Royal Swedish Academy of Sciences announced on October 3 that the Nobel Prize in Physics for 2006 would be awarded to Dr. J.C. Mather (NASA Goddard Space Flight Center) and Prof. G. F. Smoot (University of California, Berkeley).

  This is the second time that laureates have proved the Big-bang cosmology as the origins of the Universe through the observations of cosmic microwave background radiation. The first time was in 1978 when the prize was awarded to A. A. Penzias and R. W. Wilson (Both in Bell Telephone Laboratories, Inc.) <1> who discovered for the first time the cosmic microwave background while investigating the source of unexpected noise in their radio-receiver.

  According to the Big-bang cosmology, our Universe was created 13.7 billion years ago in a huge explosion and expansion from a primeval fireball with high-density and high-temperature confined in a very small volume. During expansion and cool down process of the Universe, there was an era called “decoupling of matter and radiation”, when the matter and radiation could exist independently through the Universe. The background radiation gradually cools down as the Universe expands, now corresponds to the radiation emitted by a blackbody with a temperature of 2.7K. This is so called “Cosmic microwave background radiation”<2>. In fact, Planck's Law can lead to this radiation which has its maximum at the wavelength of about 1mm (Frequency 300 GHz), decreasing smoothly in both sides of the peak: Terahertz electromagnetic wave exactly sitting in the short wavelength side of the background radiation spectrum.

  However, Penzias and Wilson’s experiments were not enough to measure the short wavelength side of the peak due to the limitation of their radio technology. Then my old acquaintance Prof. P. L. Richards, University of California, Berkeley, engaged in the measurement of the entire 2.7K thermal blackbody by far-infrared technique. Richards made the best of his advanced technique of sensitive cooling semiconductor detector as well as his knowledge about Fourier-transform spectroscopy. Dr. Mather, one of the Nobel Prize winners, was his student in the doctoral course engaged actually in the measurement under his supervision. The measurements were made using balloons at the altitude of approximately 40 kilometer. But even at such high altitude the inevitable water vapor absorption had affected the measurement, hence he could not obtain clear results. His results at that time appeared in <3>. Dr. Mather got doctorate in this research and joined NASA. The measurement was taken over to another doctoral student, Dr. Woody. He improved the device<4> and obtained clear results before finishing the doctorate<5>. However, it still remained the influence of the water vapor absorption in the measurements at the altitude of the balloon.

  Having observed these activities, Prof. Yukio Hayakawa (president of Nagoya University, afterwards) suggested undertaking of the measurement outside the Earth’s atmosphere. Thus, the project started in collaboration with Prof. Richards. During the experiments with rockets, many young students in the Hayakawa Laboratory, especially Dr. Toshio Matsumoto, the assistant professor at that time and the actual emeritus professor of JAXA, dedicated in developing improved instruments day and night. I was asked by Prof. Hayakawa to incorporate my knowledge about far-infrared region (currently named “terahertz”) into the project. The instrument in the rocket was a combination of various types of filters and cooling detectors<6>. The experiments were held three times in total during 1985 - 1989, where the background radiation was monitored at the six bands chosen along with a 2.7K Planck radiation spectrum<7,8>. The paper reported the results was attracted a worldwide attention and was cited in more than 100 papers.

  Looking at these activities, Dr. Mather worked as a driving force for a gigantic project using a satellite at NASA Goddard Space Flight Center. The satellite named COBE (Cosmic Background Explorer) carried three different high-sensitive instruments covering the large wavelength range from 1μm to 1cm to measure the spectrum of the cosmic microwave background with high precision in all directions: FIRAS (Far InfraRed Absolute Spectrophotometer, Fig. 1), DMR (Differential Microwave Radiometer, Fig. 2) and DIRBE (Diffuse InfraRed Background Experiment).

Fig. 1: FIRAS（９）	Fig. 2: DMR（９）

　FIRAS measured the spectral distribution of the cosmic background radiation in the wavelength range 100μm - 1cm at one thousand measuring points over the entire sky and determined the deviation of the spectrum from the blackbody radiation with the precision of 1/1000. Any deviation would indicate a signature of amazingly intense-radiation in the early days of the Universe. Dr. Mather was also the principal investigator for FIRAS instrument. DMR measured the difference in microwave radiation intensities between two separate points with the precision of 1/100,000. The data from DMR instrument was used for discovering of the “origin” of the cosmic structure such as anisotropic expansion, rotation, gravitation wave, large scale matter flow or cosmic string. Prof. G. F. Smoot, one of the Nobel Prize winners, was responsible for the DMR. DIRBE could measure the absolute radiation intensity between 1μm to 300μm, covering mainly the infrared region. The instrument enabled them to measure the diffused infrared radiation emitted from the source like a protogalaxy or stars in the First generation at the early time of the Universe with the highest precision ever before. We can know the mechanism of the related radiation if the radiation spectrum becomes clear in this wavelength band. Dr. M.G. Hauser was the principal researcher of the DIRBE. This year’s Nobel Prizes were awarded to the principal investigators of FIRAS and DMR <2>.

　I was paying attention especially to the Fourier-transform spectrometer using liquid helium in FIRAS at that time (Fig. 1). Performing the experiment in space with liquid helium was a real surprise for me, because even earthbound measurements at room temperatures in the far-infrared region were difficult in those days. The COBE satellite was finally launched on November 18, 1989 and the measurement began from December. The first results arrived after only nine minutes of observations: The data follow perfectly a blackbody spectrum with the temperature 2.735 ± 0.060K<10>. When the data was faxed to JAXA, I was there by chance. The Planck’s law perfectly fits to the data, numerous small squares in Fig.3 obtained by the COBE measurement. Planck’s law and the measurement result were so coincident that the data was something doubtful in early times, although we learned afterwards how precise it was.

Fig.3: COBE result（１０）

<References>

(1) A.A. Penzias and R.W. Wilson, “A Measurement of Excess Antenna Temperature at
4080 Mc/s", Astrophys. J. 142, 419 (1965).

(2) S.Gulkis, P.M.Lubin, S.S.Meyer, R.F. Silverberg (translated into Japanese by H. Okuda), COSMIC BACKGROUND EXPLORER (COBE), Science, 20(3), 90 (1990).

(3) J.C. Mather, P.L. Richards and D.P. Woody, “Balloon-Based Measurements of the Cosmic Background Radiation", IEEE Trans. Microwave Theory Tech. MTT-22, 1046(1974).

(4) D.P. Woody, J.C. Mather, N.S. Nishioka and P.L. Richards, “Measurement of the Spectrum of the Submillimeter Cosmic Background", Phys. Rev. Lett. 34, 1036 (1975).

(5) D.P. Woody and P.L. Richards, “Spectrum of the Cosmic Background Radiation", Phys. Rev. Lett. 42, 925 (1979).

(6) A.E. Lange, S. Hayakawa, T. Matsumoto, H. Matsuo, H. Murakami, P.L. Richards and S. Sato, “Rocket-borne submillimeter radiometer", Appl. Opt. 26, 401 (1987).

(7) S. Hayakawa, T. Matsumoto, H. Matsuo, H. Murakami, S. Sato, A.E. Lange and P.L. Richards, “Cosmological Implication of a New Measurement of the Submillimeter Background Radiation", Pub. Astron. Soc. Jap. 39, 941 (1987).

(8) T. Matsumoto, S. Hayakawa, H. Matsuo, H. Murakami, S. Sato, A.E. Lange and P.L. Richards, “The Submillimeter Spectrum of the Cosmic Background Radiation", Astrophys. J. 329, 567 (1988).

(9) J.C. Mather, “The Cosmic Background Explorer (COBE)", Opt. Eng. 21(4), 769 (1982).

(10) J.C. Mather, E.S. Cheng, R.E. Eplee,Jr., R.B. Isaacman, S.S. Meyer, R.A. Shafer, R. Weiss, E.L. Wright, C.L. Bennett, N.W. Boggess, E. Dwek, S. Gulkis, M.G. Hauser, M. Janssen, T. Kelsall, P.M. Lubin, S.H. Moseley,Jr. T.L. Murdock, R.F. Silverberg, G.F. Smoot and D.T. Wilkinson, “A Preliminary Measurement of the Cosmic Microwave Background Spectrum by the Cosmic Background Explorer (COBE) Satellite", Astrophys. J. Lett. 354, L37 (1990).

<Address of thanks>

I would like to take this opportunity to give a special thanks to Mr. Hiroshi Matsuo (present assistant professor at National Astronomical Observatory) for his significant information, who joined the rocket experiment as a postgraduate student in the Hayakawa laboratory.

President, Terahertz Technology Forum

Kiyomi Sakai