Effective cosmo const explained w/o vacuum energy

[marcus]

http://arxiv.org/abs/1603.04170
Effective cosmological constant induced by stochastic fluctuations of Newton's constant
Marco de Cesare, Fedele Lizzi, Mairi Sakellariadou
(Submitted on 14 Mar 2016)
We consider implications of the microscopic dynamics of spacetime for the evolution of cosmological models. We argue that quantum geometry effects may lead to stochastic fluctuations of the gravitational constant, which is thus considered as a macroscopic effective dynamical quantity. Consistency with Riemannian geometry entails the presence of a time-dependent dark energy term in the modified field equations, which can be expressed in terms of the dynamical gravitational constant. We suggest that the late-time accelerated expansion of the Universe may be ascribed to quantum fluctuations in the geometry of spacetime rather than the vacuum energy from the matter sector.
10 pages, 1 figure

In other words the cosmological constant Λ is seen as a QUANTUM GRAVITY EFFECT.
"quantum fluctuations in the geometry of spacetime" are the key here
"rather than the vacuum energy from the matter sector"
The conclusion is:
"... quantum geometry effects may lead to stochastic fluctuations of the gravitational constant"
which could reasonably average out, according to their calculation, in the observed late-time acceleration.

 

[marcus]

==quote Cesare, Lizzi, Sakellariadou, page 6==
This implies an exponential expansion at late times without the need to introduce a cosmological constant by hand or to originate it from the matter sector, e.g. as vacuum energy. Note that this is a general feature of the model that does not depend on the particular values chosen for ρ₀ or the noise strength σ. Furthermore, when σ = 0 one recovers the standard cosmology in the matter-dominated era, namely ρ ∝ 1/t² and H ∝ 1/t.
==endquote==
Their analysis shows that the size of the effective cosmological constant depends, to put it simply, on the first step of a random walk.
==page 7==
...
the effective cosmological constant receives contribution from each of them, with the dominant contribution still coming from the initial value of the white noise ξ(t_i) multiplied by the total energy density as in (17).
In other words, the effective cosmological constant depends on the initial value of the total energy density, but not on the species populating the Universe. It seems natural to fix the initial data at a time where all species were equally dominating, i.e. t_i ≈ t_Pl. This conclusion, if correct, due to the limits of our effective approach at Planckian times, implies that the final stage of evolution of the Universe is entirely determined by quantum fluctuations of the spacetime geometry at early times. Furthermore, being determined only by the initial value of the total energy density, it treats all fields on the same level and it is insensitive to further details of the Universe’s history. In this sense Eq. (18) can be interpreted as a constraint on the underlying quantum theory, at the time when the dynamics of the fast degrees of freedom of the gravitational field approach their stochastic limit.
==endquote==

 

[marcus]

==quote Cesare Lizzi Sakellariadou, outlook and conclusions section page 8==
Within an effective macroscopic description of the dynamics of the gravitational field, we have considered a stochastically fluctuating gravitational constant as a possible phenomenological feature of a theory of quantum gravity. Consistency with the Bianchi identities requires the addition of an extra source term, which does not communicate with the matter sector and may be interpreted as dark energy. We built a simple model where the latter is seen to be responsible for the present acceleration in the expansion of the Universe, which is thus seen to have a purely quantum geometrical origin. The simplifications introduced in the model and the sensitivity of the effective cosmological constant to the value of the total energy density at the Planck time prevent us from using such model to extract quantitative predictions for the present value of the cosmological constant and its probability. It is our hope that this preliminary work could open new ways towards a solution to the cosmological constant problem.
==endquote==

So at present they don't have a good enough handle on the total energy density at Planck time, to let them PREDICT the size of present-day effective Λ.
But they show its dependency on that and open a way for a theory of quantum gravity which DOES tell us the initial Planck-time energy density to make such a prediction and be observationally tested by it.

AFAIK that's a new landmark advance. Tell me if you know of any other research that has reached this stage. I don't, and I think it's pretty impressive.

Here is Sakellariadou's author profile
http://inspirehep.net/author/profile/M.Sakellariadou.1

 

[marcus]

Another Sakellariadou paper recently caught my attention---not explicitly related but could connect in some fashion. It uses Group Field Theory (GFT) condensates to model early universe cosmology in a way that is different from LQC though similar in some respects. It gets inflation for free, without introducing an "inflaton field"
http://arxiv.org/abs/1603.01764
Accelerated expansion of the Universe without an inflaton and resolution of the initial singularity from GFT condensates
Marco de Cesare, Mairi Sakellariadou
(Submitted on 5 Mar 2016)
We study the expansion of the Universe using an effective Friedmann equation obtained from the dynamics of GFT isotropic condensates. A promising feature of this model is the occurrence of an era of accelerated expansion, without the need to introduce an inflaton field with an appropriately chosen potential. Although the evolution equations are "classical", the cosmological model is entirely quantum and does not admit a description in terms of a classical spacetime. Consistency with Riemannian geometry holds only at late times, when standard cosmology is recovered. Hence the dynamics is given in purely relational terms. An effective gravitational constant is seen to arise from the collective behaviour of spacetime quanta, as described by GFT. The occurrence of a bounce, which resolves the initial spacetime singularity, is shown to be a general property of the model.
4 pages, 4 figures

 

[desuro]

a bounce,huh? amazing!

 

[marcus]

It's a way many Quantum Cosmology research papers get rid of the initial "singularity" (considered unphysical, infinite energy density etc) Should really be considered a breakdown of the classical theory rather than as something real.

This excerpt from the "outlook and conclusions" section of Accelerated expansion of the Universe without an inflation suggests that the two papers ARE connected and they are working out a single unified picture including bounce, early universe inflation, and the later accelerated expansion associated with the cosmological constant.

==quote==
The second result is the occurence of an era of accelerated expansion without the need for introducing ad hoc potentials and initial conditions for a scalar field. The picture given could replace the inflationary scenario. Since it is an inherently quantum description of cosmology, it does not share the unsatisfactory features of inflationary models, which were spelled out in the introduction.

The novel feature of this model, namely its incompatibility with Riemannian geometry, opens a window towards understanding how to bridge the gap between quantum geometry and the classical macroscopic world as described by general relativity. It also creates more perspectives from the GFT side, since after the phase transition that gives rise to the condensate of spacetime quanta, there must be yet another phase transition from a pre-geometric phase, where the geometric properties of the Universe, e.g. the volume, cannot be obtained from a metric, to a properly called geometric phase. The model we considered incidentally gives a phenomenological description of this phase transition below the critical point...

Yet another interesting result is that, even though Newton’s constant is related to, and actually constrains, the parameter of the microscopic GFT theory (as shown in [4]), the dynamics of the expansion of the Universe is actually determined by the effective gravitational constant, which is instead determined by the collective behaviour of spacetime quanta. This would possibly shed some light on the nature of the gravitational constant and whether it actually deserves the status of being a fundamental constant, along with the possibility of measuring its time variation.
==endquote==

The reference [4} is to the other paper, Effective cosmological constant induced...
So there is a connection. The two papers should be read together
http://arxiv.org/abs/1603.01764
http://arxiv.org/abs/1603.04170

the earlier paper has a reference to http://arxiv.org/abs/1602.08271
Bouncing cosmologies from quantum gravity condensates
Daniele Oriti, Lorenzo Sindoni, Edward Wilson-Ewing
which mentions this 2014 AEI Workshop
https://workshops.aei.mpg.de/qgcosmo/talk-titles-and-abstracts/
and several of the participants including Sakellariadou, in the acknowledgments

 

[marcus]

People could be wondering who Sakellariadou is. Here is a four minute Youtube where she introduces herself, talks about her teaching and research aims:

at the end she makes a plug for Kings College London, where she teaches---the video is sponsored by KCL, but nevertheless it gives a sense of the person and her academic context. You can find other stuff (faculty webpage...) by googling.
For reference, here is the raw URL for the youtube:
www.youtube.com/watch?v=6-BBlibcscg

 

[haushofer]

Nice paper. It reminds me of the idea to develop a theory of relativity based on the (A)dS group rather than Poincaré. Just as the speed of light doesn't get renormalized in ordinary QFT, the cosmological constant Lambda then also doesn't need any renormalization. Somehow that attitude makes more sense to me.

 

[windy miller]

Any layman explanation of GFT?