Extracting Physics from Gravitational Waves: Testing the Strong-field Dynamics of General Relativity and Inferring the Large-scale Structure of the Universe

243 Pages·2015·6.257 MB·English

by  Tjonnie G. F. Li (auth.)

Springer Theses
Recognizing Outstanding Ph.D. Research
Tjonnie G.F. Li

Extracting Physics
from Gravitational
Waves
Testing the Strong-field
Dynamics of General Relativity
and Inferring the Large-scale
Structure of the Universe
Springer Theses
Recognizing Outstanding Ph.D. Research
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Tjonnie G.F. Li
Extracting Physics from
Gravitational Waves
fi
Testing the Strong- eld Dynamics
of General Relativity and Inferring
the Large-scale Structure of the Universe
Doctoral Theses accepted by
VU University Amsterdam, The Netherlands
123
Author Supervisor
Dr. TjonnieG.F. Li Dr. Chris F.F.Van Den Broeck
LIGO Laboratory National Institute for Subatomic
California Institute ofTechnology Physics—Nikhef
Pasadena,CA Amsterdam
USA TheNetherlands
ISSN 2190-5053 ISSN 2190-5061 (electronic)
SpringerTheses
ISBN978-3-319-19272-7 ISBN978-3-319-19273-4 (eBook)
DOI 10.1007/978-3-319-19273-4
LibraryofCongressControlNumber:2015942505
SpringerChamHeidelbergNewYorkDordrechtLondon
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“Isn’t it a pleasure to study and practice what
you have learned?”
Confucius (551–479 BC)
’
Supervisor s Foreword
At its 100th anniversary, Einstein’s theory of general relativity (GR) still encap-
sulates our best understanding of space, time, and gravitation. Early tests of the
framework included the perihelium precession of Mercury, the deflection of star-
lightbytheSun,andShapirotimedelay.Althoughexcellentagreementwiththeory
was obtained, these checks only probed the effect on the motion of test masses of
low-order general-relativistic corrections to the gravitational field. The situation
improveddramaticallywiththediscoveryoftheHulse-Taylorbinaryneutronstarin
1974, which confirmed the existence of gravitational radiation at leading-order
(quadrupole)level.Subsequently,morerelativisticbinarysystemswerediscovered,
allowingforimpressivenewtestsofGR.Nevertheless,ifoneisinterestedinfurther
probing the dissipative dynamics of binaries, and especially the dynamics of
spacetimeitself,whatmattersistheorbitalcompactnessGM/(c2R)(withMthetotal
mass and R the separation), as well as the orbital velocity v=c. Even the recently
discovered neutron star-white dwarf system only has GM/(c2R) *2 × 10−6, and
v=c) *4 × 10−3.
By contrast, binaries consisting of neutron stars and/or black holes on the
vergeofmergerwillhaveGM/(c2R)>0.2andv=c>0.4,withcopiousgravitational
wave (GW) emission. The ability to observe the orbital motion of such systems
would give us access to the genuinely strong-field, relativistic regime of gravity.
Most importantly, we would like to probe the dynamical self-interaction of
spacetime itself, such as the scattering of gravitational waves off the spacetime
curvaturegeneratedbythebinaryasawhole.Theonlywaytogainempiricalaccess
to such phenomena is through the direct detection of gravitational waves.
A network of advanced interferometric gravitational wave detectors is currently
under construction, including the Advanced LIGO in the US, Advanced Virgo in
Italy, and KAGRA in Japan; GEO-HF in Germany is already active, and an
Advanced LIGO interferometer may be placed in India. Towards the end of the
decade, these observatories may find tens of GW signals per year from coalescing
compact binaries.
vii
viii Supervisor’sForeword
Thereisaconsiderablebodyofliterature on(families of) alternative theoriesof
gravity: models with an anomalous dispersion relation for the “graviton”,
scalar-tensor theories, parity-violating theories, models with higher curvature cor-
rections,andmore.UsingGWsignalsfromcoalescingbinariestolookforevidence
for each of these separately might be impractical; moreover, if a GR violation is
present then it may well be of a kind that has not yet been envisaged. For that
reason, it is important to test GR in a model-independent way, by developing a
method that is capable of uncovering the effects of a very wide range of ways in
which GR might break down. Moreover, in order to be useful in the immediate
future,themethodshouldbeabletodealwiththerelativequietcoalescencesignals
that we expect to see with the GW observatories currently under construction.
Ideally, it would also allow for combining information from multiple detections in
order to arrive at a statement about the validity of GR (or lack thereof) that is as
strong as possible.
Such a framework is introduced in this book. This Bayesian model selection
schemesatisfiesthedesideratalistedabove,andhassincebeenshowntoberobust
against a number of nuisance effects of a fundamental, astrophysical, and instru-
mentalnature,atleastinthecaseofcoalescingbinaryneutronstars;extendingitto
GW events involving black holes is currently in progress. Thus, the method pre-
sented here is poised to become a standard tool for empirical study of the
strong-field dynamics of spacetime.
Another field that will greatly benefit from the direct detection of GW signals
from coalescing binaries is cosmology. This is because coalescence events are
“standard sirens”, similar to the electromagnetic (EM) “standard candles” such as
Type Ia supernovae. However, an important difference between EM and GW
cosmic distance markers is that the latter are self-calibrating: the luminosity dis-
tance to binary coalescences can be inferred from the GW signals alone, without
having to compare with any other kind of source, thus obviating the need for a
cosmic distance ladder and avoiding the systematic errors that may be associated
withit.PrecisionmeasurementoftheHubbleconstantwillalreadybepossiblewith
the upcoming advanced gravitational wave detectors. Meanwhile, a conceptual
designstudyhasbeenperformed for athird-generation observatorycalledEinstein
Telescope, which will be capable of seeing about three orders of magnitude more
coalescenceeventsperyear,outtohighredshifts.Asshowninthisbook,itwillbe
possible to use the Einstein Telescope to measure the densities of matter and dark
energy, as well as the dark energy equation of state and its possible time depen-
dence, with accuracies comparable to those of future EM surveys, but in a com-
pletely independent way.
The subject matter of this book is at the forefront of the nascent fields of
observational gravitational wave physics and cosmology. At the same time the
discussion is to a large degree self-contained, including an accessible introduction
Supervisor’sForeword ix
tothetheorybehindgravitational waveemissionandwaveformmodelling,aswell
as state-of-the-art data analysis methods. As such it will be an invaluable resource
for young researchers as we enter an exciting new era in the study of gravitation.
Amsterdam, The Netherlands Dr. Chris F.F. Van Den Broeck
April 2015
Preface
Humanshavebeenwatchingtheskyforthousandsofyears.Inearlytimes,humans
tracked the motion of the Sun and the Moon to make calendars and to associate it
with earthly events such as tides and seasons. By tracking the motion of celestial
objects,theearlynotionoftheorbitoftheSun,theMoon,andtheplanetsstartedto
form. Isaac Newton (1642–1727) showed that all objects “pull” on each other
through gravitational force, and the strength of this force is determined by the
masses of the two objects.
In 1915, Albert Einstein (1879–1955) refined Newton’s law of universal grav-
itationbyintroducingthegeneraltheoryofrelativityorgeneralrelativityforshort.
General relativity describes how mass distorts spacetime and, in turn, how space-
timedictateshowmassesflowthroughit.Moreover,generalrelativitypredictsthat
gravity is mediated by a new type of radiation: gravitational radiation.
Gravitationalwavesthatcomposegravitationalradiationareripplesinthefabricof
spacetime, which periodically lengthen and shorten space, and speed up and slow
down time.
However, it was not until 1974 that the effects of gravitational radiation were
first measured, albeit indirectly, by Hulse and Taylor. The changes in the orbital
motionoftwopulsarswereremarkablyconsistentwiththeemissionofgravitational
radiation. Despite this tremendous discovery, which was awarded the 1993 Nobel
Prize in physics, we have yet to directly detect minor distortions of spacetime
caused by passing gravitational waves.
Large-scale physics experiments such as the United States-based Laser
Interferometer Gravitational Wave Observatory (LIGO) and the Italy-based Virgo
aim to, for the first time in the history of mankind, detect the influences of gravi-
tational waves directly. These detectors are set up to measure tiny changes in
distancesofaboutonethousandththediameterofaproton.In2010,bothLIGOand
Virgo were decommissioned to make way for significantly improved versions
of these detectors, the so-called Advanced LIGO and Advanced Virgo. The
Advanced detectors are scheduled to be completed in 2015, and scientists are
optimistic that the first direct detection of gravitational waves is imminent.
xi