COSMOLOGY Exploring the Cosmic Microwave Background All-Encompassing Light from the Early Universe SHAUN AKHTAR

t every moment, night and day, Smoot and John Mather received a presentation speech for Mather and the Earth is bombarded from all the Nobel Prize in Physics for their Smoot’s 2006 Nobel Prize, Dr. Per Carlson directions with microwave-range contributions to the instruments aboard stated that Mather and Smoot’s research Aradiation. !is radiation is characteristic the Cosmic Background Explorer (COBE). developed the foundation for cosmology’s of a black-body, an opaque object with a COBE collected data that conclusively establishment as a “precision science” (6). equilibrated temperature of approximately demonstrated the spectrum of the CMB !e anisotropies of the CMB can be 2.725 degrees Kelvin (1). Arno Penzias to be remarkably well-described by black- viewed in the form of an angular power and Robert Wilson, then working at the body radiation (Fig. 2), supporting the idea spectrum (Fig. 3). Radiation angular power Bell Labs accidentally discovered this that this background was a remnant of a spectrums represent the state of oscillations continuous background signal in 1964. largely homogeneous early universe. !is in the &uid of photons, electrons, protons, !ey "rst described it as an “excess antenna result con"rmed that mathematical models and neutrons immediately preceding temperature” they could not account when could be made with some con"dence about decoupling and the release of the calibrating their instruments (2). A fellow the expansion of the universe. In addition, background radiation. Fortunately for team of researchers at Princeton University, COBE revealed that the CMB contained cosmologists, the science determining led by Robert H. Dicke, soon realized that small-scale temperature &uctuations, called such oscillations is su#ciently understood this radiation was the remnants of the anisotropies, on the order of one part in one that power spectra can be drawn for “primordial "reball” that existed following hundred thousand. !e CMB’s anisotropies many scenarios involving varied initial the Big Bang (3). With these discoveries, were correctly predicted to originate conditions – namely, various parameters of research of the cosmic microwave with the interaction of early background the cosmological model. !ese parameters background (CMB) had begun. radiation with perturbations in the density include the Hubble constant, H0. !e and velocity of extant matter (5). !e Hubble constant is the rate of expansion The CMB and Cosmology subsequent study of these anisotropies of the universe, whose reciprocal provides allowed researchers to establish constraints an estimate for the universe’s current age. !e early universe was extremely on the cosmological parameters ascribed Another parameter is ρ , the critical crowded with highly energetic photons. crit to the current standard model (4). In density value determining the universe’s !ese photons had su#cient energy to break the bonds that may have formed between any electrons and nuclei. Since the universe was very dense at this point, atoms would be ionized almost immediately a$er combination. As a result, the early universe was full of free electrons and nuclei, with photons continually scattering o% both types of particles. As the universe expanded and cooled, the energies of photons decreased. Accordingly, their ability to regularly excite and ionize electrons from their host atoms was diminished. !is point is known as decoupling—the period in which the frequency of interaction between photons and electrons decreased precipitously. Since the time of decoupling, many photons have traveled unimpeded all the way to our telescopes and antennae. As the universe has continued to grow, their wavelengths have been redshi$ed into the microwave range of the spectrum. From Earth’s point of view, these photons composing the CMB appear to originate from the edge of a sphere. !is edge is called the surface of last scattering (4). Photo courtesy of NASA In 2006, astrophysicists George Figure 1: The cosmic microwave background was first detected by a horn antenna seeking satellite- created radio waves at the Bell Labs in Holmdel, New Jersey.

12 DARTMOUTH UNDERGRADUATE JOURNAL OF SCIENCE spatial geometry. A third parameter is the cosmological constant Λ, which represents the pure energy available in a vacuum. It has recently been associated with the concept of “dark energy,” which appears to be driving an acceleration in the universe’s rate of expansion (7).

Historical Breakthroughs Research into the CMB has produced both theoretical and observational breakthroughs over the past half-century. During the 1970s, Rashid Sunyaev and Yakov Zel’dovich anticipated, researched, and con"rmed Inverse Compton Scattering as the major cause of anisotropy in the CMB. Inverse Compton Scattering, which involves the transfer of energy from high- energy electrons to photons, takes place to a noticeable extent in hot electron gas. Electron gasses are heavily present in intergalactic regions within galactic clusters. Perturbations of the CMB caused by Inverse Compton Scattering, known Image courtesy of NASA Figure 2: Measurements from the Cosmic Background Explorer (COBE) showed that the intensity of the as the Sunyaev–Zel’dovich e%ect, can background radiation was a precise match to the black-body spectrum predicted by the Big Bang theory. therefore point astronomers toward the location of faraway galactic clusters. (8). Chile, and the Planck satellite, operated by temperature shi$ in passing CMB photons Understanding the apparent black-body the European Space Agency (ESA) since proportional to the relative velocity of the spectrum of the CMB thus helped select the 2009. galactic cluster. It is much less noticeable Big Bang model as the preferred model for than the more standard thermal Sunyaev– describing universal origins (9). Atacama Cosmology Telescope Zel’dovich e%ect in high-mass clusters (13). Further breakthroughs derived from ACT In 2001, the Wilkinson Microwave !e Atacama Cosmology Telescope data are expected in coming years. Anisotropy Probe (WMAP) was launched (ACT) is a six-meter re&ecting telescope as a successor to COBE. Over seven years, resting over 5,100 meters above sea level The Planck Observatory WMAP plotted a full-sky map of the in the Chilean mountains. It is focused CMB, and provided the best look yet at its on three bands in the microwave range. !e Planck satellite was launched angular power spectrum. !e results of the It is tasked with both improving current by the ESA with the goal of mapping the WMAP data correspond most closely to a estimates of cosmological parameters and CMB across 95 percent of the sky. !e cosmological model known as Λ-Cold Dark observing faraway galactic clusters and ESA boasted that Planck was intended to Matter, or Λ-CDM. !is model assumes their local environments (11). In 2011, improve upon COBE’s measurement of the presence of dark energy and “cold” researchers operating the ACT reported that temperature variations by a factor of ten, and (non-relativistic) dark matter. WMAP telescopic data had provided evidence for re"ne its angular resolution of the data by a provides not only improved estimates the existence of dark energy derived solely factor of "$y. In addition to strengthening for cosmological constants, yielding high from observation of the CMB. Analysis of the estimates for a number of cosmological con"dence intervals for these values. the background radiation’s power spectrum parameters, the Planck team hoped to Highlights of these updated parameters cannot simultaneously constrain values determine whether certain anisotropies includ: the current age of the universe at for the universe’s curvature value and rate could be attributed to gravitational waves 13.75 ± 0.11 billion years; dark energy of expansion. However, ACT detection in the early universe. Such results would density, set to 72.8 ± 0.16 percent of the of radio sources revealed gravitational support the theory of cosmic in&ation, critical density; and the age of the universe lensing, or path distortion due to the pull of which holds that the universe experienced at decoupling, at 377,730 ± 3,205 years (10). massive objects, a%ecting photons from the a period of exponentially rapid expansion CMB. !is lensing data, combined with the shortly a$er the Big Bang, converting Current Research power spectrum, is su#cient to provide an small-scale density &uctuations into the A number of new projects are estimate for the density of dark energy (12). seeds for the massive structures that were underway to investigate the properties of In March, scientists announced that ACT to later develop. Another goal of the Planck the CMB and re"ne current estimates. Two sky maps had led to the "rst measurement of project was to compare structures found of the most notable current projects include galaxy cluster motions using the kinematic in the high-resolution CMB maps with the Atacama Cosmology Telescope, located variety of the Sunyaev–Zel’dovich e%ect. surveys of millions of known galaxies, in an in elevated desert land on Cerro Toco in !is component of the e%ect induces a attempt to connect the dots in the evolution

SPRING 2012 13 Image courtesy of NASA Figure 3: Recent data, including that gathered by the Wilkinson Microwave Anisotropy Probe (WMAP), have placed constraints on the background’s angular power spectrum, and, with it, a number of cosmological parameters. of galactic clusters over time (14). wealth of observational resources at their Dec 2006. Early results from the instruments disposal for investigating this signal from 7. D. Scott, Can. J. Phys. 84, 419-435 (2006). 8. Y. Rephaeli, Annu. Rev. Astron. Astrophys 33, 541- aboard Planck were "rst released in the early universe. Further re"ned analysis 580 (1995). December of last year, through 26 papers of the CMB will provide greater insights into 9. P. J. E. Peebles, D. N. Schramm, E. L. Turner, R. G. published in Astronomy & Astrophysics. the details of the infancy of the cosmos, and Kron, Nature 352, 769-776 (1991). Detection of the Sunyaev–Zel’dovich e%ect will re&ect a greater understanding of the 10. N. Jarosik, et al., Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) indicated almost 200 potential galactic constraints that have driven our universe Observations: Sky Maps, Systematic Errors, and Basic clusters, many of which were con"rmed by over the past 13.7 billion years. Results (2010). Available at http://lambda.gsfc.nasa. additional observation. Planck results have gov/product/map/dr4/pub_papers/sevenyear/basic_ also provided great detail of the cosmic CONTACT SHAUN AKHTAR AT results/wmap_7yr_basic_results.pdf (04 April 2012). 3(!5.9!+(4!2 $!24-/54(%$5 11. ACT Atacama Cosmology Telescope, About Us infrared background, the radiation emitted (2012). Available at http://www.princeton.edu/act/ from heated dust particles surrounding about/ (4 Apr 2012). stars. However, data on the CMB remains References 12. B. D. Sherwin, et al., Phys. Rev. Lett. 107, 021302 unavailable while the project team isolates (2011). 1. D. J. Fixsen, Astrophys. J. 707, 916-920 (2009). 13. N. Hand, et al., Phys. Rev. Lett., in press (available background radiation from stronger 2. A. A. Penzias, R. W. Wilson, Astrophys. J. 142, 419- at http://arxiv.org/pdf/1203.4219v1.pdf). sources of other origins. Planck’s most 421 (1965). 14. U. Seljak, Nature 482, 475-477 (2012). exciting data therefore awaits a 2013 release 3. R. H. Dicke, P. J. E. Peebles, P. G. Roll, D. T. 15. European Space Agency, Objectives (2010). (15). Wilkinson, Astrophys. J. 142, 414-419 (1965). Available at http://sci.esa.int/science-e/www/object/ 4. A. Liddle, An Introduction to Modern Cosmology index.cfm?fobjectid=30968 (4 April 2012). (Wiley, West Sussex, ed. 2, 2010). Conclusion 5. P. D. Naselsky, D. I. Novikov, I. D. Novikov, !e Physics of the Cosmic Microwave Background As investigation of the cosmic (Cambridge Univ. Press, Cambridge, 2006). microwave background approaches its 6. P. Carlson. Speech given at the presentation of the second half-century, cosmologists have a 2006 Nobel Prize in Physics, Stockholm, Sweden, 10

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