On Monday, September 16, 2024 at 12:17:45 PM UTC-6 Jesse Mazer wrote:
The Scientific American article "Misconceptions About The Big Bang" by Charles Lineweaver and Tamara Davis at https://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf (distilled from their more technical review 'Expanding Confusion' at https://arxiv.org/abs/astro-ph/0310808 ) covers this question on p. 42-43, along with other common misconceptions: "Running to Stay Still the idea of seeing faster-than-light galaxies may sound mystical, but it is made possible by changes in the expansion rate. Imagine a light beam that is farther than the Hubble distance of 14 billion light-years and trying to travel in our direction. It is moving toward us at the speed of light with respect to its local space, but its local space is receding from us faster than the speed of light. Although the light beam is traveling toward us at the maximum speed possible, it cannot keep up with the stretching of space. It is a bit like a child trying to run the wrong way on a moving sidewalk. Photons at the Hubble distance are like the Red Queen and Alice, running as fast as they can just to stay in the same place. One might conclude that the light beyond the Hubble distance would never reach us and that its source would be forever undetectable. But the Hubble distance is not fixed, because the Hubble constant, on which it depends, changes with time. In particular, the constant is proportional to the rate of increase in the distance between two galaxies, divided by that distance. (Any two galaxies can be used for this calculation.) In models of the universe that fit the observational data, the denominator increases faster than the numerator, so the Hubble constant decreases. In this way, the Hubble distance gets larger. As it does, light that was initially just outside the Hubble distance and receding from us can come within the Hubble distance. The photons then find themselves in a region of space that is receding slower than the speed of light. Thereafter they can approach us. The galaxy they came from, though, may continue to recede superluminally. Thus, we can observe light from galaxies that have always been and will always be receding faster than the speed of light. Another way to put it is that the Hubble distance is not fixed and does not mark the edge of the observable universe. *I don't think this is the consensus view, which is that the Hubble constant IS constant, and galaxies beyond our event horizon will never be seen, if the universe in their region is expanding faster than c. AG * What does mark the edge of observable space? Here again there has been confusion. If space were not expanding, the most distant object we could see would now be about 14 billion light-years away from us, the distance light could have traveled in the 14 billion years since the big bang. But because the universe is expanding, the space traversed by a photon expands behind it during the voyage. Consequently, the current distance to the most distant object we can see is about three times farther, or 46 billion light-years." *But within the observable universe, space is expanding at a rate less than c. Correct? So the 46 BLY distance doesn't seem right. AG* On Mon, Sep 16, 2024 at 12:53 AM Alan Grayson <[email protected]> wrote: It's claimed to be 46 billion LY, but its age is only measured as 13.8 billion years. What I find puzzling about these numbers is that it seems this would imply the rate of expansion must have been greater than c during its lifetime. But AFAICT, the measured rate of expansion using Hubble's law never exceeded light speed before it reached its present size. Can anyone explain this apparent discrepancy? TY, AG -- 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 on the web visit https://groups.google.com/d/msgid/everything-list/eb57d007-3a2d-45ff-9e99-78039ee01022n%40googlegroups.com <https://groups.google.com/d/msgid/everything-list/eb57d007-3a2d-45ff-9e99-78039ee01022n%40googlegroups.com?utm_medium=email&utm_source=footer> . -- 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 on the web visit https://groups.google.com/d/msgid/everything-list/e0e2ee64-c917-4fd3-9d30-0183d9635da1n%40googlegroups.com.
