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Given these two expressions

1) $\sinh{d}=\frac{\sqrt{t^2−x^2}}{\sqrt{1−(t^2−x^2)}}$

2) $\sin{d}=\frac{\sqrt{t^2−x^2}}{\sqrt{1+(t^2−x^2)}}$

for distance $d$ from the origin $(0,0)$ to point $(x,t)$, which of these two options applies to de-Sitter space and which to anti de-Sitter space? For definiteness, assume $t$ is time and $(x,t)$ is time-like, that is $t^2-x^2$ is positive.

Does option (1) correspond to a space with the positive curvature and (2) to the negative one?

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    Is there any reason in the former expression that you're implicitly constraining $t^2-x^2 < 1$? Beyond that, I'd recommend checking some extremal limits (eg. $t^2-x^2 \to \infty$, or whatever makes sense) and see which obtains the correct behavior.2012-07-05
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    In (1), the distance is defined only for $|t^2-x^2|<1$, which is what you get in a space with a hyperbolic distance measure. For instance, in the 2D hyperbolic space, $d$ has real values only for $|x^2+y^2|<1$. As for checking the limits, I don't see how that will help me decide which one is de-Sitter and which one is anti de-Sitter.2012-07-05
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    Sorry, I recall this being an easy way to tell between positive and negative curvature -- the correct application isn't coming to mind at the moment.2012-07-05
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    Hint: Anti De Sitter Space is hyperbolic, with negative curvature...2012-07-05
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    @Sam, Perhaps you could give me a hint as to what I should say to people claiming de Sitter has a hyperbolic distance measure, e.g. page 10 of http://arxiv.org/abs/math-ph/99100412012-07-05
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    How are the coordinates $x$ and $t$ obtained? I'm most familiar with $\text{dS}^2$ and $\text{AdS}^2$ being presented as submanifolds of $\mathbb{R}^{1,2}$ and $\mathbb{R}^{2,1}$, respectively. Are your $x$ and $t$ the restrictions of coordinates $t, x, z$ on $\mathbb{R}^3$, or something like that?2015-11-17
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    @Vectornaut $t$ and $x$ are coordinates of $\mathbb R^2$, there is no $z$. The question is about 2d spaces only and I do not use the 3d models you are referring to.2016-01-03

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