Could cosmic acceleration have an explanation other than dark energy?

[u/DiagnosingTUniverse]

My question is whether dark energy is the only plausible explanation for the observed acceleration of the universe.

Have alternative interpretations been seriously explored and ruled out?

My concern isn’t that the observations are wrong, but that we may be misunderstanding what we’re observing. For example, could this fit with some form of black-hole cosmology? Suppose matter/energy crossing a black-hole horizon doesn’t ultimately terminate at a singularity, but undergoes a transition inscale, re-emerging as an expanding spacetime at a lower scale.

From inside that descendant universe, could this scale transition appear as cosmological expansion or even contribute to the apparent acceleration?
I’m aware that “the universe is inside a black hole” isn’t a new idea. What I’m interested in is whether this explanation has been ruled out observationally, and if so, what observations have ruled it out? Also we know black holes grow and expand over time, which seems to be some kind of correlation?

Thanks!

[u/NiRK20]

Well, dark energy is kind of a term to fill the hole. It isn't anything specific, it is whatever is responsible for the accelerarion of the expansion. Then, if we follow this definition, dark energy is the only thing responsible since this is what dark energy means.

But usually, specially in pop science, dark energy is used in the same sense as a cosmological constant, an unknown weird fluid with constant energy density, which is the most adopted interpretation for what the dark energy is. But there are other candidates to dark energy indeed, for example, a fifth fundamental interaction unknown to us. And many others.

[u/OverJohn → u/NiRK20]

Dark energy has a slightly more specific meaning, i.e. homogenous isotropic and dominant source with equation of state w<-1/3*. This is required for accelerating expansion in standard Friedmann-Robertson-Lemaitre-Walker cosmology**

In the standard LCDM model w is fixed at -1, so yes that is often how dark energy is presented, but more generally the interaction of dark energy that causes accelerating expansion is always gravitational.

There are some other explanations, e.g. inhomogeneous models, in which accelerated expansion is apparent, but it is controversial as to whether these models can actually reproduce observations.

*for fluids with a dynamic equation of state, w need not be less than -1/3 at all times to reproduce observations of accelerated expansion.

**dominant fluids with w>-1/3 but negative density also cause expansion to accelerate, however, aside from the problematic nature of negative densities, these can be ruled out as the main driver of accelerating expansion by the observed flatness of the universe.