How Do Astronomers Measure the Size of the Universe?

Question: I’m 15 years old. I want to know how do scientists measure the size of universe or observable universe.  – Hassan

Answer:  There are several techniques that astronomers use to measure distances in the Universe.  Most of these distance measurement techniques build upon other distance measurement techniques, giving us a distance measurement “ladder”.  There is a very nice short video explanation of how scientists measure the size of the Universe from the Royal Greenwich Observatory that really does a great job of explaining this distance ladder, and how astronomers measure the size of the Universe.  If you want even more information on cosmic distances, check out Dave Goldberg’s “Ask a Physicist” column describing how we measure the size of the universe.  I hope that this answers your question.

Jeff Mangum

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4 Responses to How Do Astronomers Measure the Size of the Universe?

  1. Dr. Len Margolin says:

    The question asks about the size of the universe. The answer talks about measuring distances. I am not an astronomer, but I am a theoretical physicist for more than 40 years and I would like to know how to measure this “size”? — Len

    • Jeff Mangum says:

      The answer is in fact described in the links provided. Using a variety of cosmological distance measurement techniques, the size of the observable Universe is thought to be about 48 billion light years.

      Jeff Mangum

      • Sekar says:

        I don’t mean to be picky, but when you say the “size of the universe”, should you not be referring to its diameter? The 48 billion light years you’ve mentioned is the radius of the (spherical observable) universe, right? Also, you’ve not included the links that you say have been provided.

        • Jeff Mangum says:

          Even though it is convenient to try to think of the universe has having some physical structure, like a sphere, this is in fact not really correct. Every galaxy in the universe is separating from every other galaxy. You can think of this as space expanding between galaxies rather than galaxies flying away from each other. So, the 48 million light years I quoted is the maximum distance that light could have travelled since time began. The light from the Cosmic Microwave Background comes from a point very near this horizon. Therefore, 48 million light years is the maximum size, independent of the type of geometrical structure you associate with the universe. Also, the links are embedded in the text of my original post.

          Jeff Mangum

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