Yesterday I wrote about the Alcock-Paczynski cosmological test (http://goo.gl/uouVEh), and how it narrowed the field of cosmological models down to two broad choices: an expanding universe with dark matter and dark energy, or a static universe that exhibits what is known as “tired light”. Now you might think that adding “tiredness” to light is no worse than inventing “dark matter” and “dark energy” to fit observational data. From a theoretical standpoint you’d be right. So why do astronomers accept dark matter and dark energy rather than tired light?
The Color of Speed
In our everyday lives, we’re familiar with the Doppler effect as it applies to sound. You might notice when a car or train passes you, its sound shifts downward as it passes. This is because the sound waves from an object are bunched together as it moves toward you, and stretched apart as it moves away from you. For light a similar thing occurs.
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