Blue-shift is similar to red-shift in that it too is a byproduct of viewing an electromagnetic source within in accelerating reference frame. In this case however, the light-source is accelerating towards the observer rather than away from them, causing the emitted electromagnetic waves to compress and hence result in blue-shift. An example of a blue-shifted light-source would be the Andromeda galaxy, which is accelerating towards our galaxy, the milky way. As an addendum, it is important to note that although the universe at global scales is exapnding at an accelerated rate, at the local level, objects may in fact accelerate towards one another due to their mutual gravitation.
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