The first thing that was needed was to find a population
of very remote objects. We would then compare the objects’
known brightnesses (or intensities, I) to their observed
brightnesses (Io) and infer the distances. While the requirements
of general relativity dictate a more complex algorithm
is employed, this is conceptually similar to employing the
familiar I = Io/(4pr2). Galaxies are visible distant objects, but
the problem is that an individual galaxy’s intrinsic brightness
is unknown and therefore they are unsuitable for this measurement.
However, there exists an astronomical event for
which the intrinsic brightness can be determined—exploding
stars of the type that opened this article. Much like the galaxies
that Hubble observed, supernovae serve as buoys in the
fabric of spacetime to reveal the speed of spatial expansion
(and any changes in it). These Type Ia supernovae are formed
in binary star systems in which one is a white dwarf. As
shown in Fig. 3, the white dwarf siphons off matter from its
companion until the white dwarf ’s mass reaches about 1.38
times the mass of our Sun. At that mass, the star explodes in a
supernova. This process results in a very bright and predictable
initial light source, corresponding to an object that, were
it to explode at a distance of 70 light-years from Earth, would
appear to be about 100 times brighter than a full Moon. For
cosmological distance measurements, Type Ia supernovae are
the preferred objects of study.