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Let $\mathcal{M}$ be a projective smooth moduli space over $\mathbb{C}$ (the specific example I have in mind is the moduli of curves $\mathcal{M}_g)$. Consider a point $[X]\in \mathcal{M}(\mathbb{C})$. Then via deformation theory in algebraic geometry, we say that the tangent space $T_{[X]}\mathcal{M}$ consists of lifts (or rather isomorphism classes of lifts) of $X$ to $\mathbb{C}[\varepsilon]/(\varepsilon^2)$. This is naturally a $\mathbb{C}$-vector space. My question is in two parts:

  1. Does this construction match with the tangent space of the analytification $\mathcal{M}^{\mathrm{an}}$ at $X$? (This question doesn't need compact, I think)

  2. Since I assume that $\mathcal{M}$ is projective, then $\mathcal{M}^{\mathrm{an}}$ is compact. If we choose a basis of the projective space, we get an induced metric on $\mathcal{M}^{\mathrm{an}}$ (thanks to the comments for pointing me to this gap!). By Hopf-Rinow, the exponential map $T_{[X]}\mathcal{M}^{\mathrm{an}}\rightarrow \mathcal{M}^{\mathrm{an}}$ is well-defined (if we assume that $[X]$ does not lie in the boundary of $\mathcal{M}^{\mathrm{an}}$. If part 1) is true, the tangent space corresponds to infinitesimal lifts of $X$. Can we explicitly say what the image of such a lift is under the exponential map?

If this isn't possible in general, is there perhaps a way to do it for the moduli of curves, moduli of abelian varieties or the Hilbert scheme?

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  • $\begingroup$ You must specify what metric you put on your $\mathcal{M}$. $\endgroup$
    – abx
    Commented Jul 30, 2020 at 16:27
  • $\begingroup$ Ah, right. If I assume thought that $\mathcal{M}$ is projective, then it should inhert the metric from the projective space, no? $\endgroup$ Commented Jul 30, 2020 at 16:29
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    $\begingroup$ You need to pick a basis for the projective space (modulo unitary transformations) to get a metric. $\endgroup$
    – Will Sawin
    Commented Jul 30, 2020 at 16:30
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    $\begingroup$ One class of varieties that has a canonical metric is hermitian locally symmetric spaces, roughly the same as Shimura varieties, such as the moduli of pp abelian varieties. Analogous to the exponential map is an arithmetic phenomenon, the Serre-Tate coordinates, which identify the formal neighborhood of a point with its tangent space. $\endgroup$ Commented Aug 5, 2020 at 15:03

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