What do the eyes of Doctor T. J. Eckleburg symbolise in The Great Gatsby?

The eyes of Doctor T. J. Eckleburg are really an advertisement for an eye doctor; they feature on a billboard that overlooks the valley of ashes, a desolate area through which the characters in the novel must travel to reach New York. However, the protagonist, Nick, delays telling the reader that they are not real, which causes the reader to form different ideas as to what the disembodied eyes are, creating an eerie atmosphere. This atmosphere is suited to the valley of ashes as this area represents the grotesque, decaying reality beneath the glamour of New York and the American Dream. The eyes overlook much of the drama in the novel, and may be compared to the eyes of God, suggesting that he is always watching the characters. The eyes might therefore be introducing religion into a story from which God is otherwise absent, suggeting to the reader that the characters are always being judged. Alternatively, the eyes might imply that God has been replaced by capitalism: in a world where religion no longer plays a large part, the closest manifestation of God is as an advertisement.