Will the Big Bang repeat? 

Completely coincidentally, Paul Tod put a new paper about Conformal Cyclic Cosmology on the arXiv just a few days ago. Among other things, he addresses the point I mentioned in the video about the eon-to-eon feedback, though the brief answer seems to be no one really knows: arxiv.org/abs/2202.10864

Excellent. I enjoyed that talk. Have to say, I rather like Penrose's CCC idea. It's a brave effort. He himself admits it is quite speculative and I am sure he'd have no problem with it being discussed in this informed and fairly detailed way.

Yes. Looking forward to watching your comments on Penrose's ideas.

I think Penrose has a point. Rescaling can be done when all matter has decayed. No matter means no time which means no distance.

I had been waiting for this episode! Penrose's contributions for the geometrification of the puzzle we have been trying to solve has been spectacular; and he seems to be such a gentleman that one can't help but root for him. Still, I don't see this video as a negative thing at all. When one gets on the podium and says "I claim X", the audience is supposed to ask how/why? Sabine is doing just that. I now hope for a response from Penrose.

Sabine, after every one of your videos, I have more appreciation for you and your ability to capture the essence of a topic in a concise and understandable manner. Thank you.

Without being able to imagine a 'beginning', Penrose's idea seems to be the most intuitive, which is why im drawn to it. I recognize that the universe is not always intuitive.

I love getting notifications on Saturday morning letting me know that Sabine has dropped a new episode. I hope your back is doing better!

I appreciate the education I receive here on this channel. Much love and care and blessings to you Sabine.

Another one I had to watch twice and REALLY enjoyed. I like Penrose's theory, and trying to wrap my brain around the scaling, and the transferring of information from eon to eon!

Thanks for another great video, Sabine. I so look forward to Saturdays and your wonderful videos!

Sabine, you are a rare source of enlightenment, entertainment, life affirming knowledge and I look forward to every opportunity I find to watch your lectures and discussions. I am improved every time I see another of your productions. Thank you for that.

This was really good. I have a lot of reservations about Penrose's model, too, the biggest being the one you brought up about the physical meaning of the math and whether it really makes sense physically. But you brought up some others I hadn't thought of, and you explained them really well.

This is by far the simplest and clearest explanation of CCC I've seen to date, and I've watched many of Penrose's lectures on it. Thank you. :) That said, the theory still raises so many questions -- or perhaps I am simply unable to get my head around the (physical?) mechanism that begins a new eon.

found your chanel the other day by grace of the algorithm and i am OBSESSED and have been bingeing for days maaany thanks!

the explanation of conformal is SO elegant! thank you for another great video!

It seems to me that we have to deal with a lot of complications because of the “fact” that the expansion of the universe is accelerating. This acceleration, is, of course, based on our current “best efforts” at measuring cosmological distance. To me, distances beyond what are directly measurable by parallax methods are suspect. Given that the strong nuclear force is an (extremely) local phenomenon, why are we convinced that gravity and/or electromagnetism not also local, with a redefinition of what “local” means on these scales. . . As for inflation. . . I’m reminded of a cartoon, where a white-coated researcher standing in front of a blackboard full of equations says “and then a miracle happens”. Thanks, Sabine, as always, for making me think.

Even though there are many holes in this idea, it is incredibly creative. Thank you for the video!

Great video! And very informative, thanks Sabine!

I always enjoy your videos. Thanks for doing such great work.

My brain was in a superposition while watching this video. It both made complete sense and was simultaneously unintelligible.

Thank you for covering my favourite crazy theory. It's fun trying to explain the conformal boundary to people, and the idea that the universe could become conformally invariant (and why).

Thanks for an extremely informative and really balanced presentation/ review.

Really interesting! Currently taking an introductory GR class right now and just a handful of pages from the Weyl Tensor description :). Been balking at getting through The Road to Reality so this was a terrific summary of Conformal Cyclic Cosmology! :). I Always get a lot out of your videos!

In R. Penrose favor I have heard him call CCC "outrageous" so he knows that it is a wild speculation. But if I remember correctly, he originally studied mathematics and they can do wild speculations and crazy generalizations. Also I have heard him explain that at the end of each cycle every particle would need to decay to photons. If only a single massive particle is left the whole thing doesn't work. Anyway thanks 💕 for a good video.

Penrose gave a talk at my university when I was an undergrad. In the Q&A I asked him about cyclic cosmologies. He said that they were only possible in perfectly uniform universes (at least that's how I remember his answer). I guess he's changed his mind.

I really appreciate this video. I'm excited to see how this theory develops, and it's extremely useful to know what are the possible weaknesses (or at least the ones Sabine mentions).

I've been waiting for this video from you, ever since I watched a talk by Penrose about this. Also, did anyone notice the cyclic nature of the title screen at 0:52? That was a nice touch!

Some thoughts I've had regarding this topic: I think the best or most useful conceptualization of time is as a measurement of relative change. Anytime time is referenced change must always also be referenced. So change is fundamental to time and given a static closed system there can be no internal clock. An implication of this seems to be that once the universe evolves to it's final state of absolute heat death. Where any remaining undecayed particles are so isolated that they can never interact with each other and so for all intents and purposes there is no longer any relative change in the universe and all that's left is besides individual isolated particles is the quantum fluctuations of empty space. When the universe arrives at that state, time has essentially stopped since there are no longer any events by which to measure time. From that point on, anything that does happen could be said to happen instantly afterward. So, imagining an external clock, if it took a near infinite amount of time to elapse on that external clock for the quantum foam to produce a fluctuation that evolved into the next iteration of a material universe. From the internal perspective, lacking the concept of time following the heat death of the previous iteration, the emergence of the next could be said to have happened instantly following the end of time of the previous. Therefore it becomes easy to imagine that the near infinite amount of "time" required for the quantum foam to produce a fluctuation that will evolve into a material universe is actually no time at all. The ultimate implication of that being that not only is the emergence of a universe out of the quantum fluctuations of empty space probabilistically likely, it's inevitable, since whatever time is required is available and actually is relatively instantaneous from the perspective of a changeless universe. All of this however assumes eternally expanding space filled with quantum foam. How that originated, why there's even that as opposed to true nothingness? Probably has to do with probability and the uncertainty principle in a way thats difficult for me as a layperson to conceptualize or explain but I feel like I understand intuitively. True nothingness is a form of absolute certainty and the uncertainty principle is such that a state of absolute true nothingness must instantly and randomly evolve into a state with non zero energy. Or something like that. @sabinehossenfelder I'd love to know if you, or anyone else reading this, thinks any of it makes any sense lol 🙏🏽🙌🏽🌌

At last, thanks for this Episode. I missed some more details about the fact, that CCC does not need an inflation part, because what we need for the inflation part of our past is naturally the expansion of the previous eon. I always found this an interesting part because the current inflation theory always seem a bit like a hat trick to me. Anyway, thanks for the episode and the much more polite than usual way to discuss/criticize a very decent man's theory indeed.

very interesting, and very good explanation, thank you!

Your content brings so much to my mornings. Thank you for what you do :)

It may not be what really happens at the end of time/matter, but it is the most philosophically pleasing I've come across for a while. Rogers explanation of CCC helped form a model of all of space, time and infinity in my mind. However, nothing is really answered unless you know how the first in the chain starts.

Thank you, Sabine. One interesting detail you didn't mentioned is that close to "eons end" universe consists mostly of photons and photons are not "bored" with infinity -- they live in their NOW..... Altogether, I am happy you have addressed CCC hypothesis and -- I immensely value your efforts to popularize science. I am also grateful for your performance art and music !!

3 mins in.. great exposition of the conformal re-scaling. Really loved that.

Sabine your awesome thank you so much for your hard work in opening and broadening mine and other people's minds. Your videos are always so well made and full of knowledge. Thank you again for all you do.

I am so glad this theory is being looked at seriously. Your one of the first people outside of Penrose himself I have seen discussing. It seems a mathematical model is not a conformal representation to what we observe in larger structures in reality. The conformal theory reminds me of fractals but I can't help but wonder if we make an error in science in seeing the laws as fixed or constant (and though he has his controversy Rupert Sheldrake comes to mind)...As such the problem with a conformal cyclical model would be that very assumption of constants ...Great problems to ponder.

I really like this idea from an aesthetic point of view. Though, as my father-in-law observed after I gave him a (very rough and ready) explanation of what CCC was, "Only a mathematician could think it worked that way." I.e. same as Sabine's question of what the conformal rescaling actually means in practice. Which is a fair objection, and one I leave to the actual physicists. The idea that reality might be so pure in its adherence to mathematical elegance is too good to be true - but this platonist lives in hope.

Thank you for very good explanation of conformal remapping. That was very helpful!

I was very interested to hear your opinions on this idea, so I'm glad you did finally get around to doing it. Thank you for this, and all you do. You are truly one of the bright spots in my otherwise dark world, and that means a lot to me. ❤❤

10:40 The fluctuations feed into each other recursively, but what is to say the gain is greater than 1 if they in fact amplify? Or what if there is some kind of destructive interference?

So happy that CCC gets more and more attention! Better later than..... Sir. Roger Penrose is our time genius, for the rest of us would be wise to listen more closely what ppl like him has in mind. Thanks Sabine :)) Word hypothesis kills me every time tho :D

Thanks for this video, Sabine!

Thank You for all your efforts!

So in layman's terms, CCC is the equivalent of "Turtles all the way down"?

You can see how much Sabine respects Penrose by the fact she practically made no joke for the whole video.

As always excellent info video.Thank You Kindly.

I loved this video! I would like to see more videos where you introduce a theory and then gives some pros and cons from your perspective. I know you mentioned you don’t like to pick apart someone else’s specific theory but I’m confident you can come up with something. 😁

This kind of reminds me the hypothesis that the higs field might have more than one stable energy level - and it might be statistically possible for the universe to switch from one to another. Such a switch would reset everything in a really hardcore way and as it is about the higs field, it would comply with Sabine's request to deal with all the massive particles in the process. :)

personally my bias is that i hope penrose is right, i've held him close to my heart ever since the penrose tiles thing, people who understand, and solve, other people's problems without any great effort are my favourite people. i do hope that penrose is right cos i think CCC is a neat way to get around infinities, the way i've understood it is that if you are left with only photons then time has no real meaning and distance has no real meaning, so however "big" the universe has become it is just a speck once more, but the point you make about the higgs and scale does put a spanner in the works. whatever the explanation of how the universe began, what happened at the start, what happens at the end, and will there be an episode II of the universe, i hope we find out before i croak.

Ooh, been waiting for this!

Great job Sabine, this is a fantastic review of Penrose's CCC. I will be using this video to send to everyone that requires an introduction to CCC. You covered every significant point of CCC in simple yet informative manner, extremely impressed. I don't agree with your final thoughts on the matter. You propose good questions at the end. However, it seems that essentially your questions stem from your assumption that all processes in metaphysics/physics follow the same rules we know for spacetime. You must recognise this is an assumption. Edit: I am happy to respond to all your thoughts you provide in the video if you give this comment a heart. It will let me know you are actually interested in my observations on your final thoughts.

The Higgs field is the only field with a positive vacuum energy (I hope I got that phrased right). Could that be a false vacuum? Maybe there's a lower energy state where the Higgs field is zero? Then as the universe expands, this false vacuum could decay into a true vacuum where all particles are massless. Such a state is also be assumed to have happened in the early universe, right?

I always see re-runs of The Big Bang Theory on TV!!

Great summary and criticism. I would love to see more videos on cosmology. Thank you.

This is the first time I have half understood CCC and the most mind-boggling explanation of the universe I have heard, apart from all the other ones.

I always wondered what happens after the heat death. When all matter finally decayed. I thought if there really is no difference in energy throughout space anymore, aka maximum entropy, then spacetime becomes irrelevant. I read about different cyclic cosmology hypothesis in the 90's. And all i could think is that maximum entropy is also minimum entropy at the same time and a "new" cosmos would "condense" out of the evenly distributed radiation. Like fog on a window. So in my mind, the requirement for another big bang is is entropy crossing a threshold that causes spacetime itself to "decay". Alright, enough crackpotting for one day, i'm an electrician, after all.

My thought on this was if the universe continues to expand, eventually all matter would be separated out and simply not interact with any higgs fields, as time continues and less matter interacts with the higgs field the mass would appear to decrease, if E=MC2 (sorry no superscript) and E is fixed, then as M approaches 0 C would approach infinity, eventually the separated matter would accelerate to such speeds it would be almost everywhere at once, this would then mean collisions can happen and thus kick off a big bang. Ahh the joys of oversimplification.

Fascinating. I’m not a physicist … though I grew up with them, and I like to keep up (as best I am able with my brain-of-small-bear) with these concepts. Your channel is a fascinating, and entertaining help in that effort. I do like … to a degree … Penrose’s CCC, and am also drawn to Smolen’s “evolutionary” universe (though with serious reservations). Please keep this up. Endlessly entertaining and informative (plus I’ve developed a mild crush, but no matter).

First time commenter. Really appreciate you taking the time to do a video on this. There is something about CCC that feels like it is going in the right direction, like you pointed out in this video, and I really feel that is where we should focus our experimental efforts. One of the things that you pointed out as a criticism is the conformal aspects of the universe being overly speculative. Something that I've always wondered is why in QFT we assume the stable state of quantum fields is basically uniform across the universe? The more we study about condensed matter physics the more it seems like energy densities not only play a role in matter physics but probably extend into quantum field states as well. Perhaps the reason we have such a hard time detecting dark matter and dark energy sources is because the densities of their energy are so small they exist in different quantum metastable states than physical matter. Theories that don't make this assumption could provide insight on experimental physics moving forward.

What would spark the need for the re-scaling? Is there a specific event that would trigger it?

Rather than rescaling, it seems to me that the "next" universe(s) could simply be the result of a fluctuation in the vacuum state left behind by the depleted prior iteration. I can't think of a test for this idea, but it's certainly simpler and requires fewer moving parts, as it were.

Interesting and worthwhile video.

Thank you for the video.

If this cycle exists, could previous hyper-advanced civilizations hypothetically leave a message in the CMB?

Or if hipsters do it, it's non-conformal cycle cosmetology

10:24 Are there solid arguments for why CCC-models would have a positive feedback loop for fluctuations. Does any of the papers in the description describe that and its effect on different fields? If so which? Thanks!

Sabine, you are amazing. it is a pleasure to listen to you. thank you

Just not next week, please, I have holidays booked.

Seems implausible that a mathematical rescaling would reduce entropy. How does rescaling reduce the number of possible states?

I already watched this video in the previous universe. Nice to see you again 😉

I definitely watched this too early in the morning. I'm going to rewatch after a bit of caffeine. Thanks! Dave J

Difficult to imagine someone of Penrose's stature would be concerned about criticism, goes with the territory, surely? I've attended a few of his lectures at the Royal Institution...still looking forward to the day when I can see Sabine doing a lecture at the same venue.

If there's infinite universes behind us, wouldn't the infinite fluctuations cancel out?

Thanks for the video :)

Awesome, Sabine

Seems like so much of CCC is untestable, it almost reminds one of a god hypothesis.

Thank you for the video, Sabine. This summed it all up nicely for the layman like me.

Thanks for making this video Dr. Hossenfelder

The cycle and creation of the universe repeating is far less likely than the creation of theories that can't be proven.

I've understood maybe about half of this but I was still enthralled the whole time. :D

I was wondering about ccc for some time since I first heard it couple of years back, thanks for break down

My favourite science explainer take on my favourite speculation in Physics/Cosmology? I'm sold

Thank you for the explanation. Many years ago I watched an interview with Mr. Penrose explaining this theory. It was my impression that as the universe aged, eventually all matter would cease to exist leaving only energy. Then there would be no scale to the universe and something like a phase change would occur presenting the new invocation of the universe. Not saying it makes any sense, just my interpretation.

While I don't claim to fully understand either CCC or Sabine's critique I like the idea of an infinitely repeating cosmos. Hopefully CCC or something similar will turn out to be how things actually work on these incredible long time scales. As I learn more about time it is a much more illusive concept then I initially thought from a pragmatic engineering perspective.

As always.... Amazing! Please.... Keep bring cosmological/particle physics articles. :-)

Excellent… so helpful

I watched a video of an interview with Penrose and was struck by inconsistencies, contradictions actually, in his explanation of CCC. I admit that he had to dumb it down for the non-Physicists so that may have been the cause of the problems. Thanks for the link to the paper, it is nice to find research papers that don't cost $35 to $45 to read. Unfortunately, it has been over 40 years since I graduated so my math is rusty, and my cosmology is rusty and outdated, meaning it is going to take many hours of study before I actually have a good understanding of the paper. But what the heck, I'm retired so this is as good a way as any to spend my time. LOL Thanks for the video, link to the paper, and motivation.

Thank you Sabine for the video. It seems that all the approaches for cosmology comes from an exterior and abstract way. Is there any internal approach, or a phenomenological one? Considering this fractallity of the universe, would not the cosmos structure be rescaled and represented in the smallest structure of matter contained in our daily experience?

Love the fact that your criticism are questions. Listening to you ask questions that I would never thought of is inspirational. Now my question is, what questions do Dr. Neil Turok's take on the cyclic universe?

Thank you, Dr. Hossenfelder. This video was highly educational for me on CCC. Since the Higgs field potential has to be in a range to be operational (Mexican hat shape), is it possible that it goes out of the range towards the end of the universe and all elementary particles lose their mass (and hence conformal)?

I am often reminded of the saying 'Not only is the universe stranger than we think, it is stranger than we can think.' I cannot add anything to these discussions but I have a feeling that great minds are approaching the answer, maybe not rapidly but certainly steadily. As for entropy, many years ago I was an engineering student and we learned that entropy was all to do with steam engines and measured in British Thermal Units per pound per degree Fahrenheit. Happy days!

Escher's Circle Limit IV is based on his correspondence with Coxeter on the Poincare' disk model of hyperbolic geometry, so not really conformal rescaling of a Euclidean plane.

For me, this theory at the very least made me thinking in more fundamental ways than I used to, about how the concept of equivalence could give rise to derived concepts that otherwise seem strange and counter-intuitive. Like the idea that if there is no theoretical way to measure duration and distance, in this case because the universe has stretched to the point it no longer contains anything, any duration or distance becomes equivalent to any other. So they might as well be concentrated in a single point. This way, I can at least wrap my insufficient mind around concepts like entanglement: e.g. if two particles are generated "with opposite spin", and any combination of down and up is allowed, because they are equivalent, the universe simply does not contain the required information to "decide" which of the possible values any one of them "should" have. So then anything that allows distinction (breaks the equivalence) between the contained possible values will count as a measurement on it and will therefore measure only one of the containing values, with the other being opposite to it.

Thanks for talking about CCC. Seems like not many people even consider it. Even if wrong, it's worth discussing if for no reason other than to develop a model closer to truth

I saw your post on SWIFT. I have a friend who posted this on facebook. It's him with a Ukrainian friend. J: my friend. N:Ukrainian girl. j: Can't somebody kill Putin. N: Our president told that Swede help us. J: Yeah, we are doing what we can but idk. N:Me either. N: My dad goes to war. N: My mother is volunteering for the front now. N: I asked them to run away with me N: They refused and told they will stay away and defend the country. I saw your post Sabine. You will not get to say that you did not know. Long time subscriber. I'm joining the Swedish armed forces as of today. I know now that the Germany will not help us if it comes to it.. I don't believe in God, but if he exits, may he have mercy on all of you.

Around 4:50 you discussed conformally rescaling the Weyl tensor as you join the old universe to the new big bang, but the Weyl tensor is actually conformally invariant, so it would actually be unaffected by the rescaling.

Excellent Sabine. Clever analyses. Many physicists are proposing theories based on already established but problematic theories such as the Big Bang, Inflation, Singularity, Copenhagen interpretation of Quantum Physics, and with lack of understanding of the nature of Entanglement, double-slit experiments, dark matter, dark energy, apparent expansion of the space, gravity at large scales and finally time.

It's great to see this level of respect and good disposition awarded to a proposition while opposing it.

Good work.