Implications of the COBE DMR Map

of the Early Universe

What COBE DMR saw:

The COBE DMR (Cosmic Background Explorer Differential Microwave Radiometer) mapped the microwave (wavelengths of 9, 5,6, and 3.3 mm) sky showed signal from the relic radiation from the early phases if the Big Bang. First it was shown to be extremely uniform meaning that the early Universe was extremely uniform. There were variations in signal from the early Universe at a level of one part in 100,000. These variations tell us about what happened before this light left the photosphere (like the surface of the Sun) and provided information about how the Universe will develop after this time. The tiny variations of intensity of CMB light (photons) provide an image of the density variations in the early universe which under the influence of gravity accrete and grow to be the large scale structure (galaxies, groups and clusters of galaxies, and clusters of clusters) present in the Universe today. These variations were so physically large that they must have been either original or put there by extraordinary physical processes such as Inflation.
The COBE DMR maps reveal the Universe when it was roughly 300, 000 years old (past the beginning of the Big Bang and time as we understand it). This seems to be quite old by human standards until we compare it to the current age of the Universe of about 14 billion (14 x 10^9) years today. Put in human terms, if the Universe were a middle-aged person today, then the epoch revealed by the COBE DMR maps corresponds to an image of an embryo at 10 hours age. Thus we have an image of the Universe at an extremely early epoch in its development.

What this meant!

Public and Media Interest

The COBE DMR discovery had an immediate public and scientific impact. The public and media interest was overwhelming. The COBE DMR results were carried on the front page of most newspapers throughout the world. Media interviews and public discussion continued for months. Magazines carried in depth stories and a number of scientific TV shows featured the results. The implications of an image of the very early Universe were well appreciated by the media and the public. NASA was properly proud of its first satellite dedicated to cosmology.
This interest, support and pride made it possible for next generation experiments to obtain public, institutional, and to some extent scientific support - though the science implications and future potential would eventually provide the continuing scientific support.

Scientific Interest and Implications

Scientists were excited and that generated public interest and that public interest fed back to give the scientists momentum to push forward on theoretical implications and the development of new CMB experiments.
First the COBE DMR results showed that the Big Bang model was on firm footing. Before the discovery there were doubters saying that no fluctuations would be found and that would mean that the Big Bang could not explain the formation of galaxies and clusters of galaxies. Had COBE DMR not found fluctuations at the part in 100,000 level but had shown they were not there more than a factor of two smaller ( one part in 200,000) then galaxies and clusters would not have had sufficient level of seeding to form. Instead COBE DMR found that they are just at the appropriate level in our Universe. (It took a couple of years for the arguments and controversy to settle down and general agreement to this point.)

Second the COBE DMR showed that the typical amplitude of these fluctuations was essentially independent of their physical size. That is, large length scale fluctuations, medium length, and smaller lengths all have about the same average (actually rms or absolute value as some are negative and some positive indicating slight over and under densities) variations. This is just the right (like Goldilocks) size dependence. If there were bigger amplitudes at smaller scales, the Universe would have a lot of black holes in it all around. If there were bigger amplitudes on larger scales then there would be giant aggregations of matter pulling everything towards them at high speeds due to the acceleration of gravity over the 14 billion years since the Big Bang started. This provides the basic input conditions for those studying how stars, galaxies, clusters, and so on formed.

Third the COBE DMR provides information on the primordial conditions in the even earlier Universe. For the scientists that are interested and study how the Universe is formed and ultra-high energy physical process, this provided the first look into regions where terrestrial accelerators and experiments did not reach and for some things not likely to reach. Of great interest is what the maps might tell us about Inflation. Inflation is a model of the Big Bang in which at a very early time the Universe undergoes a rapid and accelerating (increasingly rapid) expansion. This solves problems in the Big Bang (or set up appropriate conditions). The long period of acceleration provides a means to smooth out the early universe taking a small homogeneous region and expanding it to a point it is now a size larger than we can see today. That expansion would make space nearly flat (Euclidean - meaning there is little or no curvature to our three dimensional space). The expansion also dilutes nearly all initial impurities to the point that they are negligible today. The rapid, accelerating expansion would also generate fluctuations on the otherwise smooth Universe by generating small quantum fluctuations (rapid expansion) and pulling them out to astronomical scales but frozen in amplitude (accelerating expansion). This rapid, accelerating expansion must be driven by a large energy everywhere in space and at the end of inflation that energy is converted to the be the particles and energy the Universe is made of today. To make such fluctuations probably requires energies billions to trillions of times higher than any man-made accelerator being consider for construction. It is likely to be at the energy scale of the Grand Unification of forces - the energy at which basic forces all show that they are the same basic force and have frozen out to different forces in the cooler present Universe. We see signs that this might happen from experiments and theory. The CMB maps allow one to explore this region most directly.

Fourth the COBE DMR discovery of variations not only meant that a lot of these theories are supported by the data but also indicated that further more precise observations in terms of angular resolution (the DMR beam size was rough 7 degrees) would have signal to see and theory to be developed. Both theory and observations have made substantial progress in the 10 years since the COBE DMR announcement of variations (anisotropy). It is now clear that the CMB can provide a wealth of cosmological information and has developed already into a powerful cosmological probe. In 2000 the MAXIMA and BOOMERANG balloon-borne experiments and then in more detail in 2001 and the DASI ground-based interferometer adding the showed that the spectrum of fluctuations has features that move away from scale independence on smaller angular scales. In particular that there were characteristic fluctuations on angular scales of about 1 degree, 1/2 degree, and 1/3 degree. The first peak is strongest and the next are harmonics, as in music. Following them in 2002 the Archeops balloon-borne detector mapped nearly 1/5 the sky and 12.5% of the sky was extremely clean and provided a beautiful picture of the sky with resolution of 1/6 degree. The location of the first peak provides very strong evidence that the Universe is flat or nearly so - as predicted by Inflationary models. The existence of the first, second, and third peaks is more direct evidence of the acoustic oscillations that were predicted (originally by Andre Sakarov) as the beginning steps before structure formation starts fully. These are powerful confirmations of our cosmological model and show the power of CMB observations.
But that is not all. Earlier this year the CBI experiment provided measurements on much smaller angular scale and detected power out in the regions where the higher harmonics of the acoustic oscillations are expected (1/4 degree, 1/5 degree, 1/6 degree, 1/7 degree). This power is falling in roughly the expected way due to damping and photon leakage from the fluctuations as predicted by Prof. Joe Silk and thus called Silk damping. This was followed late this fall by even more precise results from the ACBAR experiment operating from the high, dry  (and cold) site at the NSF's South Pole Station. This is the same site as the DASI experiment which recently (September 2002) reported detection of CMB polarization. Theory predicts that CMB polarization must be present at about 10% of the level of CMB temperature variations (anisotropy), if our cosmological picture is correct. There is much more that can be learned for CMB polarization observations including direct information about the energy scale of Inflation process.
These are only some of the experiments and theory motivated and enabled by the COBE DMR discovery and maps (some from the data and from the substantial public support and interest). We are awaiting the first results from the first of the two following satellites: NASA's MAP (Microwave Anisotropy Probe) already taking data and expected to confirm established results including COBE DMR and to improve them with full sky coverage and thus more samples of our Universe and the ESA/NASA (European Space Agency/National Aeronautics and Space Administration) Planck Surveyor planned for launch in 2007 as a third generation CMB satellite.

Thus the COBE DMR not only made a major discovery about the Universe but also became the pioneer in a field which is consistently making new observations about our Universe.
It set a high standard with the quality of its observations both in terms of the input and maps and in terms of its science. Succeeding experiments have had to strive to meet and exceed this high level and as such this has spilled out into cosmology as a whole. Most cosmological experiments are now measured in comparison to Planck surveyor and held to high standards. The excitement following the announcement of the COBE DMR discovery made it more exciting and appropriate to do such experiments and for funding agencies to support them.

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