# BMS in cosmology

## Abstract

Symmetries play an interesting role in cosmology. They are useful in characterizing the cosmological perturbations generated during inflation and lead to consistency relations involving the soft limit of the statistical correlators of large-scale structure dark matter and galaxies overdensities. On the other hand, in observational cosmology the carriers of the information about these large-scale statistical distributions are light rays traveling on null geodesics. Motivated by this simple consideration, we study the structure of null infinity and the associated BMS symmetry in a cosmological setting. For decelerating Friedmann-Robertson-Walker backgrounds, for which future null infinity exists, we find that the BMS transformations which leaves the asymptotic metric invariant to leading order. Contrary to the asymptotic flat case, the BMS transformations in cosmology generate Goldstone modes corresponding to scalar, vector and tensor degrees of freedom which may exist at null infinity and perturb the asymptotic data. Therefore, BMS transformations generate physically inequivalent vacua as they populate the universe at null infinity with these physical degrees of freedom. We also discuss the gravitational memory effect when cosmological expansion is taken into account. In this case, there are extra contribution to the gravitational memory due to the tail of the retarded Green functions which aremore »

- Authors:

- Physics Division, National Technical University of Athens, 15780 Zografou Campus, Athens (Greece)
- Department of Theoretical Physics,24 quai E. Ansermet, CH-1211 Geneva 4 (Switzerland)
- (CAP),24 quai E. Ansermet, CH-1211 Geneva 4 (Switzerland)

- Publication Date:

- Sponsoring Org.:
- SCOAP3, CERN, Geneva (Switzerland)

- OSTI Identifier:
- 22572085

- Resource Type:
- Journal Article

- Resource Relation:
- Journal Name: Journal of Cosmology and Astroparticle Physics; Journal Volume: 2016; Journal Issue: 05; Other Information: PUBLISHER-ID: JCAP05(2016)059; OAI: oai:repo.scoap3.org:15760; cc-by Article funded by SCOAP3. Content from this work may be used under the terms of the Creative Commons Attribution 3.0 License. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.; Country of input: International Atomic Energy Agency (IAEA)

- Country of Publication:
- United States

- Language:
- English

- Subject:
- 71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; ASYMPTOTIC SOLUTIONS; COSMOLOGY; DEGREES OF FREEDOM; GENERAL RELATIVITY THEORY; GRAVITATIONAL WAVES; GREEN FUNCTION; INFLATIONARY UNIVERSE; LIGHT CONE; METRICS; NONLUMINOUS MATTER; SYMMETRY; VACUUM STATES

### Citation Formats

```
Kehagias, A., Riotto, A., and Center for Astroparticle Physics.
```*BMS in cosmology*. United States: N. p., 2016.
Web. doi:10.1088/1475-7516/2016/05/059.

```
Kehagias, A., Riotto, A., & Center for Astroparticle Physics.
```*BMS in cosmology*. United States. doi:10.1088/1475-7516/2016/05/059.

```
Kehagias, A., Riotto, A., and Center for Astroparticle Physics. Wed .
"BMS in cosmology". United States.
doi:10.1088/1475-7516/2016/05/059.
```

```
@article{osti_22572085,
```

title = {BMS in cosmology},

author = {Kehagias, A. and Riotto, A. and Center for Astroparticle Physics},

abstractNote = {Symmetries play an interesting role in cosmology. They are useful in characterizing the cosmological perturbations generated during inflation and lead to consistency relations involving the soft limit of the statistical correlators of large-scale structure dark matter and galaxies overdensities. On the other hand, in observational cosmology the carriers of the information about these large-scale statistical distributions are light rays traveling on null geodesics. Motivated by this simple consideration, we study the structure of null infinity and the associated BMS symmetry in a cosmological setting. For decelerating Friedmann-Robertson-Walker backgrounds, for which future null infinity exists, we find that the BMS transformations which leaves the asymptotic metric invariant to leading order. Contrary to the asymptotic flat case, the BMS transformations in cosmology generate Goldstone modes corresponding to scalar, vector and tensor degrees of freedom which may exist at null infinity and perturb the asymptotic data. Therefore, BMS transformations generate physically inequivalent vacua as they populate the universe at null infinity with these physical degrees of freedom. We also discuss the gravitational memory effect when cosmological expansion is taken into account. In this case, there are extra contribution to the gravitational memory due to the tail of the retarded Green functions which are supported not only on the light-cone, but also in its interior. The gravitational memory effect can be understood also from an asymptotic point of view as a transition among cosmological BMS-related vacua.},

doi = {10.1088/1475-7516/2016/05/059},

journal = {Journal of Cosmology and Astroparticle Physics},

number = 05,

volume = 2016,

place = {United States},

year = {Wed May 25 00:00:00 EDT 2016},

month = {Wed May 25 00:00:00 EDT 2016}

}