Barack Hussein Obama became the 44th president of the United States Tuesday, and called on Americans to join him in confronting what he described as an economic crisis caused by greed but also "our collective failure to make hard choices.""Today I say to you that the challenges we face are real," Mr. Obama said in his inaugural address minutes after he took the oath of office on the same bible used by Abraham Lincoln at his first inaugural in 1861. "They are serious and they are many. They will not be met easily or in a short span of time. But know this, America - they will be met."Mr. Obama, the first African American to serve as president, spoke to a sea of cheering people, hundreds of thousands of Americans packed on the National Mall from the Capitol to beyond the Washington monument. The multitude was filled with black Americans and Mr. Obama's triumph was a special and emotional moment for them.With his wife, Michelle, holding the Bible, Mr. Obama, the 47-year-old son of a white mother from Kansas and a black father from Africa, was sworn in just after noon, a little later than planned, and spoke immediately thereafter..In his speech, Mr. Obama promised to take "bold and swift" action to restore the economy by creating jobs through public works projects, improving education, promoting alternative energy and relying on new technology."Starting today, we must pick ourselves up, dust ourselves off, and begin again the work of remaking America," Mr. Obama said in a prepared copy of his remarks.