Observable Universe
The observable universe refers to the portion of the universe from which light has had time to reach us since the Big Bang, 13.8 billion years ago. It is the region accessible to our observations, bounded by our cosmic horizon. Beyond that, the light has not yet had time to reach us—or will never reach us, due to the accelerated expansion of the universe.
Why 46 billion and not 13.8 billion?
One might think that the observable universe spans 13.8 billion light-years—the distance that light has traveled since the Big Bang. In reality, its radius is approximately 46 billion light-years. The difference is due to the expansion of space: the regionsfrom which the oldest photons we receive today originated have moved considerably farther away since those photons were emitted, due to the expansion.
The cosmic horizon
The horizon of the observable universe is not a physical barrier but a limit to our observational capacity. Beyond it, objects likely exist—the entire universe is probably much larger, perhaps infinite—but their light has not yet had time to reach us. Furthermore, due to accelerated expansion, certain regions areare now moving away from us faster than the speed of light and will never be able to send us a signal.
What the observable universe contains
The observable universe contains at least 2,000 billion galaxies, according to the most recent estimates. Each contains, on average, hundreds of billions of stars. The scale is virtually impossible to comprehend: if the observable universe were shrunk to the size of Earth, our galaxy would be smaller than a molecule.
Importance in Astronomy
The observable universe defines the limits of all scientific investigation in cosmology. Everything we know about the universe comes from within this horizon. The most powerful instruments—such as the James Webb Space Telescope—seek to push the boundaries of what we can observe by detecting galaxies that are increasingly distant and closer to the Big Bang.
Concrete example
The galaxy GN-z11, observed by Hubble, is one of the most distant ever detected, with a redshift of z=11. Its light was emitted only 400 million years after the Big Bang. The light we receive from it today has traveled for 13.4 billion years, and the galaxy itself is nowabout 32 billion light-years away from us.
Frequently Asked Questions
Is the entire universe larger than the observable universe?
Almost certainly. Cosmic inflation suggests that the total universe could be orders of magnitude larger than the observable portion. Some models predict an infinite universe. But by definition, we cannot observe what lies beyond our horizon.
Is the observable universe the same for all observers?
No. An observer located in a galaxy 10 billion light-years away from us would have an observable universe centered on them, with a region that overlaps with oursbut also regions that we cannot see. Every point in the universe has its own observable horizon.
Is the observable universe expanding?
Yes and no. In terms of the time it takes for light to travel, we can see farther and farther over time. But because of the accelerated expansion, certain regions that were once within our observable universe are gradually slipping out of it. In the very long term, our effective observable universe will shrink.