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August 27, 2004

News release on emergent string spacetime

Posted by Urs Schreiber

My friend Rüdiger Vaas, who is working as a science journalist for the german science magazin Bild der Wissenschaft, where he is responsible for coverage of gr-qc and hep-ph, hep-th related things, informs me that his new article

Rüdiger Vaas: Time before Time (2004)

on notions of the term ‘universe’ and beginning and end of time in science and philosophy pays tribute to some of the hard lobbying work that Robert Helling and I did a while ago here on the SCT in order to promote awareness in the lay public of the fact that spacetime is an emergent phenomenon in string theory.

I know, at least in the US press string theory tends to be rather over- than under-hyped, but since the interested German reader of popular science magazines is currently bound to believe that it is established that ‘the universe is a spin-network’, I find the small reference to Robert Helling’s exposition ‘D-geometry’ on p.14 of Rüdiger’s article already worth mentioning.

Apparently the semi-popular article discussing the amazing fact that string theory identifies spacetime and its generalizations with 2-dimensional superconformal field theories of central charge c=15 is still lacking. Its dramatic implication for the general world view is a maybe a little less accesible than the claim that ‘the universe is a spin network’ (which, remarkably, is wrong even in loop quantum gravity).

Posted at August 27, 2004 7:08 PM UTC

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Re: News release on emergent string spacetime

Dear Urs and SCT readers,

thank you for mentioning my paper:
Vaas, R. (2004): Time before Time. Classifications of universes in contemporary cosmology, and how to avoid the antinomy of the beginning and eternity of the world,

It deals with a problem which, perhaps not surprisingly, also appears in the framework of string cosmology now: The question whether the (or our) universe has a beginning or not.

Perhaps all the matter-energy we can observe today has a beginning (namely in the big bang), so let’s put the question different: Is spacetime past-eternal or not?

This of course is a (well, almost) eternal question going back at least to the pre-Socratics, Asian thinking etc. It was discussed in the Middle Ages with sophisticated arguments. And it reappeared in modern relativistic cosmology with the big bang/steady state controversy.

If string theory (or M theory) is a “final theory” or a “theory of everything” in some relevant way one might hope that it has an answer also in respect of this controversy.

What I find very interesting is, that - at the moment - string cosmology offers all the well-known alternatives again. Eternal recurrence of limited concepts and possibilities, I’d like to say. Though it is on a much more sophisticated or complicated level, string cosmology repeats the problem. That might be philosophically disappointing or challenging, dependent on personal taste, but it is also promising, because if string theory could not even address these questions something elementary would be truly missing.

So perhaps it might be useful to have a look a the alternatives:


String cosmology allows, at it seems, at least perfectly what I call eternal cosmologies. There are different kinds of them, both in ancient and in modern cosmology:
- static ones (without irreversible changes on a coarse-grained level),
- evolutionary ones (with cumulative change),
- and revolutionary ones (with sharp phase-transitions).
And they could have either a linear or a cyclic time.

– Static ones, I think, are of no relevance anymore.

– Evolutionary eternal cosmologies would be, e.g., eternal, but expanding branes.

If our universe would be such a brane, how could string theory explain its existence? Is it just an assumption, without further justification, or could we imagine a string theory inspired explanation?
But what about the big bang? We might call the big bang a true phase-transition, thus:

– Revolutionary eternal cosmologies entail the big bang as an event WITHIN the history of spacetime, not as the beginning of it. Thus, there would have been a time before time.

A very fascinating model is, of course, the Cyclic Universe by Steinhardt and Turok et al.
See, e.g., the following two recent references:
Turok, N., Steinhardt, P. J. 2004: Beyond Inflation: A Cyclic Universe Scenario,
Turok, N., Steinhardt, P. J. 2004: The Cyclic Model Simplified,

Here the branes are eternal (and infinite), too. But they are colliding repeatedly, which makes new big bangs. It is an open question, whether this oscillation is truly past-eternal or not. See the references above and, e.g.,
Albrecht, A. 2004: Cosmic Inflation and the Arrow of Time. In: Barrow, J. D., Davies, P. C. W., Harper, C. L. (eds.): Science and Ultimate Reality. Cambridge: Cambridge University Press,

Even if the cycles are not past-eternal the demand for an explanation of the past-eternal branes remains.

So the Cyclic Universe has a severe limit. On the other hand it provides a very important alternative to inflation and makes predictions which can be checked in the near-future. It is very healthy to have those scenario now.
For a very critical view see, e.g.,
Linde, A. 2003: Inflationary theory versus ekpyrotic/cyclic scenario. In: Gibbons, G. W., Shellard, E. P. S., Rankin, S. J. (eds.): The Future of Theoretical Physics and Cosmology. Cambridge: Cambridge University Press, 801-838,

Another option might be intersecting brane-worlds (a string proposal to explain inflation), but I do not know enough about these scenarios yet.
Are those branes also assumed to be eternal?
Is the big bang (or only inflation) a consequence of brane intersection?

Also very interesting are the bubble nucleation and colliding bubble braneworld universes.
See, e.g.,
Bucher, M. 2001: A Braneworld Universe From Colliding Bubbles,
Blanco-Pillado, J. J., Bucher, M. 2003: The Global Structure of the Colliding Bubble Braneworld Universe. Astrophys. Space Sci. 283, 525-530,
Blanco-Pillado, J. J., Bucher, M., Ghassemi, S., Glanois, F. 2004: When do colliding bubbles produce an expanding universe? Phys. Rev. D69, 103515,

As I understand it, the big bang is explained here also as a phase transition, but either the branes are eternal, too, or they do nucleate from (or out of) something which is eternal, e.g. bubbles filled with AdS5 nucleate from ds5 or M5 through quantum tunneling.


String cosmologies, on the other hand, allows also for initial cosmologies with an absolute beginning of spacetime. This is the instanton approach by Hawking et al.
Hawking, S. W., Hertog, T., Reall, H. S. 2000: Brane New World. Phys. Rev. D62, 043501,
Hawking, S. W. 2001: The Universe in a Nutshell. New York: Bantam Books, ch. 7

But I am not sure how well-developed this is.
Are there more detailed studies?
Are there empirical predictions, e.g. regarding CMB patterns?

If there is a cosmological instanton solution within string theory?
And how does (our kind of) time emerge out of the Euclidean regime?
Might it be viewed as a decompactification of a formerly rolled-up time dimension? (Something which would at least provide a kind of mechanism which, as I understand it, the Hawking-Hartle and Hawking-Turok models do not offer.)

Another question is whether the above mentioned bubble nucleations could be also viewed as tunneling “out of nothing”, not something.


There is a third option which I called pseudo-beginning. It is a kind of compromise between eternal and initial cosmologies.

The first kind is a scenario where the arrow of time starts with the big bang out of a much simpler state without such an general arrow (see my micro/macro time distinction in my paper for details).

Rebhan, E. 2000: “Soft bang” instead of “big bang”: model of an inflationary universe without singularities and with eternal physical past time. Astron. Astrophys. 353, 1-9,
Eckard Rebhan (p. 9) wrote: “In physics time is a parameter that is used for ordering changes of states. However, when there are no changes then this order parameter loses its sense. In a very slowly changing situation it may therefore become more useful to consider the changes themselves as the order parameter that represents time instead of using an order parameter ordering no changes.”

One might interpret the pre-big bang model of Veneziano and Gasperini as a string version of such an approach, see., e.g.,
Gasperini, M., Veneziano, G. 2003: The Pre-Big Bang Scenario in String Cosmology. Phys. Reports 373, 1-212,
Veneziano, G. 1999: Challenging the Big Bang: a longer history of time. CERN Courier 39 (2), 18-20.

Here the string vacuum – where a local collapse in the Einstein frame (which corresponds to a dilaton-driven inflation in the string frame) before the big bang occurs – is quite simple, homogeneous, almost empty and does not have an overall arrow of time. However, mathematically, the origin of the pre-big bang – or, to be more precise, any pre-big bang, for the model does also imply a multiverse scenario – traces back to a maximally simple, static condition only in the infinite past (principle of asymptotic past triviality). But this can also be interpreted just as a local predecessor of a big bang and not a feature characterizing the infinite string vacuum as a whole. “The universe becomes more and more cold, empty, flat and non-interacting as time becomes more and more negative, until it reaches complete emptiness and triviality in its asymptotic past. The universe would thus obey a principle of Asymptotic Past Triviality, emerging from the simplest possible kind of initial states” (Veneziano, p. 19). “It seems indeed physically (and philosophically) satisfactory to identify the beginning with simplicity. However, simplicity should not be confused with complete triviality: a rigorously empty and flat Universe, besides being uninteresting, is also very special, i.e. non-generic. By contrast, asymptotically trivial Universes, though initially simple, are also generic in a precise mathematical sense” (Gasperini and Veneziano 2003, online p. 53).

In a sense, this principle of Asymptotic Past Triviality is simpler than assuming the existence of past-eternal branes. You get more out of less. But, of course, this is a philosophical argument which is not sufficient. More important, at least at the moment, is to test this scenario rigorously both from theoretical and empirical points of view. And there is some criticism, e.g.
Kaloper, N., Linde, A., Bousso, R. 1999: Pre-Big-Bang Requires the Universe to be Exponentially Large From the Very Beginning. Phys. Rev. D59, 043508,

Perhaps intersecting brane-worlds might also offer pseudo-beginnings IF the branes are in a kind of string vacuum state without an arrow of time. Thus, macrotime (with an arrow of time) might be generated locally due to brane intersections.

Another and more fundamental kind of pseudo-beginning is to treat spacetime not as primary, unreducible as we have discussed this already here at the SCT, see:

So it might by that string theory could build up spacetime from scratch, e.g. out of strings.

Some interesting thoughts can are published here:
Helling, R. 2000: D-Geometry,
Nishimura, J. 2003: Lattice Superstring and Noncommutative Geometry,

I would be eager to see more approaches like this for the background-dependence of most string approaches cannot be the final word.

Another issue that might be relevant is whether strings (or D-branes etc.) could be really elementary. As I heard there are even more elementary building blocks in the discussion now, i.e. string bits, and I would like to know the opinion of SCT readers about this.

IF there is, ultimately, some spacetime dust (build out of stringy elements), spacetime would be secondary and derived. Even if so, however, it still remains to be seen what this would exactly mean regarding the notion of past-eternity.

I would be grateful for comments and suggestions regarding this issues.

Best wishes,

Posted by: Rüdiger Vaas on September 19, 2004 8:55 PM | Permalink | Reply to this

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