Negative Pressure

[Aki]

what are scalar fields?

Is leaking gravity the thing about gravitons can escape to other parallel universe, and therefore gravity is the weakest force?

I saw the Elegant Universe episodes, and a brane collision might have caused the big bang, so how does that relate to dark energy?

Bo, please tell me more

[Bo]

what are scalar fields?

hmm First of all the question: "What is a field?" Without going into details: a field is in some sense a different way to describe a particle. A classical particle has a certain postion in space. We can specify the particle by specifying the position and the properties of the particle. In quantum mechanics it is a lot harder to talk about exact positions, so one uses to concept of a field. A field is defined over the complete space.
A "scalar field" is a field, with no spin. spin is a (quantummechanical) property of a particle, but this is not really the important point here
The notion of scalar fields in the context of negative pressure is the following: A scalar field, subject to some conditions, can have the properties to drive inflation (the accelerated expansion of the universe) (such a field is commonly called the inflaton field). A nice thing is that such scalar fields, arise very naturally from String theory.

Is leaking gravity the thing about gravitons can escape to other parallel universe,....

I wouldn't say 'parallel universe' but 'extradimensional' is the right word. String theory gives the possibility that most 'normal' matter is confined in our 'normal' 4 dimensional space-time, while gravity on the large scale can escape to 5th or higher dimensions, losing it's 'grip' in our 4D world.

...and therefore gravity is the weakest force?

No; this has nothing to do with it Even without leaking gravity, gravity is the weakest force.

I saw the Elegant Universe episodes, and a brane collision might have caused the big bang, so how does that relate to dark energy?

This is called the ekpyrotic scenario, see another topic

Bo

[Aki]

ah, thanks for clarifying things for me Bo.