What happens to the energy of light as it crosses the universe

Light coming from ever more distant galaxies has an ever-longer wavelength, or a ‘red shift’. According to physics, that should mean the photons of light have lost energy on their journey. Yet this overlooks the weird physics that kicks in on cosmic scales. In particular, it ignores the fact that the Universe – and thus the very fabric of space and time – is expanding. This literally stretches the light, increasing its apparent wavelength while leaving its energy unchanged. This explanation is still rejected by some, who argue it’s the cosmic expansion that’s an illusion, while the energy of light really does decrease through collisions with particles in its path. This so-called ‘tired light’ idea is intuitively much more appealing, and can be put to the test. For example, it predicts that distant objects will appear much fuzzier, as the higher-energy photons striking them will be scattered more violently. Yet images of distant galaxies show they’re relatively sharp. On the other hand, the cosmic expansion theory predicts that distant supernova explosions should appear slower than nearby ones, while tired light theory predicts no such stretching of time. Observations of supernovae have confirmed a cosmic ‘slow-mo’ effect – proving tired light wrong again.