Cosmological principle
The cosmological principle is more of a philosophical notion than theoretical cosmology. The universe is not homogenous at smaller scales but is statistically homogenous at larger scales. Inconsistencies have been found in a handful of studies. Implications include some sort of mechanical equilibrium and a non-static universe.
Expanding universe
The expansion of the universe is intrinsic. As a result, the rate at which the universe expands is not bounded by the speed of light. Objects within this metric do not move nor change.
During the inflationary epoch (10-32 seconds after the Big Bang), the universe drastically expanded from 1 nanometre to roughly 10.9 light-years. The rate of expansion slowly slowed down until 4 billion years ago before accelerating again. It now expands at around 72 kilometres per second and is accelerating due to dark energy.
Recessional velocity
Recessional velocity is the rate at which extragalactic astronomical bodies recede as seen from an observer. It can be measured by observing the wavelength shifts of spectral lines emitted by an object.
Cosmological red/blueshift
It can be used to measure the speed at which stars and galaxies are approaching or receding from us, aka radial velocity. This allows us to better understand the expansion of the universe
Redshift
Redshift is electromagnetic radiation from an object that increases in wavelength.
Blueshift
Blueshift is electromagnetic radiation from an object that decreases in wavelength.
Hubble's law
Hubble's law is the observation in physical cosmology that galaxies are moving away from Earth at velocities proportional to their distance. The apparent relative velocity is proportional to the distance.
Hubble's law
v = H0D
v = speed of recessional velocity
H0 = Hubble's constant (around 70 km/s/megaparsec)
Convening distance
It is the distance between 2 objects that do not change through time by accounting for the expansion of the universe
Proper distance
It is the distance between 2 objects that changes through a certain cosmological time due to the expansion of the universe.
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