MAP mission poster. Photo: NASA

The Microwave Anisotropy Probe (MAP) will determine conditions in the early universe by making a full sky map of the cosmic microwave background temperature. The MAP mission is set to make major advances in cosmology, the science that attempts to answer fundamental questions such as: "What is the origin and structure of the universe? How did it evolve? What is its fate?"

In 1992, NASA's Cosmic Background Explorer (COBE) satellite detected tiny fluctuations, or "anisotropy," in the cosmic microwave background. It found, for example, one part of the sky has a temperature of 2.7251 kelvins, while another part of the sky has a temperature of 2.7250 kelvins. (kelvin, is a unit of temperature: 0 kelvin is the complete absence of heat, called "absolute zero," and 273 kelvins is the same as 0 degrees Celsius). These fluctuations are related to fluctuations in the density of matter in the early Universe and thus carry information about the initial conditions for the formation of cosmic structures such as galaxies, clusters, and voids.

If viewed from afar, we would see the Earth as a uniform sphere. When viewed with improved resolution, we would see blurry images of the continents and oceans. With yet better resolution, the rich features of the Earth --deserts, mountains and forests -- would become visible. The first observations of the microwave background revealed only a uniform sky. The smallest features that COBE could distinguish were about 7 degrees wide on the sky, so COBE made the equivalent of the first detection of continents and oceans. Over the past few years, balloon-borne and ground-based experiments have made high-resolution images of small portions of the sky. MAP will make a high-resolution image of the whole sky. Analysis of the new information revealed by the MAP observations will help cosmologists to answer several key questions, such as: What is the density of atoms in the Universe? What is the density of exotic dark matter in the Universe? How old is the Universe? How did structures such as galaxies and clusters of galaxies form in the Universe? When did the first such structures form?

Comparing the advances in the observing cosmic microwave background temperature. Photo: NASA

The only widely accepted scientific theory for the origin of our Universe is the Big Bang theory. According to the theory, the Universe began about 14 billion years ago as an unimaginably hot and dense soup of exotic particles, and has since continuously expanded and cooled. (It is the space itself that expanded, and is still expanding today. The Universe did not expand into something else.)

For the first 400,000 years or so after the Big Bang, the Universe was a seething cauldron of matter (electrons, protons, neutrons, and a small percentage of heavier atomic nuclei), and light (photons). Since photons scatter or bounce off electrons, the universe was opaque. As space expanded, the Universe cooled and the electrons combined with the protons (and other atomic nuclei) to create the first atoms, primarily hydrogen. The first light of creation could finally be freed from its pinball-like interactions with the electrons at which point the Universe became transparent.

Since this time, this light has effectively moved through the cosmos unimpeded and brings to us an image of the infant Universe. It is the oldest light that can be detected. Cosmologists studying the first light from the Big Bang, called the "cosmic microwave background" (CMB) radiation, look back through time and space to about 400,000 years after the Big Bang, when the Universe was opaque.

One of the most fascinating things about the Universe is its mystery. It seems that every question that is answered with some certainty gives rise to ten more. The data collected from MAP should help to answer some of the most fundamental questions before astronomers today.

Science observations and data collection
MAP does not measure the absolute sky temperature, but rather the difference in temperature between two points in the sky approximately 140 degrees apart. MAP spins every two minutes and its spin axis maintains a fixed angle of 22.5 degrees to the Sun-Earth line. The spin axis moves around the Sun-Earth line, allowing the instrument to view 30 percent of the sky every hour. In addition, MAP rotates annually with the Earth around the Sun so MAP can see all points in the sky from many different viewpoints. It will take six months at L2 for MAP to see the entire sky. To determine the validity of the signals received, MAP will cover the sky multiple times and at multiple frequencies. Five frequency bands from 22 GHz to 90 GHz will allow emission from our Galaxy and environmental disturbances to be modeled and removed based on their frequency dependence.

During the phasing loops and until MAP is past the Moon, MAP communicates with Earth with the use of its transponders and two omni antennas located at the top and bottom of the spacecraft. On the way to L2, MAP will switch to use of the Medium Gain Antennas located at the bottom of the spacecraft. Data is transmitted to Earth once per day from L2. On orbit operations are conducted at NASA's Goddard Space Flight Center.

Control of scientific measurement errors
To realize the full value of the MAP measurements, sources of error must be controlled to an extraordinary level. This was the most important factor driving the MAP design, and led to the following design choices:

• Differential: MAP measures temperature differences on the sky using symmetric microwave receivers coupled to back-to-back telescopes. By measuring temperature differences, rather than absolute temperatures, most spurious signals will cancel. This is analogous to measuring the relative height of bumps on a high plateau rather than each bump's elevation above sea level.
  
  
• Sky scan pattern: MAP spins like a top. This observing pattern covers a large fraction of the sky (approximately 30 percent) during each one hour precession.
  
  
• Multifrequency: Five frequency bands from 22 GHz to 90 GHz will allow emission from the Galaxy and environmental disturbances to be modeled and removed based on their frequency dependence.
  
  
• Stability: The L2 Lagrange point offers an exceptionally stable environment and an unobstructed view of deep space, with the Sun, Earth, and Moon always behind MAP's protective shield. MAP's large distance from Earth protects it from near-Earth emission and other disturbances. At L2, MAP maintains a fixed orientation with respect to the Sun for thermal and power stability.

New technology associated with MAP
At the core of the MAP instrument is a complex set of state-of-the-art microwave receivers that turn the very faint microwaves from the early Universe into signals that common electronic equipment can process. The MAP microwave receivers carefully measure the difference in temperature between two directions of the sky. This is done with very low noise and with minimal additional receiver artifacts.

The MAP receivers split the incoming microwaves into separate paths, each with their own independent amplification. Any differences in these signals are due to instrumental changes, while common signals are from the sky. This automatically neutralizes the adverse affects that would otherwise occur due to naturally occurring changes of receiver gain.

This MAP receiver approach was enabled by the creation of amplifiers (which are only a little bigger than a box of matches) that were custom designed by Marian Pospieszalski and built for MAP by the National Radio Astronomy Observatory in Charlottesville, Virginia. The MAP receivers contain 80 of these amplifiers.

The microwaves are directly amplified, even at MAP's highest frequency (about 100 GHz) with high sensitivity over a 20 percent wide range of frequencies. In the past, it would have been necessary to convert the microwave signals to lower frequencies in a process that resulted in a considerable loss of sensitivity and which often introduced other problems. The MAP receivers also preserve the polarization information of the original signal, which provides additional constraints on cosmology.

Partners/Science team
Goddard; Princeton University, New Jersey; University of California at Los Angeles (UCLA); and the University of Chicago; Brown University, Providence RI; and University of British of Columbia (UBC), Vancouver, Canada are where the MAP Science Team are located. Goddard and Princeton University worked as closely cooperating partners on the procurement and/or fabrication of all flight hardware and software. UCLA, Chicago, Brown and UBC provide additional scientific guidance, expertise, and oversight of the scientific conduct of the mission. All institutions contribute to the development of the data analysis software.