The cosmic microwave background radiation (CMBR), discovered in 1964, is a telltale remnant of the early universe. Its very existence is compelling evidence that the universe has evolved from an extraordinarily hot, compact beginning. To have produced radiation with the characteristics of the CMBR, the universe must at one time have been entirely different from what astronomers see today. No galaxies, stars, or planets existed: the universe was filled with elementary particles and radiation at extremely high energies.
The universe is between 8 billion and 15 billion years old. For all of that time, it has been expanding and the CMBR has been cooling. Currently, the radiation temperature is 2.73 K, which means that most of the CMBR exists now as radio energy in the microwave band. Man-made microwaves of the same sort link communication satellites to stations on Earth. But there are two major differences between satellite microwaves and the CMBR: First, the CMBR comes from all directions rather than from only one spot in the sky. Second, the CMBR has its power distributed over a wide range of microwave frequencies rather than concentrated at a single frequency, as is the case for a radio transmitter. To get accurate information about the early universe, cosmologists must measure the CMBR over a wide range of frequencies and across most of the sky.
>From such measurements, cosmologists believe that the CMBR has been largely unchanged, except for cooling down, during the entire history of the universe. The complex evolution of matter in the universe-such as the formation of stars, galaxies, and large-scale structure-did not affect the CMBR. This radiation is a pristine cosmic remnant. It gives us a wonderful opportunity to look far back in time to study even fine details of the early universe. As cosmologists try to understand the origin and evolution of structure in the universe today, it is essential to know about physical conditions that existed long ago.
Since the discovery of the CMBR, cosmologists have made measurements of its intensity at different wavelengths-its spectrum. The Big Bang theory predicts that the remnant radiation will have a special kind of spectrum, a thermal spectrum. The thermal spectrum has a characteristic shape, and the wavelength corresponding to the "peak" depends on the temperature of the emitting body. The CMBR (at a temperature of 2.73 K) peaks at 2-mm wavelength; the Sun's thermal spectrum (6,000 K) peaks at a visible wavelength. Years of ground-based and balloon-based observations traced out a crude spectrum that tended to support the Big Bang theory. However, it became clear in the mid-1970s that truly decisive measurements of the CMBR needed to be done from space, above Earth's obscuring and bright (at these wavelengths) atmosphere. NASA's Cosmic Background Explorer (COBE) satellite, which was launched in November 1989, was specifically designed to make accurate measurements of the CMBR. The first scientific result from the COBE satellite was an exquisitely accurate measurement of the CMBR spectrum. The spectrum matched the thermal shape, just as the Big Bang theory had predicted. The data and the prediction are shown in Figure 1. This result provides strong support for the Big Bang theory.
The shape of the spectrum seen in Figure 1 has a distinguished history in physics for reasons not related to cosmology. Early in this century, Max Planck and others reluctantly introduced quantum physics to explain this same spectrum, emitted by all cavities at uniform temperature, regardless of the kind of material used to make the cavity. This same thermal spectrum now turns out to match the intensity profile of the universal CMBRa curious connection between the smallest and the largest realms of physical theory. The simplest and most compelling explanation for the thermal shape of the CMBR is that the universe was all at the same temperature for some very early part of its early history. Space was uniformly filled with hot (but rapidly expanding and cooling) ionized gas and thermal radiation. The recognition that such an epoch existed in the early history of the universe is a cornerstone of modern cosmological models.
That a single experiment can have such profound implications is
unusual. But the measurement was not easy. It required the use
of a satellite to get the instrument above the atmosphere of Earth,
and the entire instrument was cooled to a temperature of 1.5 K
to reduce radiation from the instrument itself. The design, operation,
and calibration of the COBE instrument all had their technological
heritage in ground-based and balloon-based instruments, and the
experience gained in these earlier experiments laid the foundation
for the highly successful satellite measurement.
Theoretical modeling of the thermal history of the universe has
developed concurrently with progress in the spectral measurements.
The accurate fit of the measured CMBR spectrum to a thermal shape
sets extremely tight bounds that limit the variety of hypothetical
physical processes that could have taken place in the early universe.
For example, energy-releasing processes that would have reheated
the universe at critical epochs in its history can now be ruled
out.
Cosmological theories based on the Big Bang unambiguously predict that the temperature of the universe will fall with time as the universe expands. Recent observations from the new Keck telescope have provided the first direct evidence that the CMBR temperature has indeed decreased over (relatively recent) cosmic time. Using the light from a distant, bright, quasi-stellar source, astronomers were able to measure the temperature of carbon atoms in an intergalactic cloud between Earth and the source. The shift of the spectral lines (redshift) of the cloud was also measured. The temperature of the cloud, which should have been the same as that of the CMBR, was found to be 7.6 K. This is exactly the value expected for the CMBR at that redshift.
The second important characteristic of the CMBR is the variation in intensity (or temperature) from place to place on the sky. Measurements of these variations, often called anisotropy measurements, tell us about tiny fluctuations in the uniformity of the early universe. Though small (1 part in 100,000), these fluctuations are believed to be the seeds of all complex structure in the universe today.
The main idea is that the universe as we see it today must somehow
have evolved from the early stage of thermal uniformity implied
by the CMBR spectrum measurements. Today, the universe is a rather
lively place with everything from stars and planets to quasars,
colliding galaxies, and black holes. A central unresolved question
is, How did these structures form? One of the most appealing answers
is also the simplest: objects formed because gravity pulled together
matter that existed originally in slightly denser regions. Given
enough time, the matter became compressed because, as more matter
was pulled in, the gravitational forces grew even stronger until
galaxies and other objects resulted. These objects resist further
collapse because of their rotation and/or internal motions.
This idea, called gravitational instability, probably explains
most of the objects that astronomers see in the sky today, but
it requires small initial density fluctuations to start the process
of collapse. Because the expansion of the universe greatly retards
the formation of instabilities, the seeds of structure must already
have existed at the time the CMBR last interacted with matter,
150,000 years after the Big Bang. Fluctuations that existed then
are detectable by the CMBR anisotropy measurements as tiny variations
(bumps) in radiation intensity across the sky.
The theoretical details of the fluctuations and their consequences have been developed in the 30 years that cosmologists have known about the CMBR. Until 1992, no anisotropy (except for a separate effect due to the motion of our Solar System in the cosmos) had been detected in the CMBR, though many attempts had been made. Increased receiver sensitivity forced experimenters to develop increasingly sophisticated techniques to reduce systematic errors and the effects of noise from other sources of microwave radiation. As measurements became more sensitive and no anisotropy was found, the range of theoretical models that could fit the observations became smaller and smaller. As models were increasingly constrained, many cosmologists, especially the theorists making predictions, became increasingly nervous over the lack of detected fluctuations. So it was with great excitement that the COBE science team announced in 1992 that it had detected the long-sought bumps in the CMBR. The illustration on the cover of this report is the resulting COBE map of the intensity of microwave radiation arriving from various directions in the sky. The map contains some instrumental noise, but its lumps and bumps also show evidence for the beginnings of structure in the universe.
Like the measurement of the CMBR spectrum, the COBE detection of the CMBR anisotropy could not have occurred without the experience of earlier anisotropy experiments from the ground, from balloons, and from aircraft. As valuable experience was gained from the suborbital measurements, the technology and the experimenters' understanding of how to avoid contamination from many bright local sources both evolved. Balloons, rockets, and aircraft also provide important opportunities to follow up on satellite discoveries. Recently, data from an independent balloon experiment, using a frequency above that of the COBE's receivers, exhibited the same basic CMBR pattern as seen in the COBE data, thus confirming the satellite result. Low-cost balloon experiments have also enabled important first steps toward extending these results to smaller angular scales.
The detection of large-scale anisotropy has finally allowed the field of CMBR research to become established. Whereas previous noise-limited measurements could exclude but not support certain theories, the anisotropy measured by the COBE satellite is approximately at the level needed for the origin of structure as predicted by theories. In addition, the manner in which the strength of the anisotropy varies with the angular size of the bumps is consistent with Big Bang theory. Even the idea of an inflationary epoch in the early universe (see section V) seems to fit with the COBE result, although this characteristic of the CMBR fluctuations is not yet well determined. Within the past year, both theoretical thinking and experiment planning have undergone an important transformation because of the COBE detection. Almost all areas of cosmology have been affected, and many now take the magnitude of the COBE anisotropy signal as a reference point for new developments.
While the COBE anisotropy detection is extremely important, it was made at angular scales of more than 10 degrees on the sky, scales much larger than those actually involved in the formation of galaxies and clusters of galaxies. The best direct comparison between the primordial seeds and present day structures awaits the reliable detection and detailed mapping of CMBR anisotropy on smaller angular scales, comparable to the physical scale of superclusters of galaxies.
Medium-scale (0.5- to 10-degree) anisotropy measurements also probe important details of the decoupling process. Numerical calculations show that during the time that matter was combining into atoms and interacting with the CMBR for the last time, there were acoustic oscillations in the overdense bumps that should have left strong "fingerprints" in the CMBR anisotropy. Oscillations would amplify bumps to an extent that depended on the bump size and critical details of the cosmological model. The theory's prediction of peaks in the CMBR medium-scale anisotropy offers an intriguing and unexpected opportunity. If the profile of CMBR anisotropy versus bump size can be accurately measured, cosmologists may be able to measure three important cosmological parameters: , the ratio of the mean mass density to that required to close the universe and eventually stop its expansion; B, the contribution to from ordinary (baryonic) matter; and the expansion rate of the universe (the Hubble constant, H0). Of course, the data may reveal something completely unexpected, a clue to unknown processes in the early universe. In either case, medium-scale anisotropy signals are bringing us detailed information about the conditions and dynamics that existed in the universe when it was only 150,000 years oldsomething quite unimaginable only a few years ago.
The opportunity to gain such important knowledge has recently focused a great deal of experimental and theoretical research on the issue of CMBR anisotropy on angular scales around 1 degree, at the low end of the medium scale. This scale corresponds to about 300 million light-years, which is smaller than the size of the region in the universe around us that has been well mapped in redshift surveys of thousands of galaxies. Experiments on this angular scale are relatively new compared to studies of the CMBR spectrum and the large-scale anisotropy measurements. The techniques and technology needed to overcome the new experimental challenges are just now beginning to be understood.
A current problem is to remove possible interference from weak radio sources and faint, diffuse emission from our own galaxy. To effect the separation, a wider range of frequencies and better sensitivity are being used. At angular scales around 1 degree, the greatest understanding of the structure-formation process would follow from detailed mapping of large regions of the sky; this poses difficult challenges for current experiments. Ground-based experiments must overcome fluctuations in atmospheric emission that are thousands of times larger than the expected CMBR anisotropy signal. Thus, experimenters are observing from sites with a cold, dry atmosphere like the South Pole, northern Canada, and mountain tops. Balloon experiments afford much smaller atmospheric fluctuations, but they are limited by the relatively short times available for observations. Long-duration balloon flights of many days are being planned to alleviate this problem, and they offer a powerful new opportunity to extend COBE results at relatively low cost. However, flight opportunities for long-duration balloons are currently scarce. Limited sky coverage and Earth's radiation environment are also problems for experimenters trying to map the CMBR anisotropy from ground-based and balloon-based platforms.
The situation for the measurement of medium-scale anisotropy is similar to that for measurement of the spectrum and large-scale anisotropy two decades ago. Data are beginning to indicate that something interesting is happening, but greater accuracy and broader sky coverage are needed to extract the important scientific results. Techniques for successful experiments are being developed using the experience gained with suborbital experiments, but Earth's environment poses major problems for experiments requiring high accuracy and large sky coverage. Research now under way will ultimately lead to the design of a satellite that can utilize the advantages of the space environment. Preliminary feasibility studies indicate that a midsize Explorer satellite in an orbit far from Earth is an attractive and relatively inexpensive possibility. A satellite experiment will permit the mapping of the CMBR sky pattern with sufficient detail and sensitivity to form an excellent picture of the early development of structure in the universe.
Measurements of the CMBR anisotropy on angular scales smaller
than about 0.5 degree must be made with relatively large ground-based
telescopes (high angular resolution requires large antennas).
The theoretical case is not yet as strong for small-scale measurements
because such anisotropies are presumed to have been smeared out
when matter and radiation interacted for the last time at the
epoch of photon decoupling. However, variants of the Big Bang
theory, such as theories involving cosmic strings (discontinuities
in the structure of space), predict that important clues to the
universe's history might be embedded in the CMBR at these angular
scales. A few experiments have been done from single large telescopes
and from arrays of radio telescopes, such as the Very Large Array.
Though sensitivities are comparable to the COBE detection level,
only a small fraction of the sky has been scanned. Signals are
detected, but they are thought to be due mostly to radio emission
from galaxies, quasars, or other foreground radio sources. Because
of the need for large instruments and the extreme care required
for these measurements, progress on small-scale anisotropy is
likely to be relatively slow. But it should be remembered that
only a decade ago there was almost no interest in even medium-scale
measurements. As understanding has grown, so also has the need
for more diverse experimental data. And surely, as more data are
analyzed, the simple models of structure formation must break
down at some point. Improvements in small-scale measurements are
one way to find such weaknesses in the models.