A light-year is a measure of distance, not time. It represents the distance light travels in the vacuum of space over the course of one Earth year. A light-year is a vast distance; it is approximately 9.46 trillion kilometers. This is based on the fact that light travels at a speed of about 300,000 kilometers per second.
Light is the fastest thing in the universe because it has no mass. According to Einstein's Theory of Relativity, no object with mass can travel at the speed of light.
If we were somehow able to travel at the speed of light, it would take us about 93 billion years to reach the edge of the universe. Our solar system is approximately 4.6 billion years old.
Our universe is approximately 13.8 billion years old. Our galaxy (the Milky Way) spans about 100,000 light-years. There are approximately two trillion galaxies in the universe.
The observable universe is estimated to contain around 2 trillion galaxies [37][38][39] and a total of approximately 10²⁴ stars [40][41]—that is, more stars (and potentially Earth-like planets) than there are grains of sand on all of Earth's beaches combined [42][43][44].
However, according to some other estimates, this number might be closer to just a few hundred billion galaxies rather than trillions [45][46][47]. If the cosmic inflation model is correct and the universe has expanded by more than 60 e-folds, then there could be more than 10¹⁰⁰ stars in the entire universe.
The comoving distance from Earth to the edge of the observable universe is approximately 14.26 gigaparsecs (i.e., 46.5 billion light-years or 4.40×10²⁶ meters) in any direction.[27]: 2
Therefore, the observable universe can be considered a sphere with a diameter of approximately 28.5 gigaparsecs [28] (about 93 billion light-years or 8.8×10²⁶ meters).[29]
Assuming space is largely flat (i.e., following Euclidean geometry), its comoving volume based on this size would be approximately 1.22×10⁴ Gpc³ (or 4.22×10⁵ Gly³, or 3.57×10⁸⁰ m³).[30]
These are distances at the present time (cosmic time), not at the time the light was emitted. For instance, the Cosmic Microwave Background Radiation (CMBR) we observe today was emitted at the time of photon decoupling—estimated to have occurred about 380,000 years after the Big Bang [31][32]—while the Big Bang itself took place approximately 13.8 billion years ago. The matter that emitted this radiation subsequently condensed primarily into galaxies, and the current distance of those galaxies from Earth has been calculated to be approximately 46 billion light-years.[7][9]
To estimate the distance of that matter at the time of light emission, we can first note that—according to the Friedmann-Lemaître-Robertson-Walker (FLRW) metric, which is used to model the expanding universe—if we receive light with a redshift $z$, the scale factor $a(t)$ at the time of emission is:[33][34]
$$a(t) = \frac{1}{1 + z}$$
Combining WMAP's nine-year results with other measurements reveals that the redshift at the time of photon decoupling was:
z = 1091.64 ± 0.47,[35]
which implies that the scale factor at that time was approximately 1/1092.64.
Therefore, if the current distance of the matter that emitted the oldest CMBR photons is 46 billion light-years, then at the time of photon decoupling, that distance was only about 42 million light-years.
“And we still think we know everything.”
“The universe is truly beyond human imagination.”
“Sometimes, the sheer size of the universe puts our entire existence into perspective.”
“We are living on a tiny planet, in an unimaginably vast universe.”
“The more we learn about the universe, the more we realize how little we know.”