# DeSitter

The four-dimensional de Sitter space-time satisfies the Einstein equations in vacuum with a positive cosmological constant: $G_{\mu\nu} + \Lambda g_{\mu\nu} = 0$. deSitter spacetime is the surface

$-(T)^2 + (X)^2 + (Y)^2 + (Z)^2 + (W)^2 = \ell^2$

where $\Lambda \equiv 3/\ell^2$, in the five-dimensional flat spacetime

$ds^2 = - (dT)^2 + (dX)^2 + (dY)^2 + (dZ)^2 + (dW)^2$

The surface can be covered with coordinates $t$, $\chi$, $\theta$ and $\phi$, defined by

$\begin{array}{rcl} x_0 & = & T = \ell \sinh(t/\ell) \\ x_1 & = & X = \ell \cosh(t/\ell) \cos\chi \\ x_2 & = & Y = \ell \cosh(t/\ell) \sin\chi \cos\theta \\ x_3 & = & Z = \ell \cosh(t/\ell) \sin\chi \sin\theta \cos\phi\\ x_4 & = & W = \ell \cosh(t/\ell) \sin\chi \sin\theta \sin\phi \end{array}$

In these coordinates the deSitter metric is

$ds^2 = - dt^2 + \ell^2 \cosh^2(t/\ell)\, d\chi^2 + \ell^2 \cosh^2(t/\ell)\, \sin^2\chi\, (d\theta^2 + \sin^2\theta \, d\phi^2)$

#### Static coordinates

Let

$\begin{array}{rcl} x_0 & = & \sqrt{\ell^2 - r^2} \sinh(t/\ell) \\ x_1 & = & \sqrt{\ell^2 - r^2} \cosh(t/\ell) \\ x_2 & = & r \, \sin\theta\, \sin\phi \\ x_3 & = & r\, \sin\theta \, \cos\phi \\ x_4 & = & r \,\cos\theta \end{array}$

The deSitter metric is

$ds^2 = -(1 - r^2/\ell^2) dt^2 + \frac{1}{(1 - r^2/\ell^2)} dr^2 + r^2 (d\theta^2 + \sin^2\theta \, d\phi^2 )$

These coordinates cover only the strip $-\ell \leq x_4 \leq \ell$ of the full manifold. Let $r = \ell \sin\chi$ and the metric becomes

$ds^2 = -\cos^2\chi\, dt^2 + \ell^2 dr^2 + \ell^2\sin^2\chi (d\theta^2 + \sin^2\theta \, d\phi^2 )$

#### Conformally flat coordinates

Let

$\begin{array}{rcl} x_0 & = & \ell \sinh(t/\ell) + e^{t/\ell} X_i X_i /(2\ell)\\ x_1 & = & \ell \cosh(t/\ell) - e^{t/\ell} X_i X_i /(2\ell)\\ x_2 & = & e^{t/\ell} X_1 \\ x_3 & = & e^{t/\ell} X_2 \\ x_4 & = & e^{t/\ell} X_3 \end{array}$

where $X^i X^i \equiv (X_1)^2 + (X_2)^2 + (X_3)^2$. The metric is

$ds^2 = -dt^2 + e^{2t/\ell} (dX_1^2 + dX_2^2 + dX_3^2)$

Only the region $x_0 + x_1 \geq 0$ is covered by these coordinates.