GRE考试

【单选题】From the beginning, the idea of a finite universe ran into its own obstacle, the apparent need for an edge, a problem that has only recently been grappled with.Aristotle’’s argument, that the universe is finite, and that a boundary was necessary to fix an absolute reference frame, held only until scientists wondered what happened at the far side of the edge. In other words, why do we not redefine the "universe" to include that other side 　　Riemann ingeniously replied by proposing the hypersphere, the three- dimensional surface of a four-dimensional ball. Previously it was supposed that the ultimate physical reality must be aEuclidean space of some dimension, and thus if space were a hypersphere, it would need to sit in a four-dimensionalEuclidean space that allows us to view it from the outside.But according to Riemann, it would be perfectly acceptable for the universe to be a hypersphere and not embedded in any higher-dimensional space; nature need not therefore cling to the ancient notion.According toEinstein’’s powerful but limited theory of relativity, space is a dynamic medium that can curve in one of three ways, depending on the distribution of matter and energy within it, but because we are embedded in space, we cannot see the flexure directly but rather perceive it as gravitational attraction and geometric distortion of images. Thus, to determine which of the three geometries our universe has, astronomers are forced to measure the density of matter and energy in the cosmos, whose amounts appear at present to be insufficient to force space to arch back on itself in "spherical" geometry. Space may also have the familiarEuclidean geometry, like that of a plane, or a "hyperbolic" geometry, like that of a saddle. Furthermore, the universe could be spherical, yet so large that the observable part seemsEuclidean, just as a small patch of the earth’’s surface looks flat.　　We must recall that relativity is a purely local theory: it predicts the curvature of each small volume of space--its geometry--based on the matter and energy it contains, and the three plausible cosmic geometries are consistent with many different topologies: relativity would describe both a torus and a plane with the same equations, even though the torus is finite and the plane is infinite.Determining the topology therefore requires some physical understanding beyond relativity, in order to answer the question, for instance, of whether the universe is, like a plane, "simply connected", meaning there is only one direct path for light to travel from a source to an observer.
A、simply connectedEuclidean or hyperbolic universe would indeed be infinite--and seems self-evident to the layman--but unfortunately the universe might instead be "multiply-connected", like a torus, in which case there are many different such paths.An observer could see multiple images of each galaxy and easily interpret them as distinct galaxies in an endless space, much as a visitor to a mirrored room has the illusion of seeing a huge crowd, and for this reason physicists have yet to conclusively determine the shape of the universe. The author would regard the idea that the universe inhabits a spherical geometry as________.
A、unimportant

B、unscientific
C．self-evident
D．plausible
E．unlikely

网考网参考答案：E
网考网解析：
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