Advances in Mathematical Physics
Volume 2018 (2018), Article ID 7323090, 24 pages
https://doi.org/10.1155/2018/7323090

Research Article

Generalized -Attractor Models from Elementary Hyperbolic Surfaces

Elena Mirela Babalic and Calin Iuliu Lazaroiu

Center for Geometry and Physics, Institute for Basic Science, Pohang 37673, Republic of Korea

Correspondence should be addressed to Elena Mirela Babalic; mbabalic@theory.nipne.ro

Received 15 October 2017; Accepted 31 January 2018; Published 19 March 2018

Academic Editor: Luigi C. Berselli

Copyright © 2018 Elena Mirela Babalic and Calin Iuliu Lazaroiu. 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 consider generalized -attractor models whose scalar potentials are globally well-behaved and whose scalar manifolds are elementary hyperbolic surfaces. Beyond the Poincaré disk , such surfaces include the hyperbolic punctured disk and the hyperbolic annuli of modulus . For each elementary surface, we discuss its decomposition into canonical end regions and give an explicit construction of the embedding into the Kerekjarto-Stoilow compactification (which in all three cases is the unit sphere), showing how this embedding allows for a universal treatment of globally well-behaved scalar potentials upon expanding their extension in real spherical harmonics. For certain simple but natural choices of extended potentials, we compute scalar field trajectories by projecting numerical solutions of the lifted equations of motion from the Poincaré half plane through the uniformization map, thus illustrating the rich cosmological dynamics of such models.

1. Introduction

In [1], we introduced a class of cosmological models which form a wide generalization of -attractors [2–7]. Such models are defined by cosmological solutions (with flat spatial section) of the Einstein theory of gravity coupled to two real scalar fields, the dynamics of the latter being described by a nonlinear sigma model whose scalar manifold is a noncompact, oriented, and topologically finite borderless surface endowed with a complete metric of constant Gaussian curvature . The scalar potential is given by a smooth real-valued function defined on . The rescaled scalar field metric has Gaussian curvature equal to ; hence is a geometrically finite hyperbolic surface in the sense of [8]. Using the uniformization theorem of Koebe and Poincaré [9] and the theory of surface groups (Fuchsian groups without elliptic elements), [1] discussed the basic features of cosmological dynamics and inflation in such models.

In the present paper, we consider the nongeneric situation when is elementary, focusing on special features which arise in this case. As explained in [1], an elementary hyperbolic surface is isometric with the hyperbolic disk , with the hyperbolic punctured disk or with the hyperbolic annulus of modulus , where is a real number. Each of these surfaces is planar, having the unit sphere as its end compactification. This allows one to parameterize globally well-behaved [1] scalar potentials through the coefficients of the Laplace expansion of their global extension to the end compactification, which in this case reduces to an expansion in real spherical harmonics. Both the hyperbolic metric and a fundamental polygon are known explicitly for and . In particular, one can describe explicitly the hyperbolic geometry of such surfaces and one can compute scalar field trajectories on by determining trajectories of an appropriate lift of the model to the Poincaré half plane and projecting them to or to through the uniformization map. When the scalar potential is constant, this illustrates how the different projections from to and can take the same trajectory on the Poincaré half plane into qualitatively different trajectories on the corresponding elementary hyperbolic surface. The explicit embedding of , , and into their common Kerekjarto-Stoilow compactification is also different. As a consequence, a smooth real-valued function defined on will generally induce rather different globally well-behaved scalar potentials on the disk, the punctured disk, and the annulus. When combined with the different projections from , this leads to qualitatively different dynamics on , , and . By considering some simple but natural choices of extended scalar potentials on , we use numerical methods to compute various trajectories on and , thus illustrating the rich dynamics of such models, which is not entirely visible in the gradient flow approximation.

The paper is organized as follows. Section 2 briefly recalls the definition of generalized -attractor models and the lift of their cosmological evolution equation to the Poincaré half plane. In Section 3, we consider globally well-behaved scalar potentials on topologically finite planar oriented surfaces (of which the elementary surfaces are particular cases), showing how they can be parameterized through the coefficients of the Laplace expansion of their extension to the end compactification. In Section 4, we review the classification of elementary hyperbolic surfaces and some of their basic properties. Section 5 recalls the case of the hyperbolic disk, showing how it fits into the general approach developed in [1] and how well-behaved scalar potentials can be described through the Laplace expansion of their extension to . Section 6 discusses the hyperbolic punctured disk. After giving the explicit form of the hyperbolic metric on , of a partial isometric embedding into three-dimensional Euclidean space, and of the decomposition into horn and cusp regions, we compute certain scalar field trajectories for a few globally well-behaved scalar potentials which are induced naturally from . This illustrates the rich dynamics of our models on the punctured hyperbolic disk. We also discuss inflationary regions and the number of e-folds for various trajectories and provide an explicit example of an inflationary trajectory which produces between 50 and 60 e-folds, thus showing that such models can match observational constraints. Section 7 performs a similar analysis for hyperbolic annuli, using globally well-behaved scalar potentials induced by the same choices of functions on . In particular, this illustrates how the dynamics of our models differs on and . Section 8 comments briefly on the relation of such models with observational cosmology, while Section 9 contains our conclusions and sketches a few directions for further research.

Notations and Conventions. All manifolds considered are smooth, connected, oriented, and paracompact (hence also second-countable). All homeomorphisms and diffeomorphisms considered are orientation-preserving. By definition, a Lorentzian four-dimensional manifold has “mostly plus” signature. The symbol denotes the imaginary unit. The Poincaré half plane is the upper half plane with complex coordinate :

endowed with its unique complete metric of Gaussian curvature , which is given by

The real coordinates on are denoted by and . The complex coordinate on the hyperbolic disk, the hyperbolic punctured disk, or an annulus is denoted by . We define the rescaled Planck mass through

where is the (reduced) Planck mass.

2. Generalized -Attractor Models

2.1. Definition of the Models

Let be a noncompact oriented, connected, and complete two-dimensional Riemannian manifold without boundary (called the scalar manifold) and be a smooth function (called the scalar potential). We say that is hyperbolic if the metric has constant Gaussian curvature equal to . We say that is topologically finite and that is geometrically finite if the fundamental group is finitely generated. Let , where is a fixed positive real number.

Let be any four-dimensional manifold which can support Lorentzian metrics. The Einstein-Scalar theory defined by the triplet includes four-dimensional gravity (described by a Lorentzian metric defined on ) and a smooth map , with action [1]:

Here is the volume form of , is the scalar curvature of , and is the (reduced) Planck mass.

When is diffeomorphic with and is a FLRW metric with flat spatial section, solutions of the equations of motion for action (4) for which depends only on the cosmological time define the class of so-called generalized -attractor models [1]. In this case, the map (where is a real interval) defines a curve in which obeys an invariantly defined nonlinear second-order ordinary differential equation (which is locally equivalent with a system of two second-order equations). We refer the reader to [1] for a general discussion of such models.

2.2. Lift to the Poincaré Half Plane

As explained in [1], the cosmological equations of motion can be lifted from to the Poincaré half plane by using the covering map which uniformizes to . This allows one to determine the cosmological trajectories by projecting to the trajectories of a “lifted” model defined on . The lifted model is governed by the following system of second-order nonlinear ordinary differential equations [1, eq. ]:

where and is the cosmological time while , are the Cartesian coordinates on the Poincaré half plane with complex coordinate and is the lifted potential. Let be any point of and let be chosen such that . An initial velocity vector defined at on and its unique lift through the differential of at are related through

Notice that the differential of at is a bijective linear map because is a covering map and hence a local diffeomorphism. Writing and , we have with real , . As shown in [1], a cosmological trajectory on with initial condition can be written as , where is the solution of lifted system (5) with initial conditions:

Eliminating the Planck Mass. Let and , where is the rescaled Planck mass (3). Then (5) becomes

showing that we can eliminate the Planck mass from the equations provided that we measure both and (and hence also ) in units of . The numerical solutions extracted in latter sections of this paper were obtained using system (8), after performing such a rescaling of and .

3. Laplace Expansion of Globally Well-Behaved Scalar Potentials

Let denote the Kerekjarto-Stoilow (a.k.a. end) compactification of (see [10, 11]) and identify with its image in through the embedding map . A smooth scalar potential is called globally well-behaved [1] if there exists a smooth function whose restriction to equals , in which case is uniquely determined by through continuity. Since all elementary hyperbolic surfaces are planar, their end compactification is diffeomorphic with the unit sphere , so in this case globally well-defined scalar potentials on correspond bijectively to smooth functions .

Let and be spherical coordinates on ; thus

where and . Any smooth map is square-integrable with respect to the round Lebesgue measure on and admits the Laplace-Fourier series expansion:

where are the complex spherical harmonics and

The series in (10) converges uniformly to on since is smooth (see [12]). Recall that , where are the associated Legendre functions. Since is real-valued, expansion (10) reduces to

where and are real constants. Equivalently, we have

where and are the real (a.k.a. tesseral) spherical harmonics, which correspond to orbitals. This expansion is again uniformly convergent and gives a systematic way to approximate by truncating away the contributions with greater than some cutoff value.

Some Particular Choices for . If only the modes with and ( and orbitals) are present, then we have

where are real constants and we used the expressions

The constant term in (14) corresponds to the orbital ) while the terms with prefactors , , and correspond to the orbitals , , and .

For , two simple choices are and , where is the rescaled Planck mass (3). These give the following -independent potentials, which involve only the orbitals and and are shown in Figure 1:

Notice that has a maximum at (north pole) and a minimum at (south pole) while has a minimum at (north pole) and a maximum at (south pole).

Figure 1: Plot of (blue) and (orange) as functions of . The value corresponds to the north pole of , while corresponds to the south pole.

Another simple choice is and , which corresponds to a linear combination of the and orbitals and gives the extended potential:

Unlike , this function does not have extrema at the north or south pole. Instead, it has two extrema along the equator of , having a maximum (equal to ) at the point and a minimum (equal to ) at . Notice that and are Morse functions on , so the potentials derived from them on a planar surface (whose end compactification is ) will be compactly Morse in the sense of [1].

A Universal Approach to Globally Well-Behaved Potentials. The techniques of passing to the end compactification and lifting to the Poincaré half plane introduced in [1] allow for a uniform treatment of globally well-behaved scalar potentials in generalized -attractor models for which is geometrically finite. This is summarized in the commutative diagram (18), where is the smooth embedding of into its end compactification and is the uniformizing map from :

Any smooth real-valued function defined on induces a globally well-behaved scalar potential on through the formula , while any globally well-behaved scalar potential on lifts to a smooth function defined on . When the maps and are known, can be recovered from as the composition , where is the composite map . Notice that and are smooth maps, while is holomorphic when is endowed with the complex structure which corresponds to the conformal class of the metric . The maps and differ for distinct geometrically finite hyperbolic surfaces having the same end compactification , which means that the same “universal” extended potential defined on can induce markedly different globally well-behaved potentials and lifted potentials for different hyperbolic surfaces of the same genus.

When is a planar surface, the end compactification coincides with the unit sphere and can be expanded into real spherical harmonics as explained above. This induces uniformly convergent expansions:

of and . In the next sections, we determine explicitly the maps , , and for the planar elementary surfaces , , and and the maps and induced by the choices and given above. This illustrates how the same function leads to different dynamics of the generalized -attractor models associated with distinct planar surfaces. For the three elementary surfaces, we will construct the map by first diffeomorphically (but not biholomorphically!) identifying with the complex plane of complex coordinate (or with with a point removed) and then identifying the latter with the once- or twice-punctured sphere by using stereographic projection from the north pole of :

4. Elementary Hyperbolic Surfaces

A (complete) hyperbolic surface is called elementary if it is conformally equivalent with a simply connected or doubly connected regular domain (a regular domain is called doubly connected if its complement in the Riemann sphere has two connected components, which happens iff ) contained in the complex plane. This amounts to the condition that the uniformizing surface group of is the trivial group or a cyclic subgroup of of parabolic or hyperbolic type.

Any simply connected regular domain is conformally equivalent with the unit disk (and hence with the upper half plane ). Such a domain admits a unique complete hyperbolic metric, known as the Poincaré metric. Any doubly connected regular domain is conformally equivalent to one of the following, when the latter is endowed with the complex structure inherited from the complex plane:

(i)The punctured plane (ii)The punctured unit disk (iii)The annulus of modulus , where .

When endowed with its usual complex structure, the punctured plane does not support a complete hyperbolic metric. On the other hand, the punctured disk and annulus admit uniquely determined complete hyperbolic metrics. Notice that both and are homeomorphic with (open) cylinders. Due to this fact, the hyperbolic punctured disk is also called the parabolic cylinder while the hyperbolic annuli are also called hyperbolic cylinders [8]. Summarizing, the list of elementary hyperbolic surfaces is as follows:

(1)The hyperbolic disk (which is isometric with the Poincaré half plane )(2)The hyperbolic punctured disk (uniformized to by a parabolic cyclic subgroup of )(3)The hyperbolic annuli for (uniformized to by a hyperbolic cyclic subgroup of ).

The explicit form of the hyperbolic metric is known in all cases, as is a fundamental polygon [13–15] for and . This allows one to study the cosmological dynamics of -attractor models defined by such surfaces either directly on or (as explained in [1]) by lifting to the hyperbolic disk or to the Poincaré half plane.

For elementary hyperbolic surfaces, the isometry classification of the ends is as follows [1, 8, 16]:

(i)The hyperbolic disk has a single end, known as a plane end.(ii)The hyperbolic punctured disk has two ends. One of these is a cusp end, the other being a horn end.(iii)The hyperbolic annulus has two ends, both of which are funnel ends.

All elementary hyperbolic surfaces are planar (i.e., of genus zero). As explained in [1], this implies that their end compactification [10, 11] is the unit sphere . On the other hand, the conformal boundary [1, 17, 18] differs in the three cases:

(i)For the hyperbolic disk, we have , where the circle corresponds to the plane end.(ii)For the hyperbolic punctured disk, we have , where the origin corresponds to the cusp end and corresponds to the horn end.(iii)For the hyperbolic annulus, we have , each of the circles corresponding to a funnel end.

Remark 1. By a theorem of Hilbert, a complete hyperbolic surface cannot be embedded isometrically into Euclidean 3-dimensional space. However, incomplete regions of such a surface can be embedded isometrically (and we shall see examples of such partial embedding in latter sections). Notice that one can sometimes find isometric embedding of complete hyperbolic surfaces into non-Euclidean 3-dimensional space, such as the well-known embedding of the Poincaré disk as a sheet of a hyperboloid defined inside three-dimensional Minkowski space.

5. The Hyperbolic Disk

The cosmological model defined by the hyperbolic disk coincides with the two-field -attractor model of [7], which was discussed extensively in the literature. The purpose of this section is to show how this fits into the general theory developed in [1].

5.1. Semigeodesic Coordinates

The unit disk admits a unique complete hyperbolic metric, which is given by

In polar coordinates given by and , the metric becomes

Semigeodesic coordinates for are obtained by the change of variables:

This maps the unit disk (diffeomorphically, but not conformally) to the complex plane with polar coordinates and complex coordinate

and brings the metric to the form

The single end of (which is called a plane end) corresponds to , while the center of corresponds to .

5.2. The End Compactification of  

The end compactification of the hyperbolic disk coincides with the Alexandroff compactification of the -plane, which by the stereographic projection (20) is identified with the unit sphere . The north pole corresponds to the plane end at , while the south pole corresponds to , that is, to the center of . In spherical stereographic coordinates , the Poincaré metric (25) becomes

Remark 2. The compact Riemann surface into which is embedded holomorphically [17, 18] is the Riemann sphere associated with the -plane. The coordinate is given by

where are the homogeneous coordinates of .

5.3. Globally Well-Behaved Scalar Potentials on

A potential is globally well-behaved on iff there exists a smooth function such that

that is,

The condition that is smooth on implies, in particular, that has a finite limit for and hence has a -independent limit for , that is, for . Expansion (13) gives the uniformly convergent series:

To obtain inflationary behavior with the scalar field rolling from the plane end toward the interior of , one can require that has a local maximum at the north pole of . In the simplest models, one can take to have only two critical points, namely, a global maximum at the north pole and a global minimum at the south pole. In that case, has a global minimum at (the center of ) and increases monotonically to -independent finite value as grows from zero to (toward the conformal boundary of ).

In polar coordinates , the extended potentials (16) of Section 3 correspond to the following globally well-behaved scalar potentials on :

where . These potentials are shown in Figures 2(a) and 2(b), which illustrate the characteristic stretching toward the end when the potential is expressed in semigeodesic coordinates (with respect to the fact that the two locally defined real scalar fields of the sigma model have canonical kinetic terms). The supremum of corresponds to (being equal to ), while the infimum is attained at (where vanishes). On the other hand, tends to its vanishing infimum for and has a maximum at (where it equals ). Notice that only leads to standard -attractor behavior.

(a) Plot of (blue) and (orange) as functions of . The value corresponds to the center of , while corresponds to the conformal boundary . Only the potential leads to -attractor behavior when inflation takes place near the plane end

(b) Plot of (blue) and (orange) as functions of . The value corresponds to the center of , while corresponds to the plane end. Only the potential leads to -attractor behavior when inflation takes place near the plane end

Figure 2: The potentials on the Poincaré disk.

Using the relation , choice (17) gives the following potential on :

6. The Hyperbolic Punctured Disk

The hyperbolic punctured disk (also known as the “parabolic cylinder” [8]) is the simplest example of a hyperbolic surface with a cusp end. It also has a horn end.

6.1. The Hyperbolic Metric

The punctured unit disk admits a unique complete hyperbolic metric given by [19]

In particular, and are isothermal coordinates. In polar coordinates defined through

the metric takes the form

The center of corresponds to the cusp end, while the bounding circle of corresponds to the horn end (see below). The Euclidean circle at is a horocycle of hyperbolic length . Notice that has infinite hyperbolic area.

6.2. Diffeomorphism to the Punctured Plane

One can introduce an orthogonal coordinate system on through the coordinate transformation:

This gives a diffeomorphism between and the punctured complex plane with complex coordinate:

In this coordinate system, metric (33) takes the form

The center of corresponds to , while the bounding circle of corresponds to .

6.3. Partial Isometric Embedding into Euclidean 3-Dimensional Space

One can isometrically embed the portion of the hyperbolic punctured disk into Euclidean as the open half tractricoid (the surface obtained by revolving an open half of a tractrix along its asymptote [20])   defined in cylindrical coordinates (where and ) by the parametric equations:

Indeed, it is easy to see that the Euclidean metric of induces a metric on which coincides with (38). This is the classical pseudosphere model of Beltrami (see Figure 3).

Figure 3: Upper half of a tractricoid (pseudosphere). The horocycle of length one is drawn in light blue. It separates the half tractricoid into a cusp (upper part) and a portion of a horn (lower part). The latter is bounded by a horocycle of length .

6.4. The End Compactification of

The stereographic projection (20) identifies with the one-point compactification of the -plane. This shows explicitly that is the end compactification of , where the north pole corresponds to the horn end and the south pole corresponds to the cusp end. The embedding is given by

6.5. Semigeodesic Coordinates

The further change of variables

brings metric (38) to the form

where . In particular, are semigeodesic coordinates. The center of corresponds to while the bounding circle of corresponds to . The horocycle at (i.e., ) has length .

6.6. The Hyperbolic Cusp

Let

As mentioned above, the Euclidean circle has hyperbolic length . The hyperbolic cusp (cf. [1]) corresponds to the portion of lying inside this circle (see Figure 3), which is the open punctured disk:

endowed with the restriction of the metric (33); notice that the restricted metric is not complete. In coordinates , the metric on is obtained by restricting (38) to the range , where

In semigeodesic coordinates the cusp metric is given by (42) with the restriction . Notice that has hyperbolic area equal to .

6.7. The Hyperbolic Horn

By definition, the hyperbolic horn is the annulus:

endowed with the (incomplete) restriction of metric (33). In coordinates , the horn metric is obtained from (38) by restricting the range of to . In coordinates , the metric takes form (42), with the restriction . Defining , this can be brought to the form

where the bounding circle of corresponds to .

6.8. Canonical Uniformization to

The punctured disk is uniformized to the Poincaré half plane (1) with complex coordinate by the parabolic cyclic group generated by the translation:

which corresponds to the parabolic element:

This fixes the point . The uniformization map is

The hyperbolic cusp is the projection through of the cusp domain:

which is bounded by the horocycle:

This horocycle is tangent to the conformal boundary of at the point , which projects through to the cusp ideal point of the end compactification of .

A fundamental polygon for the action of on is given by the semi-infinite vertical strip

and has vertices at the points (see Figure 4(a)):

The Poincaré side pairing maps into through transformation (48), which generates . The relative cusp neighborhood [1] with respect to is the intersection . A lifted scalar potential is given by

being invariant under translation (48). In particular, the restrictions of to the sides and agree through the Poincaré pairing.

(a) A fundamental polygon for the punctured disk on . The relative cusp neighborhood of the vertex corresponds to the shaded region

(b) A fundamental polygon for the punctured disk on . The shaded region is the relative cusp neighborhood of the vertex

Figure 4: Fundamental polygons for the uniformization of to the Poincaré half plane and to the hyperbolic disk.

6.9. Canonical Uniformization to

For completeness and comparison with [1], we also give the canonical uniformization of to the hyperbolic disk. The Cayley transformation

is an isometry from to . When uniformizing to , the fundamental polygon becomes a hyperbolic triangle with vertices at the following points, which correspond, respectively, to the points , , and of (54) through the Cayley transformation

and a free side connecting the points and (see Figure 4(b)). The sides of this triangle are a segment connecting to (which passes through the origin of ), the portion of connecting to , and Euclidean circular arc orthogonal to which connects to . The hyperbolic cusp neighborhood of the vertex is bounded by the horocycle:

which is tangent to at the point . The intersection of with is the relative cusp neighborhood with respect to (see [1]), which is the image of through the Cayley transformation.

6.10. Globally Well-Behaved Scalar Potentials on

A scalar potential is globally well-behaved on iff there exists a smooth map such that

that is,

where . Expansion (13) gives the uniformly convergent series:

For choices (16) and (17), we find

These potentials are shown in Figure 5. Notice that leads to -attractor behavior if inflation takes place near the horn end, while leads to -attractor behavior if inflation takes place near the cusp end [1]. The extended potentials have maxima and minima at the two ideal points of the end compactification of (which correspond to the north and south poles of ). On the other hand, does not have extrema at the ideal points; its extrema coincide with those of , being located inside . The minimum (equal to zero) is at the point while the maximum (equal to ) is at .

(a) Plot of (blue) and (orange) as functions of . The value corresponds to the cusp end, while corresponds to the horn end

(b) Level plot of on the punctured disk. Darker tones indicate lower values of

Figure 5: The potentials and on the hyperbolic punctured disk.

Remark 3. Using (41), we find the following expressions in semigeodesic coordinates:

Lift of the Potentials and to . Consider the well-behaved scalar potentials and on given in (62). Let and . Then the covering map (50) reads

which gives

Hence the potentials and have the following lifts to (see Figure 6):

The function (which is periodic under ) attains its minimum at the points (with ), while the maximum is attained for with .

(a) Plot of (blue) and (orange) as functions of . The values and correspond, respectively, to the horn and cusp ends

(b) Level plot of . Higher values of correspond to lighter tones. The fundamental domain is bounded by the red lines

Figure 6: The lifted potentials and .

6.11. Cosmological Trajectories on the Hyperbolic Punctured Disk

In this subsection, we present examples of numerically computed trajectories on for the vanishing scalar potential and for the globally well-behaved scalar potentials and . These were obtained as explained in Section 2.2, by numerically computing solutions of system (8) on the Poincaré half plane for the corresponding lifted potentials and then projecting these trajectories to the hyperbolic punctured disk using explicitly known uniformization map (50) (which is equivalent to (65)).

Trajectories for Vanishing Scalar Potential. To understand the effect of the hyperbolic metric on the dynamics, we start with the case of a vanishing scalar potential . Then and one immediately checks that straight lines given by constant functions , are solutions of (5) for any initial point , with initial velocity zero. This means that a scalar field starting “at rest” remains at rest for all times. On the other hand, numerical computation shows that any solution of (5) tends to the real axis for , irrespective of its initial conditions. As a consequence, any solution defined on (which is obtained by projecting a solution defined on through map (50)) will tend toward the horn end as . This shows that the hyperbolic metric acts as an effective force which repulses away from the cusp end. Notice that a global trajectory defined on can have cusp and self-intersection points and hence that it need not correspond to an embedded curve in .

Figure 7 shows five trajectories on and their projections to for and , with the initial conditions shown in Table 1.

Table 1: Initial conditions for five trajectories on the Poincaré half plane.

Trajectory

Orange

Yellow

Red

Blue

Magenta

(a) Trajectories for on the Poincaré half plane. The solutions shown in orange and yellow are stationary

(b) Projection of the trajectories shown at the left to the hyperbolic punctured disk

Figure 7: Numerical solutions for and .

Trajectories for . Five lifted trajectories (and their projections to ) for and with the initial conditions given in Table 1 are shown in Figure 8. Since has a maximum at the cusp end (center of the disk) and a minimum at the horn end, it reinforces the effect of the hyperbolic metric, together with which it produces an effective repulsion away from the cusp end. In particular, the two trajectories which start with vanishing initial velocity are no longer stationary but evolve for to the funnel end (see the orange and yellow trajectories in the figures). Any other trajectory (irrespective of its initial velocity) also evolves for to the funnel end.

(a) Trajectories for on the Poincaré half plane, drawn over a level plot of on

(b) Projection of the trajectories shown at the left to the hyperbolic punctured disk. We also show a level plot of on

Figure 8: Numerical solutions for and .

Trajectories for . Five lifted trajectories for and (and their projections to ) with the initial conditions given in Table 1 are shown in Figure 9. Since has a minimum at the cusp end (which corresponds to the center of the disk), it produces an attractive force toward the cusp end, which acts as a counterbalance to the repulsive effect of the hyperbolic metric.

(a) Trajectories for on the Poincaré half plane and a level plot of on  

(b) Projection of the trajectories shown at the left to the hyperbolic punctured disk and a level plot of on

Figure 9: Numerical solutions for and .

Trajectories for . Five lifted trajectories (and their projections to ) for and with the initial conditions given in Table 1 are shown in Figure 10.

(a) Trajectories for on the Poincaré half plane

(b) Projection of the trajectories shown at the left to the hyperbolic punctured disk

Figure 10: Numerical solutions for and .

Figure 11 shows in more detail the behavior of the magenta trajectory of Figure 10(a) near the local minimum of located at (which projects to ) and of its projection to . For this solution, evolves in a spiral around the minimum of , until it settles at the minimum.

(a) Detail of the magenta trajectory shown in Figure 10(a) near one minimum of

(b) Projection of the trajectory shown at the left to the hyperbolic punctured disk

Figure 11: Detail of the magenta trajectory.

6.12. Inflationary Regions and Number of e-Folds

Recall from [21] the expressions for the Hubble parameter and the critical Hubble parameter:

For the inflationary regions the following inequality should be satisfied:

meanwhile the number of e-folds is given by integrating over the first inflationary time interval :

Analyzing the five trajectories with the initial conditions given in Table 1, we find that only the orange and yellow trajectories start in inflationary regime for each of the three potentials , while the other three trajectories do not start in inflationary regime for any of these potentials. We also notice that the yellow trajectory in is in inflationary regime for all times. Calculating the number of e-folds, we find for the orange trajectory the values for the potential and for , while the yellow trajectory has e-folds for the potential (so it satisfies the observational requirement of – e-folds) and for . Varying the initial conditions of the yellow trajectory in the potential (namely, varying in the range ) produces many other trajectories with lying in the observationally expected range of e-folds.

7. Hyperbolic Annuli

Hyperbolic annuli (also known as “hyperbolic cylinders” [8]) have a single modulus and two funnel ends.

7.1. The Hyperbolic Metric

Let be a real number. The annulus of modulus admits a unique complete hyperbolic metric, which is given by [19]

In particular, and are isothermal coordinates. Notice that the transformation is an isometry. In polar coordinates defined through

the metric takes the form

For any , the Euclidean circle has hyperbolic circumference given by

We have

Notice that increases from to infinity as increases from to and as decreases from to . In particular, the minimum hyperbolic length is attained for the circle of Euclidean radius , which is the only closed hyperbolic geodesic of and has hyperbolic length:

known as the hyperbolic circumference of . The lines

are geodesics of infinite length. Notice that the hyperbolic area of is infinite. Relation (75) gives

7.2. Diffeomorphism to the Punctured Disk and to the Punctured Plane

The annulus is diffeomorphic (but not biholomorphic!) with the punctured unit disk with complex coordinate through the map:

whose inverse is given by

Notice that maps the Euclidean circle to and the Euclidean circle to the Euclidean circle . Composing with the map (36) gives a diffeomorphism from to the punctured complex plane with coordinate:

where

The funnel end at corresponds to while the funnel end at corresponds to .

7.3. The End Compactification of

The end compactification of is the unit sphere with polar coordinates , which is mapped to the -plane through stereographic projection (20). The funnel end at corresponds to the south pole, while the funnel end at corresponds to the north pole. The explicit embedding of into is given by

7.4. The Hyperbolic Funnel

Let be a positive real number and be defined as in (77). By definition, a hyperbolic funnel (cf. [1]) of circumference is the annulus:

endowed with the restriction of the metric (70). Since is an isometry of , the funnel is isometric with the annulus (endowed with the restriction of (70)). Hence decomposes as the disjoint union of two funnels and the closed geodesic . Notice that a funnel has infinite hyperbolic area. The funnel is diffeomorphic (but not conformally equivalent!) with the punctured unit disk through the map:

which takes the funnel end to the center of the unit disk and the circle into the bounding circle of the unit disk.

7.5. Semigeodesic Coordinates on

Consider orthogonal coordinates on given by

where and is the polar angle in the -plane. The limit corresponds to , while corresponds to . In these coordinates, the metric on becomes

The coordinates and

are semigeodesic on . In these coordinates, the metric takes the form

The limit corresponds to while corresponds to . The parameter is the hyperbolic length of the geodesic at .

7.6. Partial Isometric Embedding of the Funnel into Euclidean 3-Dimensional Space

One can isometrically embed the annulus corresponding to the range

into Euclidean as the surface of revolution defined by the parametric equations (see [20, Chap. 3C] or [22, Chap. 15]):

where

and denotes the elliptic integral of the second kind. This is one of the three types of (incomplete) classical surfaces of revolution in of constant Gaussian curvature equal to , namely, a surface of “hyperboloid type” (see Figure 14). The other two are the pseudosphere/tractricoid (which corresponds to a portion of the hyperbolic cusp) and the surface of “conical type.”

7.7. Canonical Uniformization to

The hyperbolic annulus is uniformized to the upper half plane by the hyperbolic cyclic subgroup generated by the transformation:

which corresponds to the hyperbolic element

and fixes the points and lying on . The uniformization map is

which gives

A fundamental polygon is given by (see Figure 15(a))

This is a hyperbolic quadrilateral with two free sides and vertices located at the points

The funnel is the projection of the relative funnel neighborhood [1]:

7.8. Canonical Uniformization to

When passing to the disk model, the fundamental domain is mapped by the Cayley transformation (56) to a hyperbolic quadrilateral with vertices located at the following points, which are obtained from the points defined in (97) by applying (56):

The free sides and are portions of (see Figure 15(b)), while the sides and are arc segments of Euclidean circles which are orthogonal to .

7.9. Globally Well-Behaved Scalar Potentials on

A scalar potential on is globally well-behaved iff there exists a smooth function such that

that is,

Expansion (13) gives the uniformly convergent series:

For choices (16), we find

where and we used (81). The potentials are plotted in Figure 16(a) for the case (). For choice (17), we find

Recall that has two extrema on , which are located at (maximum) and (minimum). At each of these points, relation (20) gives , so (81) gives , where

It follows that the two critical points of on are located on the real axis at

(i), where attains its maximum (which equals ),(ii), where attains its minimum (which equals zero).

The level curves of are shown in Figure 16(b) for the case (), which gives .

Lift of and to . The globally well-behaved scalar potentials (103) and (104) lift to the following potentials on (see Figures 17(a) and 17(b)):

where (see (95)). The lifted potential has local maxima at the inverse image points of and local minima at the inverse image points of (see (105)). Using (95), we find that these are located at

(i)(maxima) (where equals ),(ii)(minima) (where vanishes),

with being an arbitrary integer. Hence all extrema of lie on the half-line through the origin which makes an angle with the -axis of the -plane. The maxima and minima alternate along this half-line and the ratio between the absolute values of two consecutive maxima or two consecutive minima equals ; in particular, the extrema accumulate toward the origin along . The fundamental domain contains exactly one of the minima, namely, that located at . On the other hand, the two nonfree sides of contain the two consecutive maxima located at and ; these two maxima of are identified by the projection .

In Figure 17(b), we show the level curves of for the case (), which gives and . In this case, the absolute value of the ratio of successive minima or maxima of equals . Due to the large size of this ratio, we chose for clarity to display the level curves of on a region of the “semilogarithmic upper half plane.” The latter has complex coordinate (where and ), being related to the region

of the Poincaré half plane through the coordinate transformation:

Notice that contains only those extrema of which have absolute value larger than one. In the semilogarithmic half plane, these extrema are located at with (maxima) and with (minima), lying equally spaced on the half-line in the -plane which passes through the origin at angle with the -axis. The Cartesian coordinates and , are related through

Figure 17(b) shows the level curves of the function in the region defined by and (where ), which contains the image of the following annular region of the Poincaré half plane:

Notice that contains a copy of the fundamental domain .

7.10. Lift of the Cosmological Model to

We now present examples of trajectories on for (, modulus ) for the vanishing scalar potential and for the globally well-behaved scalar potentials and . These were obtained as explained in Section 2.2, by numerically computing solutions of the system (8) on the Poincaré half plane for the corresponding lifted potentials and then projecting these trajectories to the hyperbolic punctured disk using the explicitly known uniformization map (94), which is equivalent to (95).

Trajectories for Vanishing Scalar Potential. Figure 18 shows five trajectories (orange, yellow, red, blue, and magenta) for , (), and , with the initial conditions given in Table 1. In this case, vanishing initial velocity leads to two stationary trajectories (the orange and yellow dots), while the hyperbolic geometry produces an effective attraction toward the outer funnel end for and toward the inner funnel end for . The red, blue, and magenta trajectories start in the funnel region given by (with velocities pointing toward the outer funnel end) and hence evolve toward that end.

Trajectories for . Figure 19 shows five trajectories (orange, yellow red, blue, and magenta) for , (), and , with the initial conditions given in Table 1. In this case, the potential produces a repulsive force away from the inner funnel end and an attraction force toward the outer end, thus accentuating the effect of the hyperbolic metric on the trajectories shown.

Trajectories for . Figure 20 shows five trajectories (orange, yellow red, blue, and magenta) for , () and , with the initial conditions given in Table 1. In this case, the potential induces an attractive force toward the inner funnel end. As a consequence, each of the red, blue, and magenta trajectories turns at some point in and evolves back toward the inner funnel end.

Trajectories for . Figure 21 shows five trajectories (orange, yellow, red, blue, and magenta) for , (), and , with the initial conditions given in Table 1. Figure 22 shows a detail of the yellow trajectory on both and . It spirals in a complicated manner around a minimum point of and projects to the single minimum of on located at . For , the trajectory reaches the minimum point.

7.11. Inflationary Regions and the Number of e-Folds

Using relations (67)–(69), we find that, among the five trajectories with initial conditions given in Table 1, the orange and yellow trajectories start in inflationary regime for all three potentials , and (see Figures 12, 13, and 23), while the blue trajectory is always inflationary in potential but is never inflationary for the other potentials. The orange and yellow trajectories give, respectively, and e-folds in the potential after the first inflationary regime, which lasts for a cosmological time of  s, respectively,  s. The other trajectories do not start in the inflationary regime for any of the three potentials. It is easy to see that one can rescale the potential by a positive constant in order to reach a phenomenologically appropriate value for each of the orange and yellow trajectories in this potential.

Figure 12: Plot of (black) and (green) for the yellow trajectory in the potential . For better clarity, we truncated the plot at  s.

Figure 13: Plots of (black) and (green) for the yellow trajectory in the potential and for the orange trajectory in the potentials and , respectively. For better clarity, we truncated the plots at convenient values of .

Figure 14: Hyperbolic surface of revolution of hyperboloid type. The upper half corresponds to the region of . The circle drawn in light blue is the closed geodesic of length .

(a) Fundamental domain for on the Poincaré half plane. The shaded region is the relative funnel neighborhood corresponding to the free side on the left

(b) Fundamental domain for on the Poincaré disk. The shaded region is the relative funnel neighborhood of the free side on the left

Figure 15: Fundamental domains for the uniformization of the hyperbolic annulus to the Poincaré half plane and to the hyperbolic disk.

(a) Plot of (blue) and (orange) as functions of for . The values and correspond to the two funnel ends of

(b) Level plot of on for . Darker tones indicate lower values of . In this case, the maximum of is located at , while the minimum is located at

Figure 16: The potentials and on the annulus of modulus (, ). For and , we indicate only the dependence of , since these two potentials do not depend on the polar angle . Notice that tends to its infimum at the inner funnel end, while tends to its infimum at the outer funnel end. Similarly, and tend to their suprema at opposite funnel ends.

(a) Plot of (blue) and (orange) as functions of for . The values and correspond to the two funnel ends of

(b) Level plot of in the coordinates for . Darker tones indicate lower values of . The maxima of are indicated as blue dots, while the minima are shown as small red squares

Figure 17: The lifted potentials and for the annulus of modulus (, ).

(a) For , the five trajectories on with initial conditions given in Table 1 coincide with those shown in Figure 7(a)

(b) Projection to of the trajectories shown at the left. The projection differs from that for (cf. Figure 7(b)) since the uniformization maps are different

Figure 18: Five trajectories for , , and , with initial conditions as in Table 1.

(a) Trajectories on for , superimposed on a level plot of on

(b) Projection to of the trajectories shown at the left, superimposed on a level plot of on

Figure 19: Five trajectories for , , and with initial conditions of Table 1.

(a) Trajectories on for , superimposed on a level plot of on

(b) Projection to of the trajectories shown at the left, superimposed on a level plot of on

Figure 20: Five trajectories for , , and , with initial conditions of Table 1.

(a) Trajectories on for . The yellow trajectory is not stationary, although at the scale shown it looks like a dot

(b) Projection to of the five trajectories shown at the left

Figure 21: Five trajectories for when and .

(a) Detail of the lifted yellow trajectory shown in Figure 21(a) near one minimum of

(b) Projection of the yellow trajectory to the hyperbolic annulus. The trajectory spirals in a complicated manner around the minimum of , until it settles at the minimum point

Figure 22: Detail of the yellow trajectory shown in Figure 21.

Figure 23: Plots of (black) and (green) for the orange and yellow trajectories in the potential . For better clarity, we truncated the plot at  s. For both trajectories, inflation stops after a finite time (indicated in the text), though this is not visible at the scale of the figures.

8. On the Relation to Observational Cosmology

As explained in [1], generalized -attractor models based on any geometrically finite noncompact hyperbolic Riemann surface and with a well-behaved scalar potential enjoy universal behavior near each end of where the extended potential has a local maximum, in a certain one-field truncation near that end. Within this approximation, such models make the same predictions for the spectral index and the tensor to scalar ratio as ordinary -attractors, provided that inflation takes place sufficiently close to such an end and along a trajectory which proceeds radially from that end in canonical local semigeodesic coordinates. In the slow-roll approximation for motion along such a trajectory, one finds [1]

where is the number of e-folds during the inflationary period. For such trajectories, generalized -attractors are therefore as promising for matching observational data as ordinary -attractors, whose agreement with current observations is quite good [2–5]. For the models considered in this paper, such special trajectories are the radial trajectories on the punctured disk and on the annulus, when inflation takes place close to any of the components of the conformal boundary. We gave two explicit examples of such trajectories:

(i)The yellow trajectory for the hyperbolic punctured disk in potential (see Figure 9(b)): as discussed in Section 6.12, this trajectory produces e-folds, a value which lies in the observationally expected range of – e-folds.(ii)The orange and yellow trajectories for the hyperbolic annulus in potential (see Figure 20(b)): as discussed in Section 7.11, both of these trajectories produce around e-folds, but a constant positive rescaling of the potential allows one to bring the number of e-folds within the phenomenologically desired range .

Unlike the one-field -attractors usually considered in the literature, generalized -attractors are genuine two-field models and hence they can incorporate corrections to the traditional paradigm of inflationary cosmology, which assumes for simplicity that the inflaton is a single real scalar field. While current observational data can be successfully reproduced by various one-field models, they can also be reproduced by multifield models and it is generally deemed quite possible that, within the next decade, improved measurements could detect deviations from one-field model predictions. The recognition of this possibility has led to renewed interest in the study of multifield models [23–26] and in particular to the development of numerical methods for determining the effect of their cosmological perturbations [27–29] beyond the limitations of the SRST approximation [30, 31]. As a single example, see [32] for a recent investigation of constraints imposed on such models by Planck 2015 data [33].

In the context of the elementary generalized -attractor models considered in this paper, deviations from the one-field paradigm could be visible, for example, for trajectories which depart slightly from the radial trajectories discussed above. This will affect the two-point correlators which determine cosmological observables, thus producing subleading corrections to relations (111). Generalized -attractor models are also interesting for investigations of the postinflationary period of a given trajectory, for which generic two-field models have low predictive power since they depend on the choice of an arbitrary metric for the target manifold of the scalar fields. By contrast, generalized -attractors provide a natural class of two-field models which have universal behavior in the truncated inflationary regime near the ends, while at the same time allowing for remarkable dynamical complexity beyond that regime. As shown in the previous sections, even the simplest instances of such models (namely, those based on elementary hyperbolic surfaces) already allow trajectories of considerable complexity, due to the interplay between the effective force induced by the hyperbolic geometry and that induced by the scalar potential. Such models could therefore play the role of a natural testing ground of two-field model technology, in a mathematically tractable framework which may allow one to develop insights deeper than those afforded by current approximations and by generic numerical methods.

We end by mentioning that it is a nontrivial task to embed cosmological models with a single real scalar field within fundamental quantum theories of gravity such as string theory in a manner which is compatible with all phenomenological and self-consistency constraints. In particular, most scalar fields which arise naturally in closed string theory are complex-valued and one has to rely on special and quite finely tuned constructions when embedding single field models in string theory in a consistent and phenomenologically reasonable manner. Naturality arguments might therefore suggest that the inflaton could in fact be a complex-valued field in a fundamental theory of gravity, thus leading to a two-field cosmological model. In this context, we mention that generalized -attractors appear to have natural string-theoretic realizations which involve F-theory backgrounds with discrete fluxes, though a proper discussion of that construction (which involves the theory of modular curves and Shimura varieties) lies well outside the scope of the present paper.

9. Conclusions and Further Directions

We studied generalized -attractor models defined by elementary hyperbolic surfaces, showing how they fit into the framework developed in [1]. Following a “universal” approach to globally well-behaved scalar potentials, we showed how they can be approximated systematically using the Laplace expansion of their extension to the end compactification (which in such models is the unit sphere) and how a smooth real-valued map defined on the latter induces different potentials on each elementary hyperbolic surface. We also illustrated cosmological dynamics of generalized -attractor models by numerically extracting various trajectories for the cases of and , finding rather complex behavior even for relatively simple scalar potentials. From the universal perspective followed here and in [1], the difference between models with globally well-behaved scalar potential defined on various hyperbolic surfaces of the same genus is captured by two maps, namely, the uniformization map and the map which embeds the given surface into its end compactification. For elementary surfaces, both of these maps can be constructed explicitly and hence their effects on the cosmological dynamics can be explored systematically.

A similar approach could in principle be followed for any geometrically finite hyperbolic surface. In this regard, it would be natural to explore the large class of nonelementary planar surfaces, which form a classical subject in complex analysis and uniformization theory. For such surfaces, the uniformization map is not usually known explicitly and, at least for general values of the moduli, it must be determined numerically. Despite this fact, a fundamental domain is known for any planar surface, as are certain other properties of the hyperbolic metric and of the uniformization map [34, 35]. This allows one to approach generalized -attractor models whose scalar manifolds are given by such surfaces using the general algorithm proposed in [1, Sec. 7]. For example, it would be interesting to perform a detailed study of cosmological trajectories for some triply connected nonelementary planar surfaces such as the twice-punctured disk [36–39] and once-punctured annulus [40].

We also commented briefly on the potential relevance of such models to observational cosmology. As pointed out in Section 8, the models considered in this paper share the general features discussed in [1] and hence provide reasonable candidates for reproducing current observational constraints, similar to ordinary -attractors. In particular, they easily support cosmological trajectories which produce the expected number of – e-folds. Of course, much deeper investigation of such models is needed before the question of their phenomenological relevance can be answered fully.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

The work of Elena Mirela Babalic and Calin Iuliu Lazaroiu was supported by Grant IBS-R003-S1. The authors thank C. S. Shahbazi for participation in the initial stages of the project.

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