Heat olive oil over medium heat. saute red bell pepper and onion until soft. Add cayenne powder, cumin and garlic and saute until fragrant. Add black beans (undrained), tomatoes (with juice), and broth. Bring to a gentle boil, then add pasta. Cook for 6 to 8 more minutes, until pasta is soft. Serve with sour cream and shredded cheddar cheese, if desired.