[Math] Extensions obtained adding torsion points of an elliptic curve

elliptic-curvesgalois-groupsgalois-theorynt.number-theory

When adding to the rational the $p$-torsion points $E[p]$ of an elliptic curve we obtain an extension containing the $p$-th roots of the unity, and whose Galois group can be embedded in $GL(2, \mathbb{F}_p)$. To what extent are such extensions coming from elliptic curves?

I mean, assume $K/\mathbb{Q}$ to be an extension whose Galois group can be embedded in $GL(2, \mathbb{F}_p)$ and containing the $p$-th roots of the unity (which is required to expect a positive answer), is $K$ obtained adding to $\mathbb{Q}$ the torsion points of some elliptic curve defined over the rationals?

Note that I'm not considering a particular Galois representation, but just Galois groups that can be embedded in some way into $GL(2, \mathbb{F}_p)$. Thanks!

Best Answer

This question is discussed very carefully in Section 3 of the paper Mod $p$ representations on elliptic curves, by Frank Calegari (available here).

In particular, after noting that the answer is positive when $p \leq 5$ (as was already observed in the comments above), he proves (in Theorems 3.3 and 3.4) that if $p \geq 7$ then there exists a continuous representation $\rho: Gal(\overline{\mathbb Q}/ \mathbb Q) \to GL_2(\mathbb F_p)$ with cyclotomic determinant which does not come from the $p$-torsion of an elliptic curve. (He also notes that the same result was established by Dieulefait, in the paper Existence of non-elliptic mod $\ell$ Galois representations for every $\ell > 5$.)