The Shape of Time ( Cont.)

Part 3 Our Past Light Cone

A past light cone, as pictured above, are the paths in which spacetime of the light rays from distant galaxies reach us during present times. In other words, basically when we look at galaxies millions of miles of away from us within the universe we are looking at a much earlier version of them because light travels at an finite speed. So by the diagram above we can see that if we represent time by the vertical line within the cones, and we represent the two of the three space directions by the horizontal lines, than we can observe that the light reaching us now, at this point, must travel to the vertex, in order for us to see it. As we go down from the vertex to the bottom of the cone we observe earlier and earlier versions of galaxies out there in our universe. Also to all this, since the universe is expanding , and all the galaxies used to be much closer back then, we will only be able to observe the regions of a much higher matter density. From all this information, scientist have concluded that our past light cone must be able to pass through a specific amount of matter as it follows back on the y-axis or vertical line of the cone. This then supports that with this specific amount of matter, we can see that it is enough to curve spacetime, so the light rays from previous sources are bent toward each other.