Martin

Teranko raised the issue of whether understanding the basics of the standard cosmo model might impair one's creativity. This discussion was not appropriate to the original thread. Which is based on the premise that to discuss we need a common point of departure and if you want to talk cosmo you should first of all understand the conventional model that is in use---if you then want to crit and explore alts, that's fine. So since this was a new topic, I have set up it's own separate thread. AFAICS it is a real issue and the issue is one of degree. Just my opinion: there should be some common understanding, common concepts, some prerequisites for rational discussion. But not too much. It shouldn't be burdensome to acquire. So the question is extent. How much is right? A lot of SFN members over the years have recommended newcomers read Lineweaver's Sci Am article. I guess that's a minimum requirement. It's easy to read. I keep the link handy in my sig. It is also at princeton.edu. http://www.astro.princeton.edu/~aes/AST105/Readings/misconceptionsBigBang.pdf I would add that anyone who wants to talk cosmology with us should have some experience with either a cosmology calculator or a differential equation model. The point is that cosmology is a mathematical science. It is not done with words. Or with mental imagery. It is done by fitting data to an extremely simple numerical model of the universe. And there are literally millions and millions of points of data to be fitted. The amazing thing is how well such a simple model can fit the data. If you want hands-on experience with the mathematical model, well it is built in to some online Java calculators and you can play around with them. Google "wright calculator" http://www.astro.ucla.edu/~wright/CosmoCalc.html or google "cosmos calculator" http://www.uni.edu/morgans/ajjar/Cosmology/cosmos.html The third prerequisite would be to take 5 or 6 minutes and watch the balloon flic. Google "wright balloon model" http://www.astro.ucla.edu/~wright/Balloon2.html ===================== I don't think that needs to take more than about 45 minutes in all and I don't think it will impair anybody's artistic creativity. It gives us the nucleus of a common culture. Does anybody have another take on this. Or some other suggestions for prerequisites?

That doesn't follow. How about reading a Scientific American article about the basic expansion concepts? You seem not to understand or have some hang-ups on some misconceptions. The Lineweaver article in my sig is a good one. In fact it's so good they use it at Princeton as a kind of supplemental textbook! See where it says "cosmology SciAm" in small print? Distances to most visible galaxies are at present increasing faster than light, but we still see them. The Lineweaver article explains why. Faster than light expansion of distances is not confined to the early universe. The fact that it is currently going on today hardly prevents us from seeing the sun . So there is something wrong with your reasoning. Somehow you are picturing expansion wrongly so you draw absurd conclusions. I guess the cutoff is z = 1.7. If you spot a galaxy and it redshift is less than 1.7 then the distance to it is increasing slower than c. If the z > 1.7 then the distance is increasing faster. All nearby galaxies, that you can get a good picture of, that aren't just fuzzy blobs, are z < 1.7. But still, since the universe is a big place there are a lot more with z > 1.7. That's why I say that distance increasing faster than c is typical. When you get Charley Lineweaver's article pdf, the first page is blank white, so scroll down. The link works, I just checked. They have it at princeton.edu too. http://www.astro.princeton.edu/~aes/AST105/Readings/misconceptionsBigBang.pdf

Here's how this thread began: It is not contrary to normal science practice to expect people to have a basic knowledge of the conventional standard theory, especially if they want to deviate from it, or go beyond it. Nobody expects you to believe any particular theory. Believe what you want. Criticize the standard model all you want too. But understand what you are criticizing. We have an educational responsibility to explain the consensus cosmology that just about any professional cosmologist assumes as a working model. Because when professionals talk and write they are assuming those concepts. And they are the most vigilant critics as well---you get extra respect points if you can find some observation that calls the standard model into question or suggests some needed modification. A common point of departure is normal science practice. In fact that's the basic reason for this thread!

Thanks! Tell me how you do it, please, if it's simple to do. In this case there was a typo: the lambda term should be 0.73. Because of observed near flatness the matter term and lambda term always should add up to 1 (or so close to 1 that we don't worry about it, like 1.01). So that is always a good check. In this case .27 and .23 don't add up. This could be a real asset pedagogically. Hope it doesn't take a long time or offer opportunities to screw up.

Excellent! Pleased to meet you! It is great you've tried. The analog would be a hypersphere socalled. The 3D surface of a 4D ball. Or the same set without the ball (throw away the 4D surround and interior and just keep the set.) So yes, it is hard to picture. Glad you tried. It may get easier after a while. Depends on the person. This is an intelligent thought which however is based on a flawed assumption. You assume the Hubble rate back then was comparable to what it is now. So if light started out 40 million lightyears away, it shouldn't have any trouble getting here. You say. Just like light that leaves today from nearby galaxies. But what we talking about is a mathematical model in which the scalefactor a(t) and the Hubble rate H(t) are governed by differential equations derived from General Relativity. And they are highly dynamic. They did not at all stay the same over the course of history! Especially in the first few millions of years. In a mathematical science you make the simplest model that fits the available data and see what it says. You don't make up scenarios and argue them pro and con with verbal reasoning. GR is a theory of geometry which has been tested many times to exquisite precision. For practical purposes it is the only one, if you don't count the newer quantum versions. Well GR says that a roughly uniform universe should be expanding or it should be contracting. So Big Bang cosmology was predicted by Friedman in 1922 without any news about redshifts---just as one of the simplest solutions to Einstein's GR equation. Our dominant theory of gravity, orbits, time-behavior, light curving, etc etc. predicts that the early universe Hubble rate is way way faster than now. So it is not surprising that CMB photons can start out towards us, from a distance then of 40 million lightyears, and take 13.7 billion years to get here. They started out when expansion was 380,000 years old and the Hubble rate was over tenthousand times faster than it is today. (Assuming the General Relativity equation is right.) I wish I could show you Morgan's calculator with the input of z = 1090, the redshift of the CMB. It gives the distance then, distance now, recession rate then, recession rate now. (Recession rate is not a motion speed it is a distance expansion rate.) If you want to try, google "cosmos calculator" and put in the usual parameters 0.27 for matter, 0.73 for lambda, and 71 for today's Hubble rate. I think that is a really constructive goal. Unless enough people do that, we won't have a scientific culture---collective understanding will get fragmented IMO.

Yes it is beyond surreal :eek: About using which words? I am generally comfortable with the way they talk at Einstein Online. You remember "A Tale of Two Big Bangs". The tendency to use The Singularity as a time-mark without assuming that the classical model (which blows up there) is adequate or even applicable. Use it as a reference point without assuming that time started there there was infinite density and infinite curvature (physically meaningless) or that the classic model is tracking nature at that point. And all the models made so far could be wrong, including the various bounce ones. What I see is intelligent people doing the best they can, trying to cope, quantizing the classic cosmological model, taking hints from partially successful quantum gravity attempts. Trying get the simplest quantum theory they can that is faithful to GR. (Or other worthy goals, like unification of gravity with a theory of matter.) I have a lot of respect for this effort, and I try to reflect and follow what seems to be the main currents. I don't feel able to second-guess the future course of research. I'd certainly be happy if it turned out, with some revised theory, that the density needed for the bounce to happen was not as high as 40 percent Planck. Incredibly unimaginably high. Have to go back tomorrow. Personally I'm happy whatever words the people I know decide to use because I know that there is a certain level of sophistication in how they think. No words are perfect. Sophisticated cusses, they use the term singularity freely without believing that there is necessarily one in nature. One simply doesn't know as yet. There is no scientific reason to suppose time stops, but there is also no good reason to suppose that time-evolution does not stop.

There is a connection. At least with the standard cosmo model, which gives a pretty good fit to our universe, the two things are related. At least the overall average spatial curvature. If you think of the simple toy picture, the balloon, you can see as distances increase the surface becomes less curved. The overall average curvature, not counting black holes and stuff, which the toy picture doesn't capture. I don't want to think about how general that is---is it a general rule or not. I think it's probably not a general rule. I can imagine universes where the distance between galaxies is increasing but at the same time space is getting more pimply. So I don't want to say that what you are talking about would happen every time.

Cameron, in the context of GR and cosmology, space is not a substance that needs to be created or that expands or that contracts. What expands is the network of distances. Distances can change in accordance with the main Einstein equation. Spacetime curvature means that geometry is dynamic, you have no right to expect distances NOT to change. Since they can, they will. And the Einstein field equation says in what way. To make the balloon model work for you, you have to ignore the inside and the outside, think of all existence concentrated on the 2D surface. No air. No rubber. Pure dynamic geometry. Thanks for recalling the quote! I should always remember to include something like that. I think it's a great talk and we should urge people to watch it, and just work with the size comparison and cope with it. The typical size of an atom is one angstrom unit which is 1/10 of a nanometer. That is way way bigger than a proton. 100,000 times bigger. He is being very cautious. He is only going back to 10-15 of Planck density. Do you follow? 100,000 cubed is one million billion. He is saying at some point the size of the observable is about an angstrom and the density is one millionth of a billionth of Planck. The Ashtekar group has been running quantum gravity computer models where they find the bounce consistently happens at about 40% of Planck. Smoot is being respectably conservative and only talking about the universe back to a much lower density. He stays in the regime where classical GR has a good chance of applying. He goes nowhere near where the "singularity" befalls GR. As long as he keeps well clear of it he doesn't have to say anything about what might actually have happened instead of the celebrated Glitch. I sound like a Smoot fan. Maybe I am. Though Ned Wright shows where Smoot should have given some other researchers more credit in his Nobel acceptance speech.

I agree that the big bang actually happened. The big bang, as cosmologists normally understand it, does not involve having matter move (except for individual random motions). It is true that distant objects (distant material in general, anything emitting light) was much closer. It didn't move. But it got farther away. Google "wright balloon model" and watch it to see how. Since matter is not traveling (traveling matter has somewhere it is going, that it is getting closer to) it is irrelevant that it can only travel at less than c. In General Relativity, distances can increase at greater than c. And the longer distances typically do Again you should watch that 3 minute balloon model movie and think about it. The little wiggly things are photons of light traveling at a fixed speed over the balloon surface. Often the galaxy that they are traveling towards is getting farther away. http://www.astro.ucla.edu/~wright/Balloon2.html But notice that as expansion slows, after a long time, the photon will begin to make progress and get closer to its destination. It's a great lesson for all of us. If you hang in there and keep trying you may get there. That is a bum steer, Purin. You mean look for Metric_expansion_of_space If you google what you said, you don't get anything. If you google what I said, you get the Wikipedia article on the metric expansion of space. By and large it is a pretty good article.

One of the things that we should be able to calculate is the size of the observable, compressed to Planck density. You should also be able to calculate the critical density from today's Hubble rate of 71 km/s per Mpc. It is about 0.85 nanojoules per cubic meter. If anyone wants to see the calculation please ask. It's easy. The usual estimate of matter density is 0.27 of critical. So that comes to about 0.23 nanojoules per cubic meter. What we need to know is the ratio of Planck density to that. The cube root of that ratio will be how much we shrink the radius of the observable. Planck energy density ( http://en.wikipedia.org/wiki/Planck_units ) is c^7/(hbar*G^2) So we just type this into google: (c^7/(hbar*G^2))/ (0.23 nanojoule/cubic meter) We should get some huge number. Yes! we get 2.0 x 10123 The radius of the observable is currently 46 billion LY. It has to be shrunk by the cube root of that huge number. We should get something like the size of a proton, if I remember right. So let's take the cube root. Put this into google: ((c^7/(hbar*G^2))/ (0.23 nanojoule/cubic meter))^(1/3) We get 1.26 x 1041. So now let's shrink the radius of the observable by that factor. Put this into google: 46 billion light years/ (1.26*10^41) What I get is 3.5 x 10-15 meters. 3.5 femtometers. The proton Compton wavelength is 1.32 femtometers. Actually in the bounce cosmology models they find the bounce happens at some fraction of Planck density. So it wouldn't go quite that high and the size wouldn't be quite that small. But this is a good ballpark figure. You can interpret it by comparison with nuclear particle sizes however.

It doesn't sound like a singularity to me. It's just what you get if you take the observable universe as we now see it---matter density 0.2 nanojoules per cubic meter, including dark matter---and see how big it would be if compressed to Planck density. the difference between classical and non-singular cosmology is simply whether the model breaks down or not. You still extrapolate back and find extreme density, extreme temperature, basically same old conditions as in the classical case. If there was an initial singularity, which I see no reason to suppose (given that we have models being studied which do not break down) and if the universe is infinite spatial volume then the singularity was infinite. I wouldn't say "all bets are off". It clearly could not have been contained in a finite volume. I like Smoot's lecture a lot. As a popular public lecture for Los Angeles and Silicon Valley smart generalists, I think it is terrific. I don't recall him saying that the universe began with a singularity. Saying that there was, at one time, an inconceivably hot & dense state is something else. The aim of today's quantum cosmo model builders is to get back there and not have the model blow up, for a change . Have the model continue. They've got several approaches. They are getting increasing peer attention, conference visibility. They're running the stuff on computer. And trying to find some fine detail predictions of what we can see that differ from the classical model. The hard part is their new models predict very close to the same stuff about the CMB and overall structure. I'm sorry. Does Smoot actually say singularity? Maybe he did, so please let me know. If he just said very hot very dense, that's of course fine! If he said observable universe concentrated in volume the size of an atom, an atomic nucleus, or a proton, that's fine. But if he said that since the classic theory breaks down (has a singularity) and therefore nature herself breaks down. If he said that time evolution stops. Then that's disappointing. I guess I could forgive him---since it is a glitzy elite popular lecture, not an academic talk. But I would feel let down. I put a calculation of the size the observable would have at Planck density over in the "cosmo basics" sticky thread. Maybe it will come in handy.

Raul, have you watched the wright balloon model movie yet? Google "wright balloon model". It's just a simple 2 or 3 minute computer animation. It helps a lot of people to visualize what cosmologists are talking about. About your question, "light should have reached us earlier", I don't understand: earlier than what?

Wait, let's get the link for that. I think it was George Smoot talking to the TED club (technology entertainment design leaders and trendsters). Google "smoot TED". Hot dog. First hit =

That's a helpful kind of example because easy to picture. I don't remember ever using that kind of simplified illustration. You are saying let the distances (those that are governed by the expansion law) increase by 1/12 or 8 percent, every second. Currently in the real world, distances (largescale ones) increase 1/140 of a percent every million years. A tiny percentage in a mindboggling long time. So it's hard to picture. What you are saying is easier to picture: 8 percent increase every second. Anybody, How are we coming with RaulDuke's questions? I have a feeling that RaulDuke is OK at this point. On the other hand, Improvision seemed to be discontented. It's great to feel more like a cosmology mob than an isolate, enjoying it

LOL I don't have a next right at the moment. But it is nice to have a response so I can start a fresh post and take a new tack, if I get a notion to. Also maybe someone else with respond. For now everything seems peaceful and in order. Relatively speaking.

I agree with the general tenor of what NowThat is saying here. I think it is merely a figurative expression to say "space expands" when mathematically what we are talking about is a pattern of increasing distances. Physics does not need to have space defined, as if it were a substance. Defining space is more an issue for philosophy. Practically speaking what matters IMHO is that we can measure distances, angles, areas, volumes---that we can observe geometry. The distance function that summarizes this kind of information is called the metric. I halfway suspect that Impro knows this already and is just being "Impish" when he asks "what is the current definition of space". For a sympathetic listener it is not so hard to recognize a figure of speech and to understand what the speaker is trying to say. Personally I often try to avoid saying "space expands". If there's a perceived risk of being misunderstood, I prefer to talk about increasing distances instead. Find other ways to say the same thing.

RaulDuke started a valuable thread in which a number of persistent confusions came to light. He elicited excellent responses from several people---Airbrush, Sisyphus, NowThat, Severian and probably others I'm forgetting. The thread has grown. I'd like it if we could have a shorter focused thread that examines the causes of the confusion that people (not just RaulDuke) get into. The reaction was kind of coherent and unanimous, on most points, which is one reason I think it worked. An exception that stands out in my mind is that early on Sisyphus made the key point, which then was seemingly forgotten as a ton of other comment, often informed and valuable, poured in. The key point he made is that expansion of distances is not motion in space. In ordinary motion, there is some destination out there that you get closer to. That doesn't happen in cosmo expansion. When newcomers arrive with this misconception, we all need to hammer it into them that expansion is not motion. Don't let them slip back into the old habit of talking about expansion as if it were a kind of motion. Also there are two other common misconceptions: the explosion picture singularity pictured as a single point. The response was unanimous in this case and Raul got the message: Singular does not mean single. The root meaning of the technical term is "odd or peculiar". On my way here I saw a singular man acting in a singular fashion---this means I saw a weird man acting weirdly. It doesn't mean I saw a bachelor--a single man--who was concentrating on doing one single action. In mathematics a singularity is a situation or region where some manmade theory breaks down/blows up. It doesn't have to be a single isolated point. A singularity can be a large region, or a place where a function fails to be defined. Acts weird. Produces meaningless infinities, etc. This might be at a single isolated point (and quite often is) but that's not what it means. Here I would say Raul is slipping back into the habit of thinking of expansion as motion. What can we do? It would be good if we had a coherent strategy. I'm open to hearing other ideas but I'll lay out what seems to me so far to work the best. You've heard it before: it's the Balloon Sermon. Think of all existence, all space and matter, concentrated on the surface of the balloon. There is no outside or inside space, only the surface. And any creatures would be 2D amoebas slithering around. White dots speckle the surface, they are galaxies. Each dot stays at the same latitude and longitude, which is what we mean by not moving. That makes sense because if the balloon expands no dot gets closer to any other---in true motion there is always some place you get closer to. Wigglers called photons move across the surface always at the same speed of one millimeter per second. Google "wright balloon model" and watch and think about it some. After 60 seconds a photon will clearly be farther than 60 millimeters from its point of origin, even though it travels at a constant speed, because the distance that it has already covered expands. Likewise after 13.7 billion years a photon will be farther from its point of origin than a mere 13.7 billion lightyears. I have to go, back later. Do other people have some ideas of how we can best handle this type of situation? BTW thanks to everybody who put in a word on the other thread.

It's a good illustration of the basic idea, but one of the numbers is off. The 12 lightyears distance would not be growing at the rate c, but would be growing at the rate of 30 million times the speed of light. This is allowed by General Relativity, since we are not talking about motion, but about changing distances (dynamic geometry). So your example is not non-physical. That kind of very rapid increase is a feature of some inflation scenarios. One way to correct the post would be to say "the distance to something 12 light seconds away increases at the speed of light, etc." For reference, 12 light seconds is about ten times the distance to the moon.

This is already answered by the others but I think you should know this handy approximate rule of thumb. Multiply the circular orbit speed by the square root of two. That works for a central body setup, where most of the mass is inside the orbit. The earth's orbit speed is about 30 clicks (30 km per second). So multiply that by sqrt 2, or 1.4. That will give you the speed you would need, at this distance from sun, to coast out of the system. The sun's orbit speed around galactic center is, if I remember, around 220 clicks. So you might think something going 40 percent faster than the sun would be destined to leave the galaxy. That would indeed be true if most of the galaxy's mass was concentrated at the central bulge. But it is not, so as the probe gets farther and farther from center he sees more and more mass pulling him back. Because of that, multiplying by sqrt 2 only gives a very rough ballpark. You actually need more of a kick than that to escape from Milky Way http://en.wikipedia.org/wiki/Solar_system http://en.wikipedia.org/wiki/Escape_velocity (this says roughly about 1000 km/s, but I don't think we actually know, because of dark matter, estimates of total mass of galaxy not all that precise)

It certainly does! It creates a doppler dipole (hot spot ahead, cold spot astern) which we've discussed here at times. Something to average out or take out of the data. So we should add some small print to that effect. And background has to be smoothed out to define it's temperature, get rid of dependence on direction.

Has everybody interested in this thread already looked at the Wikipedia article? http://en.wikipedia.org/wiki/Fermi_paradox I don't always think Wiki articles are reliable or balanced---quality varies. But this one looks pretty good to me, given my limited perspective on this question. I'd like to know other people's reactions to it.

Ole Roemer, around 1675. Young Dane, working at the Paris Observatory. He timed the orbits of Jupiter satellites, Io in particular if I remember right. He found that Io eclipses happen earlier when the earth is closer to Jupiter and later when the earth is farther from Jupiter. He estimated the speed of light and came within 10 percent or so. Before Roemer nobody was sure light even had a finite speed. Galileo suspected it might have a finite speed and tried to measure it, but was unsuccessful. Ole is short for Olaus. Probably Wikipedia has an article about Roemer. At the time the speed was measured, it would have been natural to assume that the speed was a universal constant in Newton's absolute space. The speed at which Newton's particles of light traveled. I doubt anyone raised any questions about relativity or ether at that point in history.

This poll was inspired by Asprung's thread. To me it seems like a no-brainer, but I could be wrong so let's vote. It's a vote on semantic preference, what you prefer the word simultaneous to mean. View A: Simultaneous is observer-dependent. Two events are simultaneous (for the designated observer) if he experiences them as happening at the same time according to his personal clock. In the prevailing well-tested (GR) theory, how an observer's personal clock runs depends on his history--where and how fast he travels etc. View B: Simultaneous is an absolute, though practically speaking rather imprecise idea. Imprecise, because universe time (global time, cosmology time) is only approximately measurable. But in any case it is not observer-dependent. Roughly speaking, two events are simultaneous if they happen when the Background has the same temperature. Which view is preferable? Do we have to allow both? Can you suggest a third alternative? To explain a bit more, in View B, we suppose the background temp is the same all over (after allowing for some small corrections) and gradually declining. At the present epoch it is 2.728 kelvin and after a long long time it will be 2.727 kelvin, and so on. So this is a kind of absolute clock that can be read anywhere in the universe. We could make up a word like "contemperate". Two events are "contemperate" if they happened at the same background temperature, to whatever (say one part per million) accuracy that can be determined. Or maybe that is what simultaneous ought to mean.

This is not a statement about physics, though. It is an interesting alternative definition of "simultaneous". Swansont already pointed this out a while back. Normally when physicists say simultaneous they mean according to some observer's clock. Two events are simultaneous for that observer. You seem to want people to talk different. You seem to want everybody to accept a particular time, a particular slicing of spacetime into successive moments (like what is done in cosmology although in practice one can't do it with 100 % accuracy.) Then you want people to adjust, to be aware of how their own personal clocks differ from standard, and correct for it. "According to my clock these two events were simultaneous but I was moving so my clock deviated from universe time, so if I correct for that they weren't really simultaneous." That doesn't really change the physics, does it? It would just change the way we talk. But it seems to me you also have another message, Asprung. Besides the semantics reform proposal you have an idea about existence. This gets into where I'm not sure anybody knows the answer. It looks like you are saying that only the present moment in universe time (Friedman cosmology time, with the CMB temp = 2.728 kelvin) exists. To be honest, isn't that debatable? Science is primarily about prediction, not existence. It is about improving mathematical models to achieve simplicity, elegance, accurate prediction etc.... It doesn't address the question of what exists, it's more pragmatic: does the model work? Asprung, I think you made a philosophical error here. I respect you, by the way, and I think you have raised interesting issues and you are arguing in a capable fashion. But maybe not 100% right. Personally I think this what you just said is wrong. You cannot jump from existence to experience. All our experience is different according to our world line, where we go, how fast, what acceleration, what strength gravity. Each of us has a different history, which influences our personal clock, and (IMO) influences what we experience. BTW I don't understand this post: It sounds here like you don't want NowThat to use the word "simultaneous"---which is an observer-dependent idea. But why shouldn't we be allowed to use that word? The concept is clear enough. Although it is evidently different from what I think you have in mind (which I would describe as two events occupying the same universal time-slice in the Friedman model--e.g. both occurring when the Background temp is 2.6111 kelvin, or 2.728 kelvin, or whenever.)

wouldn't that increase drag? if you don't like heeling over, the easy solution is a twin hull sailboat ---socalled catamaran