Also inflation happened *before* the hot big bang. Le sam. 21 sept. 2024, 11:00, Quentin Anciaux <[email protected]> a écrit :
> Inflation lasted 10^-32 seconds... inflation is not the cause of > recessional velocity > c, it's space expansion, not inflation, as long as > it is *uniform* (the point you seem unable to grasp), object will sooner or > later recess from each other > c. > > Le sam. 21 sept. 2024, 10:53, Alan Grayson <[email protected]> a > écrit : > >> >> >> On Friday, September 20, 2024 at 10:17:34 PM UTC-6 Alan Grayson wrote: >> >> On Thursday, September 19, 2024 at 3:14:33 AM UTC-6 Jesse Mazer wrote: >> >> On Thu, Sep 19, 2024 at 2:57 AM Alan Grayson <[email protected]> wrote: >> >> >> >> On Wednesday, September 18, 2024 at 7:10:57 PM UTC-6 Alan Grayson wrote: >> >> On Wednesday, September 18, 2024 at 5:30:06 PM UTC-6 Jesse Mazer wrote: >> >> On Wed, Sep 18, 2024 at 2:01 AM Alan Grayson <[email protected]> wrote: >> >> >> >> On Tuesday, September 17, 2024 at 4:20:31 PM UTC-6 Jesse Mazer wrote: >> >> On Tue, Sep 17, 2024 at 2:40 PM Alan Grayson <[email protected]> wrote: >> >> >> >> On Tuesday, September 17, 2024 at 10:12:53 AM UTC-6 Jesse Mazer wrote: >> >> On Mon, Sep 16, 2024 at 7:41 PM Alan Grayson <[email protected]> wrote: >> >> >> >> 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 * >> >> >> Davis and Lineweaver are just reviewing the current consensus view in >> that article and paper, not suggesting any new physics. In general >> relativity's cosmological solutions there is a time-dependent "Hubble >> parameter" whose value at any given cosmological time is called the "Hubble >> constant" at that time, but which can change over the long term (see the >> first paragraph of >> https://lambda.gsfc.nasa.gov/education/graphic_history/hubb_const.html >> for example). Astrophysicist Ethan Siegel mentions in an article at >> https://bigthink.com/starts-with-a-bang/hubble-constant-changes-time/ >> that even in models that don't have accelerating expansion due to the >> cosmological constant, the Hubble constant still need not be constant in >> time. He explains this by looking at the first Friedmann equation governing >> an expanding universe, where a term equivalent to the definition of the >> Hubble constant is on the left side of the equality and the right side has >> terms for energy density, global curvature of space, and the cosmological >> constant. So, in an expanding universe that's spatially flat and has zero >> cosmological constant, if the energy density is changing as matter/energy >> becomes more spread out, the term equivalent to the Hubble constant must be >> changing as well. From the article: >> >> "Even if you had a flat Universe (which means you can eliminate the >> second term on the right-hand side) and a Universe without a cosmological >> constant (which would mean eliminating the third term on the right-hand >> side, too), you’d understand immediately that the Hubble “constant” cannot >> be a constant in time. >> ... >> In all cases except for a cosmological constant (i.e., dark energy, to >> the best of our understanding), the energy density changes as the Universe >> expands. >> If the energy density changes, that means the expansion rate changes, >> too. The Hubble constant is only a constant everywhere in space, as we >> measure it right now. It’s not a constant in the sense that it changes over >> time." >> >> Siegel has another article covering a lot of the same issues at >> https://www.forbes.com/sites/startswithabang/2018/06/29/surprise-the-hubble-constant-changes-over-time/ >> where he also mentions that it got the name "Hubble constant" because "for >> generations, the only distances we could measure were close enough that H >> appeared to be constant, and we've never updated this". >> >> >> >> >> 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* >> >> >> Galaxies within the observable universe can be receding faster than c, as >> mentioned in that Davis/Lineweaver quote earlier, and in their review paper >> at https://arxiv.org/pdf/astro-ph/0310808 in section 3.3. If this seems >> like an intuitive contradiction it may help to be more precise about how >> cosmologists define the term "observable universe": the radius of the >> observable universe is defined in terms of the *current* proper distance >> (see >> https://en.wikipedia.org/wiki/Comoving_and_proper_distances#Uses_of_the_proper_distance >> on the meaning of 'proper distance' in cosmology) of the most distant >> objects (at rest relative to the cosmic microwave background radiation) >> such that if they emitted light towards us at some point in the *past*, the >> light would have been able to reach us by now. This doesn't necessarily >> mean that if a galaxy in the observable universe emits light *today* that >> the light will ever be able to reach us. >> >> One way of visualizing this definition more easily is using the "comoving >> distance", which is equal to the proper distance at the current time but >> which is adjusted so that the comoving distance of all objects at rest >> relative to the CMBR is fixed, i.e. if a galaxy has a proper distance of 9 >> billion light years today then it had a comoving distance of 9 billion >> light years in the distant past, say a billion years after the Big Bang, >> even though its proper distance at that time was much smaller (the 'scale >> factor' in cosmological equations gives the proportionality between the >> proper distance to the comoving distance). If you have a graph of various >> galaxies plotted in terms of the comoving distance, then the size of the >> observable universe is just the maximum size of our past light cone on this >> graph--see the last two of the three graphs Fig. 1 on p. 3 of that >> Davis/Lineweaver paper at https://arxiv.org/pdf/astro-ph/0310808 where >> the lines labeled "light cone" show our current past light cone which >> defines the size of the observable universe (the third graph is visually >> simplest because they use a "conformal" time coordinate which has a varying >> relation to ordinary proper time, in such a way that all light ray >> worldlines are 45 degree angles just like in special relativity graphs--on >> that third graph the left axis shows the conformal time, the right axis >> shows the proper time). The two graphs with comoving distance also show >> that the maximum size of our past light cone is identical to the *current* >> size of our "particle horizon", which is just the future light cone of our >> location at a point arbitrarily near the Big Bang. So the observable >> universe can also be defined in terms of the particle horizon (i.e. the >> current distance to the furthest galaxy that could receive a light signal >> from our location emitted at some point in the past). >> >> And like I said above, one consequence of these definitions is that just >> because a galaxy is currently within the observable universe, that does not >> rule out the possibility that light emitted from the galaxy *today* will >> never be able to reach us. This is shown by the third conformal graph in >> Fig. 1, where the definition of conformal time is such that an infinite >> future proper time is only a finite interval of the conformal time, so the >> top of the graph shows the maximum distance any given light ray will reach >> at a proper time of infinity. This means we will never see any events >> outside our past light cone at infinity, which is labeled our "event >> horizon" on the graph. If you think of the vertical dotted lines on the >> graph as worldlines of particular galaxies, you can see there that some of >> them were at one point within our past "light cone" which has an apex at >> the current time, but their current location in spacetime (where their >> worldlines intersect with the horizontal 'now' line) is outside the "event >> horizon", our past light cone whose apex is at infinite future proper time. >> So, we will never receive light from those galaxies as they are today, but >> since we can receive light from them that they emitted in the distant past, >> their current location is considered part of the "observable universe". >> >> Jesse >> >> I don't get it, but I'll keep trying. The claim seems to be that a star >> can be receding from an observer at velocity greater than c, and still be >> in his observable universe, and this is intelligible by changing the >> definition of observable universe and Hubble's constant. Is this the claim? >> TY, AG >> >> >> One could say the definition of Hubble's constant changed, since they >> initially did think it was constant but then theoretical modeling in >> general relativity and more distant observations favored the idea of a >> parameter that could change with time. But I don't think the definition of >> "observable universe" has changed, I think it always referred to any region >> of the universe that we can see today, even if we're seeing light that was >> emitted in the distant past when the proper distance was smaller. Do you >> just mean it doesn't match the intuitive meaning you would attach to the >> term? And if so, do you have an alternate preferred definition, like those >> regions where if a light beam was emitted today we'd be able to see it >> eventually, even if not for billions of years in the future? >> >> Jesse >> >> >> I'm satisfied leaving the definition of Observable Universe fixed, but I >> can't see how anything can recede at velocity > c and remain within our >> Observable Universe. And the measured radius of 46 BLY seems too large if >> the velocity of recession is < c. I will look at your links. AG >> >> >> But according to that definition, if some object at rest relative in >> comoving coordinates (i.e. its motion away from us is purely due to >> expansion of space, so it's at rest in the local CMBR frame), then if it >> was ever observable at any point in the past, it will be considered part of >> the "observable universe" forever, even if there is some time after which >> we can no longer observe any more light from it. Again, "observable >> universe" just means regions that can be observed by us at *some* time in >> their history. >> >> Jesse >> >> >> I think observable universe means what we can observe *now*, which >> according to theory will *decrease* in the future. But your definition >> suggests any galaxy that might have been observed in the past, will >> continue to be part of the observable universe even if it goes out of view. >> I don't think this is correct. AG >> >> >> While it's true that some galaxies we can now view, have already passed >> beyond our horizon, these will wink out, and the remainder will remain >> within our event horizon until they also eventually wink out, as long as >> the universe expands. AG >> >> >> Do you mean our "event horizon" in the sense I talked about earlier of >> our past light cone at a time of +infinity, as opposed to our past light >> cone today? Either way, if part of a galaxy's worldline is within our past >> light cone at a given time, in relativistic terms we could still be getting >> some kind of causal signal from it, even if in practice the light (or other >> causal signals moving at the speed of light like gravitational waves) from >> sufficiently distant galaxies may be too redshifted to detect with current >> instruments. Redshift approaches infinity as you approach the Big Bang in >> terms of when a given signal was emitted, but in the distant future even >> signals emitted long after the Big Bang will have very large but finite >> redshifts, so you'd need to be able to detect very long radio waves to >> "see" them, and if you can't the galaxy has effectively winked out of view. >> >> Jesse >> >> >> For me, the Observable universe means just that; the universe we can >> observe. How that fits into the constraints you define above, I am not >> sure. But I can say that some galaxies we can observe today have already >> crossed our horizon, and we are observing their last emissions just before >> crossing our horizon. But eventually they will wink out if the universe >> keeps expanding, as will all other galaxies not in our local group. I have >> no idea why you claim the red shift approaches infinity as we approach the >> BB, and I don't believe it. And I still don't know why the observed >> universe has such a large radius, of 46 BLY, which seems to imply the >> expansion rate must have exceeded light speed during the lifetime of the >> universe, allegedly 13.8 BY. AG >> >> >> Further, since the expansion of observable universe has slowed due to >> gravity since Inflation (ignoring the increase in the rate of expansion >> discovered in 1998), and was never receding faster than c, ISTM the radius >> of the observable universe has an upper bound of twice the age of the >> universe, or about 2x13.8 light years. But obviously this upper bound is >> way too low compared to the claim that it is 46 BLY. I have no idea how to >> resolve this discrepancy other than to conjecture that the universe must be >> much older than 13.8 BLY. Is this what observations of the James Webb Space >> Telescope suggests, with observations of fully formed galaxies in the very >> early universe? 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