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22h ago
Ask Ethan: Why do gravitational lenses make crosses, not rings?
- The most common gravitational-lensing outcome is a quadruply imaged background source, producing an Einstein cross rather than a ring.
- Rings require near-perfect symmetry and alignment; real lenses are not spherical and backgrounds rarely align exactly along the line of sight.
- Einstein rings do exist but are rare; the study emphasizes crosses are more common due to lens and source geometry.
- Huchra’s lens (G2237+0305) illustrates a four-image Einstein cross formed by a foreground galaxy lensing a background quasar.
- Gaia and JWST surveys have expanded the catalog of Einstein crosses, aiding dark matter studies.
- The nature of lens mass distribution—ellipticity, substructure, and multiple components—drives cross-shaped appearances.
- Crosses often arise when alignment is imperfect and the lens has aspherical substructure, not a perfect sphere.
- Partial rings and arcs occur when the lens is near-symmetric or the alignment is nearly perfect but not exact.
- Astrophysicists use cross configurations as probes of dark matter substructure within lensing galaxies.
- Cosmic lensing discoveries by Hubble and JWST are expanding understanding of the universe and cosmic history.
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