Effective theories for a nonrelativistic field in an expanding universe: Induced self-interaction, pressure, sound speed, and viscosity
Abstract A massive, nonrelativistic scalar field in an expanding spacetime is usually approximated by a pressureless perfect fluid, which leads to the standard conclusion that such a field can play the role of cold dark matter. In this paper, we systematically study these approximations, incorporati...
| Main Authors: | , , |
|---|---|
| Format: | Article |
| Language: | English |
| Published: |
SpringerOpen
2020-07-01
|
| Series: | Journal of High Energy Physics |
| Subjects: | |
| Online Access: | http://link.springer.com/article/10.1007/JHEP07(2020)059 |
| _version_ | 1818436126034100224 |
|---|---|
| author | Borna Salehian Mohammad Hossein Namjoo David I. Kaiser |
| author_facet | Borna Salehian Mohammad Hossein Namjoo David I. Kaiser |
| author_sort | Borna Salehian |
| collection | DOAJ |
| description | Abstract A massive, nonrelativistic scalar field in an expanding spacetime is usually approximated by a pressureless perfect fluid, which leads to the standard conclusion that such a field can play the role of cold dark matter. In this paper, we systematically study these approximations, incorporating subleading corrections. We provide two equivalent effective descriptions of the system, each of which offers its own advantages and insights: (i) A nonrelativistic effective field theory (EFT) with which we show that the relativistic corrections induce an effective self-interaction for the nonrelativistic field. As a byproduct, our EFT also allows one to construct the exact solution, including oscillatory behavior, which is often difficult to achieve from the exact equations. (ii) An effective (imperfect) fluid description, with which we demonstrate that, for a perturbed Friedmann-Lemaître- Robertson-Walker (FLRW) universe: (a) The pressure is small but nonzero (and positive), even for a free theory with no tree-level self-interactions. (b) The sound speed of small fluctuations is also nonzero (and positive), reproducing already known leading-order results, correcting a subdominant term, and identifying a new contribution that had been omitted in previous analyses. (c) The fluctuations experience a negative effective bulk viscosity. The positive sound speed and the negative bulk viscosity act in favor of and against the growth of overdensities, respectively. The net effect may be considered a smoking gun for ultra-light dark matter. |
| first_indexed | 2024-12-14T17:03:49Z |
| format | Article |
| id | doaj.art-456f1aede0e3461d80acf4a3a16174db |
| institution | Directory Open Access Journal |
| issn | 1029-8479 |
| language | English |
| last_indexed | 2024-12-14T17:03:49Z |
| publishDate | 2020-07-01 |
| publisher | SpringerOpen |
| record_format | Article |
| series | Journal of High Energy Physics |
| spelling | doaj.art-456f1aede0e3461d80acf4a3a16174db2022-12-21T22:53:46ZengSpringerOpenJournal of High Energy Physics1029-84792020-07-012020714910.1007/JHEP07(2020)059Effective theories for a nonrelativistic field in an expanding universe: Induced self-interaction, pressure, sound speed, and viscosityBorna Salehian0Mohammad Hossein Namjoo1David I. Kaiser2School of Astronomy, Institute for Research in Fundamental Sciences (IPM)School of Astronomy, Institute for Research in Fundamental Sciences (IPM)Department of Phyics, Massachusetts Institute of TechnologyAbstract A massive, nonrelativistic scalar field in an expanding spacetime is usually approximated by a pressureless perfect fluid, which leads to the standard conclusion that such a field can play the role of cold dark matter. In this paper, we systematically study these approximations, incorporating subleading corrections. We provide two equivalent effective descriptions of the system, each of which offers its own advantages and insights: (i) A nonrelativistic effective field theory (EFT) with which we show that the relativistic corrections induce an effective self-interaction for the nonrelativistic field. As a byproduct, our EFT also allows one to construct the exact solution, including oscillatory behavior, which is often difficult to achieve from the exact equations. (ii) An effective (imperfect) fluid description, with which we demonstrate that, for a perturbed Friedmann-Lemaître- Robertson-Walker (FLRW) universe: (a) The pressure is small but nonzero (and positive), even for a free theory with no tree-level self-interactions. (b) The sound speed of small fluctuations is also nonzero (and positive), reproducing already known leading-order results, correcting a subdominant term, and identifying a new contribution that had been omitted in previous analyses. (c) The fluctuations experience a negative effective bulk viscosity. The positive sound speed and the negative bulk viscosity act in favor of and against the growth of overdensities, respectively. The net effect may be considered a smoking gun for ultra-light dark matter.http://link.springer.com/article/10.1007/JHEP07(2020)059Classical Theories of GravityEffective Field Theories |
| spellingShingle | Borna Salehian Mohammad Hossein Namjoo David I. Kaiser Effective theories for a nonrelativistic field in an expanding universe: Induced self-interaction, pressure, sound speed, and viscosity Journal of High Energy Physics Classical Theories of Gravity Effective Field Theories |
| title | Effective theories for a nonrelativistic field in an expanding universe: Induced self-interaction, pressure, sound speed, and viscosity |
| title_full | Effective theories for a nonrelativistic field in an expanding universe: Induced self-interaction, pressure, sound speed, and viscosity |
| title_fullStr | Effective theories for a nonrelativistic field in an expanding universe: Induced self-interaction, pressure, sound speed, and viscosity |
| title_full_unstemmed | Effective theories for a nonrelativistic field in an expanding universe: Induced self-interaction, pressure, sound speed, and viscosity |
| title_short | Effective theories for a nonrelativistic field in an expanding universe: Induced self-interaction, pressure, sound speed, and viscosity |
| title_sort | effective theories for a nonrelativistic field in an expanding universe induced self interaction pressure sound speed and viscosity |
| topic | Classical Theories of Gravity Effective Field Theories |
| url | http://link.springer.com/article/10.1007/JHEP07(2020)059 |
| work_keys_str_mv | AT bornasalehian effectivetheoriesforanonrelativisticfieldinanexpandinguniverseinducedselfinteractionpressuresoundspeedandviscosity AT mohammadhosseinnamjoo effectivetheoriesforanonrelativisticfieldinanexpandinguniverseinducedselfinteractionpressuresoundspeedandviscosity AT davidikaiser effectivetheoriesforanonrelativisticfieldinanexpandinguniverseinducedselfinteractionpressuresoundspeedandviscosity |