To understand the COBE results we must go right back to 1965 for the discovery of the microwave background. This happened more or less by accident! Arno Penzias and Robert Wilson found an excess noise temperature of about 3.5 ± 1 K no matter where they pointed their telescope. At the same time physicists at Princeton were planning an experiment to detect the much-diluted thermal radiation left from the Big Bang. It was quickly realized that Penzias and Wilson had detected the signal being sought by the Princeton group, who soon measured a background temperature of 3.0 ± 0.5 K. This second temperature measurement did not really settle the question as to whether the background was a true thermal spectrum. In the U.K., for example, a handful of followers of the cosmologist Fred Hoyle continued to reject the results as supporting the Big Bang model of the universe.

Initially a relatively small number of physicists understood the importance of the discovery for learning about conditions in the early universe. Almost immediately there was an understanding that there must be temperature anisotropies at some level if the radiation was coming from the Big Bang, and naturally people began to ask themselves how they could search for the temperature fluctuations. In 1967 Robert Sachs and Arthur Wolfe predicted that temperature anisotropies might be about 1%. David Wilkinson in the gravitational physics group at Princeton quickly established himself as a major player in this area, trying to find the temperature fluctuations, which many people at the time thought was a far-out thing to attempt. And of course every time an experimental group set a limit on these fluctuations, the theorists would tell them, "Ah, but now we calculate that the fluctuations are just a little smaller than your upper limit! You should try a little harder." It was a difficult business that went on for 27 years.

Most-Cited Papers by Charles L. Bennett et al., Published Since 1992 (Ranked by total citations) Rank Paper Citations 1 G.F. Smoot, et al., "Structure in the COBE Differential Microwave Radiometer first-year maps," Astrophys. J., 396(1): L1, 1992. 1,172 2 c Astrophys. J. Suppl. Ser., 148(1): 175-94, 2003. 1,008 3 C.L. Bennett, et al., "First-year Wilkinson Microwave Anisotropy Probe (WMAP) observations: Preliminary maps and basic results," Astrophys. J. Suppl. Ser., 148(1): 1-27, 2003. 539 4 C.L. Bennett, et al., "Four-year COBE DMR cosmic microwave background observations: Maps and basic results," Astrophys. J., 464(1): L1, 1996. 360 5 E.L. Wright, et al., "The interpretation of the cosmic microwave background radiation anisotropy detected by the COBE Differential Microwave Radiometer," Astrophys. J., 396(1): L13-8, 1992. 335 SOURCE: Thomson Scientific Web of Science

The observers strove harder and harder, and it began to be an important problem that these temperature fluctuations were not detected. That’s because the non-detection reached the point where it was challenging the generally held idea that gravity was the major influence on the formation of structure in the universe. In fact, this is where cold dark matter comes into play. You couldn’t make our universe with just baryons and not see fluctuations. Cold dark matter was a way of having more matter to form structures without raising the temperature fluctuations to a detectable level. Two concepts then became closely coupled: the absence of detectable fluctuations, and acceptance that baryons might be a minor component of the matter density. These difficulties continued until the COBE results of 1992.

  In a 1994 paper from COBE’s Far Infrared Absolute Spectrophotometer data, your team says the results carry information about the universe when it was just one year old, at a redshift of 3 x 10⁶. Is this an exaggerated claim?

The point is that spectrum reaches back to the epoch when the production and destruction of photons stopped. So the photons do indeed come from that early time. And, of course, this just emphasizes in a different way the richness of the data in the microwave background.

  How did the COBE results stimulate the WMAP mission?

There had been theoretical papers talking about all the features that might be included in the radiation. Back in 1969 Rashid Sunyaev and Yakov Zel’dovich in Moscow showed that the cosmic microwave background radiation temperature fluctuations would be affected by its interaction with matter. However, papers about the interpretation of fluctuations initially had no impact because of the failure to detect any fluctuations. Naturally this changed dramatically with the publication of the COBE results. All of a sudden there was a sea change. The community said, "Now that we know the fluctuations are there, we need another space mission to investigate their character more closely." It was no secret that a new mission would provide information about the cosmological parameters. Theorists were all over this, and the experimentalists immediately began talking about how a new space mission could follow up on the COBE results.

  The new mission was named the Wilkinson Microwave Anisotropy Probe for a person you mentioned above, David Wilkinson of Princeton, the world-renowned cosmologist who died in September 2002, as you received the initial data and did preliminary analyses.

He was on the COBE team with me, and we had talked about COBE’s successor. David was one of the many people who wanted the follow-up, and in the end he and I put together the team, largely a collaboration between Princeton and the Goddard Space Science Center.

  How do the WMAP results relate to the balloon experiments in Antarctica and the radio astronomy projects in locations such as Tenerife?

The first point to make here is that the ground-based and balloon-borne experiments enable the technology development and scientific maturity that motivate space missions. The CMB results from the ground, balloons, and space complement each other, which is extremely important because the data were taken in an independent manner. Independently corroborated results were not common in observational cosmology. We are all in entirely new territory in terms of the confidence we have in the value of the cosmological parameters.

WMAP launched in 2001, and it has now mapped the temperature variations, or anisotropy, of the cosmic microwave background radiation over the full sky with unprecedented accuracy and precision. These observations provide definitive answers to cosmological questions and open the door to new investigations. For example, the WMAP has determined that the content of the universe is dominated by dark matter and dark energy. The large-scale geometry of the universe is flat, in that the sum of the interior angles of a triangle adds up to 180 degrees even over vast distances. New limits are set on the mass of neutrinos and the nature (or equation of state) of the dark energy. The WMAP results also place new limits on the physics of the very early universe, usually described in terms of Inflation theory: a rapid exponential expansion of the universe within a fraction of a second. Observations are ongoing and will improve our understanding of the physics of the universe.

  The citation rate of the paper is actually accelerating!

Over the last two decades a detailed standard cosmology has finally emerged. Using only a few parameters this model describes the evolution of the universe on scales varying from a few to thousands of megaparsecs. In this model, our universe is spatially flat, homogeneous, and isotropic on large scales. It contains radiation, ordinary matter, non-baryonic dark matter, and dark energy. WMAP was designed to offer a demanding quantitative test of this model, taking care to eliminate systematic measurement errors. WMAP produced precision data specifically for the accurate testing of cosmological models.

The observed parameters now tightly constrain the geometry, the content, and the properties of the universe. Let me give you some examples. We have an age for the universe of 13.7 ± 0.2 Gyr. Not so long ago this value would swing around ~ ±3 Gyr, according to whatever astronomical observations were used for the latest estimate. When I first entered astronomy, the Hubble parameter was considered by some to be uncertain by a factor as large as 2; now we have h=0.72 ± 0.05 from WMAP data alone. Dark matter and dark energy are now widely accepted by the community.

It is gratifying that the WMAP mission achieved its goal of accurately measuring the parameters, resulting in the principal features of the Standard Model of cosmology. WMAP scientists, such as Lyman Page of Princeton and Gary Hinshaw of NASA Goddard, deserve enormous credit for their accomplishments. However, it is humbling to realize that enormous problems remain. The universe is dominated by a cold dark matter that we have not identified and by a mysterious dark energy with a nature that has hardly been constrained at all. Without determining the nature of the dark energy, we cannot hope to specify the fate of the universe. On the other extreme, we do not know what happened at the earliest times in the universe, but measurements are possible. Many of our historic cosmological questions have been answered, but deeper important challenges remain. There’s much work to be done.