## How can the universe be flat - by Christopher J. Johnson - Quora Question Review

This document contains a review of the answer by by Christopher J. Johnson on the question in Quora: "How can the universe be flat"
• The text in italics is copied from the article.
• Immediate followed by some comments

### Reflection

When scientists refer to the universe being flat, they are not saying it is flat like a pancake, but that space-time itself does not curve back in on itself.
What does it mean that: space-time itself does not curve back in on itself . IMO no-one can explain that. Part of the problem is that the combination of space and time, called space-time, is not a physical claim but a mathematical issue.
At the same this sentence resembles the sentence: That a snake can eat its own tail. That is physical problem, but only for a small period. Sooner or later the snake will die.
The overall problem is that scientists, can say nothing about the overall shape of the universe. It's pure speculation.
A negative or flat space-time geometry indicates the universe goes on in all directions (up, down, left, right, etc.) forever, whereas positive space-time curvature indicates a finite universe.
 ``` Positive Curvature Negative Curvature Flat Curvature ```
This illustration depicts two-dimensional slices of space-time, to make a four-dimensional space-time easier to comprehend.
Yes it depicts, but none of these pictures reflect a physical reality. The physical reality is always 3D.

When scientists measured the geometry of the observable universe, they found it to be flat, which means the entire universe is either extremely large compared to the observable universe, or is infinite.

How do you measure the geometry of the observable universe?
That is impossible.
It could be that the curve is too slight to measure within the observable universe, in the same sense that we cannot see the curvature of the Earth from the ground.

### Reflection 1 - Question Review

The question "How can the universe be flat?" is very difficult to answer.
The first problem is what does flat means?
The surface of any object in the universe is roughly speaking a sphere. If the object is rotating it has an equator. And all the lines perpendicular to the equator will meet at two points called the North and South pole. But that has nothing to do with the shape of the universe.
What the litterature shows there are also two different answers possible: positive curved and negative curved.
The second problem what exactly is the universe and how is the curvature of the universe measured. Specific the second part is extremely difficult, IMO tricky.

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Created: 23 May 2024

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