Cosmology in a Cold Teacup By Professor Bei-Lok Hu

You are probably familiar with the large scale NASA experiments like COBE and WMAP looking into the relatively late cosmos. But do you know that certain table-top experiments involving cold atoms in Bose-Einstein condensates (BEC) may be used to test out some basic quantum processes in the very early universe?

Prof. Calzetta of the University of Buenos Aires (a postdoc in our gravitation theory group in the mid-80's) and Prof. Bei-Lok Hu of our department recently made such a novel suggestion. Their theory explains that the salient features of an experiment carried out by Donley et al [Nature 412, 295 (2001)] at NIST-Boulder in 2001 on the controlled collapse of a BEC condensate (`Bosenova'), such as the emission of atoms in bursts and jets, are the result of quantum fluctuations parametrically amplified by the dynamics of the condensate. Quantum cosmologist believed this process had happened around the Planck time, 10^{-43} seconds from the Big Bang. Vacuum fluctuations are excited in a similar way by the enormous energy in the dynamics of spacetime, producing particle pairs which made up the matter content of the universe.

The Bosenova experiment also illustrates another important cosmological process, structure formation during a rapid quench. Galaxies were formed very late in the history of the universe, even later than the COBE or WMAP detectable eras. But their seeds came from quantum fluctuations of the scalar field which drove the universe into inflationary expansion at a very early time, $10^{-35}$ seconds from the Big Bang for a grand-unification (GUT) epoch phase transition. It is easy to see that an inflationary expansion is the time-reverse of a rapid quench in the BEC collapse. But how exactly does a primordial quantum seed grow into today's massive galaxies?

There is a constant competition between two rates, the physical frequency of a perturbation mode describing the density contrast, and the expansion rate of the universe, the Hubble constant at that time. The modes whose physical frequencies are higher than the Hubble constant are ``inside the horizon'', they oscillate. Conversely, the modes are ``outside the horizon'', they are `frozen' and amplified, a process analogous to the growth of fluctuations during spinodal decomposition. These modes which left the inflation horizon reenter the Robertson-Walker horizon during the radiation and matter dominated eras, giving rise to acoustic oscillations which grew into structures.

In the BEC collapse problem, the role of the ``Hubble'' constant is played by the inverse growth (exponential) rate of the most unstable mode of the condensate, which is determined by the instantaneous number of particles in the condensate. As these modes change from exponential growth to oscillatory behavior, they are ``thawed''. Perceived as particles being created from the condensate, they form the jets and bursts observed in the Bosenova experiment.

This remarkable discovery by Calzetta and Hu of using BEC collapse to test out quantum processes in the early universe should open up a new venue for doing `laboratory cosmology'.

You can read more about this in http://www.lanl.gov/arXiv:condmat/0207289

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Dr. Bei-Lok Hu is a professor of physics at the University of Maryland. His research interests include general relativity, gravitation and cosmology; quantum field theory and; statistical field theory. He can be reached at hub@physics.umd.edu.