mixing Heinlein, Dick, and Hamilton there are you?
worst case.
population: 50 billion+
pollution: nonexistent
energy: thermal and
solar, fusion
tech: hydroponics, thought control
peace: totalitarian world government keeping population under control by forced euthanasia of non-productive people
best case:
population: 1 billion or less on the planet, billions all over the galaxy
pollution: non-existent
energy: solar, fusion, weird stuff
tech: ftl travel (maybe even instant comms/travel), genetics/life enhancement techniques yielding what is effectively immortality.
peace: total. With lifespans stretching into centuries or more the risk of sending people into combat is too high to contemplate.