Alejandro Gangui

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

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Publications (30)85.85 Total impact

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    Alejandro Gangui
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    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;
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    Alejandro Gangui
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    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;
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    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
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    Alejandro Gangui, Jerome Martin, Mairi Sakellariadou
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    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;
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    Alejandro Gangui
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    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;
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    A Gangui
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    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.
    Science 03/2001; 291(5505):837-8. · 31.03 Impact Factor
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    Alejandro Gangui, Levon Pogosian, Serge Winitzki
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    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).
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    Alejandro Gangui, Levon Pogosian, Serge Winitzki
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    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
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    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
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    Alejandro Gangui
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    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;
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    Alejandro Gangui, Jerome Martin
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    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
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    Alejandro Gangui, Jerome Martin
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    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
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    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
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    ABSTRACT: The problem of decoherence in quantum gravity is discussed in the framework of third quantization. We propose an interpretation of the theory in which the probabilities in our observable universe have to be calculated with the use of the one-universe density matrix, which appear after tracing out irrelevant universes from the 'googolplexus' pure state. Many separate classical universes can emerge in this one-universe density matrix only after subsequent tracing in irrelevant variables.
    Classical and Quantum Gravity 12/1998; 10(12):2495. · 3.56 Impact Factor
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    Alejandro Gangui
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    ABSTRACT: We report briefly on a recent analysis of the texture-induced CMB radiation three-point correlation function of temperature anisotropies as predicted by an analytical model. We specialize our analysis to both large-scales (e.g., for COBE-DMR, where we compare our prediction with the actual four-year data) and intermediate-scales. We show how the latter case puts strong constraints on the model parameters.
    12/1997;
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    Alejandro Gangui, Edgard Gunzig
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    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;
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    Alejandro Gangui, Patrick Peter, Celine Boehm
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    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
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    A Gangui, S Mollerach
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    ABSTRACT: Using an analytical model for the Cosmic Microwave Background anisotropies produced by textures, we compute the resulting collapsed three--point correlation function and the {\it rms} expected value due to the cosmic variance. We apply our calculations to the {\sl COBE}--DMR experiment and test the consistency of the model with the observational results. We also show that an experiment with smaller angular resolution can put bounds to the model parameters. Comment: 13 pp, RevTeX, 4 PostScript figures, to appear in Phys.Rev.D
    Physical review D: Particles and fields 11/1996; 54(8):4750-4756.
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    Brandon Carter, Patrick Peter, Alejandro Gangui
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    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;
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    Alejandro Gangui, Ruth Durrer, Mairi Sakellariadou
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    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;

Publication Stats

545 Citations
85.85 Total Impact Points

Institutions

  • 1991–2002
    • 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
  • 1993–1994
    • Scuola Internazionale Superiore di Studi Avanzati di Trieste
      Trst, Friuli Venezia Giulia, Italy