A lot of the symbols and variables simply represent physical quantities that the writer of the paper assumes you already know about (why else would you be reading a technical cosmology paper

because you've read some introductory text on the subject.The best text on relativity and cosmology is Misner, Thorne, and Wheeler. It has a "mathematical track" for the technically minded, and a "nonmathematical track" for those who are merely proficient at nonhomogeneous nth order partial differential equations. Best textbook ever written on any subject, actually.