Cosmology: The Size And Shape Of Our Universe
If our Universe has a finite volume, which it apparently should have since it had a finite size at the time of its origin – the Big Bang event – 13.7 billion years ago, then it must have some sort of overall size and no doubt shape, a shape only apparent assuming one could view it from a suitable distance. So, what’s the size and shape of our Universe and what determines that?
To start this off, one can not measure the absolute size of the Universe, because 1) its expanding, so its size is constantly increasing, but more to the point, 2) we can only measure what we can actually observe. Since light has a finite velocity, even if it is the fastest thing going, there could be objects so far out that their light hasn’t had time enough to reach us yet, so we know nothing about them or the existence of the space that they reside in. However, one can surmise that if the Big Bang event happened 13.7 billion years ago, and given that the speed of light is the fastest velocity known, then presumably the Universe must have a radius of 13.7 billion light years.
But just to complicate things, we also can not know the absolute size of our Universe due to, and subsequent to, the Big Bang event because the expansion rate of our Universe keeps changing. It was usually thought that the expansion rate of our Universe must slow down under the very gravity that the Universe has. Whether or not it would ultimately stop, and reverse direction wasn’t known – the jury was out. Then, in the late 1990’s, it was discovered that the expansion rate of our Universe was accelerating, due to some mysterious antigravity force which is little understood, called ‘dark energy’. So, the now current assumption is that the Universe will keep on expanding, ever expanding, ever faster and faster due to this repulsive force, ‘dark energy’.
We can however in theory measure the shape of the Universe. The shape however depends on how much mass there is and how that arrangement of mass shapes space and time, or space-time. That is, if you could trace the pathway of a ray of light, how it travels ultimately depends on what mass it encounters and hence the pathway of light defines the shape. It all boils down to geometry. Just because the Big Bang event is viewed as a sort of explosion spewing out matter and energy in all directions, doesn’t ultimately mean the Universe is spherical in shape.
If the Universe is ultimately going to tend towards becoming infinite in size (never actually getting there but ever struggling to get there), that’s one possibility. That situation would arise if the expanding Universe were either flat (where the angles of a triangle add up to 180 degrees) or saddle-shaped (where the angles of a triangle add up to less than 180 degrees).
A saddle-shaped Universe suggests that beams of light will spread ever father and farther apart, even though encounters with clumps of matter will cause temporary and minor deviations. So, if you shine two flashlight beams parallel to each other, they will every so slowly diverge.
A Flat Universe suggests that a beam of light, while being deflected up and down, right and left, forward and backward, as it encounters various massive (light bending) objects, nevertheless ultimately and statistically tends to move in an overall straight line. The two flashlight beams, starting off in parallel, will in the long term remain parallel.
The best evidence to hand suggests that the Universe is flat. But, recall that your backyard is also flat, yet in practice it’s got to have an ever so slight curve to it, because its part of the Earth’s surface and the Earth’s surface is curved. So maybe the Universe is likewise curved, but so vast that all that we can see appears as flat as your backyard!
Of course even a flat Universe could have a finite size (actually must have since the Big Bang event spewed out a finite amount of stuff), but still have an infinite amount of space available beyond to continue to expand into. This however runs counter to the standard model of cosmology, which states that space itself is expanding. I tend to go against that grain and assume that the expansion of our Universe is through pre-existing space.
What if the mass distribution is such that a light beam will be forced to ultimately curve back around like an ant crawling around the surface of a ball? Then the Universe is a closed Universe. The Universe is really a spherical Universe.