Why is it customary to assume that the vacuum energy contribution is neglible?

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Discussion Overview

The discussion revolves around the assumptions regarding vacuum energy in the context of cosmology and general relativity. Participants explore why vacuum energy is often considered negligible, its potential role in dark energy, and the implications of these assumptions on our understanding of gravity and the universe's expansion.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants question why renormalization methods remove vacuum energy, suggesting that the infinite values are problematic but not necessarily justifying a zero assumption.
  • There is a proposal that vacuum energy could be linked to dark energy, with an observational estimate of about 10^(-9) J m^(-3), though the theoretical basis for this number is unclear.
  • One participant notes that past calculations predict vacuum energy to be about 10^120 greater than observed values, leading to significant discrepancies between theory and observation.
  • Some participants express confusion over the assumption that vacuum energy is more likely to be zero, discussing the idea that physicists expect a symmetry that would force it to be zero, despite no such symmetry being found.
  • There is a suggestion that if vacuum energy exists, it should be treated differently in equations related to general relativity, particularly regarding its placement in the equations governing the cosmos.
  • Participants discuss the implications of general relativity on the universe's expansion, noting that while GR predicts expansion, it does not inherently predict accelerated expansion.
  • One participant raises concerns about the apparent contradiction between the weakness of gravity and the excess gravitational effects not accounted for by known matter, linking it to broader questions about the nature of gravity and potential explanations like string theory.
  • Another participant clarifies that in general relativity, all forms of energy contribute to gravitation, not just mass, and emphasizes the role of various energy forms in the energy tensor.

Areas of Agreement / Disagreement

Participants express a range of views on the nature of vacuum energy and its implications, with no consensus reached on why it is assumed to be negligible or the relationship between vacuum energy and dark energy. The discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Participants highlight limitations in understanding vacuum energy, the dependence on theoretical frameworks, and the unresolved nature of certain mathematical steps in relating vacuum energy to cosmological observations.

Galteeth
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This is a question from a non-physicist. Since there is much more observed gravity then accounted for by normal matter, why do renormalization methods remove all of the vacuum's energy? I get that the infinite values make no sense, but why assume it's zero?
 
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It might be (or might not be) that vacuum energy is the dark energy that drives the accelerating expansion of the universe. This would put (observationally) the vacuum energy at about 10^(-9) J m^(-3), but nobody knows how to get this number from theory.
 
past calculations have shown vacuum energy expected to be about 10^120 greater than this figure. Even correcting for supersymmetry the result is still 10^60 too big.
this is the worst result in magnitude for any theory vs observation in physics.

In past years Robt Forward and Feinman both stated that the vacuum energy in a coffee cup could boil the seas. They must have been working from the calculated figure.
 
map19 said:
past calculations have shown vacuum energy expected to be about 10^120 greater than this figure. Even correcting for supersymmetry the result is still 10^60 too big.
this is the worst result in magnitude for any theory vs observation in physics.

In past years Robt Forward and Feinman both stated that the vacuum energy in a coffee cup could boil the seas. They must have been working from the calculated figure.

When you say "greater then this figure" what figure are you referring to?
 
The figure given by george Jones in the previous post for the observable vacuum energy.
 
map19 said:
The figure given by george Jones in the previous post for the observable vacuum energy.

Huh. But I still don't understand why it's more likely to be zero?
 
Galteeth said:
Huh. But I still don't understand why it's more likely to be zero?
The basic idea is that physicists don't like small numbers. The argument goes like so: if you have some theory that allows a continuum range of values for the cosmological constant, then it's going to be extraordinarily unlikely that the particular value it chooses will be either close to zero or to some other specific number. Physicists generally expect that it's going to be vastly more likely for it to be forced to be identically zero due to some symmetry or other. However, no such symmetry has been found.

In any case what it means is that the number we see for the cosmological constant is thought unlikely to be an accident, and must take the value it does for a particular reason.
 
"Huh. But I still don't understand why it's more likely to be zero?"

me either. It's not zero, and that shows in many different ways.
But is the dark energy, vacuum energy, casimir energy, driving the expansion ? If not, what is ?
here's a thought
When Einstein added a cosmological constant to his equation G(uv) = 8*Pi*GT(uv) he added it on the left-hand side to make the cosmos static. G(uv) + Ag(uv) = 8*Pi*GT(uv) because he thought it was a property of space.
If we accept that the vacuum contains energy the term should be on the right, as follows: G(uv) = 8*Pi*G(T(uv) + P(vac)g(uv)).
 
map19 said:
"Huh. But I still don't understand why it's more likely to be zero?"

me either. It's not zero, and that shows in many different ways.
But is the dark energy, vacuum energy, casimir energy, driving the expansion ? If not, what is ?
here's a thought
When Einstein added a cosmological constant to his equation G(uv) = 8*Pi*GT(uv) he added it on the left-hand side to make the cosmos static. G(uv) + Ag(uv) = 8*Pi*GT(uv) because he thought it was a property of space.
If we accept that the vacuum contains energy the term should be on the right, as follows: G(uv) = 8*Pi*G(T(uv) + P(vac)g(uv)).

So general relativity predicts an expanding universe, but the rate of this expansion is accelerating, which is not predicted by GR?
 
  • #10
Galteeth said:
So general relativity predicts an expanding universe, but the rate of this expansion is accelerating, which is not predicted by GR?
GR only predicts how the rate of expansion (or contraction) is related to the contents of the universe. It neither predicts nor forbids accelerated expansion.
 
  • #11
Chalnoth said:
GR only predicts how the rate of expansion (or contraction) is related to the contents of the universe. It neither predicts nor forbids accelerated expansion.

Ok. I know I am out of my league here, but i was always a bit confused by the way our understanding of gravity is presented. I had read that it was "mysterius" why gravity as a force was so weak and that one of the appeals of string theory was that it could explain this by having some of the gravity "leak" into other dimensions, yet on the other hand we have this mystery where there is much more gravity then known matter seems to account for. (I know these two things are not related, it just seems weird on a surface level.)
 
  • #12
You don't use just matter to account for gravitation. Any energy produces gravitation. In General Relativity it's the energy tensor. So you add the effect of mass-equivalent energy, heat, momentum, electromagnetic radiation, dark energy(whatever that is) and any other energy I haven't mentioned, to come up with a total.
Note that other dimensions are speculative, we don't have an experiment to show them.
 
  • #13
map19 said:
You don't use just matter to account for gravitation.
Well, energy is a property of matter. It isn't something that exists in and of itself. And it's not just energy, but also pressure, momentum, and anisotropic shears that affect gravitation.
 
  • #14
Galteeth said:
Ok. I know I am out of my league here, but i was always a bit confused by the way our understanding of gravity is presented. I had read that it was "mysterius" why gravity as a force was so weak and that one of the appeals of string theory was that it could explain this by having some of the gravity "leak" into other dimensions, yet on the other hand we have this mystery where there is much more gravity then known matter seems to account for. (I know these two things are not related, it just seems weird on a surface level.)

I suspect when the reason for gravity is known you won't need such elaborate explanations "as leaking from one dimension to another"...the value of G is in the magnitude of c^2/R which is an estimate quoted by L Smolin (The Trouble with Physics) of the present rate of cosmic acceleration.
 

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