Aug 082023

Sticking to the cat theme (in case anyone has any doubt, yes, I am quite fond of cats) here’s Midjourney’s take on some gentlecats and ladycats celebrating this momentous occasion.

 Posted by at 3:24 pm

  19 Responses to “International Cat Day”

  1. Not necessarily related to cats, but how do you feel about the modified gravity theory you have been working on? What are the significant challenges that remain?

  2. A curious question in light of the topic of the post to which you responded, but hey, why not?

    My opinion of MOG/STVG is that it is one of the few modified gravity theories (actual theories, not made-up formulas like MOND with no underlying theory) that stand a chance. MOG/STVG can run into trouble at the post-Newtonian order, when its predictions might be at odds with reality, but this remains to be explored. At the Newtonian order, the theory does a very good job making sense of galaxy rotation curves, cluster dynamics, even cosmological observations like structure formation and the CMB angular power spectrum.

  3. Interesting. Maybe it is inevitable that a good cosmological theory will require some part of its description to be “hidden from experience” or “at odds with reality”, at least for a given set of assumptions.

    Perhaps not unlike QM, where complementarity frees self-consistent models from having to provide a complete description of reality.

    We have seen that the expanding universe, in so far as it is homogeneous and finite in age, will appear to have an unobservable dark sector. Perhaps instead it has modified gravity, that might also have some aspects which are “hidden from experience” or “at odds with reality”.

    Do you think there is any reason to consider our universe as having some kind of static/expanding duality?

  4. And here is how I bring the conversation back to cats.

  5. On quantum physics, my own thinking is that no, it is not an incomplete description of reality. It is simply at odds with our naive expectations of reality being classical, inventing nonsense like “wavefunction collapse” to avoid dealing with the fundamental nonlocality of the quantum theory (talk about a cure far worse than the disease; “objective collapse” implies yanking out the entire universe and replacing it with a different one governed by a new wavefunction.)

    But physical cosmology is pretty much classical physics except for the extreme early epochs (first few hours, really). My feeling is that the solution is going to be more mundane than many expect. The big question is the “dark ages”: The first few hundred million years after the surface of last scattering before the first high redshift galaxies. Something happened that allowed a nearly homogeneous universe to sprout supermassive black holes and fairly large galaxies in what appears to be far too short a period of time for this to occur. Until/unless we learn what actually happened during the dark ages, we don’t stand a chance I think at resolving the big questions of cosmology. JWST observations are good hints but probably not enough. We need more data.

  6. Sounds reasonable to me. Though maybe there is also a fundamental nonlocality for the dark ages. I am very skeptical that the age of the distant universe would appear the same as the age of the universe local to us.

    That might not make much sense, but I imagine that if there was a time dilation such that our clocks appeared to run for example 5 times faster than in the early universe, then perhaps the age of the distant universe should appear 5 times greater. In which case 5 billion years of local structure could develop in what we might only presently be envisioning as 1 billion years of time.

  7. Time dilation won’t help us here because a) it’s already accounted for, and b) it’s not what we measure, it’s what those early, distant galaxies “measure” by forming within the allotted time.

    There are numerous proposals attempting to increase the age of the universe. The problem with most such proposals is that the increase comes in the very early universe, whereas what we actually need is a “loitering” universe, in which the extra time comes between the surface of last scattering and the earliest high-z galaxies.

  8. What I was trying to convey is that, if I were to somehow beam myself onto a planet of one of those high-z galaxies simultaneous to our present time, I would measure my local universe age as being only for example only 13.8 billion years old.

    Then maybe I look back to where I came from – and see a younger version of our previously local part of the universe, with unexpectedly way more structure than there should be. I might conclude that the apparent age of the universe is relative. Has that already been taken into account? I was under the impression it was considered wrong by cosmologists. Maybe an EFE would permit as much?

  9. That’s not really a fair question. The EFE is still subject to the underlying assumptions. If a paradigm shift is needed to reconcile the early structure in the universe, then possibly the earlier assumptions of GR + Cosmological Principle and the resulting FLRW metric may need to be adjusted. That is really my motivation for suggesting complementarity at cosmological horizons. The cosmological horizon should really constitute a vanishing of locality, so at least that is a start.

  10. Assuming that I comprehend your question, yes, it is properly taken care of in the context of general relativity and physical cosmology.

    The actual question is different. Suppose we had a clock that has been ticking since the surface of last scattering, measuring time and recording major events. How much time would this clock have measured from the surface of last scattering to the emergence of the first high-z galaxies? How many times would it have ticked?

    The standard theory predicts an insufficient number of ticks.

    It has nothing to do with what we see now or what inhabitants of that high-z galaxy see now. It’s the number of ticks that the “comoving clock” records. General relativity predicts one thing; the evidence (large galaxies in the early universe) suggests something else. And of course the answer may very well require a paradigm shift. Or not. I have no idea and I bet no one does, or will have, until and unless we get more hints from Nature, more observational data.

  11. Ok, and maybe that is what we get with only comoving clocks. Thanks for having the discussion with me.

    I do however disagree with the argument that comoving clocks are strictly needed to preserve isotropy, namely because an observer may experience an acceleration relative to the cosmological horizon (a sort of “everywhere inward” acceleration for example) that would not result in disruption of isotropy. I know that is not part of the standard model, so presumably that would result in unaccounted for time dilation.

  12. Well, I’ve given it some more thought. Let’s just assume for a minute that time dilation in the dark ages is actually in play due to an unaccounted-for isotropic acceleration between the observer and the distant universe. In this case clocks are not necessarily comoving.

    If I interpret what you said correctly, then you objected to time dilation being a solution because of the statement that there are an insufficient number of ticks of the clock to account for the observed structure formation.

    However, if clocks ticked slower in the distant universe, and in light of the above assumption, then I suggested that maybe the distant universe is older than the local universe.

    Does it then follow that if our observations are starting from the present and we are looking back in time, then an older distant universe implies there are many more ticks between the early universe structure that we are observing and the surface of last scattering?

  13. Actually, if clocks ticked slower in the early universe, that means LESS time elapsed as measured by those clocks… or the galaxies-in-formation that act as clocks. What we need is MORE time. In any case, postulating that clocks ticked differently during the dark ages does not tell us why: It’s circular reasoning.

    Anyhow, time dilation is about what distant observers see. What we need (literally) is for clocks to have counted many, many more ticks between the surface of last scattering and high-z galaxies compared to what the standard theory tells us. Nothing to do with time dilation.

  14. The time dilation I assume is not that of special relativity, but of general relativity. The result I seek is to make the universe older in order to provide the structure when we look back some 13+ billion years for example. Our 13+ billion years, while a very high fraction of the age our view of the universe, is just not very significant for a high-z galaxy that has a clock moving much slower. Hence the high-z galaxy is much older than we estimated.

  15. Time dilation is time dilation. It’s about what distant observers see. The only difference is that in general relativity, we take into account gravity and acceleration, not just inertial motion.

    Again, it is not about “our” 13+ billion years. It is that the theory predicts that “they” (i.e., the galaxies) experienced only a few hundred million years before they fully formed. NOTHING WHATSOEVER to do with “our” anything.

    And sure, one solution is that yes, there was a loitering universe. But simply stating that the universe was loitering will not explain why the universe was loitering. It’s circular reasoning.

  16. I do have a reason for it. That this theory supports a loitering universe is perhaps a consequence of it having a static nature, but one that I hope is complementary to an expanding nature.

    It occurred to me that Olbers’ Paradox of the dark night sky might have two complementary solutions – one in which the observable universe is finite in age (standard model of an expanding universe), and one in which the observable universe is finite in space (no local dark sector, just curved space with any required “dark” mass-energy located at an event horizon).

    So the gravity/acceleration that I invoke to enable the time dilation is apparent in the latter description. The horizon is located at a pole, so any direction that one looks out into space will lead back to this pole via the curvature.

  17. Is it possible the reason that the model and observation don’t line up is a difference in age and that the model measures time preceding from one pole or origin (ticks from last scattering), while we measure a different time from the observer’s pole (ticks from our present)?

  18. I don’t know how else to say this: it is NOT about a) what we measure or b) the present (or even the estimated age of the universe). It is that general relativity predicts an elapsed time of a few hundred million years from the surface of last scattering at z~1100 to z~15 and this time may not be enough to form large galaxies. A different theory may predict a different time interval.

  19. Understood. I think we are on the same page really. Everything I have talked about constitutes a different theory from GR+FLRW, so what “general relativity predicts” should give a different result as the new theory’s metric wouldn’t be FLRW.