Planck-Scale Deformations of Relativistic SymmetriesView this Special Issue
Review Article | Open Access
Ángel Ballesteros, N. Rossano Bruno, Francisco J. Herranz, "Noncommutative Relativistic Spacetimes and Worldlines from 2 + 1 Quantum (Anti-)de Sitter Groups", Advances in High Energy Physics, vol. 2017, Article ID 7876942, 19 pages, 2017. https://doi.org/10.1155/2017/7876942
Noncommutative Relativistic Spacetimes and Worldlines from 2 + 1 Quantum (Anti-)de Sitter Groups
The -deformation of the (2 + 1)D anti-de Sitter, Poincaré, and de Sitter groups is presented through a unified approach in which the curvature of the spacetime (or the cosmological constant) is considered as an explicit parameter. The Drinfel’d-double and the Poisson–Lie structure underlying the -deformation are explicitly given, and the three quantum kinematical groups are obtained as quantizations of such Poisson–Lie algebras. As a consequence, the noncommutative (2 + 1)D spacetimes that generalize the -Minkowski space to the (anti-)de Sitter ones are obtained. Moreover, noncommutative 4D spaces of (time-like) geodesics can be defined, and they can be interpreted as a novel possibility to introduce noncommutative worldlines. Furthermore, quantum (anti-)de Sitter algebras are presented both in the known basis related to 2 + 1 quantum gravity and in a new one which generalizes the bicrossproduct one. In this framework, the quantum deformation parameter is related to the Planck length, and the existence of a kind of “duality” between the cosmological constant and the Planck scale is also envisaged.
The connection between quantum groups and Planck scale physics was early suggested in . Quantum deformations of Lie algebras and Lie groups [2–8] have been broadly applied in the construction of deformed symmetries of spacetimes [9–23], especially for the Poincaré and Galilei cases, for which the deformation parameter is known to play the role of a fundamental scale. Among all these quantum kinematical algebras the well known -Poincaré algebra [9, 13, 14, 16, 18] has been frequently considered.
These deformed Poincaré symmetries were later applied in the context of the so-called doubly special relativity (DSR) theories [24–32] which introduced two fundamental scales: the usual observer-independent velocity scale as well as an observer-independent length scale , which was related to the deformation parameter in the algebra. Since from all approaches to quantum gravity [33–37] the Planck scale is thought to play a fundamental role, DSR theories seem to establish a promising link between some Planck scale effects and quantum groups [38, 39].
From a more general viewpoint, we recall that noncommutative spaces have been proposed as a suitable algebraic framework in order to describe the “quantum” structure of the geometry of spacetime at the Planck scale through a noncommutative algebra of quantum spacetime coordinates [40–44]. In this way the deformation parameter characterizes the noncommutativity of the spacetime algebra, thus generating uncertainty relations between noncommuting coordinates that can be thought to model a “fuzzy” or “discrete” nature of the spacetime at very small distances (or high energies) [45, 46]. In particular, the noncommutative spacetime deduced from the -Poincaré algebra is the so-called -Minkowski spacetime [15, 22], which is the algebra defined by the spacetime quantum group coordinates dual to the translation (momenta) generators.
In this framework, spacetime curvature (or nonzero cosmological constant) should play a relevant role concerning the possible cosmological consequences of a quantum spacetime (see, e.g., [38, 47–51] and references therein). Therefore, it seems natural to consider the construction of the -deformation for the (anti-)de Sitter (hereafter (A)dS) groups, and to analyse their possible connections with quantum gravity theories with a nonzero cosmological constant. In this respect, we recall that the Hopf structure for the -deformation of (2 + 1)D (A)dS and Poincaré () algebras were collectively obtained in , and their connection between their deformed commutation rules and 2 + 1 quantum gravity has been explored in . The results obtained in  correspond to the l.h.s. of the commutative diagram:where vertical arrows indicate a classical deformation [52, 53] that introduces the spacetime curvature (or cosmological constant ) related to the (A)dS radius by , and the horizontal ones show the quantum deformation with parameter (related to the Planck length ); reversed arrows correspond to the spacetime contraction and (classical) nondeformed limit . As a consequence, the construction of noncommutative (A)dS spacetimes in terms of intrinsic and ambient spacetime quantum group coordinates seems worth being explored in detail and, moreover, the same framework could account for new proposals of noncommutative spaces of time-like geodesics (worldlines), which, to the best of our knowledge, have not been considered in the literature yet, even for the Poincaré case.
Here we present an enlarged and updated review version of our unpublished manuscript arXiv:hep-th/0401244, in which the above-mentioned problems are faced for the three relativistic cases simultaneously, that is, by dealing explicitly with the spacetime curvature as a contraction parameter. Hence, we propose to explore the r.h.s. of the diagram (1) (dual to the l.h.s.) by computing the quantum deformation of the (2 + 1)D (A)dS groups (that is, ) which are obtained by quantizing the Poisson–Lie algebra of smooth functions on these groups (namely, Fun() coming from a suitable classical -matrix. In this way, the (2 + 1)D noncommutative spaces (e.g., ) can then be identified as certain subalgebras of the corresponding quantum groups. Moreover, we also construct and study in detail the corresponding 4D noncommutative spaces of worldlines. We stress that in our approach all the -Poincaré relations (including its noncommutative spaces) can be directly recovered from the general (A)dS expressions through the limit . Moreover, all of the resulting noncommutative spaces are covariant under quantum group (co)actions (for the construction of Poisson and quantum homogeneous spaces we refer to [54–58] and references therein).
The structure of the paper is the following. In the next section we recall the basics on the (A)dS groups in (2 + 1) dimensions and their associated homogeneous (2 + 1)D spacetimes and 4D spaces of worldlines (time-like lines). Both types of spaces are described in terms of intrinsic quantities (related to group parameters) as well as in ambient coordinates with one and two extra dimensions, respectively, which will be further used in their noncommutative versions. By starting from the classical -matrix that generates the -deformation, we construct in Section 3 the corresponding Drinfel’d-double and obtain some preliminary information on the first-order quantum deformation, from which first-order noncommutative spaces arise. On one hand, we find that at first-order in the deformation parameter the three noncommutative relativistic spacetimes are given by the same -Minkowski algebra. Moreover, we show that the deformation parameter can be interpreted as a curvature on a classical dS spacetime for the three cases, thus generalizing the results obtained in [29, 30] for -Poincaré. On the other hand, we obtain that the first-order noncommutative spaces of worldlines are in fact nondeformed ones, and a relationship with the nonrelativistic (Newtonian) kinematical groups is thus established.
As an intermediate stage in the search of the quantum (A)dS groups, we compute in Section 4 the invariant (A)dS vector fields and next the Poisson–Lie structures coming from the classical -matrix generating the -deformation. These results enable us to propose in Section 5 the noncommutative (A)dS spaces, which are written in both intrinsic and ambient coordinates. The resulting noncommutative spacetimes show how the curvature modifies the underlying first-order -Minkowski space, while for the noncommutative spaces of worldlines we find that 2D velocity/rapidity space (spanned by the dual coordinates to the boost generators) remains nondeformed for -Poincaré but becomes deformed for the (A)dS cases. Hence Lorentz invariance seems to be lost (or somewhat “deformed”) when a nonzero curvature/cosmological constant is considered.
Section 6 is devoted to study the (dual) quantum (A)dS algebras and their deformed Casimirs in two different bases. In particular, starting from the expressions given in , a nonlinear transformation involving the generators of the stabilizer subgroup of a time-like line allows us to obtain these quantum algebras in a new basis that generalizes for any the bicrossproduct basis of -Poincaré . These results are analysed in connection with 2 + 1 quantum gravity  and a “duality” between curvature/cosmological constant and deformation parameter/Planck length is suggested along the same lines of the so-called “semidualization” approach for Hopf algebras in 2 + 1 quantum gravity  associated with the exchange of the cosmological length scale and the Planck mass (see also [60, 61]). Finally, some remarks and comments concerning recent findings in this framework close the paper.
2. (Anti-)de Sitter Lie Groups and Their Homogeneous Spaces
The Lie algebras of the three (2 + 1)D relativistic spacetimes of constant curvature can collectively be described by means of a real (graded) contraction parameter , and we denote them by . If are, in this order, the generators of rotations, time translations, space translations, and boosts, the commutation relations of readwhere from now on we assume that Latin indices , Greek ones , , and is a skew symmetric tensor such that . For a positive, zero, and negative value of , provides a Lie algebra isomorphic to , , and , respectively. The case can also be understood as an Inönü–Wigner contraction : .
Parity Π and time-reversal are involutive automorphisms of defined by which together with the composition,and the identity determine a Abelian group of involutions . The automorphisms and give rise, in this order, to the Cartan decompositions:where is the Lorentz subalgebra and covers the Lie subalgebras , , and for , correspondingly. In fact, the contraction parameter is related to the -grading associated to .
When the Lie group is considered, two relevant families of symmetric homogeneous spaces  can be constructed as follows.
(i) The (2 + 1)D spacetime: this is a rank 1 space, associated with the automorphism (4) and Cartan decomposition (5), which is defined through the quotientwhere is the Lorentz subgroup spanned by and . Thus momenta characterize the tangent space at the origin. This space turns out to have constant curvature equal to the contraction parameter: for (A)dS and () for Minkowski, where is the universe radius.
(ii) The 4D space of time-like lines (or wordlines): this is a rank 2 space, related to the automorphism (3) and Cartan decomposition (6), which is given bywhere and . This space is of hyperbolic type as this has constant curvature equal to (i.e., in terms of the speed of light). The tangent space is determined by spatial momenta and boosts . In fact, can also be interpreted as a D relativistic phase space  in which position and momentum coordinates are related to the group parameters dual to and , respectively.
We display in Table 1 each of the above symmetrical homogeneous spaces for each of the three Lorentzian Lie groups.
On the other hand, the two Casimir invariants of the Lie algebra are given bywhere and . Recall that comes from the Killing–Cartan form, while is the Pauli–Lubanski vector.
2.1. Vector Model of the (2 + 1)D Relativistic Spacetimes
The action of the (A)dS groups on the homogeneous spaces that we have just described is not linear. As it is well known, this problem can be circumvented by considering the vector representation of the Lie group which makes use of an ambient space with an “extra” dimension. In particular, the 4D real matrix representation of , given byfulfils ( is the transpose of ). The exponential of (10) leads to the vector representation of as a Lie group of matrices which acts linearly in a 4D space with ambient (or Weierstrass) coordinates . The one-parameter subgroups of obtained from (10) turn out to bewhere hereafter we also express the curvature as . Hence, and for and , while the (contraction) limit gives rise to .
Any element verifies that . The (2 + 1)D spacetime is identified with the orbit of the origin of the spacetime which is contained in the pseudosphere provided by :where . The metric on comes from the flat ambient metric divided by the curvature and restricted to the above constraint:
Ambient coordinates can be parametrized in terms of three intrinsic spacetime coordinates in different ways. We shall introduce the geodesic parallel coordinates  through the following action of the momenta subgroups (12) on the origin: namely,The role of the coordinates that parametrize a generic point under (16) in the (2 + 1)D spacetime is the following. Let be a time-like geodesic and , two space-like geodesics such that these three basic geodesics are orthogonal at . Then is the geodesic distance from up to a point measured along ; is the distance between and another point along a space-like geodesic orthogonal to through and parallel to ; and is the distance between and along a space-like geodesic orthogonal to and parallel to . Recall that time-like geodesics (as ) are compact in and noncompact in , while space-like ones (as , ) are compact in but noncompact in . Thus the trigonometric functions depending on are circular in ( and hyperbolic in ( and, conversely, those depending on are circular in but hyperbolic in .
2.2. Bivector Model of the 4D Spaces of Worldlines
The action of on the space of time-like lines is also a nonlinear one. As in the previous case, this problem can be solved by introducing an ambient space, now 6D with two “extra” dimensions, on which the group acts linearly and where is embedded. Let us consider the so-called bivector representation of (2) given by where is the matrix with entries . Under this representation any generator fulfilsBy exponentiation of (18) we obtain the bivector representation of , that is, a group of matrices which acts linearly in a 6D space with ambient (or Plücker) coordinates . The origin of is and this space is identified with the intersection of the pseudosphere determined by with a quadratic cone known as Plücker or Grassmann relation (invariant under the group action); these constraints are given by where and . The metric on follows from the 6D flat ambient metric divided by the negative curvature of and subjected to both conditions (20):
Plücker coordinates can be expressed through four intrinsic quantities of . We shall consider the space and rapidity (boost) group coordinates. The action of the following sequence of one-parameter subgroups on (those defining the tangent space to ) under the representation (18),gives rise to
Under the contraction (), this parametrization reduces to that of the 4D Minkowskian space of worldlines :Such expressions indicate that the Plücker coordinates and can be interpreted as “position-like” and “momentum-like” ones, respectively, within the phase space . In the metric (21) is degenerate and readswhich corresponds to a 2D space of rank 1 with negative curvature; that is, the 2D velocity Minkowskian space (so with coordinates ) is hyperbolic. Nevertheless, we stress that this is no longer true when (in both and ) where the complete D space structure is required (so with the four coordinates ), thus precluding the possibility of using a “reduced” 2D velocity space.
3. (Anti-)de Sitter Drinfel’d-Doubles and First-Order Noncommutative Spaces
The first-order deformation terms in the coproduct of the -Poincaré algebra [9, 13–18, 22] are known to be generated by the following classical -matrix:where denotes the skew symmetric tensor product. Recall that is a solution of the modified classical Yang–Baxter equation and that is related to the usual and deformation parameters by .
Such a classical -matrix also holds for the (A)dS algebras , so that we shall consider (26) for the whole family . Hence this element gives rise to the cocommutator of any generator through the relation ; namely,Next if we denote by the quantum group coordinate dual to , such that , and write the cocommutators as , then Lie bialgebra duality provides the so-called Drinfel’d-double Lie algebra [7, 8] formed by three sets of brackets: the initial Lie algebra, the dual relations, and the crossed commutation rules; namely,The cocycle condition for the cocommutator implies the following compatibility equations among the structure constants and :
In our case, we denote by the dual noncommutative coordinates of the generators , respectively. Thus the (A)dS and Poincaré Drinfel’d-doubles are collectively given in terms of the curvature and deformation parameter by the initial Lie algebra (2), the dual commutators,together with the crossed relations
Parity and time-reversal automorphisms (3) can be generalized to the full Drinfel’d-double as follows:
Since the first-order structure of the complete quantum deformation of is described by the corresponding Drinfel’d-double, some preliminary information concerning the physical properties of the associated noncommutative spaces can be extracted from it. Notice that, in this first-order approach, all the expressions will be linear both on the generators and on the dual quantum group coordinates.
3.1. Noncommutative Spacetimes: Linear Relations
The usual way to propose a noncommutative spacetime is to consider the commutation rules involving the quantum coordinates . Therefore, from (30) we find that the three (A)dS and Minkowskian noncommutative spacetimes are simultaneously defined by the same first-order relations:which coincide with the -Minkowski space, [15–18], for any value of the curvature . As we shall see in Section 5, further corrections of (33) depending on will appear when the full quantum (A)dS groups are considered.
As it was already studied in [29, 30], the action of the generators on the noncommutative spacetime follows by replacing formally , which requires considering the commutators involving within the Drinfel’d-double. Next the change of basis given by  provides the following commutation relations:that can directly be related to the initial Lie algebra (2). Consequently, whenever is a real deformation parameter, the commutators (35) that do not depend on close the Lie algebra for the three cases. Hence we obtain the dS spacetime as the homogeneous spacesuch that the deformation parameter now plays the role of the (negative) curvature equal to .
We stress that the connection between and the dS space was so established in  and further developed in , so that the expressions (35) generalize such a link for the noncommutative (A)dS cases as well.
3.2. Noncommutative Spaces of Worldlines: Linear Relations
A similar procedure suggests that the corresponding noncommutative spaces of worldlines arise within the Drinfel’d-double through the commutators of and (dual to and ); these arewhich are trivially independent of and .
The adjoint action on the quantum coordinates , of the isotropy subgroup of a worldline spanned by and (6) gives the following nondeformed commutation rules:where the deformation parameter does not appear. To unveil this structure we rename the former generators as and from (38) we obtain the commutation relationsSurprisingly enough, these relations define just the Newtonian Lie algebras coming from the nonrelativistic limit of the three Lorentzian Lie algebras (2), keeping the constant curvature . Namely, the relations (40) close the oscillating Newton–Hooke, Galilei, and expanding Newton–Hooke algebras [19, 53, 63, 69] according to , respectively. This fact is consistent with the known result that establishes that each of the nonrelativistic Newtonian spaces of constant curvature can be obtained from the corresponding relativistic one through a contraction around a time-like line. Therefore, the classical (nondeformed) picture is preserved for worldlines.
Summing up, the first-order deformation of characterized by the chosen -matrix (26) conveys noncommutativity on the spacetime (33) but commutativity on the space of worldlines (37). Furthermore, space isotropy is ensured in both cases as the corresponding commutation relations do not involve the quantum rotation coordinate .
4. A Poisson–Lie Structure on the (Anti-)de Sitter Groups
So far we have studied the first-order quantum (A)dS deformation. However, the obtention of the complete (in all orders in and in the generators) deformation of a semisimple group in terms of local coordinates is, in general, a very involved task. A way to study the noncommutative structures is to compute the Poisson–Lie brackets (derived from (26)) for the commutative coordinates and next to analyse their possible noncommutative version.
In particular, let us consider the matrix element of the group obtained through the following product written under the representation (12):where the group coordinates are commutative ones. Left- and right-invariant vector fields, and , of deduced from (41) are displayed in Table 2. Notice that such expressions hold for any value of the curvature . In the Poincaré case with , the vector fields, coming from the smooth contraction limit , are rather simplified as shown in Table 3.