# Even a tiny cosmological constant casts a long shadow

### APA

Ashtekar, A. (2015). Even a tiny cosmological constant casts a long shadow. Perimeter Institute. https://pirsa.org/15030075

### MLA

Ashtekar, Abhay. Even a tiny cosmological constant casts a long shadow. Perimeter Institute, Mar. 19, 2015, https://pirsa.org/15030075

### BibTex

@misc{ pirsa_PIRSA:15030075, doi = {10.48660/15030075}, url = {https://pirsa.org/15030075}, author = {Ashtekar, Abhay}, keywords = {Strong Gravity}, language = {en}, title = {Even a tiny cosmological constant casts a long shadow}, publisher = {Perimeter Institute}, year = {2015}, month = {mar}, note = {PIRSA:15030075 see, \url{https://pirsa.org}} }

Abhay Ashtekar Pennsylvania State University

**Collection**

**Talk Type**Scientific Series

**Subject**

## Abstract

Surprisingly, several basic questions in classical and quantum gravity, which were resolved some 40-50 years ago for zero $\Lambda$, still remain open in the $\Lambda >0$ case. In particular, for $\Lambda >0$, we still do not have a satisfactory notion of gravitational radiation or Bondi 4-momentum in exact general relativity, nor a positive energy theorem. Similarly, the standard constructions of `in' and `out' Hilbert spaces that we routinely use (e.g. in the analysis of black hole evaporation) do not extend to the $\Lambda >0$ case. In this talk I will present some illustrative examples of these quandaries and introduce a systematic approach to resolve the open issues.