Alejandro Gangui

University of Buenos Aires, Buenos Aires, Buenos Aires F.D., Argentina

Are you Alejandro Gangui?

Claim your profile

Publications (23)39.13 Total impact

  • Source
    Alejandro Gangui
    [show abstract] [hide abstract]
    ABSTRACT: The potential role of cosmic topological defects has raised interest in the astrophysical community for many years now. In this set of notes, we give an introduction to the subject of cosmic topological defects and some of their possible observable signatures. We begin with a review of the basics of general defect formation and evolution, we briefly comment on some general features of conducting cosmic strings and vorton formation, as well as on the possible role of defects as dark energy, to end up with cosmic structure formation from defects and some specific imprints in the cosmic microwave background radiation from simulated cosmic strings. A detailed, pedagogical explanation of the mechanism underlying the tiny level of polarization discovered in the cosmic microwave background by the DASI collaboration (and recently confirmed by WMAP) is also given, and a first rough comparison with some predictions from defects is provided. Comment: Lecture Notes delivered at the Xth Brazilian School on Cosmology and Gravitation, Mangaratiba, Rio de Janeiro, July 29 - August 9, 2002. To appear in the proceedings (AIP Press), edited by M. Novello and S. Perez Bergliaffa. Updated source files with high resolution figures available at http://www.iafe.uba.ar/relatividad/gangui/xescola/
    03/2003;
  • Source
    Alejandro Gangui
    [show abstract] [hide abstract]
    ABSTRACT: In the framework of inflationary models with non-vacuum initial states for cosmological perturbations, we study non-Gaussian signatures on the cosmic microwave background radiation produced by a broken-scale-invariant model which incorporates a feature at a privileged scale in the primordial power spectrum.
    03/2003;
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: We study non-Gaussian signatures on the cosmic microwave background (CMB) radiation predicted within inflationary models with non-vacuum initial states for cosmological perturbations. The model incorporates a privileged scale, which implies the existence of a feature in the primordial power spectrum. This broken-scale-invariant model predicts a vanishing three-point correlation function for the CMB temperature anisotropies (or any other odd-numbered-point correlation function) whilst an intrinsic non-Gaussian signature arises for any even-numbered-point correlation function. We thus focus on the first non-vanishing moment, the CMB four-point function at zero lag, namely the kurtosis, and compute its expected value for different locations of the primordial feature in the spectrum, as suggested in the literature to conform with observations of large scale structure. The excess kurtosis is found to be negative and the signal to noise ratio for the dimensionless excess kurtosis parameter is equal to |S/N|≃4×10-4, almost independently of the free parameters of the model. This signature turns out to be undetectable. We conclude that, subject to current tests, Gaussianity is a generic property of single field inflationary models. The only uncertainty concerning this prediction is that the effect of back reaction has not yet been properly incorporated. The implications for the trans-Planckian problem of inflation are also briefly discussed.
    Physical Review D 10/2002; 66(8). · 4.69 Impact Factor
  • Source
    Alejandro Gangui, Jerome Martin, Mairi Sakellariadou
    [show abstract] [hide abstract]
    ABSTRACT: In the framework of Inflationary theory, the assumption that the quantum state of the perturbations is a non-vacuum state leads to a difficulty: non-vacuum initial states imply, in general, a large energy density of inflaton field quanta, not of a cosmological term type, that could prevent the inflationary phase. In this short note, we discuss in detail why this is so, keeping an eye on possible non-Gaussian features due to considering generic non-vacuum initial states.
    07/2002;
  • Alejandro Gangui
    [show abstract] [hide abstract]
    ABSTRACT: In the framework of inflationary models with non-vacuum initial states for cosmological perturbations, we study non-Gaussian signatures on the cosmic microwave background (CMB) radiation produced by a broken-scale-invariant model which incorporates a feature at a privileged scale in the primordial power spectrum.
    International Journal of Modern Physics A 01/2002; 17:4273-4280. · 1.13 Impact Factor
  • Source
    Alejandro Gangui
    [show abstract] [hide abstract]
    ABSTRACT: Topological defects are ubiquitous in condensed-matter physics but only hypothetical in the early universe. In spite of this, even an indirect evidence for one of these cosmic objects would revolutionize our vision of the cosmos. We give here an introduction to the subject of cosmic topological defects and their possible observable signatures. Beginning with a review of the basics of general defect formation and evolution, we then focus on mainly two topics in some detail: conducting strings and vorton formation, and some specific imprints in the cosmic microwave background radiation from simulated cosmic strings.
    11/2001;
  • Source
    Alejandro Gangui
    [show abstract] [hide abstract]
    ABSTRACT: The cosmic microwave background (CMB) provides a snapshot of the early Universe when matter began to become structured and has been the focus of several recent observational campaigns. In this Perspective, we discuss the results from these campaigns, which provide support for inflationary models of the Universe.
    03/2001;
  • Source
    Alejandro Gangui, Levon Pogosian, Serge Winitzki
    [show abstract] [hide abstract]
    ABSTRACT: We propose a method for a numerical computation of the angular bispectrum of the cosmic microwave background (CMB) anisotropies arising from active models such as cosmic topological defects, using a modified Boltzmann code based on CMBFAST. The method does not use CMB sky maps and requires moderate computational power. As a first implementation, we apply our method to a recently proposed model of simulated cosmic strings and estimate the observability of the non-Gaussian bispectrum signal. A comparison with the cosmic variance of the bispectrum estimator shows that the bispectrum for the simulated string model we used is not observable.
    Physical review D: Particles and fields 01/2001; 64(4).
  • Source
    Alejandro Gangui, Levon Pogosian, Serge Winitzki
    [show abstract] [hide abstract]
    ABSTRACT: We briefly review certain aspects of cosmic microwave background anisotropies as generated in passive and active models of structure formation. We then focus on cosmic strings based models and discuss their status in the light of current high-resolution observations from the BOOMERanG, MAXIMA and DASI collaborations. Upcoming megapixel experiments will have the potential to look for non-Gaussian features in the CMB temperature maps with unprecedented accuracy. We therefore devote the last part of this review to treat the non-Gaussianity of the microwave background and present a method for computation of the bispectrum from simulated string realizations.
    New Astronomy Reviews 01/2001; · 1.82 Impact Factor
  • Alejandro Gangui
    [show abstract] [hide abstract]
    ABSTRACT: These relics from the early universe could be the answer to many astrophysical conundrums. Comment: non technical review. See also the journal site at http://www.amsci.org/amsci/articles/00articles/gangui.html or http://www.iafe.uba.ar/relatividad/gangui/SCS.html
    American Scientist 05/2000; · 0.83 Impact Factor
  • Source
    Alejandro Gangui
    [show abstract] [hide abstract]
    ABSTRACT: For those angular multipoles where cosmic variance is an issue, non-Gaussianities in the Cosmic Microwave Background (CMB) anisotropies will be hard to detect. Here, we construct explicitly the best unbiased estimator for the CMB angular bispectrum.
    04/2000;
  • Source
    Alejandro Gangui, Jerome Martin
    [show abstract] [hide abstract]
    ABSTRACT: Measuring the three-point correlators of the Cosmic Microwave Background (CMB) anisotropies could help to get a handle on the level of non-Gaussianity present in the observational datasets and therefore would strongly constrain models of the early Universe. However, typically, the expected non-Gaussian signal is very small. Therefore, one has to face the problem of extracting it from the noise, in particular from the `cosmic variance' noise. For this purpose, one has to construct the best unbiased estimators for the three-point correlators that are needed for concrete detections of non-Gaussian features. In this article, we study this problem for both the CMB third moment and the CMB angular bispectrum. We emphasize that the knowledge of the best estimator for the former does not permit one to infer the best estimator for the latter and vice versa. We present the corresponding best unbiased estimators in both cases and compute their corresponding cosmic variances.
    Physical Review D 02/2000; · 4.69 Impact Factor
  • Source
    Alejandro Gangui, Jerome Martin
    [show abstract] [hide abstract]
    ABSTRACT: Recent tentative findings of non-Gaussian structure in the COBE-DMR dataset have triggered renewed attention to candidate models from which such intrinsic signature could arise. In the framework of slow roll inflation with built-in non linearities in the inflaton field evolution we present expressions for both the cosmic microwave background (CMB) skewness and the full angular bispectrum ${\cal C}_{\ell_1 \ell_2 \ell_3}$ in terms of the slow roll parameters. We use an estimator for the angular bispectrum recently proposed in the literature and calculate its variance for an arbitrary $\ell_i$ multipole combination. We stress that a real detection of non-Gaussianity in the CMB would imply that an important component of the anisotropies arises from processes {\it other} than primordial quantum fluctuations. We further investigate the behavior of the signal-to-(theoretical) noise ratio and demonstrate for generic inflationary models that it decreases in the limited range of small-$\ell$'s considered for increasing multipole $\ell$ while the opposite applies for the standard ${\cal C}_{\ell}$'s. Comment: 15 pages, 3 figures, minor corrections Ref: MNRAS 313, 323 (2000)
    Monthly Notices of the Royal Astronomical Society 08/1999; · 5.52 Impact Factor
  • Alejandro Gangui, Patrick Peter, Edgard Gunzig
    [show abstract] [hide abstract]
    ABSTRACT: Currents in cosmic strings built out offermionic modes coupled to the vortex-forming Higgsfield are reviewed. Massive modes are also exhibitedwhich illuminate the structure of bosonic currents asthe timelike charge is increased.
    International Journal of Theoretical Physics 01/1999; 38(1):205-216. · 1.09 Impact Factor
  • Source
    Alejandro Gangui, Edgard Gunzig
    [show abstract] [hide abstract]
    ABSTRACT: In this article we review recent work aimed at showing explicitly the influence of electromagnetic self corrections on the dynamics of a circular vortex line endowed with a current at first order in the coupling between the current and the self-generated electromagnetic field. Comment: Proceedings of the Eighth Marcel Grossmann Meeting on General Relativity, Gravitation and Relativistic Field Theories. 22--27 June 1997, Jerusalem
    11/1997;
  • Source
    Alejandro Gangui, Patrick Peter, Celine Boehm
    [show abstract] [hide abstract]
    ABSTRACT: The modifications of circular cosmic string loop dynamics due to the electromagnetic self-interaction are calculated and shown to reduce the available phase space for reaching classical vorton states, thereby decreasing their remnant abundance. Use is made of the duality between master-function and Lagrangian formalisms on an explicit model. Comment: 10 pp RevTeX and 6 eps figures
    Physical Review D 05/1997; · 4.69 Impact Factor
  • Source
    Brandon Carter, Patrick Peter, Alejandro Gangui
    [show abstract] [hide abstract]
    ABSTRACT: Earlier attempts to calculate the nonlinear dynamical evolution of Witten type superconducting vacuum vortex defects relied on the use of approximate conducting string models that were too simple to take proper account of the effect of current saturation. This effect is however allowed for adequately in a newly developed class of rather more complicated, though still conveniently analytic, conducting string models. These more realistic models have recently been employed by Larsen and Axenides for investigating the collapse of circular string loops in the case for which angular momentum is absent. The present work extends this investigation to the generic case of circular string loops for which angular momentum is present, so that there will be a centrifugal potential barrier. This barrier will prevent collapse unless the initial conditions are such that the relevant current saturation limit is attained, in which case the string description of the vortex defect will break down, so that its subsequent fate is hard to foresee. On the other hand if saturation is avoided one would expect that the loop will eventually radiate away its excess energy and settle down into a vorton type equilibrium state. Comment: 15 pp RevTeX plus 6 PostScript figures
    Physical review D: Particles and fields 09/1996;
  • Source
    Alejandro Gangui, Ruth Durrer, Mairi Sakellariadou
    [show abstract] [hide abstract]
    ABSTRACT: We review recent work aimed at showing how global topological defects influence the shape of the angular power spectrum of the CMB radiation on small scales. While Sachs-Wolfe fluctuations give the dominant contribution on angular scales larger than about a few degrees, on intermediate scales the main r\^ole is played by coherent oscillations in the baryon radiation plasma before recombination. In standard cosmological models these oscillations lead to the `Doppler peaks' in the angular power spectrum. Inflation-based cold dark matter models predict the location of the first peak to be at $\ell\sim 220/\sqrt{\Omega_0}$, with a height which is a few times the level of anisotropies at large scales. Here we focus on perturbations induced by global textures. We find that the height of the first peak is reduced and is shifted to $\ell\sim 350$.
    03/1996;
  • Source
    Alejandro Gangui
    [show abstract] [hide abstract]
    ABSTRACT: I present recent work aimed at showing how currently competing theories of the early universe leave their imprint on the temperature anisotropies of the CMB radiation. We consider the 3-point correlation function, as well as the inherent theoretical uncertainties associated with it, for which we derive explicit analytic formulae. We apply them in the study of non-Gaussian features that arise on large scales in the framework of both inflationary and topological defects models. For inflation, we develop a perturbative analysis for the study of spatial correlations in the inflaton in the context of the stochastic approach to inflation. We also include an analysis of a particular geometry, the so-called `collapsed' 3-point function, in the case of post recombination integrated effects, arising generically whenever the mildly non linear growth of perturbations is taken into account. We also devote a part of the thesis to the study of recently proposed analytic models of topological defects, and implement them in the analysis of both the CMB excess kurtosis (for cosmic strings) and the CMB collapsed 3-point function and skewness (for textures). Lastly, we present a study of the CMB anisotropies on the degree angular scale in the framework of the global texture scenario, and show the relevant features that arise; among these, the Doppler peaks.
    01/1996;
  • Source
    Ruth Durrer, Alejandro Gangui, Mairi Sakellariadou
    [show abstract] [hide abstract]
    ABSTRACT: The Doppler peaks (Sacharov peaks) in the angular power spectrum of the cosmic microwave background anisotropies, are mainly due to coherent oscillations in the baryon radiation plasma before recombination. Here we present a calculation of the Doppler peaks for perturbations induced by global textures and cold dark matter. We find that the height of the first Doppler peak is smaller than in standard cold dark matter models, and that its position is shifted to $\ell\sim 350$. We believe that our analysis can be easily extended to other types of global topological defects and general global scalar fields. Comment: 10pp, LaTeX, 2 PostScript figures included. Final version to appear in Phys. Rev. Letters
    Physical Review Letters 07/1995; · 7.94 Impact Factor

Publication Stats

152 Citations
39.13 Total Impact Points

Institutions

  • 2000–2002
    • University of Buenos Aires
      • Institute of Astronomy and Spatial Physics (IAFE)
      Buenos Aires, Buenos Aires F.D., Argentina
  • 1996–1997
    • French National Centre for Scientific Research
      Lutetia Parisorum, Île-de-France, France
  • 1994
    • Scuola Internazionale Superiore di Studi Avanzati di Trieste
      Trst, Friuli Venezia Giulia, Italy