The Philosophy of Time Travel by Roberta Ann Sparrow

The Tangent Universe
....The Primary Universe is fraught with great peril.
War, plague, famine and natural disaster are common.
Death comes to us all.
....The Fourth Dimension of Time is a stable construct,
though it is not impenetrable.
....Incidents when the fabric of the fourth dimension
becomes corrupted are incredibly rare.
....If a Tangent Universe occurs, it will be highly
unstable, sustaining itself for no longer than several
weeks.
....Eventually it will collapse upon itself, forming a
black hole within the Primary Universe capable of
destroying all existence.