Advances in Mathematical Physics

Volume 2015 (2015), Article ID 761302, 5 pages

http://dx.doi.org/10.1155/2015/761302

## Constant Mean Curvature Spacelike Surfaces in Lorentzian Warped Products

^{1}Departamento de Matemáticas, E.S.I. Informática, Universidad de Castilla-La Mancha, 02071 Albacete, Spain^{2}Departamento de Matemáticas, Campus de Rabanales, Universidad de Córdoba, 14071 Córdoba, Spain

Received 13 August 2015; Accepted 4 October 2015

Academic Editor: Jacopo Bellazzini

Copyright © 2015 Juan A. Aledo and Rafael M. Rubio. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

We characterize the spacelike slices of a Lorentzian warped product as the only constant mean curvature spacelike surfaces under suitable geometrical and physical assumptions. As a consequence of our study, we derive a Bernstein-type result which widely improves and extends the state-of-the-art results in this setting.

#### 1. Introduction

Spacelike hypersurfaces in -dimensional Lorentzian spacetimes are geometrical objects of great physical and mathematical interest. Roughly speaking, each of them represents the physical space in a given instant of a time function. In electromagnetism, a spacelike hypersurface is an initial data set that univocally determines the future of the electromagnetic field which satisfies the Maxwell equations [1, Theorem 3.11.1], analogously, for the simple matter equations [1, Theorem 3.11.2]. In Causality Theory, the mere existence of a particular spacelike hypersurface implies that the spacetime obeys a certain causal property. Let us remark that the completeness of a spacelike hypersurface is required whenever we study its global properties, and, also, from a physical viewpoint, completeness implies that the whole physical space is taken into consideration. To know more details on the relevance of constant mean curvature spacelike hypersurfaces in General Relativity see [2].

From a mathematical point of view, the interest of spacelike hypersurfaces is motivated by their nice Bernstein-type properties. In fact, spacelike hypersurfaces of constant mean curvature (CMC) in -dimensional spacetime are critical points of the area functional under a certain volume constraint [3]. When the ambient spacetime is the Lorentz-Minkowski spacetime , many results have been obtained from different viewpoints. For instance, as an application of the generalized maximum principle due to Omori-Yau [4, 5] and of the Calabi-Bernstein theorem, Aiyama [6] and Xin [7] (see also [8] for a first weaker version given by Palmer) obtained simultaneously and independently a characterization of spacelike hyperplanes as the only complete spacelike hypersurfaces with constant mean curvature (CMC) in the Lorentz-Minkowski spacetime whose hyperbolic image is bounded. As another application of the previous mentioned results, a characterization of the spacelike hyperplanes as the only complete CMC spacelike hypersurfaces in the Lorentz-Minkowski spacetime which lie between two parallel spacelike hyperplanes has been given by Aledo and Alías [9].

In this work, we will deal with spacelike surfaces but in a more general setting (some previous works in this scene, though with a completely different approach, are [10–12]; see Sections and ). Indeed, we will consider CMC spacelike surfaces in three-dimensional Lorentzian warped products, also called generalized Robertson-Walker spacetimes (see [13]). Note that in our study we make use (see Lemmas 1 and 7) of Lemmas and in [14], where the authors (with a completely different approach) study complete spacelike constant mean curvature hypersurfaces in warped products with parabolic fiber. Nonetheless, unlike our current study, all the principal results in [14] require the Timelike Convergence Condition and the hypothesis of boundedness of the hyperbolic angle between the normal vector field of the spacelike hypersurface and the timelike coordinate vector field of the warped product.

Finally, let us remark that although three-dimensional spacetimes may be considered unrealistic from a physical point of view, they have been deeply studied from a purely geometrical perspective. In fact, they can be used to light suitable extensions of geometrical properties to usual four-dimensional relativistic models.

The paper is organized as follows. In Section 2 we revise some notions regarding spacelike surfaces in a 3-dimensional Lorentzian space and introduce the notation to be used. We continue, in Section 3, undertaking some technical computations. Section 4 is devoted to present our main results. Thus, in Theorems 2 and 5 we characterize the CMC spacelike surfaces of a Lorentzian warped product as the only spacelike slices under suitable geometrical and physical assumptions. As a consequence of our study, in Section 5 we derive a Bernstein-type result (Theorem 8) which widely improves and extends the state-of-the-art results in this setting.

#### 2. Preliminaries

Let be a connected Riemannian surface, an open interval in , and a positive smooth function defined on . Then, the product manifold endowed with the Lorentzian metricwhere and denote the projections onto and , respectively, is called a* Lorentzian warped product* with* fiber *,* base *, and* warping function *. Along this paper we will represent this 3-dimensional Lorentzian manifold by .

The coordinate vector field globally defined on is (unitary) timelike, and so is time-orientable. We will also consider on the conformal closed timelike vector field . From the relationship between the Levi-Civita connections of and those of the base and the fiber [15, Corollary 7.35], it follows that for any , where is the Levi-Civita connection of the Lorentzian metric (1). Thus, is conformal with and its metrically equivalent 1-form is closed.

Recall that a Lorentzian manifold obeys the* Null Convergence Condition (NCC)* if its Ricci tensor satisfies , for all null vector . In the case, when is a Lorentzian warped product with a 2-dimensional fiber, it can be easily checked that obeys the NCC if and only ifwhere stands for the Gaussian curvature of (see, e.g., [11]).

Throughout this paper we will assume that is a complete Riemannian surface with* finite total curvature*; that is, its Gaussian curvature satisfies thatwhere is the area element of and the integral above is defined with a compact exhaustion procedure.

A smooth immersion of a (connected) surface is said to be a* spacelike surface* if the induced metric via is a Riemannian metric on .

Since is time-orientable we can take, for each spacelike surface , a unique unitary timelike vector field globally defined on with the same time orientation as , such that . From the wrong-way Cauchy-Schwarz inequality (see, e.g., [15, Proposition 5.30]), we have , and the equality holds at a point if and only if at . In fact, , where is the* hyperbolic angle*, at each point, between the unit timelike vectors and .

We will denote by and the* shape operator* and the* mean curvature function* associated with . A spacelike surface with constant is called a constant mean curvature (CMC) spacelike surface.

In any Lorentzian warped product there is a remarkable family of spacelike surfaces, namely, its spacelike* slices *. Note that a spacelike surface in is a (piece of) spacelike slice if and only if the function is constant. Furthermore, a spacelike hypersurface in is a (piece of) spacelike slice if and only if the hyperbolic angle vanishes identically.

#### 3. Set Up

Let be a spacelike hypersurface in a Lorentzian warped product . If we put , the tangential part of , on , it is easy to check that the gradient of on is given byNow, from the Gauss formula and using and (5), the Laplacian of giveswhere .

On the other hand, in [11, Formula 10] the Gaussian curvature of a spacelike surface in is computed and can be written as

We will use the following result [14, Lemma 12].

Lemma 1. *Let be a CMC complete spacelike surface in whose warping function is such that . If the Gaussian curvature of is bounded from below and is contained between two slices, then*

Finally, note that, given a complete spacelike surface , the projection is a covering map provided that [13]. In particular, if the fiber is simply connected then is a diffeomorphism. Then, the area elements and of and , respectively, satisfy the relationship,see [16, equation 8].

#### 4. Main Results

Next, we will characterize the spacelike slices under suitable geometrical and physical assumptions.

Theorem 2. *Let be a simply connected Riemannian surface with finite total curvature and whose Gaussian curvature is bounded from below. Assume that the Lorentzian warped product satisfies the NCC and that the warping function is such that .**Let be a CMC complete spacelike surface in . If is contained between two slices, then is a spacelike slice.*

*Proof. *Since satisfies the NCC, from (7) we get by using (3) that the Gaussian curvature of is bounded from below when is bounded from below. To see that, note that is bounded because is contained between two slices and that, from the Cauchy-Schwarz inequality, it is . Then, from Lemma 1 we get that the (constant) mean curvature of is given by (8).

With all of this, the Gaussian curvature of (7) satisfies thatNow, since has finite total curvature we have, using (9), thatTherefore, has finite total curvature and, in particular, is parabolic.

Let us consider the function on given byUsing (5), (6), and (8), the Laplacian of can be computed to obtainwhich is signed because is constant. Since is bounded (because is contained between two slices), it follows from the parabolicity of that is constant. Thenand consequently .

If we put , we get from thatthat is, the hyperbolic angle vanishes identically on and therefore is a spacelike slice.

*Remark 3. *As we commented in Section 2, we need to assure that is a diffeomorphism. In Theorem 2 this inequality holds since is bounded.

Note that the previous theorem is a wide extension of [9, Theorem 1]. To see this, it is enough to take a suitable splitting of .

A Lorentzian warped product , where is an -dimensional Riemannian manifold, is called a* steady state spacetime* (see [17] for more details). The following result constitutes a partial extension of [17, Theorem 8], when .

Corollary 4. *Let be a 3-dimensional type steady state spacetime whose fiber is a simply connected Riemannian surface with finite total curvature and whose Gaussian curvature is bounded from below. Then, the only complete CMC spacelike surfaces contained between two slices are the spacelike slices.*

Alternatively, the assumption of NCC can be changed for the one of bounded hyperbolic angle as follows.

Theorem 5. *Let be a simply connected Riemannian surface with finite total curvature and whose Gaussian curvature is bounded from below. Assume that the warping function of the Lorentzian warped product is such that .**Let be a CMC complete spacelike surface in . If the hyperbolic angle of is bounded and is contained between two slices, then is a spacelike slice.*

*Proof. *Reasoning as in the proof of Theorem 2, we get now thatwhere we have also used (15). Thenand the proof finishes as in Theorem 2.

Notice that the comment in Remark 3 is also valid for Theorem 5. On the other hand, it is worth pointing out that the boundedness of the surface and the one of the hyperbolic angle have no relation at all [12, Remark 5.3]. Hence, in Theorem 5, from the assumption of boundedness of the surface the boundedness of the hyperbolic angle cannot be derived and so this last condition must be required in order to conclude that is a spacelike surface.

The assumption of boundedness of the hyperbolic angle admits the following physical interpretation. Along there exist two families of instantaneous observers, , (the sign minus depends on the time orientation chosen here), and the normal observers , . The quantities and are, respectively, the energy and the velocity that measures for , and we have on . Therefore, the relative speed function satisfies and so it does not approach the speed of light in vacuum on .

In order to apply the previous results to more general cases we need an extra topological hypothesis.

Let us consider a GRW spacetime , whose fiber is a 2-dimensional complete Riemannian manifold. Recall that if the warping function is bounded on a complete spacelike surface , then is a covering map [13].

Now, let us take and such that . Denote by the set of all left cosets of in It is well-known that

Now, let us assume . Thus, covers -times the fiber. Moreover, taking into account the reasoning in Theorems 2 and 5, it is not difficult to see that also has finite total curvature under the same assumptions.

*Remark 6. *(a) Consider , endowed with its canonical product metric, and , an arbitrary positive smooth function. Set . For each positive integer , let be the spacelike immersion given by . This example shows that there exist surfaces with arbitrary . (b) However, we cannot force the fact that the fundamental group of the fiber is finite unless it is trivial. This is due to the fact that the fundamental group of any noncompact surface must be free (see, e.g., [18]).

We end this section pointing out that Theorems 2 and 5 extend and improve widely the conclusion given in [11, Corollary 6.11].

#### 5. Calabi-Bernstein-Type Problems

Let be a (noncompact) complete Riemannian surface, an open interval in , and a positive smooth function on . For each such that we can consider its graph in the Lorentzian warped product . The graph of inherits a metric, represented on by , which is Riemannian if and only if satisfies everywhere on , where denotes the gradient of in . In this case, the graph is a spacelike surface in . Note that for any , and so, on the spacelike graph, and can be naturally identified.

When is spacelike, the unitary normal vector field on satisfying isand the corresponding mean curvature function is

Our aim in this section is to study the* entire* solutions of the CMC spacelike surface equationunder suitable geometrical and analytical assumptions.

Note that the constraint can be written aswhere is the hyperbolic angle of . Therefore, implies that has bounded hyperbolic angle. Moreover, this constraint means that the differential equation is, in fact, uniformly elliptic.

An important fact when we deal with entire graphs in this context is that, in general, the induced metric is not complete. In this sense, we will make use of the following result [14, Lemma 17].

Lemma 7. *Let be a Lorentzian warped product whose fiber is a (noncompact) complete Riemannian surface. Consider a function , with , such that the entire graph endowed with the metric is spacelike. If the hyperbolic angle of is bounded and , then the graph is complete or, equivalently, the Riemannian surface is complete.*

Now, from Theorem 5 and taking into account that every graph in a Lorentzian product is diffeomorphic to its fiber, we can state the following.

Theorem 8. *Let be a simply connected Riemannian surface with finite total curvature and whose Gaussian curvature is bounded from below, and let be a smooth positive function such that . Then, the only bounded solutions to the uniformly elliptic equation + are the constant functions.*

Note that Theorem 8 thoroughly extends and improves [11, Theorem 7.1], as well as [12, Theorem 5.2].

#### Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

#### Acknowledgments

The authors are grateful to the referee for his/her deep reading and making suggestions toward the improvement of this paper. The first author is partially supported by the Spanish Ministerio de Economía y Competitividad Grant with FEDER funds MTM2013-43970-P and by the Junta de Comunidades de Castilla-La Mancha Grant PEII-2014-001-A. The second author is partially supported by the Spanish MICINN Grant with FEDER funds MTM2013-47828-C2-1-P.

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