182,316 research outputs found
The LISA pathfinder mission
ISA Pathfinder (LPF), the second of the European Space Agency's Small Missions for Advanced Research in Technology (SMART), is a dedicated technology validation mission for future spaceborne gravitational wave detectors, such as the proposed eLISA mission. LISA Pathfinder, and its scientific payload - the LISA Technology Package - will test, in flight, the critical technologies required for low frequency gravitational wave detection: it will put two test masses in a near-perfect gravitational free-fall and control and measure their motion with unprecedented accuracy. This is achieved through technology comprising inertial sensors, high precision laser metrology, drag-free control and an ultra-precise micro-Newton propulsion system. LISA Pathfinder is due to be launched in mid-2015, with first results on the performance of the system being available 6 months thereafter.
The paper introduces the LISA Pathfinder mission, followed by an explanation of the physical principles of measurement concept and associated hardware. We then provide a detailed discussion of the LISA Technology Package, including both the inertial sensor and interferometric readout. As we approach the launch of the LISA Pathfinder, the focus of the development is shifting towards the science operations and data analysis - this is described in the final section of the paper
Correlated noise in networks of gravitational-wave detectors: subtraction and mitigation
One of the key science goals of advanced gravitational-wave detectors is to
observe a stochastic gravitational-wave background. However, recent work
demonstrates that correlated magnetic fields from Schumann resonances can
produce correlated strain noise over global distances, potentially limiting the
sensitivity of stochastic background searches with advanced detectors. In this
paper, we estimate the correlated noise budget for the worldwide Advanced LIGO
network and conclude that correlated noise may affect upcoming measurements. We
investigate the possibility of a Wiener filtering scheme to subtract correlated
noise from Advanced LIGO searches, and estimate the required specifications. We
also consider the possibility that residual correlated noise remains following
subtraction, and we devise an optimal strategy for measuring astronomical
parameters in the presence of correlated noise. Using this new formalism, we
estimate the loss of sensitivity for a broadband, isotropic stochastic
background search using 1 yr of LIGO data at design sensitivity. Given our
current noise budget, the uncertainty with which LIGO can estimate energy
density will likely increase by a factor of ~4--if it is impossible to achieve
significant subtraction. Additionally, narrowband cross-correlation searches
may be severely affected at low frequencies f < 45 Hz without effective
subtraction.Comment: 16 pages, 8 figure
Einstein-aether theory: Dynamics of relativistic particles with spin or polarization in a G\"odel-type universe
In the framework of the Einstein-aether theory we consider a cosmological
model, which describes the evolution of the unit dynamic vector field with
activated rotational degree of freedom. We discuss exact solutions of the
Einstein-aether theory, for which the space-time is of the G\"odel-type, the
velocity four-vector of the aether motion is characterized by a non-vanishing
vorticity, thus the rotational vectorial modes can be associated with the
source of the universe rotation. The main goal of our paper is to study the
motion of test relativistic particles with vectorial internal degree of freedom
(spin or polarization), which is coupled to the unit dynamic vector field. The
particles are considered as the test ones in the given space-time background of
the G\"odel-type; the spin (polarization) coupling to the unit dynamic vector
field is modeled using exact solutions of three types. The first exact solution
describes the aether with arbitrary Jacobson's coupling constants; the second
one relates to the case, when the Jacobson's constant responsible for the
vorticity is vanishing; the third exact solution is obtained using three
constraints for the coupling constants. The analysis of the exact expressions,
which are obtained for the particle momentum and for the spin (polarization)
four-vector components, shows that the interaction of the spin (polarization)
with the unit vector field induces a rotation, which is additional to the
geodesic precession of the spin (polarization) associated with the universe
rotation as a whole.Comment: 15 page
A Level Set Approach to Eulerian-Lagrangian Coupling
We present a numerical method for coupling an Eulerian compressible flow solver with a Lagrangian solver for fast transient problems involving fluid-solid interactions. Such coupling needs arise when either specific solution methods or accuracy considerations necessitate that different
and disjoint subdomains be treated with different (Eulerian or Lagrangian)schemes.
The algorithm we propose employs standard integration of the Eulerian
solution over a Cartesian mesh. To treat the irregular boundary cells that
are generated by an arbitrary boundary on a structured grid, the Eulerian
computational domain is augmented by a thin layer of Cartesian ghost cells.
Boundary conditions at these cells are established by enforcing conservation
of mass and continuity of the stress tensor in the direction normal to the
boundary. The description and the kinematic constraints of the Eulerian
boundary rely on the unstructured Lagrangian mesh. The Lagrangian mesh
evolves concurrently, driven by the traction boundary conditions imposed
by the Eulerian counterpart.
Several numerical tests designed to measure the rate of convergence and
accuracy of the coupling algorithm are presented as well. General problems
in one and two dimensions are considered, including a test consisting of an
isotropic elastic solid and a compressible fluid in a fully coupled setting
where the exact solution is available
- …