What do experts mean when they say the universe is flat?

Answer 1

Believe it or not, the term derives from high school geometry.

Here is an example. Everyone learns that the sum of the angles of a triangle drawn on a sheet of paper is exactly 180 degrees. Right? Now, then, take a globe and from the north pole draw a line down to the equator. Now, looking at the north pole draw another line down to the equator, but start that line at the north pole exactly 90 degrees from the first line. Step back and look at what you have a triangle with its apex at the north pole and two legs which end on the equator. The portion of the equator between the two legs forms the third side.

What kind of triangle is this? It has 3 --count 'em!-- right angles: one at the north pole, and each leg hits the equator at a 90 degree angle. What's up with that? Well, what's the difference between this triangle and the one drawn on the paper? Well, duh, the paper is....TADA!...flat.

In cosmology, "flat" generally means "Euclidean." That generally means that the geometry of Euclid applies. So, for example, a triangles behave such that no matter how large they are, the sum of their interior angles is always exactly 180 degrees; two parallel lines never intersect no matter how far they are extended; and the distance from some origin point (0,0,0) in three dimensions (say) to another point (x, y, z) is always exactly sqrt(x^2 + y^2 + z^2) no matter how far away (x, y, z) is from the origin.

HTH

Charles

Answer 2

They mean that the overall curvature of spacetime, or in other words the geometry of the Universe as a whole is flat. This concept was first introduce by Einstein in his publication of Theory of General Relativity. Current WMAP measurements (of the Cosmic Backgound Radiation) indicate that our Universe is indeed flat.

This curvature or geometry of the Universe is determined by the overall energy/matter density (called Omega or Ω) of the Universe. It's been long believed by cosmologists that the spacetime curvature is spherical and the Universe's expansion (from the Big bang) under the gravitational influence of all the matter within would eventually slow down and then reverse into a contraction.

Surprisingly, recent observations of supernovae indicated that the Universe's expansion, rather than slowing down, is accelerating, leading us to the inevitable conclusion that the Universe's geometry is either flat or hyperbolic. Whether our Universe is flat or hyperbolic would depend on the value of Ω. And so far, all indication is that Ω is nearly equal to one.

However, we should not confuse Ω with Einstein's Cosmological Constant (Lambda or Λ). Ω is the density parameter in the Friedmann equations, and it is a ratio that compares the actual energy/matter density of the Universe with the "Critical Density" (which is the density for a flat Universe).

Einstein's Cosmological Constant, Λ, is a measure of the vaccum energy density; hence, it is only a contributing factor to Ω. It i now widely believed by physicists that Einstein's Λ is the hypothesized "Dark Energy" (a form of energy that permeates all of space and exerts strong negative pressure), and that dark energy is responsible for the accelerating expansion of the Universe.

Answer 3

You, know...like a pancake.

There are three possibilities: flat, spherical, and saddle shaped. The shape depends on the value of the so-called cosmological constant (G). In their infinite wisdom, when their answers failed to "come out right," some physicists and such threw in a cosmological constant to fix the problem. Einstein did it when his relativity equations predicted a shape inconsistent with what he thought the universe ought to be. [See source.]

Current thinking finds G < 1.0, but just slightly. G = 1 would result in a flat (like a pancake) shape; so current thinking believes our universe is almost, but not quite, flat.

Think of our universe lying on the skin of a very large balloon, which represents both the known and unknown universe we live in. There is some curvature of the skin, but not much because of the size of the balloon (upon which our observable universe sits). It is like us on Earth thinking the Earth's surface looks flat when it's really curved.

What we can see of the universe is only a small portion of the entire universe. We, by example, can see out about 14 billion light years from Earth, along the skin of the balloon. What lies beyond that small area along the skin and what lies inward towards the center of the balloon or outwards away from the skin...no one has a clue.

Answer 4

It means that the geometry (..shape..) of the universe is such that parallel lines can never converge and that the universe will continue to expand indefinitely.