The Cosmological Redshift



A subtle point:


Stretched Photon (Balloon Analogy)


Q: What happens to the energy of a photon when you increase its wavelength?

A: The photon loses energy!



Consequence: as the universe expands the density of "radiant energy", i.e. light, in the universe decreases faster than the density of "mass energy".)

energy density = energy per unit volume


At the present day, there is roughly 20,000 times more energy density in the form of mass/matter . We live at a time when the universe is "Matter Dominated".

In the far distant past there would have been a time when there was more energy density in the form of light/radiation. This would have been a time when the universe was "Radiation Dominated".


Q: Where are the "fossil photons" from the Big Bang?

A: Everywhere!
(constitute about 90% of the radiant energy in the universe at the present day - but you can't see them with your eyes...)


On to Cosmic Microwave Background Radiation or back to start.