I've got two cats, one older than the other. I've had them both together for several years, and aside from occasional sparring, they've been able to live together pretty peacefully. My younger cat recently had to have surgery, and she came back from the vet two days ago--and now the older cat seemingly doesn't recognize her, and hisses and growls every time she comes near. I'm sure they'll get used to each other again eventually, but is there anything I can do to help speed up the process? (They are both females and both spayed, if that matters.)

Cats recognize by scent. When your younger cat came home, she came home smelling different. Your older cat doesn't recognize her. There is nothing to do to speed up the precess, just make sure they are both getting enough attention and they will again become accustomed to each other.

Place both of them in a small cardboard box, tape it shut, gently roll it down a flight of stairs, and wait 10 minutes. They will be BFF again