On Wednesday, September 11, 2024 at 2:17:10 AM UTC-6 Quentin Anciaux wrote:
Le mer. 11 sept. 2024, 10:08, Alan Grayson <[email protected]> a écrit : On Tuesday, September 10, 2024 at 3:50:08 PM UTC-6 Quentin Anciaux wrote: Le mar. 10 sept. 2024, 23:19, Alan Grayson <[email protected]> a écrit : On Tuesday, September 10, 2024 at 2:19:42 PM UTC-6 John Clark wrote: On Tue, Sep 10, 2024 at 3:57 PM Alan Grayson <[email protected]> wrote: *>> Even if you ignore Dark Energy and postulate that the Hubble constant really is constant, every object a megaparsec away (3.26 million light-years) is moving away from us at about 70 kilometers per second. So if you try to look at objects a sufficiently large number of megaparsec away you will fail to find any because they are moving away from us faster than the speed of light.* >* That was in the past. At present, the universe is expanding at about 70 km/sec.* *Galaxies are receding from the Earth at 70 km/sec for EACH megaparsec distant from Earth they are. The further from Earth they are, the faster they are moving away from us, so if they are far enough away they will be moving faster than the speed of light away from us. * *> You're assuming the universe today is infinite,* *NO! I said IF the entire universe is infinite today then it was always infinite, and IF it was finite 10^-35 seconds after the Big Bang then it's still finite today. I also said nobody knows if the entire universe is infinite or finite. * *>* *Hubble's law applies to the past, not to the future,* *What the hell?! * *How about an intelligent reply? Obviously, if the universe is infinite today, it was always infinite. But that's what I am questioning. For galaxies to fall out of view, they have to moving at greater than c. Now they aren't receding that fast. How will they start moving that fast? You're applying Hubble's law without thinking what it says. Just because a galaxy is now receding at less than c, how will continued expansion increase that speed to greater than c? AG * The farther they are the faster they are receding from you, so as they continue to get farther away they receed faster from you till the point they receed faster than c and go out of your horizon. Quentin Prove it, if you can. I see the separation distance increasing linearly as the radius of the sphere expands, so light can reach either galaxy, from either galaxy. AG To address your point about the linear increase in distance, here's how distant galaxies can still recede faster than the speed of light, even with constant expansion: 1. Hubble’s Law: Hubble’s Law shows that the recession velocity (v) of a galaxy depends on its distance (d) from us: v = H0 * d Where H0 is the expansion rate. This means that as the distance increases, the recession velocity increases proportionally. *But that's because Hubble is looking backward in time, when recessional velocity was much greater than today, but slowing down after Inflation. AG* 2. Linear increase in distance: You're right that, with a constant expansion rate, the distance between two galaxies increases linearly with time. However, because recession velocity depends on distance, the farther apart two galaxies are, the faster they recede from each other. So, even if the distance grows linearly, the recession velocity grows proportionally with distance. *Again, that's your conclusion using Hubble's law, when looking backward in time. If separated galaxies on a sphere are separated at a rate greater than c, you should be able to prove it mathematically, based on geometry, without relying on Hubble's law. AG* 3. Hubble Distance: The key point is the Hubble distance: d_H = c / H0 At distances greater than this, the recession velocity exceeds the speed of light (c). This doesn't violate relativity, as it's the space between galaxies that expands faster than c, not the galaxies moving through space. *Looking backward in time, you get a result which follows from an initially HUGE expansion greater than c, and then a slowing down due to gravity. But using Hubble's law leads to a questionable result going forward IMO. AG * 4. Analogy of the balloon: Think of two points on the surface of an inflating balloon. As the balloon expands at a constant rate, the distance between the points increases linearly. However, if the points are far enough apart, they will move away from each other faster than a closer pair of points. Similarly, in the universe, even though the expansion rate is constant, galaxies farther apart recede faster due to their increasing distance. 5. Why light can’t reach us: For galaxies beyond the Hubble distance, the space between us expands faster than light, meaning their light can never reach us. This is why galaxies eventually move out of our observable universe. In summary, even with a linear increase in distance due to constant expansion, the recession velocity increases with distance, and for sufficiently distant galaxies, this velocity eventually exceeds c. * John* K Clark See what's on my new list at Extropolis <https://groups.google.com/g/extropolis> hwt -- 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]. 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