To map the universe

The saga of the Microwave Anisotrophy Probe, which is scheduled to begin its scientific mission next month.

ON June 30, 2001, the American launch vehicle Delta II put a spacecraft of the National Aeronautics and Space Administration (NASA) called the Microwave Anisotropy Probe (MAP) into orbit. Thus began a journey into deep space to try and answer fundamental questions about the history, content, shape, structure and ultimate fate of the universe. The spacecraft will accomplish its objectives by mapping the relic of the Big Bang, the after-glow of creation of the universe known as Cosmic Microwave Background Radiation (CMBR), across the entire sky over a two- year period. The design and objectives of the $145 million mission signify the impact that rapid strides in technology are having on astronomy and observational cosmology.

MAP is to begin its scientific mission three months from the launch date when it would have reached its home about 1.5 million km from the earth at a point called the Second Lagrange Point (L2) of the earth-moon system in order to get an unhindered view of the sky. Headed by Charles Bennett of NASA's Goddard Space Flight Centre (GSFC), the MAP project is a partnership effort between GSFC and Princeton University. The team includes scientists from the University of Chicago, the University of California, Los Angeles (UCLA), Brown University and the University of British Columbia, Vancouver.

By the third week of August, MAP was past the moon and on its way to L2. The manner in which MAP's trajectory towards L2 was achieved is interesting. Till June 30, a month from launch, MAP was moving in a highly elliptical orbit around the earth, executing what are known as "phasing loops". These phasing loops were designed to align the spacecraft for a "lunar swing-by" which provides a gravity-assisted boost for it to reach L2. The spacecraft executed three phasing loops in such a manner that it and the moon arrived at designated points in their respective orbits synchronously for an optimal lunar fly-by. Assisted by moon's gravity, MAP's own propulsion system will carry the spacecraft to its destination, which is an orbit around L2. L2 is four times farther from the earth than the moon in the direction opposite the sun. This trajectory is used to minimise the consumption of on-board fuel.

MAP will be the first spacecraft to use an orbit around L2 as its observing post. Besides an unobstructed view of the sky, an orbit around L2 provides for an exceptionally stable environment. In orbit, MAP will maintain a fixed orientation with respect to the sun for thermal and power stability. Also, at L2 a near 100 per cent efficiency in observation and data collection is possible since the sun, the earth and the moon will always be behind the on-board instruments' field of view, according to the MAP research team. More important, because of the distance from the earth, the spacecraft will be free from near-earth electromagnetic disturbances for it to map the intensity distribution of CMBR, the primary objective of the mission, with greater sensitivity.

WHAT is Cosmic Microwave Background Radiation? The Big Bang Model is the only widely accepted theory for the origin and evolution of the universe. According to this model, the universe began about 14 billion years ago as an extremely hot and dense soup of exotic particles occupying a space of a few millimetres across. From this extraordinarily dense state it has expanded into a vast and much cooler cosmos. The solar system is believed to be 4.5 billion years old and humans emerged as a species a few million years ago.

For the first 300,000 years or so after the Big Bang, the universe was a cauldron of electrons, protons, neutrons, light (photons) and a small fraction of heavier atomic nuclei such as helium and deuterium. Because of the heat, all matter was ionised. There were no atoms, and electrons were free. Since photons are scattered by electrons, light remained trapped inside the hot, dense particle cloud. This particle cloud was opaque just as a thick fog would be because water droplets scatter light effectively. According to the Big Bang theory, since its origins the universe has been expanding. As the universe expanded it cooled, and when the temperature dropped to around 3000 Kelvin (00 C corresponds to 273 K and zero K corresponds to absolute zero or total absence of heat), nuclei and electrons combined to form atoms, primarily hydrogen. Neutral hydrogen is almost transparent to light. At this epoch of evolution - the epoch of recombination (of electrons and nuclei) - light decoupled from matter. This after-glow of Big Bang, which has been expanding with the universe and effectively free-streaming and spreading through the cosmos, now fills the universe almost uniformly in all directions. It is the oldest light that can be detected.

Multiple scattering within enclosed space, with little leakage of energy, produces what is known in physics as "thermal" or "black body" radiation. The characteristic of black body radiation is that its intensity and spectrum are determined by temperature alone, and are independent of the composition of the source. As the universe expanded and cooled, this light from the Big Bang was stretched to longer wavelengths in the microwave region (about 100 gigahertz frequency or millimetre wavelengths). The spectral distribution of this "stretched" light, which peaks at wavelengths around 1-5 mm, now corresponds to a thermal or black body radiation corresponding to a chilly temperature of 2.725 K (- 270.2750 C). This relic radiation is generally also referred to as the 30K background radiation.

Since the universe is bathed in this cold after-glow with uniform intensity, to "see" it one needs radio antennae tuned to these wavelengths. Also, fortuitously, stars and galaxies do not seem to emit in this window of the electromagnetic spectrum, making the detection of CMBR possible as if by design for us to look back in time up to 300,000 years after the Big Bang. The distribution characteristics of CMBR harbours within it information about the "surface of last scatter", before light freed itself from matter to fill the universe. When we observe CMBR, we "see" the epoch of recombination. CMBR, therefore, carries the image of the early universe.

CMBR was discovered serendipitously by Arno Penzias and David Wilson of Bell Laboratories in 1965 as a noise in the amplifier system of their experiment. Indeed, about one per cent of the static in home radios would be from CMBR. The two scientists were making radio-astronomical measurements using an antenna originally designed to receive signals reflected from the 'Echo' satellites. They found a background microwave radiation at a wavelength of about 7 cm which could be mostly accounted for by other sources such as atmospheric disturbances and terrestrial emission but for a residual emission which was surprisingly 'isotropic' or uniformly distributed in all directions. That such a radiation should exist did not come as a surprise to Big Bang cosmologists. Indeed, way back in 1948 George Gamow had estimated that the temperature of the relic radiation should be around 3 K. Since at this temperature the peak would have a wavelength around a few millimetres, detection of radiation at a wavelength of a few centimetres conformed to the Big Bang theory.

How is one sure that the radiation is indeed of the black-body type as is expected from a hot Big Bang cosmological model? A great deal of effort has gone into the measurements of CMBR in the last 36 years. Following the Penzias-Wilson discovery, other groups, notably Robert Dicke's group at Princeton, surveyed the sky for radiation at other microwave frequencies, culminating in NASA's space mission called the Cosmic Background Explorer (COBE) (Frontline, July 17, 1992). Launched in 1989, COBE in 1992 produced the first detailed all-sky map of CMBR. COBE's on-board instrument called the Far Infrared Absolute Spectrophotometer (FIRAS) measured the spectrum at 43 equally spaced points along the black-body curve and showed that CMBR fits the shape of a black-body spectrum at 2.725 plus or minus 0.002 K.

One puzzle in the early years of the discovery of CMBR was that it was featureless and uniform. According to the Big Bang model, the present universe is believed to have evolved from the density and gravity variations in the early universe of the recombination epoch. Then the mark of the lumps and clusters of matter that we now see in the form of galaxies should be imprinted in some form on CMBR. But the temperature distribution of CMBR was smooth to less than one part in 10,000. The COBE measurements changed all that. Its far greater sensitivity as compared to earlier measurements - the chief reason for that being that the observations were from a space platform - showed fluctuations in the intensity distribution of CMBR from one direction to the other. That is, for example, if the temperature in one direction was 2.7256 K, in another direction maybe it was 2.7258 K and in yet another direction maybe 2.7254 K. COBE found the extent of this intrinsic "anisotropy" in CMBR to be at the level of 10 parts in a million. In the CMBR temperature projection corresponding to COBE, while green refers to the average temperature of 2.725 K, the red regions are 0.0002 K warmer than the blue regions.)

However, though COBE's measurements were sensitive enough to detect these tiny fluctuations, they were coarse. COBE could measure variations between two directions whose angular separation was as much as 70, which is 14 times the apparent size of the moon as seen from the earth. Balloon and ground-based measurements such as Maxima and Boomerang in the last few years have advanced the cosmologist's knowledge about CMBR, though these have mapped only certain regions of the sky to greater sensitivity and resolution. MAP will also improve upon the measurements of COBE both in terms of sensitivity and resolution, and it will be an all-sky survey. The sensitivity will be better than 20 microkelvin (0.00002) K per 0.30 square pixel. To realise the full value of these measurements, the sources of error must be controlled to an extraordinary degree. This was the most important factor driving the design of MAP.

Fluctuations or anisotropy in CMBR reflect density and gravitational fluctuations in the early universe -- the hotter spots were less dense than the colder spots - and the Big Bang theory has it that those density variations were responsible for the eventual formation of galaxies. The general belief is that as the universe expanded matter accumulated around denser regions until eventually there was enough for galaxies to form, then clusters of galaxies and superclusters. The less dense regions, according to the theory, became the huge voids that are seen between these superclusters. Analysis of MAP data will thus help cosmologists answer key questions: what is the density of visible matter in the universe? What is the density of dark matter in the universe and what kind is it? How did structures such as galaxies arise? When did the first structures form? Is the rate of expansion of the universe accelerating or decelerating? Will the universe continue to expand for ever or will it collapse? What is the geometry of the universe: is it flat, negatively or positively curved?

While the fluctuations might help understand how the universe evolved, they also raise a fundamental question. How did the variations come about in the first place which we see imprinted on CMBR? Cosmologists have two viable hypotheses to explain the origins of anisotropy. Both have their premise in the Big Bang hypothesis according to which the universe expanded from an infinitesmally small point in space. The less favoured one is the "topological defect model", according to which the very fabric of space and time in the early universe was laced with cracks and defects as the universe cooled. Then, as the universe expanded to cosmic proportions, these defects expanded with it, causing density and temperature variations and eventually leading to the formation of galaxies.

The more popular hypothesis is the one that related to an "inflationary universe" in which the role of quantum effects come into prominence soon after the Big Bang. Quantum fluctuations of all scales dominate this primordial phase. According to the inflation theory, when the universe was a trillionth of a trillionth of a billionth of a second old, it went through a phase of exponentially rapid expansion, from something too small to imagine to an emerging fireball at least as big as a grapefruit, if not considerably bigger. The quantum fluctuations too inflate with it. As the universe grows, so do these fluctuations. These become the seeds for accumulation of matter. The exact manner in which the matter accumulated, however, depends upon the initial conditions - the density, the speed of expansion, how much matter, what kind of matter and so on - that prevailed in the early universe. Already, results from COBE and the later balloon experiments have been interpreted to favour an inflationary universe of a certain kind.

One does not know what these initial conditions were, and results of the MAP mission will be used to derive these in the following way. Based on initially assumed different sets of early conditions, computer simulations on the evolution of the universe over a time scale of 10-15 billion years are carried out to see which set results in the distribution of galaxies as they are today. And what kind of anisotropy in CMBR does this set produce? If the inflationary model is right, it will produce one kind of density and temperature variations in CMBR and if the defect model is right then the distribution would be different. The main purpose of the CMBR anisotropy measurement is to look at the background radiation accurately and see which model fits best. MAP promises to provide good enough data to do it.

T. Padmanabhan of the Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune says: "COBE results, which were at a resolution of about 100, were good enough to rule out several cosmological models. But it did not allow us to probe the effects of the various physical processes on CMBR, which usually act at smaller scales." Along with D. Narasimha of the Tata Institute of Fundamental Research, Mumbai, Padmanabhan was the first to analyse comprehensively in 1992 the COBE data. "MAP is designed to probe exactly the angular scales in which rich structure is expected to exist, and by comparing what MAP sees with theoretical predictions one can gain valuable information about the various cosmological parameters," says Padmanabhan. He, however, adds: "The success of this programme is virtually assured because of the information we already possess from the balloon experiments Maxima and Boomerang. The downside is that there may not be any true surprises. We may have to wait for Planck mission for that." Planck is a future CMBR anisotropy mission of the European Space Agency (ESA) which aims to measure fluctuations at a millionth of a degree at even finer resolution of 10 arc minutes. Planck, together with another related mission Herschel, will be launched in 2007.

Jayant Narlikar, IUCAA Director, who does not subscribe to the Big Bang model of the universe, has this to say on the impact of MAP: "It is standard practice to interpret the data within the framework of the 'Big Bang' paradigm. This requires a series of assumptions and parameters which are based on untested extrapolations of tested physics and as such can at best be exercises in consistency. One cannot rule out entirely different interpretation of the results based on an altogether different paradigm."

Most cosmologists would consider the Steady State universe of Narlikar and associates to be a dead concept after the discovery of CMBR itself. However, the alternative paradigm offered by its proponents is called the Quasi Steady State Cosmology (QSSC). It was put forward in 1993 by Narlikar together with Fred Hoyle and Geoffrey Burbidge. This cosmology explains the microwave background not as a relic of an early very hot era but as a thermalised form of stellar radiation from stars of previous generations, a relic from radiation of stars that have burnt out. "The fluctuations that MAP will detect can be interpreted in terms of inhomogeneities in the thermalisation of the relic starlight and such calculations have successfully explained results of Maxima and Boomerang," says Narlikar.

MAP results may well prove to be the decider between these vying views of the cosmos. With MAP, the ongoing balloon experiments like Maxima, Boomerang and Dasi, and the future space missions like Herschel and Planck, microwave background radiation promises to remain centrestage in astronomy research for a long time because it is unlikely that it does not harbour any surprises as these experiments map it in finer and finer detail.