Independent Researcher
Question
Asked 16 November 2021
Are there redshift and luminosity distances for Cepheid Variable stars and RR Lyrae stars?
Do such measurements make sense? Do they exist?
Comparing redshift and luminosity distances, if that is a sensible question, may bear on the 4/3 scaling hypothesis as it relates to dark energy.
Most recent answer
Cepheid distance is well established. My concern is the measurement of the red shift wavelength for the various Cepheids. Is there a data base with actual wavelength measurements?
All Answers (4)

Dear Robert,
every measurement makes sense, only their interpretation(s) in the model(s) is up to debate.
The only problem that could occur in my mind is that Cepheids are too "near" us and the redshift measurement is not precise and/or under the resolution of actual made spectrograph, but if the star is far enough and you could make the two measurements (variability period and redshift) the comparison is useful to the famous "ladder problem" in astrophysics (and to make sure you calculate the distance right)
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Pantheory Research Org.
Our galactic stars do not relate to redshifts but luminosity distance does apply.
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Hamburg University
Hi Robert Shour ,
Cepheid and RR Lyrae variables are well known standard candles, and important tools in the cosmological distance ladder. For example, Cepheid variables, which were discovered by Henrietta Swan Leavitt, have the property that their luminosity can be directly inferred by observing their pulsation period, which then allows one to calculate their luminosity distance, given that the observing instrument (telescope) also measures their flux.
However, although nothing stops you from making redshift measurements of relatively nearby objects, this will induce an error in any cosmological parameters inferred from these measurements (such as the luminosity distance), because the peculiar velocities of these objects would be comparable to their Hubble flow, giving you highly inconsistent results. Luminosity distances calculated by interpreting the measured redshifts as cosmological redshifts, become more reliable at large distances, where the Hubble flow dominates over the peculiar velocities.
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Similar questions and discussions
Hubble parameter change with cosmological time would directly affect the measured speed of light c in the vacuum
Emmanouil Markoulakis
According to my research,
the speed of light c in the vacuum is directly determined by the zero-point vacuum energy density,
ρvac = 5.96×10^−27 kg/m^3 ≘ 5.3566×10^−10 J/m^3 = 3.35 GeV/m^3.[https://doi.org/10.1051%2F0004-6361%2F201525830] [https://en.wikipedia.org/wiki/Cosmological_constant]
Therefore any chronological change observed of the measured value of the Hubble parameter (constant?) H0, would directly and proportionally reflect also in the measured speed of light c here on Earth. Since speed of light reliable and very accurate measurement records are kept for at least 70 years now then any slight deviation of the speed of light with time would show up in these records amounting also for any uncertainties.
Of course you could argue that time period these data are referring is too small compared to cosmological time to make any conclusions however because the very high accuracy in the speed of light measurement even a very small tendency in the data would possible show up. For example, an observed consistent chronological, speed up in the speed of light compared to past years, even so slightly over the last 70 years. Which would mean state of equation ω0>-1 say for example -0.94 or -0.93 meaning acceleration of expansion is slowing down (quintessence energy and the Big Crunch) and Hubble parameter H0 is getting smaller.
In the opposite scenario, any observed degradation in the chronological measured speed of light in the vacuum value would infer to a state of equation parameter ω0<-1 thus for example -1.3 or -1.4 and phantom energy and the Big Rip, and, that the Hubble parameter H0 is getting larger.
The benefit of the above described using the speed of light as a criteria would be that we then would be sure that the Hubble tension between the 2-3 dominant measuring methods has not an artificial root but instead the cause of the discrepancy observed is physical and caused by the slight continuous variation of the speed of light constant c with cosmological time that would affect mainly the Hubble parameter measuring methodology that relies more heavily on the z-red-shift parameter measurement and therefore should be currently the less accurate method and having the largest uncertainty.
Nevertheless, if the speed of light proves to be unchanged with cosmological time (notice that early universe period up to 1Byears after the BB, is too small compared to late universe then early universe effects can be neglected) then more or less ω0=-1 a constant, and ΛCMD model is correct (i.e. Big Freeze) therefore the reason of the Hubble tension discrepancy must be artificial inherent to the method and has not physical cause. In this case IMHO, the Reiss et al. method and measurement of about 74Km/s/Mpc is the most accurate and reliable.
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