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
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.
1 The 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.
Editor’s picks for further reading
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.