# Star with negative pressure?

1. Oct 11, 2008

### astromandi

Star with negative pressure???

Is it possible to have a star with matter with unusual equation of state.
Will it be possible to have a star with negative pressure.

2. Oct 11, 2008

### lightarrow

Re: Star with negative pressure???

Maybe an inflating red giant or a supernova.

3. Oct 11, 2008

### Naty1

Re: Star with negative pressure???

Great question!!!
As you may know, gravitational attraction occurs via three sources: energy, mass pressure. All are included in Einstein's mass/energy tensor in general relativity. So do "anti-gravitational" or "negative gravitational" repulsive effects.

Matter normally has positive pressure; dark matter has negative pressure and enough of that becomes inflationary...in other words it becomes repulsive under gravity. Negative pressure appears to be the basis of the cosmogolical constant and appears to be contained in dark energy....I don't think anybody has much of a theory yet...Oddly, unlike ordinary energy it has the same value from any reference frame...hence "cosmological".

Einstein's theory that the repulsive cosmological force is tiny and becomes large enough to overcome gravity only at cosmologically large distances is still intact...hence you and I are not moving apart as a result; gravity easily holds us and our galaxy together.

So with that as background:
I'm going to guess the answer to your question is NO, for two reasons: first the force is very tiny, second it's repulsive so it seems unlikely to me it will aggregate like positive mass ordinary matter does....due to normal gravity. But who knows?

Brian Greene discusses this in Chapter 10, FABRIC OF THE COSMOS.

4. Oct 11, 2008

### HallsofIvy

Re: Star with negative pressure???

Since it is the (positive) pressure that prevents gravity from collapsing the star, a star with negative pressure would very quickly collapse.

5. Oct 11, 2008

### astromandi

Re: Star with negative pressure???

Why does it have to be a inflating star??
Can be have a star in equilibrium with material with negative pressure??

Suppose we have a star with no pressure atall in will collapse(free fall)
If there is a positive pressure then it would try to stop/resist the collapse.
If there is a negative pressure should it not enhance the collapse making it faster??
And we should never an ever-imploding system instead of inflating supernona???

6. Oct 13, 2008

### Naty1

Re: Star with negative pressure???

Depending on what is meant by negative pressure, a entity might NOT collapse..it might never form because negative pressure can be gravitationally repulsive....as apparently with dark energy.

Last edited: Oct 13, 2008
7. Oct 13, 2008

### George Jones

Staff Emeritus
Re: Star with negative pressure???

You mean dark energy; dark matter is something quite different. Dark energy stars have been pushed chiefly by George Chapline, whose theories are well outside mainstream physics.

8. Oct 13, 2008

### Naty1

Re: Star with negative pressure???

George, thanks I edited to replace matter with energy.....The concept of negative pressure itself is now common...from Guth, Brian Greene and many others....not that such acceptance will ultimately make it correct!!!

9. Oct 14, 2008

### astromandi

Re: Star with negative pressure???

Dark energy behavior would be completely different.
Since When we deal with dark energy we have matter(Dont know weather to call it matter or something else) with negative energy.

And we study its dynamics in the spatially homogeneous and isotropic universe.

Whereas when we deal with similar matter (with same equation of state) in case of the star we deal with spherically symmetric spacetime.
hence dynamics (dictated by Einsteins equation) would be entirely different.

In the FRW universe density of dark energy remains constant.
Whereas that may not be the case in spherically symmetric spacetime. As governing Einsteins equations are totally different.