my interpretation of the expansion of the universe might be naive... correct me if I make any mistake. So, space in general is expanding, or being created all over the universe. If you have a photon traveling in free space, the frequency gets shifted. But Energy=h*frequency; the energy of the photon decreases as it travels. Where did the energy of that wave go? Similarly, what about momentum? p=E/c, is that not conserved as well? I think it has something to do with reference frame. How can one resolve the paradox?