, LIGOL, Virgo and GEO.

Dark matter axions in models of string cosmology

We update the constraints on the power spectra of primordial curvature perturbation and tensor perturbation including Planck data 2015 (P15) and recently released BICEP2/Keck data (BK15), Baryon Acoustic Oscillation data and the Type Ia supernovae data. We find that the upper limits of tensor-to-scalar ratio are $0.061$, $0.064$ and $0.072$ at $95\%$ confidence level (CL) in the $\Lambda$CDM+$r$, $\Lambda$CDM+$r$+$\alpha_s$ and $\Lambda$CDM+$r$+$\alpha_s$+$\beta_s$ models respectively, where $\alpha_s$ and $\beta_s$ are the running of scalar spectral index and running of running. The inflation model with a concave potential is favored at more than $95\%$ CL. In addition, parametrizing the slow-roll parameter $\epsilon\sim 1/N^p$, where $N$ is the e-folding number before the end of inflation and taken in the range of $[50,60]$ and $[14,75]$ respectively, we conclude that the inflation model with a monomial potential $V(\phi)\sim \phi^n$ is disfavored at more than $95\%$ CL, and both the Starobinsky inflation model and brane inflation model are still consistent with the data.

A Bianchi type-I cosmological model in the presence of a magnetic flux along a cosmological string is considered. The first objective of this study is to investigate Einstein equations using a tractable assumption usually accepted in the literature. Quantum effects of the present cosmological model are examined in the framework of loop quantum cosmology. Finally we draw a parallel between the classical and quantum approaches.

We study string cosmological models with attached particles in LRS BI type space time. The dynamical and physical properties of such universes are studied, and the possibility that during the evolution of the universe the strings disappear, leaving only the particles, is also discussed. It is found that bulk viscosity plays a large role in the evolution of the universe. In these models we find critical instances of when there was a “Bounce". The studied models are found to be of an inflationary type, and since a desirable feature of a meaningful string cosmological model is the presence of an inflationary epoch in the very early stages of evolution, our models can be thought of as realistic universes.

The field equations of Kaluza-Klein (KK) theory have been used in the area of cosmology. The researchers have scrutinized KK theory in the existence of cosmic string as a source in f(R) theory of gravity which is an extension of 5-dimensional KK string cosmological models. Certain features, physical and geometrical properties of the model are also investigated.

Pre–Big Bang models in string cosmology predict a relic background of gravitational wave radiation in the early universe. The spectrum of this background shows that the energy density rises rapidly with frequency, which is an interesting target for high-frequency (i.e., kilohertz) detectors. In this paper, we discussed the constraining power of multiple configurations of current and future gravitational wave detector (GWD) networks to the stochastic background predicted in string cosmology. The constraining power is jointly determined by the overlap reduction function and the sensitivity curves of multiple detectors. And we further elaborated on the possible contribution of a future Chinese detector and a kilohertz detector to the constraining power of detector network for stochastic background in string cosmology. Our results show that the detectability of the GWD network for the string cosmology gravitational wave background will improve considerably with the joining of a Chinese detector. This is because a Chinese detector (e.g., located at Wuhan), together with KAGRA, has a better overlap reduction function than the laser interferometer gravitational wave observatory detector pair, and therefore lead to more stringent limits for stochastic background detection. And with ideal overlap reduction function, namely, colocated detectors, a kilohertz sensitivity curve has better performance than previous detectors for stochastic background detection. Finally, the results are compared with the limitations given by the observational constraint of the Big Bang nucleosynthesis bound.

We study the development of entropy fluctuations in brane inflation in a warped throat, including the brane–antibrane tachyon as the waterfall field. We find that there is a period at the end of inflation during which the entropy mode associated with the tachyon field increases exponentially. In turn, the induced entropy seeds a contribution to the curvature fluctuation on cosmological scales which grows rapidly and could exceed the primordial curvature perturbation. We identify parameter values for which in the absence of backreaction the induced curvature fluctuations are larger than the primordial adiabatic ones. In the specific model we study, however, backreaction limits the growth of the entropy fluctuations. We discuss situations in which backreaction effects are less constraining. The lesson of our investigation is that the study of the development of entropy fluctuations at the end of the period of inflation can lead to constraints on models of brane inflation and suggests that the curvaton mechanism may contribute significantly to the spectrum of cosmological perturbations.

A Bianchi type-I cosmological model in the presence of a magnetic flux along a cosmological string is investigated. The objective of this study is to generate solutions to the Einstein equations using a few tractable assumptions usually accepted in the literature. The analytical solutions are supplemented with numerical and qualitative analysis. In the frame of the present model the evolution of the Universe and other physical aspects are discussed.

Particle creation and warm inflation

This is a short review of string cosmology. We wish to connect string-scale physics as closely as possible to observables accessible to current or near-future experiments. Our possible best hope to do so is a description of inflation in string theory. The energy scale of inflation can be as high as that of Grand Unification (GUT). If this is the case, this is the closest we can possibly get in energy scales to string-scale physics. Hence, GUT-scale inflation may be our best candidate phenomenon to preserve traces of string-scale dynamics. Our chance to look for such traces is the primordial gravitational wave, or tensor mode signal produced during inflation. For GUT-scale inflation this is strong enough to be potentially visible as a B-mode polarization of the cosmic microwave background (CMB). Moreover, a GUT-scale inflation model has a trans-Planckian excursion of the inflaton scalar field during the observable amount of inflation. Such large-field models of inflation have a clear need for symmetry protection against quantum corrections. This makes them ideal candidates for a description in a candidate fundamental theory like string theory. At the same time the need of large-field inflation models for UV completion makes them particularly susceptible to preserve imprints of their string-scale dynamics in the inflationary observables, the spectral index ns and the fractional tensor mode power r. Hence, we will focus this review on axion monodromy inflation as a mechanism of large-field inflation in string theory.

The cosmic microwave background (CMB) radiation, the relic of the early phases of the expanding universe, is bright, full of information, and difficult to measure. Along with the recession of galaxies and the primordial nucleosynthesis, it is one of the strongest signs that the Hot Big Bang Model of the universe is correct. It is brightest around 2 mm wavelength, has a temperature of \(T_{\mathrm{cmb}} = 2.72548 \pm 0.00057\) K, and has a blackbody spectrum within 50 parts per million. Its spatial fluctuations (around 0.01% on 1\({}^{\circ }\) scales) are possibly the relics of quantum mechanical processes in the early universe, modified by processes up to the decoupling at a redshift of about 1,000 (when the primordial plasma became mostly transparent). In the cold dark matter (DM) model with cosmic acceleration (\(\Lambda \)CDM), the fluctuation statistics are consistent with the model of inflation and can be used to determine other parameters within a few percent, including the Hubble constant, the \(\Lambda \) constant, the densities of baryonic and dark matter, and the primordial fluctuation amplitude and power spectrum slope. In addition, the polarization of the fluctuations reveals the epoch of reionization at a redshift approximately twice that determined from the Gunn-Peterson trough due to optically thick Lyman \(\alpha\) absorption in QSO spectra. It is of historic importance, and a testament to the unity of theory and experiment, that we now have a standard model of cosmology that is consistent with all of the observations.Current observational challenges include (1) improvement of the spectrum distortion measurements, especially at long wavelengths, where the measured background is unexpectedly bright; (2) the search for the B-mode polarization (the divergence-free part of the polarization map), arising from propagating gravitational waves; and (3) the extension of fluctuation measurements to smaller angular scales. Much more precise spectrum observations near 2 mm are likely and would test some very interesting theories. Current theoretical challenges include explanation of the dark matter and dark energy; understanding, estimating, and removing the interference of foreground sources that limit the measurements of the CMB; detailed understanding of the influence of nonequilibrium processes on the decoupling and reionization phases; and searches for signs of the second order or exotic processes (e.g., isocurvature fluctuations, cosmic strings, non-Gaussian fluctuations). At this writing, we await the cosmological results of the Planck mission.

Brandenberger and Vafa have proposed the string cosmological model based on T-duality. In this model, they took the toroidal target space and introduced the new position \tilde{x} conjugate to the winding mode, in addition to the position x conjugate to the momentum mode. In this way they can describe a universe larger than the string scale with the coordinate x and one smaller with the coordinate \tilde{x}. Resultingly, they never encounter the singularity seen in the standard Big Bang scenario. The most interesting phenomenon in this model is the transition from \tilde{x}-space to x-space when the size of universe is nearly the string scale. Here, we define the dispersion of the momentum number m times the winding number w as the `correlation' of momentum modes and winding modes. Then using the statistical mechanics of strings on a torus, we calculate the correlation in low and high temperature limits, and we consider the possibility that we can observe this effect today, but we will see that this is unlikely.

Five-dimensional space-time string cosmological models generated by a cloud of strings with particles attached to them are studied in the Brans-Dicke theory. We obtain two types of interesting models by taking up the cases of geometric strings (or Nambu strings) and p-strings (Takabayasi strings), and study their different physical and dynamical properties. The roles of the scalar field in getting different phases, such as the inflationary phase and the string-dominated phase, are discussed. An interesting feature obtained here is that in one of the models there is a “bounce" at a particular instant of its evolution.