A flat universe is a universe with any number of dimensions that has zero curvature. The density parameter , the ratio between the density of the universe and the critical density of the universe is equal to 1 in a flat universe. If the density parameter was greater than 1, it would be a hyperspherical, positively curved closed universe. If it was less than 1, then it would be a hyperbolic, negatively curved open universe.
Structure[]
Flat universes are defined to have zero curvature, while staying a relatively constant shape (according to general relativity, a massive object actually distorts spacetime, causing gravity). The universe's space may globally take on the form of a Euclidean space, as it is flat everywhere. The shape of a universe including the temporal dimensions, the continuum of space and time, can be modeled as a Minkowski space with a signature determined by the number of spatial and temporal dimensions.
A flat universe will typically be infinite in size unless it has some borders or has some closed topology despite its space itself being flat, such as a toric, tubic or Klein bottle-shaped universe.
Life cycle[]
Like all naturally created universes, a flat universe begins its life with a "Big Bang" and will continually expand throughout its life. However, unlike a closed universe, which expands from a infinitesimally small center point (known as the singularity or protoverse), open and flat universes start off as an extremely dense, hot, and curved but still infinite space, and regions that would eventually become Hubble volumes and/or googological regions are very compact and small.
Expansion will then slow down and the universe will cool down, and over the course of billions of years, galaxies, stars and planets (that may have life) may form.
The ultimate fate of the universe depends on the shape of the universe and the type of dark energy throughout the universe. Flat universes with a positive cosmological constant will continually will expand forever, but the expansion will decrease over time, it's growth and temperature asymptotically approaching zero in a Big Freeze scenario or will expand while going into a state of maximum entropy and matter dilutes, this scenario is known as Heat death.
Flat universes with negative phantom energy on the other hand, will either reaccelerate, soon expanding faster than light and overwhelming the fundamental forces and causing a "Big Rip". These three possible fates are shared with open universes. Closed universes on the other hand, go out with a Big Crunch.
See Also[]
- Open universe - A universe with constant negative curvature, and is in the shape of a hyperbolic space
- Closed universe - A universe with constant positive curvature, and is in the shape of a hypersphere