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Cosmology is the Focus of LIGO's Summer 2001 Public Lecture

Cosmology is the Focus of LIGO's Summer 2001 Public Lecture

- Contributed by Fred Raab

This summer, Dr. Marc Kamionkowski, Professor of Physics and Theoretical Astrophysics at Caltech, presented a LIGO Public Lecture on "The Birth of the Universe." The event was held at the Battelle Auditorium in Richland, Washington on August 12. This was the second time the LIGO Hanford Observatory has sponsored such a lecture, timed to kick off the LIGO Science Collaboration meeting held each summer at the observatory. LIGO's Public Lectures seek to convey the excitement of leading edge science to the general public. Last year John Wheeler and Kip Thorne reprised the connections between the birth of the nuclear age and the later development of LIGO as it paralleled Wheeler's career. In this year's lecture, Prof. Kamionkowski described how the recent imaging of relic light from the big bang has impacted our concepts of the past and future of the universe.

From ancient time onward, many have wondered both how the universe began and what its eventual fate will be. In modern times this inquiry has led to new insights into the nature of space and time, and matter and energy. By the mid-1900's, far-reaching developments in General Relativity and Particle Physics pointed to the idea of a primordial "Big Bang" that evolved into our present universe. Relic light from the Big Bang was first detected in the 1960's by Penzias and Wilson at Bell Labs, who were working on the fundamentals of modern communications systems. The first images of the infant universe emerged in the 1990's from a satellite mission, the Cosmic Background Explorer (COBE). These images captured the look of our 15 billion-year old universe at the tender age of about 300,000 years. In the first 300,000 years, space expanded rapidly and the material of our existence gradually froze out as the expansion cooled the universe. First came exotic particles. Later more familiar forms of matter like protons and electrons formed. But these particles were dispersed throughout the universe in a plasma, with no atoms in sight. A plasma is a gas of charged particles and these "free" charges strongly absorb and emit light, basically "confining" the light within the plasma. Visualize what it is like trying to drive at night through a deep fog and you pretty much have the picture. Light from distant objects is scrambled, while light emitted from your headlights comes back at you as much as it penetrates forward. You may not even be able to see the road. At about 300,000 years, the expansion had cooled the universe enough that protons and electrons could "freeze" into stable hydrogen atoms. Hydrogen atoms only absorb light in a very narrow band of wavelength in the ultraviolet, so the universe went from a "fogged" state to clarity as the atoms formed. Photons of light--carrying the properties of the early universe--could now transverse the space and time of the universe. And, about 15 billion years later, COBE scientists captured some of these photons, now chilled down to radio wavelengths, and assembled them into a fuzzy portrait of the early universe.

These fuzzy COBE pictures show the sky as extremely uniform in temperature, hovering at about 3 degrees Kelvin, very close to absolute zero. But there were small blotchy patches where the temperature was higher or lower by about 10 millionths of a degree. The accompanying density fluctuations would be the seeds of structure (stars, galaxies, clusters of galaxies, etc.) in our modern universe. Scientists scrambled to prepare new experiments to probe the cosmic background with higher resolution instruments. And the clearer pictures started to roll out in the late 1990's. These better defined images made possible a measurement of the geometry of the universe.

A "flat" universe would follow the same rules of geometry we learn in high-school math. The interior angles of a triangle would add up to 180 degrees and parallel lines would keep a constant separation. In a universe with positive geometry, sheets of space would follow rules similar to rules of navigation on the curved surface of the earth. You can find a triangle on a globe whose base is on the equator with right angles in South America, Africa and at the North Pole--a total of 270 degrees--and parallel lines running North/South from the equator will intersect at the North and South Poles! Sheets of space in a universe with negative curvature would have properties similar to the surface of a potato chip, having triangles with less than 180 degrees in their interior and parallel lines that diverge. The splotches of higher and lower temperature on the sky, seen at a great distance, can tell us the geometry of the intervening space. The splotches should have a characteristic size, related to the number of years that the objects were in equilibrium (before the "fog" cleared) times the speed of light (the fastest that information could have flowed across the blotches). Divide that by the number of light years the relic photons have traveled since the fog cleared (about 15 billion years) and you have the angular size (in radians of arc) of the blotches in a flat space. Now compare that with images of the cosmic background. Blotch size in the images should agree with this calculation if the universe is flat and systematically larger or smaller blotches indicate positive or negative curvature. Using the high resolution photos from the Boomerang mission, a high altitude balloon-borne detector that circled the South Pole, and other recent images, it is evident that the universe is flat!

These images strikingly confront our concepts of the universe and our place within it. But much of the development of the universe occurred while it was very hot, long before the relic photons were emitted. In his public lecture, Prof. Kamionkowski showed how one can try to read the imprint of relic gravitational waves, which look back much farther into the early universe. The next decade should allow us to glimpse this new horizon.

To learn more, please visit LIGO's Prof. Kamionkowski Public Lecture webpage, which includes links to video and audio coverage of the lecture, transparencies presented, and more.