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1. May 2, 2016

Einstein's Cat

Galaxies that are greater than a distance of c/H metres from Earth have recessional velocities exceeding the speed of light and begin to fade. Thus, theoretical astronomers 3 trillion years in the future will see only the Milky Way in the night sky. What is the reason for this fading of galaxies?

2. May 2, 2016

Simon Bridge

You just said the reason: they retreat faster than the speed of light.

3. May 2, 2016

Einstein's Cat

However why would that fact mean a photon emitted from that galaxy would never reach Earth?

4. May 2, 2016

PeroK

As the photon moves towards Earth at $c$, the distance between Earth and the photon is increasing at greater than $c$, so (as long as space keeps expanding at its current rate or faster) the photon will simply get further and further away.

If the expansion of space was suddenly or gradually to stop, then that would be a different matter!

5. May 2, 2016

andrew s 1905

Two points. As the universe expands all galaxies that move away from us tend to fade just because of the inverse square rule for intensity (all else being constant). As for the expansion itself have a look at this http://arxiv.org/abs/astro-ph/0310808 and you will see we can still and do observe galaxies who's recession velocity is greater than c.

Regards Andrew

6. May 2, 2016

Simon Bridge

A more accessible description in discussion here:
http://physics.stackexchange.com/qu...ceding-faster-than-light-visible-to-observers

... so amend the above observation: the galaxies are receeding from faster than the speed of light and not all the photons are able to reach a slower expansion part of the Universe. But you get the idea that farther away a galaxy the harder it is for it's light to reach us... at some point it won't be able to. The recession speed being equal to the speed of light is just not the cutoff point.
https://en.wikipedia.org/wiki/Event_horizon#Cosmic_event_horizon

7. May 2, 2016

andrew s 1905

I would emphasis that it is a cut-off, as Simon points out, rather than a fading. Give or take interstellar and inter galactic absorption all photons from a galaxy (treated as a point source) emitted towards us at the same time either all make it or they all don't.

Regards Andrew

8. May 2, 2016

George Jones

Staff Emeritus
There is a fading (and not just at and beyond the Hubble radius). The expansion of the universe has, in two ways, diminished the energy flux that we receive beyond just a distance effect. The energy of light is inversely proportional to its wavelength (energy of a photon is $E=hc/\lambda)$. As the light travels to us, the expansion of the universe expands the wavelength of the light by a factor of 1+z, where z is redshift. Also, the expansion of the universe decreases the rate at which we receive photons, as compared to the rate at which photons left a source, by another factor of 1+z (gravitational time dilation). Consequently, as redshift goes continuously towards infinity (cosmological horizon, not Hubble radius), intensity continuously decreases towards zero.

9. May 2, 2016

andrew s 1905

Can you point me at an explanation of this effect as I would like to understand it. I have come across gravitational time dilation in association with mass but not the expansion of the universe before.

Also do you agree it fades towards a cut-off or are you saying a cut-off does not exist ?The paper I referenced implies there is one.

Thanks Andrew

George, I have managed to track down some papers on cosmological time dilation that fits your formula and some Sn Ia results that seem to confirm it. (e.g. http://www.ppd.stfc.ac.uk/ppd/resources/pdf/ppd_seminar_100609_talk_1.pdf [Broken] and my original link!!) I assume this is what you intended. Thanks

Last edited by a moderator: May 7, 2017
10. May 2, 2016

Einstein's Cat

What exactly is the cosmological horizon? And also could one just apply the typical equation for the Doppler effect to a ray of light emitted from a retreating galaxy to calculate the redshift as well as the calculation you suggested?

11. May 2, 2016

Simon Bridge

Not good enough? You can also google "cosmological horizon" and get a range of articles explaining it at a variety of different levels ... you can, then, pick the one most suited to your understanding. Basically, it is the radius at which the galaxies disappear due to cosmological expansion.
The same articles will likely explain why we don't just interpret the redshift observed as a Doppler effect ... one of the side effects of doing this, for instance, would be that distant galaxies do not retreat faster than light: creating some um geometry problems.

12. May 3, 2016

quarkstar

galaxy fading sounds like the discrepancy between intensity-based distance and redshift-based distance in dark energy (as an explaination). is there anybody with hard data on intentsities and redshifts or are the galaxy's general output too fuzzy for redshift lines? There must be a general output from a galaxy that has useable data lines, or Hubble et al (Silpher) couldn't use it for the original work on the expanding universe.

13. May 3, 2016

quarkstar

the question being does the fading data coincide with the intensity problems in dimness of Sn1a information?

14. May 3, 2016

quarkstar

even further would there be an inflection point that coincides with the Sn1a data inflection point in the divergence of intensity vs redshift distance conundrum? (aka dark energy) does it agree with estimates- perlmutter reiss - z~.7 ? or is the fading of galaxies being noticed observationally but without a good analytic technique to categorize it's qualities?

15. May 3, 2016

andrew s 1905

There is real data discussed in both the links I made above. Regards Andrew

16. May 3, 2016

George Jones

Staff Emeritus
It is an effect due to redshift, independent of the cause of the redshift, i.e., it is present for cosmological redshift, for redshift caused by a massive object, and even for redshift caused by relative motion between source and receiver in special relativity.

Imagine that observers A and B have identical watches. A sends a light signal to B that B sees redshifted, so that each photon B receives has lower energy (by the redshift factor) than each photon that A sends out.This also means that there is an observed frequency shift for all frequencies, including the rotational frequencies of the second hands of the watches for A and B. B uses one eye to watch the A's second hand and one eye to watch his own second hand. B observes the rotational period of A's second hand to be larger (again, by the redshift factor) than the period (1 minute) of his own second hand. Suppose that A's experimental set up sends out one photon for per revolution of his second hand, i.e., at the rate of 1 photon per minute according to A. B sees the A's rotational period to greater than 1 minute, so B receives photons at a rate of less (by a redshift factor) than one a minute. Putting stuff together, the energy flux received by B is reduced by two factors of redshift compared to the energy flux sent out by A.

Yes, this illustrated well by panel 3 in figure 1 of the paper. I hope to get back to this, and to some other points.

17. May 4, 2016

bapowell

This is incorrect. Photons emitted from galaxies with superluminal recession velocities will indeed reach earth. See the section "Superluminal recession and the Hubble sphere" here https://www.physicsforums.com/insights/inflationary-misconceptions-basics-cosmological-horizons/. The OP might find the full article of interest.

18. May 4, 2016

Einstein's Cat

19. May 4, 2016

bapowell

See George Jones' response #16 above regarding redshift.

20. May 6, 2016

PeroK

Having read your excellent Insight, I can now see that I was ... correct!