Embedding expanders in CAT(0) spaces It is well-known that expanders are hard to embed into Hilbert (or $\ell^p$) spaces - any embedding of an expander with $n$ vertices has distortion $\Omega(\log n)$.
Can anyone provide a reference (or a quick argument) that the same holds for any "nice" CAT(0) space (like the hyperbolic space) instead of Hilbert space? I'm not giving a precise definition of "nice" here, but this is discussed e.g. in Gromov's "Random Walk in Random Groups".
 A: I don't know if this is the type of result you are looking for, because it is less quantitative than in your question, but in - http://arxiv.org/pdf/1102.0729v2.pdf (in corollary 1.7), Toyoda proves that if $X$ is a geodesically complete CAT(0) space that has a group acting cocompactly and properly on it, then a family of expanders does not embed coarsely into $X$ (he actually proves a little more - check reference for exact statement). 
A: Here is a class of Hadamard spaces in which, I think, no expander embeds:
Consider a Hadamard space $X$, 
which is also "FDSCBB" in the sense 
Burago, Gromov, and Perel'man (see http://seven.ihes.fr/~gromov/PDF/3[86].pdf section 7).
By Corollary 7.10 in [BGP] The tangent cone has non-negative curvature in Alexandrov sense.
By Theorem 3.19 from [Bridson & Haeflinger] the tangent cone is also CAT(0), which together means it is Euclidean.
So given a finite subset $S\subset X$, let $x_0\in X$  be its center of mass. 
Since the tangent cone around $x_0$ is Euclidean we can have a map into Hilbert space $f:S\cup\{x_0\} \to H$ such that $f(x_0)=0$ for every $s\in S$, 
$\|f(s)\|_2= d(x_0,s)$, and the angles in the image are preserved, .i.e.  $\angle_0 (f(s),f(s')) = \angle_{x_0}(s,s')$  for every $s,s'\in S$.
The CAT(0) property implies that the mapping $f$ is 1-Lipschitz on $S$.
$f$  also have the "average distortion property", i.e.,
$$ \sum_{s,s'\in S} \|f(s)-f(s')\|_2^2 \ge \frac14 \sum_{s,s'\in S} d(s,s')^2 .$$
Together this implies the ``Poincar\'e inequality" for any graph with respect to Hilbert space transfers to $X$ with a loss of factor of 4, 
see for example http://www.cims.nyu.edu/~naor/homepage%20files/spectral-compare.pdf page 5.
We are left prove the average distortion property of $f$.
This is probably standard but since I'm not aware of it, I'll sketch the argument. 
For $\lambda\in[0,1]$,
denote by $\lambda S \subset X$ the subset where each $s\in S$ is replaced with the (unique) point $\lambda s$ which is of distance
$\lambda d(x_0,s)$ from $x_0$ along the geodesic connecting $x_0$ and $s$.
Observe that $x_0$ is also the center of mass of $\lambda S$.
Define $f_\lambda:\lambda S\to H$ by $f_\lambda(\lambda s)=\lambda f(s)$.
When $\lambda $ is small $f_\lambda$ preserves distances upto arbitrary precision times $\lambda$, and so, denoting $y_0$ the barycenter of $f_\lambda(\lambda S)$
$$ \sum_{s,s'\in S} \|f(s)-f(s')\|_2^2 =\lambda^{-2} \sum_{s,s'\in S} \|f_\lambda(\lambda s)-f(\lambda s')\|_2^2 =
\lambda^{-2} 2|S| \sum_{s\in S} \|f_\lambda(\lambda s)-y_0\|_2^2
\ge \lambda^{-2} |S| \sum_{s\in S} \|f_\lambda(\lambda s)\|_2^2=|S| \sum_{s\in S} d(x_0,s)^2
\ge \frac14 \sum_{s,s'\in S} d(s,s')^2 .$$
Update: 
Upon reading the literature more carefully, similar arguments were done before. For example, Naor & Silberman In arXiv:1005.4084 have basically the same argument. See
Proposition 4.9 and Corollary 4.10 there.
A: See the recent preprint http://arxiv.org/abs/1306.5434 (Mendel and Naor). 
