arXiv subjects

Ulf H. Danielsson

Publications and source records attributed to Ulf H. Danielsson.

At least 37 records · Page 2Linked to original sources

arXiv2008

Chain inflation revisited

This paper represents an in-depth treatment of the chain inflation scenario. We fully determine the evolution of the universe in the model, the necessary conditions in order to have a successful inflationary period, and the matching with the observational results regarding the cosmological perturbations. We study in great detail, and in general, the dynamics of the background, as well as the mechanism of generation of the perturbations. We also find an explicit formula for the spectrum of adiabatic perturbations. Our results prove that chain inflation is a viable model for solving the horizon, entropy and flatness problem of standard cosmology and for generating the right amount of adiabatic cosmological perturbations. The results are radically different from those found in previous works on the subject. Finally, we argue that there is a natural way to embed chain inflation into flux compactified string theory. We discuss the details of the implementation and how to fit observations.

hep-th↗

arXiv2008

Chain inflation and the imprint of fundamental physics in the CMBR

In this work we investigate characteristic modifications of the spectrum of cosmological perturbations and the spectral index due to chain inflation. We find two types of effects. First, modifications of the spectral index depending on interactions between radiation and the vacuum, and on features of the effective vacuum potential of the underlying fundamental theory. Second, a modulation of the spectrum signalling new physics due to bubble nucleation. This effect is similar to those of transplanckian physics. Measurements of such signatures could provide a wealth of information on the fundamental physics at the basis of inflation.

hep-th↗

arXiv2008

Deforming, revolving and resolving - New paths in the string theory landscape

In this paper we investigate the properties of series of vacua in the string theory landscape. In particular, we study minima to the flux potential in type IIB compactifications on the mirror quintic. Using geometric transitions, we embed its one dimensional complex structure moduli space in that of another Calabi-Yau with h^{1,1}=86 and h^{2,1}=2. We then show how to construct infinite series of continuously connected minima to the mirror quintic potential by moving into this larger moduli space, applying its monodromies, and moving back. We provide an example of such series, and discuss their implications for the string theory landscape.

hep-th↗

arXiv2007

Stability of flux vacua in the presence of charged black holes

In this letter we consider a charged black hole in a flux compactification of type IIB string theory. Both the black hole and the fluxes will induce potentials for the complex structure moduli. We choose the compact dimensions to be described locally by a deformed conifold, creating a large hierarchy. We demonstrate that the presence of a black hole typically will not change the minimum of the moduli potential in a substantial way. However, we also point out a couple of possible loop-holes, which in some cases could lead to interesting physical consequences such as changes in the hierarchy.

hep-th↗

arXiv2006

The world next door - Results in landscape topography

Recently, it has become clear that neighboring multiple vacua might have interesting consequences for the physics of the early universe. In this paper we investigate the topography of the string landscape corresponding to complex structure moduli of flux compactified type IIB string theory. We find that series of continuously connected vacua are common. The properties of these series are described, and we relate the existence of infinite series of minima to certain unresolved mathematical problems in group theory. Numerical studies of the mirror quintic serve as illustrating examples.

hep-th↗

arXiv2006

Transplanckian signatures in WMAP3?

In this note we investigate how a possible signal in the WMAP3 data of rapid oscillations in the primordial spectrum can be accommodated into an effective model of transplanckian physics including back reaction. The results, if due to a real effect, would indicate the presence of a low fundamental scale -- possibly the string scale -- around $2.2\cdot10^{-5}M_{pl}$.

astro-ph↗

arXiv2005

Quantum gravitational proton decay at high temperature

One of the most important challenges of contemporary physics is to find experimental signatures of quantum gravity. It is expected that quantum gravitational effects lead to proton decay but on time scales way beyond what is of any relevance to experiments. At non-zero temperatures there are reasons to believe that the situation is much more favourable. We will argue that at the temperatures and densities reached at present and future fusion facilities there is a realistic possibility that proton decay could be detectable.

gr-qc↗

arXiv2005

Inflation as a probe of new physics

In this paper we consider inflation as a probe of new physics near the string or Planck scale. We discuss how new physics can be captured by the choice of vacuum, and how this leads to modifications of the primordial spectrum as well as the way in which the universe expands during inflation. Provided there is a large number of fields contributing to the vacuum energy -- as typically is expected in string theory -- we will argue that both types of effects can be present simultaneously and be of observational relevance. Our conclusion is that the ambiguity in choice of vacuum is an interesting new parameter in serious model building.

hep-th↗

arXiv2005

4D black holes and holomorphic factorization of the 0A matrix model

In this letter, we relate the free energy of the 0A matrix model to the sum of topological and anti-topological string amplitudes. For arbitrary integer multiples of the matrix model self-dual radius we describe the geometry on which the corresponding topological string propagates. This geometry is not the one that follows from the usual ground ring analysis, but in a sense its "holomorphic square root". Mixing of terms for different genus in the matrix model free energy yields one-loop terms compatible with type II strings on compact Calabi-Yau target spaces. As an application, we give an explicit example of how to relate the 0A matrix model free energy to that of a four-dimensional black hole in type IIB theory, compactified on a compact Calabi-Yau. Variables, Legendre transforms, and large classical terms on both sides match perfectly.

hep-th↗

arXiv2004

Transplanckian energy production and slow roll inflation

In this paper we investigate how the energy density due to a non-standard choice of initial vacuum affects the expansion of the universe during inflation. To do this we introduce source terms in the Friedmann equations making sure that we respect the relation between gravity and thermodynamics. We find that the energy production automatically implies a slow rolling cosmological constant. Hence we also conclude that there is no well defined value for the cosmological constant in the presence of sources. We speculate that a non-standard vacuum can provide slow roll inflation on its own.

hep-th↗

arXiv2004

Matrix models, 4D black holes and topological strings on non-compact Calabi-Yau manifolds

We study the relation between c=1 matrix models at self-dual radii and topological strings on non-compact Calabi-Yau manifolds. In particular the special case of the deformed matrix model is investigated in detail. Using recent results on the equivalence of the partition function of topological strings and that of four dimensional BPS black holes, we are able to calculate the entropy of the black holes, using matrix models. In particular, we show how to deal with the divergences that arise as a result of the non-compactness of the Calabi-Yau. The main result is that the entropy of the black hole at zero temperature coincides with the canonical free energy of the matrix model, up to a proportionality constant given by the self-dual temperature of the matrix model.

hep-th↗

arXiv2004

Lectures on string theory and cosmology

In these lectures I review recent attempts to apply string theory to cosmology, including string cosmology and various models of brane cosmology. In addition, the review includes an introduction to inflation as well as a discussion of transplanckian signatures. I also provide a critical discussion of the possible role of holography. The material is based on lectures given in January 2004 at the RTN String School in Barcelona, but also contain some additional material.

hep-th↗

arXiv2004

Type 0A 2D Black Hole Thermodynamics and the Deformed Matrix Model

Recently, it has been proposed that the deformed matrix model describes a two-dimensional type 0A extremal black hole. In this paper, the thermodynamics of 0A charged non-extremal black holes is investigated. We observe that the free energy of the deformed matrix model to leading order in 1/q can be seen to agree to that of the extremal black hole. We also speculate on how the deformed matrix model is able to describe the thermodynamics of non-extremal black holes.

hep-th↗

arXiv2004

On Thermalization in de Sitter Space

We discuss thermalization in de Sitter space and argue, from two different points of view, that the typical time needed for thermalization is of order $R^{3}/l_{pl}^{2}$, where $R$ is the radius of the de Sitter space in question. This time scale gives plenty of room for non-thermal deviations to survive during long periods of inflation. We also speculate in more general terms on the meaning of the time scale for finite quantum systems inside isolated boxes, and comment on the relation to the Poincaré recurrence time.

hep-th↗

arXiv2004

A matrix model black hole: act II

In this paper we discuss the connection between the deformed matrix model and two dimensional black holes in the light of the new developements involving fermionic type 0A-string theory. We argue that many of the old results can be carried over to this new setting and that the original claims about the deformed matrix model are essentially correct. We show the agreement between correlation functions calculated using continuum and matrix model techniques. We also explain how detailed properties of the space time metric of the extremal black hole of type 0A are reflected in the deformed matrix model.

hep-th↗

arXiv2003

Holography, inflation, and quantum fluctuations in the early universe

In this paper the relevance of holographic entropy bounds in the context of inflation is investigated. We distinguish between entropy on large and small scales and confront the entropy of quantum fluctuations in an inflating cosmology with the appropriate entropy bounds. In conclusion we do not find any constraints on inflation from holography, but some suggestions for future studies are given.

hep-th↗

arXiv2002

Miracles and complementarity in de Sitter space

In this paper we consider a scenario, consisting of a de Sitter phase followed by a phase described by a scale factor $a(t)\sim t^{q}$, where $1/3<q<1$, which can be viewed as an inflationary toy model. It is argued that this scenario naively could lead to an information paradox. We propose that the phenomenon of Poincaré recurrences plays a crucial role in the resolution of the paradox. We also comment on the relevance of these results to inflation and the CMBR.

hep-th↗

arXiv2002

On the consistency of de Sitter vacua

In this paper the consistency of the de Sitter invariant $α$-vacua, which have been introduced as simple tools to study the effects of transplanckian physics, is investigated. In particular possible non renormalization problems are discussed, as well as non standard properties of Greens functions. We also discuss the non thermal properties of the $α$-vacua and the necessity of $α$ to change. The conclusion is that non of these problems necessarily exclude an application of the $α$-vacua to inflation.

hep-th↗