# Question on infinite curvature of the universe

1. Jan 22, 2007

### fauyd

Ok, so I dont know much about general relativity or quantum mechanics but, if gravity effects everything in the universe, and if heisenberg's uncertainty principle makes it so that you can not have truly empty space (so every point of space has to have some sort of particle occupying it, bc if you did not then you would be able to measure in some way an exact point of space down to 0 variance in measurement) then wouldnt gravity just pull and curve every bit of space in on its self? Forgive my ignorance, and if the question makes no sense then just disregard it, lol.

2. Jan 22, 2007

### cesiumfrog

if there isn't a particle there, how will you know which exact point in space you measured? Besides, there's no problem unless you also expect to measure the speed of that point in space, which you can't do anyway..

3. Jan 22, 2007

### pervect

Staff Emeritus
Cosmological measurements suggest that empty space does have gravitational effects. The main effect as it applies to GR is called the "cosmological constant" However, oddly enough, observations suggest that the net effect of the gravitational effect of the swarm of virtual particles in "empty" space is to make space expand, i.e. to make the expansion of the universe accelerate. At the quantum level, this is an unsolved problem as to why this happens.

It is possible to view the presence of the cosmological constant as implying that "empty space" has a positive energy density, a negative pressure, and what acts a lot like a negative mass.

How do you get a negative mass out of a positive energy density? Well, both energy and pressure contribute to gravity. See for instance http://www.math.ucr.edu/home/baez/einstein/

Because density + 3*pressure is negative, the effect is essentially one of "negative mass", in spite of the positive energy density. You can think of this as "empty" space gravitationally repelling itself - it "wants to" expand.