Undergrad Questioning the timescape model

[timmdeeg]

"Supernovae evidence for foundational change to cosmological models" https://arxiv.org/pdf/2412.15143
The paper claims:

We compare the standard homogeneous cosmological model, i.e., spatially flat ΛCDM, and the timescape cosmology which invokes backreaction of inhomogeneities. Timescape, while statistically homogeneous and isotropic, departs from average Friedmann-Lemaître-Robertson-Walker evolution, and replaces dark energy by kinetic gravitational energy and its gradients, in explaining independent cosmological observations.

The paper seems regarded in the community, here some citations:

https://arxiv.org/pdf/2511.17160
5.1 The timescape model The timescape cosmology [39, 40, 66–68] represents the first example in the literature of an averaged cosmological model applied to observational data [77–80]. In particular, it has been shown to provide an excellent fit to Type Ia supernova observations [77, 78], even outperforming the standard ΛCDM model in this context with recent observations [79, 80].Interestingly, the model provides an alternative perspective on the origin of dark energy,proposing that it arises purely as a geometric effect, resulting from backreaction phenomena coupled to calibration effects of the varying clock rates among cosmological observers.
5.1.1 Timescape: theoretical framework Within the timescape framework, the Universe is partitioned into two distinct domain classes,walls and voids [39, 40, 66–68]. Physically, walls correspond to marginally bound regions of the Universe, containing overdense matter structures, whereas voids are identified with underdense domains undergoing faster-than-average expansion.
Each domain is simplified and modelled by a dust-sourced FLRW spacetime, with no cosmological constant, and with a curvature depending on the domain type: spatially flat for walls and negatively curved for voids. ...

https://inspirehep.net/files/12a747b2f7cf8415b0631049b4e98ac1
The void and wall regions are each defined by locally homogeneous and isotropic FLRW solutions with their own respective scale factors av and aw, which are combined as a disjoint union to form the Buchert volume-average scale factor, a¯ = fvi a3v + fwi a3w , (1.55) where fvi and fwi correspond to the initial fraction of void and wall regions by volume in the universe.

https://arxiv.org/pdf/2504.01669
https://arxiv.org/pdf/2503.13391
https://arxiv.org/pdf/2502.20494
https://inspirehep.net/files/ee43eed060da936a2951abf3da5f1b68

Simply understood it seems that mainly the voids "undergoing faster-than-average expansion" in effect are replacing the assumption of the Cosmological Constant. But surely the model has to be confirmed by further investigations.

We know that since recombination the temperature dropped from around 3000 k then to 2,7 K now. Which corresponds to an increase of the scale factor of about 1100 times since then. So it seems that ΛCDM and timescape yield the same expansion of the universe till today inspite of causing accelerated expansion in much different ways. Coincidentally? Whereby this doesn't exclude that the expansion history is different. Would you agree with this reasoning?

Regarding the curvature it is said above "spatially flat for walls and negatively curved for voids": Our universe is spatially flat according to the power spectrum of the cosmic microwave background. How does this fit to the quote? As far as I know the spatial curvature can't change over time.

 

[PeterDonis]

So it seems that ΛCDM and timescape yield the same expansion of the universe till today inspite of causing accelerated expansion in much different ways. Coincidentally?

No. The expansion factor of 1100 is based on data, not models--all models have to account for it somehow.

Whereby this doesn't exclude that the expansion history is different.

Yes, that's how the different models can have the same expansion factor of 1100 from recombination to today despite having very different mechanisms: by varying the expansion history. The expansion factor of 1100 doesn't tell you how long it took between those two times, or how the expansion rate varied from then to now. Many very different models can all have the same expansion factor but still have varying times and expansion rates in between.

 

[timmdeeg]

No. The expansion factor of 1100 is based on data, not models--all models have to account for it somehow.

Thanks, got it.

 

[javisot]

We know that since recombination the temperature dropped from around 3000 k then to 2,7 K now. Which corresponds to an increase of the scale factor of about 1100 times since then. So it seems that ΛCDM and timescape yield the same expansion of the universe till today inspite of causing accelerated expansion in much different ways. Coincidentally?

Like timmdeeg, I would like to know if it is a coincidence that the anomalous acceleration of the expansion of the universe can be reinterpreted as a gravitational effect of some kind, or if the equivalence principle has something to do with it.