Sigma models can serve as models of string theory in curved and time-dependent backgrounds. Solution of sigma-models in such backgrounds is often very complicated, not to say impossible. On the other hand, there are many backgrounds whose properties were thoroughly investigated and it is therefore interesting to find if they can be transformed to some others. Important example of such transformation is so called Poisson Lie T-duality.

In their seminal work [1] Klimčík and Ševera set conditions for dualizability of backgrounds and gave formulas for their transformation. Since then several examples of dualizable sigma models were constructed, see e.g. [2- 4]. Unfortunately, most of the examples are not physically interesting. The purpose of this paper is to show that physical backgrounds that admit sufficiently large group of isometries are naturally dualizable and therefore equivalent in a sense to some others. In this paper we are going to investigate four-dimensional plane-parallel wave metrics [5-8] from this point of view.

The basic concept used for construction of dualizable sigma models is Drinfel’d double-Lie group with additional structure. The Drinfel’d double for a sigma model living in curved background can sometimes be found from the knowledge of symmetry group of the metric. More precisely, in the Drinfel’d double there are two equally dimensional subgroups whose Lie algebras are isotropic subspaces of the Lie algebra of the Drinfel’d double. In case that the metric has sufficient number of independent Killing vectors, the isometry group of the metric (or its subgroup) can be taken as one of the subgroups of the Drinfel’d double. The other one then must be chosen abelian in order to satisfy the conditions of dualizability. Short summary of the dualization procedure described e.g. in [9] is given in the next section.

Let G be a Lie group and its Lie algebra. Sigma model on the group G is given by the classical action 

where F is a second order tensor field on the Lie group G. The functions are determined by the composition where and are components of a coordinate map of neighborhood of element.

Equivalently the action can be expressed as 

where are right-invariant fields

. The relationship between E and F is given by the formula

where are the components of right invariant forms. The equations of motion derived from the action (1) have the following form 

where are components of the Levi-Civita connection associated with the second order tensor field F This tensor field is a composition of the metric (a symmetric part) and the torsion potential (an antisymmetric part). The condition of dualizability of sigma-models on the level of the Lagrangian is given by the formula [1] 

where are structure coefficients of the dual algebra and are left-invariant fields on the Lie group G. The algebras and then define the Drinfel’d double that enables to construct tensor F satisfying (5).

Homogenous plane wave is generally defined by the metric of the following form [5,6]

where is the standard metrics on Euclidean space and The form of this metric seems to be simple, but explicit construction of sigma models can be very complicated. Therefore, we have focused on the special case of isotropic homogenous plane wave metric 

Metric (20) has a number of symmetries important for the construction of the dualizable sigma models. It admits the following Killing vectors 

where satisfies 

The Killing vectors are generators of orthogonal rotations in. For special choice of 

there are further isometries related to the scaling of the light-cone coordinates 

The specific form of enables us to calculate the function explicitly. The Killing vectors of the metric (20) for are 

where D is the generator associated with the scaling symmetry and.

In the following we shall investigate the case. It means that the metric tensor in coordinates reads 

This metric is not flat but its Gaussian curvature vanishes. Note that it has singularity in. It does not satisfy the Einstein equations but the conformal invariance conditions equations for vanishing of the -function

where the covariant derivatives, Ricci tensor and Gauss curvature are calculated from the metric that is also used for lowering and raising indices. Torsion in this case vanishes and dilaton field is [5]

The metric (26) admits the following Killing vectors³

One can easily check that the Lie algebra spanned by these vectors is the semidirect sum where and ideal . The algebra is abelian and its generators can be interpreted as dilation in and rotation in. Generators of the algebra commute as two-dimensional Heisenberg algebra with the center.

As explained in Section 2, dualizable metric can be constructed by virtue of Drinfel’d double. For this goal the Lie algebra of the Drinfel’d double can be composed from the four-dimensional Lie subalgebra isomorphic to the four-dimensional subalgebra of Killing vectors and four-dimensional Abelian algebra⁴. Moreover, the four-dimensional subgroup of isometries must act freely and transitively [1] on the Riemannian manifold M where the metric (20, 23) is defined so that.

Using the method described in [11] for semisimple algebras we find that up to the transformation, i.e. there are six classes of four-dimensional subalgebras of the isometry algebra of the homogeneous plane wave metric isomorphic to 

•

•

•

•

•

where are arbitrary parameters.

Infinitesimal form of transitivity condition can be formulated as requirement that four independent Killing vectors can be taken as basis vectors of four-dimensional vector distribution in M. In other words, these Killing vectors must form a basis of tangent space in every point of M. It means that in every point of M there is an invertible matrix that solves the equation

where, and form a basis of the subalgebra.

Infinitesimal form of requirement that the action of the isometry subgroup is free says that if in any point of M there is a vector of the corresponding Lie subalgebra such that its action on the point vanishes then it must be null vector.

By inspection we can find that the only four-dimensional subalgebras that generate transitive actions on M are isomorphic to or . Their non-vanishing commutation relations are 

and

respectively where and are real parameters. One can also check that the action of both corresponding groups of isometries is free. In the following we shall find metric dual to (26) that follows from its Drinfel’d double description where is isomorphic either to algebra spanned by or by .

Let us start with construction of the Drinfel’d double following from the algebra isomorphic to (33) and dual Abelian algebra. Assume that the Lie algebra is spanned by elements with commutation relations 

where and are arbitrary real parameters. The basis of left-invariant vector fields of the group generated by is

where are group coordinates used in parametrization 

To be able to obtain the metric (26) by the Drinfel’d double construction first we have to transform it into the group coordinates. Transformation between group coordinates x₁, x₂, x₃, x₄, and geometrical coordinates u, , x, y is

It converts the Killing vectors into the left-invariant vector fields (36) and the metric (26) into the form

that is obtainable by (3) and (10). To get the matrix necessary for construction of the dual model we note that it is given by the value of in the unit of the group, i.e. by value of for. 

The dual tensor on the Abelian group constructed by the procedure explained in the Section 2, namely by using (3), (10) and (13) is

One can see that the dual tensor has also antisymmetric part (-field or torsion potential) 

and its torsion is

The Gauss curvature of its symmetric part vanishes but the Ricci tensor is nontrivial. Dual metric that is symmetric part of (41) does not solve the Einstein equations either but again we can satisfy conformal invariance conditions (27)-(29) by the dilaton field

If we use the subalgebra of isometries spanned by instead of that spanned by then the transformation between group coordinates x₁, x₂, x₃, x₄, and geometrical coordinates u, , x, y is 

the matrix gets again the form (40) and we get another tensor dual to (26)

Even though it is not symmetric its torsion is zero. It satisfies the conformal invariance conditions (27)-(29) with the dilaton field

Another type of metrics that have rather large group of isometries are so called Lobachevsky plane waves [7,8]. They are of general form

They satisfy Einstein equation with cosmological constant iff

The Gauss curvature of this metric is. For special forms of function H the metric (48) admits various sets of Killing vectors. All of them are subalgebras of a vector space spanned by

A bit surprisingly, all these seven independent vector fields found in [7] form a Lie algebra even though they are not Killing vectors of the same metrics (it depends on the form of). We are interested in metrics that admit at least four independent Killing vectors because they can be interpreted as dualizable backgrounds for sigma models in four dimensions.

As mentioned in the Section 3.1, for construction of dualizable metrics we need a four-dimensional subalgebra of Killing vectors that generates group of isometries that acts freely and transitively on the four-dimensional Riemannian manifolds. Here we shall investigate metrics of the form (48) where that, i.e.

It solves the Einstein equation with the cosmological constant for [12].

Isometry groups of metrics can be used for construction of their (nonabelian) T-dual backgrounds. Sufficient condition for that is that the metric have an isometry subgroup whose dimension is equal to the dimension of the Riemannian manifold and its action on the manifold is transitive and free.

We have shown that for the plane wave metrics (26) and (51) such isometry subgroups exist and the metrics can be dualized by the Poisson-Lie T-duality transformation. We have determined the metrics and B-fields dual to the plane waves. For homogeneous plane waves (26) we have also found the dilaton field that guarantees conformal invariance of the dual metric.

Metrics that possess isometry group whose dimension is greater than the dimension of the Riemannian manifold may have several duals. More precisely, if the metric admits various isometry subgroups with above given properties then we can construct several backgrounds dual to the metric. This phenomenon is another kind of plurality of sigma models different from the Poisson-Lie T-plurality described in the Section 2.

An example of this type of plurality is provided by the plane wave metric (26) with isometry subgroups generated by Killing vectors or by (see (31) producing two dual backgrounds (41) and (46)). To decide if this plurality is different from the Poisson-Lie T-plurality one has to check whether the eight-dimensional Drinfel’d double s generated by the four-dimensional abelian algebra and algebras spanned by or are isomorphic by a transformation that leave the constant matrix (40) invariant. This is, however, very difficult task that might be investigated in the future.

This work was supported by the research plan LC527 of the Ministry of Education of the Czech Republic. Consultation with P. Winternitz and L. Šnobl on classification of subalgebras are gratefully acknowledged.