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A Clever Way to Measure Distances to Galaxies

Mercury Summer 2011 Table of Contents

by Joseph B. Jensen

Coma Cluster

A Hubble Space Telescope view of a segment of the Coma Cluster of galaxies, one of the densest known galaxy collections in the universe. Courtesy NASA / ESA / Hubble Heritage Team (STScI / AURA).

In 1998, competing teams of astronomers were forced to come to grips with increasingly convincing observations that the expansion of the universe was accelerating, rather than decelerating (as they expected). This cosmic acceleration provided the first clear evidence that a completely unknown force, called "dark energy," dominated the total mass and energy of the universe. The nature of dark energy is one of the most important and compelling questions in all of science today. Its discovery resulted directly from measurements of the distances to remote galaxies.

Determining how far away galaxies are is crucial to our understanding of the universe. Only when we accurately establish the distance to a remote object can we understand many of its properties, particularly what source of energy powers its luminosity. Distance measurements tell us how big the universe is, how old it is, how it changes with time, and what its eventual fate will be. For many years, systematic errors in those measurements led to the awkward conclusion that the universe is younger than the oldest stars in it — a conflict that had astronomers stumped until the 1998 discovery of cosmic acceleration.

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