NEW REFLECTIONS ON HIGHER DIMENSIONAL LINEARIZED GRAVITY C. Garc´ıa-Quintero1, A. Ortiz and J. A. Nieto2 Facultad de Ciencias F´ısico-Matem´aticas de la Universidad Aut´onoma de Sinaloa, 80010, Culiac´an, Sinaloa, M´exico Abstract We make a number of remarks on linearized gravity with cosmological constant in any dimension, which, we argue, can be useful in a quantum gravity framework. For this purpose we assume that the background space-time metric corresponds to the de Sitter or anti-de Sitter space. Moreover, via the graviton mass and the cosmological constant correspondence, we make some interesting observations, putting special attention on the possible scenario of a graviton- tachyon connection. We compare our proposed formalism with the Novello and Neves approach. arXiv:1904.07019v1 [gr-qc] 12 Apr 2019 Keywords: linearized gravity, graviton, quantum gravity Pacs numbers: 04.20.Jb, 04.50.-h, 04.60.-m, 11.15.-q March, 2019 [email protected], [email protected] [email protected], [email protected] 1 1. Introduction It is known that there are a number of works relating tachyons with M- theory [1] (see also Ref. [2] and references therein), including the brane and anti-brane systems [3], closed-string tachyon condensation [4], tachyonic insta- bility and tachyon condensation in the E(8) heterotic string [5], among many others. Part of the motivation of these developments emerges because it was discovered that the ground state of the bosonic string is tachyonic [6] and that the spectrum in AdS/CFT [7] can contain a tachyonic structure. On the other hand, it is also known that the (5 + 5)-signature and the (1 + 9)-signature are common to both type IIA strings and type IIB strings. In fact, versions of M-theory lead to type IIA string theories in space-time of signatures (0+10), (1+9), (2+8), (4+6) and (5+5), and to type IIB string theories of signatures (1 + 9), (3 + 7) and (5 + 5) [8]. It is worth mentioning that some of these theories are linked by duality transformations. So, one wonders whether tachyons may also be related to the various signatures. In particular, here we are interested to see the possible relation of tachyons with a space-time of (4 + 6)-dimensions. Part of the motivation in the (4 + 6)- signature arises from the observation that (4 + 6) = (1 + 4) + (3 + 2). This means that the world of (4 + 6)-dimensions can be splitted into a de Sitter world of (1 + 4)-dimensions and an anti-de Sitter world of (3 + 2)-dimensions. Moreover, looking the (4+6)-world from the perspective of (3+7)-dimensions obtained by compactifying-uncompactifying prescription such that4 → 3 and 6 → 7, one can associate with the 3 and 7 dimensions of the (3 + 7)-world with a S3 and S7, respectively, which are two of the parallelizable spheres; the other it is S1. As it is known these spheres are related to both Hopf maps and division algebras (see Ref. [9] and references therein). In this work, we develop a formalism that allows us to address the (4 + 6)- dimensional world via linearized gravity. In this case, one starts assuming the Einstein field equations with cosmological constant Λ in (4 + 6)-dimensions and develops the formalism considering a linearized metric in such equations. We note that the result is deeply related to the cosmological constant Λ ≶ 0 sign. In fact, one should remember that in (1 + 4)-dimensions, Λ is positive, while in (3 + 2)-dimensions, Λ is negative. At the level of linearized gravity, one searches for the possibility of associating these two different signs of Λ with tachyons. This leads us to propose a unified tachyonic framework in (4 + 6)-dimensions which includes these two separate cases of Λ. Moreover, we argue that our formalism may admit a possible connection with the increasing interesting proposal of duality in linearized gravity (see Refs. [10-12] and 2 references therein). In order to achieve our goal, we first introduce, in a simple context, the tachyon theory. Secondly, in a novel form we develop the de Sitter and anti-de Sitter space-times formalism, clarifying the meaning of the main constraints. Moreover, much work allows us to describe a new formalism for higher dimen- sional linearized gravity. Our approach is focused on the space-time signature in any dimension and in particular in (4 + 6)-dimensions. A further motivation of our approach may emerge from the recent direct detections of gravitational waves [13-15]. According to this detection the up- −58 per bound of the graviton mass is mg ≤ 1.3 × 10 kg [15]. Since in our computations the mass and the cosmological constant are proportional, such an upper bound must also be reflected in the cosmological constant value. Technically, this work is structured as follows. In section 2, we make a simple introduction of tachyon theory. In section 3, we discuss a possible formalism for the de Sitter and anti-de Sitter space-times. In section 4, we de- velop the most general formalism of higher dimensional linearized gravity with cosmological constant. In section 5, we establish a novel approach for consid- ering the constraints that determine the de Sitter and anti-de Sitter space. In section 6, we associate the concept of tachyons with higher dimensional lin- earized gravity. In section 7, we develop linearized gravity with cosmological constant in (4 + 6)-dimensions. We add an appedix A in attempt to further clarify the negative mass squared term-tachyon association. Finally, in section 8, we make some final remarks. 2. Special relativity and the signature of the space-time Let us start considering the well known time dilatation formula dτ dt = . (1) v2 1 − c2 2 dxq2 dy 2 dz 2 Here, τ is the proper time, v ≡ ( dt ) +( dt ) +( dt ) is the velocity of the object and c denotes the speed of light. Of course, the expression (1) makes sense over the real numbers only if one assumes v<c. It is straightforward to see that (1) leads to the line element ds2 = −c2dτ 2 = −c2dt2 + dx2 + dy2 + dz2. (2) In tensorial notation one may write (2) as 3 2 2 2 (+) µ ν ds = −c dτ = ηµν dx dx , (3) where the indices µ, ν take values in the set {1, 2, 3, 4}, x1 = ct, x2 = x, x3 = y 4 (+) and x = z. Moreover, ηµν denotes the flat Minkowski metric with associated signature (−1, +1, +1, +1). Usually, one says that such a signature represents a world of (1 + 3)-dimensions. If one now defines the linear momentum dxµ pµ = m(+) , (4) 0 dτ (+) with m0 =6 0 a constant, one sees that (3) implies µ ν (+) (+)2 2 p p ηµν + m0 c =0. (5) (+) Of course, m0 plays the role of the rest mass of the object. This is because setting pi = 0, with i ∈ {2, 3, 4}, in the rest frame and defining E = cp1, the 2 constraint (5) leads to the famous formula E = ±m0c . Let us follow similar steps, but instead of starting with the expression (1), one now assumes the formula dξ dλ = , (6) u2 c2 − 1 2 dw 2 dρ 2 dζ 2 q where u =( dξ ) +( dξ ) +( dξ ) . Note that in this case one has the condition u>c. Here, in order to emphasize the differences between (1) and (6), we are using a different notation. Indeed, the notation used in (1) and (6) is introduced in order to establish an eventual connection with (4+6)-dimensions. From (6) one obtains ds2 = −c2dξ2 =+c2dλ2 − dw2 − dρ2 − dζ2. (7) In tensorial notation, one may write (7) as 2 2 2 (−) µ ν ds = −c dξ = ηµν dy dy , (8) 1 2 3 4 (−) where y = cλ, y = w, y = ρ and y = ζ. Moreover, ηµν denotes the flat Minkowski metric with associated signature (+1, −1, −1, −1). One says that this signature represents a world of (3 + 1)-dimensions. If one now defines the linear momentum 4 µ − dy Pµ = m( ) , (9) 0 dξ (−) with m0 =6 0 a constant, one sees that (9) implies µ ν (−) (−)2 2 P P ηµν + m0 c =0. (10) Since, u>c one observes that in this case the constraint (10) corresponds to (−) a tachyon system with mass m0 . Now, for the case of ordinary matter, if one wants to quantize, one starts promoting pµ as an operator identifyingp ˆµ = −i∂µ. Thus, at the quantum level (5) becomes µ ν (+) (+)2 (−∂ ∂ ηµν + m0 )ϕ =0. (11) It is important to mention that here we are using a coordinate representation for ϕ in the sense that ϕ(xµ)=< xµ|ϕ>. (+)2 (+) µ ν By defining the d’Alembert operator = ηµν ∂ ∂ one notes that last equation reads (+)2 (+)2 ( − m0 )ϕ =0. (12) Analogously, in the constraint (10) one promotes the momentum Pµ as an µ µ (+) (−) operator Pˆ = −i∂ and using ηµν = −ηµν , the expression (10) yields (+)2 (−)2 ( + m0 )ϕ =0. (13) The last two expressions are Klein-Gordon type equations for ordinary matter and tachyonic systems, respectively. In fact, these two equations will play an important role in the analysis in section 6, concerning linearized gravity with positive and negative cosmological constant. 3. Clarifying de Sitter and anti-de Sitter space-time Let us start with the constraint i j (+) d+1 2 2 x x ηij +(x ) = r0, (14) (+) where ηij = diag(−1, 1, ..., 1) is the Minkowski metric and the i index goes 2 from 1 to d. and r0 is a positive constant. The line element is given by 2 A B i j (+) d+1 2 ds ≡ dx dx ηAB = dx dx ηij +(dx ) .