Does the Big Bang Theory Break Down at the Planck Time?

[Moridin]

I know that Hawking and Penrose proposed that a singularity existed at the beginning of the big bang (and that they apparently changed their minds later). They extrapolated GR back and according to the mathematics, the density and gravitational field becomes infinite. But doesn't quantum mechanics, as currently formulated, say that GR must break down at times less than the Planck time? Does this mean that GR cannot be used to imply that a singularity occurred prior to the Planck time because the theory breaks down?

My question is essentially if the singularity proposed by the earlier mathematics means that it actually was infinitely dense etc. or that these infinites is the models way of saying that it has broken down?

 

[cristo]

The singularity in the theory simply means that the theory breaks down at that point. As you correctly observe, we need a theory of quantum gravity to be able to trace back in time all the way to the big bang.

 

[Moridin]

Thank you for your reply.

 

[Chris Hillman]

Some Misleading and Possibly Offensive Imputations of "Popular Cosmology" (Hab1:11)

My question is essentially if the singularity proposed by the earlier mathematics means that it actually was infinitely dense etc

It was always understood by cosmologists that if you run "time" backwards in FRW models, near the time when the curvature scales approach the Planck length, gtr is believed to "almost certainly" break down with unknown consequences. However, this subtlety never made it into the popsci literature or into the public conciousness.

I know that Hawking and Penrose proposed that a singularity existed at the beginning of the big bang

Actually, it was Friedmann; Gamow and his students later turned the FRW models from "toy models" into models with a sound rationale. Hawking and Penrose came along much later.

(and that they apparently changed their minds later).

At least in the case of Penrose, I suspect you are referring to speculation about "pre-Big Bang cosmology", which is two or three stages beyond the basic Big Bang scenario proposed by Gamow et al. See my recent post in another thread pointing out that the public seems to be seriously confused about the relative stature of various ideas often mentioned in the popsci literature on cosmology. This has been known to stir up strife and contention.

 

[Moridin]

Thank you for your reply and corrections.

At least in the case of Penrose, I suspect you are referring to speculation about "pre-Big Bang cosmology", which is two or three stages beyond the basic Big Bang scenario proposed by Gamow et al.

I was referring to the the fact the GR breaks down and that apparently makes inference of a singularity with that method invalid or did I misunderstand that?

See my recent post in another thread pointing out that the public seems to be seriously confused about the relative stature of various ideas often mentioned in the popsci literature on cosmology. This has been known to stir up strife and contention.

Indeed. Even though I'm a layman when it comes to this subject, I understand that multiverse, string theory, LQG etc. are just mathematical models (if that) without empirical support right now whereas the standard hot big bang cosmology has a lot of supporting evidence and is the currently accepted working model(?)

But did I get it right with the statement that as GR breaks down at the time when the curvature scales approach the Planck length, the inference to a singularity using only GR is invalid (seems like correct per definition)?

 

[Chris Hillman]

Define "invalid".

 

[Moridin]

"cannot successfully be used to imply that"

 

[Wallace]

But did I get it right with the statement that as GR breaks down at the time when the curvature scales approach the Planck length, the inference to a singularity using only GR is invalid (seems like correct per definition)?

A singularity is not a 'thing', it is a region in a theory throws up it's hands and says 'I dunno!'. The sentence 'the inference to a singularity using only GR is invalid' is therefore ill-constructed. What we can say is that as we go to T=0 the the equations of GR give you a singularity, indicating that the equations breakdown and cease to give meaningful prediction.

The inadequacy of GR is embodied by the fact that it gives a singularity, not that is fails to do so!

We know that we need a full quantum gravity theory to resolve this since we are well aware that GR does not work on very small scales where quantum effects become important. We would expect that a working quantum gravity theory would not contain a singularity as we push it further back in time, indeed it is quite probable that we would find that there is no T=0, since the Big Bang does not require this to be the case.

 

[Moridin]

Ah, now I understand. Thanks.

 

[Chris Hillman]

Pedantic caveat and a caution

We know that we need a full quantum gravity theory to resolve this since

Actually, it is not clear that a quantum theory of gravity will automatically exorcise all curvature singularities or even that such a thing would shed much light on them. This should probably regarded as a pious hope, at least until something appears which everyone agrees can be understood as a viable quantum theory of gravitation (such an assertion has been made about superstring theory, but this is a highly controversial and extreme point of view).

Morodin, I hope you recognize that just because theorists expect gtr to break down at very high energy-densities/spacetime-curvatures, this does not mean that gtr is nonsense through and through. In particular, theorists do not expect gtr to break down in the vicinity of the event horizon of astrophysical black holes. It might help to think of the example of Newtonian gravitation. We know from experiment that this theory breaks down for fast motion or modestly non-weak gravitational fields, but it is simpler than gtr so we use it when we can. Gtr has not yet been shown to break down experimentally, although everyone accepts that Nature adores a quantum and on this basis we can have confidence that gtr should break down at sufficiently high energy densities. But a quantum theory of gravitation will almost certainly be more complex and more difficult to work with than gtr, so again we will continue to use gtr in regimes where Newtonian gravitation becomes inaccurate but gtr remains accurate. It turns out that this intermediate regime covers a huge swath of modern astrophysics/cosmology (with the rest mostly covered adequately by Newtonian gravitation).

 

[Chronos]

If a balloon pops, do the receding edges of the puncture create the impression of an initial 'singularity'?

 

[Moridin]

Morodin, I hope you recognize that just because theorists expect gtr to break down at very high energy-densities/spacetime-curvatures, this does not mean that gtr is nonsense through and through.

Of course it doesn't. GR has a massive amount of evidence in its favors and agrees with observations very nicely, from gravitational lenses, GPS discrepancies, discrepancies in atomic clocks in motion around the Earth at different directions, muon-decay in the atmosphere, gravitational redshift, Shapiro effect, orbit of mercury and all sorts of other very interesting things. Any future theory that tries to explain more must naturally make the same predictions in the area where both applies, as TR does with Newtonian mechanics where Newtonian mechanics is a good approximation. I've been told that if you, say, drop a rubber ball on a table the difference in the predictions between NM and GR isn't even measurable.

 

[Chris Hillman]

OK, I retroactively declare that my caveat was addressed to hypothetical lurkers