On Sunday, December 14, 2025 at 10:46:36 PM UTC-7 Brent Meeker wrote:
You should check with Ned Wright's Tutorial page: https://www.astro.ucla.edu/~wright/cosmo_02.htm which shows how the various distance measures differ depending on the cosmological model. *I'm looking at it. How many such models are there? Gemini seems to imply the Lamda CDM model is the one to use to measure cosmic distances. AG * He also links to a Javascript calculator https://www.astro.ucla.edu/~wright/CosmoCalc.html Brent On 12/14/2025 8:40 PM, Alan Grayson wrote: On Sunday, December 14, 2025 at 9:20:55 PM UTC-7 Alan Grayson wrote: I just checked with AI again. Gemini says the following: "The Luminosity Distance ($D_L$) *is a measure that accounts for the expansion*, but precisely *because* it includes the energy loss and time dilation effects of the expansion, it gives a distance value that is *larger* than the current, real physical distance (Proper Distance, $D_P$) [1.2]. The standard candle provides $D_L$, but you need to know the redshift ($z$) and use the cosmological model to truly calculate the Proper Distance ($D_P$ ). Gemini also posted as follows: The relationship between the Proper Distance ($D_P$), Luminosity Distance ( $D_L$), and the redshift ($z$) is defined by the *Etherington's distance-duality equation*: $$D_L = D_P (1 + z)$$ - For nearby objects, $z \approx 0$, so $D_L \approx D_P$. - For very distant objects with significant redshift (e.g., $z=1$), $D_L = 2 \times D_P$. The Luminosity Distance is *twice* the "actual distance now" because of the compounding dimming effects. I might have reached the wrong conclusion because of what another AI claimed, namely Co-pilot. I will check out its comments to see if they're consistent with Gemini. AG On Sunday, December 14, 2025 at 4:44:28 PM UTC-7 Alan Grayson wrote: I plan to check it out again, but I'm pretty sure AI claimed the Hubble's law gives us the velocity and distance of galaxies in the present, NOW. WRT distance, it claims that using standard candles, we get the actual distance NOW, not the distance when the light left some galaxy 10 billion years ago. Is there concurrence among the humans here? TY, AG -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion visit https://groups.google.com/d/msgid/everything-list/167ff617-05b2-4598-8a8e-939e0cf590efn%40googlegroups.com.
