You can use both variations. Wiktionary lists "time-travel" as an alternative spelling to "time travel", so it seems "time travel" is the more common version.
Any link between time travel and chemistry.
Time travel is not possible, so it's not dangerous at all.
He never even mentions time travel.
Go to a shrine in Ilex forest and after that you time travel
phycological
Subgenre is one word, however, those who doubt how the word is constructed may use a hyphen.
Jazz
Tower defense, which is a subgenre of real-time strategy games.
It's a revenge tragedy.
nature writing
Nerdcore is a subgenre of hip hop.
The book "John Henry" falls under the genre of American folktales and its subgenre would be specifically categorized as a tall tale.
nature writingnature writing
She contributed to the development of the gothic subgenre
Miles Davis
Fiction