Discrepancy in Jorrie’s LightCone7-2017-02-08 Cosmo-Calculator

[JimJCW]

TL;DR Summary

When using Jorrie’s cosmological calculator, the input Hubble constant and the output Hubble parameter at the present time, i.e., H(z=0) are different by about 0.5%, but they should be the same.

When studying cosmology, Jorrie’s LightCone7-2017-02-08 Cosmo-Calculator, http://jorrie.epizy.com/LightCone7-2017-02-08/LightCone_Ho7.html?i=1, can be a very useful tool to calculate quantities in the lambda-cold-dark-matter model. Recently I noticed a discrepancy in the program: The entered Hubble constant and the output value of Hubble parameter at z=0 are different (by about 0.5%). You can help by doing the following:

(1) Verify that there is such a discrepancy in Jorrie’s program and

(2) Contact Jorrie about it if you know how.Jim JCW

 

[Bandersnatch]

Huh. I've never noticed - probably because the numbers look similar.

Contact Jorrie about it if you know how.

One has to stand in front of a mirror, draw the ancient sigil of the electric monkey, and repeat his name three times.
@Jorrie @Jorrie @Jorrie

 

[Arman777]

Summary:: When using Jorrie’s cosmological calculator, the input Hubble constant and the output Hubble parameter at the present time, i.e., H(z=0) are different by about 0.5%, but they should be the same.

When studying cosmology, Jorrie’s LightCone7-2017-02-08 Cosmo-Calculator, http://jorrie.epizy.com/LightCone7-2017-02-08/LightCone_Ho7.html?i=1, can be a very useful tool to calculate quantities in the lambda-cold-dark-matter model. Recently I noticed a discrepancy in the program: The entered Hubble constant and the output value of Hubble parameter at z=0 are different (by about 0.5%). You can help by doing the following:

(1) Verify that there is such a discrepancy in Jorrie’s program and

(2) Contact Jorrie about it if you know how.Jim JCW

I don't see any $H(z)$ as an output ?

 

[Bandersnatch]

I don't see any $H(z)$ as an output ?

Open the column definition and selection menu.

 

[Jorrie]

Huh. I've never noticed - probably because the numbers look similar.One has to stand in front of a mirror, draw the ancient sigil of the electric monkey, and repeat his name three times.
@Jorrie @Jorrie @Jorrie

Sorry guys, were logged out and did not receive notification.
I have also not noticed the discrepancy before, but will have a look at the code tomorrow, my valley's time...

 

[Jorrie]

Summary:: When using Jorrie’s cosmological calculator, the input Hubble constant and the output Hubble parameter at the present time, i.e., H(z=0) are different by about 0.5%, but they should be the same.

When studying cosmology, Jorrie’s LightCone7-2017-02-08 Cosmo-Calculator, http://jorrie.epizy.com/LightCone7-2017-02-08/LightCone_Ho7.html?i=1, can be a very useful tool to calculate quantities in the lambda-cold-dark-matter model. Recently I noticed a discrepancy in the program: The entered Hubble constant and the output value of Hubble parameter at z=0 are different (by about 0.5%). You can help by doing the following:

(1) Verify that there is such a discrepancy in Jorrie’s program and

(2) Contact Jorrie about it if you know how.Jim JCW

It appears as if the 0.5% low error is consistent throughout the ranges requested, so it is probably a systematic error, or whatever one calls it for a simulation...
It happened with an update of outputs way back in Jan 2017, probably just a misalignment between table lookup indexes. Will investigate and correct.

Thanks for spotting it!

 

[JimJCW]

Jorrie wrote: It appears as if the 0.5% low error is consistent throughout the ranges requestedI just compared the calculated H(z) for a given H(0) with results obtained by other means. It seems the 0.5% difference persists for all z.

I find your calculator very useful.

JimJCW

 

[Jorrie]

I haven't found the source of the misalignment yet, but will keep probing.
It seems to me that it's only the H column that is misbehaving...

 

[JimJCW]

It appears as if the 0.5% low error is consistent throughout the ranges requested, so it is probably a systematic error, or whatever one calls it for a simulation...

I haven't found the source of the misalignment yet, but will keep probing.
It seems to me that it's only the H column that is misbehaving...

I have noticed the following:

H(z) can also be calculated from the output of the calculator: H(z) = v(then) / D(then) in unit of c/Gly. The result consistently shows a 0.5% higher value than the tabulated H(z). I think this implies that the discrepancy is caused by a conversion factor somewhere.

JimJCW

 

[Jorrie]

Ok, it was just a silly conversion error. Tx for that.
Before I upload the corrected version, is there any columns that you and other fairly regular users might want to see as default at startup? Without making it too daunting for newcomers, of course...

 

[JimJCW]

Ok, it was just a silly conversion error. Tx for that.
Before I upload the corrected version, is there any columns that you and other fairly regular users might want to see as default at startup? Without making it too daunting for newcomers, of course...

Great!

Nick Gnedin has a ‘Cosmological Calculator for the Flat Universe’ on the Fermilab website, https://home.fnal.gov/~gnedin/cc/. Sometimes I use its feature, ‘Distance between the two redshifts’. For example, between ‘z=1 and z=0.1’, instead of between ‘z=1 and z=0’. It is not urgent or necessary.

JimJCW

 

[Jorrie]

Nick Gnedin has a ‘Cosmological Calculator for the Flat Universe’ on the Fermilab website, https://home.fnal.gov/~gnedin/cc/. Sometimes I use its feature, ‘Distance between the two redshifts’. For example, between ‘z=1 and z=0.1’, instead of between ‘z=1 and z=0’. It is not urgent or necessary.
JimJCW

Offending conversion factor fixed. :)
Distance between 2 redshifts does not sit nicely with a tabular/graphing calculator. I'm also not sure what Nick means by ‘Distance between the two redshifts’ - e.g. in what frame?

 

[JimJCW]

Offending conversion factor fixed. :)
Distance between 2 redshifts does not sit nicely with a tabular/graphing calculator. I'm also not sure what Nick means by ‘Distance between the two redshifts’ - e.g. in what frame?

Yes! It is fixed: http://jorrie.epizy.com/Lightcone7-2021-03-12/LightCone_Ho7.html?i=1

It seems the original version is still there: http://jorrie.epizy.com/LightCone7-2017-02-08/LightCone_Ho7.html?i=1About ‘Distance between the two redshifts’, I can see that it does not sit nicely with a tabular/graphing calculator. That’s why I mentioned that it is not urgent or necessary. Here are some more explanations:

For a given observed object (for example, located at z=1), your calculator's outputs are for an observer at the present time, i.e., at t=13.8 Gyr, a=1, and z=0:

D(then) =5.531702 Gly​

D(now) = 11.06340 Gly​

For the same observed object, for an observer at t=12.46 Gyr, a=0.909, and z=0.1, one may want to know what are the corresponding values of D(then) and D(now). Here are the proper distances in that case obtained from Nick Gnedin’s calculator:

D(then) = 4.83567 Gly​

D(now) = 8.79212 Gly​

I use these values for a problem I am involved.I would like to ask you a related question. I am considering about starting a post titled,

Journey of an observed photon from space – an exercise using Jorrie’s calculator​

I am thinking about plotting a cosmic microwave background photon’s proper distance as a function of cosmological time using your calculator’s Chart function. Do you think this is too obvious to most people or it might be interesting to some people?

JimJCW

 

[Jorrie]

I am thinking about plotting a cosmic microwave background photon’s proper distance as a function of cosmological time using your calculator’s Chart function. Do you think this is too obvious to most people or it might be interesting to some people?

Do you mean essentially plotting D(then) against time, i.e. our present the lightcone?

 

[JimJCW]

Do you mean essentially plotting D(then) against time, i.e. our present the lightcone?

Yes. I am thinking to outline the steps so everyone can reproduce the proper distance of a photon vs. cosmological time plot and visualize how complicated the journey of a photon in an expanding universe is.

JimJCW

 

[JimJCW]

Before I upload the corrected version, is there any columns that you and other fairly regular users might want to see as default at startup? Without making it too daunting for newcomers, of course...

Bandersnatch derived two equations that can handle the ‘distances between observes’ problem: A - Distances between observers using the Lightone7 calculator. They are very helpful and, with the help of MS Excel, I can produce D(now) and D(then) columns for observation times other than z=0 easily. I don’t think it should be included in your calculator though; it can be confusing to some users.

I wonder whether you know any ‘closed-form’ expressions for calculating quantities at very small scale factor (or very high z values), such as the event horizon. Your calculator can calculate quantities up to z=20000.

JimJCW