简述目前超弦理论中比较受到研究者较多关注的几个方向，特别是超弦宇宙学以及观测宇宙学的最新进展对理论提出的挑战。
A few active directions in string theory are described in this paper, in particular, string cosmology and challenges to theorists posed by recent results in observational cosmology.
葛林是哥伦比亚大学ISCAP（弦论、宇宙学和天体粒子研究中心）的教授。
He is a professor at Columbia University's Institute for Strings, Cosmology, and Astroparticle Physics.
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