16 Outlook

The past decade has witnessed a revival of interest in massive gravity as a potential alternative to GR. The original theoretical obstacles that came in the way of deriving a consistent theory of massive gravity have now been overcome, but with them comes a new set of challenges that will be decisive in establishing the viability of such theories. The presence of a low strong coupling scale on which the Vainshtein mechanism relies has opened the door to a new way of thinking about these types of effective field theories. At the moment, it is yet unclear whether these types of theories could lead to an alternative to UV completion. The superluminalities that also arise in many cases with the Vainshtein mechanism should also be understood in more depth. At the moment, its real implications are not well understood and no case of true acausality has been shown to be present within the regime of validity of the theory. Finally, the difficulty in finding fully-fledged cosmological and black-hole solutions in many of these theories (both in ghost-free massive gravity and bi-gravity, and in other extensions or related models such as cascading gravity) makes their full phenomenology still evasive. Nevertheless, the well understood decoupling limits of these models can be used to say a great deal about phenomenology without going into the complications of the full theories. These represent many open questions in massive gravity, which reflect the fact that the field is yet extremely young and many developments are still in progress.