ar X iv : a st ro - p h / 97 02 04 2 v 1 4 F eb 1 99 7 Gold in the Doppler Hills : Cosmological Parameters in the Microwave Background

Research on the cosmic microwave background (CMB) is progressing rapidly. New experimental groups are popping up and two new satellites will be launched. The current enthusiasm to measure fluctuations in the CMB power spectrum at angular scales between 0 • .1 and 1 • is largely motivated by the expectation that CMB determinations of cosmological parameters will be of unprecedented precision: cosmological gold. In this article I will try to answer the following questions: • What is the CMB? • What are cosmological parameters? • What is the CMB power spectrum? • What are all those bumps in the power spectrum? • What are the current CMB constraints on cosmological parameters? 1. What is the Cosmic Microwave Background? Thirty years ago Penzias and Wilson (1965) discovered excess noise in their horn antenna in Holmdel, New Jersey. The measured temperature of this noise was ∼ 3 K and it did not vary in intensity over the sky; it was isotropic. They received the Nobel Prize for this serendipitous discovery of the cosmic microwave background (CMB) radiation. The prediction of the existence of a CMB and of its temperature (Alpher & Herman 1948) followed by its detection, provides possibly the strongest evidence for the Big Bang. The observable Universe is expanding and cooling. Therefore in the past it was hotter and smaller. The CMB is the after-glow of thermal radiation left over from this hot early epoch. It is the redshifted relic of the Big Bang. The CMB is a bath of photons coming from every direction with wavelengths about as big as these letters. There are about 415 of them in every cubic centimeter of the Universe. These are the oldest photons one can observe (see Figure 1). Their long journey towards us has lasted 99.997% of the age of the Universe; a journey which began when the photons were last scattered by free electrons of the ubiquitous cosmic plasma, when the Universe was 1000 times smaller and the temperature 1000 times higher than the CMB is today. The CMB contains 1