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View Full Version : Why the factor 2 in BAO / CMB measurements?



john hunter
2018-Feb-10, 11:37 AM
In Aubourg, https://arxiv.org/abs/1411.1074

they say (bottom of page 9) "we find cln(1+z)/DM(z) = 151 km/s/Mpc at z=1090, a factor of two larger than any of the low redshift values"

So do we now have two cosmic co-incidences - dark energy and matter densities being a similar size and such a small, round number of 2 for the above?

Have any reasons been found or suggested for the factor of two? Very strange?!

Shaula
2018-Feb-10, 01:10 PM
Have any reasons been found or suggested for the factor of two? Very strange?!
It is not strange at all. Nor is the 'factor of two' particularly close to two. Dm varies between 64 and 67 in the low redshift domain, which is a factor of between 2.25 and 2.35. Factor of two is just a convenient, imprecise expression. The evolution of Dm with time has a minimum at about z=1 and increases quite slowly either side of that - so the ratio between Dm is generally going to be quite small.

john hunter
2018-Feb-10, 08:05 PM
It is not strange at all. Nor is the 'factor of two' particularly close to two. Dm varies between 64 and 67 in the low redshift domain, which is a factor of between 2.25 and 2.35. Factor of two is just a convenient, imprecise expression. The evolution of Dm with time has a minimum at about z=1 and increases quite slowly either side of that - so the ratio between Dm is generally going to be quite small.

You are right that cln(1+z)/DM increases quite slowly, it's plotted here https://www.desmos.com/calculator/a6lfvckbei

Is it correct to say that the 1+z of 1090 is deduced from the ratio of temperatures at recombination to temperature of the microwave background radiation, from which the ratio of scale factor a(0)/a(1) is deduced to be 1090?

If so it seems quite a coincidence that of all the temperatures that the background radiation could have been, it has the value to give the cln(1+z)/DM of about 2*(Hubbles constant). In fact for H(0) between 65 and 75 corresponding to 130 - 150 on the graph, only redshifts of 420 to 1260 do it. Whereas the redshift of the background radiation could have been any value even into millions, for example. If we use Riess's 73.24*2=146.5 (up to 150) it's only about z=1040 to 1260.