Editing Shape of the universe (section)

== Shape of the observable universe ==
{{Main|Observable universe|}}
{{See also|Distance measures (cosmology)}}

The universe's structure can be examined from two angles:
# '''Local''' geometry: This relates to the curvature of the universe, primarily concerning what we can observe.
# '''Global''' geometry: This pertains to the universe's overall shape and structure.
The observable universe (of a given current observer) is a roughly spherical region extending about 46 billion light-years in every direction (from that observer, the observer being the current Earth, unless specified otherwise).<ref>{{Cite news |last=Crane |first=Leah |date=29 June 2024 |editor-last=de Lange |editor-first=Catherine |title=How big is the universe, really? |url=https://www.newscientist.com/article/mg26234970-500-how-big-is-the-universe-the-shape-of-space-time-could-tell-us/ |url-access=subscription |work=New Scientist |page=31}}</ref> It appears older and more [[redshift]]ed the deeper we look into space. In theory, we could look all the way back to the [[Big Bang]], but in practice, we can only see up to the [[cosmic microwave background]] (CMB) (roughly {{val|370,000}} years after the Big Bang) as anything [[Recombination (cosmology)|beyond that is opaque]]. Studies show that the observable universe is [[isotropic]] and [[homogeneous]] on the largest scales.

If the observable universe encompasses the entire universe, we might determine its structure through observation. However, if the observable universe is smaller, we can only grasp a portion of it, making it impossible to deduce the global geometry through observation. Different mathematical models of the universe's global geometry can be constructed, all consistent with current observations and general relativity. Hence, it is unclear whether the observable universe matches the entire universe or is significantly smaller, though it is generally accepted that the universe is larger than the observable universe.

The universe may be compact in some dimensions and not in others, similar to how a [[cuboid]]{{Citation needed|date=August 2024}} is longer in one dimension than the others. Scientists test these models by looking for novel implications – phenomena not yet observed but necessary if the model is accurate. For instance, a small closed universe would produce multiple images of the same object in the sky, though not necessarily of the same age. As of 2024, current observational evidence suggests that the observable universe is spatially flat with an unknown global structure.