IL NUOVO CIMENTO Vol. ?, N. ? ? The Future of Cosmology George Efstathiou Institute of Astronomy, Madingley Road, Cambridge, CB3 OHA. England. Summary. — This article is the written version of the closing talk presented at the conference ‘A Century of Cosmology’ held at San Servolo, Italy, in August 2007. I focus on the prospects of constraining fundamental physics from cosmological observations, using the search for gravitational waves from inflation and constraints on the equation of state of dark energy as topical examples. I argue that it is important to strike a balance between the importance of a scientific discovery against the likelihood of making the discovery in the first place. Astronomers should be wary of embarking on large observational projects with narrow and speculative scientific goals. We should maintain a diverse range of research programmes as we move into a second century of cosmology. If we do so, discoveries that will reshape fundamental physics will surely come. 1. – Introduction It is a privilege to be invited to give the closing talk at this meeting celebrating ‘A Century of Cosmology’. I have taken the liberty of changing the title of the written version to make it shorter and snappier. Of course, I will not be able to cover all of arXiv:0712.1513v2 [astro-ph] 4 Jan 2008 cosmology and I am not a clairvoyant. What I will try to do is to review a small number of topics and use them as a guide to how our subject might develop. I am told that a good high court judge leaves everybody in the courtroom dissatisfied. I have borne this in mind in preparing this talk. Rather than congratulating our community on its remarkable progress, I have tried to pose some difficult questions. If readers are dissatisfied with some, but not all, of my answers, I will have succeeded with this talk. It is a cliche, but nonetheless true, that our subject has undergone a revolution in the last decade. When I was a graduate student, astronomers were still struggling with photographic plates and had measured redshifts for only a thousand or so galaxies. A galaxy redshift of a half was regarded as exotically high – almost primordial. Extragalac- tic astronomy from space was in its infancy – a few bright X-ray sources and little more. The contrast with the present day is striking. We now have space satellites covering radio frequencies through to gamma rays. Observational cosmology has moved with a vengeance into survey mode, with large teams dedicated to special projects such as sur- veys of galaxy redshifts, distant supernovae, weak lensing, and so on. The era of projects c Societ`aItaliana di Fisica 1 2 GEORGE EFSTATHIOU geared towards ‘precision cosmology’ is upon us, with WMAP providing the archetypal example. Many of the cosmological parameters that we have been struggling to measure for decades are now constrained accurately, possibly to higher precision than many as- tronomers actually care about. The talks at this meeting provide ample testament to how cosmology has changed. Cosmology has clearly been successful and we have learned a lot about our weird and wonderful Universe. But has hubris set in? Have our successes led to unachievable expectations concerning the future? (‘Irrational exuberance’ as Alan Greenspan famously referred to the dot-com boom). I will try to answer these questions by considering three topics in this article. The first two, the search for gravitational waves from inflation and constraints on the equation of state of dark energy, use cosmology to test fundamental physics. The third topic, the non-linear Universe, may or may not lead to new results of relevance to fundamental physics. If not, does this mean that a topic such as galaxy formation is a less worthy problem than understanding dark energy or inflation – a cosmological equivalent of weather forecasting? To what extent should we judge projects by their potential to test fundamental physics rather than to improve our understanding of complex non-linear phenomena? These are difficult, and for some people emotive, questions. But we must face up to them as we move into a second century of cosmology. 2. – The Search for Tensor Modes Inflation is a compelling theoretical idea. The almost perfect agreement between the cosmic microwave background (CMB) anisotropy measurements (particularly by WMAP) and theoretical predictions based on inflation has been interpreted by many, though not all, cosmologists as evidence that inflation actually happened. A key prediction of inflationary models is the existence of a stochastic background of gravitational waves. Such a background is potentially detectable via a ‘B-mode’ polarization signature in the CMB [1, 2]. If only we could measure this B-mode signature, so the thinking goes, we would have incontrovertible evidence for inflation that should convince the staunchest of skeptics. A detection of tensor modes would set strong constraints on the dynamics of inflationary models and would fix the energy scale of inflation via, (1) V 1/4 3.3 1016r1/4 GeV, ≈ × where r is the tensor-scalar ratio (defined so that for r 1 tensor and scalar modes contribute nearly equal amplitudes to the large angle temperature≈ anisotropies, see [3] for a precise definition). However, the problem is not a simple one for at least three reasons: [A] The expected signal will be small: Firstly, any B-mode signal from inflation will be incredibly small and difficult to detect. The direct upper limit on B-modes from WMAP polarization measurements corresponds to a 1σ upper limit of about r < 1 [4]. From parameter fitting it is possible to set a2σ indirect limit of r < 0.36 with plausible∼ theoretical assumptions [5]. A primordial B-mode of this amplitude∼ would produce an rms anisotropy signal of only 0.35µK, i.e. about a factor of 20 times smaller than the rms anisotropy in E-modes.∼ This is well below the sensitivity levels achievable by WMAP. The Planck satellite [6] scheduled for launch in 2008 will be considerably more sensitive than WMAP but will struggle to constrain B-mode anisotropies. THE FUTURE OF COSMOLOGY 3 Fig. 1. – Power spectra for a simulation matched to the Planck 143 GHz sensitivity. The blue points show individual points of the power spectra. The magenta points at ℓ > 20 show power spectra averaged in bands of width ∆ℓ ≈ 20. The red lines shows the theoretical power spectra used to generate this realisation based on the ‘minimal’ WMAP ΛCDM parameters [8], but also including a tensor mode with r = 0.15. This is illustrated in Figure 1 which shows power spectra from a simulation matched to the sensitivity level of the Planck 143 GHz polarized detectors. (This simulation has been done at half Planck resolution, but with the noise level adjusted to match the sensitivity expected for the 143 GHz detectors). The simulation employs a realistic focal plane geometry, scan strategy, low frequency detector noise and Galactic mask, as described in [7]. The B-mode spectrum for r =0.15 becomes noise dominated at ℓ > 20. Even if all sources of systematic error are kept under control, Planck will struggle∼ to detect anything more than a few multipoles in the B-mode even if the tensor amplitude is as high as 0.15. Improving∼ the sensitivity further requires large arrays of detectors. Several ground based/sub-orbital B-mode polarization experiments are either planned or in progress (examples include BICEP [9], QUIET [10], SPIDER [11] and CLOVER [12].) Groups in both the USA and Europe have considered designs for a B-mode optimised low resolution ( > 30′) space satellite. It is certainly possible to conceive of experiments with the raw sensitivity∼ to probe tensor-scalar ratios of r < 10−2. However, it is not yet clear whether systematic errors can be reduced to below this∼ level. [B] Polarized foregrounds will be dominant: In the case of temperature anisotropies, WMAP and other experiments have shown conclusively that at frequencies of around 100 GHz Galactic emission over much of the sky is small compared to the primordial anisotropy signal. It is therefore possible to estimate the temperature power spectrum accurately by masking out a relatively small area of the sky around the Galactic plane. The situation in polarization is very different, 4 GEORGE EFSTATHIOU particularly for large angle B-mode anisotropies. Outside the P06 polarization mask defined by the WMAP team, the Galactic polarized signal at 65 GHz contributes 0.7µK(1) at low multipoles [4], compared to an expected B-mode signal of 0.05µK ∼if r 0.1. Thus, one must be able to subtract polarized foregrounds in Q and U∼ maps to a precision∼ of order a percent to detect a primordial B-mode with r 0.1 at low multipoles. Foreground removal is not a well posed problem and it is not easy∼ to assess the accuracy of any given technique. Even for temperature anisotropies, no experiment has yet been done with enough frequency channels to fit and subtract a realistic parametric model of the foreground components. Luckily, foregrounds are unimportant for the temperature anisotropies over much of the sky and so it doesn’t matter critically how foreground cleaning is handled. But it certainly does matter for polarization. The ultimate limit of polarized foreground cleaning is not yet known and this should be borne in mind when assessing experiments with instrumental sensitivities capable of achieving r 0.01 or better. The dominance of polarized foregrounds at low multipoles suggests that∼ an effective strategy for detecting B-modes is to target the multipole range ℓ 50 100 at frequencies of 100 GHz in the cleanest regions of the sky. This approach∼ requires− much higher sensitivity∼ than Planck, but lowers the foreground contribution compared to any primordial signal and has the added advantage that such an experiment can be done from the ground.