What kind of reversible process in a liquid could show the path of a flying particle and quickly erase the track after its passage? It would have to be a process that magnified the tiny effect of the atomic particle itself, as the condensation of droplets in supersaturated vapor magnifies the ionization produced by a particle in a cloud chamber. It occurred to me that a superheated liquid, like a supersaturated vapor, might provide the desired unstable equilibrium that could be triggered by a small stimulus to yield a large effect. Physical chemists have long known that in a clean, smooth-walled vessel a very pure liquid may be heated above its usual boiling point without boiling. I wondered whether a flying particle might, under suitable conditions, trigger the formation of the microscopic bubbles that start the boiling process. If so, it might make a visible track in a superheated liquid.

I used a bulb (half an inch in inside diameter) filled with ether; it was connected by a capillary tube to a piston-fitted cylinder with a hand crank which could quickly lower the pressure. High-speed movies, at the rate of 3,000 pictures per second, were made of the happenings in the bulb when the pressure was reduced. Sure enough, the pictures disclosed a track of tiny bubbles when a particle darted through the superheated ether. The bubble type of chamber soon proved to be a very sensitive recorder. Even fast mu mesons, which ionize only lightly, made visible tracks in the superheated liquid.

Having demonstrated that the bubble chamber idea worked, we proceeded to the task of building one large enough for practical laboratory use. We first built a two-inch chamber of duralumin and glass, with a diaphragm, actuated by compressed air, which could fully expand the chamber in five thousandths of a second. The liquid remained sensitive for seven thousandths of a second. We then incorporated the same design features in a larger pentane-filled version in which the liquid volume is six inches long, two inches wide and three inches high. This chamber is now in use with the Cosmotron at the Brookhaven National Laboratory. We have made 400 excellent pictures of tracks of protons from this accelerator. These track photographs are as easy to read as the best cloud chamber records and are about 10 times as accurate.

From Slowdown to Speedup
By Adam G. Riess (Nobel Prize in 2011) and Michael S. Turner
Published February 2004

From the time of Isaac Newton to the late 1990s, the defining feature of gravity was its attractive nature. Gravity keeps us grounded. It slows the ascent of baseballs and holds the moon in orbit around the earth. Gravity prevents our solar system from flying apart and binds together enormous clusters of galaxies. Although Einstein’s general theory of relativity allows for gravity to push as well as pull, most physicists regarded this as a purely theoretical possibility, irrelevant to the universe today. Until recently, astronomers fully expected to see gravity slowing down the expansion of the cosmos.

In 1998, however, researchers discovered the repulsive side of gravity. By carefully observing distant supernovae—stellar explosions that for a brief time shine as brightly as 10 billion suns—astronomers found that they were fainter than expected. The most plausible explanation for the discrepancy is that the light from the supernovae, which exploded billions of years ago, traveled a greater distance than theorists had predicted. And this explanation, in turn, led to the conclusion that the expansion of the universe is actually speeding up, not slowing down. In the past few years, astronomers have solidified the case for cosmic acceleration by studying ever more remote supernovae.

But has the cosmic expansion been speeding up throughout the lifetime of the universe, or is it a relatively recent development—that is, occurring within the past five billion years or so? The answer has profound implications. If scientists find that the expansion of the universe has always been accelerating, they will have to completely revise their understanding of cosmic evolution. But if, as cosmologists expect, the acceleration turns out to be a recent phenomenon, researchers may be able to determine its cause—and perhaps answer the larger question of the destiny of the universe—by learning when and how the expansion began picking up speed.

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