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The universe is incredibly big. But how do astronomers know that? Billion-mile-long tape measures can't be found at the hardware store. Instead, astronomers use the expansion of the universe itself to establish milepost markers. The light from remote objects is attenuated and weakened as space stretches like a rubber band. The consequences are that starlight will look redder relative to a nearby star of the same temperature. When starlight is spread into its component color via spectroscopy, features in the light will be shifted to the red end of the spectrum. This "redshift" can be used to reliably calibrate distances. The challenge is the farthest objects in the universe are typically too faint for spectroscopy to work. So instead, astronomers deduce a galaxy's distance by precisely measuring its colors in visible and infrared light. This technique has found candidates for the farthest object in the universe.
via HubbleSite NewsCenter -- Latest News Releases
http://hubblesite.org/newscenter/archive/releases/2015/22/
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