In any model, the discovery of a contradiction signifies its end. The small pieces resulting from its explosion must then be partially glued back together with the goal of resolving this contradiction.
In the expansion model, the void between two densities is supposed to dilate ever more. An equation was then devised, the Friedmann equation, to describe this expansion as a function of density. This equation is said to be inspired by Einstein's theory, general relativity, but is it truly? What does it tell us?
First point: without matter, no space, no time, nothing; in short, mass is an engine and the void follows, via the space-time deformations imposed by mass. We define flat space (-time) with a zero gravitational field, without masses, but this space is degenerate; it is the realm of photons in which time flows infinitely fast, a mathematical construction in which the model places masses. Nothing, therefore, says that the Big Bang, of infinite density and punctual, could have occurred there.
Second: the more significant a mass is, the more the space around it is contracted, rather than dilated.
Third: the more significant a mass is, the more a signal emitted by this mass is redshifted.
We then understand that if space must dilate ever more with time, as per General Relativity, it is equivalent to saying that masses decrease, that the intensity of gravity decreases over time. And therefore, the redshifts of the emitted signals are less and less redshifted over time, which does not prevent them from being increasingly redshifted with distance, given that observations are made in the past.
However, in the expansion model, galaxies are supposed to be increasingly redshifted over time, an idea derived from the fact that galaxies, seen in their past, are as redshifted as they are distant.
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To resolve this contradiction, the expansion model therefore assumes that there is an unknown energy that allows densities to be pushed apart with acceleration. Do not confuse this acceleration with that attributed to dark energy: as a reminder, expansion does not seek to mimic a classic explosion in which speeds decrease, but a 'geometric explosion' in which the acceleration can vary, and, without dark energy, it is supposed to decrease as it is assumed to be struggling against gravity. But even if it were to decrease, this acceleration requires continuous energy (unlike a kinetic explosion where the energy input occurs at t=0), this continuous input then serving to create empty space between masses.
This leads us to consider two possible paths: either we must consider that there is a continuous input of energy, which is a huge thorn in the expansion model because this energy is unknown, just like the so-called dark energy. Or we consider that gravity decreases over time. Expansion model VS relaxation model.
To know which model fits reality best, a measurement of redshift drift is necessary; a second photo of the universe is needed to compare with today's. If this is complicated (it's not easy to see the observable universe move!), then we pit the two models against each other by studying both of them. Defenders of General Relativity will have already made their choice: not only is there no need to consider an unknown energy in the relaxation model, but there is also no need to measure any variation in the expansion (or dilation) rate based on density along the line of sight, a measurement never performed although necessary to validate the Friedmann equation. Indeed, measurements have never yielded anything, even at the scale of superclusters (~1Gal)!
It would therefore appear that the expansion model has exploded in mid-air!