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Trouble For Dark Energy Hypothesis?
[[ Meta-comment: This discussion started in sci.physics.research, but its "natural home" is in sci.astro.research. I've cross-posted this article to both newsgroups, and set the Followup-To: header so further discussion should be in s.a.r. ]] Gary Harnagel wrote: I'm having trouble picturing why we should see the CMBR at all. Since it's traveling at the speed of light but we're moving somewhat slower, shouldn't it have passed us long ago? I know, the FLWR metric must have something to do with it, but ... To try to answer Gary Harnagel's question: begin analogy Imagine an infinite static Euclidean universe (i.e., flat spacetime, no gravity involved) filled with (stationary) fog which both emits and scatters (visible) light, and consider a (stationary) observer in that fog. Now suppose that at a time which we will label t=0, two things happen: * all the fog suddenly condenses into larger water droplets, and * those water droplets no longer emit light. Since the scattering cross-section of large water droplets is vastly smaller than that of fog, the result is that at t0, the sea-of-droplets is mostly transparent to light (certainly much more transparent than the original fog was). In other words, at times t0 light basically travels in straight lines, with little scattering, emission, or absorption. What will our observer see at t=1 year? Since at t0 there is minimal scattering, emission, or absorption, we see that at t=1 year our observer will see (receive) those photons, and only those photons, which were (a) exactly 1 light-year away from her at t=0, and (b) travelling directly towards her at t=0. This holds in any direction our observer looks. In other words, at t=1 year our observer will see a uniform glow on her "sky". At t=2 years our observer will will see those photons, and only those photons, which were (a) exactly 2 light-years away from her at t=0, and (b) travelling directly towards her at t=0. This holds in any direction our observer looks. In other words, at t=2 year our observer will see a uniform glow on her "sky". But that glow is comprised of a *different set of photons, emitted at a different set of events* than was the glow she saw at t=1 year. Etc etc for any other time t0. end analogy As you can see, this analogy reproduces many of the features of the CMBR. It doesn't reproduce the CMBR's temperature -- for that you need a cosmological redshift between the last-scattering time (t=0 in the analogy, approximately 0.5 million years after the big bang in standard cosmology) and today. But the analogy does produce an all-sky uniform glow seen by all observers, even at far-future times. I hope this makes things a bit clearer (no pun intended). -- -- "Jonathan Thornburg [remove -animal to reply]" Dept of Astronomy & IUCSS, Indiana University, Bloomington, Indiana, USA currently visiting Max-Plack-Institute fuer Gravitationsphysik (Albert-Einstein-Institut), Potsdam-Golm, Germany "There was of course no way of knowing whether you were being watched at any given moment. How often, or on what system, the Thought Police plugged in on any individual wire was guesswork. It was even conceivable that they watched everybody all the time." -- George Orwell, "1984" |
#2
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Trouble For Dark Energy Hypothesis?
On Sunday, January 14, 2018 at 9:42:10 AM UTC-7, Jonathan Thornburg [remove -animal to reply] wrote:
[[ Meta-comment: This discussion started in sci.physics.research, but its "natural home" is in sci.astro.research. I've cross-posted this article to both newsgroups, and set the Followup-To: header so further discussion should be in s.a.r. ]] Gary Harnagel wrote: I'm having trouble picturing why we should see the CMBR at all. Since it's traveling at the speed of light but we're moving somewhat slower, shouldn't it have passed us long ago? I know, the FLWR metric must have something to do with it, but ... To try to answer Gary Harnagel's question: begin analogy Imagine an infinite static Euclidean universe (i.e., flat spacetime, no gravity involved) filled with (stationary) fog which both emits and scatters (visible) light, and consider a (stationary) observer in that fog. Now suppose that at a time which we will label t=0, two things happen: * all the fog suddenly condenses into larger water droplets, and * those water droplets no longer emit light. Since the scattering cross-section of large water droplets is vastly smaller than that of fog, the result is that at t0, the sea-of-droplets is mostly transparent to light (certainly much more transparent than the original fog was). In other words, at times t0 light basically travels in straight lines, with little scattering, emission, or absorption. What will our observer see at t=1 year? Since at t0 there is minimal scattering, emission, or absorption, we see that at t=1 year our observer will see (receive) those photons, and only those photons, which were (a) exactly 1 light-year away from her at t=0, and (b) travelling directly towards her at t=0. This holds in any direction our observer looks. In other words, at t=1 year our observer will see a uniform glow on her "sky". At t=2 years our observer will will see those photons, and only those photons, which were (a) exactly 2 light-years away from her at t=0, and (b) travelling directly towards her at t=0. This holds in any direction our observer looks. In other words, at t=2 year our observer will see a uniform glow on her "sky". But that glow is comprised of a *different set of photons, emitted at a different set of events* than was the glow she saw at t=1 year. Etc etc for any other time t0. end analogy As you can see, this analogy reproduces many of the features of the CMBR. It doesn't reproduce the CMBR's temperature -- for that you need a cosmological redshift between the last-scattering time (t=0 in the analogy, approximately 0.5 million years after the big bang in standard cosmology) and today. But the analogy does produce an all-sky uniform glow seen by all observers, even at far-future times. I hope this makes things a bit clearer (no pun intended). -- -- "Jonathan Thornburg [remove -animal to reply]" Dept of Astronomy & IUCSS, Indiana University, Bloomington, Indiana, USA currently visiting Max-Plack-Institute fuer Gravitationsphysik (Albert-Einstein-Institut), Potsdam-Golm, Germany Thanks, JT. I like your fog analogy; however, let's consider the case where the fog consists of photons which begin in some finite volume of space. They would be moving in random directions at c and, presumably, would interact in some process to create particles with mass, conserving energy and momentum. But pair production can't satisfy both energy and momentum conservation unless there is some other mass that can absorb the excess of one or the other, yes? Of course, the big bang has the same problem, as well as the problem of having equal parts matter and antimatter. Anyway, the created particles will still have kinetic energy and will disperse, but at lower speeds than the unconverted photons. So my original question is still unanswered by the fog analogy. |
#3
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Trouble For Dark Energy Hypothesis?
On 1/15/18 1/15/18 3:30 PM, Gary Harnagel wrote:
On Sunday, January 14, 2018 at 9:42:10 AM UTC-7, Jonathan Thornburg [remove -animal to reply] wrote: begin analogy Imagine an infinite static Euclidean universe (i.e., flat spacetime, no gravity involved) filled with (stationary) fog which both emits and scatters (visible) light, and consider a (stationary) observer in that fog. Now suppose that at a time which we will label t=0, two things happen: * all the fog suddenly condenses into larger water droplets, and * those water droplets no longer emit light. Since the scattering cross-section of large water droplets is vastly smaller than that of fog, the result is that at t0, the sea-of-droplets is mostly transparent to light (certainly much more transparent than the original fog was). In other words, at times t0 light basically travels in straight lines, with little scattering, emission, or absorption. What will our observer see at t=1 year? Since at t0 there is minimal scattering, emission, or absorption, we see that at t=1 year our observer will see (receive) those photons, and only those photons, which were (a) exactly 1 light-year away from her at t=0, and (b) travelling directly towards her at t=0. This holds in any direction our observer looks. In other words, at t=1 year our observer will see a uniform glow on her "sky". At t=2 years our observer will will see those photons, and only those photons, which were (a) exactly 2 light-years away from her at t=0, and (b) travelling directly towards her at t=0. This holds in any direction our observer looks. In other words, at t=2 year our observer will see a uniform glow on her "sky". But that glow is comprised of a *different set of photons, emitted at a different set of events* than was the glow she saw at t=1 year. Etc etc for any other time t0. end analogy As you can see, this analogy reproduces many of the features of the CMBR. It doesn't reproduce the CMBR's temperature -- for that you need a cosmological redshift between the last-scattering time (t=0 in the analogy, approximately 0.5 million years after the big bang in standard cosmology) and today. But the analogy does produce an all-sky uniform glow seen by all observers, even at far-future times. I like your fog analogy; however, let's consider the case where the fog consists of photons [...] That is not the analogy. As the first sentence says, the fog "both emits and scatters (visible) light". The correspondence to the cosmological model and CMBR: fog = primordial particles water droplets = atoms (visible) light = CMBR At "recombination", z ~ 1100, the cosmological expansion had reduced the average energies of primordial particles (protons, deuterons, alphas, electrons, ...) so much that they could form stable atoms (mostly hydrogen and helium). Suddenly the universe became nearly transparent to electromagnetic radiation. Today we see CMBR photons whose time of last scatter was ~ 13 gigayears ago. At and after recombination, the energy of the CMBR photons is far too low for pair production (threshold = 1.022E6 eV). During recombination the CMBR photon energy peak was a few eV; today it is 6.624E-4 eV. BTW there should also be a cosmic neutrino background, which decoupled much earlier than photons. It would be very interesting if this could be detected. Tom Roberts |
#4
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Trouble For Dark Energy Hypothesis?
On 15/01/2018 21:30, Gary Harnagel wrote:
On Sunday, January 14, 2018 at 9:42:10 AM UTC-7, Jonathan Thornburg [remove -animal to reply] wrote: [[ Meta-comment: This discussion started in sci.physics.research, but its "natural home" is in sci.astro.research. I've cross-posted this article to both newsgroups, and set the Followup-To: header so further discussion should be in s.a.r. ]] Gary Harnagel wrote: I'm having trouble picturing why we should see the CMBR at all. Since it's traveling at the speed of light but we're moving somewhat slower, shouldn't it have passed us long ago? I know, the FLWR metric must have something to do with it, but ... To try to answer Gary Harnagel's question: begin analogy Imagine an infinite static Euclidean universe (i.e., flat spacetime, no gravity involved) filled with (stationary) fog which both emits and scatters (visible) light, and consider a (stationary) observer in that fog. Now suppose that at a time which we will label t=0, two things happen: * all the fog suddenly condenses into larger water droplets, and * those water droplets no longer emit light. Since the scattering cross-section of large water droplets is vastly smaller than that of fog, the result is that at t0, the sea-of-droplets is mostly transparent to light (certainly much more transparent than the original fog was). In other words, at times t0 light basically travels in straight lines, with little scattering, emission, or absorption. What will our observer see at t=1 year? Since at t0 there is minimal scattering, emission, or absorption, we see that at t=1 year our observer will see (receive) those photons, and only those photons, which were (a) exactly 1 light-year away from her at t=0, and (b) travelling directly towards her at t=0. This holds in any direction our observer looks. In other words, at t=1 year our observer will see a uniform glow on her "sky". At t=2 years our observer will will see those photons, and only those photons, which were (a) exactly 2 light-years away from her at t=0, and (b) travelling directly towards her at t=0. This holds in any direction our observer looks. In other words, at t=2 year our observer will see a uniform glow on her "sky". But that glow is comprised of a *different set of photons, emitted at a different set of events* than was the glow she saw at t=1 year. Etc etc for any other time t0. end analogy As you can see, this analogy reproduces many of the features of the CMBR. It doesn't reproduce the CMBR's temperature -- for that you need a cosmological redshift between the last-scattering time (t=0 in the analogy, approximately 0.5 million years after the big bang in standard cosmology) and today. But the analogy does produce an all-sky uniform glow seen by all observers, even at far-future times. I hope this makes things a bit clearer (no pun intended). -- -- "Jonathan Thornburg [remove -animal to reply]" Dept of Astronomy & IUCSS, Indiana University, Bloomington, Indiana, USA currently visiting Max-Plack-Institute fuer Gravitationsphysik (Albert-Einstein-Institut), Potsdam-Golm, Germany Thanks, JT. I like your fog analogy; however, let's consider the case where the fog consists of photons which begin in some finite volume of space. They In an infinite universe (or for that matter even a finite one that is much larger in size than c*age_of_universe) there is always a surface of last scattering where photons can escape from the previously opaque plasma and travel more or less unhindered until they reach us. The fog analogy is about as good as it gets to explain this. would be moving in random directions at c and, presumably, would interact in some process to create particles with mass, conserving energy and momentum. But pair production can't satisfy both energy and momentum conservation unless there is some other mass that can absorb the excess of one or the other, yes? Of course, the big bang has the same problem, as well as the problem of having equal parts matter and antimatter. Observationally near to us it appears that there was an excess of what we call matter - otherwise there would be a lot more matter-antimatter annihilation events seen. Other alternatives would require some sorting of matter and antimatter on a galactic cluster scale. I don't think you can observationally determine if this is the case. Is there any way even in principle to determine observationally if all clusters are made of matter as opposed to some being of antimatter? Anyway, the created particles will still have kinetic energy and will disperse, but at lower speeds than the unconverted photons. So my original question is still unanswered by the fog analogy. The photons from the cosmic microwave background have been going past the our position ever since the universe first became transparent. Their energy gradually decreasing as the universe expands and cools and also by redshifting of the receding surface of emitters. There is always a layer at just the right distance for its photons to be reaching us now. I suppose if the universe was of finite extent the CMBR could go out suddenly tomorrow. It does beg an interesting question in the very long term future - will the CMBR gradually fade out to T=0 and time dilate as the surface of last scattering approaches the particle horizon or will the dark energy acceleration make it switch off sharply? -- Regards, Martin Brown |
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Trouble For Dark Energy Hypothesis?
Gary Harnagel wrote:
I'm having trouble picturing why we should see the CMBR at all. Since it's traveling at the speed of light but we're moving somewhat slower, shouldn't it have passed us long ago? I know, the FLWR metric must have something to do with it, but ... I replied with an analogy: # Imagine an infinite static Euclidean universe (i.e., flat spacetime, # no gravity involved) filled with (stationary) fog which both emits and # scatters (visible) light, and consider a (stationary) observer in that # fog. [[...]] Gary Harnagel replied: I like your fog analogy; however, let's consider the case where the fog consists of photons which begin in some finite volume of space. Stop here. In my analogy the fog consisted of matter (water) + a bunch of photons, not just photons. And the matter+photons are everywhere in space, not restricted to "some finite volume of space". The case that you're now describing (photons only, restricted to a finite spatial volume) is not a good analogy to the hot-big-bang-model CMBR formation. In the hot-big-bang model, the "fog" consists of matter (mostly a plasma of protons, alpha particles, and electrons) in radiative equilibrium with am ambient field of (approximately black-body) photons. These matter and photons are everywhere in space, not restricted to a finite spatial volume. In the hot-big-bang model the occurence analogous to my analogy's "fog condensing" is (was) the temperature dropping low enough so that the ambient matter could "condense" and form (mostly) hydrogen & helium atoms. This happened roughly 0.5e6 years after the big bang. ciao, -- -- "Jonathan Thornburg [remove -animal to reply]" Dept of Astronomy & IUCSS, Indiana University, Bloomington, Indiana, USA currently visiting Max-Plack-Institute fuer Gravitationsphysik (Albert-Einstein-Institut), Potsdam-Golm, Germany "There was of course no way of knowing whether you were being watched at any given moment. How often, or on what system, the Thought Police plugged in on any individual wire was guesswork. It was even conceivable that they watched everybody all the time." -- George Orwell, "1984" ----- End forwarded message ----- |
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Trouble For Dark Energy Hypothesis?
In article , Martin Brown
writes: I suppose if the universe was of finite extent the CMBR could go out suddenly tomorrow. It does beg an interesting question in the very long term future I have to point out that it merely asks the question. Begging the question means essentially answering a question with a statement which repeats the question, e.g. "Why did you come here?" "Because I had a desire to do so." - will the CMBR gradually fade out to T=0 and time dilate as the surface of last scattering approaches the particle horizon or will the dark energy acceleration make it switch off sharply? The particle horizon always increases in comoving coordinates and, in an expanding universe, in proper distance as well. It is always behind the surface of last scattering. The redshifts of both increase with time, but it doesn't make sense to say that the latter approaches the particle horizon. If the cosmological constant comes to dominate the expansion, which appears will happen in our universe, then the universe will asymptotically approach the de Sitter model. This model has an event horizon at a fixed proper distance. As the universe expands, objects approach it, becoming infinitely redshifted when reaching it. There is no sharp switch-off. |
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Trouble For Dark Energy Hypothesis?
In article ,
Martin Brown writes: Is there any way even in principle to determine observationally if all clusters are made of matter as opposed to some being of antimatter? I don't think there's any way to distinguish for any one cluster. However, if a matter galaxy falls into an anti-matter cluster or vice versa, there should be lots of gamma rays, which should (I expect) heat the X-ray-emitting cluster gas in an obvious pattern. Even more spectacularly, entire galaxy clusters sometimes collide. If a collision between two "opposite types" took place, I expect that would be obvious. -- Help keep our newsgroup healthy; please don't feed the trolls. Steve Willner Phone 617-495-7123 Cambridge, MA 02138 USA |
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Trouble For Dark Energy Hypothesis?
On Tuesday, January 16, 2018 at 9:57:35 AM UTC-7, Tom Roberts wrote:
On 1/15/18 1/15/18 3:30 PM, Gary Harnagel wrote: .... BTW there should also be a cosmic neutrino background, which decoupled much earlier than photons. It would be very interesting if this could be detected. Tom Roberts The neutrino flux would be red-shifted by z ~ 1100 also, but today's neutrino detectors aren't sensitive to energies below a few 100 KeV, yes? That means primordial neutrinos would have to have had energies above 100 MeV. BTW, did you get the email I sent you last November about my equipment? Gary [[Mod. note -- 1. I am not an expert on this, but I think you're right: if you want any direction-of-travel angular resolution (necessary to distinguish other sources from the Sun's neutrino flux), today's neutrino detectors have essentially no sensitivity to neutrinos with energies below a few MeV. 2. Whoever the last line is directed to, replies should be private (e.g., email); the newsgroup is for *public* conversations. -- jt]] |
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Trouble For Dark Energy Hypothesis?
In article ,
Gary Harnagel writes: The neutrino flux would be red-shifted by z ~ 1100 also, This ignores the part about the neutrinos having decoupled long before the photons. One source, which seems to be a textbook by Daniel Baumann at Cambridge: http://www.damtp.cam.ac.uk/user/db27...y/Chapter3.pdf gives a neutrino decoupling redshift of 6E9. That corresponds to an energy of about 1 Mev and a time about 1 s after the Big Bang. Cosmological neutrinos should therefore have a kinetic energy today of about 1/6 meV (i.e., milli-, not mega-). As the OP wrote, that's very far from detectable. -- Help keep our newsgroup healthy; please don't feed the trolls. Steve Willner Phone 617-495-7123 Cambridge, MA 02138 USA |
#10
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Trouble For Dark Energy Hypothesis?
Le 23/01/2018 22:33, Steve Willner a écrit :
In article , Gary Harnagel writes: The neutrino flux would be red-shifted by z ~ 1100 also, This ignores the part about the neutrinos having decoupled long before the photons. One source, which seems to be a textbook by Daniel Baumann at Cambridge: http://www.damtp.cam.ac.uk/user/db27...y/Chapter3.pdf gives a neutrino decoupling redshift of 6E9. That corresponds to an energy of about 1 Mev and a time about 1 s after the Big Bang. Cosmological neutrinos should therefore have a kinetic energy today of about 1/6 meV (i.e., milli-, not mega-). As the OP wrote, that's very far from detectable. It depends on your antena's neutrino sensitivity. Why do neutrinos react with some Chlorate compounds? Isn't it a consequence of the geometry of the collision? What about putting a ring of neutrino sensitive atoms orientable with an outer magnetic field and just trying to point to the sun? We could turn around the chlorine with its ring until we see what direction and position should the chlorine have to intercept at best neutrinos coming from a specific direction. Why does underground chlorine detectors work? Bceause among the millions of atoms, one has the right orientation to exactly trap a neutrino. Having an array of neutrino detectors at a molecular level would increase sensitivity and positioning. Or not? |
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