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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10 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

  2. John Wake says:

    I fail to understand why the size of the universe is measured by either the distance of light or the matter that is observed, I understand that you cannot measure what we deem as “nothing”, ie this “nothing” houses the light from the big bang and all the matter we observe, to me its like spilling salt in a bath tub full of water and measuring the salt we can see and claiming the size of the bath tub (poor analogy I know). The singularity known as the big bang does not answer one question of how the universe began, rather it answers one event in its history, a history that surely existed before the big bang

    • Moez Bouselmi says:

      “…a history that surely existed before the big bang.”
      But wait a minute i thought that time and space started with the big bang !

      • Jeff Mangum says:

        Comments regarding what existed before the Big Bang are based solely on speculation. Current cosmological models concentrate on using our observations of the universe to constrain models of how the universe began and has evolved. That evolution is theorized to have started with the Big Bang.

  3. Jeff Mangum says:

    Your analogy where one measures the size of a bath tub by measuring the salt floating in it is in fact a pretty good one. The salt is analogous to the stuff we can see in the universe; stars, planets, and galaxies. Think of these objects as reference points.

    Jeff Mangum

  4. Damon says:

    How can the distance to the edge of the visible universe be 46 billion light years if the universe is only 13.7 billion years old?

    A light year being the measurement of how far light can travel in one year.

    How can we possibly see light that has taken 3 times longer than the age of the universe to reach us?

    Either the speed of light is not constant?
    Or scientists are wrong about the age or size of the universe?

    Please could you explain the fault in my reasoning?

    Yours sincerly Mr D Jebb

    • Jeff Mangum says:


      The age of the CMB signals that we measure with WMAP and Planck is 13.8 billion years. Now, that radiation from the CMB has, in the meantime, continued to expand so that the structures measured in the CMB signal would be 46.5 billion light years from us at this time. We cannot (yet) see these structures as they are today, but will see that those structures (galaxies and clusters of galaxies) look like in 46.5 – 13.8 = 32.7 billion years. But, we know that the universe has continued to expand since the measured CMB signal was emitted, so we can say that the edge of the universe is 46.5 billion years from us today.

      Jeff Mangum

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