MaxEnt files: Sam's Extrig Talk.
Ruxandra Bondarescu - Home Page Ruxandra Bondarescu `

Welcome to Ruxandra Bondarescu's Page!




CV pdf. See also brief research description below.


Personal my Family My brother's page Pictures and fun

Welcome to my page. I received my PhD in physics in August 2008 from Cornell University and I am now a postdoc at Penn State. My research includes the following topics:

  • Gravitational Wave Astronomy
    Together with Ravi Kopparapu, Sam Finn, Ryan Fisher, Meagan Lang (Penn State) and Tiffany Summerscales (Andrews University), I am working on inferring gravitational waveforms from the noisy data of a detector network. Our goal is to use recovered gravitational waveforms to identify source properties. In particular, just as in electromagnetism and astronomy we study gravitational wave polarization in terms of Stokes Parameters. These parameters can be directly determined from the + and x components of an gravitational waveform as seen by a given detector network. They are frequency dependent and can be combined to measure the degree of circular, linear, and elliptical polarization of the wave. The degree of polarization can determine source orientation and degree of non-axisymmetry. Systems that are non-axisymmetric radiate angular momentum and emit circularly polarized radiation. Axisymmetric systems emit linearly polarized gravitational radiation.

  • Rmodes
    Rmodes are oscillations that occur in rotating fluids. In rapidly rotating neutron stars these modes can be unstable. The instability is driven by the gravitational radiation reaction. The most relvant mode for gravitational radiation emisson is the Rossby wave with L=m=2. This mode is unstable when gravitational driving dominates viscous dissipation. Once the amplitude L=m=2 r-mode passes its parameteric instability threshold amplitude, it excites other near-resonant modes in the system and nonlinear effects become important. Roughly speaking, the r-mode instability converts rotational energy to mode energy and gravitational radiation and the star slows down. We study how nonlinear effects can affect the limiting spin frequency of neutron stars. This project is a continuation of my thesis research and is in collaboration with Ira Wasserman and Saul Teukolsky (Cornell University). This limiting spin frequency depends on internal neutron star physics such as neutrino cooling and viscous heating. Our goal is to understand how fast neutron stars can spin and connect the limiting spin frequency due to r-modes to current observations.

  • Beams and mirror shapes for future gravitational wave interferometers.
    Thermal noise will be the dominant form of noise in the most sensitive frequency band of Advanced LIGO detectors. In the past, my collaborators and I investigated how finite mirror effects can affect the thermal noise of non-Gaussian beams. We found some resonances that led to preferred beam widths with lower thermal noise for the same diffraction loss. We showed that the coating thermal noise, which dominates in the most sensitive frequency band of Advanced LIGO, can be reduced by 12% with no additional effort by using finite mirror effects to our advantage rather then working against them and by 28% with some modifications to the mirror to match the phase front of the finite beam. This work is in collaboration with Andrew Lundgren (Syracuse University), David Tsang (Cornell University; moving to Caltech in August) and Mihai Bondarescu (University of Mississippi). Previous work by Mihai, Oleg Kogan and Yanbei Chen, in which they did not include finite mirror effects, found that the optimal non-Gaussian mirror is conical and reduces thermal noise by 60% compared to Gaussian alternatives.

  • Luke-warm Dark Matter
    Most matter in the universe is non-luminous. The observed flatness of the galactic rotation curves has been an indicator of the presence of dark haloes around galaxies. More recently, gravitational lensing observations of the Bullet cluster have provided direct proof of the presence of dark matter. In collaboration with Andrew Lundgren (Syracuse U.), Mihai Bondarescu (the University of Mississippi) and Jayashree Balakrishna (Harris Stowe State University), I work on understanding the cosmological evolution and the Bose-Einstein condensation of ultra-light dark matter particles that have a Compton wavelength of galactic dimensions. Agglomerations of these particles form stable halo structures that are supported against collapse by Heisenberg's uncertainty principle similar to boson stars and naturally exhibit no small scale structure. The particles that condense to the ground state behave like dust or non-relativistic matter, while the particles in excited states act as radiation. Constraints on the amount of radiation other than photons and neutrinos that can be present in the universe are given by WMAP measurements. The condensate can have a temperature of up to 1.5 K, which makes it luke-warm.

  • Other Exotic Dark Matter Candidates
    In the past I did some work on compact scalar objects - both boson stars (complex scalar field configurations) and soliton stars (real scalar field, which could represent an axion). Light axions could have been created by non-thermal processes in the early universe leaving them slow moving and compatible with preferred cold-dark matter models. Stars composed of such particles would be an exotic source of gravitational waves. Their detection would confirm the presence of scalar field dark matter. We studied propreties of these stars using a 3D code based on the Cactus Computational Toolkit (www.cactuscode.org), their stability under spherical and non-spherical perturbations and the gravitational waveforms they produce. This work has been done in collaboration with Jayashree Balakrishna (Harris Stowe University), Gregory Daues (NCSA), Francisco S. Guzman (Universidad Michoacana de San Nicolas de Hidalgo, Mexico), Mihai Bondarescu (AEI/Caltech/U. of Mississippi), and Ed Seidel (LSU/CCT). The luke-warm dark matter work is an indirect continuation of this topic.

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    Publications

    Work in Progress

    1. R. Kopparapu, R. Bondarescu , L. S. Finn, M. Lang and T. Summerscales, "Distinguishing GRB progenitors using gravitational wave observations. "
    2. R. Bondarescu , R. Kopparapu, L.S. Finn, M. Lang and T. Summerscales, "Gravitational Wave Astronomy with Stokes Parameters." (to be submitted to ApJ)
    3. A. Lundgren, R. Bondarescu , D. Tsang, and M. Bondarescu, "Optimizing mirrors for gravitational wave detectors including finite mirror effects."

    Refereed Publications

    7. A. Lundgren, M. Bondarescu, R. Bondarescu and J. Balakrishna,
    "Luke-warm dark matter: Bose condensation at finite temperatures. ", arXiv:1001.0051
    (submitted to ApJ Letters)

    6. R. Bondarescu, S. Teukolsky, and I. Wasserman,
    "Spinning Down Newborn Neutron Stars: Nonlinear Development of the R-mode Instability",
    arXiv:0809.3448 Phys. Rev. D 79, 104003 (2009).

    5. A. Lundgren, R. Bondarescu , D. Tsang, M. Bondarescu,
    "Finite Mirror Effects in Advanced Interferometric Gravitational Wave Detectors",
    arxiv:0710.3808, Phys. Rev. D 77, 042003 (2008).

    4. J. Balakrishna, R. Bondarescu , G. Daues, M. Bondarescu,
    "Numerical Simulations of Oscillating Soliton Stars: Excited States in Spherical Symmetry and Ground State Evolutions in 3D",
    arxiv:0710.4131, Phys. Rev. D 77, 024028 (2008).

    3. R. Bondarescu, S. Teukolsky, and I. Wasserman,
    "Spin Evolution of Accreting Neutron Stars: Nonlinear Development of the R-mode Instability",
    arXiv:0704.0799, Phys. Rev. D 76, 064019 (2007).

    2. J. Balakrishna, R. Bondarescu, G. Daues, F. Guzman, and E. Seidel,
    "Evolution of 3D Boson Stars with Waveform Extraction",
    Class. Quantum Grav. 23 (2006) 2631-2652, gr-qc/0602078.

    1. R. Bondarescu, G. Allen, G. Daues, I. Kelley, M. Russell, E. Seidel, J. Shalf, M. Tobias,
    "The Astrophysics Simulation Collaboratory Portal: a Framework for Effective Distributed Research",
    Future Generation Computing Systems 21 (2005) 259-270, Special Issue on Advanced Grid Technologies.
    PDF version of the paper   Abstract through Science Direct



    Ph.D. Thesis Ruxandra Bondarescu, Cornell University, August 2008.

    Conferences and Talks

    Physics Talks
    1. "Rossby waves in neutron stars", CAM 2007, Montreal, Canada, August 2007, (Travel Grant from APS)
    2. "Spin Evolution of Accreting Neutron Stars and the R-mode Instability", GR18, Australia, July 2007 (ICPS travel grant + Cornell Travel Grant + NSF travel grant).
    3. "Spin Evolution of Accreting Neutron Stars: Nonlinear Effects of the R-mode Instability", East Coast Gravity Meeting and The Teukolsky Birthday Symposium, Cornell, June 2007 (free; I also chaired a session ).
    4. "Spin Evolution of Neutron Stars: Nonlinear Effects of the R-mode Instability", April APS Meeting, Florida, 2007 (Travel Grant from both Cornell and APS)
    5. "R-modes in Spinning neutron stars", International Conference for Physics Students, Bucharest, Romania, August 2006.
    6. "How fast can neutron stars spin?", CAM 2005 (Travel grant awarded by APS that covered all expenses)
    7. "Evolution of 3D boson stars", April APS Meeting 2004, Tampa, Florida (Travel grant awarded by APS to attend this conference)
    8. "Boson Stars", East Coast Gravity Meeting, March 2003
    9. "Introduction to Boson Stars", West University of Timisoara (invited talk)


    Old computer science talks
    1. "Requirements for Grid Enabling an Application", presented at the GAT Workshop, Cardiff University, United Kingdom, July 2003 Slides
    2. "The Astrophysics Simulation Collaboratory Portal", presented at the GridLab All Hands conference and workshop, Eger, Hungary, April 2003 Powerpoint Slides
    3. "The ASC Portal: A Tool for Numerical Relativity", presented at Louisiana State University, February, 2003
    4. "Grid and Portal Technology Tutorial", presented at West University of Timisoara, Romania, March, 2003
    5. "Short overview of research activities at the LSU CAPITAL + Short Cactus Tutorial", presented as part of the Gravitational Waves Fun - Louisiana State University, January, 2004 (Slides)

    Teaching and TAing

    • Cornell University
      Physics 217 - Electricity and Magnetism for Honor Students: Spring 2005 (part time TA) with Prof. Lois Pollack, Fall 2005 (full time TA) with Prof. Andre Leclair
    • My brother and I organized a series of 14 Gravitational Waves lectures at LSU (December 03-January 04) based on Kip Thorne's Ph. 237. We received funding from LSU CAPITAL to organize the lectures. I organized the last 4 lectures after Mihai left with help from Greg. We did the same thing at Cornell in the Spring of 2004 (and for part of the summer) and here I was the main organizer.
      Gravitational Waves Fun at Cornell - Spring 2004
      Gravitational Waves Fun at LSU - Winter 2003
    • Department of Physics, University of Illinois at Urbana-Champaign, IL
      1. Teaching Assistant
        Course: Physics 371 (Advanced Optics- Light)
        Professor: Taekjip Ha
        Spring 2003
      2. Teaching Assistant
        Course: Physics 140 (Practical Physics: How Things Work)
        Professor: Laura H. Greene
        Fall 2002
      3. Teaching Assistant
        Course: Physics 114 (General Physics: Waves and Quantum Mechanics)
        Professor: Gary Gladding and Lance Cooper
        Spring 2002



    These are two webpages with pics from boson star simulations. They were primarly written to convince my undergraduate advisor that we had enough for a paper and it worked :).

    Boson Star Results (Last Updated Jan 25, 2004)

    Older Webpage with Results (Last Updated Dec 1, 2003)