Observable universe
The portion of the universe visible from Earth.
The observable universe is a spherical region centered on Earth, encompassing all matter whose electromagnetic radiation has had time to reach our planet since the Big Bang. Its radius is estimated at about 46.5 billion light-years (14.26 gigaparsecs), a distance determined not by technological limits but by the fundamental constraint of the speed of light. Because the universe is only about 14 billion years old, light from objects beyond this distance has not had sufficient time to travel to Earth; such objects lie beyond the particle horizon and are thus unobservable. Every location in the cosmos has its own observable universe, which may partially overlap with Earth’s.
The cosmic microwave background radiation (CMBR) originates from a comoving distance of roughly 45.7 billion light-years, while the edge of the observable universe is about 46.6 billion light-years away, a 2% difference. Using the critical density and this diameter, the total mass of ordinary matter in the observable universe can be calculated. In November 2018, astronomers reported that extragalactic background light (EBL) totals approximately 4 × 10⁸⁴ photons.
Due to the universe’s accelerating expansion, objects outside the local supercluster will appear to freeze in time, emitting progressively redder and fainter light. For example, objects with a redshift between 5 and 10 will remain observable only for 4–6 billion years. A future visibility limit exists at a comoving distance of about 62 billion light-years, beyond which light will never reach Earth. This limit implies that the number of theoretically observable galaxies in the infinite future is only 2.36 times the current count, ignoring redshift effects. A cosmic event horizon currently lies about 16 billion light-years away: signals from events within that distance can eventually reach Earth, while those beyond cannot. This reachable universe, combined with the current visibility limit, equals the future visibility limit.
- radius
- 46.5 billion light-years (14.26 gigaparsecs)
- diameter
- 93 billion light-years (28.5 gigaparsecs)
- total mass of ordinary matter
- 1.5×10^53 kg
- extragalactic background light photons
- 4×10^84
- estimated number of galaxies
- 2 trillion
- estimated number of stars
- 10^24
- comoving volume
- 3.57×10^80 m³
Lore & Background
The observable universe is centered on Earth, though every location in the universe has its own observable universe. Its boundary is set by the particle horizon, beyond which light has not had time to reach us since the Big Bang. Due to the accelerating expansion of the universe, a future visibility limit exists at a comoving distance of about 62 billion light-years. Objects beyond this limit will never be observable from Earth, even in infinite time. The current cosmic event horizon, within which signals can eventually reach Earth, is about 16 billion light-years away. The universe's total size is unknown and may be infinite.
Reader's Guide
The concept of the observable universe is fundamental to cosmology, as it defines the scope of all astronomical observations. It is not limited by technology but by the finite speed of light and the age of the universe. The observable universe contains an estimated 2 trillion galaxies and 10^24 stars, though these numbers are disputed. Its mass in ordinary matter is calculated at 1.5×10^53 kg. The accelerating expansion, attributed to dark energy, means that most galaxies beyond the local supercluster will eventually become invisible as their light redshifts and fades. The future visibility limit implies that only about 2.36 times as many galaxies will ever become observable compared to the current count. This framework helps distinguish between the observable universe and the larger, possibly infinite, universe that may exist beyond our causal horizon.
Did You Know?
- The radius of the observable universe is about 46.5 billion light-years, even though the universe is only about 13.8 billion years old, due to the expansion of space.
- The total mass of ordinary matter in the observable universe is estimated at 1.5×10^53 kg.
- Extragalactic background light amounts to 4×10^84 photons.
- The future visibility limit, beyond which objects will never be observable, is at a comoving distance of 62 billion light-years.
Frequently Asked Questions
What is the Observable universe?
It is the spherical region of space centered on Earth that contains everything detectable by electromagnetic radiation, bounded by how far light has had time to travel since the universe began expanding. In short, it is the maximum 'viewing distance' available to us given the finite speed of light.
How big is the Observable universe?
Its radius stretches roughly 46.5 billion light-years (about 14.26 gigaparsecs), giving it a diameter of around 93 billion light-years (28.5 gigaparsecs). That span marks the full extent of space from which photons have had enough time to reach Earth since the Big Bang.
What does the Observable universe contain?
Current estimates place the galaxy count at around two trillion, with roughly 10^24 stars scattered among them. The total mass of ordinary (baryonic) matter within the sphere is approximately 1.5 × 10^53 kg, and it has produced about 4 × 10^84 photons of extragalactic background light over cosmic history.
Why can't we see beyond the Observable universe?
Because light travels at a finite speed, there is a hard physical boundary beyond which no electromagnetic signal has had enough time to reach us since the universe started expanding. The universe does not 'end' at that edge—it is simply that information from farther away has not yet arrived.
Why is the Observable universe important to study?
It defines the totality of what we can empirically investigate with current physics, making it the natural boundary for cosmology, astrophysics, and astronomy. Every model of cosmic structure, evolution, and composition is ultimately built by cataloguing what lies within this sphere.
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