Observing the Big Bang
Session 120 -- Heineman Prize Lecture
Invited presentation, Saturday, January 15, 12:00-12:50, Salons III/IV Room (Crystal Gateway)

## [120.01] Observing the Big Bang

John C. Mather (NASA Goddard Space Flight Center)

The COBE satellite has measured the large scale anisotropy of the cosmic microwave background radiation, set strict limits on the distortion of its spectrum from the the predicted blackbody form, and obtained the necessary data to determine the infrared background from primeval galaxies. As a result, theoretical analysis of the formation of large scale structure of the universe can now begin with a measurement of the gravitational potential at the time of recombination. We also know that less than 0.03\% of the energy in the background radiation was added after the first year.

I will briefly describe the instruments and spacecraft, to show how differential techniques and long observations in a stable and protected environment were used to obtain orders of magnitude improvements in sensitivity and accuracy. There are three instruments to cover the wavelength range from 1.2 $\mu$m to 1 cm, and all have worked so well that their cosmological results are limited primarily by diffuse emissions from our own solar system and the Galaxy. All instruments required special designs and observing techniques to avoid systematic errors and distinguish the foreground emissions.

These include the continuum emission from diffuse hot electrons in the Galaxy, emitting by synchrotron and free-free processes, and from interstellar dust, heated by starlight to temperatures of 15 to 30 K with tiny amounts at much lower and higher temperatures. The COBE also mapped 9 interstellar molecular, atomic, and ionic far IR transitions of CO, [C~I], [C~II], and [N~II]. At shorter wavelengths, it measured the emission of the cool giant stars in the Galaxy, and the absorption, reddening, and polarization of their light by interstellar dust. The zodiacal light was observed over a wide range of angles from the Sun, and 4 comets were mapped as well.

I will conclude with a summary of the current measurement programs using ground-based and balloon-borne instruments, and a discussion of the importance and limits to accuracy of background measurements.