Robert Charles Wilson

"Two events made September 1st a memorable day for Jesse Cullum. First, he lost a pair of Oakley sunglasses. Second, he saved the life of President Ulysses S. Grant. It's the near future, and the technology exists to open doorways into the past--but not our past, not exactly. Each "past" is effectively an alternate world, identical to ours but only up to the date on which we access it. And a given "past" can only be reached once. After a passageway...

Turk and his young friend Isaac Dvali are taken up by a community of fanatics who use them to enable a passage to the dying Earth, where they believe a prophecy of human/Hypothetical contact will be fulfilled.