1. The problem statement, all variables and given/known data The assignment is to find the energy loss constant for the theory of tired light. If redshift isn't caused by the expansion of the universe but from a loss of energy, how much of a photons energy is lost per unit distance(MegaParseC) for some set Hubble constant. 2. Relevant equations The equation: dE/dr = -K*E r is the distance and K is the energy loss constant. 3. The attempt at a solution Because photon energy is determined by E=h*f, the energy loss must correspond to the decrease in frequency. The relative frequency (per MPC in this instance) loss comes from the Doppler equation(c being the speed of light): f=(c/(c+H0))*f0 So with a Hubble constant (H0) of 70km/s per MPC gives loss in frequency of ~ 0.00033 % per MPC. From here I'm kind of lost. How do i convert this to a relative energy loss (as the is no set frequency). With these formulas I'm still working with the electronvolt of the planck constant, but i know the K constant does not have an energy dimension (it is only has MPC^-1). I would be very thankful for any kind of pointers.