You know that short selling is possible? And index funds are traditionally some of the keenest participants to lend their shares out to short sellers in return for a bit of extra return (over the raw index).
As far as I can tell, there's no minimum period you have to wait after an IPO to be able to short shares. Legally, you can do it from day one.
And instead of a classic short where you have to borrow the stock, you can also write single stock futures. Futures don't require you to borrow the underlying. You just need enough collateral, but that can be anything, like T-bills or whatever.
Or you can write call options, or buy put options, to bet on a falling stock price.