2013: A Good Year for Cosmology A Brief History of Contemporary Cosmology

Transcription

1 2013: A Good Year for Cosmology A Brief History of Contemporary Cosmology Cristiano Sabiu School of Physics, KIAS The Universe on large scales seems to obey the proposed cosmological principle; it is homogeneous and isotropic. This means that no matter in which direction we look and not matter where we place ourselves in the cosmos, our surroundings will always look the same. Now, when we say look we mean that the structure of the universe is statistically measured, for instance we could count the number of objects in one direction then turn our telescope and count the number of galaxies in the opposite direction. Comparing these two measurements puts tight constraints on the cosmic isotropy. We can also look at the 3-dimensional distribution of mass-tracers (ie galaxies) and notice that translations in the space do little to alter is appearance. So our initial assumption of homogeneity works fairly well once applied to the data. The Universe also seems to obey Einstein s Gravitational theory of General Relativity (GR). On small table top experiments (Pound & Rebka, 1960) to solar (Gravity Probe A, Vessot et al., 1980) and galactic systems (Binary pulsar, Hulse & Taylor, 1975), GR holds well. However recent measurements may for the first time be casting a doubt over the validity of GR on the large cosmic scales. The origin of the accelerating expansion of the universe is one of the most salient questions in contemporary cosmology. Its origins are illusive and range from evolving scalar fields remnant from the big bang, to the standard assumption of a non-zero energy associated with the vacuum of space itself. Whatever the theoretical motivation, it is hard to reconcile the data within the standard GR framework without allowing for late-time acceleration. THE KIAS Newsletter

2 FIG. 1 The SDSS uses a dedicated 2.5 m f/5 modified Ritchey-Chretien altitude-azimuth telescope located at Apache Point Observatory, in south east New Mexico. A 1.08 m secondary mirror and two corrector lenses result in a 3 distortion-free field of view. Sloan Digital Sky Survey and the Baryon Oscillation Spectroscopic Survey Building on the legacy of the Sloan Digital Sky Survey (SDSS) and SDSS-II, the SDSS-III Collaboration is working to map the Milky Way, search for extrasolar planets, and solve the mystery of dark energy. SDSS-III began to collect data in 2008, and will continue until 2014, using the Sloan Foundation 2.5-meter Telescope at Apache Point Observatory in New Mexico. SDSS-III consists of four surveys, each focused on a different scientific theme. FIG. 2 Scales of certain physical processes, imprinted on the CMB, survive to the late-time universe and can be see in the clustering signal of galaxy positions The SDSS-III s Baryon Oscillation Spectroscopic Survey (BOSS) is mapping the spatial distribution of luminous red galaxies (LRGs) and quasars to detect the characteristic scale imprinted by baryon acoustic oscillations in the early universe. Sound waves that propagate in the early universe, like spreading ripples in a pond, imprint a characteristic scale on cosmic microwave background fluctuations. These 18 Korea Institute for Advanced Study

3 Research at KIAS fluctuations have evolved into today s walls and voids of galaxies, meaning this baryon acoustic oscillation (BAO) scale (about 150 Mpc) is visible among galaxies today. This concept is illustrated below (some of the relative scales have been exaggerated for illustration purposes). At first order the sound horizon is given by, 1 a s = * 1/2 H 0 m 0 da c s (a + a eq ) 1/2 where H 0 is the Hubble constant, today, and m is the matter fraction of the universe. a eq is the scale factor at matter radiation equality and c s is the sound speed. This scale can then be measured in the statistical spatial distribution of galaxies using either power spectra, in fourier space, or correlation functions, in configuration space. This scale has been measured over a wide redshift range and fits very well to the standard LCDM theory, see Fig 3. accounted for when making these measurements. Redshift space distortions (RSD) are induced by the large scale velocity flow of galaxies and are thus intimately connected to the growth rate of cosmic structure. Over the last 10 years, as the size of spectroscopic surveys has increased, this effect has been exploited, allowing testable predictions of general relativity on large scales. Geometric distortions are induced by distances along and perpendicular to the line of sight being fundamentally different. Measuring the ratio of galaxy clustering radially to transversely provides a probe of this, called the Alcock-Paczynski effect. Assuming the in- correct cosmological model for the coordinate transformation from redshift space to comoving Cartesian space leaves a residual geometric distortion. In observations the geometric effect is convolved with RSD, but the fixed physical scale of baryon acoustic There are however some issues that must be Planck D v (z) (r s,fid /rs ) (Mpc) dFGS SDSS- BOSS WMAP ACDM WiggleZ Redshift FIG. 3 The distance ratio, D v, is an approximate model independent cosmic observable, which allows for statistically robust combined analyses. Impressively, the line is a prediction, not a fit! -1 π (h Mpc) σ (h Mpc) FIG. 4 The measured, best fit, and LCDM-predicted versions of (, ) are plotted, using the Planck early universe prior. The blue filled contours represent the measured (, ), and the black unfilled contours represent the best fit (, ) with two different cuts on fitting scale. THE KIAS Newsletter

4 oscillations (BAO) can alleviate this covariance. These baryon acoustic oscillations have now been measured in the anisotropic redshift-space distribution of galaxies as illustrated in Fig 4. The BAO signal forms a ring in the anisotropic projection. Carefully fitting this ring structure on large scales constrains the Alcock-Paczynski effect and the geometrical information of the space-time metric while on small scale the amount of squashing and compression of the contours informs us about the linear growth rate and the gravitational dynamics of matter. COBE WMAP Plank Using the acoustic scale as a physically calibrated ruler, BOSS will determine the angular diameter distance with a precision of 1% at redshifts z = 0.3 and z = 0.55 using the distribution of galaxies. It will also measure the distribution of quasar absorption lines at z = 2.5, yielding a measurement of the angular diameter distance at that redshift to an accuracy of 1.5%. It will also measure the cosmic expansion rate H(z) with 1-2% precision at the same redshifts. These measurements will provide demanding tests for theories of dark energy and the origin of cosmic acceleration. Planck and the CMB ESA s Planck satellite was launched on the 14 th May 2009 and operated for over four years, scanning the whole sky several times at microwave and sub-millimetre frequencies. Its main goal was to take the most detailed photograph ever of the tiny fluctuations present in the Cosmic Microwave Background (CMB), the most ancient light that has travelled across the Universe. FIG. 5 The progression in the angular resolution of CMB experiments from COBE to WMAP and finally Planck. The full-sky map is from the latest release (2013) from the Planck collaboration. The first image of the entire sky showing these minute differences and based on the data collected during Planck s first 15.5 months of observations, was released in March Planck has provided the most accurate snapshot made of the matter distribution in the early Universe, only years after the Big Bang. Fluctuations in the CMB correspond to the cosmic seeds that would evolve into all the structure observed in the Universe today from stars and planets to galaxies and galaxy clusters. Planck s precise data enables cosmologists to investigate a huge variety of models for the origin and evolution of the cosmos. The new image of the CMB has confirmed that the standard model of cosmology is a very good description of the Universe. Dominated 20 Korea Institute for Advanced Study

5 Research at KIAS by the as yet unexplained dark matter and dark energy, the cosmos we live in appears to have begun almost 14 billion years ago with an early period of accelerated expansion, called inflation, during which the seeds of cosmic structure were embedded in the Universe. The data from Planck have allowed cosmologists to set very tight constraints on many parameters of the standard model (Ade et al. 2013), including the Hubble constant (H 0 ), which describes the expansion rate of the Universe today, the densities of baryonic matter, dark matter and dark energy ( b, m, ), and the spectral index (ns), which describes the relative amount of primordial fluctuations on different scales. However the cosmological parameters are just Planck s headlines: there is plenty more science within this dataset. They have gone beyond the isotropic power spectrum, to put limits upon the so-called non-gaussianities which are signatures of the detailed way in which the seeds of large-scale structure in the Universe were initially laid down. They ve observed clusters of galaxies which give us yet more insight into cosmology (and which seem to show an intriguing tension with some of the cosmological parameters). They ve measured the deflection of CMB light by gravitational lensing. They have also used the CMB maps to put limits on some of the ways in which our simplest models of the Universe could be wrong, such as interesting topology or rotation on the largest scales. The Big Picture If we now combine these datasets we can break key degeneracies in the cosmological model parameter space. The observables of the CMB and LSS are linked by cosmic evolution and alone can provide not only constraints on LCDM but tests for Einstein s gravitational theory of General Relativity. In testing Cosmology our analysis approach has been model independent, obtaining constraints on the distances D A and H -1 without even assuming a Friedmann integral relation between them and on the velocity growth factor G. While we have so far compared the values individually to the best fit LCDM predictions from the CMB, we should also look at the joint probabilities. We can test for consistency with the LCDM model by examining whether the fixed relations between these quantities in LCDM, i.e. the 1D curves in the D A H -1, D A G, and H -1 G planes, all intersect the measured confidence contours. Furthermore, we can generalize -1 (h Mpc) H G FIG. 6 Using small scale information (>20 h-1 Mpc, in dark orange) rather than our standard (>40 h-1 Mpc, in light grey), shifts the results into a region corresponding to no reasonable cosmology within general relativity. This figure assumes a Planck early universe prior, as an example. THE KIAS Newsletter

6 the test by allowing for spatial curvature or non- dark energy. For the growth factor G this comparison also allows a test of general relativity since within this theory the distance quantities (measuring the cosmic expansion) have a definite relation to the growth quantity G. As shown in Fig 6, depending on the specific cutting criteria we can find slight deviations from Einsteinian Gravity. However due to the unknown amount of non-linear gravitational contamination, it is too early to make a more solid statement on this issue σ Ω m FIG. 7 Comparison between Planck+WP, Planck SZ clusters, CFHTLS lensing and our results. When using only the f8 constraint from our analysis (orange contours), there is a degeneracy, similar to the cluster and lensing datasets. The geometric information can break this degeneracy. The tension in 8 between our measurement and Planck+WP is directly related to the large we find in our LCDM consistency check. [Figure from Beutler et al. 2013] In Beutler et. al (2013) they take both a different statistical and analysis approach from our work. They measure the clustering fourier multipoles, and within their analysis they combine with Planck to test General Relativity (GR) through the simple -parameterisation. The combined analysis reveals a 2 tension between the data and the prediction by GR. This tension can be traced back to a tension in the clustering amplitude, 8, between CMASS-DR11 and Planck, See Fig 7. Putting this into context, the standard model of cosmology seems to run into problems at large scales which could indicate a breakdown of GR. Ultimately one would hope to probe GR on these large scale using two probes; redshift space distortions (RSD) and gravitational lensing. While RSD measures the Newtonian potential, lensing measures the sum of the metric potentials. However, any modification of gravity needs to pass the very precise tests on smaller scales (Pound & Rebka experiment, Pound & Rebka measurement / prediction density [M / Mpc 3 ] scale [Mpc] FIG. 8 Summary of different tests of General Relativity (GR) as a function of distance scale (bottom axis) and densities (top axis). [Figure from Beutler et al. 2013] 22 Korea Institute for Advanced Study

7 Research at KIAS 1960, Gravity Probe A, Vessot et al. 1980, Hulse- Taylor binary pulsar Hulse & Taylor 1975). These constraints can be seen in Fig 8. While the large scale structure data provided by SDSS BOSS and the PLANCK CMB project (Ade et al, 2013) have each given us a great improvements on the precision of cosmological parameters, in combination there seems to be some tension. At the level of 1-2sigma we cannot infer anything, but it leaves the door open for some interesting theoretical studies that will motivate the next generation of cosmological surveys, like MS-DESI, LSST, Euclid and BigBOSS. In the meantime we wait tentatively for the release of PLANCK polarization data that may hold yet more answers to some of the most fundamental questions regarding gravitation, dark energy and cosmic Inflation. Reference [1] Cosmological Constraints from the Anisotropic Clustering Analysis using BOSS DR9 Eric V. Linder, MinJi Oh, Teppei Okuma, Cristiano G. Sabiu, Yong-Seon Song [2] The Baryon Oscillation Spectroscopic Survey: Testing gravity with redshift-space distortions using the power spectrum multipoles Florian Beutler et.al 2013 In preparation [3] Pound R. V. and Rebka, Jr. G. A., Phys. Rev. Lett. 4, 337 (1960). [4] Vessot etal PRL, , 1980 [5] Hulse & Taylor 1975 Astrophysical Journal, vol. 195, Jan. 15, 1975 [6] Planck 2013 results. XVI. Cosmological parameters P. Ade etal (Planck Collaboration) Cristiano Sabiu Cristiano Sabiu has been a KIAS research fellow since Oct He obtained his BSc and MSc from University of Glasgow, Scotland. In 2010 was awarded his PhD in Cosmology from University of Portsmouth, England. He held a postdoctoral research position at University College London (UCL) before coming to Korea. He is an active member in the Sloan Digital Sky Survey (SDSS) and the Dark Energy Survey (DES) THE KIAS Newsletter