Understanding Negative Pressure and its Role in the Expansion of the Universe

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Discussion Overview

The discussion revolves around the concept of negative pressure and its potential role in the expansion of the universe, particularly in relation to the idea of repulsive gravity. Participants explore the implications of negative pressure in gravitational contexts and its connection to dark energy, while questioning the validity of certain claims regarding gravitational energy and the overall energy balance in the universe.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants propose that negative pressure is linked to repulsive gravity, which they suggest could be responsible for the accelerating expansion of the universe.
  • Others argue that there is no conclusive evidence for the existence of repulsive gravity, noting that measurements indicate gravity may behave differently on large scales without confirming repulsion.
  • A participant mentions that the concept of negative gravitational energy is a matter of calculation convention, asserting that earlier beliefs about gravitational energy balancing mass-energy are incorrect.
  • Another participant introduces the idea that dark energy, often associated with negative pressure in general relativity, may be relevant to the discussion of accelerating expansion.
  • One participant raises a question about the basis for discrediting the idea that gravitational energy balances the rest of the universe's mass-energy.

Areas of Agreement / Disagreement

Participants express differing views on the existence of repulsive gravity and the interpretation of negative gravitational energy. There is no consensus on these topics, and the discussion remains unresolved regarding the implications of negative pressure and its relationship to dark energy.

Contextual Notes

Participants highlight limitations in understanding the relationship between negative pressure, repulsive gravity, and gravitational energy calculations. The discussion reflects ongoing uncertainties and assumptions that have not been fully addressed.

zeromodz
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I read somewhere that negative pressure is responsible for repulsive gravity that is increasing the expansion of the universe. Can repulsive gravity exist? and if so how is it related to negative pressure?

I also heard that the negative gravitational energy compensates for all other energies in the universe making the total energy in the universe zero.
 
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zeromodz said:
Can repulsive gravity exist? and if so how is it related to negative pressure?
There is no evidence for repulsive gravity, there are possible measurements that suggest the normal law of gravity isn't quite right on large scales (gravity is weaker than it shoudl be) but nothing conclusive.

I also heard that the negative gravitational energy compensates for all other energies in the universe making the total energy in the universe zero.
This isn't 'negative gravity' it's just how you do the calculation. If you start with the mass in one point and lift the masses outward it's easier to make that energy negative, so when the mass is attracted back inward, that movement is positive - but it's purely a question of convention. It was once thought that the energy of the gravity expansion balanced the mass-energy of the rest if the universe - that isn't true.
 
mgb_phys said:
It was once thought that the energy of the gravity expansion balanced the mass-energy of the rest if the universe - that isn't true.

On what premise have we discredited this? Did we disprove this somehow?
 
mgb_phys said:
There is no evidence for [..]
It sounds like the OP is referring to dark energy which, if I understand correctly, can be represented as negative pressure in GR's source tensor, has a repulsive consequence of accelerating intergalactic expansion, and still figures prominently in the mainstream picture.
 
ok - I thought they mean the pioneer anomoly
 

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