A Gallery of Lenses (4) ...

The preceding examples have illustrated gravitational lensing by large mass concentrations. However, gravitational lensing is also important for smaller objects. This lensing by smaller objects is called microlensing.
Gravitational Microlensing and MACHOs
The following animation illustrates the use of gravitational microlensing to search for dark, compact objects in the halo of the galaxy. Such objects are called Massive Compact Halo Objects (MACHOs). They could be a source of part of the missing baryonic mass.

In MACHO searches one looks for an unexpected brightening of a star that could be causes by gravitational lensing from an unseen foreground object. For example, candidate events for MACHOs have been found by using the Magellanic Clouds as the background stars. Such brightening could in principle also be caused by intrinsic variability of the star, but this interpretation is ruled out by three features:

  • The MACHO candidates correspond to events where a star brightens and then dims only once. Variable stars are expected to change their light output periodically, not just once.
  • The light output from the MACHO candidates is observed to vary in the same way for two different wavelengths (red and blue light). This is expected for a gravitational lens, but for a variable star one would generally expect the variation at different wavelengths to not be the same.
  • The detailed shape of the light curve, in particular that it has a symmetric peak rather than being skewed to one side or the other, is as expected from mathematical models of lensing. Most light curves for variable stars are less symmetric than those observed for MACHO candidates.

    • These checks make a strong case that the brightening observed is actually caused by a massive lensing object lying between us and the brightening star. However, note that even if this is a correct interpretation, it is not certain that any MACHO thus detected is in the halo of our galaxy. For example, it is possible that the brightening star and the lensing mass both lie in the Large Magellanic Cloud.