Heads of state and big-name billionaires opened the Paris climate summit with a bang on Monday, promising billions of dollars to develop new green technology to solve a key sticking point of the negotiations: financing a low-carbon future for developing nations like India. Yet campaigners and experts warned that it will be hard to deploy any new technologies quickly in places where they are needed most unless negotiators at the two-week U.N. talks can work out a deal on how rich countries will help finance this. “I think the elephant in the room is still finance,” said Yvo de Boer, former head of the U.N. climate change secretariat. He and others were encouraged by Monday’s announcements, which opened the taps for funding a wave of research in hopes of breakthroughs such as the artificial photosynthesis Bill Gates envisions to produce liquid hydrocarbons that challenge fossil fuels. France and India launched a plan for a trillion-dollar alliance to deliver solar energy to poor nations.