#### Alladi Ramakrishnan Hall

#### Cosmological Horizon and energy flux in de Sitter background

#### Sk Jahanur Hoque

##### IMSc

*One of the physical realizations of gravitational waves is that they carry energy. Though there is no gauge invariant local definition of gravitational stress energy tensor, in the special context of two widely separated scales, Isaccson defined an effective gravitational stress tensor for the ripples. Energy computed from effective stress tensor is well understood for weak gravitational waves in Minkowski background. In de Sitter background space-like character of future null infinity $\mathcal{J} ^{+}$, makes the definition of energy subtle. A spatially compact source in de Sitter back ground also provides a distinguished null hypersurface, its cosmological horizon, $\mathcal{H}^{+}$. In this talk I will discuss how Isaacson prescription can be adopted to compute energy flux in de Sitter background. I will compare energy flux computed by effective stress tensor at $\mathcal{H}^{+}$ with that given by Ashtekar et al at $\mathcal{J}^{+}$.*

Done