E8 Gravitational Lenses

Topic

The central feature of this model is gravitational lensing—the deflection of light as it passes near a mass—a phenomenon first confirmed by observation in 1919.

Gravitational lensing is classified into three types based on intensity: strong lensing, which produces arcs, complete Einstein rings, and multiple images of a single source; weak lensing, which distorts the shape of sources only in a purely statistical manner; and microlensing, which causes a transient amplification of the brightness of individual stars. The mass of the lens and the geometry of the alignment determine how a single physical phenomenon produces these three distinct observational effects.

The Einstein deflection angle—the angle at which the gravitational lens deflects light—increases with the square root of both the lensing mass and the distance involved; thus, a greater mass or a greater distance results in a greater deflection. However, a complete Einstein ring forms around the lens only when the source, the lens itself, and the observer are perfectly aligned along a single line. This perfect alignment is an all-or-nothing condition; any misalignment, however slight, causes the ring to break up into arcs or separate multiple images.

When the source behind the gravitational lens is itself variable—as is the case with certain distant galactic nuclei—the multiple images produced by the lens reach the observer with varying time delays, because each image travels a different optical path. This time delay between images makes it possible to measure the Hubble constant (the current rate of the universe's expansion) in a way that is completely independent of the traditional cosmic distance ladder; the value obtained in this manner currently stands in tension with the value inferred from the cosmic microwave background. Thus, from the gravitational lensing with which this document begins to the tension in measuring the universe's expansion rate with which it concludes—taking in the three ways it manifests and the all-or-nothing condition governing the formation of a complete Einstein ring—gravitational lensing transforms the simple deflection of light by gravity into one of the most independent (and currently most debated) methods for measuring the expansion rate of the universe itself.

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