Table of contents

Disorder on the string theory landscape may significantly affect dynamics of eternal inflation leading to the possibility for some vacua on the landscape to become dynamically preferable over others. We systematically study effects of a generic disorder on the landscape, starting by identifying a sector with built-in disorder—a set of de Sitter vacua corresponding to compactifications of the type IIB string theory on Calabi–Yau manifolds with a number of warped Klebanov–Strassler throats attached randomly to the bulk part of the Calabi–Yau. Further, we derive a continuum limit of the vacuum dynamics equations on the landscape. Using methods of the dynamical renormalization group we determine the late-time behavior of the probability distribution for an observer to measure a given value of the cosmological constant. We find the diffusion of the probability distribution to significantly slow down in sectors of the landscape where the number of nearest-neighboring vacua for any given vacuum is small. We discuss the relation of this slowdown with the phenomenon of Anderson localization in disordered media.

We study the perturbation of holographic dark energy and find it to be stable. We study the fate of the universe when interacting holographic dark energy is present, and discuss a simple phenomenological classification of the interacting holographic dark energy models. We also discuss the cosmic coincidence problem in the context of holographic dark energy. We find that the coincidence problem cannot be completely solved by adding an interacting term. Inflation may provide a better solution of the coincidence problem.

We study modulated inflation from the kinetic term. Using the Mukhanov–Sasaki variable, it is possible to determine how mixing induced by the kinetic term feeds the curvature perturbation with the isocurvature perturbation. We show explicitly that the analytic result obtained from the evolution of the Mukhanov–Sasaki variable is consistent with the δN-formula. From our results, we find analytic conditions for the modulated fluctuation and the non-Gaussianity parameter.

The observed cosmic acceleration today could be due to an unknown energy component (dark energy), or a modification to general relativity (modified gravity). If dark energy models and modified gravity models are required to predict the same cosmic expansion history H(z), they will predict different growth rates for cosmic large scale structure, f_g(z). If gravity is not modified, the measured H(z) leads to a unique prediction for f_g(z), f_g^H(z), if dark energy and dark matter are separate. Comparing f_g^H(z) with the measured f_g(z) provides a transparent and straightforward test of gravity. We show that a simple χ² test provides a general figure of merit for our ability to distinguish between dark energy and modified gravity given the measured H(z) and f_g(z). We find that a magnitude-limited NIR galaxy redshift survey covering >10 000 (deg)² and a redshift range of 0.5<z<2 can be used to measure H(z) to 1–2% accuracy via baryon acoustic oscillation measurements, and f_g(z) to the accuracy of a few per cent via the measurement of redshift-space distortions and the bias factor which describes how light traces mass. We show that if the H(z) data are fitted by both a DGP gravity model and an equivalent dark energy model that predict the same H(z), a survey area of 11 931 (deg)² is required to rule out the DGP gravity model at the 99.99% confidence level. It is feasible for such a galaxy redshift survey to be carried out by the next generation space missions from NASA and ESA, and it will revolutionize our understanding of the universe by differentiating between dark energy and modified gravity.

We discuss the equations governing the cosmological evolution in a recently suggested new model of quantum initial conditions for the Universe. The effective Friedmann equation incorporates the effect of the conformal anomaly of quantum fields and, interestingly, shows that the vacuum Casimir energy of those fields is completely screened and does not gravitate. The cosmological evolution also features a new mechanism for a cosmological acceleration stage. This stage is followed by a big boost singularity—a rapid growth up to infinity of the scale factor acceleration parameter. We also briefly discuss the relation between our model, the AdS/CFT correspondence and RS and DGP braneworlds.

We study cosmological expansion in F(R) gravity using the trace of the field equations. High frequency oscillations in the Ricci scalar, whose amplitude increases as one evolves backward in time, have been predicted in recent works. We show that the approximations used to derive this result very quickly break down in any realistic model due to the non-linear nature of the underlying problem. Using a combination of numerical and semi-analytic techniques, we study a range of models which are otherwise devoid of known pathologies. We find that high frequency asymmetric oscillations and a singularity at finite time appear to be present for a wide range of initial conditions. We show that this singularity can be avoided with a certain range of initial conditions, which we find by evolving the models forwards in time. In addition we show that the oscillations in the Ricci scalar are highly suppressed in the Hubble parameter and scale factor.

We consider asymptotically stable scalar–tensor dark energy (DE) models for which the equation of state parameter w_DE tends to zero in the past. The viable models are of the phantom type today: however, this phantomness is milder than in general relativity if we take into account the varying gravitational constant when dealing with the SNIa data. We study further the growth of matter perturbations and we find a scaling behaviour on large redshifts which could provide an important constraint. In particular, the growth of matter perturbations on large redshifts in our scalar–tensor models is close to the standard behaviour , while it is substantially different for the best-fit model in general relativity for the same parametrization of the background expansion. As for the growth of matter perturbations on small redshifts, we show that in these models the parameter can take absolute values much larger than in models inside general relativity. Assuming a constant γ when γ'₀ is large would lead to a poor fit of the growth function f. This provides another characteristic discriminative signature for these models.

During a strongly first-order phase transition gravitational waves are produced by bubble collisions and turbulent plasma motion. We analyze the relevant characteristics of the electroweak phase transition in the nMSSM to determine the generated gravitational wave signal. Additionally, we comment on correlations between the production of gravitational waves and baryogenesis. We conclude that the gravitational wave relic density in this model is generically too small to be detected in the near future by the LISA experiment. We also consider the case of a 'standard model' with dimension-six Higgs potential, which leads to a slightly stronger signal of gravitational waves.

A class of non-canonical inflationary models is identified, where the leading-order contribution to the non-Gaussianity of the curvature perturbation is determined by the sound speed of the fluctuations in the inflaton field. Included in this class of models is the effective action for multiple coincident branes in the finite n limit. The action for this configuration is determined using a powerful iterative technique, based upon the fundamental representation of SU(2). In principle the upper bounds on the tensor–scalar ratio that arise in the standard, single-brane DBI inflationary scenario can be relaxed in such multi-brane configurations if a large and detectable non-Gaussianity is generated. Moreover models with a small number of coincident branes could generate a gravitational wave background that will be observable in future experiments.

Interactions incorporating the vacuum polarization effects in curved backgrounds modify the null cone structure in such a way that the photon trajectories would not be the space–time geodesics any longer. The gravitational birefringence introduced as a direct consequence of these effects will allow shifts in the photon velocities leading to polarization dependent superluminal propagation. Taking these effects into account, we study Fermat's principle in the context of the 1+3 (threading) formulation of the space–time decomposition. We find an expression for the modified space–time refractive index and show that it is proportional to the light cone correction to first order. Consequences of this modification for spatial light paths are considered.

We calculate the contribution of fluctuations with a thermal origin to the inflationary non-Gaussianity. We find that even a small component of radiation can lead to a large non-Gaussianity. We show that this thermal non-Gaussianity always has positive f_NL. We illustrate our result in the chain inflation model and the very weakly dissipative warm inflation model. We show that is general in such models. If we allow a modified equation of state, or some decoupling effects, a large thermal non-Gaussianity of order f_NL>5 or even f_NL∼100 can be produced. We also show that the power spectrum of chain inflation should have a thermal origin. In the appendix, we give a clarification on the different conventions used in the literature related to the calculation of f_NL.

We consider the impact of thermal inflation—a short, secondary period of inflation that can arise in supersymmetric scenarios—on the stochastic gravitational wave background. We show that while the primordial inflationary gravitational wave background is essentially unchanged at cosmic microwave background scales, it is massively diluted at solar system scales and would be unobservable by a Big Bang Observer (BBO) style experiment. Conversely, bubble collisions at the end of thermal inflation can generate a new stochastic background. We calculate the likely properties of the bubbles created during this phase transition, and show that the expected amplitude and frequency of this signal would fall within the BBO range.

In this paper we find that Starobinsky's inflationary solution is also valid in the Dvali–Gabadadze–Porrati (DGP) model where a 3-brane is embedded in five-dimensional Minkowski bulk. We show that such a solution is typically not supported by the self-accelerated branch of the model, giving therefore a natural selection of the conventional branch of solutions. In the absence of brane-induced Einstein–Hilbert term the SA branch is always selected out. We then study the linearized modes around all such de Sitter brane solutions finding perturbative stability for a range of parameters of the brane QFT.

We study the dynamical behaviour of the interacting holographic dark energy model whose interaction term is Q = 3H(λ_dρ_d+λ_cρ_c), where ρ_d and ρ_c are the energy densities of dark energy and cold dark matter respectively. To satisfy the observational constraints from type Ia supernovae, the cosmic microwave background shift parameter and baryon acoustic oscillation measurements, if λ_c = λ_d or λ_d,λ_c>0, the cosmic evolution will only reach the attractor in the future and the ratio ρ_c/ρ_d cannot be slowly varying at present. Since the cosmic attractor can be reached in the future even when the present values of the cosmological parameters do not satisfy the observational constraints, the coincidence problem is not really alleviated in this case. However, if and they are allowed to be negative, the ratio ρ_c/ρ_d can be slowly varying at present and the cosmic attractor can be reached near the present epoch. Hence, the alleviation of the coincidence problem is attainable in this case. The alleviation of the coincidence problem in this case is still attainable when confronting this model with Sloan Digital Sky Survey data.

The standard model of big bang nucleosynthesis (BBN) relies on a nuclear reaction network operating with thermal reactivities for Maxwellian plasma. In the primordial plasma, however, a number of non-thermal processes triggered by energetic particles of various origins can take place. In the present work we examine in-flight nuclear reactions induced in the plasma by MeV protons generated in D(d, p)T and ³He(d, p)⁴He fusions. We particularly focus on several low threshold endoergic processes. These are reactions omitted in the standard network—proton-induced break-ups of loosely bound D, ⁷Li, ⁷Be nuclei—and the ³H(p, n)³He charge-exchange reaction important for the interconversion of A = 3 nuclei in the early universe. It is found that the break-up processes in the plasma take the form of Maxwellian processes at temperatures T>70 keV, while in the lower temperature range they proceed as non-thermal reactions. It is shown that at T<70 keV the in-flight reaction channels can enhance the break-up reactivities by several orders of magnitude. The levels of these reactivities however remain insufficiently high to affect BBN kinetics and change the standard prediction of light element abundances. The abundances are found to be: Y_p = 0.2457, D/H = 2.542 × 10⁻⁵, ³He/H = 1.004 × 10⁻⁵, ⁷Li/H = 4.444 × 10⁻¹⁰. Future steps in the study of non-thermal processes in the primordial plasma are briefly discussed.

In the first part of this paper, we outline the construction of an inflationary cosmology in the framework where inflation is described by a universally evolving scalar field ϕ with potential V (ϕ). By considering a generic situation that inflaton attains a nearly constant velocity, during inflation, (where is the e-folding time), we reconstruct a scalar potential and find the conditions that have to be satisfied by the (reconstructed) potential to be consistent with the WMAP inflationary data. The consistency of our model with the WMAP result (such as n_s = 0.951_−0.019^+0.015 and r<0.3) would require 0.16<α<0.26 and β<0. The running of the spectral index, , is found to be small for a wide range of α.

In the second part of this paper, we introduce a novel approach of constructing dark energy within the context of the standard scalar–tensor theory. The assumption that a scalar field might roll with a nearly constant velocity, during inflation, can also be applied to quintessence or dark energy models. For the minimally coupled quintessence, (where A(Q) is the standard matter–quintessence coupling), the dark energy equation of state in the range −1≤w_DE<−0.82 can be obtained for 0≤α<0.63. For α<0.1, the model allows for only modest evolution of dark energy density with redshift. We also show, under certain conditions, that the α_Q>0 solution decreases the dark energy equation of state w_Q with decreasing redshift as compared to the α_Q = 0 solution. This effect can be opposite in the α_Q<0 case. The effect of the matter–quintessence coupling can be significant only if , while a small coupling |α_Q|<0.1 will have almost no effect on cosmological parameters, including Ω_Q, w_Q and H(z). The best fit value of α_Q in our model is found to be , but it may contain significant numerical errors, namely α_Q = 0.06 ± 0.35, which clearly implies the consistency of our model with general relativity (for which α_Q = 0) at 1σ level.

The clumpy maser discs observed in some galactic nuclei mark the outskirts of the accretion disc that fuels the central black hole and provide a potential site of nuclear star formation. Unfortunately, most of the gas in maser discs is currently not being probed; large maser gains favor paths that are characterized by a small velocity gradient and require rare edge-on orientations of the disc. Here we propose a method for mapping the atomic hydrogen distribution in nuclear discs through its 21 cm absorption against the radio continuum glow around the central black hole. In NGC 4258, the 21 cm optical depth may approach unity for high angular resolution (VLBI) imaging of coherent clumps which are dominated by thermal broadening and have the column density inferred from x-ray absorption data, ∼10²³ cm⁻². Spreading the 21 cm absorption over the full rotation velocity width of the material in front of the narrow radio jets gives a mean optical depth of ∼0.1. Spectroscopic searches for the 21 cm absorption feature in other galaxies can be used to identify the large population of inclined gaseous discs which are not masing in our direction. Follow-up imaging of 21 cm silhouettes of accelerating clumps within these discs can in turn be used to measure cosmological distances.

Cosmological models involving an interaction between dark matter and dark energy have been proposed in order to solve the so-called coincidence problem. Different forms of coupling have been studied, but there have been claims that observational data seem to narrow (some of) them down to something annoyingly close to the ΛCDM (CDM: cold dark matter) model, thus greatly reducing their ability to deal with the problem in the first place. The smallness problem of the initial energy density of dark energy has also been a target of cosmological models in recent years. Making use of a moderately general coupling scheme, this paper aims to unite these different approaches and shed some light on whether this class of models has any true perspective in suppressing the aforementioned issues that plague our current understanding of the universe, in a quantitative and unambiguous way.

We study the anisotropy signature which is expected if the sources of ultrahigh energy, >10¹⁹ eV, cosmic rays (UHECRs) are extra-galactic and trace the large scale distribution of luminous matter. Using the PSCz galaxy catalog as a tracer of the large scale structure (LSS), we derive the expected all sky angular distribution of the UHECR intensity. We define a statistic that measures the correlation between the predicted and observed UHECR arrival direction distributions, and show that it is more sensitive to the expected anisotropy signature than the power spectrum and the two-point correlation function. The distribution of the correlation statistic is not sensitive to the unknown redshift evolution of UHECR source density and to the unknown strength and structure of inter-galactic magnetic fields. We show, using this statistic, that recently published >5.7 × 10¹⁹ eV Auger data are inconsistent with isotropy at CL, and consistent with a source distribution that traces LSS, with some preference for a source distribution that is biased with respect to the galaxy distribution. The anisotropy signature should be detectable also at lower energy, >4 × 10¹⁹ eV. A few-fold increase of the Auger exposure is likely to increase the significance to >99% CL, but not to>99.9% CL (unless the UHECR source density is comparable to or larger than that of galaxies). In order to distinguish between different bias models, the systematic uncertainty in the absolute energy calibration of the experiments should be reduced to well below the current .

We investigate the static and spherically symmetric solutions of Einstein's equations for a scalar field with a non-canonical kinetic term, assumed to provide both the dark matter and dark energy components of the Universe. In particular, we give a prescription to obtain solutions (dark halos) whose rotation curve v_c(r) is in good agreement with observational data. We show that there exist suitable scalar field Lagrangians that allow us to describe the cosmological background evolution and the static solutions with a single dark fluid.

We consider a variant of the seesaw mechanism by introducing extra singlet neutrinos and a singlet scalar boson, and show how low scale leptogenesis is successfully realized in this scenario. We examine whether the newly introduced neutral particles, either singlet Majorana neutrinos or singlet scalar bosons, can be dark matter candidates. We also discuss the implications of dark matter detection through scattering off the nucleus of the detecting material on our scenarios for dark matter. In addition, we study the implications for the search for invisible Higgs decay at the Large Hadron Collider, which may serve as a probe for our scenario for dark matter.

We derive an expression for the entropy of a dark matter halo described using a Navarro–Frenk–White model with a core. The comparison of this entropy with that of dark matter in the freeze-out era allows us to constrain the parameter space in mSUGRA models. Moreover, combining these constraints with the ones obtained from the usual abundance criterion and demanding that these criteria be consistent with the 2σ bounds for the abundance of dark matter: 0.112≤Ω_DMh²≤0.122, we are able to clearly identify validity regions among the values of tanβ, which is one of the parameters of the mSUGRA model. We found that for the regions of the parameter space explored, small values of tanβ are not favored; only for are the two criteria significantly consistent. In the region where the two criteria are consistent we also found a lower bound for the neutralino mass, m_χ≥141 GeV.

We assume the validity of the standard model up to an arbitrary high-energy scale and discuss what information on the early stages of the Universe can be extracted from a measurement of the Higgs mass. For , the Higgs potential can develop an instability at large-field values. From the absence of excessive thermal Higgs field fluctuations we derive a bound on the reheat temperature after inflation as a function of the Higgs and top masses. Then we discuss the interplay between the quantum Higgs fluctuations generated during the primordial stage of inflation and the cosmological perturbations, in the context of landscape scenarios in which the inflationary parameters scan. We show that, within the large-field models of inflation, it is highly improbable to obtain the observed cosmological perturbations in a Universe with a light Higgs. Moreover, independently of the inflationary model, the detection of primordial tensor perturbations through the B mode of CMB polarization and the discovery of a light Higgs can simultaneously occur only with exponentially small probability, unless there is new physics beyond the standard model.

We use the techniques of effective field theory in an expanding universe to examine the effect of choosing an excited inflationary initial state built over the Bunch–Davies state on the CMB bi-spectrum. We find that, even for Hadamard states, there are unexpected enhancements in the bi-spectrum for certain configurations in momentum space due to interactions of modes in the early stages of inflation. These enhancements can be parametrically larger than the standard ones and are potentially observable in future data. These initial state effects have a characteristic signature in l-space which distinguishes them from the usual contributions, with the enhancement being most pronounced for configurations corresponding to flattened triangles for which two momenta are collinear.