How big is a Gigaparsec?

A gigaparsec is a unit of distance equal to one billion parsecs, or about 3.26 billion light-years. A light-year is the distance light travels in one year. In meters, a gigaparsec is about 3.09 × 10²⁵, where 10²⁵ means a 1 followed by 25 zeros. Astronomers use it for the largest distances in the universe.

Definition

The parsec is defined by the International Astronomical Union (IAU), which sets the units astronomers use. A parsec is exactly 648,000 divided by pi astronomical units. One astronomical unit is exactly 149,597,870,700 meters, close to the average distance from Earth to the Sun. That makes a parsec about 3.26 light-years, or 30.9 trillion kilometers.

The prefix giga-, from the International System of Units, means one billion (10⁹). A gigaparsec is therefore about 206 trillion astronomical units, or 3.09 × 10²² kilometers. Light takes about 3.26 billion years to cross it, almost a quarter of the 13.8-billion-year age of the universe.

Size comparisons

• If a parsec were shrunk to 1 millimeter, a gigaparsec would be 1,000 kilometers (620 miles) long.
• About 33,000 disks of the Milky Way, each about 100,000 light-years across, would fit end to end across a gigaparsec.
• The Pisces-Cetus Supercluster Complex, a band of galaxy clusters that includes the Milky Way, is about 0.3 gigaparsecs long. About three of them would span a gigaparsec.
• The Hercules–Corona Borealis Great Wall, a proposed structure of distant galaxies, is estimated at 2–3 gigaparsecs across.
• The observable universe, the part of space from which light has had time to reach Earth, has a radius of about 14.3 gigaparsecs. It is about 29 gigaparsecs across.

Who uses gigaparsecs

The IAU notes that the parsec is the usual unit for the structure of the Milky Way, while the light-year appears mostly in popular writing. Astronomers add the same prefixes used with meters. Many studies of galaxies give distances in megaparsecs, millions of parsecs.

The gigaparsec is used for the largest scales: the size of the observable universe, the biggest structures, and limits on the size of the whole universe. For example, a 2004 study searched maps from NASA's WMAP spacecraft for signs that space wraps around on itself. For a wide range of possible shapes, it ruled out a universe smaller than 24 gigaparsecs across.

Expansion across a gigaparsec

The universe is expanding, and the rate is given in kilometers per second per megaparsec. This rate is called the Hubble constant. Using the Planck satellite, astronomers measured it as about 67.4 kilometers per second per megaparsec.

By Hubble's law, a distant galaxy moves away at the Hubble constant times its distance. A gigaparsec is 1,000 megaparsecs, so a galaxy that far away recedes at about 67,400 kilometers per second. That is about 22% of the speed of light, which is about 300,000 kilometers per second.

Sources

• IAU 2015 Resolution B2 on Recommended Zero Points for the Absolute and Apparent Bolometric Magnitude Scales. International Astronomical Union, 2015.
• Measuring the Universe. International Astronomical Union.
• The International System of Units (SI). In SI Brochure, 9th edition. Bureau International des Poids et Mesures, 2019.
• Planck 2018 results. VI. Cosmological parameters. Planck Collaboration, Astronomy and Astrophysics, 2020.
• Speed of light in vacuum. NIST CODATA, 2022.
• Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe. Davis and Lineweaver, Publications of the Astronomical Society of Australia, 2004.
• Constraining the Topology of the Universe. Cornish et al., Physical Review Letters, 2004.
• A Map of the Universe. Gott et al., The Astrophysical Journal, 2005.
• More about clustering on a scale of 0.1 c. Tully, The Astrophysical Journal, 1987.
• New data support the existence of the Hercules-Corona Borealis Great Wall. Horváth et al., Astronomy and Astrophysics, 2015.
• Our Milky Way Galaxy: How Big is Space?. NASA Science.