- Circa 2018,
it was found that
the Hubble constant
by direct observations for the local
observable universe
was H_0 ≅ 73 (km/s)/Mpc
and by global indirect measurements from the
Planck spacecraft (2009--2013)
was H_0 ≅ 67 (km/s)/Mpc.
These determinations now disagree by more than 2 standard deviations (i.e., σ's).

So there is a tension that is approaching a falsification of the (

**BASE**) Λ-CDM model and perhaps of the cosmological-constant dark energy. This tension is the Hubble tension. - There are various extensions to the
Λ-CDM model collectively called
extended-Λ-CDM models.
One or many or none of these may be adequate to account for the Hubble tension. But being adequate is

**NOT**enough to confirm a theory to high confidence.The Λ-CDM model had an elegant simplicity that the extended-Λ-CDM models lack when viewed by Occam's razor. The extended-Λ-CDM models all seem ad hoc: just fix-ups.

An extended-Λ-CDM model that can account for the Hubble tension and is motivated by an elegant theoretical reason is strongly desired.

- One possible extension
is to allow
dark energy or
cosmological constant (AKA Lambda, Λ)
to vary with cosmic time while
still being constant in space.
But we would like to have elegant theory of that variation.

- Right now we need more guidance from observation or some elegant new
theory to make progress
in solving
Hubble tension.
Among other projects, the Roman Space Telescope (RST, c.2025--c.2030) and the Euclid (c.2021--c.2027) will supply cosmological statistics which should give us a much better idea of how dark energy behaves as a function of cosmic time.

- The solution cosmological model to the
Hubble tension
might require a new name if it is too different
from the Λ-CDM model.
Perhaps, we will just call it the standard model of cosmology (SMC) which name is currently used sometimes for the Λ-CDM model itself.

Actually, using SMC is a good idea since it allows us to update our standard model of cosmology without having to update the name.

- Advances in cosmology
are very exciting---but updating
Introductory Astronomy Lectures (IAL)
is a nuisance.

Caption: The Hubble tension illustrated by the leftmost and rightmost data points.

Note the Hubble constant H_0 is the relative rate of the expansion of the universe. It is given in units of (km/s)/Mpc. So a Hubble constant of H_0 = 73 (km/s)/Mpc means that every megaparsec (Mpc) further out you go, the recession velocity relative to you of astronomical objects participating in the expansion of the universe increases by 73 (km/s).

Features:

Image link: Wikimedia Commons: File:Recent Hubble's Constant Values.png.

Local file: local link: hubble_tension.html.

File: Cosmology file: hubble_tension.html.