Katherine Pavlis Porter came of age in the 1960s, and you can see the echoes of abstract expressionism in her work, but also a reaction against it. She has a more refined, Kandinsky-like linearity that makes her paintings on paper edgy but not soppy. Her show, ''Living on This Earth,'' at Alpha Gallery, is the first she's had in Boston in more than a decade. These works are dense, bright, and full of tension. They're bold, but unlike the abstract expressionists' paintings, not heroic or ego-driven. They're more interrogatory than declarative, and there's so much more delicious creative energy in questions than there is in answers. Over fields of color, Porter uses a lexicon of images: concentric circles; stacked, brief horizontal bars in undulating columns that vaguely recall spines; lines that loop down the page like loosely coiled springs. She grounds ''Mythology'' on fields of orange and blue, like fire against a dark sky. Large circles hover over the surface like different lenses through which to view the world and its creation. She builds space and then fractures it, or pulls it inside out, with all her lines and shifting colors.