WMAP Mission

Overview ( 1 )

The Wilkinson Microwave Anisotropy Probe (WMAP) mission, operational from 2001 to 2010, meticulously mapped the cosmic microwave background radiation across the celestial sphere to unveil insights into the early universe's conditions. This radiation, emitted approximately 375,000 years post-Big Bang, revealed minute temperature variations (anisotropy) discerned by WMAP from opposing directions, offering profound insights into the universe's fundamental fabric. Propelled into space on a lunar-assisted trajectory, WMAP journeyed to the Sun-Earth L2 libration point, where it conducted observations shielded from solar, lunar, and terrestrial influences. Equipped with passively cooled microwave radiometers and employing differential measurement techniques, WMAP meticulously gauged subtle temperature fluctuations in the microwave sky. With five frequency bands spanning 22 to 90 GHz, the instrument adeptly differentiated between galactic foreground signals and cosmic background radiation. Employing a differential approach akin to comparing adjacent objects, WMAP assessed temperature disparities across the celestial vault. Nestled in a stable thermal environment near L2, the observatory executed rapid sky scans, facilitating comprehensive sky coverage with minimal observational hindrances. These meticulous measurements, refining COBE's pioneering work, deciphered intricate temperature fluctuations with unprecedented fidelity, unveiling nuances pivotal for elucidating the universe's dimensions, constituents, age, geometry, and ultimate destiny. Such revelations furnished invaluable insights into the universe's primal architecture, shedding light on the genesis of galaxies and offering empirical validations for cosmological theories.

Overview ( 2 )

The Wilkinson Microwave Anisotropy Probe (WMAP), honoring Dr. David Wilkinson's contributions to cosmic background radiation research, endeavors to precisely measure the relative Cosmic Microwave Background (CMB) temperature across the sky with high angular resolution and sensitivity. Its design emphasizes controlling systematic errors, aiming for accuracy. WMAP employs differential microwave radiometers to detect temperature differences between two celestial points while orbiting around the L2 Sun-Earth Lagrange point, 1.5 million kilometers away, ensuring a stable observational environment shielded from solar, lunar, and terrestrial interference. With systematic scans covering about 30% of the sky daily and a full-sky survey completed every six months, WMAP utilizes five frequency bands from 22 to 90 GHz to discern foreground signals from the Milky Way effectively. The spacecraft features back-to-back telescopes directing microwave radiation from two sky locations, processed by ten differential receivers beneath the optics assembly. Extensive radiators facilitate cooling for sensitive amplifiers, while essential services for mission operations, including command systems and propulsion, are housed in the lower section. Continuous shading is maintained by a deployable sun shield, ensuring uninterrupted observations while supporting the solar panels.

Specifications

Here are the high-level technical specifications for the WMAP instrument and observatory: The WMAP instrument operates at frequencies ranging from 22 to 90 GHz, corresponding to wavelengths between 13.6 mm and 3.3 mm. It comprises a total of 40 channels distributed across these frequencies, with varying resolutions from 0.93 to <0.23 FWHM degrees and a sensitivity of approximately 35 µK per 0.3° x 0.3° pixel. Utilizing a differential pseudo-correlation method with polarization, the radiometer system employs dual Gregorian reflectors measuring 1.4 meters by 1.6 meters each, all enclosed within a composite/aluminum structure. The focal plane covers a field of view measuring 3.5° x 3.5°, with a pointing accuracy of 0.6° control in elevation and 1.8' knowledge. On the other hand, the WMAP observatory, with a mass of 840 kg, operates primarily on a single-string redundancy system with a planned lifetime of 27 months extendable to over three years due to fuel constraints. Positioned in a 1-10° Lissajous orbit around the Sun-Earth L2 Lagrange point, it utilizes a lunar-assist trajectory with phasing loops for optimal operation. Communication is facilitated by two omnidirectional antennas during phasing loops and a medium-gain antenna for downlinks after lunar flybys. Data transmission occurs once daily via the Deep Space Network, providing a data rate of 667 kbps to a 70 m dish for 16 minutes per day. Propulsion is achieved through blow-down hydrazine propulsion with eight thrusters, while attitude control relies on a combination of star trackers, gyros, Sun sensors, and reaction wheels for precise scanning. Thermal management involves passive cooling with radiators, aided by solar array shading, and the spacecraft structure is composed of composite/aluminum materials. With a power supply generating 419 W from a 3.1 m2 GaAs/Ge array angled 22.5° off full Sun, the observatory operates without eclipses, supported by a 23 A-hr NiH battery.

Optics

The WMAP instrument is comprised of two symmetrical telescopes producing focal planes A and B on opposite sides of the spacecraft axis. Each focal plane contains ten corrugated feeds transporting power to the amplification electronics. The reflector design features two off-axis Gregorian telescopes with primary reflectors measuring 1.4 m x 1.6 m, ensuring sufficient focal plane area while meeting spacecraft constraints. The primary reflectors, shaped elliptically, produce convex focal planes with plate scales of ~15'/cm. Constructed from composite materials and coated with aluminum and silicon oxide, the reflectors are mounted on a carbon-composite truss structure to minimize mass and cool-down shrinkage. Diffraction shields limit diffracted signals, while solar panels and insulation act as a Sun shield. The feed layouts for each focal plane include pairs at 22, 30, 40, 60, and 90 GHz, with designs optimized for minimal aperture and length. The feeds are corrugated to produce symmetric beams with low sidelobes, ensuring frequency-independent beam characteristics. Extensive modeling and testing, including chamber and rooftop measurements, validate the optical performance of the system, crucial for processing flight data accurately.

Receivers

The WMAP Microwave System comprises ten 4-channel differencing assemblies, each receiving signals from a pair of feeds and producing output proportional to the temperature difference between two sky sightlines. This setup improves sensitivity and stability by dividing the system into cold and warm sections. The cold portion, situated in the Focal Plane Assembly (FPA) box, is passively cooled by radiators, while the warm section, housed in the Receiver Box (RXB) below the FPA, completes signal amplification. Differencing assemblies operate at various frequencies, and each pair of feeds undergoes differencing via hybrid Tee circuits, amplification in cold and warm HEMT amplifiers, phase switching, and subsequent signal combining. Custom-built HEMT amplifiers, phase switches, and bandpass filters enhance performance, with gain fluctuations minimized by warm amplifier stages. The Analog Electronics Unit (AEU) processes electrical signals from the differencing assemblies into digital form for spacecraft data handling. It includes high-pass and low-pass filtering, synchronous demodulation, and voltage-to-frequency conversion. The DEU provides timing signals for phase switching, demodulation, and signal integration, enabling data co-addition before compression and telemetry. The Power Distribution Unit (PDU) regulates instrument power, with feedback control to ensure proper HEMT device operation. On/off commands allow selective power removal from individual radiometers to maintain operational integrity.

Frequency

WMAP's frequency coverage is essential for distinguishing Galactic foreground signals from Cosmic Microwave Background (CMB) anisotropy. Operating across five frequency bands ranging from 22 to 90 GHz, WMAP observes variations in antenna temperature attributable to CMB anisotropy and known Galactic foreground emissions, including synchrotron radiation, free-free radiation, and thermal radiation from interstellar dust. The K and Ka bands, operating at lower frequencies, provide valuable insights into galactic emissions. WMAP employs two techniques to evaluate and remove Galactic foreground: template fitting using existing Galactic maps and linear combinations of multi-frequency observations. These methods account for uncertainties and spectral variations, enabling accurate extraction of CMB signals. The choice of five frequency bands with comparable sensitivity ensures robust detection of synchrotron, free-free, dust, and CMB anisotropy signals, thereby enhancing our understanding of the cosmic microwave background.

Resolution

WMAP's angular resolution is optimized to map the entire sky with precision, prioritizing regions where high cosmological returns are expected. The WMAP optics, featuring back-to-back 1.4 m x 1.6 m primary reflectors, achieve remarkable angular resolutions, especially evident in the highest frequency (90 GHz) channel, where it reaches below 0.25°. The angular resolution across the five frequency bands is as follows: At 22 GHz, it is 0.93°; at 30 GHz, 0.68°; at 40 GHz, 0.53°; at 60 GHz, 0.35°; and at 90 GHz, it is less than 0.23°. These values represent the full width at half maximum (FWHM) of the central beam lobe, ensuring precise measurements essential for understanding cosmic microwave background anisotropy. The accuracy of these measurements is further validated through observations of celestial bodies like Jupiter during the mission.

Sensitivity

The WMAP specification calls for an equal noise sensitivity per frequency band of ~35 µK per 0.3° x 0.3° square pixel. The mission duration required to meet this specification is two years of continuous observation. If Galactic emission is negligible at high latitudes above 40 GHz, as was the case for COBE, the sensitivity achievable by combining the three highest frequency channels is ~20 µK per 0.3° x 0.3° pixel. The sensitivity for each frequency band is as follows: 22 GHz: ~35 µK, 30 GHz: ~35 µK, 40 GHz: ~35 µK, 60 GHz: ~35 µK, 90 GHz: ~35 µK. These values highlight the uniformity in sensitivity across the frequency bands, ensuring consistent and reliable data collection. Additionally, the sensitivity to the angular power spectrum is illustrated in a plot showcasing the anisotropy signal amplitude as a function of angular scale. Various cosmological models are represented, providing insights into the universe's structure across different angular resolutions.

Orbit

To optimize observing efficiency while minimizing environmental disturbances, WMAP operates from a Lissajous orbit around the L2 Sun-Earth Lagrange point, situated 1.5 million km from Earth. The trajectory to reach this observation point involved three lunar phasing loops followed by approximately 100 days of cruising to L2. L2, one of the Lagrange points, represents an optimal location for conducting observations of the Cosmic Microwave Background (CMB). Positioned at a distance of 1.5 million km from Earth, it offers significant advantages, including minimal interference from Earth's microwave emission, magnetic fields, and other disruptions. Additionally, it provides a stable thermal environment and near 100% observing efficiency, as the Sun, Earth, and Moon remain consistently outside the instrument's field of view. The trajectory to L2 involves three or five lunar phasing loops, depending on the lunar cycle at the time of launch. WMAP utilized three loops, followed by a lunar flyby to aid in reaching L2. The journey from the phasing loops to L2 takes approximately 100 days. Launch windows for this trajectory occur roughly 20 minutes per day for seven consecutive days, twice each month. Once in orbit around L2, the satellite maintains a Lissajous orbit, ensuring that the WMAP-Earth vector stays between 1 and 10 degrees off the Sun-Earth vector for communication purposes while avoiding eclipses. Periodic station-keeping maneuvers, conducted about four times per year, are necessary to maintain the orbit's stability. This trajectory and orbit configuration enable WMAP to conduct uninterrupted and precise observations of the cosmic microwave background, crucial for advancing our understanding of the universe's origins and evolution.

L2

Lagrange points, named after Italian-French mathematician Joseph-Louis Lagrange, represent five special positions where a small mass can orbit in a consistent pattern with two larger masses. These points result from a gravitational balance where the pull of two large masses precisely matches the centripetal force required for a smaller object to move with them. Of these five points, three are unstable (L1, L2, and L3), situated along the line connecting the two large masses, while two are stable (L4 and L5), forming the apex of equilateral triangles with the large masses at their vertices. Notably, L4 precedes Earth's orbit, while L5 follows. The L1 point, positioned between Earth and the Sun, offers an uninterrupted solar view and hosts the Solar and Heliospheric Observatory Satellite SOHO. In contrast, the L2 point, which was occupied by WMAP and currently hosts Planck, as well as the future James Webb Space Telescope, is conducive to astronomy due to its proximity to Earth for communication, unobstructed solar power, and clear deep-space views. L1 and L2 are unstable over approximately 23 days, necessitating regular course corrections. In contrast, the L3 point, hidden behind the Sun at all times, is unlikely to find practical use, though it has inspired science fiction narratives. Stability characterizes the L4 and L5 points, provided the mass ratio between the large bodies exceeds 24.96. These points host stable orbits, known as Trojans, seen with asteroids and moons in various celestial systems. For instance, the Earth-Moon system's Trojan points were confirmed with the discovery of Trojan asteroid 2010 TK7 by NASA's WISE telescope. The configuration and dynamics of Lagrange points are best understood by adopting a rotating frame of reference, where forces on a body at rest can be inferred from an effective potential, akin to reading wind speeds from a weather map. This perspective illustrates how satellites at Lagrange points tend to drift but stabilize due to Coriolis forces, akin to a marble on a hill or saddle. This understanding provides crucial insights into celestial mechanics and spacecraft positioning.

Scan Strategy

The WMAP mission mandates comprehensive sky coverage to facilitate robust constraints on cosmological models and ensure accurate determination of low-order spherical harmonic moments. Observing the full sky is crucial for maximizing the number of independent samples and enhancing sensitivity to the angular power spectrum. WMAP accomplishes this by scanning the entire sky every six months, yielding fourfold redundancy over a two-year period. Its scan strategy is meticulously designed to minimize systematic errors and optimize observational efficiency. By rapidly scanning a large fraction of the sky and observing each pixel through multiple azimuth angles on various time scales, WMAP ensures thorough coverage while maintaining continuous shadow for passive cooling and shielding against stray signals. Differential measurements between two points on the sky demand a substantial angular separation between observing beams, ensuring sensitivity to large angular scales essential for comparing results with COBE and normalizing the angular power spectrum. WMAP's scan pattern, resembling a Spirograph® pattern, fills an annulus centered on the local solar vector, enabling more than 30% of the sky to be observed daily and ensuring coverage of the ecliptic poles every day. This scanning geometry achieves a reasonable level of azimuth coverage in each sky pixel, promoting stable sky map solutions, minimal striping, and enhanced polarization sensitivity. While not as complete as COBE's azimuth coverage, WMAP's strategy prioritizes systematic error avoidance while meeting scientific goals. Continuous revolution of the annular scan pattern around the sky ensures full sky coverage every six months, offering redundancy for stability checks and consistency verification across independent intervals.

Map Making

WMAP employs a differential approach, measuring temperature disparities between points separated by approximately 141° on the celestial sphere. These temperature differences serve as the basis for reconstructing maps of relative sky temperature, a process reminiscent of the algorithm utilized by COBE-DMR but with modifications. The iterative algorithm employed by WMAP is conceptually akin to a least squares fit, albeit without the need for matrix evaluations or inversions. In essence, WMAP measures the temperature difference, DT = T(A) - T(B), between two feeds observing different points on the sky. Given the temperature estimate T(B) from a previous iteration, WMAP derives the temperature T(A) for each map pixel as the sum of DT and T(B). This iterative process relies on observations of each pixel from various neighboring points on its ring, necessitating a meticulously planned scan strategy. WMAP's strategy ensures comprehensive sky coverage while avoiding proximity to disruptive celestial bodies like the Sun, Earth, and Moon. Extensive simulations, incorporating realistic sky signals, instrument noise, and calibration methods, validate the efficacy of this algorithm. These simulations demonstrate that after 40 iterations, map artifacts induced by the reconstruction process exhibit peak-peak amplitudes of less than 0.2 µK, even in the presence of Galactic features with peak brightness exceeding 60 mK.

Map Calibration

WMAP's Microwave System comprises ten 4-channel 'differencing assemblies,' each pair of which captures signals from feeds observing distinct sections of the sky. These assemblies output signals proportional to the temperature disparity between the two observed sky regions. The system's radiometric gain, which relates input temperature differences (in mK) to output voltage (in digital counts), requires precise calibration due to intrinsic random asymmetries in each assembly, leading to output offsets even when inputs are equal. To achieve accurate calibration, signals must be compared against known microwave sources. Fortuitously, the sky itself serves as an ideal, ever-present calibration source: the dipole effect. This phenomenon arises from the Earth's, solar system's, and galaxy's motion through the universe, akin to the Doppler shift of sound while passing a stationary source in a moving vehicle. Specifically, the Cosmic Microwave Background (CMB) exhibits a temperature disparity of 6.706 mK between opposite directions in the sky, depicted in the false-color temperature map provided by the COBE mission. Calibrating the data to reflect true temperature differences on the sky involves comparing raw output data with the signal expected from the known dipole anisotropy. This calibration process ensures the fidelity of WMAP's temperature measurements across the celestial sphere.

Pixelization

WMAP utilizes the "HEALPix" format for pixelization of its sky maps, a method developed by Gorski, Wandelt, and Hivon. This format enables the division of the celestial sphere into increasingly finer resolution patches of equal area. Software tools for handling HEALPix format maps, available from the HEALPix website, facilitate data manipulation and analysis. For instance, WMAP's sky maps are rendered in this format, allowing for precise characterization of features across the celestial sphere. An Earth topography map, composed of approximately 3,145,728 pixels with a resolution of about 7 arcminutes, illustrates the method's effectiveness. Similarly, a model of the Cosmic Microwave Background (CMB) radiation temperature anisotropy, comprising roughly 12,582,912 pixels with a resolution of approximately 3.4 arcminutes, demonstrates the format's ability to capture intricate details in cosmological datasets.

Early Universe

The cosmic microwave background (CMB) radiation, discovered in 1965 by Arno Penzias and Robert Wilson, remains a crucial window into the early universe. This radiant heat, a remnant of the Big Bang, offers invaluable insights into the physical conditions prevailing during the universe's infancy. Despite its apparent uniformity, subtle fluctuations in the CMB temperature reveal profound secrets about the universe's evolution. NASA's Cosmic Background Explorer (COBE) satellite, launched in 1992, marked a significant milestone by detecting minute temperature variations in the CMB. These fluctuations, known as anisotropy, signify variations in the density of matter during the universe's formative stages. However, COBE's relatively coarse angular resolution limited its sensitivity to large-scale fluctuations. In contrast, the Wilkinson Microwave Anisotropy Probe (WMAP), launched in June 2001, revolutionized our understanding of the CMB. With superior resolution, sensitivity, and accuracy compared to COBE, WMAP generated detailed maps of CMB temperature fluctuations. These finer fluctuations provide crucial insights into fundamental cosmological questions, potentially guiding astrophysicists toward new frontiers of inquiry. WMAP's achievements represent a significant leap forward in our quest to unravel the mysteries of the universe's origins and evolution.

Overview ( 3 )

The Big Bang theory provides a framework for understanding the universe's structure and behavior, but its details are shaped by various free parameters that must be empirically determined. These parameters govern fundamental aspects such as the universe's fate—whether it will expand indefinitely or collapse—and its composition, including the presence of enigmatic dark matter. They also dictate the universe's geometry, the formation of galaxies, and the nature of its expansion, among other factors. Given that the cosmic microwave background (CMB) radiation originates from the universe's early epochs, it serves as a unique window into these parameters' properties. This ancient radiation, pervading the entire sky, bears the imprint of the universe's primordial conditions, offering invaluable insights into its evolution. By meticulously analyzing the patterns of CMB radiation, WMAP provides precise measurements that elucidate cosmological parameters. To grasp WMAP's methodology for determining these parameters, a foundational understanding of the early universe's physical evolution and cosmological descriptions of microwave background radiation's statistical properties is necessary.

WMAP

The Big Bang theory suggests that in the early universe, temperatures exceeded 2967° Kelvin within the first 380,000 years, ionizing most of the hydrogen and creating a turbulent sea of energetic protons and electrons. This ionized gas emitted, scattered, and reabsorbed photons, forming the cosmic microwave background (CMB) radiation—a remnant of this primordial state. As long as the gas remained ionized, strong interactions among particles caused them to behave as a single fluid, influencing the propagation of sound waves through the medium. Consequently, the CMB radiation serves as a fossil record of these primordial sound waves. As the universe expanded and temperatures dropped, electrons and protons combined to form neutral hydrogen, rendering it nearly transparent to cosmic background radiation. This enabled the unimpeded propagation of radiation throughout the universe, analogous to the appearance of the sky on cloudy days, where water droplets scatter optical light but water vapor remains transparent. When WMAP observes the microwave background sky, it effectively looks back to the era when free electrons could scatter cosmic background radiation—the so-called "surface of last scatter." Any features imprinted on this surface, such as variations in brightness, persist to this day, providing direct insights into the physical conditions of the universe merely 375,000 years after the Big Bang.

Spectrum

When WMAP observes the cosmic microwave background (CMB) radiation across the entire sky, it detects nearly uniform radiation with slight variations, akin to ripples on a pond. These fluctuations, or temperature variations, arise from several factors, including density and velocity variations of the gas at the "surface of last scatter" and fluctuations in the gravitational potential of the universe along the photon path. By examining these microwave background fluctuations, scientists can glean valuable insights into the early universe. These fluctuations are measured in terms of an "angular fluctuation spectrum," which represents the amplitude of temperature fluctuations across different angular sizes. This spectrum, depicted as a wiggly line on a graph, shows a plateau at large angular scales and coherent peaks at progressively smaller scales. These features arise from various physical processes that generate differing amounts of energy at different angular scales, and they change based on the details of these physical processes, which are governed by fundamental cosmological parameters. On the largest length scales, fluctuations in the gravitational potential during the time of last scatter primarily cause temperature variations. Photons encountering varying gravitational potentials appear dimmer or cooler depending on whether they are climbing out of a potential well or descending into it. These gravitational variations are largely influenced by the density of dark matter, whose origin remains uncertain, though many cosmologists speculate that inflation shortly after the Big Bang may have played a role. At smaller angular scales, the imprint of sound waves traveling through ionized hydrogen gas becomes apparent. Before the epoch of last scatter, photons, electrons, and protons behaved as a single fluid. Consequently, regions of higher density, indicative of compression in a sound wave, appear brighter or hotter, while regions of lower density appear dimmer or cooler. When radiation decouples from the gas at the time of last scatter, these waves freeze out, resulting in observable temperature variations. Most cosmologists posit that gravitational fluctuations shortly after the Big Bang induced sound waves in the hydrogen gas. These waves, with a limited lifespan of 375,000 years, can only travel a finite distance, setting a fundamental length scale in the early universe known as the "sonic horizon." The peaks observed in the angular fluctuation spectrum correspond to these sound waves, with the first peak representing very low-frequency waves that were just starting their compression period at freeze out, while subsequent peaks represent higher-frequency waves caught in periods of rarefaction and compression. The relative heights and locations of these peaks provide valuable information about the gas properties at the time of last scatter.

Geometry

A prominent feature of the microwave background fluctuations is the presence of randomly distributed spots, each approximately 1 degree across. These spots result from sound waves traveling through the hot ionized gas in the universe at a known speed, corresponding to the speed of light divided by the square root of 3, for a specific duration of 375,000 years. Using the relationship distance = rate * time, we can deduce the distance traveled by the sound waves and thus determine the actual size of a typical hot or cold spot. By comparing the observed apparent size of these spots to their known actual size, we can derive information about both the distance to the last scattering surface and the curvature of the light path between us and this surface, which is contingent upon the geometry of the universe. With independent knowledge of the Hubble constant, we can ascertain the distance to the last scattering surface and subsequently employ the spot size to uniquely determine the universe's geometry. The apparent size of the average hot or cold spot serves as a diagnostic tool for discerning the universe's geometry. In a flat universe, straight light paths from opposite sides of a typical hot spot converge on us, depicted by the red lines in the accompanying figure. Conversely, in a universe with negative curvature, the light paths curve outward, as indicated by the gray lines. By measuring the apparent (angular) size of these spots, we can infer the trajectories of light reaching us. The main peak in the temperature spectrum, occurring around l~220, aids in determining the average apparent spot size: a flat universe exhibits this peak at this location, while a negatively curved or "open" universe shows a peak further to the right, and vice versa for a closed universe.

Electrons

The first three peaks in the temperature spectrum provide valuable insights into the matter composition of the universe. These peaks' locations and relative heights are pivotal in constraining the universe's matter content. The behavior of sound waves within the early universe fluid hinges on the relative densities of photons, atoms, and dark matter. A higher abundance of atoms, comprising protons and electrons, intensifies their response to the gravitational influence exerted by dark matter. Consequently, this amplifies the first and third acoustic peaks, which arise from the photon-atom fluid's convergence into dense regions of compressed dark matter. Conversely, the second (and fourth) acoustic peaks, generated by the fluid's outward movement, experience suppression. By scrutinizing the relative amplitudes of the even and odd peaks in the temperature spectrum, we can discern the relative densities of protons and dark matter. Furthermore, augmenting the electron-to-photon ratio (ns), along with protons, diminishes the fluid's speed of sound. Slower fluid motion prompts successive oscillations to manifest at reduced linear and angular scales, shifting the peak spacing in the temperature spectrum toward higher values.

Content

The cosmic microwave background radiation traveling from the surface of last scatter to our observation point undergoes gravitational fluctuations along its trajectory. Photons encountering the gravitational potential wells of clusters experience energy gain, which they subsequently lose while ascending out of these wells. In a flat universe dominated by dark matter, these effects nullify each other, resulting in no net gravitational impact from the matter along the photon path. However, the presence of dark energy alters this scenario. With time, gravitational potential wells diminish in depth due to the decay of dark energy. Consequently, a photon descending from a deep potential well encounters a slightly shallower one. Conversely, a photon traversing a low-density region experiences energy loss while descending a shallower hill than it ascended. Models incorporating a cosmological constant exhibit additional fluctuations on large angular scales due to these effects. Large angular scale measurements are particularly sensitive to variations in the gravitational potential at low redshift. A similar phenomenon occurs at high redshift (z ~ 500 - 1300) when photons and neutrinos significantly contribute to the total energy density of the universe. During this epoch, gravitational potential fluctuations decay, leading to heightened fluctuations on angular scales smaller than approximately two degrees. This scale corresponds to the horizon at z ~ 500. The ratio of photon energy density to matter density dictates the prominence of radiation during this period and influences the amplitude of temperature fluctuations. Given that measurements from the FIRAS instrument have determined the photon energy density, the amplitude of fluctuations on this angular scale enables the determination of the matter energy density.

Polarization

WMAP doesn't just scrutinize temperature fluctuations in the microwave background; it also delves into its polarization. Theoretical models anticipate a detectable polarization amplitude within WMAP's sensitivity range. Through multiple scans of the sky, WMAP's detectors have indeed discerned a subtle microwave background polarization signal. Scattered light, whether sunlight reflected off haze or microwave background photons reflected off free electrons in the early universe, often exhibits polarization. This phenomenon arises because the scattering cross-section between electrons and photons hinges on the polarization of the incident photon, correlating with the photon's electric field direction. Consequently, we can investigate the properties of the electrons encountered by photons en route from the surface of last scatter to our instruments. One might interject here, noting the absence of free electrons once the universe cooled below 3000 K. However, this assumption is incorrect. The emergence of stars leads to radiation that ionizes hydrogen, releasing free electrons. These liberated electrons induce detectable polarization fluctuations in the microwave background.

Structure

The Big Bang theory, while successful in explaining many cosmic phenomena, leaves unanswered questions about the formation of structures like stars, galaxies, and large-scale cosmic structures. Initially, the theory assumes a uniform distribution of matter and radiation throughout the universe, coupled with the universal validity of general relativity. However, this basic premise fails to elucidate the emergence of celestial bodies and cosmic structures. A striking example of these structures is depicted in the renowned "Deep Field Image" captured by the Hubble Space Telescope, revealing a rich tapestry of galaxies. Cosmologists hypothesize that the galaxies we observe today arose from minute density fluctuations in the early universe, governed by gravitational interactions. These fluctuations manifest as temperature variations in the cosmic microwave background radiation, a phenomenon meticulously measured by the WMAP satellite. By scrutinizing these fluctuations, WMAP sheds light on the nascent stages of structure formation, offering crucial insights into cosmic evolution within the framework of the Big Bang theory.

WMAP Team

The WMAP mission involves a collaborative effort between several prestigious institutions, including the Goddard Space Flight Center and Princeton University, with scientific oversight from a team comprising members from Johns Hopkins University, UCLA, University of Chicago, University of British Columbia, and Brown University. Dr. Charles L. Bennett of Johns Hopkins University serves as the Principal Investigator for the project. The current WMAP Science Team consists of researchers from various institutions, including Johns Hopkins University, University of Oxford, NASA Goddard Space Flight Center, University of British Columbia, Princeton University, University of Texas at Austin, Columbia University, University of Chicago, University of Toronto, and University of California, Los Angeles (UCLA). Past members of the WMAP team have contributed from institutions such as Princeton University, Cornell University, JPL/California Institute of Technology, University College London, and University of Barcelona. Additionally, major hardware suppliers for the WMAP mission include Boeing for the launch vehicle, NRAO for High Electron Mobility Transistor (HEMT) amplifiers, Princeton University for microwave feeds and differencing assemblies, and Goddard/ Litton for various instrument electronics components. Other suppliers include Lockheed Martin, Ithaco Inc., Kearfott Inc., Adcole Aerospace, Eagle-Picher Industries, Inc., Tecstar Inc., and Structural Composites Industries for different aspects of the mission hardware.

David T. Wilkinson

David T. Wilkinson, a distinguished cosmologist and esteemed member of the WMAP team, is honored through the namesake of the Wilkinson Microwave Anisotropy Probe (WMAP). He passed away in September 2002, leaving behind a legacy of groundbreaking contributions to the field of cosmic microwave background research. Dr. Wilkinson's illustrious career spanned over four decades, during which he played a pivotal role in shaping our understanding of the early universe. As an originator of the MAP Mission and a key figure in the development of the Cosmic Background Explorer (COBE) satellite, launched in 1989, his impact on the field was profound. Beyond his scientific achievements, Dr. Wilkinson was renowned for his dedication to teaching and was recognized with Princeton University's President's Award for Distinguished Teaching. His passing marked a significant loss to the scientific community, leaving behind a legacy of integrity, excellence, and camaraderie.