Title: On Infinities of Cosmological Spacetimes
Abstract:
The mathematical structures encoding infinities of spacetime play deep explanatory roles in general relativity. The notion of null infinity, in particular, makes power emitted by gravitational radiation a gauge-invariant quantity and has substantial bearing on phenomena like gravitational wave memory, relevant as a future LISA observable. However, null infinity is a feature of asymptotically flat spacetimes, incompatible with our observations of a positive cosmological constant. In this talk, I discuss the delicate de-idealization of insights derived under the assumption of asymptotic flatness to more realistic cosmological settings. I begin with a brief recapitulation of cornerstone results, ranging from the discovery of asymptotic symmetry groups in the 1960s to the more recent infrared triangle at the heart of the Celestial Holography programme. The second part of the talk describes the technical obstructions to de-idealizing asymptotic flatness, arising from the discontinuous $\Lambda \to 0$ limit. I argue that asymptotic flatness is not one idealization but a concord of several that fragment into distinct, incompatible splinters in the presence of a cosmological constant. The route one pursues as a de-idealization strategy constrains how observations concerning gravitational waves and large-scale structure statistics are to be interpreted. I conclude by raising some concerns about how asymptotic structure bears on adjacent debates in the philosophy of physics literature, particularly on the notions of subsystem embeddings.