|Cosmic Microwave Background|
The idea for what we now call Big Bang Cosmology first came to light in the work of the Belgian astronomer Georges Lemaître (1-3), who based the idea on Albert Einstein's theory of general relativity, combined with Alexander Friedmann's demonstration that the universe cannot be static (4), and Edwin Hubble's demonstration that the universe appears to be expanding (5). Einstein did not initally agree with the idea of an expanding universe, and added a term now called the cosmological constant to his equations of general relativity, in order to force his theory to allow for a static universe. However, once Hubble made known his evidence for an expanding universe, Einstein recanted the cosmological constant, and accepted the expanding universe. Today, Georges Lemaître is generally recognized as the "father" of the expanding universe cosmology.
Through the 1950's & 1960's, The Big Bang (as it was derisively titled by Fred Hoyle) was one of several cosmological models jousting for supremacy. As time passed, the others slipped behind, as observational evidence cosistently pushed the Big Bang forward. Today, Big Bang cosmology is standard, accepted by the vast majority of astronomers, astrophysicists & cosmologists, as the theory which best matches observation.
The cosmic microwave background radiation (CMBR) was one of the early predictions of Big Bang Cosmology (BBC). Physicist Richard C. Tolman had shown by theory in 1934 that an expanding universe would be filled with a thermal radiation (6), and in 1948 George Gamow (7) and Ralph Alpher & Robert Herman (8,9) predicted that our universe of today should be filled with background thermal radiation, at a temperature of about 5 Kelvins (5K). Numerous estimates of what the temperature of such a background should be followed, ranging from as low as about 3K to as high as 50K. But too little was known about the theory of the expanding universe, and its physical consequences, to do any better than that at the time.
Aware of the earlier predictions, a number of cosmologists were designing experiments in the late 1950's and early 1960's, in an attempt to actually detect the predicted CMBR. However, the discovery was actually made by two scientists who were not looking for it, and were unaware of the predictions.
Arno Penzias & Robert Wilson were trying to study the radio emission from the halo of the Milky Way, and were trying to account for all sources of noise and confusion. They eventually identified or compensated for all of these except one. Only after a chance meeting with well known physicist R.H. Dicke, did they all realize that it was the CMBR that Dicke had searched for in vain earlier, and was about to look for again. The three of them published their results in a pair of papers in 1965 (10,11), initally presenting the effective temperature of this emission as 3.5±1.0 Kelvins. Penzias & Wilson shared the 1978 Nobel Prize in Physics for their discovery.
The significance of the discovery of the CMBR is that it was predicted by the basic physical theory of the expanding universe, first laid down decades before, by Georges Lemaître. While the cosmology wars of the 1940's and 1950's pitted several competing models against one another, only the Big Bang had predcited the necessity of the CMBR as a consequence of the theory. So it's only natural that its discovery gave support to that theory. The supporters of alternative cosmologies tried to bring such a background radiation out of their models after the fact, but always with unsatisfying, arbitrary tricks. Only the Big Bang actually requires that there be a CMBR. The existence of the CMBR was one of several strong reasons that developed during the 1960's & 1970's, which drove Big Bang cosmology into its premiere position amongst scientists.
In the case of the CMBR, its major selling point was that just any old CMBR would not do. Big Bang cosmology required that this CMBR be both thermal and isotropic. "Thermal" means that the spectrum (power as a function of wavelength or frequency) has to match the theoretical spectrum expected from a source of a given temperature, radiating energy solely because of its temperature. Such a source is called a blackbody, and it is said to have a blackbody spectrum. "Isotropic" means that it has to look essentially the same in all directions, which implies that the CMBR is not only thermal, but shows essentially the same temperature, no matter which direction one looks in.
Subsequent observations confirmed, as well as it could be confirmed by limited, ground based observations, that the CMBR discovered by Penzias & Wilson indeed had the required thermal spectrum, and was indeed isotropic (12-15 for example). These two facts alone are strong indicators that the CMBR is the predicted relic of Lemaître's primeval atom, and argues against alternative cosmologies that are still popular in some corners, though very much the minority view today.
Observing the CMBR was quite an effort for anybody. Typical experiments could only see a small part of the sky at a time, and effects of the earth's atmosphere and terrestrial noise limited the effectiveness & sensitivity of observations. Was the CMBR really thermal? And how isotropic was it? It couldn't be perfectly smooth, perfectly isotropic. By this time, theorists of the expanding universe knew that there had to be some primordial structure in the early universe, to act as a seed for large scale structure seen in our universe of today. Everything from galaxy cluster & superclusters, down to the galaxies themselves, had to come about as a result of prior structure, and that prior, primordial structure, should have left behind a signature in the CMBR. So where was that signature? A CMBR without any structure in it at all could be a real problem for BBC, so cosmologists set out to answer the question.
The response to this need was the Cosmic Background Explorer (COBE) satellite. COBE was launched into orbit on November 18, 1989, though it had been conceived many years before that. The primary mission of COBE was to measure the CMBR over the entire sky. All of the CMBR observations before then, from the ground, combined, had only observed a small part of the total sky. This alone would be a major accomplishment. In addition, it would measure the CMBR extended from the microwave to the far infrared region, where its power should show a maximum. By measuring the full spectrum, from microwave to far infrared, COBE could prove by the measured shape of the spectrum that the CMBR was truly thermal. And, by actually measuring that far infrared peak, it could pin down the exact temperature.
COBE was a success at both of these tasks. The full spectrum measurement showed that the CMBR was in fact thermal, its temperature was determined to be 2.725±0.002 Kelvins (16-19).
COBE also was able to demonstrate the existence of anisotropy in the distribution of the CMBR around the sky. Although definitely thermal in every direction, there were subtle differences in temperature when one part of the sky was compared to the other. The differences were only on the order of a few microKelvins, but that was enough. These anisotropies represent the primordial structure that cosmologists were looking for, the seed that resulted in the structure we see in the universe today (20).
Prior to COBE it was understood that the CMBR had to have a thermal spectrum, in all directions, and this fact was soundly confirmed by the COBE results. It was also understood that the CMBR should be essentially isotropic, and this too was confirmed by COBE. The value of the COBE data was that they provided the first true all sky maps of the CMBR, finally proving that the expectation of theory was matched with fidelity by observation.
But note, it was understood that the CMBR should be essentially isotropic, but not exactly isotropic. This raises the obvious question of how anisotropic one might expect the CMBR to be? This is somewhat model dependent, so it really requires an observation of the anisotropies to pin down the free parameters in the models. COBE provided the first cut at that exercise, and its results were invaluable. COBE demonstrated that there were anisotropies, although perhaps smaller in temperature amplitude than had been expected. Also, the large beams of the COBE DMR made it impossible to see any but the largest angular scale anisotropies. It quickly became evident that much smaller angular scales were required, if one was to pin down the details of Big Bang cosmology via the anisotropies in the CMBR.
The cosmological value of the anisotropies in the CMBR is not it how large they are in temperature units, but how large they are (or how small they are) in angular size on the sky, and how anisotropies of one angular size correlate statistically with anisotropies of other angular sizes. That correlation, which we call the angular power spectrum, can be related directly to key parameters of basic Big Bang cosmology, such as the curvature of space time, the age of the universe, the rate of expansion of the universe, the matter density of the universe, and more. The angular size to which COBE was sensitive was too large to carry out this exercise of fitting the theory to the observsations.
So, the success of COBE sparked a flurry of activity in the following decade, to measure the anisotropies in the CMBR at ever smaller angular scales. BOOMERanG, MAXIMA, DASI, VSA, and CBI are the better known experiments, all of which have published significant results that are referenced on their webpages. In addition to these, the Microwave Anisotropy Probe (MAP) has been gathering data since April 2001. It has already produced one full sky map, and its first data will probably be published before 2002 is gone. MAP will do essentially the same thing COBE did, but with much higher angular resolution. Meanwhile, the European Space Agency is plotting to launch their own MAP-like mission in 2007, Planck. Similar to MAP, Planck will have the added ability to measure the polarization of the CMBR. That could lead to the detection of signatures left behind by gravitational waves roaming the universe, in the era when the CMBR was born.
COBE set the stage by showing that anisotropies did exist in the CMBR. But the angular resolution of COBE was not enough to establish even the first peak in the anisotropy power spectrum. Since that feature would be revealing of the "flatness" of he universe, the post-COBE world of precision cosmology was hot on its trail. Data from BOOMERanG and MAXIMA, amonst others, established that first peak unambiguously. Along with DASI they went on to detect 2nd & 3rd peaks, though perhaps only marginally. But the results from VSA & CBI, announced in spring 2002, established the unambiguous nature of the higher order peaks in the CMB angular power spectrum. We now know that there are are anisotropies in the CMB on size scales down to the upper limit of the size
There is no doubt that Big bang Cosmology is by far the most widely accepted amongst scientists, especially those who specialize in studying astronomy, asrtophysics, and cosmology. This is not just because of some strange commitment to the status quo, or some requirement to be scientifically politically correct. It is because, if you stack the scientific evidence, the observed facts, up against the theoretical explanations offered, then Big Bang cosmology is the one that quite simply works bets, overall. And part of that evidence is the CMBR.
This does not mean that there are no cosmologists who disagree with Big Bang cosmology, there certainly are. Jayant Narlikar, former student of Sir Fred Hoyle, continues the make a case (21) for the Quasi Steady State Cosmology (QSSC) long championed by Hoyle, and his students Narlikar & Wickramasinghe (22,23). Aside from QSSC, there is also the argument of Halton Arp, that redshifts are the result of some unexplained intrinsic process, and that they do not represent a cosmological expansion (24-26). And Australian astronomer David Crawford holds to a form of "tired light" cosmology, where the redshift comes from an intrinsic energy loss mechanism, and not universal expanasion (27).
Those are the alternatives that still seem to have enough content to remain in the regular scientific publications. There are yet others, rarely seen now, such as the esoteric idea of Plasma Cosmology first put forth by Nobel prizewinner Hannes Alfven (28,29), ideas now known to be of such low merit that they are simply ignored.
All of these alternative have to somehow deal with the CMBR, and our increasingly detailed knowledge of it. And they are increasingly unable to do so. The CMBR is one of the major factors that discriminate between Big Bang cosmology, which explicitly requires a CMBR, and alternatives, which allow it, but don't need it. It was after all, Big Bang cosmology which first predicted the existence of a CMBR. And the detailed consistency between the CMBR and Big Bang models of the universe continues to push the Big Bang forward in the field of cosmology, as its rivals slowly fall back.
Formerly known as the Microwave Anisotropy Probe (MAP), this COBE follow on mission was renamed the Wilkinson Microwave Anisotropy Probe (WMAP), in honor of David Wilkinson, cosmologist & MAP team member, who died in September 2002. Launched June 30, 2001, the spacecraft reached its destination (the L2 Lagrange point) on October 1, 2002, and finished its first all sky map in April, 2002. I said earlier that the results would probably be published before 2002 was out, and that was almost correct. First results were announced on February 11, 2003, and published in preprint form on February 12, 2003 (30-42). These results were eventually published in the Astrophysical Journal Supplement 148(1): 1-241, September 2003. The linked preprints have been replaced with the published versions of the papers.
The significant difference between COBE and WMAP is resolution. The COBE DMR maps has a resolution on the sky of about 7°, but WMAP has a resolution that is about 1° down to ¼°. Following 10 years after COBE, WMAP is the only post-COBE experiment to date, which has mapped the whole sky.
Unlike COBE, which was relatively insensitive to anisotropies in the CMBR, the WMAP observatory is designed to measure those anisotropies. That makes WMAP the first all-sky anisotropy mapper. The anisotropies are used to derive several cosmological parameters. It is possible to build a physical model of the universe, by a detailed analysis of anisotropies in the CMBR. So the significant result from WMAP is that, unlike COBE, it can contribute to the developments of such parameters as the Hubble constant, the age of the universe, the presence of dark matter, and the presence of a cosmological constant.
The result that seems to have gotten the most press is a derived age for the universe of 13.7±0.2 billion years. Other results support inflationary cosmology, the presence of non-baryonic dark matter, and a non-zero cosmological constant. I'll discuss these, and other results, in the next section as well.
This section is a short note on what has come to be known as the concordance cosmology. The esoteric worls of CMBR cosmology meets the more well known areas of cosmology, like the HST Key Project, in the ability to derive cosmological parameters from the angular power spectrum of the CMBR. What is remarkable is that these very different approaches to cosmology are yielding essentially the same results. It really amounts to a new era in cosmology; instead of various methods & observations pointing off in different directions, now all of the methods and observations point towards the same basic model of the universe. This creates a strong confidence in the reality of Big Bang cosmology in general, and the specific details of a workable Big Bang model of the universe. The CMBR angular power spectrum, measurements of the Hubble constant via the extragalactic distance scale using cepheid variable stars, and more recent implications of high redshift type Ia supernovae, all lead to the same small set of conclusions.
It certainly appears that we are zeroing in on a real cosmology, and this may be one of the key scientific features of the beggining of the 21st century. After decades of confusion, cosmology is settling in as an exact science. Our ability to make precise measurements of the CMBR is a major feature of that new scientific environment.
Places to go on the web, where you can deal with the CMBR and Big Bang cosmology in far more detail than I can handdle here on my one small webpage.
Tim Thompson July 2002, updated January 2004