In this example, the earth isn't in an accelerating reference frame, only the astronaut is. There isn't a "master reference frame", but that doesn't mean that acceleration is relative.

Despite the upvotes on reddit, this really is a topic that can only be done justice by good book or instructor. Any self-respecting library should have a copy of Relativity: The Special and General Theory by Einstein, it's written specifically for the layman. Come to think of it,... I should probably pick it up again myself.