Dark energy

[Moonshane]

Just thinking out loud. What I have read so far (and assuming that I understand it) suggests that dark energy is the 'force' which is driving expansion of the universe from the inside out. However, with the multiverse idea, is it possible that the 'dark energy' is actually other, presumably more massive, universes that are 'pulling' our universe's expansion and therefore dark energy is not therefore required in our own universe?

[PhotoGav]

This assumes uniform multiverses surrounding our own universe as, as far as I understand it, our universe is expanding uniformly, not in a bulging manner as non uniform 'point sources' of external pull would create.

Dark energy - where does it come from and how can there be ever more of it as the universe expands to create the acceleration in expansion that is observed?!

Ouch... my brain just ached.

[Moonshane]

Good point ☺There goes my Nobel Prize ?

[Unklewhale]

I suspect that somewhere in the next ten years, some fundamental knowledge step is going to be taken.

[Davey-T]

According to latest research whatever is pushing / pulling the Universe is getting stronger and even the Hubble Constant is increasing so can no longer be considered constant.

Dave

[PhotoGav]

Is the change constant? As for the Hubble Constant being constant in the first place, I’m yet to find two concurring values! Now the CMB lot are having a go they have a value for it that is lower than everyone else - more evidence of a constantly increasing constant?! It is all fascinating though isn’t it. When I suggested that the CMB is actually originating from outside our universe (imagine a bubble in a bath tub of water) and not necessarily evidence of ‘the Big Bang’, I was not told to shut up!!

So many Nobel Prizes up for grabs...!

[andrew s]

3 hours ago, Davey-T said:

According to latest research whatever is pushing / pulling the Universe is getting stronger and even the Hubble Constant is increasing so can no longer be considered constant.

Dave

The rate of expansion is increasing yes but the Hubble constant/parameter is getting smaller !

At the time the CMB was emitted it was in the 10s of k and has been reducing ever since.

This is counter intuitive but is due to it being  the ratio of the derivative of the scale factor to the scale factor.

I imagine you are in a car moving at a constant speed the equivalent of the Hubble constant is your speed/distance you have travelled. So at the beginning distance is small so you get a high ratio as you go further it get smaller. Even if you accelerate the ratio gets smaller.

Regards Andrew 

[Davey-T]

If we don't know what dark energy is or why / how it's accelerating the expansion of the universe then there's no reason to suppose it will go on expanding faster and faster, it could just as easily slow / stop or reverse.

Dave

[SIDO]

Is not the evolution of multiverse theory dependent on the weak nature of dark energy were multiverse theory is needed to hold up the current understanding of dark energy and universal expansion so these two theories are then interdependent? So then all universes in the multiverse must operate on those same physical principles I presume, this would mean all multiuniversal expansion would also be uniform in nature. I'm just guessing of course but here's an interesting link...

https://www.quantamagazine.org/why-the-dark-energy-problem-spawned-the-multiverse-hypothesis-20180312/

[Star101]

19 hours ago, Moonshane said:

Just thinking out loud. What I have read so far (and assuming that I understand it) suggests that dark energy is the 'force' which is driving expansion of the universe from the inside out. However, with the multiverse idea, is it possible that the 'dark energy' is actually other, presumably more massive, universes that are 'pulling' our universe's expansion and therefore dark energy is not therefore required in our own universe?

The problem with "more massive, universes that are 'pulling' our universe" is that would, I expect, mean we accelerate in the direction of "pull". Yet my understanding is the expansion is in ALL directions!, uni-formerly.

 

[ollypenrice]

I think there's a more fundamental misconception at work in your original question, Shane. The force driving the expansion, if it exists, does not drive from the 'inside out' because there is not an outside (or a direction towards an outside) in any conventional sense. Multiverse models do not resemble a bag of balls or bubbles, each ball being a universe adjacent to another universe, as your question seems to imply. As I understand it (which is hardly at all!) the hypothetical 'other universes' do not share our universe's system of dimensions.

The popular expansion analogy of the surface of a balloon expanding as the balloon inflates carries the risk of the reader overlooking the key word surface.  In the analogy the balloon as a sphere in three dimensions does not exist. In practice it is very hard to rid ourselves of this vital concept but only the rubber surface exists and there are no other dimensions in the analogy. It is, therefore, flatly impossible to paint an arrow on this surface which points to anything resembling an outside. Indeed any and all arrows painted on the surface are equivalent in that they point to nowhere in particular.

Olly

[Moonshane]

Great post Olly. You are quite right. 

[vlaiv]

What is wrong with "good old vacuum negative pressure" approach to dark energy?

 

[andrew s]

3 minutes ago, vlaiv said:

What is wrong with "good old vacuum negative pressure" approach to dark energy?

 

Only that calculations based on QM are a factor of 10^120 out.

My preferred solution is the cosmological constant which fits well with the LCMB concordance model.

Regards Andrew 

[vlaiv]

4 minutes ago, andrew s said:

Only that calculations based on QM are a factor of 10^120 out.

My preferred solution is the cosmological constant which fits well with the LCMB concordance model.

Regards Andrew 

It is the same thing, isn't it? Constant vacuum energy - one that does not dilute with expanding "box" is equal to negative pressure of the vacuum.

Explaining it via energy of ground states of all quantum fields in QFT and integrating those fields "over estimates" result, but I'm not that confident in either rationale nor calculation that is used to produce that result.

[andrew s]

6 minutes ago, vlaiv said:

It is the same thing, isn't it? Constant vacuum energy - one that does not dilute with expanding "box" is equal to negative pressure of the vacuum.

Explaining it via energy of ground states of all quantum fields in QFT and integrating those fields "over estimates" result, but I'm not that confident in either rationale nor calculation that is used to produce that result.

No not at all. The cosmological constant is a net curvature of spacetime while as you say the vacuum energy is due to quantum zero point field fluctuations. 

I don't have a personal view on the calculation but it has been published in serious peer reviewed journals. It has got closer it was out by 10^500.

They do have the same effect as they are targeted at the same phenomenon but one is about right the other seriously not.

Regards Andrew