There are some good online tutorials and calculators to help one get a
feeling for the numbers.
https://www.astro.ucla.edu/~wright/cosmo_01.htm
https://www.cosmologycalculator.com/cosmocalc.htm
https://cosmologycalculator.com/D1_preface_documentation.pdf
Brent
On 12/5/2025 5:23 AM, 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 galaxyneeded to travel through spaceto 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, andis 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 viewpointthe 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 isnow 11 billionlight years away from us
not 7.8billion, and when that light was first emitted the galaxy and
theEarthwere 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 tocalculatedistance 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 stretch
2) 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 shiftthat 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>*
9d6
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