The past light-cone is shown in comoving spatial coordinates and physical time.
For the Gamow (i.e. dominated by radiation) universe, its size has a finite limit for the begin of time, which marks the horizon. This is the region from which the event B can be influenced. The events on the fireball have partly disconnected horizons. This yields the question why the the fireball is homogeneous up to 1 by 100000 (horizon problem).deutsch
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