The origins of the universe is unknown in cosmology. The hot big bang model only covers the history of the universe from 10^{-43} seconds forward. Prior to that is described as a singularity. However its important to note that the singularity is not a blackhole style. Instead singularity in this case simply means a point in time where our mathematics can no longer accurately describe it. Numerous youtube videos and pop media articles would have you believe our universe exploded from some super particle. This was never predicted by the hot big bang model.

The observable universe which is the portion we can see is a finite, sphere with a radius of 46 Gly, which is equal to 46 billion light years. The 46 Gly particle horizon refers to the today's distance of objects, whose radiation emitted in the past we receive today. The overall size of the universe is not known, it could be infinite or finite. If its infinite now then it would be infinite in the past, a finite value can never become infinite. So why is geometry so important to cosmology if we know the size of the observable universe? The answer to that question lies in how geometry affects the following aspects, Light paths, rate of expansion or collapse and overall shape.

In regards to light paths and geometry a closed universe described as a sphere will have two beams of light emitted at different angles eventually converge. An open hyperbolic universe such as a saddlebag will have those same two light beams diverge. A flat universe will have parallel light paths (provided the beams at emission were parallel to begin with)

You will notice on each image there is a triangle, this triangle represents how the geometry affects our measurements. In a flat curvature the three angles of a equilateral triangle will add up to 180^{0}. A positive curvature will add up to greater than 180^{0}, a negative curvature will add up to less than 180^{0}

Image from http://universeadventure.org

The topography of the universe is determined by a comparison of the actual density (total density) as compared to the critical density. The critical density is represented by the following formula

$\rho_{crit} = \frac{3c^2H^2}{8\pi G}$

$\rho$=energy/mass density

c=speed of light

G= gravitational constant.

density is represented by the Greek letter Omega $\Omega$ so critical density is $\Omega crit$

total density is

$\Omega$_{total}=$\Omega$_{dark matter}+$\Omega$_{baryonic}+$\Omega$_{radiation}+$\Omega$_{relativistic radiation}+${\Omega_ \Lambda}$

$\Lambda$ or Lambda is the value of the cosmological constant often referred to as "dark energy" more accurately it is the vacuum pressure that attributes to expansion.

the subscript "_{0}"for $\Omega$ shown in the image above denotes time in the present.

Energy-density is the amount of energy stored per unit volume of space or region. Energy per unit volume has the same physical units as pressure, and in many circumstances is an exact synonym.

$\Omega=\frac{P_{total}}{P_{crit}}$

or alternately

$\Omega=\frac{\Omega_{total}}{\Omega_{crit}}$

for a technical descriptive showing the related mathematics see

http://cosmology101.wikidot.com/geometry-flrw-metric/