2021 Fall Physics Colloquium Series: Prof. Stephen A. Fulling, Texas A&M University
|Date||September 22, 2021|
|Time||4:00 - 5:00 pm|
|Location||Baylor Sciences Building, Room D.110|
A quantum field can be thought as a collection of infinitely many harmonic oscillators. The spectrum of one oscillator consists of equally spaced energy levels, so its Nth eigenstate can be thought of as containing N "particles"
or excitations. If the frequency of the oscillator depends on time, these particles can be created and destroyed -- if they remain meaningful at all. For a (free, bosonic)
field in ordinary flat space-time, we just do this for each normal mode and declare the problem solved. In the expanding early universe, or any model where the geometry of space is time-dependent, particles are created. Naively it appears that an infinite density of
particles can be produced -- and then largely disappear if the expansion slows down. The resolution of this problem is to concentrate on field observables (such as energy density) instead of counting particles. The instantaneous frequency of each mode is less important than a loosely defined "effective frequency" that more correctly describes the excitations of such a time-dependent oscillator. Even more interesting things happen in a space-time with more complicated geometry such as black holes, but that is a story for another day.
3:40 p.m., ground-floor outside Patio D
|Publisher||Department of Physics|
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