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Negative pressure in the universe

Dear Mr. Miller, I am in the theoretical astrophysics group at Fermilab. You recently sent a question to Glenn Blanford about negative pressure in the universe. Here is an attempt at an answer:

I recently read the article in Fermi News entitled Depatment of (Missing) Energy ( see FermiNews98-05-15.pdf). In the article a property called "negative presure" was discused. My question is: Is this negative pressure the result of a particle that would be the opposit of a graviton. having a force against gravity forcing objects away from each other. I would think that this force would be extreamly weak, just like the gravitational force, but perhaps its force would grow with the size of the universe.

I don't think of the new form of energy that people have proposed as being "anti-gravity." Rather, it is yet another form of energy. We know there is ordinary stuff like electrons and protons. They fall into a category called "matter" or "non-relativistic matter." Things that move fast like light fall into a category called "radiation" or "relativistic matter." Both of these (matter and radiation) have positive pressure. This new form of energy is different in that it has to have negative pressure. Just as matter and radiation affect the gravitational field, so too this new form of energy impacts on the gravitational field. On cosmological distances, it causes the universe to accelerate. It is this effect -- the impact the new form of energy has on gravity -- that is most important. It is true -- as you say -- that this new form of energy could mediate a new force, just as for example light [or more correctly, photons] mediate the force of electromagnetism. However, I don't think this force is particularly important for cosmology. It's something to think about though.

Thanks for your question. I hope this helps. Please feel free to contact me if you have any further questions.

Best,

Scott

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last modified 8/31/1998   physicsquestions@fnal.gov