On the number of light rings in curved spacetimes of ultra-compact objects
In a very interesting paper, Cunha, Berti, and Herdeiro have recently claimed that ultra-compact objects, self-gravitating horizonless solutions of the Einstein field equations which have a light ring, must possess at least two (and, in general, an even number of) light rings, of which the inner one...
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| Format: | Article |
| Language: | English |
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Elsevier
2018-01-01
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| Series: | Physics Letters B |
| Online Access: | http://www.sciencedirect.com/science/article/pii/S0370269317309139 |
| _version_ | 1818130951053508608 |
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| author | Shahar Hod |
| author_facet | Shahar Hod |
| author_sort | Shahar Hod |
| collection | DOAJ |
| description | In a very interesting paper, Cunha, Berti, and Herdeiro have recently claimed that ultra-compact objects, self-gravitating horizonless solutions of the Einstein field equations which have a light ring, must possess at least two (and, in general, an even number of) light rings, of which the inner one is stable. In the present compact paper we explicitly prove that, while this intriguing theorem is generally true, there is an important exception in the presence of degenerate light rings which, in the spherically symmetric static case, are characterized by the simple dimensionless relation 8πrγ2(ρ+pT)=1 [here rγ is the radius of the light ring and {ρ,pT} are respectively the energy density and tangential pressure of the matter fields]. Ultra-compact objects which belong to this unique family can have an odd number of light rings. As a concrete example, we show that spherically symmetric constant density stars with dimensionless compactness M/R=1/3 possess only one light ring which, interestingly, is shown to be unstable. |
| first_indexed | 2024-12-11T08:13:11Z |
| format | Article |
| id | doaj.art-57371a77d6bf43aa9ab1e7e789eb563f |
| institution | Directory Open Access Journal |
| issn | 0370-2693 1873-2445 |
| language | English |
| last_indexed | 2024-12-11T08:13:11Z |
| publishDate | 2018-01-01 |
| publisher | Elsevier |
| record_format | Article |
| series | Physics Letters B |
| spelling | doaj.art-57371a77d6bf43aa9ab1e7e789eb563f2022-12-22T01:14:50ZengElsevierPhysics Letters B0370-26931873-24452018-01-01776C1410.1016/j.physletb.2017.11.021On the number of light rings in curved spacetimes of ultra-compact objectsShahar Hod0The Ruppin Academic Center, Emeq Hefer 40250, IsraelIn a very interesting paper, Cunha, Berti, and Herdeiro have recently claimed that ultra-compact objects, self-gravitating horizonless solutions of the Einstein field equations which have a light ring, must possess at least two (and, in general, an even number of) light rings, of which the inner one is stable. In the present compact paper we explicitly prove that, while this intriguing theorem is generally true, there is an important exception in the presence of degenerate light rings which, in the spherically symmetric static case, are characterized by the simple dimensionless relation 8πrγ2(ρ+pT)=1 [here rγ is the radius of the light ring and {ρ,pT} are respectively the energy density and tangential pressure of the matter fields]. Ultra-compact objects which belong to this unique family can have an odd number of light rings. As a concrete example, we show that spherically symmetric constant density stars with dimensionless compactness M/R=1/3 possess only one light ring which, interestingly, is shown to be unstable.http://www.sciencedirect.com/science/article/pii/S0370269317309139 |
| spellingShingle | Shahar Hod On the number of light rings in curved spacetimes of ultra-compact objects Physics Letters B |
| title | On the number of light rings in curved spacetimes of ultra-compact objects |
| title_full | On the number of light rings in curved spacetimes of ultra-compact objects |
| title_fullStr | On the number of light rings in curved spacetimes of ultra-compact objects |
| title_full_unstemmed | On the number of light rings in curved spacetimes of ultra-compact objects |
| title_short | On the number of light rings in curved spacetimes of ultra-compact objects |
| title_sort | on the number of light rings in curved spacetimes of ultra compact objects |
| url | http://www.sciencedirect.com/science/article/pii/S0370269317309139 |
| work_keys_str_mv | AT shaharhod onthenumberoflightringsincurvedspacetimesofultracompactobjects |