Relationship between energy density and cosmological constant

In summary, the energy density of the universe is 1/16th the cosmological constant, and while this may seem like a peculiar coincidence, it is not a constant relationship and changes with time due to the expansion of the universe. Therefore, there is no theoretical reason for this precise relationship.
  • #1
novice_hack
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TL;DR Summary
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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  • #2
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
 
  • #3
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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  • #4
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.
 
  • #5
Greetings,

Why do you find the value of 16 to be possibly special? Just because it happens to be an integer?ES
 
  • #6
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.
 
  • #7
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.
 
  • #8
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
 

FAQ: Relationship between energy density and cosmological constant

What is the relationship between energy density and cosmological constant?

The cosmological constant, also known as dark energy, is a constant term in Einstein's field equations that represents the energy density of the vacuum of space. This means that as the energy density of the universe increases, so does the value of the cosmological constant.

How does the energy density affect the expansion of the universe?

The energy density of the universe plays a crucial role in determining the rate of expansion of the universe. As the energy density increases, the expansion of the universe accelerates due to the repulsive force of dark energy. This is why the cosmological constant is often referred to as the "driving force" behind the expansion of the universe.

Is there a direct proportionality between energy density and cosmological constant?

While there is a relationship between energy density and cosmological constant, it is not a direct proportionality. The value of the cosmological constant also depends on other factors such as the geometry of the universe and the amount of matter present. However, in general, a higher energy density does lead to a higher value of the cosmological constant.

How does the cosmological constant affect the fate of the universe?

The value of the cosmological constant has a significant impact on the fate of the universe. If the value is positive, as it is currently believed to be, the expansion of the universe will continue to accelerate and eventually lead to a "Big Rip" where all matter is torn apart. If the value is negative, the expansion will slow down and eventually reverse, resulting in a "Big Crunch" where the universe collapses in on itself.

Can the energy density and cosmological constant change over time?

Yes, both the energy density and cosmological constant can change over time. In fact, the energy density of the universe has been decreasing as it expands, while the value of the cosmological constant has remained relatively constant. However, there are theories that suggest the cosmological constant may change in the future, leading to different possible fates for the universe.

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