Hi, Im reading Tasi lectures on cosmology and in a lot of places states that certain magnitudes (example, comoving curvature R) work in someway (example "does not evolve") outside the horizon k<aH, where k is some fourier frequency of the magnitude we may be talking about. I understand the general idea (outside the horizon things cannot affect what happens inside the horizon -or something like that-. k is a frequency so 1/k > (aH)^-1 -that is wavelength > hubble radius should behave in a special way-) but: 1 i dont see how that intuitive condition transform in "for k<aH" in mathematical language. I get what it tries to say but I cant see the full symmetry between the words and the math 2 what "does not evolve" mean in this context (or whatever other condition is defined on any other magnitude) 3 why does it have to mean that? I insist, i get the idea that things otuaide my horizon cant affect me, my question is how is that idea translated to mathematical language. I dont know if the question is clear, i hope you can help me! Thanks in advance for your usual useful help!