Why care about general relativity?
We have entered an era in which our ability to benefit from more and more accurate timekeeping relies on our understanding of general relativity. Applications of quantum theory are ubiquitous. Even the kilogram is set to change to be defined in terms of Planck's constant.
The first practical applications of general relativity, where the general relativistic effect is not something to be corrected for, but it is the measurement we are interested in, lie at the interface between general relativity and geophysics.
My most recent work
describes how the slow down of time (a general
relativistic effect) measured by atomic clocks can be used to monitor volcanoes and the solid-Earth tide (see our technical article ). This work was featured in the media, e.g., see the Phys.org article or
Einsteins Tanz auf dem Vulcan from welt.de.
I have also been involved in a number of ongoing and planned missions including the Space Time Explorer and QUantum Equivalence Test (STE-QUEST), and the
Einstein Gravitational Redshift Probe (E-GRIP). The latter is a new Swiss mission that aims to launch an improved version of the Hydrogen Maser designed for the Atomic Clock Ensemble in Space mission in an eccentric orbit around Earth.
The elliptic orbit improves its sensitivity to a host of
higher order relativistic effects and
alternative theories of gravity .
At ground level, portable atomic clocks can add detail to
satellite maps and help us in understanding the interior structure
of the Earth. Our proposal that the best optical atomic clocks
are ready and should be used to locally measure the geoid
received significant media attention. The goid is the surface of constant geopotential
that extends the mean sea level to continents. In terms of clock language, it is the surface
of constant clock tick rate. See
e.g., the MIT Technology Review of my paper or the Welt der Phyisk review.
1. Shattering Neutron Star Crusts.
We investigate the resonant excitation of neutron star modes by tides and
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 explain precursor flares that happen before short GRBs.
Such flares that have already been observed by Swift-BAT, Fermi and Suzaku.
For more see the right pitch to shatter a neutron star or the original paper & Dave's summary with more pictures and links to other press releases. Our paper is also mentioned in the GRB section of wikepedia , and
featured as extraordinary research by the
Physical Review Letters . Some other news stories about it can be found in
arstechnica and New Scientist.
2. Nonlinear effects in the r-mode instability of neutron stars.
This was my PhD work in collaboration with Profs. Saul Teukolsky and Ira Wasserman.
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 L=m =2 r-mode is unstable when gravitational driving dominates viscous dissipation. Once the amplitude the r-mode
passes its parameteric instability threshold amplitude, it excites other near-resonant modes in the system and
nonlinear effects become important. They convert
rotational energy to mode energy and gravitational radiation and thus slow the star down. Stars that spin fast enough to have active r-modes are either old
recycled neutron stars that are spun up by a binary companion (See our R-modes in Low Mass X-ray Binaries paper ) or very young pulsars ( newborn neutron stars paper ).
Our most recent work on the topic is featured in How fast
can neutron stars spin? - it is published in ApJ .
Luke-warm Dark Matter.
Ultralight dark matter particles Bose-condense into halos and could
be luke-warm while remaining consistent with existent data from WMAP, Planck,
and Big Bang Nucleosynthesis.
For more see
blog post and/or the techical article we wrote .
Beams of the future.
Thermal noise will be the dominant form of noise in the most sensitive frequency band of Advanced LIGO detectors.
We look for alternative mirror and beam shapes that minimize the thermal
noise while keeping the diffraction loss constant, and find that thermal noise
can be reduced by up to 60% relative to the currently used Gaussian beams. See this Phys. Rev. D article and Mihai, Oleg and Yanbei's paper for more details.
Black Holes? Boson Stars? or Soliton Stars?
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) are alternative to black holes.
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 (compact scalar objects) 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 quasi-normal mode structure of the gravitational waves they could produce. The results were published in CQG ( boson stars paper ) and Phys. Rev. D (soliton stars paper ).
Before the Cloud.
My first paper described the Astrophysics Collaboratory Portal. Our group
led by Prof. Edward Seidel and Gabrielle Allen won three of the four prize
at SC2002: the Bandwidth Challenge, the HPC Challenge for the most distributed application, and the HPC challenge for the most heterogeneous set of platforms; here we write about this award winning work. Similar principles are behind cloud computing.
PDF version of the paper Abstract through Science Direct.