Relationship between energy density and cosmological constant

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Discussion Overview

The discussion revolves around the relationship between the energy density of the universe and the cosmological constant, specifically questioning whether there is a theoretical reason for the observed numerical ratio of approximately 1/16th between these two values. The scope includes theoretical implications and potential physical significance of this relationship.

Discussion Character

  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants note that the energy density of the universe is approximately 1/16th of the cosmological constant and question if there is a theoretical reason for this relationship.
  • Others argue that the relationship is not constant and changes over time, as the energy density of the universe decreases with expansion while the energy density of the cosmological constant remains unchanged.
  • A participant suggests that the value of 16 might be special or indicative of something significant, prompting further inquiry into its physical relevance.
  • Another participant challenges the premise of the question, asserting that the ratio is time-dependent and not a fixed value that requires explanation.

Areas of Agreement / Disagreement

Participants express differing views on whether the ratio of energy density to cosmological constant has any theoretical significance, with some asserting it is merely a coincidence and others suggesting it could be special. The discussion remains unresolved regarding the nature of this relationship.

Contextual Notes

The discussion highlights the dependence of the ratio on time, as well as the ambiguity surrounding the significance of the number 16, which is not universally accepted as a constant value.

novice_hack
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TL;DR
According to the wiki entry on Planck units, https://en.wikipedia.org/wiki/Planck_units, the energy density of the universe, 1.8 × 10−123, is 1/16th the cosmological constant, 2.9 × 10−122. Is there a theoretical reason for this precise relationship?
According to the wiki entry on Planck units, https://en.wikipedia.org/wiki/Planck_units, the energy density of the universe, 1.8 × 10−123, is 1/16th the cosmological constant, 2.9 × 10−122. Is there a theoretical reason for this precise relationship?
 
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novice_hack said:
According to the wiki entry on Planck units, https://en.wikipedia.org/wiki/Planck_units, the energy density of the universe, 1.8 × 10−123, is 1/16th the cosmological constant, 2.9 × 10−122. Is there a theoretical reason for this precise relationship?
Greetings,

Any reason there should be a physcially significant reason?

The number you quoted for energy-density is actually specified as density in units of Planck mass per unit Planck volume.ES
 
novice_hack said:
Is there a theoretical reason for this precise relationship?
The relationship is not a constant; it changes with time, because the energy density of the universe decreases as it expands, while the energy density of the cosmological constant does not.
 
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EigenState137 said:
Greetings,

Any reason there should be a physcially significant reason?

The number you quoted for energy-density is actually specified as density in units of Planck mass per unit Planck volume.ES
PeterDonis said:
The relationship is not a constant; it changes with time, because the energy density of the universe decreases as it expands, while the energy density of the cosmological constant does not.
Ok. Call it mass density. I am asking whether there is some reason that there would be a relationship between the two. Maybe the answer is 'no'. It just seems curious to me that the one value is 16 times the other value. It seems like the kind of thing for which there might be some theoretical explanation. Perhaps it is just a peculiar coincidence.
 
Greetings,

Why do you find the value of 16 to be possibly special? Just because it happens to be an integer?ES
 
I am assuming from your questions that either you think the answer to the question is 'no' or that you don't know the answer.
 
novice_hack said:
I am asking whether there is some reason that there would be a relationship between the two.
And I have already told you that there is no such "relationship", because the ratio between the cosmological constant and the matter density (or mass density, or energy density, or whatever you want to call it) in the universe changes with time. So your question is based on a false premise, that the current value of 16 (which isn't exactly 16 anyway) is an unchanging value that needs an explanation.
 
novice_hack said:
I am assuming from your questions that either you think the answer to the question is 'no' or that you don't know the answer.
Greetings.

If that post is addressed to me, what I am asking is exactly what I posted. Why do you consider the numerical value of 16 to be possibly special and perhaps indicative of something physically significant?

That question has nothing to do with what @PeterDonis has already explained to you that the ratio is time-dependent.ES
 

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