Dark Energy

[TheFaithfulStone]

the big difference is that where lambda is actually a geometric quantity (in the context of general relativity), qunitessence is a field (in fact: a scalar field; meaning it does not carry spin)

 

Okay, I understand that a scalar field is basically a continuous field with a scalar associated with each point, meaning that quintessence says that the relative "strength" of dark energy is variable across space, right?

 

But what does "geometric quantity in the context of general relativity" mean?

 

TFS

[Kayra]

But what does "geometric quantity in the context of general relativity" mean?

 

TFS

 

It means I do not know wnough to even be previewing this thread :)

[Bo]

Okay, I understand that a scalar field is basically a continuous field with a scalar associated with each point, meaning that quintessence says that the relative "strength" of dark energy is variable across space, right?

almost right:) ; more precise: space and time. (by the way: if the qunitessence field mimics lambda; then it must be almost constant)

 

But what does "geometric quantity in the context of general relativity" mean?

TFS

with this I mean that if you do general relativity (which is a geometrical theory of spacetime); you can have a nonzero lambda, without any other matter or stuff. the nonzero lambda is just a property of spacetime in this case. For example if I look at the surface of a sphere; then I noticed this surface is curved and I can relate this curvature to the radius of the sphere.

Then I say that the radius has a geometric origin. (in this case: lambda is proportional to the inverse radius of a five dimensional hyperboloid)

 

Bo

[TheFaithfulStone]

Then I say that the radius has a geometric origin. (in this case: lambda is proportional to the inverse radius of a five dimensional hyperboloid)

 

That's not much help, unfortunately. :phones:

 

Okay. In the Lambda model, Lambda is just a property of space time itself - like the curvature of the surface of a sphere. Or a tangent to a point on an arc or something. In other words, there's only one answer, and it is dependent upon the geometric properties of the universe?

 

TFS

[Bo]

In other words, there's only one answer, and it is dependent upon the geometric properties of the universe?

Exactly. but as I said, there are particles/fields/stuff that exactly behaves like a cosmological constant. And in the end you cannot distinguish between a 'geometrical' lambda and a field mimicing lambda. (lambda is in a sense the energy of the vacuum; this energy can both come from the structure of space time (the geometrical part) or from field contributions to the vacuum (which is a quantum effect))

Bo

[TheFaithfulStone]

So... if you can't distinguish between the two, how could you possible know what the right answer is?

 

TFS

[Bo]

in a sense you cannot. The only difference is that quintessence type models are dynamical; and thus at a certain time do and at another time do not contribute to the effective lambda. This *might* be possible to measure. Also from a theoretical point of view quintessence-type models pop-up from e.g. string theory. (while a positive, geometrical lambda is almost impossible to achieve in string theory)

 

Bo

[TheFaithfulStone]

Awesome.

 

Got it.

[HappytheStripper]

To me dark energy is the periphery of the knowledge of modern physics; it is essentially a concept introduced to explain phenomena that cannot be otherwise explained. Its a theory, and one must accept it at that level only.!:cup:

 

I agree .. well said hallenrm .. :)