Cosmology & the Early Universe

The Cosmic Microwave Background

13.8 billion years ago the universe was ~1100 times smaller and hotter than it is today. Atoms were ionized; massively overdense structures such as planets, stars, and galaxies had not yet formed; and the universe was a much simpler place. Today we can observe the early universe and measure its physical properties by studying the relic thermal radiation from that era: the Cosmic Microwave Background (CMB). The frequency spectrum of the CMB and the very slight variations in its temperature from point to point on the sky encode a wealth of information about the early universe and fundamental physics. The CMB also acts as a "backlight" to the entire photon-observable universe, illuminating the evolution of cosmic structures. Through a variety of mechanisms these cosmic structures leave an imprint on the CMB.

When the CMB photons that we see today were emitted their spectral signature was that of a 3,000 K black body, qualitatively similar to the broad-band spectrum of our sun which has an effective temperature ~5,800 K. During the subsequent 13.8 billion years the cosmological expansion of space time has redshifted the spectrum of the CMB into that of a ~3 Kelvin black body. This signal was first discovered (serendipitously) in 1964 by Arno Penzias and Bob Wilson who noticed a 3 Kelvin excess signal while characterizing a 4 GHz horn antenna built by AT&T Bell Labs for satellite communications experiments. This 3 Kelvin signal, at the accuracy that Penzias and Wilson achieved, had the same intensity in all directions on the sky--- a key property which relic thermal radiation from the universe is expected to have. In the early 1990s the COBE satellite accurately measured the CMB spectrum from 30 GHz through 700 GHz. These data showed the most perfect thermal black body spectrum yet measured in nature. Fitting the observed CMB spectrum to the theoretical black body curve yields a temperature of 2.725 +/- 0.001 K. This places the spectral peak of the CMB intensity (per unit frequency) at 160 GHz, corresponding to a wavelength of 1.9 mm.

Even more precise measurements reveal that there are slight (~100 micro-Kelvin or less) variations in the thermodynamic temperature of the CMB from point to point on the sky. These variations, called anisotropies, present a snapshot of the seeds from which the present day structures in the universe formed. Theoretical models predict that the anisotropies will have a number of features, including: an angular power spectrum with a strong peak at an angular scale corresponding to the size of the sound horizon when the CMB was formed; a harmonic series of peaks marching out to smaller angular scales; and a reduction in the amplitude of anisotropies going from larger to smaller angular scales.

Above: Sensitive 60 GHz HEMT amplifier developed at NRAO's Central Development Laboratory for the WMAP satellite Above: The Cosmic Background Imager, a compact CMB interferometer operated from the Atacama desert of Chile
050128quad-from-crane.jpg Boomerang Telescope.jpeg
Above: the QUaD telescope in its ground screen at the South Pole Above: the balloon-borne BOOMERANG experiment at launch.
Figure 1  

CMB anisotropies were first detected by COBE in 1992. Precise measuring their angular power spectrum has been the focus of intense work by numerous research groups. Most of these investigations used ground- and balloon-based radio and millimeter wave telescopes; a few of these experiments are shown in Figure 1. Since the angular resolutions of greatest interest are modest it was possible to use physically small antennas, enabling small groups to field experiments using a wide variety of innovative experimental techniques. The Wilkinson Microwave Anisotropy Probe (WMAP) satellite, using cryogenic HEMT amplifiers developed and built at NRAO, produced the definitive all-sky anisotropy map down to about 10' resolution. The COBE and WMAP all-sky maps of the CMB are shown in Figure 2.

dmr-4year-clipped.png 101080 7yrFullSky WMAP 1280B.png
COBE 4-year map of CMB anisotropies (Galactic plane excised, 10 degree resolution). WMAP map of CMB anisotropies (~10 arcminute resolution).
Figure 2  

The CMB intrinsic anisotropy power spectrum produced from 7 years of WMAP data is shown in Figure 3. Comparing these data and other CMB data like them to theoretical models has yielded a wealth of fundamental insights about our Universe, including:
  • Confirming the Cosmological Constant (or Dark Energy), Omega_Lambda = 0.73 +/- 0.03, independent of standard candle supernova measurements
  • measuring the equation of state of Dark energy and finding it to be within 15% (1 sigma) of the expectation for a cosmological constant (w= -1.10 +/- 0.14)
  • measuring the dark matter and baryonic matter cosmic densities
  • measuring the line-of-sight optical depth from our current vantage point to the epoch when the CMB formed (tau=0.088 +/- 0.015). This measures the total free electron content along the line of sight and therefore directly constrains models of reionization, implying reionization occured at a redshift z=10.6 +/- 1.2 under the simplest scenarios.
  • measurement the primordial helium abundance, Yp=0.326 +/- 0.075
  • confirming several predictions of inflation, including: flatness; approximate scale-invariance of the primordial spectrum of density fluctuations (with the expected slight red tilt); and the existence of a specific correlation between the CMB temperature and polarization signals on angular scales just larger than the first acoustic peak.
The interested reader can refer to Komatsu et al. (2009, 2011) and Spergel et al. (2003) for excellent discussions of these constraints and the nuances of the cosmological parameter analysis.

111133 7yr PowerSpectrumM.jpg fig4wmap7actSptOthers.png
WMAP 7-year CMB power spectrum Small angular scale CMB power spectrum results from the ACT, SPT, QUaD, and ACBAR experiments (from Shirokoff et al. 2010).
Figure 3  

There is a great deal of current interest in making even more precise measurements of CMB anisotropies. Ongoing efforts have two main thrusts: measuring the polarization of CMB anisotropies, and refining the smaller angular scale (high angular resolution) measurements of CMB anisotropies. The current state of the art in extending CMB anisotropy measurements to small angular scales is shown in Figure 3. Accurate small-angular-scale anisotropy measurements are sensitive to large scale structures at low redshift through both the Sunyaev-zel'dovich Effect (SZE, discussed below) and gravitational lensing of CMB anisotropies, which tends to slightly wash out features in the angular power spectrum. Gravitational lensing by galaxy clusters and superclusters also affects the polarization of the CMB by converting "E-modes" (the only polarization pattern generated causally at the last scattering surface) into the orthogonal B-mode polarization pattern. Inflationary theories predict a large-scale (> 2 degrees) B-mode polarized anisotropy signal. If detected this would provide further evidence of an epoch of inflationary cosmic expansion; its amplitude would provide important constraints on the fundamental physics governing inflation. The intrinsic CMB polarization anisotropies are typically 2 or more orders of magnitude weaker than the total intensity anisotropies, depending on angular scale. Because of this it is essential to precisely understand CMB foregrounds arising from synchrotron, dust, and other radiation emanating from our own and other galaxies. ALMA, EVLA, and other large telescopes will play vital roles in these studies.

The cosmology and high energy physics communities are eagerly awaiting power spectrum results from the PLANCK satellite. PLANCK, launched in 2009, has mapped the whole sky at 3 times the angular resolution of WMAP, with 10 times the sensitivity, and in 9 frequency bands ranging from 30 GHz to almost 900 GHz. Like WMAP it is sensitive to both total intensity and polarization. Early science results from PLANCK include discovery of 20 new clusters of galaxies via the SZE, catalogs of newly discovered extragalactic sources and cold dust clouds in our own Galaxy, and the most precise measurements of "spinning dust" emission in individual objects to date. The final PLANCK CMB power spectrum will be sensitive enough to test a wide variety of basic cosmological assumptions, break parameter degeneracies (reducing dependence on external datasets), and dramatically improve cosmological parameter constraints. The PLANCK maps of Galactic emission will also be vital for current and next generation polarization experiments which would otherwise be foreground-limited.

Galaxy Clusters and the Sunyaev-Zel'dovich Effect

Galaxy clusters are the largest virialized structures in the universe, the astrophysics of which is interesting in its own right. Furthermore their abundance as a function of mass and redshift offers important constraints on cosmology. The CMB provides a valuable means of studying both through the Sunyaev-Zel'dovich Effect, or SZE. The SZE is a distortion in the spectrum of the CMB created by the hot (5 - 10 keV) plasma which fills the gravitational potential wells of clusters. This hot plasma is known as the "Intra-Cluster Medium" or ICM. At radio and long millimeter wavelengths the SZE appears as a decrement or shadow in the CMB at the location of the cluster in the sky. The depth or intensity of this shadow directly measures the line-of-sight integral of the thermal pressure in the ionized ICM.

One interesting and useful property of the SZE is that the depth of the decrement depends only on the physical state of the ICM and not, say, on the distance between the observer and the cluster. This distinguishes the SZE from, for instance, measurements of the optical or x-ray surface brightness emitted by cosmological objects, both of which suffer (1 + z)^4 surface brightness dimming due to cosmic expansion. The ICM can also be studied by its thermal bremstrahlung emission in the X-ray band. Comparing these two data sets yield complementary information, with the x-rays generally more sensitive to the plasma electron density, and the SZE generally more sensitive to the hottest thermal electrons. Comparing the SZE and the X-ray surface brightnesses also gives a direct measurement of the Hubble constant when the cluster redshift is known.

Advances in millimeter wave instrumentation in the past few years--- principally large, low-noise bolometer arrays--- have made it possible to conduct large (thousands of square degrees) sky surveys sufficiently sensitive to find galaxy clusters using the Sunyaev-Zel'dovich Effect. These surveys have so far discovered dozens of massive, high-redshift galaxy clusters which were missed by previous optical and x-ray based techniques. By expanding the surveys, as well as analyzing already-collected but not yet processed data, hundreds more will be found. Due to the redshift independence of the SZE these samples are not biased towards low redshift but will instead find clusters at whatever redshift they exist. Since the SZE is proportional to the thermal pressure in the ICM, which is in turn typically in rough virial equilibrium, the SZE data also provide a relatively straightforward means to estimate the total mass of each cluster. The two leading SZE survey instruments are the South Pole Telescope (SPT) and the Atacama Cosmology Telescope (ACT).

High angular resolution is a new frontier in observations of the SZE. Traditionally SZE observations have been done with small single-dish telescopes or hyper-compact CMB interferometers having beam sizes of an arcminute or more. Bolometer arrays on much larger single dishes (such as the 100-m GBT, the 45-meter Nobeyama telescope, and the 30-meter IRAM antenna)--- as well as more conventional compact interferometers with modern digital backends such as CARMA--- are making detailed studies of the ICM using the Sunyaev-Zel'dovich Effect possible. Two recent results from the MUSTANG bolometer camera on the GBT are shown in Figure 4. These observations reveal local departures from equilibrium that were only hinted at in previous X-ray observations. These disturbances-- in these cases due to ongoing sub-cluster mergers-- can bias SZE surveys, the interpretation of which is already limited by astrophysical assumptions that need to be made about the ICM. High resolution SZE observations are also capable of probing in detail the astrophysical mechanisms responsible for the observed structure and evolution of the ICM. These investigations require larger and more sensitive bolometer arrays on large single-dish telescopes like the GBT. In the southern hemisphere ALMA, particularly at its longest wavelengths (~1 cm), will also be very capable.

korngut1347jybm.png macs0744wxray.png
Image of the SZE in galaxy cluster RXJ1347-1145 at 10" resolution made with the GBT (from Mason et al. 2010) Composite SZE (pink) and X-ray (blue) image of galaxy cluster MACSJ0744+3927 showing a newly revealed shock near the core of this cluster (from Korngut et al. 2011).
Figure 4  

Further Reading

More information about the SZE and CMB cosmology can be found in the following resources and references cited therein:
  • "Cosmic Microwave Background Mini-Review" D. Scott & G. Smoot Review of Particle Physics (2010; also arXiv:1005.0555)
  • "The Sunyaev-Zel'dovich Effect" M. Birkinshaw, Physics Reports 310, 97 (1999)
  • "Cosmology with the Sunyaev-Zel'dovich Effect" J. Carlstrom, G. Holder, & E. Reese ARA&A 40, 643 (2002)
  • "Modern Cosmology" Scott Dodelson (Academic Press, New York, 2003)

-- BrianMason - 2012-04-19
Topic attachments
I Attachment Action Size Date Who Comment
050128quad-from-crane.jpgjpg 050128quad-from-crane.jpg manage 1 MB 2012-04-26 - 13:49 BrianMason QUaD telescope in its ground shield at the South Pole
101079_1024.jpgjpg 101079_1024.jpg manage 303 K 2012-04-19 - 16:20 BrianMason stacked hot & cold spots, in temperature and polarization, from WMAP-7 data. from http://map.gsfc.nasa.gov/media/101079/ retrieved 4/19/2012
101080_7yrFullSky_WMAP_1280B.pngpng 101080_7yrFullSky_WMAP_1280B.png manage 359 K 2012-04-19 - 15:55 BrianMason WMAP 7 year all-sky map of the CMB (foreground-subtracted) http://map.gsfc.nasa.gov/media/101080/ (retrieved 4/19/2012)
111133_7yr_PowerSpectrumM.jpgjpg 111133_7yr_PowerSpectrumM.jpg manage 98 K 2012-04-20 - 14:57 BrianMason WMAP 7-year power spectrum , from http://map.gsfc.nasa.gov/media/111133/ (retrieved 4/20/2012)
Amp_V4.jpgjpg Amp_V4.jpg manage 57 K 2012-04-20 - 16:43 BrianMason WMAP V-band (60 GHz) HEMT amplifier, from http://map.gsfc.nasa.gov/mission/observatory.html retrieved 4/20/2012
Boomerang_Telescope.jpegjpeg Boomerang_Telescope.jpeg manage 39 K 2012-04-26 - 13:51 BrianMason BOOMERANG being launched
cbi-frontview.jpgjpg cbi-frontview.jpg manage 106 K 2012-04-20 - 16:42 BrianMason the Cosmic Background Imager, from http://www.astro.caltech.edu/~tjp/CBI/pictures/ retrieved 4/20/2012
cmb_fluctuations_big.gifgif cmb_fluctuations_big.gif manage 380 K 2012-04-19 - 15:53 BrianMason COBE map, from http://cosmos.lbl.gov/Images/
dmr-4year-clipped.pngpng dmr-4year-clipped.png manage 68 K 2012-04-20 - 16:21 BrianMason COBE DMR 4-year map with Galactic plane excised from http://lambda.gsfc.nasa.gov/product/cobe/dmr_image.cfm (retrieved 4/20/2012)
fig4wmap7actSptOthers.pngpng fig4wmap7actSptOthers.png manage 48 K 2012-04-20 - 14:52 BrianMason current state of the art in small scale CMB meas'ts, from http://pole.uchicago.edu/public/data/shirokoff10/ (Shirokoff et al. 2010), retrieved 4/20/2012
firas_spectrum.jpgjpg firas_spectrum.jpg manage 54 K 2012-04-19 - 15:58 BrianMason spectrum of the CMB monopole from COBE FIRAS, from http://lambda.gsfc.nasa.gov/product/cobe/cobe_images/ (retrieved 4/19/2012)
korngut1347.pngpng korngut1347.png manage 221 K 2012-04-20 - 17:43 BrianMason MUSTANG image of the SZE in RXJ1347 from Korngut et al. (2011)
korngut1347jybm.pngpng korngut1347jybm.png manage 153 K 2012-04-20 - 17:48 BrianMason image of the SZE in RXJ1347-1145 made with MUSTANG on the GBT (Korngut et al. 2011, jy/bm)
korngut1347snr.pngpng korngut1347snr.png manage 65 K 2012-04-20 - 17:48 BrianMason image of the SNR , rxj1347 (Korngut et al. 2011)
macs0744jybm.pngpng macs0744jybm.png manage 293 K 2012-04-20 - 17:49 BrianMason image of the SZE in MACS0744+3927 made with MUSTANG (Korngut et al. 2011)
macs0744wxray.pngpng macs0744wxray.png manage 249 K 2012-04-20 - 17:49 BrianMason 0744 mustang + xray, korngut et al. 2011
Topic revision: r9 - 2012-05-01, BrianMason
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