The Whitney Museum’s new Downtown building is rising fast on Gansevoort Street next to the High Line’s southern end, just east of the Hudson River Park. Designed by architect Renzo Piano, it is slated to open in 2015. The new building will include more than 50,000 square feet of indoor galleries and 13,000 square feet of outdoor exhibition space on a series of rooftops facing the High Line. The building will also sport a huge space for temporary exhibitions — roughly 18,000 square feet — which will be the city’s largest column-free museum gallery.