You're not the underdog in the hand if it's the first hand of the tournament. If everyone has equal stacks, AA has pot odds on any number of all-ins. Since it's an all-in situation for everyone, just compare how much money you're getting back for what you're putting in and compare with the odds of you winning. For you and 2 callers if you're 50% to win and getting 3-1 pot odds, easy choice. Even for 40% with you and 3 callers getting 4-1 it works out. Whether you're willing to risk the tournament right there is up to how you want to play the tournament, but since everyone in the hand is going to need to rely on their pre-flop odds, your best bet is to call.