Not to the same extent, part of the point of AT is that everything shares the same feed, while the AP relays only offer the view of co-operating servers.
Now Mastodon does offer its own streaming API that lets you see every post on a given server, and at least the fedibuzz relay offers that as well, so if you use that API endpoint you’ll see every post of every server it knows about in real time.
They already exist, they range from smaller webring like relays to ones that tie the major nodes together.
Neat! Does that mean there’s an ActivityPub equivalent to the ATProto Firehose?
Not to the same extent, part of the point of AT is that everything shares the same feed, while the AP relays only offer the view of co-operating servers.
Now Mastodon does offer its own streaming API that lets you see every post on a given server, and at least the fedibuzz relay offers that as well, so if you use that API endpoint you’ll see every post of every server it knows about in real time.