Caption: Artist's conception on a quasi logarithmic scale of the observable universe (that part of the universe inside of our past light cone). Going outward from the center at the Sun, are the Solar System inner planets, the Solar System outer planets, the Kuiper belt, the Oort cloud, Alpha Centauri, the Perseus Arm, the Milky Way, the Andromeda galaxy (M31, NGC 224), nearby galaxies, the large scale structure (which is sometimes now called the cosmic web), the cosmic microwave background (CMB), and the Big Bang's invisible plasma on the edge.

Features:

1. The observable universe is all of the universe within our past light cone: i.e., it is the part of the universe from which a light could have reached us (if unimpeded) since the time of Big Bang which was age of the universe ago.

2. The observable universe is a sphere centered on us, the observers of the observable universe.

3. Aristotle (384--322 BCE) was WRONG: the universe is NOT a sphere centered on the Earth. Aristotle was HALF-RIGHT: observable universe is a sphere centered on the Earth. We can't see out, we can't get out.

4. The color in the image eventually gets redder as you move away from the center to suggest the progressive cosmological redshift.

   Note the original cosmic background radiation at the recombination era (∼ 378,000 years after the Big Bang) that evolved to be CMB (which is invisible in the visible band (fiducial range 0.4--0.7 μm)) had a blackbody radiation temperature of ∼ 3000 K (Wikipedia: Cosmic microwave background: Relationship to the Big Bang), and so would have looked white hot or maybe even blue hot to the human eye (see Wikipedia: Red heat).

5. Within the Λ-CDM model (which is favored circa 2020, but perhaps NOT forever), the comoving radius of the observable universe = 14.25 Gpc = 46.48 Gly (current value) and the age of the universe = 13.797(23) Gyr (Planck 2018).

   Note the comoving radius of the observable universe is a true physical distance: i.e., one measurable at an instant in time with a ruler. But, in fact, we CANNOT measure it with a ruler. It is NOT a direct observable and its value can only be determined from a cosmological model.

6. Current observations support the hypothesis cosmological principle for the observable universe: i.e., that the observable universe is homogeneous and isotropic in average properties on a large enough scale.

   What is the large enough scale? Circa 2023, a commonly suggested value for the cosmological principle scale is ∼ 400 Mpc = 0.4 Gpc (see Wikipedia: Cosmological principle: Observations).

   Since 0.1 Gpc is much less than comoving radius of the observable universe = 14.25 Gpc = 46.48 Gly (current value), we can see the average behavior of the observable universe and our view of it should be typical (which as a hypothesis is called Copernican principle).

   Everyone (i.e., us humans and the aliens) from their observing stations should see the same observable universe when averaged on a large enough scale.

7. Now the cosmological principle is posited from each instant in cosmic time since the Big Bang, but the observable universe does evolve.

   So it looked different in the past. And we actually see it looking different due to the finite vacuum light speed c = 2.99792458*10**8 m/s.

   The farther, we look out, the farther we look back in cosmic time. The time difference between now and when a light signal started out toward us is the lookback time.

   So cosmologists have it over historians: cosmologists can actually see the past---the average past, NOT the past of where we are in the observable universe: i.e., in the Milky Way in the Local Group in the Virgo Supercluster in the Laniakea Supercluster.