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Everything posted by Martin

  1. Never heard the joke but I assume that one of the statisticians aimed at the average of the ducks---their mean location---and so hit no ducks at all.
  2. Many SFN posters must be familiar with the usual models of stellar explosion---this "pair instability" mechanism is different, and far more powerful. If anyone is interested in a simple explanation of how these hyper-powerful explosions work, here's the SciAm article http://www.scientificamerican.com/article.cfm?id=the-biggest-bang-theory Here is the technical article: http://arxiv.org/abs/1001.1156 from a team led by Avishay Gal-Yam. Usual supernovae result in two ways: One way is from a binary star where one partner is burnt-out and modest-size (like around 1.4 solar mass). The small burnt-out partner explodes when the large active partner dumps mass on it and it reaches a critical mass. The other way is from from core-collapse. That's the scenario everybody knows (it would be in Wikipedia). A large star in the range 10-100 solar fuses all the elements up to iron and develops an iron core. Since iron can't generate energy by fusing, the star has a dead iron core, which (as it cools and loses pressure) becomes subject to sudden violent collapse. Read Wikipedia to see how the core-collapse (to neutron matter or even to a black hole) by sudden release of gravitational energy can lead to explosion. Some of the infall matter bounces. What I want to discuss is a different kind of core-collapse, when the core is oxygen, that can happen in stars of some 140+ solar mass. Say it burns to where there is a 50 solar mass core of oxygen. Ordinarily the oxygen core would keep on fusing and releasing energy as photons and exerting pressure outwards to support the rest of the star. It would be generating so it would not collapse. The iron core only collapses because it is dead and has no energy to support the weight. But something tricky happens that DEFLATES the photon pressure from the oxygen core. It produces photons but they are so energetic that PAIRS of photons collide and produce electron-antielectron pairs. The formation of these matter-antimatter pairs EATS UP some of the photons being produced by the core. Outward pressure is canceled. And the oxygen core starts to collapse. This accelerates the process of soaking up photons and forming matter-antimatter pairs. So if the star is massive enough, like 140 solar, there can be a sudden and near total collapse. Then the matter-antimatter annihilates and blows away all the outer layer material with a huge release of pure energy---very little if anything left behind.
  3. The technical article was just posted. It confirms earlier findings and gives more detail. http://arxiv.org/abs/1009.5733 ====quote from 1009.5733 abstract === "...The combined data set strongly confirms the 5.37-day, 12.9-day, 3.15-day, and 67-day planets previously announced by Bonfils et al. (2005), Udry et al. (2007), and Mayor et al (2009). The observations also indicate a 5th planet in the system, GJ 581f, a minimum-mass 7.0 M_Earth planet orbiting in a 0.758 AU orbit of period 433 days and a 6th planet, GJ 581g, a minimum-mass 3.1 M_Earth planet orbiting at 0.146 AU with a period of 36.6 days. The estimated equilibrium temperature of GJ 581g is 228 K, placing it squarely in the middle of the habitable zone of the star and offering a very compelling case for a potentially habitable planet around a very nearby star." ==endquote== SciAm has a news item. http://www.scientificamerican.com/article.cfm?id=habitable-exoplanet-gliese-581 It's a bit puzzling why they say "squarely in the middle of the habitable zone". Maybe someone here can comment. Water freezes at 273 kelvin, so the equilibrium temperature they give, 228 kelvin, seems quite cold. With a mass of 3 Earths it could retain a nice dense atmosphere, with greenhouse effect. Might be geologically active---hot springs--some hospitable local environments. Any ideas? ============================= I see! it is a small reddish star so the habitable zone is in close to the star and the planet is almost certainly not rotating but is instead TIDALLY LOCKED. Always the same face to the star. That means that it would have a whole range of temperature from quite hot on the front to quite cold on the back====and there would be some GOLDILOCKS LONGITUDES where the temperature is just right! ==quote from SciAm== Nevertheless, Earthlings would not mistake Gliese 581g for their home planet—in addition to its so-called super-Earth dimensions, it orbits a star far smaller and dimmer than the sun, and its average surface temperatures would vary dramatically, from well below freezing on its night side to scorching hot on the day side. But somewhere between those temperature extremes, which Vogt estimated might range from –35 to 70 degrees Celsius, would exist stable climatic bands, which Vogt called "eco-longitudes," within which liquid water might persist. Because the planet is probably tidally locked, showing only one hemisphere to its star just as the moon does to Earth, the temperate band between permanent daylight and permanent night might afford life a toehold. "There is a continuum of temperatures in between that are stable," Vogt said. ==endquote== ==from page 31-32 of the technical article== An equally important consideration is the actual surface temperature Ts . The equilibrium temperature of the Earth is 255 K, well-below the freezing point of water, but because of its atmosphere, the greenhouse effect warms the surface to a globally-averaged mean value of Ts = 288 K. If, for simplicity, we assume a greenhouse effect for GJ 581g that is as effective as that on Earth, the surface temperatures should be a factor 288/255 times higher than the equilibrium temperature. With this assumption, in the absence of tidal heating sources, the average surface temperatures on GJ 581g would be 236 – 258 K. Alternatively, if we assume that an Earth-like greenhouse effect would simply raise the equilibrium temperature by 33 K, similar to Earth’s greenhouse, the surface temperature would still be about the same, 242 – 261 K. Since it is more massive than Earth, any putative atmosphere would likely be both denser and more massive... ==endquote==
  4. http://physicsworld.com/blog/2010/09/by_hamish_johnstonstephen_hawk.html ==quote== M-theory, religion and science funding on the BBC By Hamish Johnston This morning there was lots of talk about science on BBC Radio 4’s Today programme – but I think it left many British scientists cringing under their duvets. Stephen Hawking was on the show explaining why M-theory – an 11-dimensional structure that underlies and unifies various string theories – is our best bet for understanding the origin of the universe. Hawking explained that M-theory allows the existence of a “multiverse” of different universes, each with different values of the physical constants. We exist in our universe not by the grace of God, according to Hawking, but simply because the physics in this particular universe is just right for stars, planets and humans to form. There is just one tiny problem with all this – there is currently little experimental evidence to back up M-theory. In other words, a leading scientist is making a sweeping public statement on the existence of God based on his faith in an unsubstantiated theory. This, and other recent pronouncements from Hawking in his new book The Grand Design were debated in a separate piece on Today by brain scientist Susan Greenfield and philosopher AC Grayling. Neither seemed too impressed with many of Hawking’s recent statements and Greenfield cautioned scientists against making “Taliban-like” statements about the existence of God. That brings me to another bit of news making the headlines... to save money, the government will soon be “rationing funds by quality”. So what does this have to do with Stephen Hawking and M-theory? Physicists need the backing of the British public to ensure that the funding cuts don’t hit them disproportionately. This could be very difficult if the public think that most physicists spend their time arguing about what unproven theories say about the existence of God. ==endquote==
  5. http://www.nytimes.com/2010/09/08/books/08book.html "Grand Design" is co-written with a younger guy, Leonard Mlodinow. It has about 100 pages of text, filled out with a lot of color illustrations, cartoons, and such. The NYTimes reviewer found the text superficial, even for pop physics. The review says the real news about the book is how "disappointingly tinny and inelegant" it is. The book is selling extremely well. #1 at Amazon.com. One reason may be what science writer Tim Ferris calls "Godmongering". Science books that raise issues about God tend to grab attention and may sell better on that account. This time Hawking-Mlodinow play the "God is unnecessary" card. They seem to be saying "Science can explain why the universe came into existence and has the laws it does, so we don't need God." Sounds like a hook to get readers. ==sample excerpt from NYT review== The real news about “The Grand Design,” however, isn’t Mr. Hawking’s supposed jettisoning of God, information that will surprise no one who has followed his work closely. The real news about “The Grand Design” is how disappointingly tinny and inelegant it is. The spare and earnest voice that Mr. Hawking employed with such appeal in “A Brief History of Time” has been replaced here by one that is alternately condescending, as if he were Mr. Rogers explaining rain clouds to toddlers, and impenetrable. “The Grand Design” is packed with grating yuks. “If you think it is hard to get humans to follow traffic laws,” we read, “imagine convincing an asteroid to move along an ellipse.” (Oh, my.) This is the sort of book that introduces the legendary physicist Richard Feynman as “a colorful character who worked at the California Institute of Technology and played the bongo drums at a strip joint down the road.” Mr. Hawking has written “The Grand Design” with Leonard Mlodinow, a fellow physicist who has also worked on “Star Trek: The Next Generation.” They’re an awkward pair, part “A Beautiful Mind,” part borscht belt. This book is provocative pop science, an exploration of the latest thinking about the origins of our universe. But the air inside this literary biosphere is not especially pleasant to breathe. ==endquote==
  6. Need Improvement, your addition to this thread, based on what you call the Hebrew Bible (the Old Testament?) has been moved to Speculations. Please continue your discourse there. The Astro/Cosmo forum is more for mainstream conventional science discussion.
  7. Some people, I understand, approach the question you have raised by assigning an entropy to the gravitational field, as well as to the matter. The entropy of the gravitational field is calculated in a way which at first seems odd or unintuitive. The lowest entropy state is the most evenly spread out! Because while a gas wants to spread out evenly, by its random molecular bouncings, the gravitational field wants to cluster and curdle and collapse. So for him, being uniformly spread out is low entropy. And being in various coagulated states is higher entropy. I remember attending a lecture by Roger Penrose where he was explaining this. There was a pretty woman sitting next to me. But I cannot satisfactorily recall Sir Roger's discussion for you. Swansont is certainly right about the radiation. For things to coagulate some gravitational energy must be blown off. Also in an expanding universe that radiation energy that gets blown off (to allow things to stick together) will eventually get redshifted to zero. There should be a source on entropy in cosmology. I don't have one handy. Someone mentioned Wald in this context. Do you have a book by Wald handy so you can check in the index.
  8. Yes. You say several things here---all correct as far as I know. AFAICS dark energy (the estimated 75%) is much more of a puzzle than dark matter (the 20%). The two are extremely different, as I suspect you realize, and you are talking just about dark matter. In my experience when people talk as if they think dark matter is somehow dubious or mysterious, they are usually relying on out-of-date information or on pop-sci media like "discovery channel". One can see and make maps of the concentration of DM around clusters of galaxies, using the "weak lensing" effect. Maps of DM concentration can then be compared and correlated with what one can see has happened to the clusters and what is currently observable. So astronomers have a pretty good handle on DM. Another good perspective is to google "Smoot TED". You get a 20 minute talk by Nobel laureate George Smoot where he shows movies of computer reconstructions of the collapse and condensation of DM in the early universe. The formation of cobwebby patterns and voids, as the stuff fell together. These were calculated using a dynamical model of how DM behaves in an expanding geometric context, under the effect of its own gravity. Indeed the concentrations of DM would have helped pull together ordinary matter to make clusters of galaxies, as you said.
  9. The research area you are asking about is "quantum cosmology". A natural place to do a keyword search of the professional literature is the Spires database at Stanford/SLAC Here are the papers on that topic which have appeared since 2006 http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+DK+QUANTUM+COSMOLOGY+AND+DATE%3E2005&FORMAT=www&SEQUENCE=citecount%28d%29 The papers are listed in order of citation count---the number of times the paper has been cited as a reference in other research---a rough measure of how important or influential they have been. This is not just about your particular idea. It is about the general question of what was going on, what might have led up to the big bang, started the expansion. I think your idea, in one form or another is probably part of the mix. The big question now is how can we observe effects in the cosmic microwave background that can serve to test the various preliminary ideas people have, and the various mathematical models of how the big bang was caused. The most recent paper I know of about testing was given last month in Paris by Aurelien Barrau at a major conference called the ICHEP. It may not mean anything to you but this gets into empirical stuff, space instruments, observational cosmology. Using a proposed model to predict details in the CMB which they can then look for with improved space instruments and check if they are in the data. The business needs to get off of a purely speculative track and contact the real sky. Here are a couple of recent papers by Barrau: http://arxiv.org/abs/0911.3745 http://arxiv.org/abs/1003.4660 If you go to those links and click on "pdf" you get the papers. They reflect what he had to say at the ICHEP conference this summer. The conference slides are here: http://indico.cern.ch/contributionDisplay.py?contribId=126&sessionId=47&confId=73513 It may not mean much or be very comprehensible (depending on your background) but in any case it gives a taste of how the observational side of things is beginning to creep in to pre-bang cosmology.
  10. "The bottom line is that CDF and D0 can now exclude (at 95% confidence level) the existence of a Standard Model Higgs particle over a fairly wide mass range in the higher mass part of the expected region: from 158 to 175 GeV. If the SM Higgs exists, it appears highly likely that it is in the region between 114 GeV (the LEP limit) and 158 GeV." http://www.math.columbia.edu/~woit/wordpress/?p=3073 New Higgs Results from the Tevatron Press release from Fermilab: http://www.fnal.gov/pub/presspass/press_releases/Higgs-mass-constraints-20100726.html So either the Higgs of the Standard Model does not exist or its mass is probably somewhere in the range of 114-158 GeV. That seems like progress. The Fermilab ring collider is now quite old but still producing results. Has not, as yet, been made totally redundant by the new LHC.
  11. There are now 6 known double quasars. http://hoggresearch.blogspot.com/2010/07/two-new-binary-quasars.html A July 23, 2010 blog entry: "...Given that there are only four binary quasars known previously, this was a pretty good day's work." A quasar is already a pretty interesting object, but having a binary pair of quasars going around each other adds additional zest. Here is an earlier article on binary quasars: http://www.sciencedaily.com/releases/2010/02/100203131413.htm A binary quasar is two supermassive black holes, each radiating a vast power output, that orbit each other. They can result when two galaxies (each with a central supermassive black hole) collide and merge into one galaxy, without their central black holes merging. The supermassive black hole in our (Milky) galaxy is only a few million solar masses, and does not radiate (it is not a quasar.) Many observed supermassive black holes are much larger---like billions of solar masses. Those that radiate do so because as they suck in the material around them it gets glowing hot. The hot material radiates and also gets ionized--- being charged some of it can then get diverted by the quasar's magnetic field and ejected in polar jets. Hogg's discovery has not been confirmed, and it has not been published yet. He only just now announced it on his blog. I know some of his other work and have a high regard for him, so I'll gamble that this will be confirmed and we will see something in the professional literature about it before too long.
  12. When you don't specify that it is visible luminosity, you mean wattage across the whole EM spectrum (including wavelengths the eye does not detect). That is measured by a bolometer---basically a broadband light meter--then adjusted for distance. Typical device you isolate the light from the star and see how much absorbing it heats the bolometer. You give the result in watts or in multiples of the sun's standard luminosity which is around 4x1026 watts. "Magnitude" is a logarithmic way of reporting luminosity. Like you suggest it is just a way of talking about it. For me, the absolute bolometric luminosity (or simply luminosity) is the only real measure. The rest is just ways of talking that have accumulated over time. Astronomers have a long history of accruing different ways of saying the same thing---each one appropriate to some context where it is convenient to talk that way. Like "apparent visible magnitude" is convenient for people out stargazing. They get so they can judge it by eye. Because stars on the "main sequence" that is typical stars not near the end of life follow a very regular pattern you can GUESS a star's luminosity simply from its color---its spectral lines (you know what I mean, the type of rainbow it makes). That is not measuring. That is guessing, based on past experience with other stars of the same color pattern where we already made a real measurement of luminosity. Estimating luminosity from the color pattern of the star is a whole other thing. To me, well I take this very simple view that the basic concept is actual energy output= wattage = luminosity, and there is only ONE way to determine it. Measure with a bolometer, plus then you have to know the DISTANCE. So the really interesting quantity to estimate is distance. There is a ladder of different ways of estimating distance and you may know something about that. For the nearest stars, parallax. Then the open cluster method. And Cepheids. And supernovae. And the HR diagram being used along the way as a kind of check. I'm probably forgetting some of the rungs in the distance ladder. That is one of the most interesting basic topics in astro. EDIT: I found the Wikipedia on luminosity. WiPi is not always so good or reliable, but it can sometimes be a big help and this article looked OK to me. I didn't read it all but this part seemed all right: The luminosity of stars is measured in two forms: apparent (counting visible light only) and bolometric (total radiant energy); a bolometer is an instrument that measures radiant energy over a wide band by absorption and measurement of heating. When not qualified, luminosity means bolometric luminosity, which is measured in the SI units watts, or in terms of solar luminosities, ; that is, how many times as much energy the object radiates than the Sun, whose luminosity is 3.846×1026 W. Luminosity is an intrinsic measurable property independent of distance, and is appraised as absolute magnitude, corresponding to the apparent luminosity in visible light of a star as seen at the interstellar distance of 10 parsecs, or bolometric magnitude corresponding to bolometric luminosity. In contrast, apparent brightness is related to the distance by an inverse square law. Onto this brightness decrease from increased distance comes an extra linear decrease of brightness for interstellar "extinction" from intervening interstellar dust. Visible brightness is usually measured by apparent magnitude. Both absolute and apparent magnitudes are on an inverse logarithmic scale, where 5 magnitudes increase counterparts a 100:th part decrease in nonlogaritmic luminosity. By measuring the width of certain absorption lines in the stellar spectrum, it is often possible to assign a certain luminosity class to a star without knowing its distance. Thus a fair measure of its absolute magnitude can be determined without knowing its distance nor the interstellar extinction, and instead the distance and extinction can be determined without measuring it directly through the yearly parallax. Since the stellar parallax is usually too small to be measured for many far away stars, this is a common method of determining distances. In measuring star brightnesses, visible luminosity (not total luminosity at all wave lengths), apparent magnitude (visible brightness), and distance are interrelated parameters. If you know two, you can determine the third. Since the sun's luminosity is the standard, comparing these parameters with the sun's apparent magnitude and distance is the easiest way to remember how to convert between them.
  13. Yes! take the square root of 10 million and it is 3150. So the corresponding distance is 3150 AU. Good way to think of it, which for some reason did not occur to me. Thanks. I am picturing vast schools of "solar goldfish" with photoelectric scales, swimming around in that immense spherical region approximately 3000 AU from the star. Basking in the fierce UV radiation. A new idea of "habitable zone"----1/10 if a lightyear in diameter---for things that can live in vacuum and like to eat UltraViolet light. They would need to have evolved or been constructed elsewhere because the star is necessarily young, with a brief life expectancy.
  14. http://news.yahoo.com/s/ap/20100721/ap_on_sc/eu_most_massive_star I see the stars "birth weight" is estimated to have been as much as 320 solar http://www.eso.org/public/news/eso1030/ It has already blown off a considerable amount of its initial mass and its mass is now estimated to be 265 solar. The estimate of luminosity may be lower than what I originally saw in the news. EDIT: The estimate of luminosity (wattage) of "near" 10 million times the sun seems to be OK. Several sources gave it. For me it is a stretch to try to imagine. Thanks to Sisyphus in post #3 for suggesting a way to imagine it---as bright as the sun is to us, but much farther away.
  15. Those are nice questions. I can't answer without doing some hunting. For now I will just tell you my superficial reaction which can easily be wrong. I know that DM density maps are made using weak gravitational lensing. They look like topographical maps of hilly terrain. The DM is mapped for galaxy clusters. The DM makes up most of the mass of the clusters. I have not heard of clusters of galaxies ROTATING. I thought that the galaxies in the cluster just have some random velocities, like stars in a star-cluster. Or like gnats in a cloud of gnats. I didn't hear about any organized pattern. But one can tell at least the radial component of these random velocities, of the galaxies in a cluster. For the cluster to be stable and not fly apart, these random velocities must be less than the escape velocity. So one can check that against what is estimated about the mass, including the large DM component. I hope someone else will find an example of one of those DM density maps superimposed on a visible cluster of galaxies. Weak lensing is a great way of mapping DM concentration! Maybe also someone else knows more certain details and can fill in. I have to go out immediately. But liked the questions and wanted to reply right away. Hello Fisica and welcome.
  16. Thanks Timo, I was glad to see the nominations, which already included some folks I think do a great job (and liven the place up!) Looking forward to vote.
  17. I inadvertently lost the subforum link to where one can nominate. Can you give me the link so I can go there and see who has been nominated?
  18. Hopefully other people will comment. Just for starters I think you need to define some reference frame from which you are looking at the early universe matter. Have to go. May be able to get back later.
  19. http://arxiv.org/abs/1006.2799 "On 15 June 2010 the Kepler Mission released data on all but 400 of the ~156,000 planetary target stars to the public. At the time of this publication, 706 targets from this first data set have viable exoplanet candidates with sizes as small as that of the Earth to larger than that of Jupiter. Here we give the identity and characteristics of 306 of the 706 targets. The released targets include 5 candidate multi-planet systems. Data for the remaining 400 targets with planetary candidates will be released in February 2011." In the past exoplanets have been detected by watching the to-and-fro wobble of stars. Kepler spacecraft detects planets by their cutting out a bit of the stars light as they pass in front. Using a large CCD retina it can watch a lot of stars at once. It doesn't have to do the delicate Doppler shift analysis to sense the star's motion. It can only "see" planets which pass in front of their stars. But it is a good cheap mass way to get a census of exoplanets. Once detected by Kepler, some will be studied by other (e.g. spectroscopic) methods. http://kepler.nasa.gov/
  20. It is a good thing to be asking about. Many people here can help by giving some more details. I can only start us off. There are several kinds of supernova. You are talking about the core-collapse type. There is another sort called "Type Ia" which involves a binary star, a red giant with a small white dwarf companion. It is the small companion which explodes. I think you don't want to learn about all types of supernovas, and the different mechanisms by which they explode. You just want to look at the typical core-collapse case. In that case you have a big star with has done all the fusion it can, meaning that it's largely IRON at the center. Iron is the final endpoint of the series of energy-producing fusion reactions. HERE'S the section on core collapse in the Wikipedia article on supernovae ==quote== The core collapses in on itself with velocities reaching 70,000 km/s (0.23c),[62] resulting in a rapid increase in temperature and density. The energy loss processes operating in the core cease to be in equilibrium. Through photodisintegration, gamma rays decompose iron into helium nuclei and free neutrons, absorbing energy, whilst electrons and protons merge via electron capture, producing neutrons and electron neutrinos, which escape. In a typical Type II supernova the newly formed neutron core has an initial temperature of about 100 billion kelvin (100 GK), 6000 times the temperature of the sun's core. A further release of neutrinos carries away much of the thermal energy, allowing a stable neutron star to form (the neutrons would "boil away" if this cooling did not occur).[63] These 'thermal' neutrinos form as neutrino-antineutrino pairs of all flavors, and total several times the number of electron-capture neutrinos.[64] About 10^46 joules of gravitational energy—approximately 10% of the star's rest mass—is converted into a ten-second burst of neutrinos, which is the main output of the event.[57][65] These carry away energy from the core and accelerate the collapse, while some neutrinos may later be absorbed by the star's outer layers to provide energy to the supernova explosion.[66] The inner core eventually reaches typically 30 km diameter,[57] and a density comparable to that of an atomic nucleus, and further collapse is abruptly stopped by strong force interactions and by degeneracy pressure of neutrons. The infalling matter, suddenly halted, rebounds, producing a shock wave that propagates outward. Computer simulations indicate that this expanding shock does not directly cause the supernova explosion;[57] rather, it stalls within milliseconds[67] in the outer core as energy is lost through the dissociation of heavy elements, and a process that is not clearly understood is necessary to allow the outer layers of the core to reabsorb around 10^44 joules of energy, producing the visible explosion.[68] Current research focuses upon a combination of neutrino reheating, rotational and magnetic effects as the basis for this process.[57] ==endquote== Elements lighter than iron can fuse and release energy. Iron is the complete dead end of fusion. Once you get up to iron you can't get any more fusion energy. A star needs to be producing heat at the core in order to support its outer layers. It is a war of heat pressure against the squeeze of gravity. Running out of fusion fuel in the core is dangerous. You mentioned the electron degeneracy pressure. So you are thinking about the Chandrasekhar limit. Around 1.4 solar masses. As long as the core has fusion fuel (elements lighter than iron) it can make heat and the heat pressure can support the outer layers. But finally there is just this big inert mass of iron in the center of the core. Approaching the fatal Chandrasekhar limit. Suddenly it is collapsing in free fall! Suddenly the iron is all converting to neutron matter which occupies a billionth of the volume. That's effectively nothing! So the whole ball is in free fall towards its own center. Falling not with the Earth acceleration we are used to but with enormously greater acceleration. Now what energy could "rebound" from that huge collapse and blow off the outer layers? One possible mechanism is a wind of neutrinos. When protons swallow electrons--to become neutron matter--they BURP neutrinos. And in this collapse to neutron matter such a huge wind of neutrinos is produced that, as I recall, it is able to blow away stuff. Even though neutrinos in ordinary numbers are not very interactive and pass thru stuff without exerting any force, in such large numbers they act like a blast of wind. That's how I was taught, and Wikipedia seems to confirm it. But probably the whole core-collapse scenario is not perfectly understood. There may be other mechanisms besides this neutrino blastwave.
  21. Michel and Johan, I was delighted to find you interested by Verlinde ideas and have two points: when a good physicist has a creative idea, it is often wrong. Verlinde can be on a wrong track. But, for whatever it's worth, some other smart people have now followed him and written papers pushing this line of investigation further. I'm intrigued and stimulated by the development on that front but try to be prepared in case it turns out wrong. Also Verlinde has a few pages of "blog" about his "gravity as entropic force" idea which gives some additional intuition, and opinion. I will try to find the URL. It is a helpful supplement to the paper. http://staff.science.uva.nl/%7Eerikv/page20/page18/page18.html there is also the wikipedia on Erik Verlinde, which gives link to somebody else's blog commenting on this, I don't know how usefully.
  22. OK, that is a good answer. I was just curious what is bugging you. It will have to go on bugging. When you ask what is geometry made of, you are at limit of knowledge and you stand around waiting, with the rest of us. Like at a bus stop, except we don't queue up in a line like Brits. There are very interesting papers recently by Eric Verlinde (Amsterdam) and Thanu Padmanabhan (from Poona, India). They say that geometry (the grav field) behaves according to thermodynamics and as if geometry had a temperature and entropy. They derive Einstein field equation from the laws of thermo. They say if you can heat something then it must have atoms. This was Ludwig Boltzmann's great insight. If you can heat something then it must have microscopic degrees of freedom that you can't see---so he realized there were atoms and molecules explaining the behavior of a gas, even before these were seen. Look up E. Verlinde on arxiv. Look up T. Padmanabhan. It is completely leading edge by so-far reputable people. Geometry is real. The metric is just our mathematical description of a real thing. (Like our math description of an electron or a photon.) But there may be deeper microscopic degrees of freedom---"atoms of geometric relationship"---which we cannot yet see, and do not yet know. Atoms of angle, of area, of curvature. And these can be heated and they can have entropy. (You know that DeSitter space has a temp? That a black hole has a Hawk temp? that an accelerating frame has an Unruh temp? Geometry is able to have temperature, in principle that you can measure.) Verlinde has derived Newtonian gravity law from "entropic dynamics". Gravity for him is an "entropic force". Read the January paper. It is all simple math and clear reasoning. This is where respected reputable people say things that sound crazy. So look up Verlinde and Padma on arxiv. Or don't. Don't think about it. Just wait. I think you will continue to be bugged. Why? The situation is clear: there must be something deeper than geometry, from which it emerges, but we do not yet know the correct way to imagine it. Personally the situation gives me pleasure:D, it is is the right position for humans to be in---to desire and hope, but not have. But you are equally right to be bugged by the situation. Verlinde is a former string theorist who now says adamantly that string is not the way to go and his way etc etc. Which adds a slight frisson. A whipped cream topping with chopped nuts, so to speak. Oddly enough an American, Ted Jacobson, published a proof around 1995 where he derived the Einstein equation (governing geometry) from thermodynamics. Just google "jacobson thermodynamics" and take the first hit. Padmanabhan is so excited that when he gives slide lectures about it he absolutely refuses to cite the work of Jacobson and Verlinde. He cites only his own papers! This in itself is a bit sensational, a histrionic gesture outside of normal behavior. He just gave a talk yesterday about it at Perimeter Institute, available online video. Such is the extraordinary ferocity that comes out in the good times. Passions are aroused. Whole thing is fun to watch. Better than if the bus would actually arrive.
  23. Nowadays there is a widely shared expectation among cosmologists that conditions referred to in #4 did not occur. There are various models of U around time of start of expansion, which so far fit data about equally well. No scientific reason to believe time began at a big bang singularity. We simply do not have answers: various models must be tested, some ruled out, and so forth. The idea that we "know" the U began with a "singularity" some 13.7 billion years ago is a delusion which has been fostered by commercial popularizations. You can sell books and illustrated magazines by peddling halfbake speculation that excites people. There are a few honest public outreach sources. The German research outfit called Max Planck Institute has something called "Einstein Online". They have an essay called "A Tale of Two Big Bangs" which goes into the different ideas people have and the sources of misunderstanding. It says "most cosmologists would be surprised if it actually turned out that the U began in an infinitely dense, infinitely hot, infinitely curved state." In other words this business is still being investigated but the smart money is betting on there NOT being a singularity in the sense of your definition #4. But keep an open mind. People are still working on models, running computer simulations, trying to figure out ways to test, and so on. If you want a taste of the professional literature (mostly too technical for general reader) here is a search covering research just since 2007. http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=dk+quantum+cosmology+and+date+%3E2006&FORMAT=WWW&SEQUENCE=citecount%28d%29 If you want the public outreach account, google "einstein online cosmology". That will get you here: http://www.aei.mpg.de/einsteinOnline/en/spotlights/cosmology/index.html Then click where it says "A Tale of Two Big Bangs". Clear, fairly up-to-date, and not so oversimplified as to be meaningless.
  24. Well you got me puzzled and intrigued. It's really up to AJB to clarify and settle any mismatch. He answered up front exactly right AFAICS, and he studies this stuff at grad level. If I remember right. But I have to admit that what you say interests me. I don't have time now to read a lot of past posts, but maybe you can summarize in brief. You are fine with the idea that geometry is dynamic---that distances change in response to matter. Then somebody says "I heard about something expanding, what is expanding?" Well it is actually that distances are increasing according to some diff. eqn. rule, the Einst. Field Eqn. (which governs the gravitational field=the metric that defines distance). But what can I say? He wants to know what is expanding? OK, so I give him a cheap answer. I say it is the geometry which is expanding. And you don't like this! Well, can you suggest a different simplified short answer that the guy can remember easily? Or is there some other problem? This is really AJB's baby, but I am curious what's bugging you.
  25. Nano and Johan, please, if you want to understand "what is expanding", focus on the main thing AJB said. Read it over, decide what you don't understand, and ask him more questions. This is exactly the right answer, it only needs more explanation filled in to suit the needs of whoever is reading. There is no fixed geometry. There are no particles swimming in the fixed Euclid geometry you are told about in highschool. Geometry (distance, area, angle relations) is dynamic and changing. Distances can increase between objects which are not in any ordinary sense going anywhere. The immediate local geometry around the earth and solar system is approximately Euclidean, so we don't see that kind of thing. But you have no right to assume that the distance between two objects which you consider for some reason to be stationary will not increase or decrease. Distances must change dynamically according to the law of GR, the einstein field equation. This is our law of gravity. It has been tested repeatedly and verified out to great accuracy, we have no alternative description of gravity that works as well---and it says that gravity = geometry. The grav. field is no more and no less than the geometry of the world. So it is dynamic because as we know it must be affected by matter. As matter moves the geometry must change. It is the geometry that is expanding, to answer your question. This is what AJB post says. Please take a minute or two and assimilate what this means, and ask further questions. Don't get off on some other topic like photons or the double slit experiment quite yet.
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