shawnhcorey, 5 months ago

@boud @cartographer @stephenserjeant @ClaireLamman @cosmology

Because of the expansion of the universe, there is a maximum distance we can see. Beyond that, we do not know if the universe is finite.

Take the speed of light and divide it by Hubble's constant. You get about 14.6 giga-light-years. This is the maximum distance light can travel to reach us. Beyond that, we cannot measure anything about our universe.

boud, 5 months ago

@shawnhcorey

• The maximum distance that we can see to is an effect of the finite age of the Universe, not expansion.

• c/H_0 is not a serious last scattering distance, it's only order-of-magnitude.

• The possible finiteness of the Universe of a bit smaller or bigger than CMB diameter may be measurable directly (old but short review [1]); or by #TopologicalAcceleration .

@cartographer @stephenserjeant @ClaireLamman @cosmology

[1] Roukema (2000) BASI, 28, 483 https://arXiv.org/abs/astro-ph/0010185

shawnhcorey, 5 months ago

@boud @cartographer @stephenserjeant @ClaireLamman @cosmology

The age and expansion of our universe are the same thing.

boud, 5 months ago

@shawnhcorey

No. In standard #TexLaTeX notation:

• the age of the Universe (within the #FLRW family of models) is a duration of time, normally written $t_0$, approx 13.8 Gyr for #LCDM.

• "expansion" is ambiguous, but can either mean:

** the scale factor, which is now $a=1$ by definition, or

** the Hubble parameter $H := da/dt = \dot{a}$ where $t$ is the #FLRW time coordinate.

The Hubble constant is $H$ now = $H_0 = H(t_0)$.

@cosmology