And yet, astronomers are still left wondering how much more universe is out there, beyond what they observe. Our cosmos keeps going. We just may never know how far. In theory, our universe can have one of three possible shapes, each one dependent on the curvature of space itself: saddle shaped negative curvature , spherical positive curvature or flat no curvature. Few have championed a saddle-shaped universe, but a spherical cosmos makes sense to us earthlings.
Earth is round, as are the sun and planets. But starting in the late s, a series of orbiting observatories built to study the CMB made increasingly precise measurements showing that space has no curvature at all. A flat universe agrees with both observation and theory, so the idea now sits at the heart of modern cosmology. The problem is that, unlike a spherical universe, a flat one can be infinite — or not. The uniformity of the universe means that galaxy groups are spread out more or less evenly on the cosmic scale.
The flatness of the universe means that the geometry of spacetime is not curved or warped on the cosmic scale. This means that the universe does not wrap around and connect to itself like the surface of a sphere, which would lead to a finite universe.
The flatness of the universe is actually a result of the uniformity of the universe, since concentrated collections of mass cause spacetime to be curved. Moons, planets, stars, and galaxies are examples of concentrated collections of mass, and therefore they do indeed warp spacetime in the area around them.
However, these objects are so small compared to the cosmic scale, that the spacetime warping which they cause are negligible on the cosmic scale. If you average over all of the moons, planets, stars, and galaxies in the universe in order to get a large-scale expression for the mass distribution of the universe, you find it to be constant. The second observation is that our corner of the universe is not special or different.
Since the part of the universe that we can see is flat and uniform, and since our corner of the universe is not special, all parts of the universe must be flat and uniform. The only way for the universe to be flat and uniform literally everywhere is for the universe to be infinite and have no edge.
This conclusion is hard for our puny human minds to comprehend, but it is the most logical conclusion given the scientific observations. If you flew a spaceship in a straight line through space forever, you would never reach a wall, a boundary, an edge, or even a region of the universe without galaxy groups. But how can the universe have no edge if it was created in the Big Bang?
If the universe started as finite in size, shouldn't it still be finite? The answer is that the universe did not start out as finite in size. The Big Bang was not like a bomb on a table exploding and expanding to fill a room with debris. The Big Bang did not happen at one point in the universe.
It happened everywhere in the universe at once. And that's where things get tricky, because scientists aren't certain if such a drop-off exists. One form of the question asks, "Could you go somewhere that you could look 'beyond' the universe," the way one might peer beyond a cliff edge or look out a window to see the outside of a building? The answer to that query is "probably not.
One reason involves the "cosmological principle," said Robert McNees, an associate professor of physics at Loyola University Chicago. The cosmological principle states that the distribution of matter in any part of the universe looks roughly the same as in any other part, regardless what direction you look in; in scientists' terms, the universe is isotropic.
The cosmological principle is, in part, a consequence of the idea that the laws of physics are the same everywhere. The implication though, is that there is no "edge"; there is no place to go where the universe just ends and one could look in some direction and see what's beyond it. One analogy often used to describe this edgeless universe is the surface of a balloon.
An ant on such a surface can walk in any direction and it would look like the surface was "unbounded" — that is, the ant might come back to where it started but there would be no end to the journey. So even though the surface of a balloon is a finite number of square units, there's no edge to it, no boundary since you can go forever in any one direction.
In addition, there's no "center," so there's no preferred point on the balloon's spherical surface. Using the balloon analogy again, if one were to add more air to the balloon, the ant would observe other things on the balloon's surface getting farther away.
And the greater the distance between the ant and some object, the faster that object would be receding. But no matter where the ant skittered, the speed at which those objects were receding would follow the same relations — if the ant came up with an equation describing how fast the farthest objects were receding, it would work the same way anywhere on the balloon's surface.
However, balloons, when blown up, are expanding into a three-dimensional space.
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