Rhys Davis, holothete, consciously runs a self-evolving computer simulation of the entire history of the universe. Because Davis becomes visibly distressed, a "linker," who is in circuit although not participating in the program, cancels the operation and Davis goes catatonic.
The explanation comes at the end of the story. However, Anderson plants a clue earlier. Davis is not overtly religious but was taken to church as a boy by his parents and says that he walks in awe. It seems that he became distressed while experiencing the heat death of the universe but also saw something that made him cry out, "'"The living God!"'" (p. 40)
In the darkness after the universe, a smiling and loving spirit, appearing as a flame, refers to us as "'...my forebears...'" (p. 46) So this "living God" is not our Father but - the Son of Man. I do not use this phrase lightly but I do know that this was not its original meaning. See here. A "son of man" is a man but "one like a son of man" is supernatural humanoid being whereas the "God" encountered by Davis is literally our descendant.