The data behind the dark energy/cosmological constant hypothesis is there, but it is thin. If inferred dark energy is decreasing, that greatly impacts the models that can fit the phenomena and the implications dark energy phenomena have for the long term outlook of the universe.
It also presents yet another cosmology scale independent challenge to the ΛCDM "Standard Model of Cosmology" (33 more of which can be found here).
The potential energy from a time-dependent scalar field provides a possible explanation for the observed cosmic acceleration. In this paper, we investigate how the redshift vs brightness data from the recent Pantheon+ survey of type Ia supernovae constrain the possible evolution of a single scalar field for the period of time (roughly half the age of the universe) over which supernova data are available.
Taking a linear approximation to the potential, we find that models providing a good fit to the data typically have a decreasing potential energy at present (accounting for over 99% of the allowed parameter space) with a significant variation in scalar potential (⟨Range(V)/V(0)⟩≈0.97) over the period of time corresponding to the available data (z<2.3). Including quadratic terms in the potential, the data can be fit well for a wide range of possible potentials including those with positive or negative V2 of large magnitude, and models where the universe has already stopped accelerating.
We describe a few degeneracies and approximate degeneracies in the model that help explain the somewhat surprising range of allowed potentials.
Mark Van Raamsdonk, Chris Waddell, "Possible hints of decreasing dark energy from supernova data" arXiv:2305.04946 (May 8, 2023).
From the body text:
One main conclusion is that there is a lot of room for such models, and models that are consistent with observations are not necessarily close to ΛCDM. Within the space of models we consider, we have found that in models providing a good fit to the data, the scalar potential typically changes by an order one amount compared to its present value during the time scale corresponding to the supernova data, roughly half the age of the universe. In the context of models with a linear potential, a large majority of the models (> 99%) in the distribution defined by the exp(−χ^2/2) likelihood have a potential that is presently decreasing with time.
This strong preference for decreasing dark energy is somewhat surprising; one might have expected that if ΛCDM is actually the correct model, extending the parameter space to the linear potential models would yield a distribution with a similar fraction of models with V1 < 0 and V1 > 0. The dominance of V1 > 0 models in our distribution could thus be a hint that the correct model is not ΛCDM but one with a decreasing dark energy.
For models with a quadratic potential, we find a variety of qualitatively different possibilities, including models where the scalar field is now descending a downward-pointing parabola and models where the scalar is oscillating in an upward-facing parabola. Again, the best fit model has a decreasing dark energy at late times.
3 comments:
Does MOND predict/explain this at all?
MOND has nothing to say, one way or the other, about dark energy.
Despite the name, MOND is generally viewed by astronomers as GR except to the extent that it deviates from GR in very weak fields where GR is generally viewed as indistinguishable from Newtonian gravity for the purposes for which it is used. Also, MOND advocates generally assume contrary to Newtonian gravity, that photons are subject to MONDian gravitational fields in the same way that they are in GR except that the gravitational field strength is greater in regions which are in the MOND regime.
Continued . . .
So, if you think GR with a cosmological constant is correct, except for a MOND modification, then this contradicts MOND, but not in any MOND-specific way.
Deur's approach, in contrast, not only recaps MOND effects in galaxies, but also extended the theory to galaxy clusters and cosmology scale questions, including dark energy. In Deur's analysis, the phenomena attributed to dark energy are really due to depleted gravitational fields outside galaxies and galaxy clusters because the gravitational fields in those structures are being diverted to more tightly binding objects within the galaxies and galaxy clusters in what is seen as dark matter phenomena. Thus, in Deur's analysis, there should not be an infinitely increasing amount of dark energy as time passes and a decreasing apparent dark energy effect is broadly consistent with his prediction although the data are quite theory agnostic as they can be fit to many different models with declining dark energy over time.
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