我如何将* big.Int转换为golang中的字节数组

Im trying to do calculations on a big int number and then convert the result to a byte array, but I cannot figure out how to do so this is where Im at so far. anyone got any ideas

sum := big.NewInt(0)

for _, num := range balances {
    sum = sum.Add(sum, num)
}

fmt.Println("total: ", sum)

phrase := []byte(sum)
phraseLen := len(phrase)
padNumber := 65 - phraseLen