# Monstrous moonshine

In mathematics, **monstrous moonshine**, or **moonshine theory**, is the unexpected connection between the monster group *M* and modular functions, in particular, the *j* function. The term was coined by John Conway and Simon P. Norton in 1979.

It is now known that lying behind monstrous moonshine is a vertex operator algebra called the moonshine module (or monster vertex algebra) constructed by Igor Frenkel, James Lepowsky, and Arne Meurman in 1988, having the monster group as symmetries. This vertex operator algebra is commonly interpreted as a structure underlying a two-dimensional conformal field theory, allowing physics to form a bridge between two mathematical areas. The conjectures made by Conway and Norton were proven by Richard Borcherds for the moonshine module in 1992 using the no-ghost theorem from string theory and the theory of vertex operator algebras and generalized Kac–Moody algebras.

In 1978, John McKay found that the first few terms in the Fourier expansion of the normalized J-invariant (sequence in the OEIS),

Conway and Norton computed the lower-order terms of such graded traces, now known as McKay–Thompson series *T*_{g}, and found that all of them appeared to be the expansions of Hauptmoduln. In other words, if *G*_{g} is the subgroup of SL_{2}(**R**) which fixes *T*_{g}, then the quotient of the upper half of the complex plane by *G*_{g} is a sphere with a finite number of points removed, and furthermore, *T*_{g} generates the field of meromorphic functions on this sphere.

Based on their computations, Conway and Norton produced a list of Hauptmoduln, and conjectured the existence of an infinite dimensional graded representation of *M*, whose graded traces *T*_{g} are the expansions of precisely the functions on their list.

Borcherds proved the Conway–Norton conjecture for the Moonshine Module in 1992. He won the Fields Medal in 1998 in part for his solution of the conjecture.

Frenkel, Lepowsky, and Meurman then showed that the automorphism group of the moonshine module, as a vertex operator algebra, is *M*. Furthermore, they determined that the graded traces of elements in the subgroup 2^{1+24}.*Co*_{1} match the functions predicted by Conway and Norton (Frenkel, Lepowsky & Meurman (1988)).

Richard Borcherds' proof of the conjecture of Conway and Norton can be broken into the following major steps:

Thus, the proof is completed (Borcherds (1992)). Borcherds was later quoted as saying "I was over the moon when I proved the moonshine conjecture", and "I sometimes wonder if this is the feeling you get when you take certain drugs. I don't actually know, as I have not tested this theory of mine." (Roberts 2009, p. 361)

More recent work has simplified and clarified the last steps of the proof. Jurisich (Jurisich (1998), Jurisich, Lepowsky & Wilson (1995)) found that the homology computation could be substantially shortened by replacing the usual triangular decomposition of the Monster Lie algebra with a decomposition into a sum of *gl*_{2} and two free Lie algebras. Cummins and Gannon showed that the recursion relations automatically imply the McKay Thompson series are either Hauptmoduln or terminate after at most 3 terms, thus eliminating the need for computation at the last step.

Conway and Norton suggested in their 1979 paper that perhaps moonshine is not limited to the monster, but that similar phenomena may be found for other groups.^{[a]} While Conway and Norton's claims were not very specific, computations by Larissa Queen in 1980 strongly suggested that one can construct the expansions of many Hauptmoduln from simple combinations of dimensions of irreducible representations of sporadic groups. In particular, she decomposed the coefficients of McKay-Thompson series into representations of subquotients of the Monster in the following cases:

Queen found that the traces of non-identity elements also yielded *q*-expansions of Hauptmoduln, some of which were not McKay–Thompson series from the Monster. In 1987, Norton combined Queen's results with his own computations to formulate the Generalized Moonshine conjecture. This conjecture asserts that there is a rule that assigns to each element *g* of the monster, a graded vector space *V*(*g*), and to each commuting pair of elements (*g*, *h*) a holomorphic function *f*(*g*, *h*, τ) on the upper half-plane, such that:

This is a generalization of the Conway–Norton conjecture, because Borcherds's theorem concerns the case where *g* is set to the identity.

Like the Conway–Norton conjecture, Generalized Moonshine also has an interpretation in physics, proposed by Dixon–Ginsparg–Harvey in 1988 (Dixon, Ginsparg & Harvey (1989)). They interpreted the vector spaces *V*(*g*) as twisted sectors of a conformal field theory with monster symmetry, and interpreted the functions *f*(*g*, *h*, τ) as genus one partition functions, where one forms a torus by gluing along twisted boundary conditions. In mathematical language, the twisted sectors are irreducible twisted modules, and the partition functions are assigned to elliptic curves with principal monster bundles, whose isomorphism type is described by monodromy along a basis of 1-cycles, i.e., a pair of commuting elements.

In the early 1990s, the group theorist A. J. E. Ryba discovered remarkable similarities between parts of the character table of the monster, and Brauer characters of certain subgroups. In particular, for an element *g* of prime order *p* in the monster, many irreducible characters of an element of order *kp* whose *k*th power is *g* are simple combinations of Brauer characters for an element of order *k* in the centralizer of *g*. This was numerical evidence for a phenomenon similar to monstrous moonshine, but for representations in positive characteristic. In particular, Ryba conjectured in 1994 that for each prime factor *p* in the order of the monster, there exists a graded vertex algebra over the finite field **F**_{p} with an action of the centralizer of an order *p* element *g*, such that the graded Brauer character of any *p*-regular automorphism *h* is equal to the McKay-Thompson series for *gh* (Ryba (1996)).

In 2007, E. Witten suggested that AdS/CFT correspondence yields a duality between pure quantum gravity in (2+1)-dimensional anti de Sitter space and extremal holomorphic CFTs. Pure gravity in 2+1 dimensions has no local degrees of freedom, but when the cosmological constant is negative, there is nontrivial content in the theory, due to the existence of BTZ black hole solutions. Extremal CFTs, introduced by G. Höhn, are distinguished by a lack of Virasoro primary fields in low energy, and the moonshine module is one example.

Under Witten's proposal (Witten (2007)), gravity in AdS space with maximally negative cosmological constant is AdS/CFT dual to a holomorphic CFT with central charge *c=24*, and the partition function of the CFT is precisely *j-744*, i.e., the graded character of the moonshine module. By assuming Frenkel-Lepowsky-Meurman's conjecture that moonshine module is the unique holomorphic VOA with central charge 24 and character *j-744*, Witten concluded that pure gravity with maximally negative cosmological constant is dual to the monster CFT. Part of Witten's proposal is that Virasoro primary fields are dual to black-hole-creating operators, and as a consistency check, he found that in the large-mass limit, the Bekenstein-Hawking semiclassical entropy estimate for a given black hole mass agrees with the logarithm of the corresponding Virasoro primary multiplicity in the moonshine module. In the low-mass regime, there is a small quantum correction to the entropy, e.g., the lowest energy primary fields yield ln(196883) ~ 12.19, while the Bekenstein–Hawking estimate gives 4π ~ 12.57.

Later work has refined Witten's proposal. Witten had speculated that the extremal CFTs with larger cosmological constant may have monster symmetry much like the minimal case, but this was quickly ruled out by independent work of Gaiotto and Höhn. Work by Witten and Maloney (Maloney & Witten (2007)) suggested that pure quantum gravity may not satisfy some consistency checks related to its partition function, unless some subtle properties of complex saddles work out favorably. However, Li–Song–Strominger (Li, Song & Strominger (2008)) have suggested that a chiral quantum gravity theory proposed by Manschot in 2007 may have better stability properties, while being dual to the chiral part of the monster CFT, i.e., the monster vertex algebra. Duncan–Frenkel (Duncan & Frenkel (2009)) produced additional evidence for this duality by using Rademacher sums to produce the McKay–Thompson series as 2+1 dimensional gravity partition functions by a regularized sum over global torus-isogeny geometries. Furthermore, they conjectured the existence of a family of twisted chiral gravity theories parametrized by elements of the monster, suggesting a connection with generalized moonshine and gravitational instanton sums. At present, all of these ideas are still rather speculative, in part because 3d quantum gravity does not have a rigorous mathematical foundation.

In 2010, Tohru Eguchi, Hirosi Ooguri, and Yuji Tachikawa observed that the elliptic genus of a K3 surface can be decomposed into characters of the *N* = (4,4) superconformal algebra, such that the multiplicities of massive states appear to be simple combinations of irreducible representations of the Mathieu group M24. This suggests that there is a sigma-model conformal field theory with K3 target that carries M24 symmetry. However, by the Mukai–Kondo classification, there is no faithful action of this group on any K3 surface by symplectic automorphisms, and by work of Gaberdiel–Hohenegger–Volpato, there is no faithful action on any K3 sigma-model conformal field theory, so the appearance of an action on the underlying Hilbert space is still a mystery.

By analogy with McKay–Thompson series, Cheng suggested that both the multiplicity functions and the graded traces of nontrivial elements of M24 form mock modular forms. In 2012, Gannon proved that all but the first of the multiplicities are non-negative integral combinations of representations of M24, and Gaberdiel–Persson–Ronellenfitsch–Volpato computed all analogues of generalized moonshine functions, strongly suggesting that some analogue of a holomorphic conformal field theory lies behind Mathieu moonshine. Also in 2012, Cheng, Duncan, and Harvey amassed numerical evidence of an umbral moonshine phenomenon where families of mock modular forms appear to be attached to Niemeier lattices. The special case of the *A*_{1}^{24} lattice yields Mathieu Moonshine, but in general the phenomenon does not yet have an interpretation in terms of geometry.

The monster group was investigated in the 1970s by mathematicians Jean-Pierre Serre, Andrew Ogg and John G. Thompson; they studied the quotient of the hyperbolic plane by subgroups of SL_{2}(**R**), particularly, the normalizer Γ_{0}(*p*)^{+} of the Hecke congruence subgroup Γ_{0}(*p*) in SL(2,**R**). They found that the Riemann surface resulting from taking the quotient of the hyperbolic plane by Γ_{0}(*p*)^{+} has genus zero if and only if *p* is 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 41, 47, 59 or 71. When Ogg heard about the monster group later on, and noticed that these were precisely the prime factors of the size of *M*, he published a paper offering a bottle of Jack Daniel's whiskey to anyone who could explain this fact (Ogg (1974)).