WHEN Jay-Z takes the stage on Friday night at the Barclays Center in Brooklyn, it will be more than the triumphant homecoming of a native son, performing eight sold-out nights in a sparkling new arena near the housing project where he grew up. It is also the official opening of an 18,000-seat performance space that will fundamentally transform the music scene in New York City.


The arena’s opening is being closely watched in the music business, and perhaps nowhere more so than in Manhattan, at Madison Square Garden, which for generations has been the only game in town for arena acts and has earned a reputation as a career-defining concert hall.


Though Garden officials and Barclays executives are both careful to say that the arenas are not in direct competition for big-name bands, concert promoters and booking agents say such competition is inevitable, and Barclays has already positioned itself as an alternative, booking several marquee performers who have played the Garden in the past.


“Barclay’s is going to say, ‘Hey, if you want to play New York City, you have two choices,’ ” said Jim Glancy, a partner in the Bowery Presents, a New York concert promoter.


With three professional sports teams and St. John’s basketball, the Garden’s calendar has long been extremely crowded, making it a headache for tour planners seeking an open night in the city; its calendar has been even tighter over the last two years as it has undergone renovations.


And while the Garden is considered a prestigious place to play — a high-profile arena in the media capital of North America — it is not always a lucrative one. The costs of doing a show there are high and profit margins low, promoters say. What’s more, it is hard to get two nights in a row there, even if the band or performer is capable of selling out two dates.


“I consider it a godsend Barclays arena is there,” said Randy Phillips, the chief executive of AEG Live, one of the largest promoters in the country. “Prior to this we were really kind of held hostage on a tour to the availability of Madison Square Garden.”


The new $1 billion arena rises at the intersection of Flatbush and Atlantic Avenues like a modern sculpture, evoking a crashed alien spacecraft with its rusted-steel-and-glass facade and swooping lines. Inside, it is a sleek study in gray and black broken only by bright digital banners, with steeply raked rows of black seats that descend from the street level into the arena’s bowl. With clear sightlines and acoustic panels over hard surfaces to minimize reverb and noise, the space seems intimate yet open. For some, it is a symbol of Brooklyn’s cultural and economic renaissance, a sign the borough has come back from the long slide that started when the Dodgers left in 1957. But it is also a symbol of the borough’s growing stature as center for the arts.


Several big acts have added a night in Brooklyn in addition to planned dates at the Garden, among them Neil Young and Crazy Horse, the Who, Lady Gaga, Justin Bieber and Leonard Cohen. Others who have played the Garden in the past are foregoing it altogether and playing Barclays instead, among them Bob Dylan, Green Day, John Legend, Rihanna and Rush.


Brett Yormark, the chief executive of the Brooklyn Nets, who will call the arena home, and of Barclays Center, said he did not see it in competition with the Garden. He said there was a demand in New York City for far more arena concerts than the Garden could accommodate.


“We knew there was a void, and a new venue in Brooklyn could be supported by a lot of events that weren’t coming here,” he said. “I don’t look at this as cannibalization in any way. I look at it as truly additive to the market.”


Melissa Ormond, the president of Madison Square Garden Entertainment, acknowledged that the Garden’s tight calendar has meant it has few dates to offer artists, and that Barclays has benefited, as have the suburban arenas. But she said in an e-mail statement: “We always respect any competition, but the Garden will always be the Garden. We’ve been privileged to be at the center of New York City for more than 130 years, with 400 annual events, the most of any arena in the country. It’s the pinnacle of an artist’s career to play Madison Square Garden.”