The BBC is running a story entitled Hints of ‘time before Big Bang’ based on Sean Carroll’s latest efforts to promote the multiverse. The writer attended Sean’s talk at the recent AAS meeting and presumably also read Sean’s new Scientific American article, and here’s what he got out of them:
A team of physicists has claimed that our view of the early Universe may contain the signature of a time before the Big Bang…
Their model may help explain why we experience time moving in a straight line from yesterday into tomorrow…
Their model suggests that new universes could be created spontaneously from apparently empty space. From inside the parent universe, the event would be surprisingly unspectacular.
Describing the team’s work at a meeting of the American Astronomical Society (AAS) in St Louis, Missouri, co-author Professor Sean Carroll explained that “a universe could form inside this room and we’d never know”.
The inspiration for their theory isn’t just an explanation for the Big Bang our Universe experienced 13.7 billion years ago, but lies in an attempt to explain one of the largest mysteries in physics – why time seems to move in one direction…
“Every time you break an egg or spill a glass of water you’re learning about the Big Bang,” Professor Carroll explained…
If the Caltech team’s work is correct, we may already have the first information about what came before our own Universe.
Besides the “Does Time Run Backwards in Other Universes?” material from his paper with Jennifer Chen discussed in Scientific American, what’s new here is his recent paper with two Caltech collaborators about the possibility of explaining an asymmetry of marginal statistical significance observed in the CMB by invoking a more complicated version of inflation, adding a “curvaton” field to the usual inflaton. In their model, this asymmetry comes from a perturbation to the curvaton field of size larger than the horizon. Such a thing could in principle make testable predictions, but doesn’t necessarily come from the existence of a multiverse or tell us anything about it. The authors throw in one clause of a sentence about how it might occur as
a remnant of the pre-inflationary epoch or as a signature of superhorizon curvaton-web structures.
and that’s the basis of the BBC article. I have no idea what’s going on with the business about universes forming inside of rooms and us not knowing anything about this.
Sean gives more details about this in a new blog posting.
Update: The author of the piece, Chris Lintott, has a blog, and a posting about the article, where he writes:
What made me want to write the story in the first place, though, was exactly what Sean said above – to an outsider to the field the idea that it is even imaginable that we might be able to make concrete predictions from ideas about multiverses which have haunted the pages of New Scientist and its ilk for decades is stunning. That’s what I wanted to get across.
He doesn’t seem to realize that there’s nothing here different than the things he’s thinking of that “have haunted the pages of New Scientist and its ilk for decades.”
Update: This story is getting the full media treatment, including haunting the pages of New Scientist, which has the sense to strip out the nonsense and hype about the multiverse and the arrow of time. Slashdot emphasizes the part about:
Describing the team’s work at a meeting of the American Astronomical Society (AAS) in St Louis, Missouri, co-author Professor Sean Carroll explained that ‘a universe could form inside this room and we’d never know.'”
AdS/CFT gives a very non-trivial calculational method for dealing with the strong coupling behavior of certain gauge theories, and as such is a success. Whether it can be turned into a reliable method for dealing with the physical case of QCD remains to be seen.
In this case the problem is not finding the right theory that describes the real world, we have strong evidence for QCD. But this still leaves the problem of actually calculating things, and that’s also part of physics.
Thanks for the reply.
To my knowledge, the AdS/CFT model applied to QCD goes by the name of holographic QCD. It is claimed to be a computational tool that is superior to Chiral Perturbation Theory when it comes to the strong coupling regime. But, again to my knowledge, holographic QCD has not been proven to yield predictions that are consistent with observations.
Is my understanding correct?
I’d like to try to answer your question concerning holographic ideas in QCD. I hope you can follow my attempt to give a basic explanation. Perhaps Peter won’t mind that this doesn’t have anything to do with the big bang (it seems that this comment thread is already very tangled, so I don’t think I am messing it up too much).
AdS/QCD or whatever one calls it is essentially a bare-strong-coupling scheme. There is another such scheme, devised a long time ago, which is to do the strong-coupling expansion (NOT Monte-Carlo/numerical methods) on the lattice. Both of these have yielded some insights into quark confinement and chiral-symmetry breaking, and led to interesting lines of inquiry, but both have a serious problem.
In such schemes, there is a dimensionless parameter, called the bare coupling. This is not the same as the physical or effective coupling. To understand how real QCD works, this bare coupling must be taken to zero as an ultraviolet cut-off is removed. Unfortunately, nobody can do this yet.
Here is a semi-technical explanation, which you may want to skip. The reason the bare coupling must be taken to zero has to do with how QCD renormalization works. If we want to keep experimental parameters (like cross sections or masses) fixed as we remove the cut-off, the bare coupling “runs”, i.e. becomes a function of the cut-off. It so happens that the bare coupling runs toward zero as the cut-off (in momentum units) diverges. SIDE REMARK: You may already know that the renormalized/effective coupling also runs, as we look at different energy scales (not cut-offs), and it runs in the same way as the bare coupling.
No calculation in the holographic approach to QCD can be taken seriously, unless someone figures out how to deal with arbitrarily small bare coupling (which means large curvature). Real QCD means infinitesimal bare coupling. My impression is that few string theorists (perhaps none) are working on this problem, because it is extremely hard.
In the light of the above, I think AdS/QCD and the older strong-coupling lattice approach should be viewed only as imperfect models of the strong interaction. This does not mean that they are unworthy of study. They are not, however, real QCD.
To Peter Orland,
Thanks for your reply and explanation.
I am truly curious to see some real debates on the physics related to the direction of time paradox, so pardon me for interrupting your lively discussions about String (an issue that I consider moot).
The central argument in this paradox concerns the entropy of the pre-inflation universe. It is commonly stated (by Penrose, for example) that it has to be even lower than just after inflation ends. The reasoning that leads to this conclusion involves the statement that the pre-inflation universe contains the same number of quantum states as the current universe, because “according the rules of quantum mechanics, the total number of microstates in a system never changes” (a quote from the SciAm article). But this is very suspect. Quantum mechanics is known to be incompatible with gravity (or, equivalently, dynamic space-time). Trying to trace all microstates backward through inflation is plain nuts. Furthermore, the current universe has undergone 14 billion years worth of quantum de-coherence. Maybe someone has a way of estimating the effect of all the quantum de-coherence on the microstate count, and I sure would love to read it, but I doubt the answer can be as simple as “no effect” because quantum de-coherence is in essence the APPARENT decoupling of entangled states when the particle in question interacts with cold, localized, massive subsystems (yes, like those objects that formed AFTER the inflation). The wave function is truncated in an (albeit extremely accurate) approximation. The new EFFECTIVE quantum description of the system remains valid for all practical purpose, but the number of microstates does not seem to be conserved, or maybe someone can enlighten me otherwise.
Oh, by the way, I am aware of the fact that the universe always has had exactly ONE state. Talking about entropy, however, requires the invocation of ensembles (or something similar, although the multiverse concept may be a step too far). Counting the microstates, therefore, involves solving the effective quantum Hamitonian after de-coherence. Now I really do not see how the count can be possibly conserved.
Even if the microstate count in the pre-inflation universe is indeed high enough, we still cannot safely jump to the conclusion that “among all the different ways the microstates of the universe can arrange themselves, only an incredibly tiny fraction correspond to a smooth configuration of ultradense dark energy packed into a tiny volume” (again, a quote from SciAm). That statement is true if one assumes that there is no interactions among the incredibly dense collection of ultra-heavy particles packed into a universe the size of a penny, but that is such a dumb assumption that I do not presume anyone can seriously make it. In fact, smoothness seem almost inevitable with any meaningful collisions going on, so if anyone understands how that SciAm quote was arrived at, please enlighten me by all means.
Dear M. Wang,
Extrapolating thermodynamic and quantum concepts all the way to the Universe scale is highly speculative and should not be taken seriously. Likewise, assuming that both statistical and quantum physics continue to be valid in pre-inflationary Universe lacks any foundation.
The meaning of the word “speculation” appears to have changed in recent times.