Liu, ZG (reprint author), Univ Chinese Acad Sci, Sch Phys, Beijing 100049, Peoples R China.
; Piao, Yun-Song] Univ Chinese Acad Sci, Sch Phys, Beijing 100049, Peoples R China
; [Li, Hong] Chinese Acad Sci, Inst High Energy Phys, Key Lab Particle Astrophys, Beijing 100049, Peoples R China
; [Li, Hong] Chinese Acad Sci, Natl Astron Observ, Beijing 100012, Peoples R China
; [Piao, Yun-Song] Chinese Acad Sci, Inst Theoret Phys, State Key Lab Theoret Phys, Beijing 100190, Peoples R China
Recent B-mode polarization observation seems to imply the tensor tilt n(T) greater than or similar to 1 at large angular scale if the primordial signal is dominated. We show that for a primordial universe, which is in a slowly expanding genesis phase before the slow-roll inflation, the primordial tensor spectrum will get a large-scale cutoff, i.e., n(T) greater than or similar to 1 at large scales while n(T) similar or equal to 0 at small scale. We find that this inflationary scenario not only may be consistent with the observation, but also predicts a large-scale anomaly in BB power spectrum, i.e., due to the large suppression of tensor perturbation amplitude we will hardly see the reionization bump at low-l, which may be falsified by the Planck polarization data.