MadSci Network: Physics
Query:

Re: How/where is Energy from 'Fabric of Space' accounted for?

Date: Mon May 21 06:36:40 2007
Posted By: Ken Rines, Grad student, Astronomy, Harvard-Smithsonian Center for Astrophysics
Area of science: Physics
ID: 1163106108.Ph
Message:

In the simplest case (with no dark matter or dark energy), the fate of the universe depends on the balance between 'kinetic energy' of the expansion of the universe and the gravitational 'potential energy' of all galaxies and stars pulling on each other. The kinetic energy is positive and the potential energy is negative. If there is more kinetic energy than potential energy at the time of the Big Bang, the 'total energy' of the universe is positive and the universe will continue to expand forever. If there is more potential energy at the time of the Big Bang, it will eventually halt the expansion of the universe (that is, make the kinetic energy zero), and then the universe would recollapse onto itself. The expansion of the universe (the 'kinetic energy') is the expansion of space, so spacetime has a positive energy at the time of the Big Bang.

One strange feature of 'dark energy' is that it violates the principle of conservation of energy. If you have normal matter and you double the volume it occupies, the density is halved (because no matter is created or destroyed in the process). With dark energy, if you double the volume of the universe, the density of dark energy remains the same, so the total amount of dark energy doubles! This violation of conservation of energy is actually okay with general relativity: energy is conserved locally, but energy doesn't have to be conserved for the universe as a whole.

You can find other discussions of these issues here and here.


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