All could be antonyms

adv could be

• certainly — You use certainly to emphasize what you are saying when you are making a statement.
• definitely — You use definitely to emphasize that something is the case, or to emphasize the strength of your intention or opinion.
• surely — firmly; unerringly; without missing, slipping, etc.
• improbably — not probable; unlikely to be true or to happen: Rain is improbable tonight.
• unlikely — not likely to be or occur; improbable; marked by doubt.
• impossibly — not possible; unable to be, exist, happen, etc.