Light responds to gravitation. Light has momentum. Given these two postulates it follows that if a photon passes a massive body both the photon and the body will be effected, though admittedly the effect on the massive body will be extremely slight. Put another way, a photon has its own gravitational field. Now a photon traveling through the universe is not going to travel in a straight line, it's course will be constantly altered by the gravitational fields it goes through. Since it is constantly changing directions it seems that some of its energy would be radiated away as gravitational waves. Since its velocity is fixed this would mean that it would shift to a lower energy frequency. The longer the photon traveled through the universe the more energy it would lose and the more red shifted it would become. How do we know that the red shift in light from distant stars is shifted due to the star moving away and not due to losing energy in the form of gravity waves?