$1)$ Why is S&P $500$ a good proxy for the implied equity risk premium $($ERP$)$ estimation?

$2)$ Based on the Gordon model, given the current price at $2,300,$ please estimate the most current ERP.

Expected Dividends in $201850$

g: $4\mathrm{.}5\%$

Rf: $0\mathrm{.}5\%$