Table Of ContentDECONSTRUCTING COSMOLOGY
The advent of sensitive high-resolution observations of the cosmic microwave
background radiation and their successful interpretation in terms of the standard
cosmological model has led to great confidence in this model’s reality. The
prevailing attitude is that we now understand the Universe and need only work
out the details. In this book, Sanders traces the development and successes of
Lambda-CDM,andarguesthatthistriumphalismmaybepremature.Themodel’s
two major components, dark energy and dark matter, have the character of the
pre-twentieth-century luminiferous aether. While there is astronomical evidence
for these hypothetical fluids, their enigmatic properties call into question our
assumptions of the universality of locally determined physical law. Sanders
explains how modified Newtonian dynamics (MOND) is a significant challenge
for cold dark matter. Overall, the message is hopeful: the field of cosmology has
not become frozen, and there is much fundamental work ahead for tomorrow’s
cosmologists.
ROBERT H. SANDERS is Professor Emeritus at the Kapteyn Astronomical
Institute of the University of Groningen, the Netherlands. He received his
Ph.D. in astrophysics from Princeton University under the supervision of Lyman
Spitzer.AfterworkingatColumbiaUniversityandtheNationalRadioAstronomy
Observatory, he moved to Europe. He spent his career studying active galactic
nuclei (in particular, the Galactic Center), on the hydrodynamics of gas in
galaxies, and, for several decades, on the problem of the “missing mass” in
astronomical systems. His previous books are The Dark Matter Problem: A
Historical Perspective (2010) and Revealing the Heart of the Galaxy: The Milky
WayandItsBlackHole(2013).
DECONSTRUCTING COSMOLOGY
ROBERT H. SANDERS
KapteynAstronomicalInstitute,TheNetherlands
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Contents
Acknowledgments pagevii
Introduction 1
1 CreationMythology 6
2 ThreePredictionsofPhysicalCosmology 12
2.1 TheBasisofPhysicalCosmology 12
2.2 TheExpansionoftheUniverse 14
2.3 TheBackgroundRadiation 18
2.4 AnisotropiesintheBackgroundRadiation 23
3 TheVeryEarlyUniverse:Inflation 28
3.1 Fine-TuningDilemmasandtheInitialSingularity 28
3.2 AnEarlyDeSitterPhase 30
3.3 ThePhysicalBasisofInflation 32
4 PrecisionCosmology 37
4.1 StandardCDMCosmology 37
4.2 PrimordialSoundWaves 41
4.3 TheΛCDMParadigm 47
5 TheConcordanceModel 49
5.1 Consistency 49
5.2 WMAPandPlanck 51
5.3 TheDensityofBaryons 53
5.4 SupernovaCosmology 55
5.5 HubbleTrouble? 57
5.6 BaryonAcousticOscillations 58
v
vi Contents
5.7 The“AxisofEvil” 61
5.8 SummingUp:AbsenceofDiscord 63
6 DarkEnergy 66
6.1 TheEvidenceforDarkEnergy 66
6.2 TheNatureofDarkEnergy 70
6.2.1 Zero-PointEnergy 70
6.2.2 DynamicDarkEnergy 71
6.3 DarkEnergyandFundamentalPhysics 75
7 DarkMatter 77
7.1 EvidenceforDarkMatterinGalaxiesandGalaxySystems 77
7.2 CosmologicalEvidenceforDarkMatter 81
7.3 TheNatureofDarkMatter 82
7.4 TheScienceofDarkMatterDetection 85
7.4.1 IndirectDetectionofDarkMatter 85
7.4.2 DirectDetectionofDarkMatter 89
7.4.3 TheLHCandDarkMatter 91
7.5 TheSociologyofDarkMatterDetection 93
8 MOND 96
8.1 GalaxyPhenomenologyRevealsaSymmetryPrinciple 96
8.2 AnEmpiricallyBasedAlgorithm 101
8.2.1 GalaxyRotationCurves 101
8.2.2 TheBaryonicTully–FisherRelationship 104
8.2.3 ACriticalSurfaceDensity 107
8.3 CosmologyandtheCriticalAcceleration 110
8.4 ProblemswithMOND 111
9 DarkMatter,MONDandCosmology 115
9.1 ThePuzzle 115
9.2 ParticleCosmicDarkMatter 117
9.2.1 Neutrinos 117
9.2.2 SoftBosons 119
9.3 NewPhysics 120
9.4 Reflections 122
10 Plato’sCaveRevisited 126
Notes 130
Index 141
Acknowledgments
I express my gratitude first of all to my old friend Frank Heynick of Brooklyn,
NY. Frank spent some time and effort in reading an initial draft of this book
and giving me his very useful advice as an intelligent, interested non-expert.
Thisledtoconsiderableimprovementstothecontentandreadability.Ialsothank
PhillipHelbigforalaterreadingofthecompletemanuscript.NotonlydidPhillip
(with his eagle eyes) spot my many typographical and grammatical errors, but
he also made substantial comments and criticisms on scientific and philosophical
issues.
I have had numerous useful conversations with my colleague Saleem Zaroubi
on a number of aspects of the cosmological paradigm. It was very beneficial
to have the highly informed opinions of a thoughtful supporter of the standard
model. Thanks to Saleem, I also benefitted from a short but useful conversation
with Naoshi Sugiyama on the phase focussing of primordial sound waves (the
reasonweseedistinctacousticpeaksintheCMBangularpowerspectrum)andits
consistencywiththeinflationaryparadigm.Ialsogratefullyacknowledgeahelpful
conversation with Alexi Starobinsky over the early history of the idea of cosmic
inflationasitdevelopedintheSovietUnion.
I thank Moti Milgrom for his extensive and very helpful comments on the
MOND chapters. Moti’s characteristically deep and insightful criticisms greatly
improved the presentation and gave me a new appreciation and understanding of
thesignificantspace–timescaleinvarianceofthedeepMONDlimit.
I am very grateful to Stacy McGaugh for making figures available in legible
form.Stacydeservesmuchcreditforhispersistentemphasisovertheyearsonthe
beautifulsimplicityandimportanceofthebaryonicTully–Fisherrelationasatest
ofCDMandofMOND.
Iwishtoacknowledgethemanyconversations,lettersandemailsover30years
withJacobBekenstein.Jacobdidnothavethechancetoreadoverthismanuscript
but he certainly influenced the content and range of this book. His profound
vii
viii Acknowledgments
understanding of physics in general and relativity in particular benefitted all who
hadtheprivilegeofknowingandinteractingwithhim.Hisexplanationswereclear
and intuitive (in a subject that is not so intuitive), and with me he was always
patientandconsiderate.Hewillbegreatlymissed.
And finally I thank Vince Higgs of Cambridge University Press for his help,
guidanceandadvicethroughoutthisproject.
Introduction
Under the sub-subject of cosmology, Amazon.com currently lists 5765 items.
Among them there are textbooks, serious scientific discussions, popular books,
books on history, philosophy, metaphysics and pseudoscience, mega-bestsellers
likethosebyStephenHawking,BrianGreeneandLisaRandall,andworksnoone
hasheardofbyauthorsasobscureastheirbooks.Itwouldalmostseemasthough
thenumberofbooksonthesubjectisexpandingfasterthantheUniverse;thatsoon
thenatureofthemissingmasswillbenomystery–thedarkmatterisintheform
ofpublishedbutlargelyunreadcosmologybooks.Doestheworldneedyetanother
bookaboutthissubject?WhyhaveIdecidedtocontributetothisobviousgluton
thebookmarket?WhydoIfeelthatIhavesomethingtoaddofuniquevalue?
Theideaforthecurrentprojecthaditsdimoriginsintheyear2003whenIwas
invited to lecture on observational cosmology at a summer school on the Aegean
islandofSyros.IwassurprisedatthisinvitationbecauseIamneitheranobserver
noracosmologist;Ihavealwaysworkedonsmaller-scaleastrophysicalproblems
thatIconsideredsoluble.InthiscareerchoiceIwasnodoubtinfluencedbymyfirst
teachersinastrophysics,whowereexcellentbuttraditionaland,tomyperception
at least, found cosmology to be rather fanciful and speculative (although I never
heardthemexplicitlysaysoandalmostcertainlytheywouldnotsaysonow).
ButIdecidedthatthisinvitationwasanopportunitytolearnsomethingnew,so
I prepared a talk on the standard cosmological tests (e.g., the Hubble diagram,
the angular size–redshift relation and the number counts of faint galaxies) in
the context of the current cosmological paradigm that is supported by modern
observations, such as the very detailed views of tiny anisotropies in the cosmic
microwave background radiation (CMB). The issue I considered was the overall
consistency of these classical tests with the standard model – Lambda-CDM
(ΛCDM).
I was actually more interested in finding inconsistency rather than consistency.
This is because of my somewhat rebellious nature, as well as my conviction that
1
2 Introduction
science primarily proceeds through contradiction and conflict rather than through
agreement and “concordance.” However, somewhat to my chagrin, I found that
the classical tests were entirely consistent with the standard paradigm, although
withmuchlowerprecisionthanthatofthemodernCMBobservations.Thereisno
conspicuous inconsistency between the model and those observations considered
tobecosmological.
And yet worries persist (not only by me) about Lambda-CDM, not because
of any direct observational contradiction on a cosmological scale but because of
the unknown nature of the two dominant constituents of the world – dark energy
and dark matter. These two media are deemed necessary because the Universe,
and more significantly, objects within it, do not behave as expected if the laws
of physics on the grand scale take on the same form as that established locally.
The universality of terrestrial and Solar System physical law deduced from local
phenomenaisanassumption,andonethatisdifficulttotest.Therequiredexistence
ofthesetwoinvisiblemedia–aethers,undetectableapartfromtheirdynamicalor
gravitationaleffects–isperhapsahintthattheassumptionoftheuniversalvalidity
oflocalphysicsmaynotbevalid.
These two components – dark energy, designated by Λ, and dark matter,
abbreviatedas“CDM”(forcolddarkmatter)–comprise95%oftheenergydensity
of the Universe, and yet we have no clear idea of what they are. Dark energy is
elusive; the only evidence is, and very possibly can only be, astronomical. Is this
simply a cosmological constant in Einstein’s equations, or is it a vacuum energy
density–thezero-pointenergyofaquantumfield?Isittheevolvingenergydensity
of a background field – a field that is not usually included in the formulation
of general relativity? Or does apparent requirement of this medium signal the
breakdownofgeneralrelativityitselfonacosmologicalscale?
Dark matter appears to be required to explain the details of the observed
temperature fluctuations in the cosmic microwave background radiation and,
more fundamentally, the formation of structure in an expanding universe of finite
lifetime. This same dark matter presumably clusters on the scale of galaxies and
accountsforthetotaldynamicalmassofthesesystems–amassthatoftenexceeds
the detectable baryonic mass by a large factor. There is the additional problem
that the dark matter particles, which should be detectable locally, have so far
not made an appearance in various increasingly sensitive experiments designed
tocatchthem,butthis,Ihaveargued,isnot(andcannotbe)afalsification.1
For the two components taken together, there is a further complication of
naturalness:thedensityofthesetwo“fluids”decreasesdifferentlyastheUniverse
expands (they have different equations of state). The dark matter dilutes with
the expanding volume element; the dark energy does not dilute at all (or at least
verydifferently)withexpansion.Whyshouldthesetwocomponentswithdifferent
