![]() The galaxies move away from each other as space stretches. On average, all galaxies are moving away from each other, but within a galaxy, the stars are not moving away from each other because gravity keeps them bound together.Īnimated Figure 13.5: An analogy for the expansion of space: a stretchy band. So our Universe is expanding, but our Galaxy and Solar System are not. That is because the internal (gravitational) forces within galaxies overwhelm the expansion. Furthermore, we see that the galaxies themselves are not stretching, they are not growing, only the space between them is. As the band stretches, the galaxies all move away from each other, but they are moving due to the stretching of the space, they are not moving through the space. One way to imagine space is like a stretchy band with galaxies stuck onto it (Animated Figure 13.5). Without general relativity, we are not used to thinking of space as having properties of its own, so we will use several analogies. It is worth elaborating on how an expanding space differs from a static space with objects moving through it. General relativity provides a framework for understanding how that snapshot relates to the ongoing evolution of the Universe. An analogous scenario is true for the Universe on grander scales: when we look at any given galaxy we are essentially seeing a snapshot of it. If you looked at a photograph showing a ball in that position, you would assume that it must be either rising or falling. Given the success of general relativity in explaining many individual phenomena (gravitational lenses, precession of orbits, etc.), it is natural to think of space overall as either expanding or contracting.Įven in a Newtonian analog the idea of a static Universe is difficult to make work: think how surprised you would be to see a ball simply hovering some distance above Earth’s surface. In fact, if general relativity is an accurate theory of the behavior of space and time, it is very difficult to contrive a situation in which the Universe could be static. General relativity provides a context in which the notion of expanding space makes sense. If we abandon the scenario in which galaxies are moving through space, we are left with one where space itself expands and carries galaxies along as it does so. From the perspective of general relativity it is much more natural to explain the redshift-distance relation in terms of galaxies that are embedded in a space that is uniformly expanding. The actual Big Bang theory is something quite different. How could they all conspire to move in the same way? Many people, when they hear the term ‘Big Bang,’ imagine some sort of explosion, like a giant firecracker, but that is problematic because it is not consistent with a number of observed properties of the Universe. After all, the galaxies are billions of light-years away from each other. It is very difficult to explain these observations in terms of independent motions of galaxies through space. The observation of galactic redshifts from the 1920s and thereafter was the main motivation for the Big Bang theory.
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