Table Of ContentSpringer Theses
Recognizing Outstanding Ph.D. Research
Stéphane Ilić
The Large Scale
Structures
A Window on the Dark
Components of the Universe
Springer Theses
Recognizing Outstanding Ph.D. Research
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Stéphane Ilic´
The Large Scale Structures
A Window on the Dark Components
of the Universe
Doctoral Thesis accepted by
the University of Paris-Sud, Orsay Cedex, France
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Author Supervisor
Dr. Stéphane Ilic´ Prof.MathieuLanger
Département de Physique Département de Physique
Institutd’Astrophysique Spatiale Institutd’Astrophysique Spatiale
Université Paris-Sud Université Paris-Sud
Orsay Cedex Orsay Cedex
France France
ISSN 2190-5053 ISSN 2190-5061 (electronic)
ISBN 978-3-319-07745-1 ISBN 978-3-319-07746-8 (eBook)
DOI 10.1007/978-3-319-07746-8
Springer ChamHeidelberg New YorkDordrecht London
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‘‘When I reach for the edge of the universe, I do so
knowing that along some paths of cosmic discovery,
there are times when, at least for now, one must be
content to love the questions themselves.’’
Neil deGrasse Tyson
Dedicated to the ones no longer with us
to gaze upon this beautiful Universe
Supervisor’s Foreword
Modern cosmology is a young science. In less than a century of existence, it has
witnessed many twists and turns that have led to a tremendous global progress.
The parallel development of technological advances, observational efforts and
theoretical modelling has allowed the establishment of a standard cosmological
model,calledLCDM.Itisstriking thatwenamethecosmologicalmodelafterits
two most mysterious, but dominant, components. Indeed, on the one hand, CDM
stands for Cold Dark Matter, the existence of which we detect indirectly through
its many gravitational effects, but of which we do not know the intimate nature.
We know nevertheless that it makes up slightly more than a quarter of the total
energybudgetoftheUniverse.Intensiveexperimentalsearchesarededicatedtoits
production in particle colliders, to its direct detection in underground laboratories
and indirect detection through astrophysical signatures. On the other hand, rep-
resenting 68 % of the energy budget, L stands for the famous cosmological
constant which is thought to be responsible for the accelerated expansion of the
Universe, evidenced in the late 1990s and crowned by the Nobel Prize in Physics
in2011.DeterminingtheintimatenatureoftheDarkComponentsoftheUniverse
is one of the most enthralling challenges of current cosmology. Many different
approacheshavetobeexploredtopindowntheexactamountandthepropertiesof
thesecomponents.Onesuchfruitfulapproachtakesadvantageofthestructuration
of matter on large scales, affected both by Dark Energy and Dark Matter.
On the Dark Energy side, the measurement of the so-called integrated Sachs–
Wolfe(iSW)effectprovidesawayoftestingboththecosmologicalmodelandthe
properties of Dark Energy. It is essentially the net gain of energy that photons of
the Cosmic Microwave Background (CMB) acquire while travelling across time-
varyinggravitationalpotentials.Beforethisthesis,theobservationalsituationwas
somewhat unclear: many different studies, in some cases using the same data,
reported iSW detections with statistical significances ranging from negligible to
above four standard deviations. To clarify the picture, in his thesis Stéphane Ilic´
hasadoptedastrategy inthreesteps:(i)hedevelopedtheformalismandtoolsfor
thecross-correlationofthe Cosmic InfraredBackground,tracingthegravitational
potentials underlying the distribution of galaxies, with the CMB, and showed
rigorouslythattheiSWisthusdetectable,inprinciple,withaveryhighstatistical
ix
x Supervisor’sForeword
significance; (ii) he devised a complete stacking strategy to detect the iSW
imprinted in the CMB by the largest cosmic superstructures (super-clusters and
super-voids),including athoroughexaminationofstatisticalandselectioneffects,
andapplieditsuccessfullytobothWMAPandPlanckCMBdata;(iii)hecomputed
the exact iSW effect expected from such superstructures within the framework of
General Relativity and the LCDM model, and showed that any possible discrep-
ancy reported so far between data and theory might be due rather to our poorer
understanding of the mildly nonlinear regime of structure formation than to a
failure of the LCDM model itself.
On the Dark Matter side, it is well known that the annihilation of Weakly
InteractingMassiveParticlesandthedecayoflightexoticparticlesmayhavelefta
subtleimprintontheCMB,especiallyinitsE-modepolarisation.Ontheonehand,
the related injection of energy into the Intergalactic Medium modifies the ioni-
sation state of the baryonic gas. This will be very difficult, if not impossible, to
detect, as the related signatures in the CMB appear on the largest scales where
uncertainties due to Cosmic Variance dominate. On the other hand, the thermal
state of the intergalactic gas is affected as well, in a potentially more significant
way.Stéphaneshowedhow measurementsoftheintergalactic temperature canbe
exploited to learn more about Dark Matter. Through an analysis of recent data
extractedfromtheLymanalphaforest,Stéphaneshowedthatpowerfulconstraints
can be put both on the Dark Matter particle properties and on the redshift and
duration of the Second Reionisation of Helium.
Research on the exploitation of the distribution of matter in the Universe on
large scales is rapidly evolving, on both sides of theory and observation. New
results will be soon within reach, but this thesis sets a standard in the rigour and
thoroughnessneededtounderstandproperlywhatthelarge-scalestructurestellus
about the Dark Sectorof the cosmologicalmodel.It has been a great pleasure for
MarianDouspisandmyselftoguideStéphaneIlic´ inthisdoctoralproject,andhis
results are a source of real satisfaction.
Orsay Cedex, April 2014 Prof. Mathieu Langer
Abstract
DarkEnergyisoneofthegreatmysteriesofmoderncosmology.Itisanunknown
component supposedly filling the whole universe and responsible for the current
acceleration of the expansion of the Universe. Its study is a major focus of my
thesis:thewayIchoosetostudyandcharacterisethisDarkEnergyisbasedonthe
large-scale structure of the Universe through a probe called the integrated Sachs–
Wolfeeffect(iSW).Thiseffectistheoreticallydetectableinthecosmicmicrowave
background (CMB): this light, which originated in the early Universe (380,000
years after the Big Bang), travelled through large cosmic structures (galaxies and
clusters)beforereachingus,allofthem underlainbygravitational potentials. The
acceleration of the expansion (and Dark Energy) has the effect of stretching and
‘‘flattening’’ these potentials during the crossing of photons. Depending on the
properties of the Dark Energy, these photons change frequency and should
thereforeappearascolderorhotterspotsintheCMB.TheiSWeffecthasadirect
butveryweakimpactonthepowerspectrumofthetemperaturefluctuationsofthe
CMB. It therefore requires the use of external data to be detectable. A
conventional approach to this problem is to correlate the CMB with a tracer of
the distribution of matter in the Universe (usually galaxy surveys), and therefore
the underlying gravitational potential. This has been attempted numerous times
with surveys covering a large range of wavelengths, but the measured correlation
has yet to give a definitive and unambiguous result on the detection of the iSW
effect.Thisismainlyduetotheshortcomingsofcurrentsurveysthatarenotdeep
enoughand/orcovertoosmallafractionofthesky.Apartofmythesisisdevoted
tothecorrelationoftheCMBwithanotherdiffusebackground,namelythecosmic
infrared background (CIB), which is composed of the integrated emission of the
non-resolved distant star-forming galaxies. I was able to show that it is an
excellenttracerofthegravitationalpotentials,beingfreefrommanyshortcomings
ofcurrentsurveys.Theresultsofmystudyshowthatthelevelsofsignificancefor
the expected CIB-CMB correlation exceed those of current surveys, and compete
withthosepredictedforthefuturegenerationofverylargesurveys(Pan-STARRS,
LSST, Euclid). Next, my thesis focuses on the individual imprint in the CMB of
thelargeststructuresintheUniversebytheiSWeffect.Accordingtoanarticleby
[1],theiSWeffect wasdirectlydetectedinthestackingofpatches oftheCMBat
the positions of superstructures. However, the high measured amplitude of the
effectseemstobeatoddswithpredictionsfromthestandardmodelofcosmology.
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