Douglas Scott
Professor
University of British Columbia
No one knows. There are many ideas such as 'inflation", or that it grew out of some early "stringy' state for the Universe. At the moment we have no real answers, but the exciting thing is that there are possibly ways to test these different ideas (through variations in the Cosmic Microwave Background, for example).

Dr. Michael M. Davis
Astronomer
SETI Institute
Boy, I wish I knew the answer to this one. I'd be famous!

One suggestion, in the book "A Brief History of Time" by Stephan Hawking, is that time itself is logarithmic, and that we never get to time zero by working our way back along the time axis. Talk about exotic concepts!

Yervant Terzian
Professor
Cornell University
We do not know. Some think that we shall have a Theory of Everything and we shall know. Others think we shall never know.

Astronomer*
Science can not attribute a cause to this event.

Astronomer
University of Sussex, UK
No one knows the answer to this, so don't believe anyone who tells you they do!

Karen Vanlandingham
Assistant Professor
Columbia University's Biosphere 2 Center
Don't know.

Astronomer*
No one knows, but we speculate some kind of quantum mechanical fluctuation in an already existing void or vacuum.

Eilat Glikman
Graduate Student
Columbia University
Your guess is a good as mine! We have a pretty good idea of what caused all the particles and forces that were produced milliseconds after the big bang, but I have yet to hear a theory of what cased the big bang. In fact the question is slightly ill-posed. Its like asking what happened before time began, or what is north of the north pole. Kind of like an MC Echer paining.

Professor*
Nobody knows.

Ed Churchwell
Professor/Astronomer
University of Wisconsin
That is still not known, although some theories have been advanced, none are generally accepted by everyone.

Prof. Wayne G. Roberge
Theoretical Astrophysicist
Rensselaer Polytechnic Inst.
Most likely our Universe began as a fluctuation in a larger "Multiverse". There are probably an infinite number of Universes like ours, except that the laws of physics are probably different.

Don Brownlee
Astronomer
University of Washington
Pressure caused it to expand. What happened before is a ???

Bob Mathieu
Professor of Astronomy
Department of Astronomy, University of Wisconsin - Madison
That's a good question on which theoretical physicists are working right now. So I'm
afraid that I don't have a good answer yet!

Professor Rex A. Saffer
Physicist, Astronomer, Educator
Dept. of Astronomy & Astrophysics, Villanova University
That is a very good question, one that no one really knows the answer. It is a topic of much current controversy and research. One theory says that the universe was much like a supercooled liquid, one that is well below the freezing point but in which the material is still liquid. A small disturbance, however it is caused, can then cause the whole structure to "freeze out" quickly, changing the behavior of the material. In our universe, a similar kind of process might have caused the release of a great deal of energy, which then caused the universe to begin expanding as if a great explosion (or Big Bang) had taken place. But this is a very simplified picture - the real answer is very complicated and not really understood very well.

Steven Balbus
Astronomy Professor
University of Virginia
Nobody knows with any certainty.

James Pantaleone
Professor of Physics
University of Alaska Anchorage
The universe is finite, but is has no edge. The situation is like the surface of a balloon. The balloon has finite area yet that surface has no edge. As one blows up the balloon, the area expands just like the universe is expanding.

David Batuski
Astronomer
University of Maine
It was not really an explosion. More like a rapid creation of space to separate things. The 'cause,' the best we know now, appears to have been a random quantum mechanical fluctuation.

Astronomy Professor*
To some extent, this is not a question science can pursue... at least at present. Our models can take us back to a time very close to the Big Bang but the ultimate cause may forever remain unknown.

John Huchra
Astronomer/Professor
Harvard-Smithsonian
Good question. Want to win a Nobel Prize? There are various theories, but no clear winner.

*Respondents opted for anonymity and we respect their wishes.