Or the universe might even recollapse if dark energy density falls to a negative value.

当然宇宙也有可能因为暗能量密度降为负值，而再度塌陷。

Satellite retroreflector is used as laser ranging cooperative target to improve the intensity of reflected beam, which has close relationship with processing parameters of the retroreflector.

卫星角反射器为激光测距系统中的合, 其主要在于提高收机范围内反射光束的能量密度。