Question

A gravitational lens should produce a halo effect and not arcs. Given that the light travels not only to the right and left of the intervening massive object but also to the top and bottom, why do we typically see only arcs?

Short answer

Answer: Gravitational lenses typically produce arc-like images instead of a halo effect due to the elliptical mass distribution of the lensing objects, the geometry of the lensing system, and the rarity of perfect cosmic alignments. These factors contribute to the formation of arcs rather than complete rings in most gravitational lensing situations.

Step-by-step solution

Recall the definition of gravitational lensing

Gravitational lensing is a phenomenon that occurs when light from a distant object, such as a galaxy, is bent around a massive object, such as a galaxy cluster, due to its gravitational field. This bending of light causes the light to take different paths, resulting in distorted and multiple images of the distant object.

Understand the arc effect in gravitational lensing

In gravitational lensing, arcs are formed due to the mass distribution of the lensing object. When the mass of the intervening object is not symmetrical or the background object is not perfectly aligned with the lensing object, the light's path becomes distorted, causing the formation of arcs instead of a complete ring.

Consider mass distribution and geometry

The intervening massive object, such as a galaxy cluster, generally has an elliptical mass distribution. This uneven distribution of mass results in stronger gravitational lensing effects along the major axis of the elliptical mass profile, causing arc-like images to be more prominent. When a background source is closer to the major axis of the foreground mass distribution, the tangential stretching of its light is more pronounced, forming an elongated, arc-like image.

Remember the cosmic alignment

In order for a perfect halo or ring effect to be visible, the alignment of the lensing object, the distant object, and the observer must be nearly perfect. This ideal alignment is quite rare in the universe, making the exact halo effect less common. Instead, what we typically see are arc-like images due to the slightly misaligned positions of objects in cosmic gravitational lensing situations.

Conclusion

In conclusion, we typically see arc-like images in gravitational lensing instead of a halo effect due to the elliptical mass distribution of the lensing objects, the geometry of the lensing system, and the rarity of perfect cosmic alignments. The combination of these factors results in the formation of arcs rather than complete rings in most gravitational lensing situations.