 When we observe light from distant galaxies, we are seeing the light from the stars in those galaxies and that light has absorption lines. The same lines measured in a lab give us the wavelength of the light at the time it was emitted. A stretching of the wavelength creates a shift in the spectral lines to the red. For our nearby galaxies, light travels for a relatively short period of time, so the stretching due to space expansion is small. Our use of the Doppler effect that shifts spectral lines as the basis for determining radial velocities provides excellent measurements, but as the distance increases to hundreds of millions and billions of light-years, space expansion becomes the dominant factor. In either case, we continue to measure redshift z as the difference between the wavelength emitted and the wavelength observed, divided by the wavelength emitted. In this hypothetical example, we have an object with a redshift equal to 6. Once a model for the change in the cosmic scale factor over time is specified, redshift gives us a great deal of information. For now, we'll assume a flat matter-dominated universe. First, redshift gives us an object's receding velocity. With our model, we have the object moving away at 6 times the speed of light. Redshift also gives us the actual cosmic scale factor at the time the light was emitted. It gives us the age of the universe at the time the light was emitted. And it gives us the amount of time the light was traveling. Redshift gives us the distance to the object at the current time, and it gives us the distance to the object at the time the light was emitted. You can see why astronomers rely so heavily on redshift measurements.