Observable universe (noun, “ub-SERV-uh-bul YOO-nih-vers”)
The observable universe is a huge bubble-shaped region of space around Earth. It contains all the matter and energy that we could, in theory, observe from Earth. The universe itself may go on forever. But the edge of the observable universe is like a cosmic horizon line that we cannot see beyond.
Why can we observe only a set region of space? Because the way we observe distant objects in the universe is by looking at the light they give off. This might be visible light. Or it might be radio waves, X-rays or other types of radiation.
Light travels through space at a constant speed, crossing one light-year (5.88 trillion miles) per year. And the universe is 13.8 billion years old. So only the light from objects within a certain distance from Earth has had time to reach us over the history of the universe.
You might think, OK, light travels one light-year per year. The universe has been around 13.8 billion years. So the observable universe should extend 13.8 billion light-years from Earth. But the observable universe is actually bigger. That’s because the universe is expanding. It started expanding the moment it was born in the Big Bang. And space continues to expand at faster and faster rates today.
As a result, the observable universe has been stretched out to 46.5 billion light-years from Earth in every direction. That’s enormous. Scientists think that vast bubble of space contains at least 100 billion galaxies.
We cannot actually observe much of the observable universe right now. This might seem contradictory. But this region is “observable” only in the sense that its light could have reached us by now. That doesn’t necessarily mean we have tech to detect it. Building better telescopes grants us access to more of our observable universe.
In a sentence
The Giant Arc contains dozens of galaxies spanning a distance of about one-fifteenth the radius of the observable universe.
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