The Cosmos

10
Oct

How large is the observable universe?

If you were born on an isolated desert island in the middle of the ocean and had no communication with the outside world, your knowledge of geography would be limited. Peering through binoculars, gazing out in any direction, your view would be bounded by the sea’s horizon. Although you might speculate about what lies beyond the edge, you’d lack tangible evidence to support your hypothesis.

Confined to our planet and its environs, we face the same situation: We can see a portion of the universe, but we can only speculate about its full extent. We might surmise through its flat geometry that it continues indefinitely in all directions, like a prairie stretching out as far as the eye can see. (Flat in this context refers to a straight three-dimensional space, like an endless box.) However, our understanding of the actual universe is bounded by the edge of the observable universe. We cannot know for sure what lies beyond the enclave our instruments can detect.

Accordingly, we might wonder: How large is the part of the universe we’re potentially able to observe directly? At first glance, the answer might seem like a simple calculation. The speed of light is approximately 186,282 miles per second, or about 5.9 trillion miles per year. The time that has elapsed since the Big Bang is 13.75 billion years. Multiple the two figures and—voilà—we find that over the entire history of the universe, light could have travelled 13.75 billion light-years, or 81 billion trillion miles. But, in fact, that answer would be wrong.

Let’s think about when the light was produced. From the time of the Big Bang to the era of recombination (when neutral hydrogen atoms formed) some 380,000 years later, the universe was opaque to light. Photons bounced between charged particles and didn’t travel very far. The reason is that charged particles interact with photons—either absorbing or emitting them. Only after the era of recombination could light journey through space. That is because photons can pass through neutral hydrogen gas without being diverted. Therefore, any estimate of the size of the observable universe must assume that the farthest light we see was released after that pivotal era when space became transparent. (We may someday be able to detect neutrinos and other particles from before that era, pushing the timeline earlier and enlarging the realm of what is observable, but for now we are still limited.) The difference between the two times doesn’t change the calculation much, but is important to note.

Another adjustment is far more important. Since the primordial burst of creation, space has been stretching as the universe expands. A galaxy’s distance from us today is far greater than it was when it released the light. We can think, by analogy, of a relay race in which a girl tosses a ball to her teammate and then runs away from him. If the coach later asks the teammate what is the farthest throw he has caught he would give a very different answer than if he is asked where is the farthest player he has caught a ball from. Similarly, the distances traveled by the photons hurled by light sources do not reflect the much greater extent of the sources’ current positions. Thus, we could potentially observe light sources that are much farther out than 13.75 billion light-years, if their light was released when they were close enough to Earth.

Yet another factor that expands the limit of the observable universe is its acceleration. Not only is the universe expanding; its growth has been speeding up. Data from the Hubble Space Telescope, the WMAP (Wilkinson Microwave Anisotropy Probe) satellite and other instruments have been used to pin down the rate of acceleration, along with the current expansion rate, the age of the universe, and other important cosmological parameters.

Taking advantage of this wealth of information, in 2005 a team of astrophysicists led by J. Richard Gott of Princeton performed a detailed calculation of the radius of the observable universe. Their answer was 45.7 billion light-years—more than three times bigger than our first, naïve estimate! Within this sphere lie hundreds of billions of galaxies, each with hundreds of billions of stars.1

Observable_Universe_500

Image credit: Andrew Colvin

Gott’s team calculated this radius by figuring out how far away from us a source would be today if the light we now observe from it was emitted during the recombination era. In our relay race analogy, that’s determining where someone must have stood if she threw a ball and we caught it, and then using her running speed to figure out where she must be right now.

Interestingly, as the universe expands, the size of the observable portion will grow—but only up to a point. Gott and his colleagues showed that eventually there will be a limit to the observable universe’s radius: 62 billion light-years. Because of the accelerating expansion of the universe, galaxies are fleeing from us (and each other) at an ever-hastening pace. Consequently, over time, more and more galaxies will move beyond the observable horizon. Turning once again to our relay race analogy, we imagine that if the players get faster and faster as the race goes on, there will be more and more who were so far away when they first threw the ball that the light would never have had time to reach us.

Naturally not everything within the observable universe has been identified. It represents the spherical realm that contains all things that could potentially be known through their light signals. Or to draw from a famous comment by former Secretary of Defense Donald Rumsfeld, the observable universe contains “known unknowns,” such as dark matter, that could eventually be analyzed. Beyond the observable universe lie “unknown unknowns”: the subject of speculation rather than direct observation.

1The 45.7 billion light-year radius includes only light sources. If neutrinos and other particles that could penetrate the opaque conditions of the early universe are included the value becomes 46.6 billion light-years.

Go Deeper
Editor’s picks for further reading

arXiv: The Long-Term Future of Extragalactic Astronomy
In this article, astrophysicist Avi Loeb investigates how our view of the universe will change in the distant future.

Edge of the Universe: A Voyage to the Cosmic Horizon and Beyond
In his latest book, Paul Halpern investigates what may lie beyond the boundaries of the observable universe.

James Webb Space Telescope: The End of the Dark Ages: First Light and Reionization
Learn more about the era of recombination and observations of the very early universe in this NASA resource.

Tell us what you think on Twitter, Facebook, or email.

phalpern-big

Paul Halpern

    Paul Halpern is Professor of Physics at the University of the Sciences in Philadelphia. A prolific author, he has written thirteen science books, including "Einstein's Dice and Schrödinger’s Cat: How Two Great Minds Battled Quantum Randomness to Create a Unified Theory of Physics" (Basic Books). His interests range from space, time and higher dimensions to cultural aspects of science. The recipient of a Guggenheim Fellowship, Fulbright Scholarship, and an Athenaeum Literary Award, he has appeared on the History Channel, the Discovery Channel, the PBS series "Future Quest," and "The Simpsons 20th Anniversary Special." Halpern's books include "Time Journeys," "Cosmic Wormholes," "The Cyclical Serpent," "Faraway Worlds," "The Great Beyond," "Brave New Universe," "What's Science Ever Done for Us?," "Collider," and most recently "Edge of the Universe: A Voyage to the Cosmic Horizon and Beyond" (Wiley 2012). More information about his writings can be found at phalpern.com.

    • http://www.facebook.com/don.lincoln Don Lincoln

      Nice job Paul.

      • Paul Halpern

        Thanks.

    • http://www.facebook.com/jonathan.dearman.1 Jonathan Dearman

      I was actually trying to discuss this with friends a few months ago, but couldn’t find any supporting info within the timeframe of the conversation. Info like this is always fun to blow someone’s mind. :)

      • Paul Halpern

        Yes, it is truly mind-blowing to think about such immensity!

    • http://www.facebook.com/michael.fuller.338 Michael Fuller

      Instead of thinking of how large it is, think of how small it may actualy be. Let that sink in a bit

      • Paul Halpern

        Interesting thought. Relative to the actual universe, which could well be infinite, the observable universe could indeed represent an infinitesimal portion.

    • http://www.facebook.com/chris.cantrell.07 Chris Cantrell

      While working on a project in high school, it occurred to me that the Universe could be hyperspherical. If this were the case, would there not be a horizon blocking our view at some distance from Earth? Also, it would seem that Universe-wide gravitational lensing would play a role; is there a way to measure any lensing from the far reaches of the Universe?

      • Paul Halpern

        Chris, although there were earlier speculations that the universe could be hyperspherical recent observations by the WMAP satellite point to a flat universe. Gravitational lensing is an important phenomenon, and yes scientists have measured the distortion of the light of distant objects such as quasars due to lensing.

    • Bly

      Fascinating. Good stuff, man.

      • Paul Halpern

        Thanks.

    • Gary

      Very nice analysis. What I love about the universe is that it is so mind boggling. How would gravitation bending light waves affect calculations?

      • Paul Halpern

        Thanks. If the universe is flat, as surmised, light waves would follow conventional, straight paths unless they encountered matter (galaxies, dark matter), in which case their paths would be deflected. Cosmological calculations typically look at the larger picture and don’t take individual galaxies into account. However, you are right that it would be interesting to see what effect galaxies and other forms of matter would have on light paths.

        • Gary

          When you say the universe is flat does that mean all solar systems would rotate in the same plane or could some rotate perpendicular to ours for example?

          • Paul Halpern

            “Flat” is a term used by cosmologists to mean “not curved in any of the three dimensions of space.” It is like an infinite box, rather than a plane. So other solar systems generally rotate along different planes, not the same plane.

            • Gary

              Question. I think it may be a dumb one but nonetheless I’m curious. When looking at Polaris the light took 434 years to reach earth. If I look with a very powerful telescope would the light I see still be 434 years old?

            • Paul Halpern

              Yes, meaning that it was generated 434 years ago. The expansion of the universe does not effect the motion of stars in our galaxy, but rather the motion of galaxies in space,

            • http://smj.int-domains.com/ Jay Ell III

              That’s really freaky!!!! So, even if I had a super powerful telescope and was able to look deep into a star (and burn my eyes) that would still be “images” of the distance from that original star to my point of observation?

    • http://www.facebook.com/kennylmacdonald Kenneth Lawrence Macdonald

      The best part about the “observable” universe is that we are always in the center of it.

      • Paul Halpern

        Yes, very true. Unlike the actual universe, which apparently has no center (if it is indeed infinite)!

        • http://www.facebook.com/kennylmacdonald Kenneth Lawrence Macdonald

          Or all center….whoa!

        • Annihilation

          Actually, there is no “actual” universe outside of the central viewpoint of an observer. There is only a subjective universe and never an objective one, and the the universe only exists when being observed. This is the fundamental flaw of modern physics.

    • Robert Riede

      Coming from the German speaking territories, it’s rather interesting, Gott calculated the radius of the observable universe…..

      Sorry for that and great piece of information, Paul.

      • Paul Halpern

        Yes, that is his surname. Brilliant physicist and a very nice person.

    • MastrBlastr

      Hey, I’m only interested in Uranus. didn’t scientists, a few years ago, determine that Uranus is not only BIG butt Gasseous, too!
      and nothiing else has been discovered since then, WHY?????

      • Bob

        This is an interesting question, almost on the same level as “how is babby formed?”

    • cmr

      Question: How do you know that the universe is spreading out rather than contracting? I know about the red shift, but if the universe was being sucked into a giant black hole, the closer you got to the black hole, the stronger would be the gravitional pull on it. This would cause anything closer to the black hole than earth is to move away from us faster than we are moving and we would be moving toward the black hole faster than anything that was further away from us. But those stars who were on the same plane as us would be the only ones that would appear to be moving closer to us. Everything else would appear to be moving away from us and, thus, would have a red shift.

      • Paul Halpern

        The expansion of the universe is approximately the same in all directions. If matter from the universe were falling into a black hole, we would see only motion in that direction.

        Moreover, there are no black holes of the size needed to gobble up the whole universe. Even supermassive black holes have radii on par with the solar system–much, much smaller than the size of galaxies, let alone the observable universe.

      • http://www.facebook.com/randy.moore.549436 Randy Moore

        I kind of like this analogy. There is another assumption about a universe being made out of antimatter travelling in the opposite direction from ours. Supposedly it originated after the Big Bang and accounts for something. But the author already discussed about something being observed if you want to be sure. Does anyone have any takes about a Quasar’s mass being less than required to produce its luminosity? The one I read mentioned black holes, the universe’s background radiation and time travel.

    • Bilal

      Paul, Why is there difference between Age of the Universe (about 13.75 Billion Years) and the Observable Universe.

      Because I am unable to understand that, the oldest object we can see is about 13.75 Billion light years away, so we conclude that the Age of Universe is 13.75 Billion years. But why is Observable Universe 92 Billion Light Years then? Doesn’t it mean that the oldest object should be 92 Billion years old?

      • Paul Halpern

        The universe itself is 13.75 billion years old. That means that no object in the universe can be older than 13.75 billion years. However, because the universe is expanding, the oldest objects are currently much farther than 13.75 light-years away. To reiterate, they are not more than 13.75 billion years old (in age) but can be more than 13.75 billion light-years away (in distance) because of the universe’s expansion.

        • Bilal

          Thanks Paul, I am starting to get a hold of this. But more questions, :)

          Please tell me, how farther is the farthest object we have ever seen.

          How do we determined that the oldest object is 13.75 Billion years old. I have read many pages on it but I am unable to grasp it still.

          Regards

          • Paul Halpern

            The farthest object ever seen is the MACS 1149-JD galaxy, just announced last month: http://www.nasa.gov/mission_pages/spitzer/news/spitzer20120919.html Its light was released 13.2 billion years ago!

            The universe is 13.75 billion years old. Galaxies didn’t start forming until a few hundred million years after the birth of the universe.

    • Anonymous

      That was a good go through that made me think about the “size” of the universe and whether the same value could be arrived at in other ways – which would be quite interesting.

      In the Theory of Something (ToS) I use 46.1 billion light-years for the radius of our universe which is just between the 45.7 and 46.6 billion light-years you have in the foot note above for the observable universe! That is remarkable since the ToS university radius can be calculated from the dark energy density and gravitational constant, which sounds very different from how you describe the calculation by Gott’s team. Yet we arrive at almost the same value!

      In ToS section 5.5.1, I derive Newton’s gravity law from electricity – see http://bit.ly/A-Anonymous for a quick understanding – and out comes the gravitational constant G= 4*c^4*R/DarkEnergyOfUniverse.

      Resolving the radius R of the universe and using the dark energy density, one simply gets R = c^2 * SQRT(3/pi/G/Ude) which gives the
      46.1 billion light-years. Ude is the 72.8% dark energy portion (WMAP seven-year analysis) of the critical density (converted to energy) to make up a flat universe.

      Paul, do you know if Gott’s team effectively is using the same input (but derived in another way) or are we onto something really big here (arriving at the same radius of the universe in a completely other way)?

    • MrRipck

      Rethink your theory by not putting us in the middle.

      • Paul Halpern

        We are the observers, after all, so we are in the middle of the sphere of observation.

        • http://smj.int-domains.com/ Jay Ell III

          I too found it interestingly that we were put right in the center of the observable Universe …. making us the origin of the Big Bang.

          Paul, that being so, it automatically invalidates the investigation.

          The center of the radius for measuring the Universe would be the dead center of the big bang and, yet, since we are not at the boundary of the Universe, that measurement would never be precise anyway; just another note in discoveries for human kind.

          That said, we could ponder that the Universe cannot and will not; ever, be precisely measured. BTW: why would that be important after all?

          Yet, I liked the point about the fuzziness right after the Big Bang. If you strike a match, first comes the light; long after comes the smoke. So, I like to believe that we have something to consider here about the ability of light to have being able to propagate from the very beginning.

          In regards to the WMAP, does it have a 360 view horizon or just focused forward like a binoculars? If like a binoculars, it would only view portion of the Universe. If a 360, it would be limited by it’s range. So, the WMAP is just a sample; not a current final word on the subject?

          • Annihilation

            That was the worst train of logic in the observable universe. I’m a biocentrist but for you I suggest suicide.

        • http://smj.int-domains.com/ Jay Ell III

          I came up with two other hypothesis:

          a) If the theory that the Universe is STILL expanding is true, the original “debris” is still flying away from the poo (point of origin) which would mean that there was no recombining at all; and

          b) Since we have telescopes on all sides of Earth and space, if we could observe the “direction” in which the Universe is still expanding, we could, somehow, try to position the right hardware to the right point of origin …. and I theorize that it would be impossible to “reach” it even with the most powerful hardware available today. We might, then, realize that the Universe is truly infinite.

          I am basing the above in the assumption that there was a Big Bang as the origin of it all.

        • alex

          Exactly. That’s why it’s called the ‘observable universe’, we are observing it. Ever creature that can observe would have their own ‘observable universe’, don’t you think Mr. Halpern? By the way, great article! I’m saving this in my files.

    • Cathy W

      You explained that beautifully; I’ve been holding on to this email/link until I just had the time to read your article. For a long time, I’ve wondered about our size. Thanks for the well-written answer to my curiosity:) cw

      • Paul Halpern

        Glad you enjoyed the piece!

    • Ben Kschenka

      Thanks Mr Halpern, that was quite clear for the most part but would you mind clarifying the following:

      We are at the center of the radius because we are the observer. If someone in a galaxy 7 parsecs from us were making the same observations would they determine the universe to have a radius of 14 billion parsecs and would they see the Milky Way as being mid-way along their radius? If this is so it seems counter-intuitive to the big bang concept i.e the universe expanding outward from a singularity.

    • http://twitter.com/jeffreyremick Jeffrey Remick

      This is, roughly, the size of a marble… correct?

    • http://www.facebook.com/robert.parent.56 Robert Parent

      Question from an amateur: What if the “Big Bang” was more like a solar flare spit out in one direction and expanding conically forever until it finally just dissipates? How could we know the time frame of the big event?

    • Matthewcobb

      Do you think that the size of the observable universe plays a role in Mach’s Principal? Namely, that the large scale structure of the observable universe determines local physical law? I have a paper on this that I am working on, and the results are quite consistent with increases in galactic mass currently attributed to so called dark matter. Let me know. MatthewCobb at DiscorsiScientific. The law I am working under is the most general law of relativity. Thanks.

    • stephen mann

      The universe has been likened to an expanding balloon with its galaxies upon that’s surface, ever pulling further apart. The rate at which this occurs is called “Hubble’s constant”, although this figure has changed in the last twenty years due to newer satellite-based observations. Now, the universe’s diameter is given as 93.2 billion light years. The older estimate for this was 27.6 billion (based upon its age). The expansion of “spacetime” has lately been factored in. The ratio between the universe’s size and age then becomes about 3.38 and is, in effect, “Hubble’s constant”.

      If I could step outside of the universe, it would look like a transparent sphere covered with spiderweb (or the galaxies). These would pull apart in three dimensions while its diameter increased with time, its “fourth dimension”. Because the ratio of size to time is close to that of pi (3.142), I suggest that 13.8 billion l.y.’s are its radius while the figure of 93.2 billion l.y.’s is its circumference rather than diameter. This distinction resolves for me the confusion between the universe’s expansion in space versus its in time…

    • David Snyder

      I have read the article, read through the comments, but still I am confused as to how the observable universe can be over 90 billion light years in diameter. I understand that the universe is expanding, and accelerating in that expansion, never the less, that speed at which everything is moving outward from the origin of the big bang must be insignificant compared to the speed of light. So, if the oldest object we can see sent out its light 13.2 billion years ago, then it should only be (just a guess) 14ish billion light years away now. Maybe a little more or less, right? Shouldn’t the matter in the universe have a diameter of much less than 28 billion light years? And the only thing at a distance of 13 or 14 billion light years from the point of origin would be the very light that was sent outward at that 380,000 year mark, not matter?

      • Odin Matanguihan

        Not a physicist here. But my understanding from this article is, that the observable universe is 14ish billion light years in radius at the time that the light was emitted. In the the time that it took for the light to reach us, the universe has since expanded. The part I’m not sure of is, whether they mean that the universe has already expanded to 90 billion light years, or whether they expect 90 billion light years to be the limits of observation. (That is, anything released from 90 billion light years away would have been receding so fast that the light is never going to reach us)

      • Kenneth Cole

        Initially, the expansion of the universe vastly outstripped the speed of light. After about 1 second, it was larger than our solar system. I believe it is still expanding faster than the speed of light.

    • Major

      What phenomenal fine tuning… Makes you wonder

    • Steve

      Have physicists ever given any thought to the possibility that our atoms – since they have electrons that orbit their nuclei – similar to our planets orbiting its nuclei (the sun) make up yet another universe in a much smaller dimension than ours – and we could make up another universe much larger dimension than ours? I mean if the universe is infinite – could this not be possible? This idea came to me upon watching the movie “The Incredible Shrinking Man” many years ago. At the conclusion of the movie, the film showed pictures of galaxies in our universe. It led one to believe that this man had actually shrunk down so small, that he actually became part of our atomic substructure – which could be compared to our solar system on a much smaller scale. Could planet earth be an electron of an atom in a much larger dimension?

    • James Peters-Gill

      Just wondering – How can the observable universe’s radius me more than double 13.75 billion light-years? Assuming that at the start of the universe, an object was 13.75 billion light-years away when it emitted the light and has been traveling at the speed of light directly away from us ever since (which, according to Einstein, is the universal speed limit), it would only be 27.5 billion light years away. Surely, this is the maximum distance possible? In fact, not even this is possible seen as at the time of the big bang, the universe wasn’t large enough to contain such distance, and also, the fact that only after the era of recombination could light journey through space.

      I’m sure you’re right. Just a little confused as to how it works.

    • Avraam J. Dectis

      .
      So in other words, there could very well be a hell and heaven dimension and it could also be a good idea to be on the side of the angels to ensure that your escort to the better place shows up when needed.
      .

    • eric pham

      By mathematic calculation the radius of the universe is about 142 billion light years for all object moving slower than the speed of light. This calculation base on premise that energy = mass * c^2 and the limit of all material things are smaller than infinity.

      • eric pham

        actually it is round up to only 71 billion light years and not 142 as I post above.

    • http://www.senttell.com SentTell

      What are the chances that there is more out there than the observable universe?

    • Matthew Bateman

      Why do we always assume that the universe is expanding due to the Big Bang.And for that matter why do we assume there even was a Big Bang?
      Why couldn’t these things of happened in just a hit and miss self creation of particles that slowly just came into existence,and sure why not also say that these same particles that formed matter and gases and whatnot are just expanding in there domain.These same things are also what are giving off these radiation signatures that we read as background radiation.
      Also isn’t it only obvious that there must be a super massive blackhole in the center of each galaxy.Just like pulling the plug on a sink or bathtub ever notice the resemblance.
      It really disgusts me that these scientists know for a certainty that there was a Big Bang……Let me ask you this….how do you know for sure that there was?And why are they so hung up on this explanation.Because they are closed minded morons they obviously can’t imagine it being created in any other way.