I just saw that Penrose recently published a paper on Conformal Cyclic Cosmology and it made me wonder, what exactly is the consensus view these days? I first heard of Penrose in connection with Orch-OR, which I'm not a fan of, but the new paper made me wonder, what camp is winning at the moment?

Personally, my money is on a Big Rip, but I'm curious what others believe and why.


closed as primarily opinion-based by John Rennie, userLTK, Kyle Kanos, glS, Jon Custer Aug 27 '18 at 16:29

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    $\begingroup$ physics does not go by consensus but by validation with data. $\endgroup$ – anna v Aug 27 '18 at 5:21
  • $\begingroup$ Really? This is all speculative, foundational stuff and I'm sorry, but I couldn't disagree more (in this case). If you're talking about calculating how many calories are required to heat the water in your bath tub, then you're right, but this is a question about theoretical physics. Or would you rather I changed the word Consensus to something like "Most Popular"? $\endgroup$ – Thor Aug 27 '18 at 5:28
  • $\begingroup$ @Thor most popular would be better than consensus, because there's no consensus on this particular question nor is there likely to be any consensus anytime soon. Where I disagree with Anna is that consensus does follow validation, but that's just semantics. I think heat death is the most commonly believed in outcome, but I'm just guessing. There are other options that you didn't list like false vacuum, Higgs instability (which may be the same thing). Collisions between parallel Universes. :-) I'll stop now, but I think heat death is the actual most common, in my non expert opinion. $\endgroup$ – userLTK Aug 27 '18 at 7:13
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    $\begingroup$ @Thor yes, most popular or fashionable is fine for speculations. Physics is not done by physicists voting, although it might seem so to outsiders. $\endgroup$ – anna v Aug 27 '18 at 7:19
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    $\begingroup$ Well, let's not forget that science is also a social activity - so much so, that, e.g., publishing your results is widely regarded as an integral part of scientific research. I also think data is the driving force and that the social aspect of research is added noisy in the search for knowledge - but it's undeniable that consensus directs (by means of all sorts of institutional pressures) the overall direction of research of the majority of scientists. Therefore, asking what the consensus is is not a misguided question, but a relevant one. Think of climate change research. $\endgroup$ – stafusa Aug 27 '18 at 10:12

Consensus in cosmology happens when there is observational data that constrains serious practicioners into a particular corner of model space. It will alwys become looser the less constrained a domain is, and this is why it is unsurprising that people are not that strongly in agreement about the long-term future.

In terms of popularity I think we have the ordering Heat death > Big crunch > Big rip > CCC. The reason is that when cosmologists allow themselves to extrapolate the consequences of their theories they prefer theories that do not introduce extra complications. One might view this as an application of Occam's razor. Note that predicting odd early phenomena in many ways is more accepted, since it looks like at least in principle one can detect early phenomena.

Assuming the expansion continues like a FLRW universe is the simplest assumption, and given current acceleration that gives endless expansion leading to a heat death state. While there are some people who think collapse has not been entirely ruled out by the data, they seem to be a clear minority.

Big rip theories are very underconstrained: while you get a big rip if $w<-1$ and current data suggest $w\approx -1$, there is a host of related scenarios that include fields that change and allow the effective $w$ to change - selecting between them is hard since observational data will not constrain them much. So this makes people less keen on them.

CCC goes even further, since it predicts some radical and so far never observed phenomena like charge conservation violation (in order to get rid of all fermions). It is very much an interesting minority idea.


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