On Friday, December 5, 2025 at 6:24:26 AM UTC-7 John Clark wrote:
On Thu, Dec 4, 2025 at 10:09 AM Alan Grayson <[email protected]> wrote: >>> *Distances to galaxies is measured using standard candles. So the attenuation in brightness compared to intrinsic brightness is a true measure of distance even though the universe is expanding. So there doesn't seem to be any problem with Hubble's values for distances.* *>> Distance between what, and when? If Hubble gives a figure of 10 billion light years that is the distance the light from a distant galaxy needed to travel through space to reach us, it tells us what the galaxy look like 10 billion years ago, but because space had been expanding while light had been making its journey it is NOT the distance the Earth is from that galaxy now, and is NOT the distance between the two when the light was first emitted.* *> But when we use standard candles to measure distance from Earth to some galaxy, don't we get the ACTUAL distance NOW, since light attenuates in intensity due to expansion, just as its wavelength increases (and its energy decreases)? AG * *Cosmology is complicated, and you have to be clear about what sort of distance you're talking about. For example if we look at a galaxy with a red shift of 1 we know that that from our viewpoint the light has been traveling for 7.8 billion years (although from the viewpoint of a photon the travel time was zero) but during that time space has been expanding so if you could somehow pause the expansion of the universe you'd find that the galaxy is now 11 billion light years away from us not 7.8 billion, and when that light was first emitted the galaxy and the Earth were only 5.5 billion light years apart, if the Earth had existed back then. * *And just to make things more complicated, if you used the standard candle method to calculate distance that works fine in a lab you would conclude that the galaxy was 22 billion light years away; but when you're talking about cosmological distances there are two other things you need to take into account:* *1) Photons lose energy as their wavelengths stretch2) Because of time dilation photons arrive less frequently* *These two effects combine to reduce brightness by a factor of (1+z)^2, where z is the redshift.* *I think it's interesting that a** galaxy with a redshift of 1.8 is about 16 billion light-years away, but you could never reach a galaxy with a redshift that large or larger. With our largest telescopes we can see galaxies with a red shift of 11, but we could never reach a galaxy with a red shift that big even if you could move at the speed of light because the the space between us is expanding faster than the speed of light, we can still see it because we are looking at old light, but we could never know what the galaxy looks like now. * *John K Clark See what's on my new list at Extropolis <https://groups.google.com/g/extropolis>* *AG: Here's what Gemini says:* *AG: When we use standard candles to measure distances to galaxies, does the result include the expansion of space while the lighting is traveling toward the Earth?* *GM; That's an excellent and crucial question in cosmology!* *The distance calculated directly from a standard candle's apparent brightness (using the inverse square law of light <https://phys.libretexts.org/Courses/University_of_California_Davis/Physics_156%3A_A_Cosmology_Workbook/01%3A_Workbook/1.06%3A_Distances_as_Determined_by_Standard_Candles>) does not directly include the effects of space expansion during the light's travel time.* -- 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/16a97be1-cb55-4186-b3a9-1e5a523fc283n%40googlegroups.com.
