在用定义为径向距离除以尺度因子的径向坐标r和宇宙时间t的r-t图中,大爆炸时向我们发射、现在到达我们这里的光子的轨迹,r_(t),是大爆炸时我们向外发射光子轨迹,r+(t)的镜象。

• In the r-t diagram, where t is the cosmic time and r is the radial co- ordinate defined as the proper radial distance divided by the scale factor, the locus of the incoming photon emitted at the big bang and reaching us now, r_(t), is a mirror image of the locus of the outgoing photon emitted at the big bang with r = 0, r+(t).