IDEAL characterization of isometry classes of FLRW and inflationary spacetimes

Apr 18, 2017
37 pages
Published in:
  • Class.Quant.Grav. 35 (2018) 3, 035013
  • Published: Jan 8, 2018
e-Print:

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Abstract: (IOP)
In general relativity, an IDEAL (Intrinsic, Deductive, Explicit, ALgorithmic) characterization of a reference spacetime metric g 0 consists of a set of tensorial equations T[g]  =  0, constructed covariantly out of the metric g, its Riemann curvature and their derivatives, that are satisfied if and only if g is locally isometric to the reference spacetime metric g 0. The same notion can be extended to also include scalar or tensor fields, where the equations are allowed to also depend on the extra fields ϕ. We give the first IDEAL characterization of cosmological FLRW spacetimes, with and without a dynamical scalar (inflaton) field. We restrict our attention to what we call regular geometries, which uniformly satisfy certain identities or inequalities. They roughly split into the following natural special cases: constant curvature spacetime, Einstein static universe, and flat or curved spatial slices. We also briefly comment on how the solution of this problem has implications, in general relativity and inflation theory, for the construction of local gauge invariant observables for linear cosmological perturbations and for stability analysis.
Note:
  • v4: Fixed minor typos relative to published version. v3: 42 pages; restructured order of sections, fixed some inconsistent formulas; close to published version
  • (a) Constant curvature spacetime, with spacetime sectional curvature K: \begin{align} \newcommand{\e}{{\rm e}} \newcommand{\CC}{{\rm CC}} \displaystyle \CC^m_{K} = \left\{\begin{array}{@{}ll@{}} \{(m,K,\cosh(\sqrt{K}t)) & \mid K>0, ~ I=\mathbb{R} \}, \nonumber \\ \{(m,0,1) & \mid K=0, ~ I=\mathbb{R} \}, \nonumber \\ \{(m,K,\cos(\sqrt{-K}t)) & \mid K<0, ~ I=\mathbb{R} \}. \end{array}\right. \nonumber \end{align} \tag
    • [1]
      • (b) Einstein static universe, with spatial sectional curvature K\ne 0:
        • \begin{align} \newcommand{\e}{{\rm e}} \newcommand{\ESU}{{\rm ESU}} \displaystyle \ESU^m_{K} = \{(m,K,1) \mid m>1, ~ I=\mathbb{R} \}. \nonumber \end{align} \tag{ 2 }
          • (c) Spatially flat constant scalar curvature spacetime, with spacetime scalar curvature m(m+1)K and such that \frac{f^{\prime2}}{f^2}(I) = J:
            • \begin{align} \newcommand{\e}{{\rm e}} \newcommand{\CS}{{\rm CS}} \newcommand{\CSC}{{\rm CSC}} \displaystyle \CSC^{m,0}_{K,J} = &\left\{(m,\alpha,f) \mid m>1, ~ f^\prime\ne 0,\right. \nonumber \\ &\left.\left(\frac{f^{\prime\prime}}{f} - \frac{f^{\prime2}}{f^2}\right) + \frac{(m+1)}{2} \left(\frac{f^{\prime2}}{f^2} - K\right) = 0 \right\}. \nonumber \end{align} \tag{ 3 }
              • (d) Generic constant scalar curvature spacetime, with spacetime scalar curvature m(m+1)K, normalized radiation density constant Ω and such that \frac{\alpha}{f^2}(I) = J:
                • \begin{align} \newcommand{\e}{{\rm e}} \newcommand{\CS}{{\rm CS}} \newcommand{\CSC}{{\rm CSC}} \displaystyle \CSC^m_{K,\Omega,J} = \left\{(m,\alpha,f) \mid m>1, ~ \alpha \ne 0, ~ f^\prime \ne 0,\right. \nonumber \\ \left. \frac{f^{\prime2}}{f^2} + \frac{\alpha}{f^2} = K + \Omega \frac{\vert \alpha\vert ^{(m+1)/2}}{f^{m+1}} \right\}. \nonumber \end{align} \tag{ 4 }
                  • (e) Spatially flat FLRW spacetime with normalized pressure function P defined on an open interval J, with 0 < \frac{f^{\prime2}}{f^2}(I) = J and
                    • \begin{align} \newcommand{\e}{{\rm e}} \newcommand{\del}{\partial} \displaystyle P(u) \left[\del_u P(u) - \frac{1}{2\kappa}\right] \ne 0 \nonumber \end{align} \tag{ 5 } everywhere on J:
                      • \begin{align} \newcommand{\e}{{\rm e}} \newcommand{\FLRW}{{\rm FLRW}} \displaystyle \FLRW^{m,0}_{P,J} = \left\{(m,0,f) \mid \left(\frac{f^{\prime\prime}}{f} - \frac{f^{\prime2}}{f^2}\right)+\frac{m}{2}\frac{f'^2}{f^2} = -\kappa P\left((\,f^\prime/f)^2\right) \right\}. \nonumber \end{align} \tag{ 6 }
                        • (f) Generic FLRW spacetime with normalized energy function E defined on an open interval J, with 0 \not\in \frac{\alpha}{f^2}(I) = J and
                          • \begin{align} \newcommand{\e}{{\rm e}} \newcommand{\del}{\partial} \displaystyle \del_u \left[u \del_u E(u) - \frac{(m+1)}{2} \right] \ne 0 \nonumber \end{align} \tag{ 7 } everywhere on J:
                            • \begin{align} \newcommand{\e}{{\rm e}} \newcommand{\FLRW}{{\rm FLRW}} \displaystyle \FLRW^m_{E,J} = \left\{(m,\alpha,f) \mid m>1, ~ \alpha \ne 0, ~ \frac{f^{\prime2}}{f^2} + \frac{\alpha}{f^2} = \kappa E(\alpha/f^2) \right\}. \nonumber \end{align} \tag{ 8 }
                              • (a) Constant scalar, with scalar value Φ, on a constant curvature spacetime with scalar curvature K:
                                • \begin{align} \newcommand{\e}{{\rm e}} \newcommand{\CS}{{\rm CS}} \newcommand{\CC}{{\rm CC}} \displaystyle \CC^m_K \CS_\Phi = \{(m,\alpha,f,\Phi) \mid (m,\alpha,f) \in \CC^m_K \}. \nonumber \end{align} \tag{ 9 }
                                  • (b) Constant energy scalar, with energy density \rho > 0 and J = \phi(I), on an Einstein static universe with spatial sectional curvature K = \frac{2}{m(m-1)} \kappa \rho, or equivalently with cosmological constant \Lambda = \frac{(m-1)}{m} \kappa \rho:
                                    • \begin{align} \newcommand{\e}{{\rm e}} \newcommand{\CES}{{\rm CES}} \newcommand{\ESU}{{\rm ESU}} \displaystyle \ESU^m_{K} \CES_{\rho,J} = \{(m,K,1, \sqrt{2\rho/m} t) \mid I = J/\sqrt{2\rho/m} \}. \nonumber \end{align} \tag{ 10 }
                                      • (c) Spatially flat massless minimally-coupled scalar spacetime, with cosmological constant Λ, J = \phi(I) and J^\prime = \frac{f^\prime}{f}(I) \not\ni 0 and \frac{2\Lambda/\kappa}{m(m-1)} < \frac{1}{\kappa} (J^\prime){\hspace{0pt}}^2:
                                        • \begin{align} \newcommand{\e}{{\rm e}} \newcommand{\MMS}{{\rm MMS}} \displaystyle \textstyle \MMS^{m,0}_{\Lambda,J,J^\prime} = &\left\{(m,0,f,\phi) \mid \phi^\prime < 0, ~ \frac{f^\prime}{f} \ne 0, \right. \nonumber \\ &\textstyle \left. \frac{f^{\prime2}}{f^2} = \frac{\kappa \phi^{\prime2} + 2\Lambda}{m(m-1)}, ~ \left(\frac{f^{\prime\prime}}{f} - \frac{f^{\prime2}}{f^2}\right) + m\frac{f^{\prime2}}{f^2} = \frac{2\Lambda}{(m-1)} \right\}. \nonumber \end{align} \tag{ 11 }
                                          • (d) Generic massless minimally-coupled scalar spacetime, with cosmological constant Λ, normalized scalar energy constant \Omega > 0, J = \phi(I) and J^\prime = \frac{f^\prime}{f}(I) \not\ni 0:
                                            • \begin{align} \newcommand{\e}{{\rm e}} \newcommand{\MMS}{{\rm MMS}} \displaystyle \textstyle \MMS^m_{\Lambda,\Omega,J,J^\prime} =& \left\{(m,\alpha,f,\phi) \mid \alpha \ne 0, ~ \frac{f^\prime}{f} \ne 0, \right. \nonumber \\ &\textstyle \left. \phi^\prime = -\sqrt{\Omega} \frac{\vert \alpha\vert ^{\frac{m}{2}}}{f^m}, ~ \frac{f^{\prime2}}{f^2} + \frac{\alpha}{f^2} = \frac{2\Lambda + \kappa \Omega \vert \alpha\vert ^m/f^{2m}}{m(m-1)} \right\}. \nonumber \end{align} \tag{ 12 }
                                              • (e) Spatially flat nonlinear Klein-Gordon spacetime, with non-constant scalar self-coupling potential V\colon J\to \mathbb{R}, with J = \phi(I), and expansion profile \Xi\colon J \to \mathbb{R}, satisfying \Xi(u) \ne 0, \newcommand{\del}{\partial} \frac{1}{\kappa}\del_u \Xi(u) > 0 and \mathfrak{H}_V(\Xi) = 0 in the notation of (, 18: #cqgaa9f61eqn019):
                                                • \begin{align} \newcommand{\e}{{\rm e}} \newcommand{\NKG}{{\rm NKG}} \newcommand{\del}{\partial} \displaystyle \textstyle \NKG^{m,0}_{V,\Xi,J} = \left\{(m,0,f,\phi) \mid \frac{f^\prime}{f} = \Xi(\phi), \phi^\prime = -\frac{(m-1)}{\kappa} \del_\phi \Xi(\phi) \right\}. \nonumber \end{align} \tag{ 13 }
                                                  • (f) Generic nonlinear Klein-Gordon spacetime, with non-constant scalar potential V\colon J \to \mathbb{R}, with J = \phi(I), and expansion profile (\Pi, \Xi) \colon J\to \mathbb{R}^2, satisfying \Pi < 0, \Xi \ne 0, \kappa\frac{\Pi^2+V}{m(m-1)} \ne \Xi^2 and \mathfrak{G}_V(\Pi, \Xi) = 0 in the notation of (, 20: #cqgaa9f61eqn021):