But the part of the universe we can observe appears to be fairly flat. Of course, the observable universe may be many orders of magnitude smaller than the whole universe. Measurements from the Wilkinson Microwave Anisotropy Probe (WMAP) have shown the observable universe to have a density very close to the critical density (within a 0.4% margin of error). And if the universe’s density is less than the critical density, then the universe is open and has negative curvature, like the surface of a saddle. A universe with density greater than the critical density has positive curvature, creating a closed universe that can be imagined like the surface of a sphere. According to him, space and timewhich must be put together as space-timeare curved near heavy masses. You can imagine a flat universe like a sheet of paper that extends infinitely in all directions. Einstein had a different interpretation of the law of gravitation. If the density is equal to the critical density, then the universe has zero curvature it is flat. The density of matter and energy in the universe determines whether the universe is open, closed, or flat. We spent some time looking at Einsteins special relativity, so now its time for the general variety. Mass also has an effect on the overall geometry of the universe. Relativity comes in different flavors, as it happens. Overall Curvature of Space Closed universe (top), open universe (middle), and flat universe (bottom). So, locally, spacetime is curved around every object with mass. According to Einstein’s theory of general relativity, massive objects warp the spacetime around them, and the effect a warp has on objects is what we call gravity.
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