Welcome to Ruxandra Bondarescu's Page!

CV pdf. See also the research description below.

Personal my Family My brother's page Pictures and fun

Welcome to my page. I am now a fellow at the Institute for Theoretical Physics, University of Zurich. Overall, my scientific interests are in astrophysics, cosmology and geophysics. My research so far has covered a variety of topics including gravitational wave astronomy, neutron star physics, dark matter cosmology and numerical relativity. A description of some of the topics I have worked on is provided below.

In November 2011 I started a blog: http://ruxandrab.blogspot.com/. I summarize seminars I go to. I write about places I visit and about other ideas or thoughts I want to share.

Gravitational Wave Astronomy: Stokes Parameters

I work on understanding our ability to measure gravitational wave polarization with a given detector network. One can combine hp and hx in Stokes parameters that measure circular and linear polarization. Circularly polarized gravitational waves carry angular momentul, while linearly polarized waves do not. The degree of circular polarization is a measure of the nonaxisymmetry of the source. This is particularly interesting for short and long Gamma ray burst phenomena, where tracking the polarization of the gravitational waves could provide insight into the evolution of the central engine. Polarization is a property of the radiation itself and does not depend on the source model. This work is in collaboration with Sam Finn and Ravi Kopparapu, both at Penn State University.

Resonant Shattering of Neutron Star Crusts

This is work in collaboration with David Tsang (Caltech), Jocelyn Read (Mississippi) and Tony Piro (Caltech). We investigate the resonant excitation of neutron star modes by tides. We find that the driving of the L=2, m=2 crust-core interface mode can lead to shattering of the NS crust seconds before the merger of a NS-NS or NS-BH binary. This mechanism can lead to precursor flares before short GRBs like the flares that have already been observed by Swift-BAT, Fermi and Suzaku. We describe how a larger sample of precursor detections could be used alongside coincident gravitational wave detections of the inspiral by Advanced LIGO class detectors to probe the NS structure. These two types of observations nicely complement one another, since the former constrains the equation of state and structure near the crust-core boundary, while the latter is more sensitive to the core equation of state.

Rmodes

My PhD thesis in collaboration with Profs. Saul Teukolsky and Ira Wasserman was on r-modes. 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. My current project is to understand how fast neutron stars can spin in LMXBs 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/AEI Hannover), David Tsang (Caltech) 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 7-year+BAO+H0 measurements. We found that a natural temperature for this condensate would be around 0.9 K, which makes it luke-warm.

Other Exotic Dark Matter Candidates

In the past I studied compact scalar objects - boson stars (complex scalar field configurations; potential particle candidates include WIMPs) and soliton stars (real scalar field configurations; the most prominent scalar particles candidates are axions). 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 scalar 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 a continuation of this work.

Publications

1. L. S. Finn, R. Bondarescu , and R. Kopparapu, "Gravitational Wave Astronomy with Stokes Parameters." (to be submitted to MNRAS)

Refereed Publications

10. A. P. Lundgren, M. Bondarescu, and R. Bondarescu ,
"The Physics of the Far Future: Non-equilibrium Thermodynamics in deSitter Space",
http://arxiv.org/abs/1201.1298 (submitted to Physical Review Letters).

9. R. Bondarescu, M. Bondarescu, G. Hetenyi, L. Boschi, P. Jetzer,
and J. Balakrishna, "Geophysical Applicability of Atomic Clocks: Direct Continental Geoid Mapping",
Geophysical Journal International 191, pages 78-92, 2012. Journal Article
http://arxiv.org/abs/1209.2889. Description in the News.

8. D. Tsang, J. S. Read, T. Hinderer, A. L. Piro, and R. Bondarescu ,
"Resonant Shattering of Neutron Star Crusts", arXiv:1110.0467, Phys. Rev. Lett. 108 , 011102 (2012).

7. A. P. Lundgren, M. Bondarescu, R. Bondarescu and J. Balakrishna,
"Luke-warm dark matter: Bose condensation at finite temperatures. ", arXiv:1001.0051
ApJL 715 , L35 (2010).

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. P. 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.

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