MAJOR SPOILERS ahead for Crisis on Infinite Earths Part 4.
The CW's Crisis on Infinite Earths crossover confirms the DC Extended Universe (DCEU) is part of the Arrowverse with a cameo from Justice League star Ezra Miller, who plays Barry Allen aka. The Flash. The network launched its small-screen shared DC universe in 2012, while Warner Bros. worked to launch its own DC Comics franchise on the big screen. That big screen series, come to be known unofficially as the DCEU expanded in a big way in 2016 with the release of Batman V Superman: Dawn of Justice, which introduced key heroes like Ben Affleck's Batman, Gal Gadot's Wonder Woman, Jason Momoa's Aquaman and Miller's The Flash. The heroes united in 2017's Justice League, but The Flash movie is still to come in 2022.
Last month, The CW's Crisis on Infinite Earths event kicked off and proceeded to confirm all manner of DC movies and TV shows as canon within the Arrowverse. DC Universe's Titans, the classic 60s Batman series, the short-lived Birds of Prey TV show, Smallville and more have all been confirmed to be included in the Arrowverse's multiverse. In Crisis On Infinite Earths Part 4, though, The CW makes arguably its biggest confirmation - and has its most surprising cameo - yet.
Partway through Crisis On Infinite Earths Part 4, Miller's DCEU version of the Flash appears in the Speed Force while The CW's Flash (Grant Gustin) is searching for his friends. After a short conversation, Miller's Flash disappears, and their brief exchange ends with the DCEU Barry Allen saying, "I told Victor this was possible." This is a clear reference to fellow Justice League member Victor Stone aka Cyborg (Ray Fisher).
Ever since Miller was cast as the DCEU's Flash in 2014, fans of Gustin's version of the Scarlet Speedster have wanted the duo to meet on screen. Now, The CW has brought the fan service moment to life in Crisis on Infinite Earths. The brief conversation between the two Flash heroes includes Miller's character asking Gustin's if he's cosplaying, treating the CW superhero like a fan. When Arrowverse Barry introduces himself as The Flash, DCEU Barry looks contemplative, hinting this is how the Justice League character gets his superhero name. They compliment each other's costumes, and go to shake hands, both saying, "I'm Barry Allen," at the exact same time. The exchange ends with Arrowverse Flash alluding to the destruction of the multiverse, saying, "This should be impossible now." Miller's Flash seems confused, but then fades away, making the reference to Cyborg.
Arrowverse Flash has a point that he shouldn't be able to run into another Flash from the multiverse considering the Anti-Monitor had destroyed every single Earth. However, it's possible Barry ran across a remnant or memory of the DCEU's Flash within the Speed Force since it connects to the multiverse - it just wasn't his own memory. It's unclear how exactly that works, and it's possible the Arrowverse producers were focused more on bringing the fan service moment to life than on explaining how and why it's possible. For the fans, seeing two Flashes interact on screen was no doubt an exciting moment in DC TV history, and they can have even more fun speculating about what it means for not only the Arrowverse, but the DCEU and The Flash movie.