From Heroes Wiki
Future Hiro warns Peter that he may be risking a rift in speaking with him.
- In a blog entry called "New York or Bust", Hiro writes that he is disappointed in Future Hiro. "Even though he/I will/was very cryptic he/I should've not interacted with someone from my current timeline. I'm disappointed. He/I should've known/should know the negative ramifications of time travel. (>.<)"
- Hiro expresses his concern in a blog entry that he "might be risking a rift" by going back in time to save Charlie.
- Hiro sends an email to Hana stating that going back to give Peter Petrelli a message is a rift, but that the "rift is not a risk. It can't be."
- After traveling to April 24, 2006, Hiro believes that just by being there he is messing with the space-time continuum. Later when Hiro calls Yamagato Industries and accidentally talks to himself, Hiro believes that chatting on the phone with his past self is "unanimously considered a bad idea." (Chapter 7)
"You're talking out of your ass. You don't know how any of this works."
- In an interview for Heroes Unmasked, Masi Oka explains that Future Hiro gives the message to Peter as a way to communicate to his past self, presumably to avoid a rift.
Please refer to Theory:Rift for fan-created theories and other speculation.