Table Of ContentSpringer Theses
Recognizing Outstanding Ph.D. Research
Benjamin Wallisch
Cosmological Probes
of Light Relics
Springer Theses
Recognizing Outstanding Ph.D. Research
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Benjamin Wallisch
Cosmological Probes
of Light Relics
Doctoral Thesis accepted by
the University of Cambridge, Cambridge, UK
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Author Supervisor
Dr. Benjamin Wallisch Prof. DanielBaumann
Schoolof Natural Sciences Institute for Theoretical Physics
Institute for AdvancedStudy University of Amsterdam
Princeton, NJ, USA Amsterdam, TheNetherlands
Department ofPhysics
University of California
SanDiego LaJolla, CA, USA
ISSN 2190-5053 ISSN 2190-5061 (electronic)
SpringerTheses
ISBN978-3-030-31097-4 ISBN978-3-030-31098-1 (eBook)
https://doi.org/10.1007/978-3-030-31098-1
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For my parents
’
Supervisor s Foreword
The oldest light in the universe was released 13.8 billion years ago, when the first
stable atoms coalesced out of the primordial plasma. Since then, this ancient light
haspropagatedfreelythroughthevastnessofspace,carryingwithitatreasuretrove
of information about the physics of the early universe. By decoding the pattern of
fluctuations in the cosmic microwave background (CMB), cosmologists have
obtained amazing insights into the origin, structure and evolution of our universe.
We have learned that the universe today isfilled mostly with dark matter and dark
energy, and that ordinary atoms are only a small fraction of the cosmic energy
budget. Moreover, we now understand in great detail how all of the structures we
see around us (galaxies, stars, planets) grew out of the tiny inhomogeneities
observedintheCMB.Finally,wehavestrongevidencethattheveryearlyuniverse
went through a brief period of very rapid expansion called inflation, during which
quantum fluctuations were amplified and the primordial density fluctuations were
created.
Although the standard cosmological model is consistent with all current obser-
vations,itistheoretically unsatisfyinginanumberofways.Wedonotunderstand
thenatureofdarkmatteranddarkenergy,andwedonothaveafundamentaltheory
explainingtheoriginoftheinflationaryexpansion.Similarly,althoughtheStandard
Model of particle physics explains all available data, we have good reasons to
believethatitisincomplete.Variousfine-tuningproblemssuggestnewphysicsthat
is either too heavy or too weakly coupled to be produced by collider experiments.
Benjamin Wallisch’s remarkable Ph.D. thesis is an important step in using
cosmology as a probe of physics beyond the Standard Model (BSM). The focus
ofthethesiswasonverylightparticlesthatcanhaveescapeddetectioninterrestrial
experiments because they are so weakly interacting with ordinary matter. These
particles can nevertheless be produced in large numbers in the early universe,
making cosmology a unique way to look for them. Benjamin showed that
free-streaming relativistic particles (such as neutrinos and light axions) leave a
uniquesignatureintheCMBandtheclusteringofgalaxies.Hethenforecastedhow
well this effect can be measured by current and future observations. A specific
prediction of Benjamin’s work is a subtle imprint of the cosmic neutrino
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viii Supervisor’sForeword
background on the statistical distribution of galaxies in the sky. Together with
members of the BOSS collaboration, Benjamin then analysed data from the Sloan
DigitalSkySurvey(SDSS)andprovidedthefirstdetectionofthiseffect.Thisnew
measurement constitutes an interesting confirmation of the standard cosmological
modelwhichlinkstheproductionofneutrinosonesecondafterthebigbangtothe
clustering of galaxies billions of years later.
Benjamin further showed that future cosmological observations will be highly
sensitive to this type of BSM physics. He demonstrated that any light particle that
has ever been in thermal equilibrium with the Standard Model will be detectable
with these observations. This new experimental target has since been adopted by
the CMB-S4 collaboration as one of the main science objectives of the next gen-
erationofCMBexperiments.Eventheabsenceofadetectionofnewthermalrelics
wouldbeinformative,sinceitwouldputstrongconstraintsontheircouplingstothe
StandardModel.Benjamininvestigatedthespecialcaseofaxionsandshowedthat
the constraints achievable from cosmology will surpass existing bounds from
laboratory experiments and astrophysical observations by orders of magnitude.
BenjaminWallischhasproducedaPh.D.thesisofthehighestquality.Hiswork
is a rare combination of theoretical insights, numerical computations and data
analysis. It isatestimony toBenjamin’s unique talent that hewas able tocombine
these distinct aspects of the research effortlessly. It has been a great privilege to
have been Benjamin’s Ph.D. supervisor, and I wish him all the best in his future
career.
Amsterdam, The Netherlands Prof. Daniel Baumann
June 2019
Abstract
One of the primary targets of current and especially future cosmological observa-
tions are light thermal relics of the hot big bang. Within the Standard Model of
particle physics, an important thermal relic are cosmic neutrinos, while many
interesting extensions of the Standard Model predict new light particles which are
even more weakly coupled to ordinary matter, and therefore hard to detect in
terrestrialexperiments. Ontheotherhand,these elusive particles maybeproduced
efficientlyintheearlyuniverseandtheirgravitationalinfluencecouldbedetectable
in cosmological observables. In this thesis, we describe how measurements of the
cosmic microwave background (CMB) and the large-scale structure (LSS) of the
universe can shed new light on the properties of neutrinos and on the possible
existence of other light relics.
These cosmological observations are remarkably sensitive to the amount of
radiation in the early universe, partly because free-streaming species such as neu-
trinos imprint asmallphase shiftinthe baryon acousticoscillations (BAO), which
we study in detail in the CMB and LSS power spectra. Building on this analytic
understanding,weprovidefurtherevidenceforthecosmicneutrinobackgroundby
independently confirming its free-streaming nature in different, currently available
datasets. In particular, we propose and establish a new analysis of the BAO
spectrum beyond its use as a standard ruler, resulting in the first measurement of
this imprint of neutrinos in the clustering of galaxies.
Future cosmological surveys, such as the next generation of CMB experiments
(CMB-S4),havethepotentialtomeasuretheenergydensityofrelativisticspeciesat
thesub-percentlevelandwill,therefore,becapableofprobingphysicsbeyondthe
StandardModel.Wedemonstratehowthisimprovementinsensitivitycanindeedbe
achievedandpresentanobservationaltargetwhichwouldallowthedetectionofany
extralightparticlethathaseverbeeninthermalequilibrium.Interestingly,eventhe
absenceofadetectionwouldresultinnewinsightsbyprovidingconstraintsonthe
couplingstotheStandardModel.Asanexample,weshowthatexistingboundson
additionalscalarparticles,suchasaxions,maybesurpassedbyordersofmagnitude.
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Parts of this thesis have been published in the following journal articles:
[1] D.Baumann,D.Green,J.MeyersandB.Wallisch,PhasesofNewPhysicsinthe
CMB,JCAP01(2016)007.arXiv:1508.06342[astro-ph.CO].
[2] D.Baumann,D.GreenandB.Wallisch,NewTargetforCosmicAxionSearches,
Phys. Rev. Lett. 117 (2016) 171301. arXiv:1604.08614 [astro-ph.CO].
[3] D. Baumann, D. Green and B. Wallisch, Searching for Light Relics with
Large-Scale Structure, JCAP 08 (2018) 029. arXiv:1712.08067 [astro-ph.CO].
[4] D.Baumann,F.Beutler,R.Flauger,D.Green,A.Slosar,M.Vargas-Magaña,
B. Wallisch and C. Yèche, First Constraint on the Neutrino-Induced Phase
Shift in the Spectrum of Baryon Acoustic Oscillations, Nat. Phys. 15 (2019)
465. arXiv:1803.10741 [astro-ph.CO].
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