New light on Big Bang

The Nobel Prize in Physics has been awarded to two scientists for discovering the true form of cosmic microwave background radiation.

THE universe is about 14 billion years old. That is what the widely accepted theory about its origin and evolution, the Big Bang Model, tells us. That is, the universe exploded into being from a point (hence the phrase Big Bang) at a certain instant in time as an extremely hot and dense soup of exotic particles occupying a tiny volume of space. The solar system is about 4.5 billion years old and the human species appeared a few million years ago.

For about 400,000 years after the Big Bang, the universe was still a dense cauldron of electrons, protons, neutrons, photons (light) and a small fraction of heavier atomic nuclei such as helium and deuterium. At that extreme temperature, all matter was ionised. There were no atoms, and electrons were not free. Since photons are scattered by electrons, light remained trapped inside the hot and dense particle cloud, just as thick fog appears opaque because water droplets scatter light effectively.

As the universe expanded, it cooled and when the temperature dropped to around 3,000 Kelvin (0o) Celsius corresponds to 273 K, and 0 K to absolute zero or total absence of heat), nuclei and electrons combined to form atoms, mainly hydrogen. Neutral hydrogen is transparent to light. At this epoch, when electrons and nuclei combined, light decoupled from matter and began to propagate freely into space. This "after glow" of the Big Bang has been expanding with the universe and free-streaming and spreading uniformly in all directions. This is the oldest light in the universe - the relic of its origin - and it is filled with it.

The condition of the universe immediately after the Big Bang, with the light trapped within, can be imagined to be a situation where multiple scattering occurred within an enclosed space with no energy leakage. Such a condition results in what physicists call "thermal" or "black body" radiation. The essential feature of black body radiation is that its intensity and spectrum are independent of the composition of the source and are determined by temperature alone. Cosmologists, therefore, believe that the "after glow" of the Big Bang must have the characteristics of black body radiation.

With the expansion and cooling of the universe, however, this light from the Big Bang was stretched to longer wavelengths in the microwave region (about 100 gigahertz frequency or millimetre wavelengths). The spectrum of this "stretched" light peaks at about 1-5 mm wavelength. If we assume that this is indeed a black body radiation, it corresponds to a temperature of about 3 K (-270oC). This relic radiation is, therefore, generally referred to as the 3 K background radiation or the cosmic microwave background (CMB) radiation. Put another way, if the Big Bang scenario of the universe is the correct one, the CMB should have the black body spectral form and it should be isotropic (that is, uniformly distributed in all directions).

This year's Nobel Prize in Physics has been awarded to John C. Mather of the National Aeronautics and Space Administration's Goddard Space Flight Centre (NGSFC) and George F. Smoot of the University of Berkeley "for their discovery of the black body form and anisotropy of the cosmic microwave background radiation". That is, the two scientists established the true form of the CMB and also measured its small variations in different directions. They did this using the detailed observations made by the COBE (Cosmic Background Explorer) satellite launched in 1989. "These," the Nobel Foundation release on the award observed, "have played a major role in the development of modern cosmology into a precise science."

The CMB itself was discovered, serendipitously though, in 1964 by Arno Penzias and Robert Wilson in trying to understand the source of unexpected noise in their radio receiver. Indeed, the CMB is part of the "blizzard"-like noise that we receive on television sets when normal transmission stops. However, this background radiation had already been predicted by R.A. Alpher, R.C. Herman and George Gamow in 1948. But, more significantly, R.C. Tolman had also shown in 1934 that in an expanding universe cooling black body radiation retained its black body form.

Following the discovery of the CMB, Big Bang cosmology became the generally accepted paradigm rather than the other competing theory of a `steady state universe'. Soon several important measurements were made by David Wilkinson and others, mostly using balloon-borne, rocket-borne or ground-based instruments. But these had limitations because the intensity of CMB is maximum around 1 mm where atmospheric absorption is also strong. While most experiments supported the black body form, few measurements were available on the high-frequency or low-wavelength side of the spectral peak and, therefore, the black body nature of the radiation could not be established. There were also some measurements that found significant deviations from the black body form. The question that remained to be settled was whether the observed radiation had the black body form.

The Big Bang scenario suggests that the CMB should be isotropic. But the fact that there are galaxies and clusters of galaxies suggests that this large-scale structure of the universe could have arisen only if small anisotropies had existed in the "after glow". Minuscule density fluctuations present in the early universe could have initiated the aggregation of matter in some spots and these would have grown because of gravitation leading to galaxy formation. The imprint of these density fluctuations or the primordial seeding should still be there and should show up as small temperature variations in the CMB.

In 1967, R.K. Sachs and A.M. Wolfe carried out a detailed relativistic calculation to show how three-dimensional density fluctuations can result in two-dimensional large angle (more than 1o) temperature anisotropies in the CMB. But the early observations, being largely earthbound, could not measure in all directions, to measure the extent of isotropy. And, also the measurements were coarser and could not pick up the variations. The CMB distribution measured by these was smooth, which could not have resulted in the galaxies, the stars, the planets and life in the universe.

It had been recognised for long that measurement of the long-frequency part of the CMB (wavelengths smaller than 1 mm) is best carried out from space using a satellite probe so that atmospheric absorption is avoided. Smoot points out, "Even at wavelengths where the radiation does come through, the atmosphere emits its own radiation, which confuses matters quite a lot. So it was really important to get up into space where it is cold and quiet." A satellite instrument also gives a complete coverage of the sky and a long observation time, the latter necessary to reduce systemic errors in the measurements.

In 1974, NASA issued an invitation to astronomers to submit new proposals for new space-based experiments. This led to the COBE project being initiated. John Mather was the prime moving force behind the huge collaborative experiment to measure the CMB, in which over 1,000 scientists, engineers and others were involved and which took 15 years to be realised when the satellite was launched 17 years ago on November 18, 1989. NASA's original plan was to launch COBE by one of the space shuttles. However, after the shuttle tragedy of 1986, the experiment proposal had to be renegotiated for being launched by a different rocket launcher. The satellite, weighing 2.3 tonnes, orbited at an altitude of about 900 km and its axis of rotation was roughly at right angles to the direction of the sun.

COBE carried three instruments covering the wavelength range 1 micrometre to 1 cm to measure the anisotropy and spectrum of the CMB as well as the diffuse infrared background radiation. These were: Diffuse InfraRed Background Experiment (DIRBE), Differential Microwave Radiometer (DMR) and Far InfraRed Absolute Spectrophotometer (FIRAS). Mather, the project leader, was also in charge of the FIRAS instrument and Smoot was responsible for DMR, the instrument that would look for anisotropies in the CMB. Mike Hauser was in charge of the other instrument, DIRBE. The objective of DMR was to search for anisotropies at three wavelengths: 3 mm, 6 mm and 10 mm with an angular resolution of 7{+0}. FIRAS was designed to measure the spectral distribution of the CMB in the range of 0.1-10 mm and compare it with the black body form expected from the Big Bang Model. DIRBE was meant to measure the infrared background. FIRAS and DIRBE were cooled to 1.8 K with liquid helium.

The COBE mission proved to be an enormous success. All instruments worked well with the first results from COBE coming after nine minutes of observations: the spectrum measured by COBE had a perfect black body shape. At that time this was a surprising discovery because some earlier measurements had shown significant departures from the black body form. The observations fitted the Big Bang scenario very well: The COBE-curve turned out to be - and still is regarded as - one of the most perfect black body spectra ever measured, corresponding to an estimated body temperature of 2.725 K (-270.275oC) with an error window of only 0.002 K. The temperature fluctuations too were, as expected, of the order of 10{+-}{+5} (Fig. 3).

In an interview on April 29, 1992, in The Times, Stephen Hawking said that COBE results were "the greatest discovery of the century, if not of all times". The absolute value of the black body temperature is one of the best-determined cosmological parameters. The anisotropy measurements were, in particular, received with great enthusiasm by the scientific community. In fact, the Indian scientists T. Padmanabhan and D. Narasimha were the first to carry out a detailed analysis of the COBE anisotropy data.

When the COBE experiments were being planned, the temperature variations in the CMB necessary to explain galaxy formation were about one-thousandth of a degree. But the challenge proved to be more severe even as COBE was being built when the ideas about dark matter were gaining ground (Frontline, October 6). The influence of dark matter meant that the temperature variations would be of the order of a hundred-thousandth of a degree. The instrument had to be redesigned, but there was still a lot of room for uncertainty in the data. Though the results from COBE (published in 1992) could be correlated with ground-based measurements, the latter were even more uncertain. However, when the correlations were actually done, the directions in space where COBE recorded temperature variations turned out to be exactly the same as those detected from the earth using balloons and other methods.

The results of the COBE were soon confirmed by a number of balloon experiments such as Maxima and Boomerang and, more recently, by the Wilkinson Microwave Anisotropy Probe (WMAP) launched in 2001 (Frontline, September 14, 2001) with a 1{+0} resolution temperature map. Comparison of variation in the temperature within different angles offers a tool to estimate the relationship between visible matter, dark matter and dark energy in the universe. Temperature variation measurements have become particularly significant in the context of this new perspective of the universe with dark matter and dark energy to determine indirectly their densities.

"The [COBE] map, that shows the hot and cold spots, shows the universe as it was approximately 390,000 years after the Big Bang and we certainly think that there is further information hidden in the CMB," says Mather. He believes that this can be unravelled with even more precise measurements. "One of the continuing investigations is to get the polarisation of this radiation. That has already been measured with WMAP... [This] tells us that the first luminous objects after the Big Bang were quite early, when the universe was less than a 20th of the present size. Much more is thought to be lurking there in the radiation if we could measure even better, traces from the gravitational waves of the earliest universe, for instance," adds Mather.

Such high-precision measurements pose an enormous challenge because, as he points out, the signature would be extremely faint. "The spectrum measurement was made to a part in hundred-thousandth accuracy. Now the polarisation may be a hundredth of that, so we are getting down to signals that are measured in nanoKelvins," Mather points out.

By enabling such precise measurements of a number of other cosmological parameters besides the CMB, COBE can be said to have heralded cosmology as a precise science. This becomes particularly relevant in the context of the emerging discipline of astro-particle physics, the interface between cosmology and physics of elementary particles, which aims at a quantitative understanding of what happened during the moments before the CMB was emitted. This is an epoch about which very little is understood at present and is sought to be explained unsatisfactorily by scenarios such as the "inflationary universe", when the universe underwent an exponential expansion within fractions of a second after the Big Bang.

In this context, the proposed launch, on the one hand, of the European satellite Planck, a future CMB anisotropy mission which aims to measure fluctuations at a millionth of a degree at an even finer resolution of 10 arc minutes, and investigations into the nature of dark matter and other exotic particles that make up the universe with the new Large Hadron Collider at CERN, the European Centre for Nuclear Research, on the other, are significant. Given the scientific platform that COBE has established, these will hopefully provide some clues, if not definite answers, to the many puzzling cosmological questions.