Inflation or more generally negative pressure

[sanctus]

I am just wondering if my interpration of negative pressure is right. When deriving inflation you get to the point where you have the equation required by inflation:

 

[math] P< -\frac{\rho}{3} [/math]

 

then the book says (Dodelson in my case):

Since the energy density [[imath]\rho[/imath]] is always positive, the pressure [P] must be negative.

 

At first negative pressure seemed to me as unusual as a negative energy density, but then I came up with the following and was wondering if my interpretation is right.

 

Pressure is a quantity that measures the force acting on area, i.e. a square meter on a beach has a force acting on it equal to [imath]1 atm \cdot 1 m^2 [/imath], since it is positive it means that that the force is towards the considered surface (ie pushing the sand down).

Hence if the pressure is negative it just means that the force acts in the other direction, in the beach example it would mean that the pressure if negative would try to make the sand float upwards.

 

Is this right? I think so, because writing it down made it sound even more logic...