NNSC-Cobordism of Bartnik Data in High Dimensions

In this short note, we formulate three problems relating to nonnegative scalar curvature (NNSC) fill-ins. Loosely speaking, the first two problems focus on: When are (n−1)-dimensional Bartnik data (Σⁿ⁻¹ᵢ, γᵢ, Hᵢ), i=1,2, NNSC-cobordant? If (𝕊ⁿ⁻¹, γₛₜd, 0) is positive scalar curvature (PSC) cobordant...

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Published in:Symmetry, Integrability and Geometry: Methods and Applications
Date:2020
Main Authors: Hu, Xue, Shi, Yuguang
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Language:English
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author Hu, Xue
Shi, Yuguang
author_facet Hu, Xue
Shi, Yuguang
citation_txt NNSC-Cobordism of Bartnik Data in High Dimensions. Xue Hu and Yuguang Shi. SIGMA 16 (2020), 030, 5 pages
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description In this short note, we formulate three problems relating to nonnegative scalar curvature (NNSC) fill-ins. Loosely speaking, the first two problems focus on: When are (n−1)-dimensional Bartnik data (Σⁿ⁻¹ᵢ, γᵢ, Hᵢ), i=1,2, NNSC-cobordant? If (𝕊ⁿ⁻¹, γₛₜd, 0) is positive scalar curvature (PSC) cobordant to (Σⁿ⁻¹₁,γ₁, H₁), where (𝕊ⁿ⁻¹, γₛₜd) denotes the standard round unit sphere, then (Σⁿ⁻¹₁,γ₁, H₁) admits an NNSC fill-in. Just as Gromov's conjecture is connected with the positive mass theorem, our problems are connected with the Penrose inequality, at least in the case of n=3. Our third problem is on Λ(Σⁿ⁻¹, γ) defined below.
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fulltext Symmetry, Integrability and Geometry: Methods and Applications SIGMA 16 (2020), 030, 5 pages NNSC-Cobordism of Bartnik Data in High Dimensions Xue HU † and Yuguang SHI ‡ † Department of Mathematics, College of Information Science and Technology, Jinan University, Guangzhou, 510632, P.R. China E-mail: thuxue@jnu.edu.cn ‡ Key Laboratory of Pure and Applied Mathematics, School of Mathematical Sciences, Peking University, Beijing, 100871, P.R. China E-mail: ygshi@math.pku.edu.cn Received January 22, 2020, in final form April 13, 2020; Published online April 20, 2020 https://doi.org/10.3842/SIGMA.2020.030 Abstract. In this short note, we formulate three problems relating to nonnegative scalar curvature (NNSC) fill-ins. Loosely speaking, the first two problems focus on: When are (n − 1)-dimensional Bartnik data ( Σn−1 i , γi, Hi ) , i = 1, 2, NNSC-cobordant? (i.e., there is an n-dimensional compact Riemannian manifold ( Ωn, g ) with scalar curvature R(g) ≥ 0 and the boundary ∂Ω = Σ1 ∪ Σ2 such that γi is the metric on Σn−1 i induced by g, and Hi is the mean curvature of Σi in ( Ωn, g ) ). If ( Sn−1, γstd, 0 ) is positive scalar curvature (PSC) cobordant to ( Σn−1 1 , γ1, H1 ) , where ( Sn−1, γstd ) denotes the standard round unit sphere then ( Σn−1 1 , γ1, H1 ) admits an NNSC fill-in. Just as Gromov’s conjecture is connected with positive mass theorem, our problems are connected with Penrose inequality, at least in the case of n = 3. Our third problem is on Λ ( Σn−1, γ ) defined below. Key words: scalar curvature; NNSC-cobordism; quasi-local mass; fill-ins 2020 Mathematics Subject Classification: 53C20; 83C99 Dedicate this paper to Professor Misha Gromov on the occasion of his 75th birthday. Bartnik data ( Σn−1, γ,H ) consists of an (n − 1)-dimensional orientable Riemannian mani- fold ( Σn−1, γ ) and a smooth function H defined on Σn−1 which serves as the mean curvature of Σn−1. One basic problem in Riemannian geometry is to study: under what conditions is it that γ is induced by a Riemannian metric g with nonnegative scalar curvature, for example, defined on Ωn, and H is the mean curvature of Σ in ( Ωn, g ) with respect to the outward unit normal vector? Indeed, this problem was proposed by M. Gromov recently (see [8, Problem A] and [9, Sections 3.3 and 3.6]). On the other hand, when n = 3, for each Bartnik data ( Σ2, γ,H ) may be associated with certain quasi-local masses, for instance, when the Gaussian curvature K of γ is positive, (S2, γ) can be isometrically embedded into R3 with mean curvature H0 (with respect to the outward unit normal vector of the embedded image in R3), with this embedding we may define Brown–York mass for ( S2, γ,H ) [4, 5] as mBY ( S2; γ,H ) = 1 8π ˆ S2 (H0 −H) dσγ . This paper is a contribution to the Special Issue on Scalar and Ricci Curvature in honor of Misha Gromov on his 75th Birthday. The full collection is available at https://www.emis.de/journals/SIGMA/Gromov.html mailto:thuxue@jnu.edu.cn mailto:ygshi@math.pku.edu.cn https://doi.org/10.3842/SIGMA.2020.030 https://www.emis.de/journals/SIGMA/Gromov.html 2 X. Hu and Y. Shi If ( S2, γ,H ) admits an NNSC fill-in and H > 0, it was shown that mBY ( S2; γ,H ) ≥ 0 [22]. There are several pieces of interesting work on NNSC fill-ins relating to positivity of Brown– York mass (for instance see [13, 14]). Obviously, positivity of Brown–York mass is one necessary condition for the existence of such a fill-in, but it is far from sufficient. It was shown that for Bartnik data ( S2, γ,H ) with positive Gaussian curvature and H > 0, let H0 be the mean curvature of isometric embedding of ( S2, γ ) in R3, if mBY ( S2; γ,H ) = 0 and H 6= H0 then there is a constant ε depending only on ( S2, γ,H ) such that for any H̃ > H − ε, ( S2, γ, H̃ ) admits no NNSC fill-ins [14, Theorem 3]. If K > −κ2 where κ is a constant, then ( S2, γ ) can be isometrically embedded into the hy- perbolic space with constant sectional curvature −κ2, and we can make use of such embedding to define a generalized Brown–York mass, moreover if H > 0 we were able to prove its posi- tivity [25]. Clearly, this positivity of generalized Brown–York mass is also a kind of necessary condition for the Bartnik data with K > −κ2 and H > 0 to admit NNSC fill-ins. For Bartnik data ( Σ2, γ,H ) , we can define its Hawking mass as following: mH(Σ, γ,H) = √ Area(Σ) 16π ( 1− 1 16π ˆ Σ H2 dσγ ) . It should be interesting to explore similar relation between Hawking mass or other quasi-local masses of the Bartnik data with its NNSC fill-ins. Unfortunately, it is not easy to obtain a lower bound of the Hawking mass which depends only on ( Σ2, γ ) . In the investigation of above Gromov’s NNSC fill-in problem, we often need to deal with NNSC-cobordisms of Bartnik data which may have its own interests. More specifically, given Bartnik data ( Σn−1 i , γi, Hi ) , i = 1, 2, we say ( Σn−1 1 , γ1, H1 ) is NNSC-cobordant to ( Σn−1 2 , γ2, H2 ) if there is an orientable n-dimensional manifold ( Ωn, g ) with ∂Ωn = Σn−1 1 ∪ Σn−1 2 , R(g) ≥ 0, γi = g|Σi, i = 1, 2, H1 is the mean curvature of Σn−1 1 in ( Ωn, g ) with respect to inward unit normal vector, and H2 is the mean curvature of Σn−1 2 in ( Ωn, g ) with respect to outward unit normal vector. Our first problem is: Problem 1. Given Bartnik data ( Σn−1 i , γi, Hi ) , i = 1, 2, when are they NNSC-cobordant? By using surgery arguments (see [10, 21]), it is not difficult to show that if Bartnik data( Σn−1 i , γi, Hi ) , i = 1, 2 can be filled in with positive scalar curvature metrics, then ( Σn−1 1 , γ1,−H1 ) is NNSC-cobordant to ( Σn−1 2 , γ2, H2 ) . Another possible relevant notion to this is so called “PSC- concordant”. Namely, two PSC-metrics γ0 and γ1 on Σn−1 are said to be PSC-concordant if there is a PSC-metric g on the cylinder Σ×I which are the product γ0 +dt2 near Σ×{0} and γ1 +dt2 near Σ×{1} (see [28]), in that case, ( Σn−1, γ0, 0 ) is NNSC-cobordant to ( Σn−1, γ1, 0 ) . By index theory, it is known that there are countable infinity distinct PSC-concordant classes for S4k−1, for any positive integer k ≥ 2. When two PSC-metrics γ0 and γ1 are isotopic, i.e., they can be connected by a continuous path γt, t ∈ [0, 1], and for each t ∈ [0, 1], γt is a PSC-metric. Then we may use quasi-spherical metric to show that if H1 is not too large then ( S2, γ0, H0 ) is NNSC- cobordant to ( S2, γ1, H1 ) , here H0 can be any given smooth positive function (see [1, 22, 23]). On the other hand, when H1 is large enough we are able to show ( S2, γi, Hi ) , i = 0, 1, cannot be NNSC-cobordant [2]. Let γ0 be a Riemannian metric on S2 with its first eigenvalue λ1(−∆0 + K) > 0, here ∆0 is the Laplacian operator of γ0, then it was shown in [18] that ( S2, γ0, 0 ) is NNSC-cobordant to ( S2, γrou, H ) provided mH ( S2, γrou, H ) > √ Area(S2,γ0) 16π , here γrou denotes the round metric on S2. For a generalization to the case of Bartnik data with constant mean curvature surfaces see [6, Theorem 1.1], and higher-dimensional analogues see [7, Theorems 1.1 and 1.2], and [19, Proposition 2.1]. An NNSC fill-in by a conformal blow-down argument which may have deep NNSC-Cobordism of Bartnik Data in High Dimensions 3 relation to Problem 1 please see the proof of Theorem 1.2 in [11]. For deep discussion on PSC- concordant relation for two PSC-metrics on a manifold from topological point of view, please see [29, 30] and references therein. As we mentioned above, one obstruction of the above NNSC fill-in problem is from pos- itivity of certain quasi-local mass (for instance, Brown–York mass, see [22, 26]). It may be reasonable to think that there may be a potential obstruction of NNSC-cobordism problem which is from Penrose-type inequality (for Penrose inequality, see [3, 12], for local Penrose inequality, see [15, 20, 24, 27]). For instance, we observed that if ( S2, γ2, H2 ) is with posi- tive Gaussian curvature and H2 > 0, and ( Σ2 1, γ1, H1 ) is NNSC-cobordant to ( S2, γ2, H2 ) , then mBY ( S2; γ2, H2 ) ≥ mH ( Σ2 1, γ1, H1 ) provided mH ( Σ2 1, γ1, H1 ) ≤ 0 [2]. To our knowledge, even the following simple case is still unknown: Problem 2. Given Bartnik data ( Sn−1, g1, H ) and ( Sn−1, g0, 0 ) , both are with positive scalar curvature, what is the largest inf Sn−1 H so that ( Sn−1, g0, 0 ) is NNSC-cobordant to ( Sn−1, g1, H ) ? Remark 1. • By the arguments of [26, Theorem 1.4] and some gluing technique, we are able to show that for any PSC-metric g1 on Sn−1, no matter whether g1 is PSC-concordant to g0 or not, there is a constant H so that ( Sn−1, g0, 0 ) is NNSC-cobordant to ( Sn−1, g1, H ) and the ambient manifold bounded by these Bartnik data is diffeomorphic to Sn−1 × [0, 1] provided g0 is the standard round metric on Sn−1 [2]. • If g0 is the standard round metric on Sn−1, then by gluing arguments, the largest inf Sn−1 H in Problem 2 is the corresponding number for ( Sn−1, g1, H ) to admit NNSC fill-ins.1 • As we know, ´ Σ2 H dµ1 and ´ Σ2 H 2 dµ1 are closely related to Brown–York mass and Haw- king mass respectively, they are also involved in classical Minkowski’s inequality for a con- vex surface and Willmore functional for a surface in R3, so, it may also be interesting to ask what the possible largest values of ´ Sn−1 H dµ1 and ´ Sn−1 H 2 dµ1 are, especially for n = 3. For an orientable closed null-cobordant Riemannian manifold ( Σn−1, γ ) , define Λ ( Σn−1, γ ) by Λ ( Σn−1, γ ) = sup {ˆ Σ H dµγ ∣∣∣ (Σn−1, γ,H ) admits an NNSC fill-in } . In the case of n = 3 and H > 0, the above Λ was introduced in [16, 17], and also some interesting properties were discussed therein. An open problem on an estimate of Λ ( Σn−1, γ ) was proposed in [9, p. 31], and a partial result in the case of H > 0 was obtained in [26, Theorem 1.3]. Suppose ( S2, γ ) is a 2-dimensional surface with positive Gaussian curvature, then it can be isometrically embedded into R3, let H0 be the mean curvature of the embedding image with respect to the outward unit normal vector, then we have: Problem 3. Is Λ ( S2, γ ) = ´ S2 H0 dµγ? The affirmative answer implies the positivity of Brown–York mass without assumption of positivity of the mean curvature. 1We are grateful to the referee for pointing this fact to us. 4 X. Hu and Y. Shi Acknowledgements The authors would like to thank Dr. Georg Frenck for his comments to clarify some notions in this note, and are also deeply grateful to anonymous referees for invaluable suggestions on the exposition. The research of the first and the second author was partially supported by NSFC 11701215, NSFC 11671015 and 11731001 respectively. References [1] Bartnik R., Quasi-spherical metrics and prescribed scalar curvature, J. Differential Geom. 37 (1993), 31–71. [2] Bo L., Shi Y., PSC-cobordism of Bartnik data in 3-dimensions, in preparation. [3] Bray H.L., Proof of the Riemannian Penrose inequality using the positive mass theorem, J. Differential Geom. 59 (2001), 177–267, arXiv:math.DG/9911173. [4] Brown J.D., York Jr. J.W., Quasilocal energy in general relativity, in Mathematical Aspects of Classical Field Theory (Seattle, WA, 1991), Contemp. Math., Vol. 132, Amer. Math. Soc., Providence, RI, 1992, 129–142. 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institution Digital Library of Periodicals of National Academy of Sciences of Ukraine
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last_indexed 2025-12-17T12:03:33Z
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publisher Інститут математики НАН України
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spelling Hu, Xue
Shi, Yuguang
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2020
NNSC-Cobordism of Bartnik Data in High Dimensions. Xue Hu and Yuguang Shi. SIGMA 16 (2020), 030, 5 pages
1815-0659
2020 Mathematics Subject Classification: 53C20; 83C99
arXiv:2001.05607
https://nasplib.isofts.kiev.ua/handle/123456789/210580
https://doi.org/10.3842/SIGMA.2020.030
In this short note, we formulate three problems relating to nonnegative scalar curvature (NNSC) fill-ins. Loosely speaking, the first two problems focus on: When are (n−1)-dimensional Bartnik data (Σⁿ⁻¹ᵢ, γᵢ, Hᵢ), i=1,2, NNSC-cobordant? If (𝕊ⁿ⁻¹, γₛₜd, 0) is positive scalar curvature (PSC) cobordant to (Σⁿ⁻¹₁,γ₁, H₁), where (𝕊ⁿ⁻¹, γₛₜd) denotes the standard round unit sphere, then (Σⁿ⁻¹₁,γ₁, H₁) admits an NNSC fill-in. Just as Gromov's conjecture is connected with the positive mass theorem, our problems are connected with the Penrose inequality, at least in the case of n=3. Our third problem is on Λ(Σⁿ⁻¹, γ) defined below.
The authors would like to thank Dr. Georg Frenck for his comments to clarify some notions in this note, and are also deeply grateful to anonymous referees for invaluable suggestions on the exposition. The research of the first and the second author was partially supported by NSFC 11701215, NSFC 11671015, and 11731001, respectively.
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Інститут математики НАН України
Symmetry, Integrability and Geometry: Methods and Applications
NNSC-Cobordism of Bartnik Data in High Dimensions
Article
published earlier
spellingShingle NNSC-Cobordism of Bartnik Data in High Dimensions
Hu, Xue
Shi, Yuguang
title NNSC-Cobordism of Bartnik Data in High Dimensions
title_full NNSC-Cobordism of Bartnik Data in High Dimensions
title_fullStr NNSC-Cobordism of Bartnik Data in High Dimensions
title_full_unstemmed NNSC-Cobordism of Bartnik Data in High Dimensions
title_short NNSC-Cobordism of Bartnik Data in High Dimensions
title_sort nnsc-cobordism of bartnik data in high dimensions
url https://nasplib.isofts.kiev.ua/handle/123456789/210580
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