The cosmological "constant" is in trouble again

No long ago, I made a post about the Λ-CDM v. the Timescape models of the universe where I talked about two papers that came out claiming the apparent acceleration of the expansion rate of the universe might be an illusion caused by time dilation due to growing cosmic voids and changing clumpiness over time. This alternate view is called the Timescape model, and it does away with the cosmological constant (Λ) altogether. I found that interesting because Λ is often cited as the most fine-tuned of all the fine-tuned constants, but if Timescape is true, then there is no Λ, much less a fine-tuned one.

Now, Λ faces a new challenge. There's a new paper that came out claiming that Λ isn't a constant after all. It changes over time. It was stronger in the past.

Λ resurfaced over two decades ago when it was discovered that the expansion of the universe is accelerating. Since the cause of the acceleration was unknown, it was called "dark energy." It was assumed, at least, that the accleration was constant. But now, it looks like the accleration hasn't been constant.

Several YouTube science channels have talked about this recently, including Becky, Maggie, and Sabine. They all discuss the threshold of statistical significance (σ). At this point, the threshold for "definitely a fact" hasn't been met, but I think it's been met for reasonablness. One of them, I think, said it amounted to around a 99% probability, which is good enough for most ordinary people even if it falls short of astrophysics standards.

If Λ is not constant, then it can't very well be a fine-tuned constant. But I don't think that means there isn't some sort of fine-tuning involved. It may just be a fine-tuning of the initial conditions. Or maybe it's a fine-tuning of the second or third derivitive of the expansion rate. In other words, while the rate of acceleration may not be constant, the rate of change of acceleration could be constant. The initial conditions, plus the rate of change of acceleration could both be fine-tuned. I guess we'll have to wait and see.