James Pantaleone
Professor of Physics
University of Alaska Anchorage
We have no idea what led up to the Big Bang. Some people make guesses, but there is currently no way to test these guesses.

Steven Balbus
Astronomy Professor
University of Virginia
We don't know, and we don't even know if the question makes sense!

Professor Rex A. Saffer
Physicist, Astronomer, Educator
Dept. of Astronomy & Astrophysics, Villanova University
I have no idea - no one else does either, at least not as far as I know.

Astronomer*
This question is generally a metaphysical one since we have little hope of observing anything from before the Big Bang. The best scientific answer is a quantum fluctuating vacuum, but this is a speculation.

Steven Novotny
Air Force Officer/Graduate Student
University of Florida Astronomy Department
By definition the big bang was the beginning, so there was nothing before the big bang.
However, some models of the universe propose that the universe is on a cycle that makes it expand, contract, and then expand again. The points where it begins expanding are the big bangs. So, according to these types of models, in the past there may have been an infinite number of universes like our own which have expanded and contracted and started over eventually making our own.

Martin Duncan
Professor of Physics
Queen's University, Canada
We don't know.

David Batuski
Astronomer
University of Maine
Nothing! (Or no one knows.)

Astronomy Professor*
Again this question is beyond the boundaries of science to consider. Questions of a first cause or a prime mover are religious and philosophical in nature. I am not sure science will ever be able to answer this question.

John Huchra
Astronomer/Professor
Harvard-Smithsonian
Another good question. I don't know. The leading theory has it that there was a "universe" filled with just energy. But I really don't know.

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*Respondents opted for anonymity and we respect their wishes.