Bill Wheaton, Caltech
2002 February
MAP at L2:
A little over twelve years ago, in November 1989, NASA launched COBE, the Cosmic Background Explorer. As the first space mission to explore the Cosmic Microwave Background (CMB), COBE was a major contributor to our understanding of the early history of the universe. The June launch of MAP, the Microwave Anisotropy Probe, continues this ongoing revolution in cosmology. Last November, after a journey of 1.5 million km and five months to the outer Earth-Sun Lagrange point L2 (on the extension of the Earth-Sun line beyond Earth), MAP settled into its proper station for the next several years of observing. Cosmology is a big subject in every sense, one that needs continual going over and probably even some due scepticism, given its richness in theoretical elaboration and dearth of observational detail. Yet with the radical improvement in our observational capabilities during the past decades, the observations are rapidly becoming more and more constraining to our theoretical and physical understanding. While it will not be possible to cover the whole vast subject here, still we should make some effort to explore the questions MAP is trying to address, and set the stage for the results that are even now being accumulated by the investigators. Thus we will make a beginning, and hope to work away at the subject in succeeding months, a little at a time.
At the start of our story was Einstein, who pondered the overall structure of the Universe as soon as he had developed his stunning General Theory of Relativity (GTR). GTR is a theory of gravitation, but also a theory of the structure and geometry of space and how that structure changes with time: the stuff of cosmology indeed. In GTR, gravity results from a curvature of space and time (4-dimensional space-time, that is), a curvature induced by the presence of energy and of stress. Just as a two-dimensional surface embedded in our ordinary three-dimensional space may be flat like a tabletop or curved like the side of a race car, so it is with space-time. In a flat space, the ordinary rules of Euclidean geometry hold. For example, the circumference of a circle is pi times its diameter, and the sum of the interior angles of a plane triangle equals 180°. And as small regions of Kansas might appear pretty flat to a human surveyor, so the closely Euclidean behaviour of the relatively small regions of space and time which we normally experience does not guarantee that the larger form is flat.
In his study of the differential equations his theory implied for the structure of space-time, and their possible solutions, Einstein found that in order to get a static homogeneous cosmos, he had to add an ad hoc repulsive term, the cosmological constant, to support everything against the steady attraction of gravity. But then just few years later Hubble discovered the recession of external galaxies and the expansion of the universe. As a result, the need for the cosmological constant seemed to vanish for a long time, since the dynamically expanding solutions appeared to be the appropriate ones. This family of solutions called for an early, dense hot phase in cosmic history, giving rise to the idea of the Big Bang.
Long years after the earliest moments, the remains of the explosion cooled sufficiently that protons could easily capture electrons, and make neutral hydrogen atoms. By sometime around 400,000 years, the free electrons were virtually all gone. Then what had been a white-hot (about 3000 K), opaque mixture of gas, plasma, and light became almost perfectly transparent neutral hydrogen, with some helium and a trace of other light elements mixed in. The light was still there too, left with a nearly perfect "black body" spectrum, characteristic of the temperature when the fog of white-hot plasma cleared, but now "decoupled" from the matter, so that the two components went their separate ways for a long while, intermingled yet hardly interacting. (There would have been neutrinos, too, and probably other cryptic components not yet known to us as well, but they did not much enter into the conception.) For billions of years the expansion continued, the gas cooled, and the light was red-shifted by gravity so that it appeared still with the nearly-perfect black body spectral form it had had from the start, but at a progressively lower and lower temperature. Today that radiation has cooled to a temperature of 2.725 K, measured by COBE with an uncertainty of only about 0.001 K. From a blinding white-hot flash, the radiation has become microwaves, the merest whispers of the creation. The existence and properties of the CMB are probably the single strongest confirmation we have that our basic picture of the early universe is substantially correct. Aside from the "dipole effect", due to the special velocity of the Sun and Earth (since individual stars and galaxies are not generally perfectly at rest with respect to it), the temperature is nearly constant in all directions.
The radiation is also very nearly perfectly smooth, with only the subtlest spatial irregularities being detected by COBE. Lately exciting observations, taken from balloons floating above Antarctica, have confirmed and extended the COBE measurements, though only for small regions of the sky. Yet the Cosmic Microwave Background is nothing less than the actual flash of the Big Bang: if you like, the legendary Fiat Lux! declared In the Beginning. (No wonder it inspires awe among us!) So in those tiny ripples observed in the photosphere of the expanding explosion, are still to be seen the marks of the earliest processes at work, the sound waves, shocks, and turbulence of that beginning. To observe it clearly, we must cool our instruments to liquid helium temperatures, seek stability to the micro-kelvin level, escape from all confusing foreground radiations, and find the nearly perfect electronic silence at L2: a million miles beyond the Earth and Moon. From the point of view of the mission designer, this is what MAP is all about.
We will return to these matters again; until then, further details may be found at:
Bill Wheaton