compounds = ?

Context:

Penrose is at his best when he explains this deep and beautiful mystery, and the book may be worth reading for this chapter alone. However, he compounds the shortcomings of his cyclic universe model when he argues that it can solve the low-entropy problem. At this point, another idea is introduced: like vac-uum cleaners, black holes appear to reduce
disorder by swallowing matter. By the end of one “aeon,” Penrose argues, most matter has ended up in giant black holes. Very little entropy remains, and the next aeon can com-mence in perfect order.