Can the Universe be finite in size? If so, what is ``outside'' the Universe? The answer to both these

questions involves a discussion of the intrinsic geometry of the Universe.

There are basically three possible shapes to the Universe; a flat Universe (Euclidean or zero curvature), a

spherical or closed Universe (positive curvature) or a hyperbolic or open Universe (negative curvature).

Note that this curvature is similar to spacetime curvature due to stellar masses except that the entire mass

of the Universe determines the curvature. So a high mass Universe has positive curvature, a low mass

Universe has negative curvature.

All three geometries are classes of what is called Riemannian geometry, based on three possible states for

parallel lines

l never meeting (flat or Euclidean)

l must cross (spherical)

l always divergent (hyperbolic)

or one can think of triangles where for a flat Universe the angles of a triangle sum to 180 degrees, in a

closed Universe the sum must be greater than 180, in an open Universe the sum must be less than 180.

Standard cosmological observations do not say anything about how those volumes fit together to give the

universe its overall shape--its topology. The three plausible cosmic geometries are consistent with many

different topologies. For example, relativity would describe both a torus (a doughnutlike shape) and a

plane with the same equations, even though the torus is finite and the plane is infinite. Determining the

topology requires some physical understanding beyond relativity.

Like a hall of mirrors, the apparently endless universe might be deluding us. The cosmos could, in fact,

be finite. The illusion of infinity would come about as light wrapped all the way around space, perhaps

more than once--creating multiple images of each galaxy. A mirror box evokes a finite cosmos that looks

endless. The box contains only three balls, yet the mirrors that line its walls produce an infinite number of

images. Of course, in the real universe there is no boundary from which light can reflect. Instead a

multiplicity of images could arise as light rays wrap around the universe over and over again. From the

pattern of repeated images, one could deduce the universe's true size and shape.

Topology shows that a flat piece of spacetime can be folded into a torus when the edges touch. In a

similar manner, a flat strip of paper can be twisted to form a Moebius Strip.

The 3D version of a moebius strip is a Klein Bottle, where spacetime is distorted so there is no inside or

outside, only one surface.

The usual assumption is that the universe is, like a plane, "simply connected," which means there is only

one direct path for light to travel from a source to an observer. A simply connected Euclidean or

hyperbolic universe would indeed be infinite. But the universe might instead be "multiply connected,"

like a torus, in which case there are many different such paths. An observer would see multiple images of

each galaxy and could easily misinterpret them as distinct galaxies in an endless space, much as a visitor

to a mirrored room has the illusion of seeing a huge crowd.

One possible finite geometry is donutspace or more properly known as the Euclidean 2-torus, is a flat

square whose opposite sides are connected. Anything crossing one edge reenters from the opposite edge

(like a video game see 1 above). Although this surface cannot exist within our three-dimensional space, a

distorted version can be built by taping together top and bottom (see 2 above) and scrunching the

resulting cylinder into a ring (see 3 above). For observers in the pictured red galaxy, space seems infinite

because their line of sight never ends (below). Light from the yellow galaxy can reach them along several

different paths, so they see more than one image of it. A Euclidean 3-torus is built from a cube rather than

a square.

A finite hyperbolic space is formed by an octagon whose opposite sides are connected, so that anything

crossing one edge reenters from the opposite edge (top left). Topologically, the octagonal space is

equivalent to a two-holed pretzel (top right). Observers who lived on the surface would see an infinite

octagonal grid of galaxies. Such a grid can be drawn only on a hyperbolic manifold--a strange floppy

surface where every point has the geometry of a saddle (bottom).

Its important to remember that the above images are 2D shadows of 4D space, it is impossible to draw the

geometry of the Universe on a piece of paper (although we can come close with a hypercube), it can only

be described by mathematics. All possible Universes are finite since there is only a finite age and,

therefore, a limiting horizon. The geometry may be flat or open, and therefore infinite in possible size (it

continues to grow forever), but the amount of mass and time in our Universe is finite.

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