575 research outputs found
Derivative of the light frequency shift as a measure of spacetime curvature for gravitational wave detection
The measurement of frequency shifts for light beams exchanged between two
test masses nearly in free fall is at the heart of gravitational wave
detection. It is envisaged that the derivative of the frequency shift is in
fact limited by differential forces acting on those test masses. We calculate
the derivative of the frequency shift with a fully covariant, gauge-independent
and coordinate-free method. This method is general and does not require a
congruence of nearby beams' null geodesics as done in previous work. We show
that the derivative of the parallel transport is the only means by which
gravitational effects shows up in the frequency shift. This contribution is
given as an integral of the Riemann tensor --the only physical observable of
curvature-- along the beam's geodesic. The remaining contributions are: the
difference of velocities, the difference of non-gravitational forces, and
finally fictitious forces, either locally at the test masses or non-locally
integrated along the beam's geodesic. As an application relevant to
gravitational wave detection, we work out the frequency shift in the local
Lorentz frame of nearby geodesics.Comment: 4 pages, 1 figur
Measuring test mass acceleration noise in space-based gravitational wave astronomy
The basic constituent of interferometric gravitational wave detectors -- the
test mass to test mass interferometric link -- behaves as a differential
dynamometer measuring effective differential forces, comprising an integrated
measure of gravity curvature, inertial effects, as well as non-gravitational
spurious forces. This last contribution is going to be characterised by the
LISA Pathfinder mission, a technology precursor of future space-borne detectors
like eLISA. Changing the perspective from displacement to acceleration can
benefit the data analysis of LISA Pathfinder and future detectors. The response
in differential acceleration to gravitational waves is derived for a
space-based detector's interferometric link. The acceleration formalism can
also be integrated into time delay interferometry by building up the
unequal-arm Michelson differential acceleration combination. The differential
acceleration is nominally insensitive to the system free evolution dominating
the slow displacement dynamics of low-frequency detectors. Working with
acceleration also provides an effective way to subtract measured signals acting
as systematics, including the actuation forces. Because of the strong
similarity with the equations of motion, the optimal subtraction of systematic
signals, known within some amplitude and time shift, with the focus on
measuring the noise provides an effective way to solve the problem and
marginalise over nuisance parameters. The -statistic, in
widespread use throughout the gravitation waves community, is included in the
method and suitably generalised to marginalise over linear parameters and noise
at the same time. The method is applied to LPF simulator data and, thanks to
its generality, can also be applied to the data reduction and analysis of
future gravitational wave detectors.Comment: 10 pages, 3 figures, 1 tabl
Spacetime Metrology with LISA Pathfinder
LISA is the proposed ESA-NASA gravitational wave detector in the 0.1 mHz -
0.1 Hz band. LISA Pathfinder is the down-scaled version of a single LISA arm.
The arm -- named Doppler link -- can be treated as a differential
accelerometer, measuring the relative acceleration between test masses. LISA
Pathfinder -- the in-flight test of the LISA instrumentation -- is currently in
the final implementation and planned to be launched in 2014. It will set
stringent constraints on the ability to put test masses in geodesic motion to
within the required differential acceleration of 3\times10^{-14} m s^{-2}
Hz^{-1/2} and track their relative motion to within the required differential
displacement measurement noise of 9\times10^{-12} m Hz^{-1/2}, around 1 mHz.
Given the scientific objectives, it will carry out -- for the first time with
such high accuracy required for gravitational wave detection -- the science of
spacetime metrology, in which the Doppler link between two free-falling test
masses measures the curvature. This thesis contains a novel approach to the
calculation of the Doppler response to gravitational waves. It shows that the
parallel transport of 4-vectors records the history of gravitational wave
signals. In practice, the Doppler link is implemented with 4 bodies in LISA and
3 bodies in LISA Pathfinder. To compensate for noise sources a control logic is
implemented during the measurement. The closed-loop dynamics of LISA Pathfinder
can be condensed into operators acting on the motion coordinates, handling the
couplings, as well as the cross-talks. The scope of system identification is
the optimal calibration of the instrument. This thesis describes some data
analysis procedures applied to synthetic experiments and shows the relevance of
system identification for the success of LISA Pathfinder in demonstrating the
principles of spacetime metrology for all future space-based missions.Comment: PhD thesis defended at University of Trento on 26th March 2012.
Advisors: Stefano Vitale, Mauro Hueller. Committee: Eugenio Coccia (Univ. of
Rome, Tor Vergata), Philippe Jetzer (Univ. of Z\"urich), Eric Plagnol
(APC-CNRS, Paris), Rita Dolesi (Univ. Of Trento
Detection principle of gravitational wave detectors
With the first two detections in late 2015, astrophysics has officially
entered into the new era of gravitational wave observations. Since then, much
has been going on in the field with a lot of work focussing on the observations
and implications for astrophysics and tests of general relativity in the strong
regime. However much less is understood about how gravitational detectors
really work at their fundamental level. For decades, the response to incoming
signals has been customarily calculated using the very same physical principle,
which has proved so successful in the first detections. In this paper we review
the physical principle that is behind such a detection at the very fundamental
level, and we try to highlight the peculiar subtleties that make it so hard in
practice. We will then mention how detectors are built starting from this
fundamental measurement element.Comment: 12 pages, proceedings of the "Fifth Joint Italian-Pakistani Workshop
on Relativistic Astrophysics", 21-23 July 2016, Lecce, Ital
Space tests of the strong equivalence principle:BepiColombo and the Sun-Earth Lagrangian points opportunity
The validity of General Relativity, after 100 years, is supported by solid
experimental evidence. However, there is a lot of interest in pushing the
limits of precision by other experiments. Here we focus our attention on the
equivalence principle, in particular the strong form. The results of ground
experiments and lunar laser ranging have provided the best upper limit on the
Nordtvedt parameter {\eta} that models deviations from the strong equivalence
principle. Its uncertainty is currently {\sigma}[{\eta}] =4.4
. In the first part of this paper we will describe the experiment, to
measure {\eta}, that will be done by the future mission BepiColombo. The
expected precision on {\eta} is . In the second part we will
consider the ranging between the Earth and a spacecraft orbiting near the
Sun-Earth Lagrangian points to get an independent measurement of {\eta}. In
this case, we forecast a constraint similar to that achieved by lunar laser
ranging.Comment: 11 pages, 2 figure
Space-borne gravitational wave detectors as time-delayed differential dynamometers
The basic constituent of many space-borne gravitational missions, in
particular for interferometric gravitational waves detectors, is the so-called
link made out of a satellite sending an electromagnetic beam to a second
satellite. We illustrate how, by measuring the time derivative of the frequency
of the received beam, the link behaves as a differential, time-delayed
dynamometer in which the effect of gravity is exactly equivalent to an
effective differential force applied to the two satellites. We also show that
this differential force gives an integrated measurement of curvature along the
beam. Finally, we discuss how this approach can be implemented to benefit the
data analysis of gravitational wave detectors.Comment: 5 pages, 1 figur
Data from a dynamic simulation in a free-floating and continuous regime of a solar greenhouse modelled in TRNSYS 17 considering simultaneously different thermal phenomena
This dataset supports the research article âComplete green- house dynamic simulation tool to assess the crop thermal well-being and energy needsâ[1] . In the agricultural sector, the use of energy can be very intensive [2] and the sim- ulation of solar greenhouses is a very complex work [3] . This dataset provides the results of thermal modeling and dynamic simulation of a solar greenhouse considering si- multaneously several thermal phenomena. The analysis was performed by TRNSYS 17 software (TRaNsient SYstem Sim- ulation)
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