Register to reply 
Equation of state 
Share this thread: 
#1
May510, 03:49 AM

P: 115

Cosmological constant dark energy has equation of state parameter = 1. What is the equation of state parameter of gravitational energy?



#2
May610, 12:13 AM

Sci Advisor
P: 4,798




#3
May1110, 12:57 AM

P: 115




#4
May1110, 03:56 AM

Sci Advisor
P: 4,798

Equation of state
One way you could do it, though, is through the conservation of the stressenergy tensor. The stressenergy tensor is a 2ndrank tensor (sort of like a matrix) that includes components such as energy, momentum, pressure, and shear (shear includes things like twisting forces). This tensor is always conserved in a very particular way: its socalled covariant derivative is zero. If you examine this conservation law in flat spacetime, you get an energy transport equation: the time rate of change of energy in a region of space is equal to the flow of energy into/out of that region. However, in curved spacetime things get a little bit more complicated. What you could do is make up a new "total energy density" which is always conserved: as in the flat spacetime example, the total energy density within a region of spacetime only changes if it flows from one place to another. You could do this by adding an extra term to the equations that exactly cancels the extra terms you get from curved spacetime, and call this your gravitational potential energy. 


#5
May1110, 10:29 AM

P: 115

Thanks. 


#6
May1110, 01:28 PM

P: 134

Would't that leave you with a completely new stressenergy tensor with two components(simplifying a lot): one negative "dark" energy (pressure) and one "gravitational" positive energy pressure?
It's interesting, if a bit radical. In the past there have been attempts along those lines with negative and positive gravity or with inertia versus gravity(de Sitter) but they were all turned down because they seemed to bring up unphysical concepts like "negative energy".Since 1998 with the accelerated expansion surprise and "dark energy" there is more going in that direction. 


#7
May1110, 01:40 PM

Sci Advisor
P: 4,798

[tex]\frac{d\rho}{dt} = \bigtriangledown \vec{p}c[/tex] That is, for the energy density in a region of space can only change if some of that energy flows into or out of the region. When we have curved spacetime, we instead have: [tex]\frac{d\rho}{dt} = \bigtriangledown \vec{p}c + [/tex]curvaturerelated terms I won't bother to look up the precise form of the curvaturerelated terms on the right hand side. But suffice it to say it's always possible to write energy conservation in this way. So I could simply make up "gravitational potential energy" and set it to the negative of those curvaturerelated terms. Then I would have conservation of total energy again. This has nothing to do with making up a new tensor, just making up a new quantity. 


#8
May1110, 02:04 PM

P: 134

Ok, and wouldn't adding that new made up quantity change anything in the tensor,at least conceptually?
About dark energy sign, I know itis supposed to have conventionally positive density to attain accelaration, but as the OP spoke in terms of negative pressure and we were calling "gravitational energy" positive I switched the signs,but I coul have made the gravitational energy negative and dark energy positive since the point seeemed to be that they are opposite and cancel each other out. Let's not get hung up about the signs here. 


#9
May1110, 02:20 PM

Sci Advisor
P: 4,798




Register to reply 
Related Discussions  
DE equation of state parameter  Advanced Physics Homework  0  
Alternative equation of state?  Cosmology  2  
Equation for S from state func and C  Advanced Physics Homework  1  
Equation of state of an electron gas  Atomic, Solid State, Comp. Physics  2  
The equation of state  Cosmology  2 