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Q&A: Relativity, Expansion, and the Speed of Light

From Keith 

I’m currently reading The Creator and the Cosmos and have a question regarding the expanding universe. As the expansion of the universe increases, Hugh said that eventually the rate of expansion would exceed the speed of light and we wouldn’t be able to see light from those distant stars any longer. But I thought that per the theory of relativity, the relative speed of massive objects could never exceed the speed of light. What am I missing?


Good question, Keith. Einstein’s theory of special relativity does say that relative to a fixed point on a fixed space surface no object can be accelerated from a velocity less than the speed of light to a velocity greater than the speed of light. However, if the space surface is not fixed but rather expanding then that rule no longer necessarily applies.

Neither Einstein’s theory of special relativity nor his theory of general relativity place any limit on how rapidly the cosmic space surface can expand. Thus, it is possible for a distant point on the cosmic space surface relative to our observing position on Earth to be moving away from us at greater than light’s velocity. Consequently, light emanated by a galaxy located at that point will never reach our telescopes.

Dark energy, which comprises about three quarters of everything in the universe, is the energy embedded in the space surface of the universe. As the space surface enlarges and as the universe expands, dark energy becomes more potent in its capacity to accelerate the universe’s rate of expansion. Thanks to dark energy the universe expands more rapidly with every passing year.

Right now, the universe expands slowly enough that astronomers can look far enough away, and, therefore, far enough back in time, to witness the cosmic creation event. However, light from the cosmic creation event is measured to be moving relative to us at almost 100 percent of the velocity of light. Therefore, it won’t be long on an astronomical timescale before the cosmic space surface expands so rapidly that we will no longer be able to observe the radiation from the cosmic creation event. In fact, within several hundred billion years the universe will be expanding so rapidly that even the light from presently very nearby galaxies will not ever reach our telescopes.