Did the Universe Begin? II: Singularity Theorems

Our best theory of gravity is classical General Relativity.  "Classical" is physics-speak for not taking into account quantum mechanics.  So we know that classical GR has to break down during the Planck era, if not later.

Classical Big Bang cosmology predicts that there is an initial singularity at the first moment of time.  In fact, there are some theorems to that effect.  These are the Penrose and Hawking singularity theorems, which will be the subject of this post.

In GR, attractive gravity is caused by the energy or pressure of matter.  Tension (which is negative pressure) produces antigravity (repulsion rather than attraction).

Very crudely speaking, the singularity theorems say that if you assume that matter obeys some energy condition restricting the amount of energy and/or pressure, then you can deduce that under certain conditions there has to be a place where your spacetime has an edge and cannot be extended any further.  This we call a singularity.  Typically, some component of the curvature becomes infinite at the singularity.

There are several different singularity theorems, pioneered by Hawking and Penrose.  One of them says that singularity theorem says that all expanding cosmologies like our own have to begin with a singularity.  Roughly speaking, it says that if there is only gravity and no antigravity, then tracing the universe backwards in time there is no way to stop it from crunching down to zero size.   Hence there must exist an initial singularity (at least somewhere, perhaps everywhere).

However, this Hawking-Penrose theorem uses something called strong energy condition, which says that the repulsive antigravity from tension is not allowed to be greater than the gravity from energy.  It turns out that the strong energy condition can be violated by lots of different types of otherwise reasonable physics theories.  In particular, it was violated during inflation, and it is violated by the cosmological constant today.  So no one really takes this theorem very seriously anymore.

There is another singularity theorem (proven originally by Penrose) which is better, because it only uses the null energy condition, which says that nearby lightrays are always focused by gravity.  This turns out to be a much weaker condition, which is satisfied by most respectable classical matter theories (although it is violated quantum mechanically).  However the Penrose singularity theorem only says that there has to be a singularity if space at one time is infinite.

If space at one time is finite in size (for example, if it is shaped like a 3-sphere) then there might be a "bounce" where the universe contracts to a small size and then starts expanding again.  The de Sitter cosmology is an example of this, although there are also examples of finite cosmologies that begin with singularities.  We don't really know whether space is finite or infinite, since inflation stretched it out so much that even if it were a giant sphere, the radius is so large that it seems flat today.

A few years ago I wrote an article in which I argued that the conclusions of the Penrose singularity theorem should continue to hold in quantum gravitational situations.  Even though the null energy condition can be violated by quantum fields, it turns out that you can get the same conclusions if you instead assume something called the "Generalized Second Law" (GSL), which says that the Second Law of thermodynamics applies to black holes and similar types of horizons.

(I described the application of this result to time travel in a  recent Scientific American blog post.  Technically, you have to use the time-reverse of the GSL, which I mentioned in the comments here, but if the GSL is true, its time-reverse should also be.  This may seem weird because normally we think of the Second Law as something which only works in one time direction, but I promise you that one can make sense of it.)

The advantage of using the GSL is that it makes it more plausible that the conclusions of the Penrose singularity theorem apply even in fully quantum-gravitational situations, e.g. during the Planck era.  In my article, I showed that the results apply "semiclassically", meaning when the quantum corrections to spacetime are small but still taken into account.   I then argued (and not everyone would find this part of my article persuasive) that under certain assumptions one might expect the result to hold even in full quantum gravity, when these quantum fluctuations are large.  But remember, all statements about quantum gravity are speculative.

I am a little reluctant to even bring up my own work, since personally I think it is more persuasive that clearly established (but incomplete) physics predicts a beginning, than that speculative new physics says this.  I think of it more as laying the groundwork for a possible future understanding, then a totally conclusive result.  Still, I think that the Penrose theorem is connected to enough other deep principles of physics that something like it will probably be true and important in the final theory of physics.  Other physicists think that singularities are so disturbing that any "complete" theory of physics should eliminate them.

Funny story.  One time I was arguing with an atheist grad student about God and the question of the universe's beginning came up.  I mentioned my own work (and I am quite sure that I put in some caveats about the potential limitations, since I always do this).  A few weeks later I found him posting on some atheist website cocky statements along the lines of "Theists believe that the universe had a beginning because of the GSL, but this is silly for the following reasons...".  And this at a time when practically no one else had even heard of my work!  So just in case it isn't clear: many smart people believed in God before I came along, and the case for Theism is hardly dependent on my tiny contributions to physics!

In conclusion, to the extent that the singularity theorems are relevant, they tend to point to a Beginning, although it might be possible to evade this conclusion either by (a) having space be finite, or else (b) through quantum gravity effects, if my speculative arguments for a quantum singularity theorem are wrong.

About Aron Wall

I am a postdoctoral researcher studying quantum gravity and black hole thermodynamics at the Institute for Advanced Study in Princeton. Before that, I read Great Books at St. John's College (Santa Fe), got my physics Ph.D. from U Maryland, and did my first postdoc at UC Santa Barbara.
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4 Responses to Did the Universe Begin? II: Singularity Theorems

  1. Roy Carvalho says:

    Dear Aron: Singularities and Infinities are the heart of the Big Bang cosmology. These are not only mathematical conundrums, they are unphysical. Planck in 1900 solved the ultraviolet catastrophe by finding the Quantum of Least Action, the smallest physical quantity. Let us set h=1 and use integers to build physical objects. Look at the Kretschman Invariant. It has radius in the denominator. Set r=0 and there's your singularity. But r=0 is arbitrary and unphysical. Get rid of r and replace it with something physical, n-sub-h, a large finite number of quantum particles. This is an object, a Supermassive Black Hole, made of a large finite number of quantum particles. Neutrinos, by Pauli Exclusion and Heisenberg Uncertainty, will assume an unbreakable degenerate state. There are more reasons why neutrinos can be considered the ultimate fundamental particle and I would be happy to discuss this with you.

  2. Aron Wall says:

    Dear Roy,

    Thanks for your comment. With respect to singularities, you're in good company; there are a lot of physicists who agree with you that singularities must be unphysical. But I think this a somewhat of a prejudice and we shouldn't just assume it without proof. Sure, there are other examples in physics where singularities in one theory get resolved by a better theory. But in the case of e.g. the ultraviolet catastrophe, the infinity would have spoiled the experimentally measurable atomic theory. I don't think the same can be said for singularities inside of black holes.

    (One should perhaps distinguish between a singularity as (a) infinite curvature or (b) an edge of spacetime. In a theory of quantum gravity one might perhaps have (b) without (a), especially if spacetime is actually discrete.)

    There is a famous calculation due to Chandrasekhar which calculates whether a star can be held up by the degeneracy pressure due to the Pauli Exclusion principle. This calculation does not support the idea that neutrino degeneracy pressure prevents the singularity. Anyway, in the Standard Model of particle physics, the neutrino is no more or less fundamental than the electrons or quarks.

    I'm guessing you don't actually have a well-defined mathematical model for the behavior of these "large finite number of quantum particles". I was just arguing in another comment that not everything in life can be mathematized, but I think that if you are trying to do physics, then at the end of the day you need to have some type of mathematical model, not just a word-picture. It's unlikely---forgive me for being blunt---that a person will be able to construct this model unless they have solid grounding in the best current models of physics.

  3. sp says:

    It seems to me that there are two appraoches going on here. One is to try and make singularity theoroems more general. The other is to try apply what we think we know about quanutm gravity candidate theories to the big bang and see what happens. What I think is often an exciting devleopment in theorietical physics is when two very different appraoches come to the same answer. However it seems to me these different appraoched are not converging as we see many claims that singualrities are resolved in these quantum gravity candidate inspired models. Do you agree?

  4. Aron Wall says:


    There are certainly several such claims, but I personally don't take them as seriously as I take the GSL.

    Take for example loop quantum cosmology. In my own view, loop quantum gravity is not a mature enough subject to be able to start talking about cosmology yet. Basically loop quantum cosmology is a descendent of "minisuperspace" models, where you only try to quantize finitely many degrees of freedom of the universe (e.g. just the overall scale factor as a function of time). The original quantum-GR minisuperspace models indicated there was still a beginning, but apparently if you quantize in a sort of LQG-inspired way you can construct bouncing cosmologies. Anyway, there are severe problems trying to even define the dynamics of loop quantum gravity, and I doubt that LQC has much relevance to what the fully quantized theory (the one with local degrees of freedom) would say.

    String theorists do talk about "singularity resolution", meaning that certain spacetimes which are singular from the perspective of point particles turn out to be OK from the perspective of propagating strings. But there are other types of singularities which are not known to be resolved in string theory. For example, I don't think there is any widely-accepted string model which resolves the singularity inside of a black hole. To the best of my knowledge (although I am not really an expert in this area) the type of singularities which string theory resolves are not the same type as those that the Penrose singularity theorem would predict.

    (I know that some people explore string theory models of pre-Big-Bang spacetimes,e.g. the "string gas csomology" of Brandenberger and Vafa. I don't understand these models in any detail, but I know that Brandenberger-Vafa take space to be finite, so that doesn't contradict the conclusions of the Penrose theorem. In any case, this is just one possible scenario, not a statement about what always happens in string theory.)

    So my tentative expectation is that my results are compatible with string theory. I am fortified in this conclusion by the fact that you can also use the GSL to predict several other types of constraints on valid spacetimes (e.g. no traversable wormholes or warp drives). Interestingly, it turns out that these conditions on the spacetime are also needed for the consistency of AdS/CFT (a version of the holographic principle) which is a pretty well-explored aspect of string theory these days. So I think everything fits into a consistent picture, although I repeat once more that all statements about Planck era quantum cosmology are speculative.

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