Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.stecf.org/coordination/esa_eso/cosmology/report.pdf Äàòà èçìåíåíèÿ: Thu Sep 4 17:58:31 2008 Äàòà èíäåêñèðîâàíèÿ: Tue Oct 2 00:34:33 2012 Êîäèðîâêà: IBM-866 Ïîèñêîâûå ñëîâà: ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï

Report by the ESA-ESO Working Group on Fundamental Cosmology Abstract
In September 2003, the executives of ESO and ESA agreed to establish a number of working groups to explore possible synergies between these two major European astronomical institutions on key scientific issues. The first two working group reports (on Extrasolar Planets and the HerschelíALMA Synergies) were released in 2005 and 2006, and this third report covers the area of Fundamental Cosmology. The Working Group's mandate was to concentrate on fundamental issues in cosmology, as exemplified by the following questions: (1) What are the essential questions in fundamental cosmology? (2) Which of these questions can be tackled, perhaps exclusively, with astronomical techniques? (3) What are the appropriate methods with which these key questions can be answered? (4) Which of these methods appear promising for realization within Europe, or with strong European participation, over the next 15 years? (5) Which of these methods has a broad range of applications and a high degree of versatility even outside the field of fundamental cosmology? From the critical point of view of synergy between ESA and ESO, one major resulting recommendation concerns the provision of new generations of imaging survey, where the image quality and near-IR sensitivity that can be attained only in space are naturally matched by ground-based imaging and spectroscopy to yield massive datasets with well-understood photometric redshifts (photo-z's). Such information is essential for a range of new cosmological tests using gravitational lensing, large-scale structure, clusters of galaxies, and supernovae. All these methods can in principle deliver high accuracy, but a multiplicity of approaches is essential in order that potential systematics can be diagnosed í or the possible need for new physics revealed. Great scope in future cosmology also exists for ELT studies of the intergalactic medium and spacebased studies of the CMB and gravitational waves; here the synergy is less direct, but these areas will remain of the highest mutual interest to the agencies. All these recommended facilities will produce vast datasets of general applicability, which will have a tremendous impact on broad areas of astronomy.


Background
Following an agreement to cooperate on science planning issues, the executives of the European Southern Observatory (ESO) and the European Space Agency (ESA) Science Programme and representatives of their science advisory structures have met to share information and to identify potential synergies within their future projects. The agreement arose from their joint founding membership of EIROforum (www.eiroforum. org) and a recognition that, as pan-European organisations, they serve essentially the same scientific community. At a meeting at ESO in Garching during September 2003, it was agreed to establish a number of working groups that would be tasked to explore these synergies in important areas of mutual interest and to make recommendations to both organisations. The chair and co-chair of each group were to be chosen by the executives but thereafter, the groups would be free to select their membership and to act independently of the sponsoring organisations. During the second of these bilateral meetings, in Paris during February 2005, it was decided to commission a group to address the current state of knowledge and future prospects for progress in fundamental cosmology, especially the nature of `dark matter' and `dark energy'. By summer 2005, the following membership and terms of reference for the group were agreed:

Membership
John Peacock (Chair) jap@roe.ac.uk Peter Schneider (Co-Chair) peter@astro.uni-bonn.de George Efstathiou gpe@ast.cam.ac.uk Jonathan R. Ellis johne@mail.cern.ch Bruno Leibundgut bleibund@eso.org Simon Lilly simon.lilly@phys.ethz.ch Yannick Mellier mellier@iap.fr Additional major contributors: Pierre Astier, Anthony Banday, Hans Bohringer, Anne è è Ealet, Martin Haehnelt, Gunther Hasinger, Paolo Molaro, Jean-Loup Puget, Bernard Schutz, Uros Seljak, Jean-Philippe Uzan. ST-ECF Support: Bob Fosbury, Wolfram Freudling Thanks are also due to many colleagues who provided further useful comments on draft versions of the report at various stages. ii


Terms of Reference
(1) To outline the current state of knowledge of the field (this is not intended as a free-standing review but more as an introduction to set the scene); (2) To review the observational and experimental methods used or envisaged for the characterisation and identification of the nature of Dark Matter and Dark Energy; (3) To perform a worldwide survey of the programmes and associated instruments that are operational, planned or proposed, both on the ground and in space; (4) For each of these, to summarise the scope and specific goals of the observation/experiment; also to point out the limitations and possible extensions; (5) Within the context of this global effort, examine the role of ESO and ESA facilities. Analyse their expected scientific returns; identify areas of potential overlap and thus assess the extent to which the facilities complement or compete; identify open areas that merit attention by one or both organisations and suggest ways in which they could be addressed; (6) Make an independent assessment of the scientific cases for large facilities planned or proposed. (7) The working group membership will be established by the chair and co-chair. The views represented and the recommendations made in the final report will be the responsibility of the group alone.

Catherine Cesarsky (ESO)

‡ Alvaro Gimenez (ESA)

September 2006

iii


Contents
Membership Terms of Reference 1 2 3 Executive summary Introduction The cosmological context 3.1 3.2 3.3 3.4 3.5 4 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The global contents of the universe . . . . . . . . . . . . . . . . . . . . The perturbed universe . . . . . . . . . . . . . . . . . . . . . . . . . . Statistical methodology . . . . . . . . . . . . . . . . . . . . . . . . . . Photometric redshifts . . . . . . . . . . . . . . . . . . . . . . . . . . . ii ii 1 6 9 9 11 17 20 22 25 25 27 33 35 35 38 40 45

The Cosmic Microwave Background 4.1 4.2 4.3 Current status . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions on the CMB . . . . . . . . . . . . . . . . . . . . . . . . .

5

Large-scale structure 5.1 5.2 5.3 5.4 Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Current status . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future LSS experiments . . . . . . . . . . . . . . . . . . . . . . . . . ESO's capability for LSS studies . . . . . . . . . . . . . . . . . . . . .

iv


6

Clusters of galaxies 6.1 6.2 6.3 Cosmological galaxy cluster surveys . . . . . . . . . . . . . . . . . . . Systematic uncertainties . . . . . . . . . . . . . . . . . . . . . . . . . Prospects with a 100k cluster survey . . . . . . . . . . . . . . . . . . .

47 48 51 53 57 57 58 60 69 73 73 79 81 85 86 88 92 95 95 96 99

7

Gravitational Lensing 7.1 7.2 7.3 7.4 Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Current status . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Systematic uncertainties and errors . . . . . . . . . . . . . . . . . . . . Future prospects for weak lensing surveys . . . . . . . . . . . . . . . .

8

Supernovae 8.1 8.2 8.3 Current status . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Systematic uncertainties . . . . . . . . . . . . . . . . . . . . . . . . . Future applications of supernovae to dark energy . . . . . . . . . . . .

9

The intergalactic medium 9.1 9.2 9.3 Method and systematic uncertainties . . . . . . . . . . . . . . . . . . . Major cosmological results from the IGM . . . . . . . . . . . . . . . . Future Prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

10 Variability of fundamental constants 10.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2 Constraints on variations in the fine-structure constant . . . . . . . . . . 10.3 Constraints on variations in the proton-electron mass ratio .......

10.4 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

v


11 Gravity-wave cosmology with LISA and its successors

102

11.1 LISA overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 11.2 Science goals for LISA . . . . . . . . . . . . . . . . . . . . . . . . . . 102 11.3 The future of gravity-wave astronomy . . . . . . . . . . . . . . . . . . 105 12 Conclusions 107

12.1 The next decade in cosmology . . . . . . . . . . . . . . . . . . . . . . 107 12.2 The international perspective . . . . . . . . . . . . . . . . . . . . . . . 108 12.3 Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 12.4 The longer-term outlook for cosmology . . . . . . . . . . . . . . . . . 114 Bibliography List of abbreviations 115 121

vi


1 Executive summary
This report is written for ESA and ESO jointly, in order to summarize current understanding of the fundamental properties of the universe, and to identify the key areas in which Europe should invest in order to advance this understanding. There is an increasingly tight connection between cosmology and fundamental physics, and we have concentrated on this area. We thus exclude direct consideration of the exciting recent progress in astrophysical cosmology, such as the formation and evolution of galaxies, the processes of reionization and the first stars etc. However, many of our recommended actions will produce vast datasets of general applicability, which will also have a tremendous impact on these broader areas of astronomy. This is an appropriate time to take stock. The past 10-15 years have seen huge advances in our cosmological understanding, to the point where there is a well-defined standard model that accounts in detail for (nearly) all cosmologically relevant observations. Very substantial observational resources have already been invested, so the next generation of experiments is likely to be expensive. Indeed, the scale of future cosmological projects will approach that of particle physics, both in financial and in human terms. We therefore need to identify the problems that are both the most fundamental, and which offer the best prospects for solution. In doing this, it is hard to look too far ahead, as our views on priorities will doubtless evolve; but planning and executing large new experiments will take time. We intend this report to cover the period up to about 2020. The standard model consists of a universe described by Einstein's theory of general relativity, with a critical energy density dominated today by a component that is neither matter nor radiation, but a new entity termed `dark energy', which corresponds to endowing the vacuum with energy. The remaining energy consists of collisionless `cold dark matter' (about 22%) and ordinary `baryonic' material (about 4%), plus trace amounts of radiation and light neutrinos. The universe is accurately homogeneous on the largest scales, but displays a spectrum of inhomogeneities whose gravitationallydriven growth is presumed to account for the formation of galaxies and large-scale structure. The simplest consistent theory for the origin of these features is that the universe underwent an early phase of `inflation', at which time the density in dark energy was very much higher than at present. Given this background, there follows a natural set of key questions: (1) What generated the baryon asymmetry? Why is there negligible antimatter, and what set the ratio of baryons to photons? (2) What is the dark matter? Is it a relic massive supersymmetric particle, or something (even) more exotic? 1


(3) What is the dark energy? Is it Einstein's cosmological constant, or is it a dynamical phenomenon with an observable degree of evolution? (4) Did inflation happen? Can we detect relics of an early phase of vacuum-dominated expansion? (5) Is standard cosmology based on the correct physical principles? Are features such as dark energy artefacts of a different law of gravity, perhaps associated with extra dimensions? Could fundamental constants actually vary? Whereas we have not attempted to rank these prime science questions in importance, we took into consideration the likelihood that substantial progress can be made with astronomical techniques. Additional information may in some cases be provided by particle-physics experiments. For example, in the case of the baryon asymmetry (1), we may hope for major progress from particle-physics experiments studying CP violation at the LHC, or at a neutrino factory in the longer term. The nature of dark matter (2) will be investigated by accelerators such as the LHC or underground dark matter experiments, while astronomical observations will constrain its possible properties; similarly, tests of the law of gravity (5) will also be conducted in the laboratory as well as on cosmological scales. Empirical studies of the properties of dark energy (3) and the physics of inflation (4) are possible only with the largest possible laboratory available, namely the universe as a whole. On the other hand, there may be some synergy with searches for the Higgs boson at the LHC; these could provide a prototype for other scalar fields, which can be of cosmological importance. Given their fundamental nature, studies of dark energy and inflation are of the utmost interest to the science community well beyond astrophysics. Of all these cosmological issues, probably the discovery of a non-vanishing dark energy density poses the greatest challenge for physics. There is no plausible or `natural' model for its nature, and we must adopt empirical probes of its properties. For example, undoubtedly one of the most important questions is whether the dark energy is simply the cosmological constant introduced by Einstein, or whether it has an equation of state that differs from w = -1, where w is the ratio of the pressure to the energy density. Several highly promising methods for studying the value of w have been identified that can be actively pursued within the next decade and which will lead to qualitatively improved insights. However, new ingredients such as w will often be almost degenerate in their effect with changes in standard parameters; numbers such as the exact value of the dark matter density must be determined accurately as part of the road to w. Progress in answering the foregoing questions will thus require a set of highaccuracy observations, probing subtle features of cosmology that have been largely negligible in past generations of experiment. Our proposed approach is to pursue multiple independent techniques, searching for consistency between the resulting estimates of 2


cosmological parameters, suitably generalised to allow for the possible new ingredients that currently seem most plausible. If these estimates disagree, this could indicate some systematic limitation of a particular technique, which can be exposed by internal checks and by having more than one external check. In addition, there is also the exciting possibility of something unexpected. The first step in these improvements will be statistical in nature: because the universe is inhomogeneous on small scales, `cosmic variance' forces us to study ever larger volumes in order to reduce statistical errors in measuring the global properties of the universe. Thus, survey astronomy inevitably looms large in our recommendations. Remarkably, the recent progress in cosmology has been so rapid that the next generation of experiments must aspire to studying a large fraction of the visible universe í mapping a major fraction of the whole sky in a range of wavebands, out to substantial redshifts. The key wavebands for these cosmological studies are the cosmic microwave background (CMB) around 1 mm, for the study of primordial structure at the highest redshifts possible; optical and infrared wavebands for the provision of spectroscopy and photometric redshift estimates, plus data on gravitational-lensing image distortions; the X-ray regime for the emission signatures of intergalactic gas, particularly in galaxy clusters. In principle the radio waveband can also be of importance, and the ability to survey the universe via 21-cm emission as foreseen by the Square Kilometre Array will make this a wonderfully powerful cosmological tool. However, according to current estimates, the SKA will become available close to 2020. We believe that great progress in cosmology is however possible significantly sooner than this, by exploiting the opportunities at shorter wavelengths. The principal techniques for probing inflation and the properties of dark matter and dark energy involve the combination of the CMB with at at least one other technique: gravitational lensing; baryon acoustic oscillations; the supernova Hubble diagram; and studies of the intergalactic medium. The CMB alone is the richest source of direct information on the nature of the initial fluctuations, such as whether there exist primordial gravitational waves or entropy perturbations. But the additional datasets allow us to study the cosmological model in two further independent ways: geometrical standard rulers and the growth rate of cosmological density fluctuations. The majority of these techniques have common requirements: large-area optical and near-IR imaging with good image quality, leading to photometric redshifts. This leads to the strongest of our recommendations, which we now list: § ESA and ESO should collaborate in executing an imaging survey across a major fraction of the sky by constructing a space-borne high-resolution wide-field optical imager and providing the essential multi-colour component from the ground, plus also a near-IR component from space. The VST KIDS project will be a pathfinder for this sort of data, but substantial increases in grasp and improve3


ments in image quality will be needed in order to match or exceed global efforts in this area. § Near-IR photometry is essential in order to extend photometric redshifts beyond redshift unity. VISTA will be able to perform this role to some extent with regard to KIDS. However, imaging in space offers huge advantages in the near-IR via the low background, and this is the only feasible route to quasi all-sky surveys in this band that match the depth of optical surveys. We therefore recommend that ESA give the highest priority to exploring means of obtaining such near-IR data, most simply by adding a capability for near-IR photometry to the above satellite for high-resolution optical imaging. § In parallel, ESO should give high priority to expanding its wide-field optical imaging capabilities to provide the complementary ground-based photometric data at wavelengths < 700 nm. The overall optical/IR dataset (essentially 2MASS with a 7 magnitude increase in depth plus an SDSS imaging survey 4 magnitudes deeper and with 3 times larger area) would also be a profound resource for astronomy in general, a legacy comparable in value to the Palomar surveys some 50 years ago. § Photometric redshift data from multi-colour imaging of this sort enable two of the principal tests of dark energy: 3D gravitational lensing, and baryon oscillations in projection in redshift shells. Photometric redshifts are also essential in order to catalogue clusters at high redshift, in conjunction with an X-ray survey mission such as eROSITA. The same is true for identifying the clusters to be detected by Planck using the Sunyaev-Zeldovich effect. § Calibration of photometric redshifts is key to the success of this plan, thus ESO should plan to conduct large spectroscopic surveys spread sparsely over 10, 000 deg2 , involving > 100, 000 redshifts. This will require the initiation of a large key programme with the VLT, integrated with the imaging survey. Ideally, a new facility for wide-field spectroscopy would be developed, which would improve the calibration work, and also allow the baryon oscillations to be studied directly and undiluted by projection effects. § A powerful multi-colour imaging capability can also carry out a supernova survey extending existing samples of high-redshift SNe by an order of magnitude, although an imager of 4m class is required if this work is to be pursued from the ground. In order to exploit the supernova technique fully, an improved local sample is also required. The VST could provide this, provided that time is not required for other cosmological surveys, in particular lensing. § Supernova surveys need to be backed up with spectroscopy to assure the classification for at least a significant subsample and to check for evolutionary effects. 4


The spectroscopy requires access to the largest possible telescopes, and a European Extremely Large Telescope (ELT) will be essential for the study of distant supernovae with redshifts z > 1. § A European ELT will also be important in fundamental cosmology via the study of the intergalactic medium. Detailed quasar spectroscopy can limit the nature of dark matter by searching for a small-scale coherence length in the mass distribution. These studies can also measure directly the acceleration of the universe, by looking at the time dependence of the cosmological redshift. § ELT quasar spectroscopy also offers the possibility of better constraints on any time variation of dimensionless atomic parameters such as the fine-structure constant and the proton-to-electron mass ratio. There presently exist controversial claims of evidence for variations in , which potentially relate to the dynamics of dark energy. It is essential to validate these claims with a wider range of targets and atomic tracers. § In the domain of CMB research, Europe is well positioned with the imminent arrival of Planck. The next steps are (1) to deal with the effects of foreground gravitational lensing of the CMB and (2) to measure the `B-mode' polarization signal, which is the prime indicator of primordial gravitational waves from inflation. The former effect is aided by the optical lensing experiments discussed earlier. The latter effect is potentially detectable by Planck, since simple inflation models combined with data from the WMAP CMB satellite predict a tensor-toscalar ratio of r 0.15. A next-generation polarization experiment would offer the chance to probe this signature in detail, providing a direct test of the physics of inflation and thus of the fundamental physical laws at energies 10 12 times higher than achievable in Earth-bound accelerators. For reasons of stability, such studies are best done from space; we thus recommend such a CMB satellite as a strong future priority for ESA. § An alternative means of probing the earliest phases of cosmology is to look for primordial gravitational waves at much shorter wavelengths. LISA has the potential to detect this signature by direct observation of a background in some models, and even upper limits would be of extreme importance, given the vast lever arm in scales between direct studies and the information from the CMB. We thus endorse space-borne gravity-wave studies as an essential current and future priority for ESA.

5


2 Introduction
The current human generation has the good fortune to be the first to understand a reasonable fraction of the large-scale properties of the universe. A single human lifespan ago, we were in an utterly primitive state where the nature of galaxies was unknown, and their recessional velocities undreamed of. Today, we know empirically how the current universe emerged from a hot and dense early state, and have an accurate idea of how this process was driven dynamically by the various contributions to the energy content. Proceeding initially on the assumption that general relativity is valid, cosmology has evolved a standard model in which all of astronomy is in principle calculable from six parameters. This success is impressive, but it is bought at the price of introducing several radical ingredients, which require a deeper explanation: § § § § An asymmetry between normal matter and antimatter. A collisionless component of `dark matter', which has been inferred purely from its gravitational effects. A homogeneous negative-pressure component of `dark energy', which has been inferred only from its tendency to accelerate the expansion of the universe. A set of density fluctuations with a power-law spectrum, which are acausal in the sense of having contributions with wavelengths that exceed ct at early times.

It is assumed that an understanding of these ingredients relates to the initial conditions for the expanding universe. Since about 1980, the standard assumption has been that the key feature of the initial conditions is a phase of `inflation'. This represents a departure from the older singular `big bang' expansion history at high energies (perhaps at the GUT scale of 1015 GeV). With this background, we can attempt a list of some of the big open questions in cosmology, which any complete theory must attempt to address: (1) What generated the baryon asymmetry? (2) What is the dark matter? (3) What is the dark energy? (4) Did inflation happen? (5) Are there extra dimensions? 6


(6) Do fundamental constants vary? There are many further features of the universe that one would wish to understand í notably the processes that connect initial conditions to complex small-scale structures: galaxies, stars, planets and life. Nevertheless, the remit of the current Working Group is restricted to what we have termed Fundamental Cosmology, on the assumption that the complex nonlinear aspects of small-scale structure formation involve no unknown physical ingredients. We are therefore primarily concerned with what astronomy can tell us about basic laws of physics, beyond what can be probed in the laboratory. Not all the key questions listed above are amenable to attack by astronomy alone. For example, an important source of progress in the study of the baryon asymmetry will probably be via pure particle-physics experiments that measure aspects of CP violation. Given a mechanism for CP violation, it is relatively straightforward in principle to calculate the relic baryon asymmetry í the problem being that the standard model yields far too low a value (see e.g. Riotto & Trodden 1999). Experiments that measure CP violation and are sensitive to non-standard effects are thus automatically of cosmological interest. This applies to experiments looking at the unitarity triangle within the quark sector (BaBar, BELLE, LHCb), and to a future neutrino factory that could measure CP violation in the neutrino sector í which would be studying physics beyond the standard model by definition. Results from the LHC are expected to be relevant also to many other fundamental cosmological problems. For example, the discovery of a Higgs boson might provide some insight into the nature of dark energy or the driving force for inflation. Likewise, the discovery of supersymmetry or extra dimensions at the LHC might provide direct evidence for a dark matter candidate whose properties could be determined and compared with astrophysical and cosmological constraints. Underground direct dark matter searches may detect the constituent of the dark matter even before the LHC has a chance to look for supersymmetric signatures. Current direct detection limits for Weakly Interacting Massive Particles (WIMPs) that form the dark matter constrain a combination of the particle mass and the spin-independent WIMP-nucleon scattering cross-section. The best sensitivity is currently achieved at masses around 100 GeV, where the cross-section must be below 10-46 m2 . The simplest supersymmetric extensions of the standard model predict cross-sections somewhere in the four orders of magnitude below this limit, so success in direct WIMP searches is certainly plausible, if hard to guarantee. A future linear electron-positron collider would also be of cosmological relevance, since it could study in more detail the spectrum of particles accompanying such dark matter candidates. However, it is beyond our remit to consider the capabilities and relative priorities of different particle accelerators. Within the compass of astronomy, then, one can consider the following observables and techniques, together with their associated experiments. This really focuses on a small list of observable signatures (tensor modes and the `tilt' of the density power spectrum, plus possible non-Gaussian and isocurvature contributions in order to probe 7


inflation; the equation of state parameter w(z) as a route to learn about dark energy): - - - - - - - Cosmic Microwave Background (CMB) anisotropies Large-Scale Structure (LSS) from large galaxy redshift surveys Evolution of the mass distribution from cluster surveys Large Scale Structure from gravitational lensing surveys The Hubble diagram for Supernovae of Type Ia Studies of the intergalactic medium (IGM) Gravitational waves

Subsequent sections in this report are constructed around asking what each of these techniques can contribute to the study of fundamental cosmology. Before proceeding, we now lay out in a little more detail some of the relevant pieces of cosmological background that will be common themes in what