In 2019, Supergirl could finally share the same Earth as her fellow Arrowverse series. At the conclusion of the Elseworlds crossover, The CW announced that 2019's event would be Crisis on Infinite Earths. In DC Comics, a Crisis is an event where worlds die and the universe is redefined. Is such a cataclysm headed for The CW's shared superhero universe - with the end result of Supergirl's Earth folding into the same Earth as Arrow, The Flash, and DC's Legends of Tomorrow?
Supergirl originally premiered on CBS in the fall of 2015. Though it was produced by Greg Berlanti, who also created the other Arrowverse series, the Girl of Steel's adventures were originally kept separate from the happenings on The CW - until The Flash crossed over into Supergirl for the season 1 episode "World's Finest". This cemented that Supergirl was indeed part of the Arrowverse, but she existed on Earth-38, while the other series occupy Earth-1.
Supergirl's move to The CW for season 2 (and production relocating to Vancouver from LA) made further crossovers easier; Kara Danvers not only had a musical two-parter with Barry Allen but she'd also meet the rest of her fellow superheroes in the next three crossover events: 2016's Invasion, 2017's Crisis On Earth-X, and 2018's Elseworlds.
What Is Crisis On Infinite Earths?
Crisis on Infinite Earths was a 1985-1986 DC Comics event that the publisher used to streamline its Multiverse into one prime universe. By 1985, DC's numerous characters were scattered across multiple alternate realities and there were different versions of their core heroes, like Batman and Superman; the Crisis wiped many characters and their realities from existence and rebooted their universe into one timeline where all superheroes lived on one Earth.
The Arrowverse finds itself already in a similar predicament; after The Flash established the existence of the Multiverse, fans have witnessed many different alternate versions of the Arrowverse's heroes living on different Earths. While this has been great fun, it has also created continuity issues and has resulted in a fractured, confusing superhero shared universe.
Crisis on Infinite Earths could address those issues and it has long been an event the Arrowverse was building to; the Crisis was teased in the pilot episode of The Flash in 2014 when the Scarlet Speedster saw the headline of The Central City Citizen newspaper dated April 25, 2024: "Flash Missing, Vanishes In Crisis". With 2019's Crisis on Infinite Earths event, the Arrowverse is moving up that timeline by 5 years, and their aim could be the same as the 1985 comics: to reboot the entire Arrowverse's history and reality.
How Supergirl's Earth Could Merge With The Main Arrowverse
Crisis on Infinite Earths could result in Supergirl's Earth-38 being merged into Earth-1. In Elseworlds, the Monitor "tested" worlds by seeing if they were "strong enough" to survive a coming threat - but those worlds were left destroyed and its superheroes dead. Earth-1 proved strong enough to the Monitor's satisfaction - but this could also mean Supergirl's Earth-38, which is gripped in anti-alien paranoia, isn't strong enough and could be destroyed in Crisis on Infinite Earths.
Destroying Earth-38 could solve some big Arrowverse issues. Originally, The CW's superhero universe was built around Green Arrow, but now that the Arrowverse has grown significantly and it's been established that Supergirl, Superman, and Batman exist, and it has also begun to make less sense that the greatest superheroes of the DC Universe don't live on the same Earth as Green Arrow and the Flash. (However, Batman does exist on both worlds - the Earth-1 Batman is missing but Earth-38's Batman is apparently active). One of the biggest unanswered questions of the Arrowverse is why Earth-1 has no Supergirl or Superman.
Crisis on Infinite Earths can fix this since the event is literally about the Multiverse coming to an end and all realities merging into one Prime reality and timeline. Therefore, Crisis on Infinite Earths could literally rewrite Arrowverse history so when Krypton explodes, Kal-El and Kara Zor-El's escape pods would crash on Earth-1 so that they both always lived on Earth-1. Rebooting the universe could be the reason why the Arrowverse will have a Crisis in 2019 - and it could all end with Supergirl becoming Earth-1's favorite Kryptonian.