Conifold [5] | n. |
A space that is smooth apart from a finite number of isolated crepant singularities—which can be smoothed without changing the canonical (1st Chern) class. |

adj. |
relating to, or involving conifolds. | |

[D. M.]The Web |
The web of all superstring compactifications (on Calabi-Yau 3-folds). |

Conifolds and The Web of Worlds

are now >21 years old [1,2,3,5]!

A little history

In 1986/87, in my final Ph.D. graduate student year at University of Maryland at College Park, I was working on Ref. [–3] and with Paul S. Green from the Math Department of University of Maryland at College Park on Ref. [–2], and—unbeknownst to each other’s efforts—Philip Candelas *et al*. were working on Ref. [–1]. We have independently noticed that constructions of Calabi-Yau 3-folds can be turned one into another, and started observing the regularities—some of this made it into Section III of Ref. [–1]. Philip and I met in the summer of 1987 at the ICTP in Trieste, where he showed me the voluminous computer listing, realizing that we were working on the same topic. In the Fall 1987, I joined Philip’s research group, within Steven Weinberg’s *Theory Group* at the University of Texas at Austin, but also continued collaboration with Paul S. Green.

In 1987, Paul noticed Miles Reid’s article [0], describing a transition consisting of replacing a (homology class of) 2-sphere(s) with a (homology class of) 3-sphere(s) or *vice versa*, and his two-fold conjecture:

(**1**) that all Calabi-Yau 3-folds are so connected, and

(**2**) that this transition is never obstructed, thus giving rise to an indefinite connected web of Calabi-Yau 3-folds.

Paul and I then proved [1,2] Reid’s conjecture within almost all the then known constructions:

- all
**7868**Calabi-Yau complete intersections of hypersurfaces in products of projective spaces [–3,–2,–1], and - all (
*number unknown*) Calabi-Yau*double solids*

are all connected one to another through Reid’s transition. The same is then true of all compactifications of higher-dimensional theories, including but not limited to superstrings. In turn, Reid’s conjecture is correspondingly applicable to all such compactifications, and would imply that there are indefinitely many topological types of superstring theories compactified to effectively 3+1-dimensional spacetime.

The

double solidsare suitably branched covers of almost Fano 3-folds [1]. Whereas Fano 3-folds have been classified by Mori and Mukai [Man. Math.36(1981) 147], and come in 87 topological types,almostFano 3-folds (with a semi-positive canonical class, such that the anticanonical sheaf has at least one global section) have not been classified, nor is it known if there are any Calabi-Yau double solids from Ref. [1] that are not in the list of 7868 Calabi-Yau complete intersections in products of projective spaces.

Much of 1987/88 was devoted to studying the implications of this connectedness to the geometry of the moduli space. We proved that the conifolds are at finite distance from smooth models [3], in the Weil-Petersson metric that turns out to be equal to Zamolodchikov’s worldsheet field theoretic one [4]. This all becomes part-and-parcel of the special geometry (á la Bryant-Griffiths) description of the space of complex structures of Calabi-Yau 3-folds [5].

The term ** conifold** was invented for the purposes of Ref. [5]: “we shall adopt the term ‘conifold’ because it is shorter than ‘nodal variety’ and more euphonious than ‘nodifold’.” For the record: Philip suggested the term to me during the work on the article, and Paul coined the cited explanation of the term.

Whereas we have initially had in mind only

nodes(A_{1}singularities, in Arnold’s classification), it soon became obvious that the generalization tocrepant singularitiesis both natural and necessary. (Paraphrasing Miles Reid: When a singularity is smoothed, it is replaced by local surgery with an ‘exceptional set’ and this most likely contributes to thecanonical class. If it does, there is adiscrepancybetween the canonical classes before and after smoothing; those singularities the smoothing of which incurs no discrepancy arecrepant. Got it? As it has no discrepancy, it is not discrepant, and so iscrepant. I guess,crepantsounds better than ‘undiscrepant’.)

The works [1,2,3,5] all describe the simplest case of Reid’s transition (a.k.a., the * conifold, extremal, geometric transition*, sometimes understood with some distinctions) whereby the topology type of the Calabi-Yau 3-folds changes—at the conifold,

I devised this diagram for the purposes of Ref. [5], where it was rendered in the limited LaTeX picture environment; this image is from Ref. [6]. Earlier, Ref. [–1] visualized the transition by presenting what looks very much like the 2-dimensional analogue of the transition—and which illustrations in fact do describe the analogous transition whereby a Riemann surface reduces genus by one, owing to a local surgery that replaces a circle (*i.e.*, a 1-sphere) by a singular node and then by a double point—which in fact is a 0-sphere. While this real 2-dimensional analogy captures some of the properties of the complex 3-dimensional transition, it fails to be faithful in other aspects. In particular, the real 2-dimensional “S^{1}•S^{0}” transition changes π_{1} and for genus-0 also π_{0}, and it also changes the canonical class. The complex 3-dimensional, “S^{3}•S^{2}” transition changes neither of those—and this is faithfully depicted in the above, “tents” diagram. In fact, this depiction of the transition permits a much more detailed analysis; see Chapter D of Ref. [6].

Reid’s “S^{3}•S^{2}” transition necessarily changes the Euler number of the Calabi-Yau 3-folds involved in the transition. This, in turn, equals Witten’s supersymmetry index for the supersymmetric worldsheet theory of the superstrings compactified on the Calabi-Yau 3-fold. Refs. [1,2,5] duly cautioned that these are phase transitions and so inherently non-perturbative, and Refs. [7,8] demonstrate the inadequacy of the known worldsheet conformal field theory approach. The lack of understanding of the physical mechanism that would make Witten’s supersymmetry index change prevented general acceptance of these results at the time.

Attitudes started changing only after Brian Greene, David Morrison and Andrew Strominger showed—in the more restrictive context of Type II theories (and where Witten’s supersymmetry index is a continuous characteristic function, interpolating between the Euler number and the Arithmetic genus, and so is not a definite number, much less an integer)—that the topology change occurs via certain massive black hole-like states turning massless and so providing the requisite additional degrees of freedom corresponding to “vanishing cycles” [9]. (This result was published back-to-back with Strominger’s brand-new analysis of how the black hole-like states turning massless on conifolds correct the conformal field theory analysis [10].) In the heterotic string case, the transition remains not fully understood except for the suggestion that it must involve “nontrivial interacting fixed points’ [11], and even used to study associated Type II string theory transitions in 3-dimensional spacetime [12]. By 2002, Maximilian Kreuzer and Harald Skarke provided a computer-generation of 473,800,776 toric constructions of Calabi-Yau 3-folds, which contain 30,108 distinct Hodge diamonds, but presumably at least an order of magnitude more topological types of Calabi-Yau 3-folds; all of these are connected [13] in a sense defined by Kreuzer and Skarke; this may well equal Reid’s transition, but is far from obvious. Thus, proving—*rigorously*—that Reid’s “deformation^{}•small resolution^{}” transition indeed suffices to connect all these Calabi-Yau 3-folds remains an open problem: Singularities are also classified (by Arnold and Gusein-Zade) in terms of their severity and number of parameters they admit. Clearly, the simplest are isolated and their small resolution requires a string of S^{2}’s instead of a single S^{2}; Reid’s basic idea, nevertheless, is the same. Interestingly, conifold transitions can also be used to construct still new Calabi-Yau 3-folds, both simply-connected [14], and multiply-connected [15].

** To the best of my knowledge**, there do exist

(

(

Note the curious 7-year rhythm: 1988: [1,2]—1995: [9,10]—2002: [13]… and 2009: **???**

Well, late 2009 and early 2011 seem to have brought finite quotient conical (“hyperconifold”) singularities in the fold, and thereby the possibility to connect multiply connected Calabi-Yau spaces with the simply connected ones [16,17], the latter of which showing that “hyperconifold” transitions can be mirrors of “ordinary conifold transitions” (funny how “preposterous” and “nonsense” turns into “ordinary” :-).

- References
`[-3]`*Calabi-Yau Manifolds—Motivations and Constructions*,- Tristan Hübsch:
*Comm. Math. Phys*.**108**(1987) 291–318. `[-2]`*Calabi-Yau Manifolds as Complete Intersections in Products of Projective Spaces*,- Paul S. Green and Tristan Hübsch:
*Comm. Math. Phys*.**109**(1987) 99–108. `[-1]`*Complete intersection Calabi- Yau manifolds,*- Philip Candelas, A.M. Dale, C. Andrew Lutken and Rolf Schimmrigk:
*Nucl. Phys*.**B298**(1988) 493. `[0]`*The moduli space of 3-folds with K = 0 may nevertheless be irreducible,*- Miles Reid:
*Math. Ann*.**278**(1987) 329–334. `[1]`*Connecting Moduli Spaces of Calabi-Yau Threefolds*,- Paul S. Green and Tristan Hübsch:
*Comm. Math. Phys*.**119**(1988) 431–44. `[2]`*Possible Phase Transitions Among Calabi-Yau Compactifications*,- Paul S. Green and Tristan Hübsch:
*Phys. Rev. Lett*.**61**(1988) 1163–1166. `[3]`*Finite Distances Between Distinct Calabi-Yau Manifolds*,- Philip Candelas, Paul S. Green and Tristan Hübsch:
*Phys. Rev. Lett*.**62**(1989) 1956–1959. `[4]`*Relation Between the Weil-Petersson and Zamolodchikov Metrics*,- Philip Candelas, Tristan Hübsch and Rolf Schimmrigk:
*Nucl. Phys*.**B329**(1990) 583–590. `[5]`*Rolling Among Calabi-Yau Vacua*,- Philip Candelas, Paul S. Green and Tristan Hübsch:
*Nucl. Phys*.**B330**(1990) 49–102. `[6]`*Calabi-Yau Manifolds—A Bestiary for Physicists*,*(errata)*- Tristan Hübsch:
*World Scientific*, Singapore; 1st (hardcover) ed. 1992, 2nd (paperback) ed. 1994. `[7]`*Chameleonic Sigma-Models*,- Tristan Hübsch:
*Phys*.*Lett*.**B247**(1990) 317-322. `[8]`*Elusive Conifold Compactifications*,- Tristan Hübsch:
*Class. Q. Grav*.**8**(1991) L31-L35. `[9]`*Black hole condensation and the unification of string vacua*,- Brian R. Greene, David R. Morrison and Andrew Strominger:
*Nucl. Phys*.**B451**(1995) 109–120. `[10]`*Massless black holes and conifolds in string theory*,- Andrew Strominger:
*Nucl. Phys*.**B451**(1995) 96–108. `[11]`*Chirality changing phase transitions in 4-D string vacua*,- Shamit Kachru and Eva Silverstein:
*Nucl. Phys*.**B504**(1997) 272–284. `[12]`*Non-Abelian conifold transitions and N=4 dualities in three-dimensions*,- Kentaro Hori, Hirosi Ooguri, Cumrun Vafa:
*Nucl. Phys*.**B504**(1997) 147–174. `[13]`*Complete classification of reflexive polyhedra in four dimensions*,- Maximilian Kreuzer and Harald Skarke:
*Adv. Theor. Math. Phys.***4**(2002) 1209–1230. `[14]`*Constructing new Calabi-Yau 3-folds and their mirrors via conifold transitions*,- Victor Batyrev and Maximilian Kreuzer: arXiv:0802.3376.
`[15]`*New Calabi-Yau Manifolds with Small Hodge Numbers*,- Philip Candelas and Rhys Davies: arXiv:0809.4681.
`[16]`*Quotients of the Conifold in Compact Calabi-Yau Threefolds, and New Topological Transitions*,- Rhys Davies:
*Adv. Theor. Math. Phys.***14**(2010) 965–990, arXiv:0911.0708. `[17]`*Hyperconifold Transitions, Mirror Symmetry, and String Theory*,- Rhys Davies:
*Nucl. Phys.***B850**(2011) 214–231, arXiv:1102.1428. - This list is necessarily ultra-local and incomplete; you should search for other sources too; there are plenty… Google currently claims some >50k (and growing) web-pages containing the term.

**Calabi-Yau sites**

Calabi-Yau Data (Maximilian Kreuzer)

Calabi-Yau Home Page
(Rolf Schimmrigk & Andreas Wißkirchen)

© Tristan Hübsch, 2012

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