Under the Equal Pay Act, does someone hired on the same day, with the same responsibilities as me have to be paid the same amount?
Only if the reason for paying you differently is done on the basis of your gender. For purposes of other laws, any illegal discriminatory basis may apply. In general, however, there is no blanket rule.
Disclaimer: These questions are derived from questions I sometimes get, with different facts, laws, and so forth. Every situation is different, so talk to a lawyer if you have questions.