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Recently there was a picture from the JWST that showed a galaxy that was 12.5 billion light years away. Its light had been lensed by a galaxy that was 5 billion light years away. In a static universe this would cause no problem. However in an expanding universe I fail to see how this would be possible.

My problem is that we have three galaxies A, B and C. Galaxy A is 12.5 billion light years away from galaxy C and galaxy B is 5 billion light years from galaxy C. When the light from galaxy A was passing galaxy B it was 7.5 billion light years from galaxy A. However 5 billion years ago galaxy C was also 7.5 billion light years from galaxy A.

James K
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Michael Mcgarry
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    Why do you think it is impossible? – Jon Custer Nov 12 '22 at 16:29
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    I think you should rephrase your question. Clearly it doesn't contradict the idea of an expanding universe, so you would want to understand why. But to answer that, we need to know why you think it does. – ProfRob Nov 12 '22 at 18:17
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    I think this will be seen as a homework-like question which doesn't mean it's real homework necessarily. To make this a good Stack Exchange question and keep it from being closed, please add to it a description of what you have tried so far, and how much you currently understand about the problem and how to solve it. Folks generally won't simply start posting answers to textbook problems on-demand, there needs to be some context and some effort shown first. Thanks, and Welcome to Stack Exchange! – uhoh Nov 13 '22 at 07:40
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    There are several ways to describe cosmological distances. The numbers you quoted are lookback distances. I assume you're talking about the famous first official image from Webb. There's a version with spectra of a few of the lensed galaxies here. The lensing object is the massive galaxy cluster SMACS 0723, at a lookback distance of ~4.6 billion lightyears. – PM 2Ring Nov 13 '22 at 16:40

2 Answers2

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Does gravitational lensing disprove the cosmic expansion theory? In a static universe this would cause no problem.

No, it's the other way around. The light from those very remote galaxies that we can see thanks to gravitational lensing is consistent with an expanding universe but inconsistent with a steady-state universe. The light from those remote galaxies is redshifted to a greater extent than is the light from the galaxy that created the gravitational lens. Redshifting due to cosmic expansion is inconsistent with static universe theories but is an essential consequence of any variant of big bang theory. That we see redshifting of remote galaxies is one of the many signs that the universe is expanding rather than static.

David Hammen
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    The theory that light is redshifted by the expansion of the universe is just that, a theory, and it is the only sign that the universe is expanding. – Michael Mcgarry Nov 13 '22 at 15:58
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    Do you know what a scientific theory is, @MichaelMcgarry? It is a self-consistent hypothesis that is backed by fact. The observation of cosmic microwave background radiation in the 1960s was the nail in the coffin for the steady state universe hypothesis. – David Hammen Nov 13 '22 at 16:37
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    If light were redshifted by any other means than expansion the end result would still be microwave background radiation. – Michael Mcgarry Nov 13 '22 at 17:06
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    @Michael What alternative redshift mechanism do you propose that results in equal redshifts at equal distances, once Doppler effects due to velocity are taken into account, as I mention here? – PM 2Ring Nov 13 '22 at 17:35
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    Your answer does not address the question of the OP (which appears to be related to the travel time of the light signals only, considering the quoted distances) – Thomas Nov 13 '22 at 18:00
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    "The observation of cosmic microwave background radiation in the 1960s was the nail in the coffin for the steady state universe hypothesis." The recent LOFAR observation that the cosmological redshift in the low frequency radio region is significantly less than in the optical region (by an amount corresponding to a velocity of about 10,000 km/sec for a redshift of about z=1.2) could actually be the nail in the coffin for the Big-Bang theory (so far only contrived attempts at an explanations for this exist) https://arxiv.org/abs/1811.08104 – Thomas Nov 13 '22 at 18:11
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    @PM2Ring I have myself suggested a redshift mechanism due to the electric microfield in the intergalactic plasma https://www.plasmaphysics.org.uk/research/redshift.htm This mechanism would not work anymore though once the wavelength becomes larger than the average distance of charges in the plasma (which should be in the region of 1m if estimates of the intergalactic plasma density are correct), and evidence for this was actually found recently in LOFAR observations, where the redshift of radio recombination lines was found to be significantly less https://arxiv.org/abs/1811.08104 – Thomas Nov 13 '22 at 18:21
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12.5 billion years ago the light left galaxy A.

At this time the light left galaxy A, galaxy A was about 2.5 billion light years from Galaxy C (= the milky way) and about 1 billion light years from galaxy B (the lensing galaxy).

However it took longer than a billion years for the light to reach the vicinity of galaxy B, since space expanded significantly while the light was travelling. I think it took about six billion years for the light to reach Galaxy B.

So about 6 billion years ago, the light passed Galaxy B and was bent by the curvature of of space around the galaxy. A the time Galaxy B was about 2 billion light years from us.

The light then continued to travel through expanding spacetime, taking another 6.5 billion years to travel the distance. Now we see the lensed Galaxy A at a distance of about 28 billion light years and galaxy B at about 8 billion. Again the distance in light years is not the same as the light travel time, because of the expansion of space.

There is no reason to suppose that Galaxy A and B were in the same location 12.5 billion years ago. Galaxy A was about 2.5 billion light years away and Galaxy B was a billion light years closer.

PM 2Ring
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James K
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    (I've tried to get the numbers right here, I may have failed, but the qualitative description is I hope still correct) – James K Nov 13 '22 at 16:40
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    You say that distance in light years is not the same as light travel time but you also say that we are 13.8 billion light years from the big bang and it has taken us 13.8 billion years to get here. Which means the average speed at which we have covered that distance is exactly the speed of light. – Michael Mcgarry Nov 13 '22 at 17:11
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    No, we are more like 46 billion light years from objects that we see 13.8 billion years ago. And the big bang happened everywhere. So we are both 46 billion light years from the big bang and 0 light years from it, and every other number, because it didn't happen at a place but at all places 13.8 billion years ago. – James K Nov 13 '22 at 17:21
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    @Michael Did the Big Bang happen at a point? Also https://astronomy.stackexchange.com/q/874/16685 – PM 2Ring Nov 13 '22 at 17:31
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    No the big bang did not happen at a point it didn't happen at all. Redshift denotes distance not movement and everything in the cosmos looks exactly as it should. – Michael Mcgarry Nov 13 '22 at 19:04
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    It is apparent that your primary purpose for this question is to be to promote a specific fringe position. It does not appear to be a good-faith effort to learn. – James K Nov 13 '22 at 20:00
  • @MichaelMcgarry The Big-Bang model has numerous free parameters that make it possible to match almost any observation (e.g. 'inflation'). The exact figures don't really matter. – Thomas Nov 14 '22 at 08:23
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    @Thomas What you last wrote about pertains to the tiny portion of the Big Bang at the very start. That part remains highly debated. From the last scattering (the source of the CMBR) and on, it is extremely solid, and there are a very small number of free parameters, most of which are nicely tied down by what we see in the CMBR and in standard candles. There are some small problems with the Hubble constant, but those problems are small. – David Hammen Nov 14 '22 at 16:15
  • @DavidHammen Check out https://astro.ucla.edu/~wright/ACC.html . I count 10 parameters that you can change here. By changing only $\Omega_M$ between 0.1 and 10, I can change the light travel time (for z=3) between 12.9 Gyr and 4.7 Gyr. Now you could consider these values for $\Omega_M$ as unlikely for some reason, but this is not the point. The point is that the equations allow these values. So the BB model can be consistent with pretty much any observation (somewhat similar to the ancient epicycle theory for the geocentric universe, which in principle also could fit any observations). – Thomas Nov 14 '22 at 21:18