It’s been a while, but I’ve finally gotten around to jotting down a few thoughts about the Sean Carroll vs. William Lane Craig debate. I previewed the debate here (part one, two, three, four). I thoroughly enjoyed the debate. Future posts will discuss a few of the philosophical questions raised by the debate, but I’ll briefly discuss some of the science in this point. (I didn’t manage to record my talk a few weeks ago, but this post summarises it.)
Firstly, I want to refer you to the much greater expertise of Aron Wall of UC Santa Barbara. I’ll list them all because they’re great.
- Did the Universe Begin? I: Big Bang Cosmology
- Did the Universe Begin? II: Singularity Theorems
- Did the Universe Begin? III: BGV Theorem
- Did the Universe Begin? IV: Quantum Eternity Theorem
- Did the Universe Begin? V: The Ordinary Second Law
- Did the Universe Begin? VI: The Generalized Second Law
- Did the Universe Begin? VII: More about Zero Energy
- Did the Universe Begin? VIII: The No Boundary Proposal
- Did the Universe Begin? IX: More about Imaginary Time
- Fuzzing into existence
(I’m on the “astrophysics” end of cosmology. The beginning of the universe probes the “particle and plasma and quantum gravity and beyond” end of cosmology. I know the field, but not as well as someone like Wall or Carroll.)
No one expects the beginning of the universe!
Regarding the scientific question of the beginning of the universe, here is how I see the state of play. Cosmologists don’t try to put a beginning into their models. For the longest time, even theists who believed that the universe had a beginning acknowledged that the universe shows no sign of such a beginning. We see cycles in nature – the stars go round, the sun goes round, the planets go round, the seasons go around, generations come and go. “There is nothing new under the sun”, says the Teacher in Ecclesiastes. Aristotle argued that the universe is eternal. Aquinas argued that we cannot know that the world had a beginning from the appearance of the universe, but only by revelation.
So when a cosmic beginning first raised its head in cosmology, it was a shock to the system. Interestingly, theists didn’t immediately jump on the beginning as an argument for God. Lemaître, one of the fathers of the Big Bang theory and a priest, said:
“As far as I can see, such a theory [big bang] remains entirely outside any metaphysical or religious question.”
In 1951, Pope Pius XII declared that Lemaître’s theory provided a scientific validation for existence of God and Catholicism. However, Lemaître resented the Pope’s proclamation. He persuaded the Pope to stop making proclamations about cosmology.
The philosophical defence of the argument from the beginning of the universe to God (the Kalam cosmological argument) starts essentially with Craig himself in 1979, half a century after the Big Bang theory is born.
In fact, the more immediate response came from atheist cosmologists, who were keen to remove the beginning. Fred Hoyle devised the steady state theory to try to remove the beginning from cosmology, noting that:
“… big bang theory requires a recent origin of the Universe that openly invites the concept of creation”. His steady-state theory was attacked “because we were touching on issues that threatened the theological culture on which western civilisation was founded.” (quoted in Holder).
Tipping the Scales
But what of the beginning in the Big Bang model? Singularities in general relativity weren’t taken seriously at first. Einstein never believed in the singularities in black holes. Singularities were believed to be the result of an unphysical assumption of perfect spherical symmetry. In Newtonian gravity, a perfectly spherical, pressure-free static sphere will collapse to a singularity of infinite density. However, this is avoided by the slightest perturbation of the sphere, or by the presence of pressure. A realistic Newtonian ball of gas won’t form a singularity, and the same was assumed of Einstein’s theory of gravity (General Relativity).
The next 80 years of cosmology sees the scales tipping back and forth, for and against the beginning.
For: The Friedman-Lemaitre-Robertson-Walker (FLRW) model for the universe has a beginning. Spacetime cannot be extended through that boundary. A beginning is there for a wide range of universes, including ours.
Against: Bouncing cosmology. Perhaps the pressure of matter will prevent the universe from collapsing to zero size, causing the size of the universe to bounce. As we look back in time, the universe’s current expansion is preceeded by one or more bouncing eras.
For (Tolman, 1930’s): the second law of thermodynamics says that entropy will continue to increase through the bounce. Each bounce will be bigger than the last, and there must have been a first bounce.
Against: The FLRW model is perfectly (and hence unrealistically) symmetric. Inhomogeneities will prevent the universe from collapsing to a point. A Universe with lumps and bumps will collapse to a non-zero size, before which it was contracting.
For (Hawking and Penrose, 1960-70’s): the existence of an initial singularity is not prevented by inhomogeneities. In fact, singularities exist under a surprisingly broad range of conditions. In particular, gravity must be attractive and general relativity must continue to hold.
Against (Guth, et al. 1980’s): Gravity is not always attractive. The theory of inflation (and particle physics models) shows that there are forms of energy with repulsive gravity, which can avoid the singularity theorems. In particular, evidence for inflation is evidence that such forms of energy were relevant to the early universe.
For (Borde, Guth and Vilenkin [BGV] 2003): a stronger singularity theorem that inflation alone doesn’t seem to be able to avoid. In particular, a classical spacetime which is expanding on average must have a beginning.
Against: quantum gravity is expected to take over from classical gravity in the very early universe. In particular, we expect spacetime to be an approximation to something else, something which behaves according to quantum laws (and in particular, the uncertainty principle) but behaves like classical spacetime on large scales. We don’t have a theory of quantum gravity, so we don’t know whether the singularity theorems hold.
For: the second law of thermodynamics states that entropy increases with time, as physical systems tend towards thermal equilibrium. If the universe were infinitely old, then it would have reached thermal equilibrium by now. Thus, it must be finitely old.
Against: The Ekpyrotic model of the history of the universe shows how, in an infinite universe, the entropy can keep increasing but never reach a maximum, being continuously diluted by the expansion. (Wall calls this “shell games with infinity”).
For: The BGV theorem also applies to the Ekpyrotic model, showing that it too must have a beginning.
Against: other models which propose a collapsing or “boring” (static) phase prior to the current expansion of the universe avoid the BGV theorem and permit an eternal universe.
For: Quantum instabilities might not allow the universe to remain still for all of eternity, and may cause a collapsing universe to crunch rather than bounce while avoiding the ordinary second law of thermodynamics. (I won’t even pretend to understand the generalised second law. See Wall.) For example, Aguirre and Kehayias say:
Although we have analyzed only one version of the emergent universe, we would argue that our analysis is pointing to a more general problem: it is very difficult to devise a system—especially a quantum one—that does nothing ‘‘forever,’’ then evolves. A truly stationary or periodic quantum state, which would last forever, would never evolve, whereas one with any instability will not endure for an indefinite time.
See also Vilenkin and Zhang. There can be singularities in quantum gravity (e.g. string) models, but in the absence of a complete theory of quantum gravity, we don’t know the physics of the bounce.
Against: the quantum eternity theorem shows that beginnings cannot be found in quantum mechanics as we know it, unless the universe has zero energy. When we wonder “what will quantum gravity – the uniting of quantum mechanics and general relativity – say about the beginning?”, the two pieces suggest opposite answers. General relativity suggests a beginning (with loopholes). Quantum mechanics suggests no beginning (with loopholes). What happens when these two wacky characters share an apartment? We don’t know.
??? Beginning in the middle. There are a number of models, including one by Carroll, that have a universe with an infinitely long time dimension. However, the “beginning of time” in the thermodynamic sense (the minimum of entropy) is in the middle. There are an infinite number of one-second-long intervals, but “half” of them are not in the past of the other half. Is this an eternal universe, or just two universes, each with a beginning? There is no “edge” in the model (so to speak), like a singularity, where our picture of earlier states causing later states breaks down. And yet, the first half of the universe does not “give rise” to the second half in any meaningful sense.
???: Hartle-Hawking “No boundary condition” and Vilenkin’s “tunnelling from nothing“. Again, read Wall on these ideas. Popular images of the Hartle-Hawking cosmology seem to suggest that the universe has a finite age, without having a beginning point. However, these pictures come with significant caveats:
quantum cosmologists have made proposals that postulate a ‘framework’ (as we have seen it may well not deserve the name ‘spacetime’!), which replaces the Big Bang singularity of classical general relativistic cosmology, and from which classical spacetime somehow ‘emerges’. Probably the best-known example is the Hartle-Hawking no-boundary proposal. But this ‘emergence’ is not a process in time: Hartle and Hawking’s proposed ‘framework’, and others such as Vilenkin’s, stands in no temporal relation to classical spacetime, or any of its parts (regions or points), even very early ones.
… one often sees a picture in which a cone-like spacetime structure (representing a cosmological solution of classical general relativity) is attached to a spherical shape that represents a Euclidean 4-manifold. This erroneously suggests that the bottom sphere is straightforwardly earlier than the classical cosmology represented by the open cone in the top half of the figure. But the 4-manifold is not earlier: there is no temporal relation between the two halves represented in the figure (or their parts)! (Butterfield & Isham, 1999)
Similarly, “tunnelling from nothing” is a metaphor, a cute nickname that shouldn’t be taken too seriously. Vilenkin says:
I have recently [in 1982] suggested a cosmological model in which the Universe is created by quantum tunnelling from “nothing” to de Sittter space, where by “nothing” I mean a state with no classical space-time.
It’s not “non-being”, and has no relevance whatsoever to the question of why anything at all exists. It’s tempting to think that this describes the transition from a quantum-gravity regime to a classical regime, except that this would suggest that at one time, there was no such thing as time and then at a later time – hey presto! – there’s time. Which is incoherent. If some sense can be made of this scenario, we might be faced with time-as-we-know-it having a beginning, like a smooth line emerging from a fractal. There wouldn’t be a classical universe with a singular boundary as a beginning point, but neither would there be an eternal universe.
Conclusion
It’s a bit of a mess. There are enough surprising hints for the beginning of the universe to be taken very seriously, but enough unknowns to keep us busy. If offered even money, I’d bet on the proposition that there has only been a finite amount of classical time in the history of the universe.
Nice survey of the back-and-forth discoveries over the last century or so.
http://www.strangenotions.com/cosmology-and-causation/
I don’t understand why the focus on models that seems to me to have very little conversation in the current literature like Ekpyrotic ( you know that Steinhardt and Turok are now pushing a new model based on Higgs instability rather than this model right?) or Vilenkin’s tunneling form nothing which I cant see giving any predictions for the power of gravity wave spectrum.
When I survey the literature I find most cosmologists agree we need to apply a quantum theory of gravity to the big bang before we can answer questions about the beginning. Since we don’t have such a theory, we just don’t know. Of course one could just leave it there.
But we do have a number of attempts to apply candidate theories such string theory and loop quantum, gravity to the big bang. These models very frequently result in a non singular bounce. I would say there is overwhelming consensus in the loop community about this and whilst the string theory community is more divided its still a common result. So I don’t think anyone can give an account of the history of this issue without talking about that. I know neither Sean Carroll nor Alex Vilnkin are fans of these bouncing cosmologies and so if one is getting their cosmology from watching atheists theist debates one might miss this. But if you look at the arxiv everyday I find it impossible to miss this trend.
“why the focus on models that seems to me to have very little conversation in the current literature like Ekpyrotic”
Because it’s a historical overview. Tolman isn’t much discussed, either.
You say: “When I survey the literature I find most cosmologists agree we need to apply a quantum theory of gravity to the big bang before we can answer questions about the beginning. Since we don’t have such a theory, we just don’t know.”
I said: “What happens when these two wacky characters [GR and quantum mechanics] share an apartment? We don’t know.”
Seems about the same.
I can’t adequately evaluate the claims of String theory and LQG when it comes to the beginning of the universe. Given that there is no evidence for these theories, it’s debatable whether they tell us anything anyway.
There is no more evidence for the ekpyrotic model than there is for string/M theory or LQG and of course the ekpyrotic model is based on M theory. So if one gets a mention so should the other.
What it seems to me has occurred is that there are many models based upon string theory for the early universe and the Ekpyortic is the only one that Vielnkin has analysed and concluded has a beginning. I have not seen him review other models like string gas cosmology, the pre big bang, brane decay models etc . So of course writers sympathetic to WLC and his cohorts like to talks about the ekpyrotic model as if it’s the only one based upon string theory, it isn’t.
Nor do they mention other models such as VSl , CCC , Higgs instability which of course Vielnkin has never tackled.
There is no evidence that the universe can tunnel form nothing, or can exist in imaginary time , but they get a mention.
Every single on of the theorems you mention are exptrapolations of some theoretical framework where experiments have never trodden.
As far I’m aware there are no experiments we can even conceive of, let alone do , that can tell us there was a singularity in our past. However there are experiments that at least have the potential to give us a signal of a bounce. Hence I find the latter scenario more worthy of discussion. But if we are to only discuss things that have already been verified, then better to drop both because neither have been.
Moreover, the BGv theorem was invented in to the context of eternal inflation and many authors have disputed the findings of the theorem including Nomura, Wetterich, Susskind and now it seems Guth himself. Who is right? Again no experiment can tell us as far as I’m aware. So again if we are relying on experimental evidence I see no reason to cite the BGV paper and not Nomura’s etc. If we are looking at this as historians of science surely the fact that Guth no longer thinks the universe is likely to have had a beginning deserves some mention.
To sum up, I suspect that Sean Caroll is right when he says most cosmologist are atheists. I suspect most simply aren’t remotely interested in the god question and not worthy of their time. But the only two that I know of that like to debate theists are Lawrence Krauss and Sean Carroll. They have their own pet models they like to promote. WLc has his champion cosmologists Alev Vilenkin who has consistently argued the universe must have had a beginning and when you talk to many theists they seem to think he is the world’s only cosmologist (“Vilenkin himself says…”).
Meanwhile in the real world of cosmology Carroll and Vielnkin are just two of many respected cosmologists. Others are busy trying to build and test models based upon developed candidate theories of quantum gravity which most commonly predict a bouncing universe. Right now these models don’t have any observational evidence that can confirm they are true , but they have a chance and they seem based on our best ideas of quantum gravity. Neither is the case for singularity theorems.
I was in Cambridge when Neil Turok was promoting the Ekpyrotic model in seminars and the weekly cosmology lunch. I don’t need WLC to teach me cosmology. Of course it isn’t the only one based on string theory. It is an interesting case of a beginning appearing as a surprise in a model that was aiming to be past eternal, even if no longer defended.
Every approach to the beginning of the universe is going to be an extrapolation. There is a difference between extrapolating a tested theory, guessing at the generic properties of a quantum field theory (such as the no boundary condition), and the predictions of theories with no tested predictions (like string theory).
My point is that none of the considerations above are decisive. Hence the cycle of “for” and “against” and proliferation of question marks.
“most cosmologist are atheists. I suspect most simply aren’t remotely interested in the god question and not worthy of their time.” I get the same impression.
“Most commonly predict a bouncing universe”. Meaning that the are most commonly used to search for a bouncing mechanism, or that they predict that most collapsing universes bounce? Are you counting universes in state space or abstracts on arXiv? (Both answers are interesting!)
I’m glad we agree the Ekpyrotic is not the only string based model. So again the question remains: why discuss the one string model that Vilenkin showed was incomplete to the past and pass on the others where that has not been demonstrated? There are many models that are cyclic too that people have proposed in recent years , CCC, VSl and Higgs instability ( Steinhardt’s and Turoks’s latest) that as far I’m aware have not been shown to be past incomplete . But the one cyclic/string base model(Ekpyrotic) that has is the one that gets a mention. How can that not create distorted picture?
Yes it’s true that Gr and quantum field theory are well tested in low curvature regimes, they have never been tested in high curvature regimes; this is the regime we are interested in. Moreover even within these paradigms as I pointed people in inflation community don’t even agree with Vilenkin interpretation of the BGV theorem, and that now includes Guth, again surely that deserves a mention?
String and loop quantum gravity are not tested by experiment. That is why we cannot say the conclusions of the models based on them are correct, they may be totally wrong. But virtually no one trusts the extrapolation of GR and quantum field theory all the way to the singularity. The existence of the singularity is telling you that the theory breaks down. Loop and strings do not necessarily break down at these high curvatures so they might be right.
Again there is a broad consensus in the loop community that in both cosmological singularities and black hole singularities there is a bounce. In string theory this is less of a consensuses but it’s a common prediction there to. This is what I mean by the most common prediction. You don’t see other narratives common to both of the two leading candidate theories. Since around 2000 people have made models of the big bang inspired from quantum gravity theories and the bounce conclusion has been found in all three of these candidates. That’s what I mean by being the most common prediction. I cannot understand how this doesn’t get a mention.
I can understand why Lawrence Krauss doesn’t mention this because he’s pushing his “universe from nothing “idea. An idea which of course he got from Vilenkin. I can understand why WLC doesn’t like to focus on this as it invalidates the BGv theorem. I can understand why Sean Carroll doesn’t want to focus on this either he has his own model based on baby universes. But I don’t understand why someone I hope to be more balanced would not mention this.
The bouncing cosmology is back on the agenda. It may go away again, we shall to see. but what I think is really interesting is that if we can probe the gravity wave spectrum we might be able to put some of these bouncing models to the test. Surely that is something to get excited about and communicate to the public?
The conclusion of the BGV theorem is not under serious dispute (see http://arxiv.org/abs/1305.3836 for example).
Moreover, I believe the BGV theorem also entails geodesic incompleteness of spacetimes containing a “bounce,” about which I have been in discussion with Vilenkin via email correspondence (for a summary of the argument, see http://www.jackspell.com/?p=5).
Reblogged this on robertbyron22 and commented:
An interesting introduction into current debates on the origin of the universe. Just marking for future reference.
Bump … Sorry about the slow reply …
“why discuss the one string model that Vilenkin showed was incomplete to the past?”. Historically important. The post is a historical overview. After discussing the BGV theorem here (http://arxiv.org/pdf/hep-th/0702178v1.pdf), Guth applies it specifically to the Ekpyrotic universe.
“How can that not create distorted picture?” Because it’s not the final word of my post. My final points, you’ll notice, are Against, ??? and ???.
Your point about GR etc is well made. We can’t discuss a beginning without some sort of extrapolation.
“there is a broad consensus in the loop community that in both cosmological singularities and black hole singularities there is a bounce.” Then get a blog and tell us about it. Seriously. Join the discussion. I’m not qualified to evaluate LQG or its claims about cosmologies. If you are, heck, write a post about it and I’ll post it here.
Given that I’m not an LQG expert, I’m wary of propaganda. Various inflation theorists are very one-eyed in their presentation of the merits of the theory (and other cosmologists are equally one-eyed about their criticisms.) Aron Wall states that “I would say that LQG really doesn’t exist yet as a well-defined theory.” (http://www.wall.org/~aron/blog/my-take-on-loop-quantum-gravity). You have piqued my interest, however. I might give this a read: http://arxiv.org/pdf/1406.2790v2.pdf
[…] « Did the Universe Have a Beginning? – Carroll vs Craig Review (Part 1) […]
Hi Luke , just noticed your reply. I agree that its historically important to note the claims that the Ekpyrotic universe was eternal into the past were contradicted by BGV. But what I am saying is that’s important to also note that many models that are not covered by BGv. I have never seen Vilenkin address any model generated by string theory other than the ekpyrotic or any other non string cyclic models such as VSl, CCC, Higgs cyclic etc. By only mentioning the ones that have been addressed a distorted picture emerges, I really hope you or maybe your readers can see that.
Im not an expert in LQG but I can see what is l generating papers on the arxiv and in the journals and what is clear is that LQC has clearly led to a lot of papers, I think a few hundred a year which is a lot for any theory of cosmogony, way more than George Ellis’s emergent universe that you mentioned. I think Ellis used LQG but LQC does not predict an emergent universe as far I’m aware.
I would write to Abhay Ashtekar if you want a guest post, he’s the undisputed leader of the field, very nice chap and generally considered the point man for communicating the field to the public. He has given some talks , interviews and so on.
Ashtekar gave a talk a the joint MIt/Tufts seminar last year and Guth and Vielnkin were both there. Since then Guth has come out saying the universe might be eternal into the past and Vilenkin admitted that his theorem may be violated in quantum gravity. Previously he has said his theorem proved a beginning, he seems to have backed off from that now.
I will try and take a look at your articles when I get the time, it looks very interesting, but the intro to you review has two remarks that grabbed my attention.
Firstly it says there has been a recent proliferation of bouncing models. Yes exactly; we see them in every attempt to apply quantum gravity models to the big bang , I suspect that’s telling us something profound although I might be wrong. This trend deserves a mention I guess is my main point and Im glad its piqued your interest also.
The two atheist cosmologists that have debated Craing don’t mention this trend, maybe because they have their own models to push and books to sell.
Secondly they seems to present bouncing cosmologies as alternatives to inflation. If that’s repeated in the full article ( I will try and read when I get the time) its certainly wrong. Some bouncing cosmologies are alternatives to inflation. For example string gas cosmology, but others such as LQc include inflation. In fact what I find especially interesting is the possibility that they predict inflation naturally without fine tuning, see here for example:
http://arxiv.org/abs/1301.1264
Click to access 1011.4249.pdf
Having got a bit further the article says it focuses on bouncing cosmologies that are alternatives to inflation. but as I said this is only one class of bouncing models, other do include inflation ( see references above)
I Believe In The Bible And The Book Of Genesis That Supports That Our Univ
erse Had A Beginning In Creation Because In My Opinion There Was A Time
That Exists Before The Beginning Of The Universe!
PS The Book Of Genesis 1:1 Proofs That God Had Created Our Universe
In Six Days!
WLC has an interesting extra follow-up response to Sean Carroll’s Quantum Eternity point here:
http://www.reasonablefaith.org/does-quantum-mechanics-indicate-an-eternal-universe
Just wanted to share this with all of you 🙂
Interesting …
[…] Did the Universe Have a Beginning? – Carroll vs Craig Review (Part 1) […]
[…] on the current history of the singularity dilemma check out astrophysicist Luke Barnes blog post here. To summarize the key points, in response to those arguing against the physical existence of a […]
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[…] Luke Barnes reviews 16 different models, theorems and ideas, showing there is plenty of evidence both ways. But in the end, he concludes: […]
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