Re: CMB energy

When you throw a frictionless ball up in the air at initial vertical velocity v_0, it starts off with kinetic energy 0.5 m v_0^2 .

So why does the ball lose (kinetic) energy as it goes up? Why does 0.5 m v^2 get smaller? Where does the energy go?

Well, sure but how can one definne potential energy for a photon?