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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