德西特空間

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數學與物理學中，一個n維德西特空間（de Sitter space，標作dSn）為一最大對稱的勞侖茲流形，具有正常數的純量曲率。

主要應用是在廣義相對論作為最簡單的宇宙數學模型。

「德西特」是以威廉·德西特（1872–1934）為名，他與阿爾伯特·愛因斯坦於1920年代一同研究宇宙中的時空結構。

以廣義相對論的語言來說，德西特空間為愛因斯坦場方程式的最大對稱真空解：具正宇宙學常數$\Lambda$對應正真空能量密度和負壓。

In mathematical physics, n-dimensional de Sitter space (often abbreviated to dSn) is a maximally symmetric Lorentzian manifold with constant positive scalar curvature. It is the Lorentzian analogue of an n-sphere (with its canonical Riemannian metric).

The main application of de Sitter space is its use in general relativity, where it serves as one of the simplest mathematical models of the universe consistent with the observed accelerating expansion of the universe. More specifically, de Sitter space is the maximally symmetric vacuum solution of Einstein's field equations with a positive cosmological constant $\Lambda$ (corresponding to a positive vacuum energy density and negative pressure). There is cosmological evidence that the universe itself is asymptotically de Sitter, i.e. it will evolve like the de Sitter universe in the far future when dark energy dominates.

de Sitter space and anti-de Sitter space are named after Willem de Sitter (1872–1934),professor of astronomy at Leiden University and director of the 萊頓天文台. Willem de Sitter and Albert Einstein worked closely together in Leiden in the 1920s on the spacetime structure of our universe. de Sitter space was also discovered, independently, and about the same time, by 圖利奧·列維-齊維塔.

## 定義

de Sitter space can be defined as a submanifold of a generalized 閔考斯基時空 of one higher dimension. Take Minkowski space R1,n with the standard metric:

$$ds^2 = -dx_0^2 + \sum_{i=1}^n dx_i^2.$$

de Sitter space is the submanifold described by the hyperboloid of one sheet $$-x_0^2 + \sum_{i=1}^n x_i^2 = \alpha^2,$$ where $\alpha$ is some nonzero constant with its dimension being that of length. The metric on de Sitter space is the metric induced from the ambient Minkowski metric. The induced metric is nondegenerate and has Lorentzian signature. (Note that if one replaces $\alpha^2$ with $-\alpha^2$ in the above definition, one obtains a hyperboloid of two sheets. The induced metric in this case is positive-definite, and each sheet is a copy of hyperbolic n-space. For a detailed proof, see Minkowski space#Geometry.)

de Sitter space can also be defined as the quotient O(1, n) / O(1, n − 1) of two indefinite orthogonal groups, which shows that it is a non-Riemannian symmetric space.

Topologically, de Sitter space is R × Sn−1 (so that if n ≥ 3 then de Sitter space is simply connected).

## Properties

The isometry group of de Sitter space is the 勞侖茲群 O(1, n). The metric therefore then has n(n + 1)/2 independent 基靈矢量場s and is maximally symmetric. Every maximally symmetric space has constant curvature. The 黎曼曲率張量 of de Sitter is given by

$$R_{\rho\sigma\mu\nu} = {1 \over \alpha^2}\left(g_{\rho\mu}g_{\sigma\nu} - g_{\rho\nu}g_{\sigma\mu}\right)$$

(using the sign convention $<pre> R^{\rho}{}_{\sigma\mu\nu} = \partial_{\mu}\Gamma^{\rho}_{\nu\sigma} - \partial_{\nu}\Gamma^{\rho}_{\mu\sigma} + \Gamma^{\rho}_{\mu\lambda}\Gamma^{\lambda}_{\nu\sigma} - \Gamma^{\rho}_{\nu\lambda}\Gamma^{\lambda}_{\mu\sigma} </pre>$ for the Riemann curvature tensor). de Sitter space is an Einstein manifold since the Ricci tensor is proportional to the metric:

$$R_{\mu\nu} = R^\lambda{}_{\mu\lambda\nu} = \frac{n - 1}{\alpha^2}g_{\mu\nu}$$

This means de Sitter space is a vacuum solution of Einstein's equation with cosmological constant given by

$$\Lambda = \frac{(n - 1)(n - 2)}{2\alpha^2}.$$

The scalar curvature of de Sitter space is given by

$$R = \frac{n(n - 1)}{\alpha^2} = \frac{2n}{n - 2}\Lambda.$$

For the case n = 4, we have Λ = 3/α2 and R = 4Λ = 12/α2.

## Coordinates

### Static coordinates

We can introduce static coordinates $(t, r, \ldots)$ for de Sitter as follows:

$$\begin{align} <pre> x_0 &= \sqrt{\alpha^2 - r^2}\sinh\left(\frac{1}{\alpha}t\right) \\ x_1 &= \sqrt{\alpha^2 - r^2}\cosh\left(\frac{1}{\alpha}t\right) \\ x_i &= r z_i \qquad\qquad\qquad\qquad\qquad 2 \le i \le n. </pre> \end{align}$$

where $z_i$ gives the standard embedding the (n − 2)-sphere in Rn−1. In these coordinates the de Sitter metric takes the form:

$$ds^2 = -\left(1 - \frac{r^2}{\alpha^2}\right)dt^2 + \left(1 - \frac{r^2}{\alpha^2}\right)^{-1}dr^2 + r^2 d\Omega_{n-2}^2.$$

Note that there is a cosmological horizon at $r = \alpha$.

### Flat slicing

Let

$$\begin{align} <pre> x_0 &= \alpha \sinh\left(\frac{1}{\alpha}t\right) + \frac{1}{2\alpha}r^2 e^{\frac{1}{\alpha}t}, \\ x_1 &= \alpha \cosh\left(\frac{1}{\alpha}t\right) - \frac{1}{2\alpha}r^2 e^{\frac{1}{\alpha}t}, \\ x_i &= e^{\frac{1}{\alpha}t}y_i, \qquad 2 \leq i \leq n </pre> \end{align}$$

where $r^2 = \sum_i y_i^2$. Then in the $\left(t, y_i\right)$ coordinates metric reads:

$$ds^{2} = -dt^{2} + e^{2\frac{1}{\alpha}t} dy^{2}$$

where $dy^2 = \sum_i dy_i^2$ is the flat metric on $y_i$'s.

Setting $\zeta = \zeta_{\infty} - \alpha e^{-\frac{1}{\alpha}t}$, we obtain the conformally flat metric:

$$ds^2 = \frac{\alpha^2}{(\zeta_\infty - \zeta)^2}\left(dy^2 - d\zeta^2\right)$$

### Open slicing

Let

$$\begin{align} <pre> x_0 &= \alpha \sinh\left(\frac{1}{\alpha}t\right) \cosh\xi, \\ x_1 &= \alpha \cosh\left(\frac{1}{\alpha}t\right), \\ x_i &= \alpha z_i \sinh\left(\frac{1}{\alpha}t\right) \sinh\xi, \qquad 2 \leq i \leq n </pre> \end{align}$$

where $\sum_i z_i^2 = 1$ forming a $S^{n-2}$ with the standard metric $\sum_i dz_i^2 = d\Omega_{n-2}^2$. Then the metric of the de Sitter space reads

$$ds^2 = -dt^2 + \alpha^2 \sinh^2\left(\frac{1}{\alpha}t\right) dH_{n-1}^2,$$

where

$$dH_{n-1}^2 = d\xi^2 + \sinh^2(\xi) d\Omega_{n-2}^2$$

is the standard hyperbolic metric.

### Closed slicing

Let

$$\begin{align} <pre> x_0 &= \alpha \sinh\left(\frac{1}{\alpha}t\right), \\ x_i &= \alpha \cosh\left(\frac{1}{\alpha}t\right) z_i, \qquad 1 \leq i \leq n </pre> \end{align}$$

where $z_i$s describe a $S^{n-1}$. Then the metric reads:

$$ds^2 = -dt^2 + \alpha^2 \cosh^2\left(\frac{1}{\alpha}t\right) d\Omega_{n-1}^2.$$

Changing the time variable to the conformal time via $\tan\left(\frac{1}{2}\eta\right) = \tanh\left(\frac{1}{2\alpha}t\right)$ we obtain a metric conformally equivalent to Einstein static universe:

$$ds^2 = \frac{\alpha^2}{\cos^2\eta}\left(-d\eta^2 + d\Omega_{n-1}^2\right).$$

These coordinates, also known as "global coordinates" cover the maximal extension of de Sitter space, and can therefore be used to find its 彭羅斯圖.

### dS slicing

Let

$$\begin{align} <pre> x_0 &= \alpha \sin\left(\frac{1}{\alpha}\chi\right) \sinh\left(\frac{1}{\alpha}t\right) \cosh\xi, \\ x_1 &= \alpha \cos\left(\frac{1}{\alpha}\chi\right), \\ x_2 &= \alpha \sin\left(\frac{1}{\alpha}\chi\right) \cosh\left(\frac{1}{\alpha}t\right), \\ x_i &= \alpha z_i \sin\left(\frac{1}{\alpha}\chi\right) \sinh\left(\frac{1}{\alpha}t\right) \sinh\xi, \qquad 3 \leq i \leq n </pre> \end{align}$$

where $z_i$s describe a $S^{n-3}$. Then the metric reads:

$$ds^2 = d\chi^2 + \sin^2\left(\frac{1}{\alpha}\chi\right) ds_{dS,\alpha,n-1}^2,$$

where

$$ds_{dS,\alpha,n-1}^2 = -dt^2 + \alpha^2 \sinh^2\left(\frac{1}{\alpha}t\right) dH_{n-2}^2$$

is the metric of an $n - 1$ dimensional de Sitter space with radius of curvature $\alpha$ in open slicing coordinates. The hyperbolic metric is given by:

$$dH_{n-2}^2 = d\xi^2 + \sinh^2(\xi) d\Omega_{n-3}^2.$$

This is the analytic continuation of the open slicing coordinates under $\left(t, \xi, \theta, \phi_1, \phi_2, \ldots, \phi_{n-3}\right) \to \left(i\chi, \xi, it, \theta, \phi_1, \ldots, \phi_{n-4}\right)$ and also switching $x_0$ and $x_2$ because they change their timelike/spacelike nature.

## 外部鏈接

- Simplified Guide to de Sitter and anti-de Sitter Spaces A pedagogic introduction to de Sitter and anti-de Sitter spaces. The main article is simplified, with almost no math. The appendix is technical and intended for readers with physics or math backgrounds.