Research Highlights
The Halo Boundary of Galaxy Clusters in SDSS
April 24, 2017 | Read more
A cluster formed in a Lambda-Cold Dark Matter simulation of structure formation.

Credit: Benedikt Diemer, Philip Mansfield
PFC@KICP fellow Chihway Chang and PFC co-Investigator Andrey Kravtsov have participated in a recent study, which presents strong evidence for the physical edge of galaxy clusters using public data from the Sloan Digital Sky Survey.

The existence of such physical edges associated with sharp density drops due to the density caustics formed by accreting matter was predicted by PFC researchers Benedikt Diemer and Andrey Kravtsov in 2014, as part of Diemers PhD research. In a follow-up study, Diemer, Kravtsov and a former PFC@KICP fellow Surhud More (currently at Institute of Physics of the Universe, Tokyo, Japan) have shown that the-edges can be considered to be natural physical boundary of dark matter halos that provide the gravitational "back-bone" for the structures observed in the galaxy distribution.

In the recent study, co-led by Chihway Chang and Eric Baxter - a former PFC student and currently a postdoctoral fellow at the University of Pennsylvania - the density drop associated with the halo edges was detected in the galaxy distribution around cluster centers.

Cosmological simulations show that massive galaxy clusters we see today have been accreting galaxies into their deep gravitational potential over the cosmic time. The process of galaxies "falling into" the cluster's potential well is a fairly clean and universal process that depends only on basic quantities of the cluster such as mass and accretion rate. One of the result of this simple picture is a sharp feature in the number density of galaxies around clusters - an imprint of the caustic formed by the infalling galaxies as they reach the first apocenter of their orbit, or the "edge" of the galaxy cluster. Researchers called the distance of the edge the "splashback" radius, as galaxies literally "splashing back" to that radius after they accrete onto cluster.

This figure shows the fraction of red and blue galaxies around galaxy clusters. The sharp change in the red fraction indicates that galaxy tend to turn red once they enter the edge of the cluster, which is marked by the grey vertical band. (Figure modified from the paper "The Halo Boundary of Galaxy Clusters in the SDSS".)
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Together with collaborators in UPenn and UIUC, that included PFC@KICP Andrey Kravtsov, Chihway Chang and Eric Baxter, examined distribution of galaxies around a sample of clusters identified within the SDSS. The existence of the edge in the galaxy distribution within clusters was confirmed. In addition, the analysis revealed that properties of galaxies around cluster are sensitive to existence of the edge. Outside the splashback radius, the mix of red and blue galaxies was approximately independent of the distance from the cluster center, while inside the splashback radius the mix is abruptly changes towards a larger fraction of red galaxies. This indicates that the edge is a real dynamical feature and that majority of galaxies get transformed by the cluster environment from blue to red in less than one orbital period.

Related PFC@KICP references:

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New measurement of H0
October 25, 2016 | Read more
This April, the SHOES (Supernova for the Equation of State) team released new measurements of the local value of the Hubble constant ("A 2.4% Determination of the Local Value of the Hubble Constant") The Hubble constant reveals the current rate of expansion of the universe, and when compared against measurements of the Hubble constant extrapolated from analysis of the Cosmic Microwave Background, provides an end-to-end test of our standard model of cosmology.

Figure: Complete distance ladder. The simultaneous agreement of pairs of geometric and Cepheid-based distances (lower left), Cepheid and SN Ia-based distances (middle panel) and SN and redshift-based distances provides the measurement of the Hubble constant.
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KICP (and Hubble) Fellow Dan Scolnic, led the supernova component of the analysis. Supernovae are the critical tool for two out of the three rungs of the "distance ladder" used to measure the Hubble constant. The three rungs are shown in Figure. In the first rung, geometric measurements of the parallax to Milky Way Cepheids and the distance to masers of nearby galaxies are used to calibrate the intrinsic luminosity of Cepheid variables. In the second rung, the brightness of cepheids are used to calibrate the luminosities of Type Ia supernovae in nearby galaxies. And in the third and last rung, the brightness of nearby supernovae are used to calibrate the luminosity of supernovae in the Hubble flow and break the degeneracy between the local expansion of the universe and the luminosity of supernovae. Scolnic used advances from PFC supported work done at KICP, including a recent effort to recalibrate all optical photometric surveys onto a single homogeneous system (2015ApJ...815..117S) and work with KICP research scientist Rick Kessler to remove systematic biases of the supernovae distances (2016ApJ...822L..35S). This latter work was done with the prominent SNANA supernova simulation software, developed by Kessler, which has been a principal tool for analyses of data from all current supernova survey including The Dark Energy Survey, led by Frieman.

The SHOES team determined the most precise value of the local value of the Hubble constant to date -- 73.24 ± 1.74 Mpc. What is particularly interesting about this value is that it is 3.4σ higher than the value determined from analysis of the Planck data assuming the ΛCDM model. There has been tremendous excitement that either the Planck or SHOES analysis contain significant systematic errors that have been seriously underestimated, or that there is need for a departure from the standard model of cosmology. This departure may be due to an additional source of dark radiation in the early universe or an evolving equation-of-state of dark energy.

Work is ongoing by the SHOES team to further reduce uncertainties in its measurements. In the upcoming year, an independent measurement of the local value of the Hubble constant from the CHP team (led by the KICP's Wendy Freedman) will be released. This will be an important addition to the current debate about the tension between the different probes of the Hubble constant. On the CMB side, a new analysis from former KICP Fellow Silvia Galli is expected to investigate possible discrepancies, or lack thereof, in the Planck data. Furthermore, new analysis from the SPT team, led by KICP Fellow Jason Henning, should provide an independent measurement of the CMB that will check the accuracy of the Planck measurements.

Currently this end-to-end test of the standard model of cosmology hints at problems in ΛCDM. Still, it is remarkable that measurements are as close as they are. Whether these results hint at new physics, or simply provide motivation for more accurate experiments, this topic will be the focus of much of the cosmology community over the next five years. The PFC at KICP will be a leader in this conversation.

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Observational detection of the splashback radius
April 25, 2016 | Read more
Top panels: The surface number density profiles, of galaxies in the SDSS photometric sample with different magnitude thresholds around the RedMaPPer clusters. The dashed lines correspond to subhalo surface density profiles predicted by cosmological simulations. Bottom panels: The logarithmic slope of the surface density profiles are shown using solid and dashed lines for the observed galaxy and the subhalo surface density profiles, respectively. The observed slope of the surface density profile has a shape which is similar to that expected from simulations. The radius at which logarithmic slope trend sharply changes corresponds to the splashback radius -- caustic in the dark matter distribution. Note that although the surface density profiles both in observations and simulations exhibit similar steepening, the corresponding radii of the steepest slope (i.e., the splashback radii) are at slightly different locations.
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Kravtsov has collaborated with former KICP fellow Surhud More and others to detect the splashback feature in the distribution of galaxies around clusters ("Detection of the Splashback Radius and Halo Assembly Bias of Massive Galaxy Clusters"). The splashback is a sharp discontinuity in the density of matter around galaxy clusters formed by the dark matter particles reaching their first apocenter after accretion. The feature was predicted to exist by Kravtsov and former PFC at KICP graduate student Benedikt Diemer based on analyses of cosmological simulations. Galaxies serve as dynamical tracers and should thus exhibit the same splashback features as dark matter. Kravtsov and collaborators have examined projected radial profile of galaxy surface density around redMaPPer clusters identified in the SDSS survey and have unambiguously detected the splashback. Moreover, they showed that subsamples of clusters split by their galaxy concentration exhibit splashback at different radii, as predicted by cosmological simulations.

At the same time, location of the splashback radius around observed clusters is found to be somewhat smaller than predicted (see Figure). Although observational bias cannot be fully excluded at this point, if the difference is real, this will have very interesting implications for the nature of dark matter. For example, for models in which dark matter particles are self-interacting with anisotropic cross-section, dark matter subhalos will experience net drag due to subhalo dark matter interacting with ambient dark matter. Such drag would reduce the splashback radius and make galaxy distribution more concentrated.Learn more >>

Creating the next generation of Teacher-Scholars
December 13, 2015
Graduating KICP Students from Summer 2014 through Summer 2015. From left to right then top to bottom: Eric Baxter, Nicole Fields, Alan Zablocki, Kyle Story, Yin Li, Vinicius Miranda, Jennifer Helsby, Benedikt Diemer, Ke Fang, Tyler Natoli, Alan Robinson, and Youngsoo Park.
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The KICP prides itself on preparing the next generation of teacher-scholars. Nowhere is this more evident than in the KICP faculty's commitment to educating and mentoring its graduate students. By any measure, graduate students at the KICP have had an outstanding year. In the period lasting from the Summer of 2014 until the end of 2015's Summer, thirteen KICP graduate students will have defended their dissertations and moved on the next stages of their careers. Each of these students has benefitted from the PFC's presence at the KICP. Eleven of the graduating students have been supported at some point with PFC funds, several have participated the Space Explorers program, many have given Astronomy Conversations at the Adler, all have worked in PFC supported research areas, and all have interacted and collaborated with the many visitors and speakers enabled by the Conferences, Workshops, and Visitors MA.

KICP Graduates from July 2014 through those anticipated (*) by September 2015.
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A new definition of Halo Radius
August 25, 2015 | Read more
Figure: density map of dark matter distribution in a slice around a cluster-sized halo formed in a cosmological simulation of the ΛCold Dark Matter cosmology. The image shows a sharp boundary around a cluster that is associated with the apocenter of the recently accreted matter. This splashback radius represents a natural physical boundary of CDM halos, as argued in a recent paper by Kravtsov, graduate student Diemer, and former fellow S. More.
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In a new study Kravtsov, student Diemer, and former fellow Surhud More built upon a paper published by Diemer Kravtsov during the previous year, to show that the sharp breaks in the halo density profile represent a natural physical boundary of halos. This is because the density profile jumps are associated with the apocenters of the orbits of the most recently accreted matter, which accumulates near the splashback radius. This radius represents the largest extent reached by halo matter that orbited within the halo at least once. More, Diemer Kravtsov proposed the splashback radius as the new way to define halo boundary and mass. The splashback radius and associated sharp feature in density distribution around a cluster-sized system are illustrated in Figure, where we also feature the use of such visualizations at the Adler Science Visualization Lab. The paper describing results and arguments has now been accepted for publication in the Astrophysical Journal.Learn more >>

Dark Matter Map from Early Dark Energy Survey Data
July 19, 2015 | Read more
Large-scale map of the projected mass distribution reconstructed from weak lensing measurements of several million galaxies (z>0.6) in DES Science Verification data. Dark red regions are the most overdense, dark blue are the most underdense. The map has been smoothed with a 20 arcmin Gaussian. Grey circles indicate the positions of foreground galaxy clusters (z<0.5); size of the circle indicates the optical richness of the cluster, from richness 20 to 160.
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KICP Associate Fellow Vikram, Chang, and DES collaborators (including Frieman) created the largest contiguous map (see Figure) of the large-scale mass distribution reconstructed using weak gravitational lensing measurements of several million high-redshift galaxies from DES Science Verification data. Such mass maps provide a powerful tool for cosmology, since they show the total matter distribution, including both luminous and dark matter. Comparison of the mass map with the distribution of foreground galaxy clusters shows a strong correlation: more clusters are found where the projected mass density is high and few where it is low. Cross-correlation of the mass map with the foreground galaxy distribution found in the same DES data shows a cross-correlation amplitude consistent with expectations from N-body simulations. Including photometric redshift information shows that a number of the over- and underdense structures seen in the projected mass maps are associated with large coherent structures in three dimensions ("superclusters" and voids). This result is based on data taken over a 139 sq. deg. region shortly after the Dark Energy Camera was commissioned and represents only 3% of the final area that will be covered by the survey.Learn more >>

Milky Way Satellite Galaxies Discovered in First-year Dark Energy Survey Data
March 22, 2015 | Read more
Figure 1: All-sky map of Milky Way satellite galaxies known prior to 2015 (blue) and new satellite galaxy candidates reported by Bechtol and collaborators (red) in Galactic coordinates. The gray-scale represents the stellar density mapped by SDSS and DES. The location of the DES footprint (red outline) opens discovery space in the southern hemisphere near to the Magellanic Clouds.
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KICP Fellow Keith Bechtol, Associate Fellow Alex Drlica-Wagner, and collaborators on the Dark Energy Survey (DES) including Frieman and U. Chicago undergraduate Williams used the first season of DES data to discover eight new Milky Way companions. Milky Way satellite galaxies include the lowest luminosity, least chemically evolved, and most dark matter dominated galaxies in the known universe. These extreme objects have reshaped how we define a "galaxy" and provide a unique opportunity to study the low luminosity threshold of galaxy formation and test the Cold Dark Matter paradigm at the smallest scales. DES is making important contributions to this program thanks to a combination of depth (r~24), sky coverage (5000 square degrees), and survey footprint located in the less explored southern Galactic cap (Figure 1). Ultra-faint satellite galaxies are identified as arcminute-scale statistical overdensities of individually resolved stars. The least luminous and most distant of the new satellites have only tens of stars detected in the DES images. The DES team employed an array of search strategies: visual inspection of coadd images, peak-finding in binned maps of the stellar density field, and a maximum-likelihood matched filter algorithm to identify and characterize candidates. The congregation of satellites near the Magellanic Clouds suggests that some may be associated with the Magellanic system. The most distant of the new objects, Eridanus II, is located just beyond the virial radius of the Milky Way, which typically separates the outer gas-rich dwarf irregulars from inner gas-poor dwarf spheroidals. If Eridanus II has experienced recent star formation, it would be the lowest luminosity star-forming galaxy known and may have implications for the minimum halo mass required to sustain star formation over billion-year timescales. The nearest of the new satellites, Reticulum II, has been dynamically and chemically confirmed as a dark matter dominated galaxy through spectroscopic follow-up observations.

Figure 2: Upper limits on the velocity-averaged dark matter annihilation cross section for dark matter annihilation into b quarks. The limits for each DES candidate galaxy, as well as combined limits (dashed red) from the eight new candidates are shown. Current best limits derived from a joint analysis of fifteen confirmed satellite galaxies (black) along with the thermal relic cross section (dashed gray) are shown for comparison. The addition of new galaxy targets will further enhance the sensitivity of indirect dark matter searches.
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Milky Way satellite galaxies are also excellent laboratories to probe the particle nature of dark matter. In regions of high dark matter density, including dwarf galaxies, dark matter particles may annihilate into energetic Standard Model particles at rates that would be detectable by gamma-ray telescopes. The DES Collaboration teamed up with the Fermi Large Area Telescope Collaboration to search for excess gamma-ray emission in the directions of the new galaxy candidates from DES. KICP member Dan Hooper and KICP Fellow Tim Linden performed a similar, independent analysis. Although an intriguing hint of a gamma-ray excess was observed toward Reticulum II, no statistically significant signal was found from the new DES satellites as a whole (Figure 2). As more satellite galaxies are found in optical data, it should become possible to rigorously test the dark matter interpretation of excess gamma-ray emission observed towards the Galactic center. The second season of DES observations, completed in February 2015, increases the survey coverage from 1800 to over 4000 square degrees and is anticipated to yield additional Milky Way satellite discoveries.

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CMB B-mode detections
September 8, 2014 | Read more
Ushering in the B-mode era. The past year saw the first detection of the B-modes of CMB polarization with SPTPol and then BICEP2. Left Panel: SPTPol cross-correlated their polarization data with the cosmic infrared background fluctuations measured by Herschel, detecting the B-mode power due to lensing. Right Panel: BICEP2 successfully cleaned leakage from the temperature and measured the B-mode of CMB polarization with errors in the 100 nK range, a full order of magnitude more sensitive than previous results.
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The past year ushered in a new era in cosmology with the discovery of B-modes in the polarization of the CMB. Fig. depicts two detections, first by SPTPol and then by the BICEP2 telescope. The SPTPol detection used data from the Herschel satellite to construct an estimator for the projected gravitational potential. This potential lenses the E-modes, thereby creating a B-mode signal from a pure E-mode sky at last scattering. The left panel of the figure depicts the cross correlation between the estimate of B combining SPTPol E and the gravitational potential with the B-mode from SPTPol. The clear signal at precisely the expected amplitude marks yet another giant step in the quest to mine the CMB for cosmological information. The twin goals of the coming decade are to map the B-mode signal, obtaining an unprecedented map of the potential and to find evidence for primordial gravitational waves by observing B-modes on large scales.

BICEP2 announced in March 2014 that the collaboration had indeed detected a large scale B-mode signal (right panel of Fig.). The jury is still out as to the extent to which this signal is due to the primordial signal as opposed to foregrounds such as Galactic dust. Upcoming measurements, including those from the Keck Array and SPT, will likely provide a definitive answer within the next two years.Learn more >>

Dark Energy constraints from SDSS-SNLS Joint Lightcurve Analysis
July 23, 2014 | Read more
Constraints on dark energy equation of state parameter from the JLA supernova analysis (blue), CMB (green), and the combination of supernovae, CMB, and large-scale structure (grey).
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Members of the KICP Supernova Hub, working as part of the SDSS-SNLS Joint Lightcurve Analysis collaboration, published the tightest constraints to date on dark energy parameters using data from over 700 type Ia supernovae in combination with CMB and large-scale structure constraints. The dark energy equation of state parameter w was constrained with 5% precision, including systematic errors, and is consistent with a cosmological constant. This result relieved tensions in the ΛCDM model found by previous measurements (2014arXiv1401.4064B).Learn more >>

Dark Energy Survey and South Pole Telescope Results
April 2, 2014
SPT CMB flux decrement vs. frequency for 602 DES-selected clusters over 150 sq. deg. Points are measurements for low (blue), medium (green), and high-richness (black) clusters. Red curves are expectation from the Sunyaev-Zel'dovich (SZ) effect. Agreement between the points and curves indicates that dusty sources contribute at most a tiny fraction of the SZ signal for these clusters.
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The Dark Energy Survey, seeded by the KICP, carried out its first observing season from late Aug. 2013 to early Feb. 2014 using the Dark Energy Camera on the Blanco 4-meter telescope at CTIO in Chile. Over 14,000 survey-quality exposures were taken over 105 nights, covering 2000 sq. deg. of sky, plus imaging of 30 sq. deg. every few nights to discover supernovae. The DES demonstrated the discovery of high-redshift supernovae, high-redshift galaxy clusters, and high-redshift quasars, and measurement of galaxy clustering and weak lensing by galaxies and clusters. DES and SPT collaborators working in the KICP DES-SPT Joint Analysis Hub measured the angular cross-correlation of DES galaxies with CMB lensing from SPT and measured the Sunyaev-Zel'dovich signal around several hundred stacked DES clusters (see Figure).

Galactic Center Dark Matter Search
January 21, 2014 | Read more
The total gamma-ray flux from the 10ox10o box surrounding the position of the galactic center (left) compared to the residual emission after all components besides the dark matter template have been subtracted (right). The spherically symmetric gamma-ray signal predicted from dark matter forms the largest residual observed in the field of view, and contributes approximately 30% of the total gamma-ray emission within the inner 1o.
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Hooper and Linden have been highly involved in interpreting data from the Fermi Gamma-Ray Space Telescope within the context of searches for dark matter. This work has included studies of the Galactic Center and Inner Galaxy (2014arXiv1402.6703D), (2013PDU.....2..118H), (2013APh....46...55H), (2011PhRvD..84l3005H), dwarf galaxies (2013arXiv1309.4780H), the extragalactic gamma-ray background (2014JCAP...02..014C), as well as searches for nearby dark matter subhalos (2014PhRvD..89a6014B) and (2012PhRvD..86d3504B) and mono-energetic gamma-ray lines (2012PhRvD..86h3532H) and (2012PhRvD..86d3524B). Gamma-ray searches for annihilating dark matter are particularly interesting at the present time, as Fermi is currently sensitive to dark matter candidates with cross sections near those predicted for a generic thermal relic. For many models, Fermi data can be used to provide the strongest constraint on the dark matter annihilation cross section, as well as perhaps the greatest prospects for discovery.

Over the past several years, Hooper, Linden and their collaborators have reported an excess of gamma rays observed from the region surrounding the Galactic Center (2014arXiv1402.6703D),(2013PDU.....2..118H), (2013APh....46...55H), (2011PhRvD..84l3005H). Recently, this signal has been characterized in greater detail and has been found to exhibit spectral and morphological features consistent with that anticipated from annihilating dark matter particles (2014arXiv1402.6703D). Furthermore, no plausible astrophysical sources or mechanisms capable of accounting for the observed excess have been proposed (see, for example, (2013PhRvD..88h3009H)). In many respects, this signal appears to be the most compelling evidence of particle dark matter reported to date.Learn more >>

The SPT Cluster Catalog and Cosmological Constraints
November 28, 2013 | Read more
Mass estimates vs. redshift for four cluster samples: (1) optically confirmed SZ-selected galaxy clusters from the SPT-SZ 2500 deg^2 survey; (2) optically confirmed SZ-selected galaxy clusters from the 950 deg^2 ACT survey; (3) SZ-selected galaxy clusters from the all-sky Planck survey (Planck Collaboration et al. 2013); and (4) X-ray-selected galaxy clusters from the ROSAT all-sky survey (Piffaretti et al. 2011). High-resolution SZ surveys such as the SPT survey, uniquely have a nearly redshift independent selection function. The redshift dependent selection in the Planck survey is due to beam dilution; the redshift dependence of the ROSAT catalog is due to cosmological dimming. The SPT points in the figure therefore graphically illustrate the abundance evolution of massive galaxy clusters.
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The abundance evolution of clusters of galaxies is sensitive to multiple cosmological parameters, including the dark energy density and its equation of state. The SPT finds clusters through the small spectral distortion they impart on the cosmic microwave background (CMB), commonly called the Sunyaev-Zel'dovich (SZ) effect. The SZ effect offers an effective way to find the most massive, distant clusters in the universe because the brightness of the effect is independent of redshift. The SPT-SZ 2500 deg^2 survey was completed in November 2011, and the collaboration is in the process of finalizing the cluster catalog and completing a large multi-wavelength observational program to measure the cluster redshifts and improve the cluster mass calibration. The catalog consists of 450 clusters, 75% of which are newly discovered clusters. The SPT-SZ data is expected to produce constraints on the dark energy equation of state comparable to current constraints from the combination of CMB+BAO+SNe data. This will be an important achievement since the cluster based constraint provides a completely independent systematic test of the standard dark energy paradigm by measuring the effect of dark energy on the growth of structure. The combination of the growth and geometrical based constraints is a test of the underlying framework of dark energy vs. modifications to gravity.Learn more >>

Primordial perturbations in SPT
November 21, 2013 | Read more
Figure: The constraints on the inflationary models in the ns - r plane, where ns is the slope of the primordial perturbations and the ratio of the amplitude of tensor to scalar perturbations. We show the two-dimensional constraints on r and ns as colored contours at the 68% and 95% confidence levels for three datasets: WMAP7(grey contours), WMAP7 + SPT (red contours), and WMAP7+SPT+H0+BAO (blue contours). Adding the SPT bandpowers partially breaks the degeneracy between ns and r in the WMAP7 constraint, which can be seen clearly moving between the grey and red contours.
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Primordial perturbations in SPT. Inflation explains many of the puzzles of the standard cosmology and provides a natural mechanism for producing the seeds of structure in the universe. Most inflationary models predict a nearly scale-invariant spectrum, so that the amplitude of the primordial scalar perturbations that evolve into large scale structure is nearly the same on all length scales. This is characterized by a power spectrum proportional to kns where k is the wavenumber of the Fourier modes. Every inflationary model makes predictions for both ns and r, the relative amplitude of the tensor and scalar perturbations. For most models, ns is not exactly equal to one, an important distinction since the phenomenological choice before inflation was the Harrison-Zel'dovich spectrum with ns=1. So a small deviation from unity is another indication of the success of inflation.

Detailed measurements of the tail of the spectrum of anisotropies in the cosmic microwave background by the South Pole Telescope (SPT) provide the most precise measurement yet of ns. As depicted in Figure the slope of the scalar spectrum is now constrained to deviate from unity at 4-sigma when combining SPT data with the larger scale experiment WMAP. When combined with other cosmological measurements, the significance increases to over 5-sigma.Learn more >>

Dark Energy Camera sees "First Light"
August 3, 2013
Figure: 4-ton DECam mounted at the prime focus of the 4-m Blanco telescope at CTIO.
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The Dark Energy Survey (DES) is a 5000 deg^2 optical imaging survey in the grizY-bands to a depth of 24th magnitude for galaxies, and a time-domain survey of 10 supernova fields visited on a weekly cadence with a total area of 30 deg^2. DES involves 525 observing nights on the Blanco 4-m Telescope at the Cerro Tololo Inter-American Observatory (CTIO) spread over 5 years, beginning in September 2013. A new 570 megapixel, 3 deg^2 field-of-view camera was built specifically for the project. The DES aims to better understand the mechanisms responsible for the apparent accelerated expansion of the Universe using a suite of complimentary methods which trace the evolution of the cosmic scale factor and the growth of structure over the last 8-10 billion years.

The Dark Energy Survey seeks to increase our understanding of the mysterious Dark Energy by using four separate probes:
  • Galaxy Angular Clustering (Baryon Acoustic Oscillations) A map of 300 million galaxies to z>1
  • Galaxy Cluster Abundances by Mass and Redshift ~100,000 galaxy clusters to z>1
  • Weak Gravitational Lensing Tomography Shape measurements for ~200 million galaxies to z>1
  • Type Ia Supernova Distances Light curves for ~4000 supernovae to z~1

A new imager for the Blanco telescope, the Dark Energy Camera (DECam), was designed to meet the ambitious DES science goals (see Figure). "First light" images with the newly installed camera were taken on 12 September 2012, and extensive "Science Verification" studies were conducted through 22 February 2013. During that time, a contiguous region of 160 deg^2 was imaged to full survey depth. Several smaller patches were observed to varying depths, including a sample of prominent galaxy clusters, as well as calibration fields overlapping with SDSS Stripe 82, Chandra Deep Field South, and the VVDS, COSMOS, and CFHTLS surveys, which will be used to train photometric redshift estimators and evaluate the DECam photometric, astrometric, and object detection performance.

COUPP-60 installed and taking data at SNOLAB
June 25, 2013
Figure: Installation of the COUPP-60 vessel at SNOLAB.
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The COUPP-60 bubble chamber has been fully installed at the depth of SNOLAB. It is presently taking physics data, with first dark matter limits expected during fall of 2013. The active volume presently contains 40 kg of CF3I, a superheated compound selected to provide maximum sensitivity to both spin-dependent and -independent WIMP-nucleus couplings. It will be soon upgraded to a target mass of 80 kg, using either the same target fluid or C3F8, better matched to detect low-mass WIMPs in the region ~10GeV/c2. COUPP chambers feature the best discrimination against minimum ionizing particles in the field (10-10 rejection factor, to be compared to 10-3 for two-phase xenon detectors). The acoustic rejection of alpha particles is demonstrated to be better than 10-3, and expected to cap at 10-5. The chamber is surrounded by a large water tank serving a triple function, temperature control, neutron moderation and Cerenkov light muon-veto. The design of a next chamber, housing 500 kg of target mass, is well-advanced. The COUPP and PICASSO collaborations have recently merged, with the goal of starting installation of the COUPP 500 detector at SNOLAB during 2015.

Size-virial relation of galaxies
April 10, 2013 | Read more
Figure: Relation between the half-mass radius of stellar distribution.
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Kravtsov used the halo abundance matching approach and halo mass function extracted from cosmological simulations to estimate virial radii for a representative sample of galaxies spanning nine orders of magnitude and stellar mass. He showed that the radius within which half of the stellar mass of each galaxy scales linearly with halo radius for both elliptical and disk galaxies (see Fig.). Such scaling was expected for disk galaxies in models in which disk has acquired its specific angular momentum during halo collapse via tidal torques, but existence of similar scaling for spheroidal galaxies is a surprise. The results of this study imply that galaxy sizes and radial distribution of baryons are shaped primarily by properties of their parent halos and that the sizes of both late-type disks and early-type spheroids are controlled by halo angular momentum. These results show a remarkable agreement of properties of observed galaxy population with the basic expectations of how galaxies and their host dark matter halos are expected to form in the standard Cold Dark Matter scenario of structure formation.Learn more >>

The South Pole Telescope Measures Small-scale Structure of the Cosmic Microwave Background to Unprecedented Precision
April 23, 2012 | Read more
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The 10-meter South Pole Telescope (SPT) is a millimeter/submillimeter-wave telescope designed to make low-noise, high-resolution images of the cosmic microwave background (CMB) radiation. The first camera on the SPT was a three-color, ~1000-element bolometer array, one of the largest and most sensitive millimeter-wave receivers ever constructed. This ultra-sensitive receiver, combined with the large collecting area, large field of view, and low-noise design of the telescope---and with the pristine observing conditions available from the South Pole---enabled the SPT team to map 2500 square degrees of the southern sky at previously unattainable levels of sensitivity and angular resolution. The primary science goal of this first receiver was to make a census of distant, massive galaxy clusters and use that information to constrain the properties of Dark Energy, the mysterious agent behind the accelerating expansion of the universe. The same data used for the cluster survey, however, also provides the most precise measurement yet of the small-angular-scale CMB.
In a paper published in December in the Astrophysical Journal (R. Keisler et al., 2011 ApJ 743 28), the SPT team used approximately one third of the total 2500-square-degree survey data in just one of the three colors to make the most precise measurement yet of the region of the CMB angular power spectrum known as the damping tail. The CMB power spectrum (which measures fluctuations in CMB temperature as a function of angular scale) is predicted to have a characteristic series of peaks, caused by oscillations in the tightly coupled plasma of charged particles and photons in the early universe.

Figure 1.
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The first peaks were discovered by ground- and balloon-based experiments and have now been measured extremely well by the WMAP satellite. Subsequent peaks are more difficult to measure, both because they trace smaller angular scales, which are difficult to resolve with single-dish microwave telescopes, and because the amplitude of these peaks is predicted to decrease. This decrease in amplitude is due to the not-quite-perfect coupling between matter particles and photons, and this region of the CMB power spectrum is commonly known as the damping tail. The combined data from all previous instruments had clearly begun to resolve the third peak and hint at further peaks, but the power spectrum measured by SPT clearly shows at least seven peaks (see Figure 1).

Figure 2.
The new SPT measurements have led to improved cosmological constraints as well as hints of some surprising results. The added cosmological power is due to the sensitivity of the amplitude and shape of the damping tail to physics from the very early universe through the time of emission of the CMB. Any process that changes the amplitude of small-scale fluctuations or that affects the balance of free and bound electrons near the time of the emission of the CMB will show its effects in the damping tail. For example, the period of exponential inflation that we believe expanded a tiny patch of spacetime into our observable universe leaves an imprint on the CMB in the ratio of large-to-small-scale fluctuations, characterized as ns, the spectral index. When combined with WMAP data (see Figure 2), SPT data improves the constraint on ns and gives strong evidence that a period of inflation did, in fact, occur.

A slightly surprising result from the SPT measurement is that there appears to be stronger damping than the simple 'concordance' cosmological model would predict. This excess damping could be explained by a higher fraction of primordial helium than is indicated by local measurements, a non-standard number of relativistic particle species in the early universe, or a non-standard shape of the primordial power spectrum. If any of these scenarios is borne out, it would have profound implications for fields of physics well beyond CMB studies and cosmology.
The SPT team is currently working on a measurement of the CMB power spectrum from the full 2500-square-degree survey, and the team recently installed a new, even more sensitive camera on the telescope (one capable of measuring not just the intensity of the CMB radiation but also its polarization properties). Results from the SPT in the near future will further exploit the promise of cosmological observations to directly constraining fundamental physics.
The SPT is a collaboration among scientists at several institutions including the University of Chicago / KICP, Argonne National Laboratory, Cardiff University, Case Western Reserve University, Harvard University, Ludwig-Maximilians-Universit, Smithsonian Astrophysical Observatory, McGill University, University of California at Berkeley, University of California at Davis, University of Colorado at Boulder and the University of Michigan.
The SPT is funded primarily by the NSF Office of Polar Programs. Partial support is also provided by the NSF-funded Physics Frontier Center of the Kavli Institute for Cosmological Physics, the Kavli Foundation and the Gordon and Betty Moore Foundation.

KICP members participating in the South Pole Telescope collaboration include KICP faculty John Carlstrom (PI), Mike Gladders, Wayne Hu, Andrey Kravtsov, and Steve Meyer; senior researchers Clarence Chang, Tom Crawford, Erik Leitch, and Kathryn Schaffer; postdocs Brad Benson, F. William High, Steven Hoover, Ryan Keisler, Jared Mehl, and Tom Plagge; and graduate students Lindsey Bleem, Abby Crites, Monica Mocanu, Tyler Natoli, and Kyle Story.

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