Lambda-CDM vs. Timescape

A long long time ago, Albert Eistein felt like he needed to have a cosmological constant in his field equation to counter the force of gravity because without it, the universe should be collapsing or expanding. When it was discovered later on (I think by Edwin Hubble) that the universe was expanding, Einstein was all like, "The cosmological constant was the biggest blunder of my career."

Once it became widely accepted that the universe was expanding, it was always assumed that the rate at which it expands should be slowing down because of gravity. The big question was whether it was slowing down fast enough to cause the universe to recollapse or if the universe would expand forever. In 1998, some astrophycisists tried to answer this question, but what they found, instead, was that the expansion of the universe was accelerating.

The accelerating expansion of the universe gave rise to the Λ-CDM model of the universe (aka Lamda-Cold Dark Matter), which has been the prevailing view for the last two decades. Lambda is the old cosmological constant that has resurfaced. Eistein's "blunder" wasn't a blunder after all. Lambda was back. Nobody knew what was causing the expanasion of the universe to accelerate, so they called it "dark energy." Lots of ideas have been suggested for what might be causing it.

Well, recently, there has been a new plot twist. Two papers were published suggesting that the accelerating expansion might just be an illusion brought on by time dilation. You see, as the universe expands, it becomes more clumpy. Galaxies merge, while voids become bigger. Since time passes more slowly near massive objects (because of the higher gravity), and time passes more quickly in the voids, this may cause it to appear that the expansion of the universe is accelerating when it really isn't.

This other model of the universe is called Timescape, and it does away with the cosmological constant because the expansion of the universe isn't really accelerating. If Timescape turns out to be right, it would undermine the Λ-CDM model. You wouldn't need a cosmological constant.

What I find interesting about this development is that the cosmological constant is often cited as the best example of fine-tuning. They say that the cosmological constant is so fine-tuned that if it were to differ by one part in 10¹²⁰, then the universe would either have collapsed too quickly for life to have ever emerged, or it would have expanded so fast that stars and galaxies could not have formed, in which case, life would not have been possible. The cosmological constant has to be fine-tuned to an incredibly precise value for life to even be possible in the universe, they say.

But if Timescape is true, then there isn't a cosmological constant at all, much less a fine-tuned one. I find that very interesting.

Several YouTube science channels have commented on the two new papers. Two of the best videos I've seen on it were by Dr. Becky and PBS Spacetime. I recommend watching these videos because they explain the papers really well. They both express skepticism about the Timescape model and say the evidence still favours Λ-CDM.

I suppose only time will tell. It's like whenever somebody claims to have discovered some artifact that over-turns everything we thought we knew about the ancient world. You have to wait and see. Sometimes, it pans out, and sometimes it doesn't. Most of the time, it doesn't. But it's always very interesting when it does.