arXiv subjects

Jose J. Blanco-Pillado

Publications and source records attributed to Jose J. Blanco-Pillado.

At least 37 records · Page 2Linked to original sources

arXiv2021

Internal Excitations of Global Vortices

We investigate the spectrum of linearized excitations of global vortices in $2+1$ dimensions. After identifying the existence of localized excitation modes, we compute the decay time scale of the first two and compare the results to the numerical evolution of the full non-linear equations. We show numerically how the interaction of vortices with an external source of radiation or other vortices can excite these modes dynamically. We then simulate the formation of vortices in a phase transition and their interaction with a thermal bath estimating the amplitudes of these modes in each case. These numerical experiments indicate that even though, in principle, vortices are capable of storing a large amount of energy in these internal excitations, this does not seem to happen dynamically. We then explore the evolution of a network of vortices in an expanding (2+1) dimensional background, in particular in a radiation dominated universe. We find that vortices are still excited after the course of the cosmological evolution but again the level of excitation is very small. The extra energy in the vortices in these cosmological simulations never exceeds the $1\%$ level of the total mass of the core of the vortex.

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arXiv2021

Comparison of cosmic string and superstring models to NANOGrav 12.5-year results

We compare the spectrum of the stochastic gravitational wave background produced in several models of cosmic strings with the common-spectrum process recently reported by NANOGrav. We discuss theoretical uncertainties in computing such a background, and show that despite such uncertainties, cosmic strings remain a good explanation for the potential signal, but the consequences for cosmic string parameters depend on the model. Superstrings could also explain the signal, but only in a restricted parameter space where their network behavior is effectively identical to that of ordinary cosmic strings.

astro-ph.CO↗

arXiv2021

Universal Class of Type-IIB Flux Vacua with Analytic Mass Spectrum

We report on a new class of flux vacua generically present in Calabi-Yau compactifications of type-IIB string theory. At these vacua, the mass spectrum of the complete axio-dilaton/complex structure sector is given, to leading order in $α'$ and $g_s$, by a simple analytic formula independent of the choice of Calabi-Yau. We provide a method to find these vacua, and construct an ensemble of $17,054$ solutions for the Calabi-Yau hypersurface $\mathbb{WP}_{[1,1,1,6,9]}^4$, where the masses of the axio-dilaton and the $272$ complex structure fields can be explicitly computed.

hep-th↗

arXiv2021

Towards a complete mass spectrum of type-IIB flux vacua at large complex structure

The large number of moduli fields arising in a generic string theory compactification makes a complete computation of the low energy effective theory infeasible. A common strategy to solve this problem is to consider Calabi-Yau manifolds with discrete symmetries, which effectively reduce the number of moduli and make the computation of the truncated Effective Field Theory possible. In this approach, however, the couplings (e.g., the masses) of the truncated fields are left undetermined. In the present paper we discuss the tree-level mass spectrum of type-IIB flux compactifications at Large Complex Structure, focusing on models with a reduced one-dimensional complex structure sector. We compute the tree-level spectrum for the dilaton and complex structure moduli, \emph{including the truncated fields}, which can be expressed entirely in terms of the known couplings of the reduced theory. We show that the masses of this set of fields are naturally heavy at vacua consistent with the KKLT construction, and we discuss other phenomenologically interesting scenarios where the spectrum involves fields much lighter than the gravitino. We also derive the probability distribution for the masses on the ensemble of flux vacua, and show that it exhibits universal features independent of the details of the compactification. We check our results on a large sample of flux vacua constructed in an orientifold of the Calabi-Yau $\mathbb{WP}^4_{[1,1,1,1,4]}$. Finally, we also discuss the conditions under which the spectrum derived here could arise in more general compactifications.

hep-th↗

arXiv2021

Exciting the Domain Wall Soliton

Many solitonic configurations in field theory have localized bound states in their spectrum of linear perturbations. This opens up the possibility of having long lived excitations of these solitons that could affect their dynamics. We start the study of these effects in the simplest configuration of a domain wall kink solution in the $λϕ^4$ theory in $1+1$ dimensions. We show that this solution has a single bound state and numerically study its slow decay rate in flat space. We then investigate the amplitude of this excitation by simulating a cosmological phase transition that leads to the formation of these kinks in an expanding universe. We find that kinks get formed with a $20\%$ excess of energy with respect to their lowest energy configuration. We also explore the kink solution interacting with a thermal bath and extract the amplitude of the localized excitation as a function of temperature. We note that this amplitude increases with temperature but again the extra energy in the kink never goes over the $20\%$ level. Finally, we argue that this extra energy may have important consequences in the subsequent evolution of defects in numerical simulations.

hep-th↗

arXiv2021

No black holes from cosmic string cusps

Recent work by Jenkins and Sakellariadou claims that cusps on cosmic strings lead to black hole production. To derive this conclusion they use the hoop conjecture in the rest frame of the string loop, rather than in the rest frame of the proposed black hole. Most of the energy they include is the bulk motion of the string near the cusp. We redo the analysis taking this into account and find that cusps on cosmic strings with realistic energy scale do not produce black holes, unless the cusp parameters are extremely fine-tuned.

astro-ph.CO↗

arXiv2019

Direct determination of cosmic string loop density from simulations

We determine the distribution of cosmic string loops directly from simulations, rather than determining the loop production function and inferring the loop distribution from that. For a wide range of loop lengths, the results agree well with a power law exponent -2.5 in the radiation era and -2 in the matter era, the universal result for any loop production function that does not diverge at small scales. Our results extend those of Ringeval, Sakellariadou, and Bouchet: we are able to run for 15 times longer in conformal time and simulate a volume 300-2400 times larger. At the times they reached, our simulation is in general agreement with the more negative exponents they found, -2.6 and -2.4. However, our simulations show that this was a transient regime; at later times the exponents decline to the values above. This provides further evidence against models with a rapid divergence of the loop density at small scales, such as ``model 3'' used to analyze LIGO data and predict LISA sensitivity.

astro-ph.CO↗

arXiv2019

Energy-conservation constraints on cosmic string loop production and distribution functions

A network of cosmic strings would lead to gravitational waves which may be detected by pulsar timing or future interferometers. The details of the gravitational wave signal depend on the distribution of cosmic string loops, which are produced by intercommutations from the scaling network of long strings. We analyze the limits imposed by energy conservation, i.e., by the fact that the total amount of string flowing into loops cannot exceed the amount leaving the long strings. We show that some recent suggestions for the cosmic string loop production rate and distribution are ruled out by these limits. As a result, gravitational waves based on such suggestions, in particular "model 3" used in LIGO data analysis, are not to be expected.

astro-ph.CO↗

arXiv2019

Slepian models for Gaussian Random Landscapes

Phenomenologically interesting scalar potentials are highly atypical in generic random landscapes. We develop the mathematical techniques to generate constrained random potentials, i.e. Slepian models, which can globally represent low-probability realizations of the landscape. We give analytical as well as numerical methods to construct these Slepian models for constrained realizations of a full Gaussian random field around critical as well as inflection points. We use these techniques to numerically generate in an efficient way a large number of minima at arbitrary heights of the potential and calculate their non-perturbative decay rate. Furthermore, we also illustrate how to use these methods by obtaining statistical information about the distribution of observables in an inflationary inflection point constructed within these models.

hep-th↗

arXiv2019

Eternal Inflation in Swampy Landscapes

The much-discussed swampland conjectures suggest significant constraints on the properties of string theory landscape and on the nature of the multiverse that this landscape can support. The conjectures are especially constraining for models of inflation; in particular, they exclude the existence of de Sitter (dS) vacua. If the conjectures are false and dS vacua do exist, it still appears that their construction in string theory requires a fair amount of fine-tuning, so they may be vastly outnumbered by AdS vacua. Here we explore the multiverse structure suggested by these considerations. We consider two scenarios: (i) a landscape where dS vacua are rare and (ii) a landscape where dS vacua do not exist and the dS potential maxima and saddle points are not flat enough to allow for the usual hilltop inflation, even though slow-roll inflation is possible on the slopes of the potential. We argue that in both scenarios inflation is eternal and all parts of the landscape that can support inflation get represented in the multiverse. The spacetime structure of the multiverse in such models is nontrivial and is rather different from the standard picture.

gr-qc↗

arXiv2019

Probing the gravitational wave background from cosmic strings with LISA

Cosmic string networks offer one of the best prospects for detection of cosmological gravitational waves (GWs). The combined incoherent GW emission of a large number of string loops leads to a stochastic GW background (SGWB), which encodes the properties of the string network. In this paper we analyze the ability of the Laser Interferometer Space Antenna (LISA) to measure this background, considering leading models of the string networks. We find that LISA will be able to probe cosmic strings with tensions $Gμ\gtrsim \mathcal{O}(10^{-17})$, improving by about $6$ orders of magnitude current pulsar timing arrays (PTA) constraints, and potentially $3$ orders of magnitude with respect to expected constraints from next generation PTA observatories. We include in our analysis possible modifications of the SGWB spectrum due to different hypotheses regarding cosmic history and the underlying physics of the string network. These include possible modifications in the SGWB spectrum due to changes in the number of relativistic degrees of freedom in the early Universe, the presence of a non-standard equation of state before the onset of radiation domination, or changes to the network dynamics due to a string inter-commutation probability less than unity. In the event of a detection, LISA's frequency band is well-positioned to probe such cosmic events. Our results constitute a thorough exploration of the cosmic string science that will be accessible to LISA.

astro-ph.CO↗

arXiv2019

Gravitational backreaction simulations of simple cosmic string loops

We present the results of computational gravitational backreaction on simple models of cosmic string loops. These results give us insight into the general behavior of cusps and kinks on loops, in addition to other features of evolution. Kinks are rounded off via an asymmetric and divergent correction to the string direction. The result is that cusps emerge in the place of kinks but the resulting smooth string section has a small amount of energy. Existing cusps persist, but quickly lose strength as backreaction removes energy from the string surrounding the cusp. Both kinks and cusps have their location in space shifted slightly with each oscillation.

gr-qc↗

arXiv2019

Flyover vacuum decay

We use analytic estimates and numerical simulations to explore the stochastic approach to vacuum decay. According to this approach, the time derivative of a scalar field, which is in a local vacuum state, develops a large fluctuation and the field \flies over" a potential barrier to another vacuum. The probability distribution for the initial fluctuation is found quantum mechanically, while the subsequent nonlinear evolution is determined by classical dynamics. We find in a variety of cases that the rate of such flyover transitions has the same parametric form as that of tunneling transitions calculated using the instanton method, differing only by a numerical factor O(1) in the exponent. An important exception is an "upward" transition from a de Sitter vacuum to a higher-energy de Sitter vacuum state. The rate of flyover transitions in this case is parametrically different and can be many orders of magnitude higher than tunneling. This result is in conflict with the conventional picture of quantum de Sitter space as a thermal state. Our numerical simulations indicate that the dynamics of bubble nucleation in flyover transitions is rather different from the standard picture. The difference is especially strong for thin-wall bubbles in flat space, where the transition region oscillates between true and false vacuum until a true vacuum shell is formed which expands both inwards and outwards, and for upward de Sitter transitions, where the inflating new vacuum region is contained inside of a black hole.

hep-th↗

arXiv2019

Racetrack Potentials and the de Sitter Swampland Conjectures

We show that one can find de Sitter critical points (saddle points) in models of flux compactification of Type IIB String Theory without any uplifting terms and in the presence of several moduli. We demonstrate this by giving explicit examples following some of the ideas recently presented by Conlon in [1], as well as more generic situations where one can violate the strong form of the de Sitter Swampland Conjecture. We stabilize the complex structure and the dilaton with fluxes, and we introduce a racetrack potential that fixes the Kähler moduli. The resultant potentials generically exhibit de Sitter critical points and satisfy several consistency requirements such as flux quantization, large internal volume, and weak coupling, as well as a form of the so-called Weak Gravity Conjecture. Furthermore, we compute the form of the potential around these de Sitter saddle points and comment on these results in connection to the refined and more recent version of the de Sitter Swampland Conjecture.

hep-th↗

arXiv2018

Gravitational back-reaction near cosmic string kinks and cusps

We find the leading-order effect of gravitational back-reaction on cosmic strings for points near kinks and cusps. Near a kink, the effect diverges as the inverse cube root of the distance to the kink, and acts in a direction transverse to the worldsheet. Over time the kink is rounded off, but only regions fairly close to the kink are significantly affected. Near cusps, the effect diverges inverse linearly with the distance to the cusp, and acts against the direction of the cusp motion. This results in a fractional loss of string energy that diverges logarithmically with the distance of closest approach to the cusp.

gr-qc↗

arXiv2018

Inflation in Random Landscapes with two energy scales

We investigate inflation in a multi-dimensional landscape with a hierarchy of energy scales, motivated by the string theory, where the energy scale of Kahler moduli is usually assumed to be much lower than that of complex structure moduli and dilaton field. We argue that in such a landscape, the dynamics of slow-roll inflation is governed by the low-energy potential, while the initial condition for inflation are determined by tunneling through high-energy barriers. We then use the scale factor cutoff measure to calculate the probability distribution for the number of inflationary e-folds and the amplitude of density fluctuations $Q$, assuming that the low-energy landscape is described by a random Gaussian potential with a correlation length much smaller than $M_{\rm pl}$. We find that the distribution for $Q$ has a unique shape and a preferred domain, which depends on the parameters of the low-energy landscape. We discuss some observational implications of this distribution and the constraints it imposes on the landscape parameters.

hep-th↗

arXiv2017

New limits on cosmic strings from gravitational wave observation

We combine new analysis of the stochastic gravitational wave background to be expected from cosmic strings with the latest pulsar timing array (PTA) limits to give an upper bound on the energy scale of the possible cosmic string network, $Gμ< 1.5\times 10^{-11}$ at the 95% confidence level. We also show bounds from LIGO and to be expected from LISA and BBO. Current estimates for the gravitational wave background from supermassive black hole binaries are at the level where a PTA detection is expected. But if PTAs do observe a background soon, it will be difficult in the short term to distinguish black holes from cosmic strings as the source, because the spectral indices from the two sources happen to be quite similar. If PTAs do not observe a background, then the limits on $Gμ$ will improve somewhat, but a string network with $Gμ$ substantially below $10^{-11}$ will produce gravitational waves primarily at frequencies too high for PTA observation, so significant further progress will depend on intermediate-frequency observatories such as LISA, DECIGO and BBO.

astro-ph.CO↗

arXiv2017

Stochastic gravitational wave background from smoothed cosmic string loops

We do a complete calculation of the stochastic gravitational wave background to be expected from cosmic strings. We start from a population of string loops taken from simulations, smooth these by Lorentzian convolution as a model of gravitational back reaction, calculate the average spectrum of gravitational waves emitted by the string population at any given time, and propagate it through a standard model cosmology to find the stochastic background today. We take into account all known effects, including changes in the number of cosmological relativistic degrees of freedom at early times and the possibility that some energy is in rare bursts that we might never have observed.

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