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Laura Mersini-Houghton

Publications and source records attributed to Laura Mersini-Houghton.

At least 37 records · Page 2Linked to original sources
arXiv2015

Cosmic bubble and domain wall instabilities II: Fracturing of colliding walls

We study collisions between nearly planar domain walls including the effects of small initial nonplanar fluctuations. These perturbations represent the small fluctuations that must exist in a quantum treatment of the problem. In a previous paper, we demonstrated that at the linear level a subset of these fluctuations experience parametric amplification as a result of their coupling to the planar symmetric background. Here we study the full three-dimensional nonlinear dynamics using lattice simulations, including both the early time regime when the fluctuations are well described by linear perturbation theory as well as the subsequent stage of fully nonlinear evolution. We find that the nonplanar fluctuations have a dramatic effect on the overall evolution of the system. Specifically, once these fluctuations begin to interact nonlinearly the split into a planar symmetric part of the field and the nonplanar fluctuations loses its utility. At this point the colliding domain walls dissolve, with the endpoint of this being the creation of a population of oscillons in the collision region. The original (nearly) planar symmetry has been completely destroyed at this point and an accurate study of the system requires the full three-dimensional simulation.

hep-th↗
arXiv2015

Cosmic bubble and domain wall instabilities III: The role of oscillons in three-dimensional bubble collisions

We study collisions between pairs of bubbles nucleated in an ambient false vacuum. For the first time, we include the effects of small initial (quantum) fluctuations around the instanton profiles describing the most likely initial bubble profile. Past studies of this problem neglect these fluctuations and work under the assumption that the collisions posess an exact SO(2,1) symmetry. We use three-dimensional lattice simulations to demonstrate that for double-well potentials, small initial perturbations to this symmetry can be amplified as the system evolves. Initially the amplification is well-described by linear perturbation theory around the SO(2,1) background, but the onset of strong nonlinearities amongst the fluctuations quickly leads to a drastic breaking of the original SO(2,1) symmetry and the production of oscillons in the collision region. We explore several single-field models, and we find it is hard to both realize inflation inside of a bubble and produce oscillons in a collision. Finally, we extend our results to a simple two-field model. The additional freedom allowed by the second field allows us to construct viable inflationary models that allow oscillon production in collisions. The breaking of the SO(2,1) symmetry allows for a new class of observational signatures from bubble collisions that do not posess azimuthal symmetry, including the production of gravitational waves which cannot be supported by an SO(2,1) spacetime.

astro-ph.CO↗
arXiv2015

Back-reaction of the Hawking radiation flux on a gravitationally collapsing star II

A star collapsing gravitationally into a black hole emits a flux of radiation, known as Hawking radiation. When the initial state of a quantum field on the background of the star, is placed in the Unruh vacuum in the far past, then in the exterior Hawking radiation corresponds to a flux of positive energy radiation travelling outwards from near the surface to future infinity. Based on pair creation, the evaporation of the collapsing star can be equivalently described by the absorption of an ingoing negative energy flux of radiation travelling towards the center of the star. Here, we are interested in the evolution of the star during its collapse. Thus we include the backreaction of the negative energy Hawking flux in the interior geometry of the collapsing star when writing the full 4-dimensional Einstein and hydrodynamic equations. Hawking radiation emitted before the star passes through its Schwarzschild radius slows down and reverses the collapse of the star. The star evaporates without forming an horizon or a singularity. This study provides a more realistic investigation than the one first presented in [1], since the backreaction of Hawking radiation flux on the collapsing star is studied in the case when the initial state of the field is in Unruh's vacuum.

hep-th↗
arXiv2014

Cosmic bubble and domain wall instabilities I: parametric amplification of linear fluctuations

This is the first paper in a series where we study collisions of nucleated bubbles taking into account the effects of small initial (quantum) fluctuations in a fully 3+1-dimensional setting. In this paper, we consider the evolution of linear fluctuations around highly symmetric though inhomogeneous backgrounds. We demonstrate that a large degree of asymmetry develops over time from tiny fluctuations superposed upon planar and SO(2,1) symmetric backgrounds. These fluctuations arise from zero-point vacuum oscillations, so excluding them by enforcing a spatial symmetry is inconsistent in a quantum treatment. We consider the limit of two colliding planar walls, with fluctuation mode functions characterized by the wavenumber transverse to the collision direction and a longitudinal shape along the collision direction $x$, which we solve for. Initially, the fluctuations obey a linear wave equation with a time- and space-dependent mass $m_{eff}(x,t)$. When the walls collide multiple times, $m_{eff}$ oscillates in time. We use Floquet theory to study the fluctuations and generalize techniques familiar from preheating to the case with many coupled degrees of freedom. This inhomogeneous case has bands of unstable transverse wavenumbers $k_\perp$ with exponentially growing mode functions. From the detailed spatial structure of the mode functions in $x$, we identify both broad and narrow parametric resonance generalizations of the homogeneous $m_{eff}(t)$ case of preheating. The unstable $k_\perp$ modes are longitudinally localized, yet can be described as quasiparticles in the Bogoliubov sense. We define an effective occupation number to show they are created in bursts for the case of well-defined collisions in the background. The transverse-longitudinal coupling accompanying nonlinearity radically breaks this localized particle description, with nonseparable 3D modes arising.

hep-th↗
arXiv2014

Antioscillons

This proceeding is based on a talk I gave at the 13th Marcel Grossmann Meeting in Stockholm Sweden, on the role of topology on bubble collisions.

hep-ph↗
arXiv2014

Backreaction of Hawking Radiation on a Gravitationally Collapsing Star I: Black Holes?

Particle creation leading to Hawking radiation is produced by the changing gravitational field of the collapsing star. The two main initial conditions in the far past placed on the quantum field from which particles arise, are the Hartle Hawking vacuum and the Unruh vacuum. The former leads to a time symmetric thermal bath of radiation, while the latter to a flux of radiation coming out of the collapsing star. The energy of Hawking radiation in the interior of the collapsing star is negative and equal in magnitude to its value at future infinity. This work investigates the backreaction of Hawking radiation on the interior of a gravitationally collapsing star, in a Hartle-Hawking initial vacuum. It shows that due to the negative energy Hawking radiation in the interior, the collapse of the star stops at a finite radius, before the singularity and the event horizon of a black hole have a chance to form. That is, the star bounces instead of collapsing to a black hole. A trapped surface near the last stage of the star's collapse to its minimum size may still exist temporarily. Its formation depends on the details of collapse. Results for the case of Hawking flux of radiation with the Unruh initial state, will be given in a companion paper II.

hep-th↗
arXiv2014

Localization on the Landscape and Eternal Inflation

We investigate the validity of the assertion that eternal inflation populates the landscape of string theory. We verify that bubble solutions do not satisfy the Klein Gordon equation for the landscape potential. Solutions to the landscape potential within the formalism of quantum cosmology are Anderson localized wavefunctions. Those are inconsistent with inflating bubble solutions. The physical reasons behind the failure of a relation between eternal inflation and the landscape are rooted in quantum phenomena such as interference between wavefunction concentrated around the various vacua in the landscape.

hep-th↗
arXiv2012

The End of Eternal Inflation

We propose a new measure for eternal inflation that includes both conditions, large field fluctuations and smooth homogeneous domains, in the self reproducing probability estimate. We show that due to the increasing inhomogeneities in the background spacetime fractal, self-reproduction stops within a finite time t_f, thus inflation can not be eternal.

hep-th↗
arXiv2011

Is Eternal Inflation Eternal?

In this paper we explore the relationship between the existence of eternal inflation and the initial conditions leading to inflation. We demonstrate that past and future completion of inflation is related, in that past-incomplete inflation can not be future eternal. Bubble universes nucleating close to the initial conditions hypersurface have the largest Lorentz boosts and experience the highest anisotropy. Consequently, their probability to collide upon formation is one. Thus instead of continuing eternally inflation ends soon after it starts. The difficulty in actualizing eternal inflation originates from the breaking of two underlying symmetries: Lorentz invariance and the general covariance of the theory which lead to an inconsistency of Einstein equations. Eternal inflation may not be eternal.

gr-qc↗
arXiv2009

Investigating Dark Energy with Black Hole Binaries

The accelerated expansion of the universe is ascribed to the existence of dark energy. Black holes accretion of dark energy induces a mass change proportional to the energy density and pressure of the background dark energy fluid. The time scale during which the mass of black holes changes considerably is too long relative to the age of the universe, thus beyond detection possibilities. We propose to take advantage of the modified black hole masses for exploring the equation of state $w[z]$ of dark energy, by investigating the evolution of supermassive black hole binaries on a dark energy background. Deriving the signatures of dark energy accretion on the evolution of binaries, we find that dark energy imprints on the emitted gravitational radiation and on the changes in the orbital radius of the binary can be within detection limits for certain supermassive black hole binaries. In this talk I describe how binaries can provide a useful tool in obtaining complementary information on the nature of dark energy, based on the work done with A.Kelleher.

gr-qc↗
arXiv2008

Birth of the Universe from the Multiverse

It is fair to say that the deepest mystery in our understanding of nature is the birth of our universe. Much of the dilemma over the last decades comes from the extraordinarily small probability that the universe started with the high energy Big Bang as compared to the chance of nucleating any other event. How can Big Bang cosmology be $10^{10^{123}}$ times less likely than nucleating the present cold universe, while accumulating such exquisite agreement with astrophysical data? Why don't we see the other nucleations that, if left to chance, seem to overwhelmingly outnumber us? Here I discuss the point of view that the selection of the initial conditions can be meaningfully addressed only within the framework of the multiverse and that the reason why Big Bang inflation was preferred over other events lies in the quantum dynamics of the landscape of the initial patches. The out-of-equilibrium dynamics selected the 'survivor' universes be born at high energies and the 'terminal' universes at low energies. I briefly review the testable predictions of this theory, in particular the giant void observed in 2007. The second part focuses on the extended framework, in particular a set of postulates needed for defining the multiverse.

hep-th↗
arXiv2008

Probing Dark Energy with Black Hole Binaries

The equation of state (EoS) of dark energy $w$ remains elusive despite enormous experimental efforts to pin down its value and its time variation. Yet it is the single most important handle we have in our understanding of one of the most mysterious puzzle in nature, dark energy. This letter proposes a new method for measuring the EoS of dark energy by using the gravitational waves (GW) of black hole binaries. The method described here offers an alternative to the standard way of large scale surveys. It is well known that the mass of a black hole changes due to the accretion of dark energy but at an extremely slow rate. However, a binary of supermassive black holes (SBH) radiates gravitational waves with a power proportional to the masses of these accreting stars and thereby carries information on dark energy. These waves can propagate through the vastness of structure in the universe unimpeded. The orbital changes of the binary, induced by the energy loss from gravitational radiation, receive a large contribution from dark energy accretion. This contribution is directly proportional to $(1+w)$ and is dominant for SBH binaries with separation $R \ge 1000$ parsec, thereby accelerating the merging process for $w > -1$ or ripping the stars apart for phantom dark energy with $w < -1$. Such orbital changes, therefore $w$, can be detected with LIGO and LISA near merging time, or with X-ray and radio measurements of Chandra and VLBA experiments.

gr-qc↗
arXiv2008

Thoughts on Defining the Multiverse

The possibility that the multiverse corresponds to physical reality deserves serious investigation. Having three different important theories,(quantum mechanics, string theory and inflation), predict the existence of the multiverse is hardly coincidental. I argue that the existence of the multiverse must be expected from the underlying fundamental theory or else we can not meaningfully address the puzzle of the initial conditions. In this view, the extension of our current cosmology to a multiverse framework becomes an extension of the Copernican principle to nature. In order to discuss the ontology of the multiverse I propose to apply: - the principle of 'No Perpetual Motion' as a criterion for the parameter of time; and, - the principle of 'Domains Correlations' as a criterion for determining the background spacetime in which the multiverse is embedded.

gr-qc↗
arXiv2007

Nontrivial Geometries: Bounds on the Curvature of the Universe

Probing the geometry of the universe is one of the most important endevours in cosmology. Current observational data from the Cosmic Microwave Background anisotropy (CMB), galaxy surveys and type Ia supernovae (SNe Ia) strongly constrain the curvature of the universe to be close to zero for a universe dominated by a cosmological constant or dark energy with a constant equation of state. Here we investigate the role of cosmic priors on deriving these tight bounds on geometry, by considering a landscape motivated scenario with an oscillating curvature term. We perform a likelihood analysis of current data under such a model of non-trivial geometry and find that the uncertainties on curvature, and correspondingly on parameters of the matter and dark energy sectors, are larger. Future dark energy experiments together with CMB data from experiments like Planck could dramatically improve our ability to constrain cosmic curvature under such models enabling us to probe possible imprints of quantum gravity.

astro-ph↗
arXiv2007

The Arrow of Time Forbids a Positive Cosmological Constant $Λ$

Motivated by the mounting evidence for dark energy, here we explore the consequences of a fundamental cosmological constant $Λ$ for our universe. We show that when the gravitational entropy of a pure DeSitter state ultimately wins over matter, then the thermodynamic arrow of time in our universe must reverse in scales of order a Hubble time. We find that due to the dynamics of gravity and nonlocal entanglement, a finite size system such as a DeSitter patch with horizon size $H_0^{-1}$ has a finite lifetime $Δt$. This phenomenon arises from the dynamic gravitational instabilities that develop during a DeSitter epoch and turn catastrophic. A reversed arrow of time is clearly in disagreement with observations. Thus we are led to conclude: Nature forbids a fundamental $Λ$. Or else general relativity must be modified in the IR regime when $Λ$ dominates the expansion of the Universe.

gr-qc↗
arXiv2006

Cosmological Avatars of the Landscape II: CMB and LSS Signatures

This is the second paper in the series that confronts predictions of a model of the landscape with cosmological observations. We show here how the modifications of the Friedmann equation due to the decohering effects of long wavelength modes on the wavefunction of the Universe defined on the landscape leave unique signatures on the CMB spectra and large scale structure (LSS). We show that the effect of the string corrections is to suppress $σ_8$ and the CMB $TT$ spectrum at large angles, thereby bringing WMAP and SDSS data for $σ_8$ into agreement. We find interesting features imprinted on the matter power spectrum $P(k)$: power is suppressed at large scales indicating the possibility of primordial voids competing with the ISW effect. Furthermore, power is enhanced at structure and substructure scales, $k\simeq 10^{-2-0} h~{\rm Mpc}^{-1}$. Our smoking gun for discriminating this proposal from others with similar CMB and LSS predictions come from correlations between cosmic shear and temperature anisotropies, which here indicate a noninflationary channel of contribution to LSS, with unique ringing features of nonlocal entanglement displayed at structure and substructure scales.

hep-th↗
arXiv2006

Cosmological Implications of the String Theory Landscape

Progress in string theory has resulted in a whole landscape of vacua solutions.In this talk I describe a proposal for exploring the cosmological implications of the landscape, based on the dynamics of the wavefunction of the universe propagating on it. The landscape is taken as the phase space of the initial conditions since every of its vacua can potentially give rise to a universe. A superselection rule on the landscape phase space emerges when we include the backreaction of massive long wavelengths and the quantum dynamics of gravitational and matter degrees of freedom. The quantum dynamics of the system selects only high energy patches which survive the collapse induced by the gravitational instability of massive perturbations.

hep-ph↗