How Everything Works
How Everything Works How Everything Works
 

QUESTIONS AND ANSWERS
 
Question 817

When I read of scientists discovering galaxies "on the edge of the universe," perhaps 15 billion light years away, I wonder if they are including the distance the objects must have traveled in the time it took for the light to reach their telescopes. Very distant objects are said to be receding from any other point in space at a higher rate than closer objects. If a galaxy is discovered 15 billion light years away today, the light left that galaxy 15 billion years ago while receding at a high rate. Where is it today, really? Twice as far away? — DK, Missouri City, TX
This seemingly simple question has a surprisingly complicated answer. You might expect that if the earth and one of these distant galaxies had been very near one another at the creation of the universe and had both been moving away from one another at almost the speed of light, that after 15 billion years each would have moved almost 15 billion light years in opposite directions and would thus be separated by almost 30 billion light years. That's not the case. That simple view ignores the important effects of special relativity on rapidly moving objects.

To understand these effects, suppose that there was an observer who was stationary at the creation and watched the earth and galaxy head off in opposite directions at almost the speed of light. From that observer's perspective, the two objects are heading away from one another at almost twice the speed of light. After 15 billion years, this observer sees the galaxy as almost 30 billion light years away from the earth.

Now suppose that there was another observer who was on the earth at the creation. From this person's perspective, the galaxy recedes from the earth at almost the speed of light, but no more. Nothing can move faster than speed of light! After 15 billion years, this observer sees galaxy as almost 15 billion light years away from the earth.

These two observations don't seem to agree. The problem lies in how the two observers perceive time and space. According to special relativity, observers who are moving relative to one another don't perceive time and space in the same way. Their perceptions will be so different that they will not even agree about just when 15 billion years has passed.

With this long introduction, here is the answer to your question: no distant galaxy in the observable universe can ever be farther from us than the distance light has traveled since the creation of the universe. Since that creation was about 15 billion years ago, the most distant possible galaxy is almost 15 billion light years away.

         

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