How big is Observable Universe? | Scale of the Universe

The observable universe is the part of the universe from which light has had time to reach Earth since the Big Bang. It is a sphere centered on Earth, about 93 billion light-years across, where a light-year is the distance light travels in one year. That diameter is about 8.8 × 10²⁶ meters, where 10²⁶ means a 1 followed by 26 zeros.

Size

The universe is about 13.8 billion years old. So it might seem that no light could reach Earth from farther than 13.8 billion light-years. But space has been expanding while the light traveled. The matter that gave off the oldest light that could reach Earth is now about 46 billion light-years away.

That distance is the radius of the observable universe, which astronomers call the particle horizon. It is worked out rather than measured directly. Cosmologists calculate it from the universe's expansion rate and contents, which spacecraft such as WMAP and Planck have measured.

Published calculations put the radius between about 46 and 46.6 billion light-years, so the diameter is 92–93 billion light-years. The radius also grows with time, as light from farther away has time to arrive.

Size comparisons

About 3 × 10⁵⁹ planets the size of Earth would fit inside it, by volume.
If the Milky Way, about 100,000 light-years across, were shrunk to the size of a basketball, the observable universe would be about 220 kilometers (140 miles) across.
About 930,000 disks of the Milky Way would fit across it side by side.
It is 10–14 times as wide as the Hercules–Corona Borealis Great Wall, a proposed structure estimated at 6.5–9.8 billion light-years across.

What it contains

The oldest light that telescopes can actually see is the cosmic microwave background. This faint glow was released when the universe was about 380,000 years old, and it now has a temperature of about 2.7 degrees above absolute zero. It comes from a sphere slightly inside the particle horizon, now about 14.0 gigaparsecs (45.7 billion light-years) away, where a gigaparsec is about 3.26 billion light-years.

Within that sphere, a 2005 study estimated that roughly 170 billion bright galaxies have formed. Each is at least about three-quarters as bright as the Milky Way.

Beyond the observable universe

The edge of the observable universe is a limit on what can be seen, not an edge of space. Measurements by the Planck satellite, combined with surveys of galaxies, show that space is flat to within about 0.2%. A 2011 analysis of the universe's curvature found that the whole universe holds at least 251 times the volume of the observable part, and it may be infinite.