Mastodawn

I need some help from the #Astronomy and #Cosmology folks.

I found this redshift calculator at https://spacetimemesh.com/redshift-calculator/, and either I’m confused (very possible), or they have one of the labels wrong.

It seems to me that “light travel distance/how far light actually travelled” suggests how much space the light from the distant object travelled through to get to us. But shouldn’t that also be the lookback time?

I suspect that what they’re labelling as “light travel distance” is the distance of the object at the time the light left it, 5.55 Gly. Then “lookback time,” 7.95 Gly, is how long the light has travelled to us as the universe expanded. Finally “comoving distance,” 11.09 Gly, is the current distance (assuming god-vision 😉) of the object.

Without getting too picky with my terminology, is my interpretation right? ELI5 explanations are welcome 😊

I know these numbers depend on the model and parameters used, I'm not interested in getting into that particular aspect.

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Alexander Knochel 1d ago

@MichaelPorter I suspect the light travel distance is just wrong? It goes to zero as z->0 but also goes to zero as z-> inf ...

I first thought it might be sth like the ("God's ruler") current distance travelled by a photon started at the big bang assuming standard cosmology without inflation, but that's not compatible with z=0.
It could be something like the distance travelled from z to the present, but measured with the scale factor back then at z?

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Alexander Knochel 1d ago

@MichaelPorter p.s. yes it looks like the number given by calculator as travel distance is the "comoving distance" value multiplied by the scale factor we had back in time corresponding to z

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Christoph Wendel 1d ago

@MichaelPorter You may find these two explanations helpful:

Hogg, David W.
Distance measures in cosmology
https://ui.adsabs.harvard.edu/abs/1999astro.ph..5116H/abstract

Davis, Tamara M. ; Lineweaver, Charles H.
Expanding Confusion: Common Misconceptions of Cosmological Horizons and the Superluminal Expansion of the Universe
https://ui.adsabs.harvard.edu/abs/2004PASA...21...97D/abstract

Distance measures in cosmology

Formulae for the line-of-sight and transverse comoving distances, proper motion distance, angular diameter distance, luminosity distance, k-correction, distance modulus, comoving volume, lookback time, age, and object intersection probability are all given, some with justifications. Some attempt is made to rationalize disparate terminologies, or at least abuse bad usage.

ADS