# Lambda-CDM

Science published last week (02/02/2018) a paper from O. muller et Al. (https://arxiv.org/pdf/1802.00081.pdf)

"A whirling plane of satellite galaxies around Centorus A challenges cold dark matter cosmology".

The authors report measurements of the velocities of 16 dwarf galaxies around Centorus A, a nearby galaxy (a nice picture here : https://en.wikipedia.org/wiki/Centaurus_A).

What they find is surprising : the galaxies are coherently rotating around centorus A, as you can see for yourself on this plot :

On the left panel, blue triangles indicate approaching galaxies and the red ones indicate receding galaxies. The upper right panel simply shows a zoom at Centaurus A, and the bottom right panel is a 3-D map of the 16 dwarf galaxies and their velocities.

This is surprising because, after our own Milky-Way and (possibly) Andromeda, this is the third case of such corotating satellite galaxies, a dynamic that is not favored at all by the best current cosmological model, $$\Lambda$$-CDM (Dark Energy and Cold Dark Matter) which predicts that less than 5 per 1000 galaxies should exhibit such behavior.

# Expanding Universe

When discussing with friends the endeavor of physics in cosmology, it sometimes appears that its purpose is unclear and carries some sort of esoteric meaning. As this blurring often seems to be fed by elusive statements from prosperous cosmologists, I find it clarifying to show cosmology at work, that is, studying the physical history of the universe.

A paper recently published by Riess & al.  A 2.4% Determination of the Local Value of the Hubble Constant (https://arxiv.org/pdf/1604.01424.pdf) is a great illustration of what this is about.

Here is the picture :

This is the comparison of various attempt to measure the current expansion rate of the universe, also called the Hubble constant and noted H0 on the x-axis.

So for instance, a value of 70 kilometers per second per MegaParsec means that the Andromeda galaxy, that is next to our Milky Way at 0.78 MegaParsec, is receding from us by 55 km each second.

Let's first focus on the red and the blue points highlighted by the vertical dashed lines :

The red point, noted  « Here » is the most precise of the direct measurement. It is obtained by looking at the position and velocities of surrounding galaxies. The bue point « Planck 15 + Lambda-CDM » is a combination of the map of the temperature of the celestial sky and the best values of the cosmological parameters to represent it (try it for yourself here :  the map of the temperature of the universe),

When expressing how much they agree with each other in term of their uncertainties (the red and blue lines), it is found that the likelihood that they are both in agreement is not even one per thousand !

Because physics is all about evaluating how reliably a picture reproduces a scene, the paper then thoroughly explores what could possibly go wrong :

• Are the experiments careful enough with their measurements? - These are the other points on the plots -
•  Should the model be re-evaluated ? - These are the arrows -

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