Martin

A good tutorial article available online "Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe" http://arxiv.org/astro-ph/0310808 It is by Lineweaver and Davis.

this might interest Dave especially one of the first things you learn in a basic astro course is that the cosmological redshift is not a doppler shift it is caused by the stretching out of space while the light was in transit Eric Linder has some online cosmology notes that explains this he teaches at UC Berkeley and has a textbook out that is an Intro to Cosmology it you try to explain it as a doppler shift you get into trouble because the recession speed of the thing changes substantially over time what matters to the wavelength is how much space expanded (the Friedmann model, the FRW, friedmann robertson walker metric) while the light was on its way. so the first thing they give you is a different, non-doppler, formula for the redshift. ----------------- then there is the Hubble law v = Hd the H is the value of the hubble parameter today the v is the recession velocity of the thing at this moment the d is the distance at the present moment the definitions depend on the FRW metric, which is the standard, and the Friedmann equations H in fact appears as (a'/a) in one of the friedmann equations (the one that defines rho crit, the critical density) because of the linearity of the Hubble law, you can see that if d is large enough you simply have to get v exceding light. but it is just a recession velocity, nobody's spaceship catches up to a photon!

Here are some online cosmology calculators Ned Wright who teaches cosmology at UCLA, and also Siobahn Morgan who teaches astronomy at Iowa check out their homepages. ned has a cosmology FAQ that is world famous! has been translated into various languages.

Martin: 3) the sky is full of things receding from us faster than c galaxies have been observed which are receding at several times c. so this has not been proven impossible ... [Tycho?]: Um, what. I call bullshit on this one, this goes against everything I know on relativity. If you can prove me wrong, by all means do so. JaKiri: That is most definitely wrong; especially given the universe is expanding at sublight speeds (at the moment, as far as my memory tells me) [Tycho?]: Yeah, it definately is, otherwise relativity would have been thrown out the window when one of its principal predictions was broken. Which it hasn't been. Dave: I have to say it goes against everything I've ever been taught, so I'm in opposition. Not quite sure where he got that one from. -------------- Hi Tycho, Jakiri, Dave, I am telling you a feature of the standard model cosmology that mainstream professional cosmologists pretty much all use. It is based on 1915 General Relativity and the Friedmann model derived from it in the 1920s by making simplifying assumptions (isotropy, homogeneity). It leads to the definition of the Hubble parameter (now pretty accurately measured) and to the socalled "big bang model" or the prevailing LambdaCDM model. there is a pretty general consensus on the basic features One of these basic features is FTL recession speeds. It is a popular confusion that these are forbidden since special relativity does not apply over great distances of curved space. it is a local theory. Nothing can catch up to and pass a photon! Nothing can come whizzing past earth at FTL speed! But that does not mean that the distance to a remote galaxy cannot be increasing at a FTL rate. in fact, General Relativity predicts FTL recession speeds the popular confusion about this has been dealt with in a remedial article by Tammy Davis and Chuck Lineweaver. Lineweaver was one of the principal investigators in charge of COBE (the first map of the CMB) and is a world class authority Tammy is one of his PhD students. They are mainstream and you can trust them. At least they took the trouble to try to counter the confusion that so many people have about superluminal expansion speeds. Their article is called "Expanding Confusion" I will get the arxiv number. To my mind one of the best ways to get over the misconception that you cant have superlum. recession speeds is to use an algorithm published by the magazine Sky and Telescope-----S&T is probably familiar to many people here. this program calculates distances and recession speeds for a given redshift z. You put in the z (and parameters like H and Lambda for dark energy) and it calculates the recession speed at present and also what it was when the object emitted the light which we are now receiving. An astro professor at the univ. of iowa named Siobahn Morgan has put the Sky and Telescope program into a java online calculator at her website for her astronomy students to use and it will give you all the superluminal recession speeds you want Even for z = 2 the thing is now receding FTL even as we receive the light from it that it emitted a few billion years ago. the calculator will supply the particulars. I will get the URL for it The consensus model of cosmology that virtually everybody uses---the mainstream professional people I mean----would break down if you didnt allow FTL recession speeds recession speeds are a rate that already large distances are increasing they are a different kind of speed from local encounter speeds (which are governed by special rel)

when you ask about potential energy you need to specify what kind (do you mean gravitational potential energy? do you mean chemical potential energy? electrical?) supposing you mean gravitational and the beaker is on earth, in earth's gravity. Maybe it is at your house on a tabletop. then there is a very tiny increase in potential energy (too small to be practical to measure) because for one thing the water expands so it rises slightly in the beaker and its center-of-mass is now higher there is also a very tiny, unmeasurable, relativistic effect, the mass of the beaker of water increases very slightly with temperature according to E = m c2 It sounds ridiculous and too small to even mention but the heat energy you put into the water does increase its mass everso slightly so that increases the gravit. potential of the beaker of water if you lowered it by string and pulley from tabletop down to floor level you could theoretically get a little bit more energy from it if the water had been warmed first. ------ there are also intra molecular forces and intermolecular the molecules of water are not only moving more rapidly (K.E.) they are also farther apart (the water has expanded) You mentioned this---so you doubtless understand that heating the water puts in a small amount of P.E. by way of widening average separations. But again it is, IMHO, better to neglect these amounts because they are so tiny. ------- In my opinion the most sensible answer is to say no, there is no (appreciable) increase in P.E. Because these effects are all so small. I could be wrong tho, and maybe someone will improve on this.

1) using the metric system it is not possible to measure the speed of light because the meter (since 1983 convention) has been defined as the distance light travels in vacuum during a certain small fraction of a standard second 1/2997692458 of a second so the speed in metric units is always equal' date=' by definition, to 299792458 meters (defined this way) per second 3) the sky is full of things receding from us faster than c galaxies have been observed which are receding at several times c. so this has not been proven impossible on the contrary ------- it can be argued, I guess i would support this viewpoint, that the first measurement of a fundamental universal constant of nature was made by Olaus Roemer in Paris around 1675 he measured the speed of light (to within an accuracy of about 10 percent) by timing orbits of Jupiter's moon Io he measured it in terms of the distance of the earth to the sun which was known in conventional earth units (miles, leagues, furlongs etc) at that time only very roughly (to within about 10 percent) and in terms of traditional earth-rotation-based time. arivero just referred to Roemer's measurement, which reminded me there is a plaque on the front of the old Observatoire de Paris building commemorating Roemer's measurement of the speed of light in 1675 what is the greatest or most memorable thing that ever happened in paris? maybe Francois Villon's poems, maybe the paintings of the Impressionists, maybe the making of a favorite movie, you have to decide for yourself. But when villon and the impressionists are forgotten people will probably still remember that a young Danish astronomer at the Observatoire measured the speed of light-----if there is still a civilization then which is capable of remembering anything. It was a first. (although galileo had tried to do it in a farcical manner some 50 years earlier)

J'Dona in cosmology Hubble's law is more important than this locally applying speedlimit rule of special rel. hubble's law says v =H D that is, the current recession speed (at this very instant) is equal to the distance D at this present instant, multiplied by the current value of hubble parameter H and also at every past instant-----the recession speed then is equal to the distance then multiplied by the value of H then. this law is taken seriously and it means that for a big enough D you can have quite big v. people who think that the hubble law is about the redshift and that the redshift is a doppler effect are mistaken when you see the law in a textbook it does not have doppler in it or z in it It simply and always says this: v = HD and the values of the three things in the law are their current values. Ned Wright spends a fair amount of time being clear about this. to be rigorous about it you need to know the distance-measure that cosmologists use (FRW---friedmann robertson walker) and a few technical details like that.. but the upshot is if you accept the hubble law (about the most basic thing in cosmology) you can no longer believe that the special rel "speed limit c" applies to expansion---recession-speeds are the rates that distances increase as space expands nothing ever catches up to and passes a photon space can stretch out without anything ever catching up to a photon so really there is no contradiction

I said "Roser Pello" to google and it gave me this picture (brunette) http://webast.ast.obs-mip.fr/people/roser/ in the old picture she was blonde, i liked it better and it was taken right after they found the galaxy so she was radiantly happy I am disappointed that someone, perhaps roser, has changed the picture at the french astronomy (midi-pyrenees observ.) website http://webast.ast.obs-mip.fr/people/roser/

here is a link to the Astronomy reference sticky-thread which is a catchall for astronomy links http://www.scienceforums.net/forums/showthread.php?p=56565#post56565

I think that result was that it is at least that big but it might be much larger, perhaps infinite I dont see a contradiction. if you see one please explain. I think the idea that the u is very large (radius at least 50 billion ly) is compatible with the idea of constant speed of light and its age being 13.7 billion years. dont know anything about non-constant speed of light theories---too marginal. I dont think it has been established that the U began with a very small volume. At moment of big bang it may have had infinite spatial extent (just a very high density) we should have some links to standard cosmology sources and FAQ Ned Wright teaches undergrad and grad course in cosmology at UCLA and he has a pretty nice website with a tutorial and introductory level FAQ just google "Ned Wright" Charlie Lineweaver a worldclass cosmologist in, I think, new zealand also has a basic article with good diagrams called "inflation and the cosmic microwave background" it is PDF downloadable at arXiv and also HTML online at California Inst. of Technology for that,just google "Lineweaver" another cosmologist with worldwide rep who has stuff on the web is Michael Turner----I dont know if google will find you his articles because i havent tried. He has a good onecalled "Understanding the new cosmology" the main thing is that the popular science writers spread confusion because they simplify the message down to where it no longer makes sense and the images they use can be misleading, so it seems to me you have to go to people who use at least a few equations and who arent trying to sell a lot of books to the public if google doesnt get you what you want ask some more here and i or somebody else will get links to some cosmology FAQ

thanks for the reply I will watch this thread to learn more about it

what is GMail?

It would depend some on how far the spaceship was from the sun (as well as how much water and what temp it starts out at) ice can exist in a near vacuum as long as it is in the shade or far enough from the sun several of the moons of jupiter are known to have icy crusts (some are believed to have liquid water deep under the surface) it is believed possible that some ice might exist in craters at the north or south ends of the earth's moon because there would not be enough sunlight and heat radiation to warm it up conditions in a crater at the lunar north pole would resemble those on a colder body more distant from the sun Here is what I picture happening, maybe someone else can make this more precise (it depends on equilibrium temperature and vapor pressure at that temp, so is a bit technical) A. if the spaceship were at Venus' distance from the sun and you gently squeezed a glob of cold water out into space then you could watch it simultaneously boil and freeze for a while until there was only a remnant consisting of ice but the chunk of ice would not last because it would be close enough to the sun that the sunlight would warm it, and it would evaporate B. if the spaceship were at Saturn's distance from the sun and you gently squeezed a glob of cold water out into space then you could watch it simultaneously boil and freeze for a while until there was only a remnant consisting of ice (just like in the other case) but this time the ice-chunk would not appear to evaporate. It would seem to be permanent because it would be far enough from the sun that its equilibrium temperature would be very low it would be so cold that its vapor pressure would be negligible

[edited part out that seemed too effusive, lacking in reserve] i am looking forward to following your next argument although I may disagree with the conclusions. go for it

that's my understanding too! those microscopic BH may have evaporated in a twinkling if there ever were any! but I was hoping jana would go into that. I could use a refresher on the details

No need for apologies on that account! I am glad to take plenty of time to assimilate what you are saying and others may want to as well, rather than trying to play lightningchess with ideas of the universe Take your time. I am looking forward to hearing more. BTW the nonstring quantum gravity pictures I've encountered are almost never holographic. In my experience people talk about 't Hooft's holographic principle more as a conjecture than as an necessary axiom to be satisfied by QG. It seems to me that you are breaking new ground here---opening up a new front in the string/nonstring debate. it is really quite majorleague in a sense. You are arguing that any decent QG theory MUST (because of your reasoning about black holes) be holographic in the sense that the state of the universe must be described on a 2D manifold. To quote your essential point: " If we then take seriously the idea a la bekenstein and hawking that the degrees of freedom of black holes are proportional to their event horizons, then the true fundamental degrees of freedom of any QG theory are area- and not volume-extensive. This argument eliminates all current approaches to quantum gravity except string theory.."

a new misplaced thread in cosmology today about the composition of comets traffic has been picking up, or so it seems to me (with not very much experience of this board as yet) I wonder how many of the "guests" are actually humans and might join if they like what they are reading, and how many are bots sent by the Great Powers to study us [edit: I recently looked at "who's online" and saw that one of the guests was reading who's online, and I can't picture a cyborg doing that, so I reckoned that guest was probably an actual person]

BTW in an earlier post jana said that he (or she) was a student at the University of Toronto planning to take a course for seniors offered this fall on string theory. To me jana seems already quite knowledgeable and well versed about string theory (i hope this is a correct impression). So with luck this could be an informative thread.

the "Big Bang" origin of the universe envisions a brief time of very high (planck scale) temperature during which particles could collide (some people think) with enough energy to form microscopic black holes since we dont see a lot of microscopic black holes around us, perhaps they evaporated or dispersed in some fashion. to me this is pretty exotic stuff and i would be happy if someone would discuss it jana brought this up in another thread so I am hoping jana will give us an informal tutorial on how high energy collisions can form microscopic black holes

this is a topic that a new member, jana, raised in another thread it really deserves a thread of its own I am hoping jana will provide a tutorial on this that several people besides me can enjoy

I do realize that lots of people suppose that in the early universe under conditions of (very high) planckian temperature and density collisions between particles may have tended to form black holes. But frankly it is not intuitive to me how extended objects like strings could do this. Since I think you know more than I do about what people working in string theory believe, i would like to ask you about that in particular. Strings are extended objects which limits (by the string scale) how close they can come. Please explain how two strings form a black hole by colliding with each other at high energy. The black hole would presumably be a great deal smaller than string scale! Sorry if this seems naive. I dont doubt that in a string theory context this is provided for, but I could use an intuitive explanation of this (having substantial areas of ignorance about string theory!) and perhaps others can as well. If you are up for it, let's start a separate thread that can serve as a tutorial (by you for me and whoever else is interested) on this separate topic. "Black hole formation in high energy collisions." you might also want to talk about how all these microscopic black holes created at planck temperature in the first zillionth of a second subsequently evaporated or dispersed