Can anyone explain what is meant by the critical density of the Universe?
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The critical density is the calculated density at which the universe will NOT expand forever, but will eventually come to a halt & go into the "big crunch". The actual density of the universe is not known with sufficient accuracy to determine if its density is or is not critical. This depends on the total amount of "dark matter" in the universe and whether or not neutrinos do, in fact, have mass.
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If you consider that density is defined as mass per unit volume, to claim that something has infinite density and zero mass is entirely false.  What I think you meant to say is infinite mass and infinitesimally small volume which would mean that density is infinite, too.\n\nHey, Jesse P.  I'm not sure if you were talking to me when you said that "because mass is zero infinite density is possible", but if that's the case, I was merely saying that to have zero mass would be impossible and make density non-existent.  There is no answer for density if mass=0 since mass is in the denominator and we know about dividing by zero.  Thus, it has to be infinitesimally small, but not zero.  I agree that it may be easier to understand conceptually to think of it as zero, but my point is valid.  I guess the absolutely accurate thing to say is that density has a limit of infinity as mass approaches zero.
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Other pages in this Astrophysics/Cosmology section are: \n\nGravitational Entropy (June '98) \nA Debate on Open Inflation (November '98). The lecture is saved as a postscript document. \nQuantum Cosmology, M-theory and the Anthropic Principle (January '99). \nRotation, Nut Charge and Anti de Sitter Space (February '99). \nLinks \n\nHome \n--------------------------------------------------------------------------------\n\nPhysics Colloquiums - Inflation: An Open and Shut Case (April '98).\nSlides and audio for this talk can be downloaded from the ITP at UCSB.\n\nThis talk will be based on joint work with Neil Turok, at Cambridge. Neil has always been interested in what might be called, alternative cosmology. He pushed the idea that topological defects like cosmic strings or textures, were the origin of the large scale structure of the universe. And he was a proponent of what is called, open inflation. This is the idea that the universe is infinitely large, and of low density, despite having been through a period of exponential expansion, in the very early stages. My opinion was, that these were all nice ideas, but that nature probably hadn't chosen the use any of them. I included open inflation in that list, because I believed strongly that the universe came into being, at a finite size, and I felt that implied that the universe now, was still of finite size, or closed. However, after Neil gave a seminar on open inflation in Cambridge, we got talking. We realized it was possible for the universe to come into existence, at a finite size, but nevertheless, be either a finite, or an infinitely large universe now. My talk will be about this idea, and new developments that have occur since then. One of these, which is so recent that it is not yet fully worked out, is that it seems there is an observational signature, of the kind of inflation Neil and I are proposing. The present measurements are not sensitive enough to see this effect, but it should be possible to test for it, in the observations the Planck satellite will make.\n\nAnother recent development, is that observations of supernovas, have suggested that the universe may have a small cosmological constant, at the present time. Even before these observations, Neil and I had realized that if there was a four form gauge field, one could invoke anthropic arguments to make it cancel the cosmological constant, that one would expect from symmetry breaking. But the anthropic argument would not require it to cancel exactly. So there could be a small residual cosmological constant. This is exciting. I shall describe the observational evidence later.\n\nAs you probably know, the universe is remarkably isotropic on a large scale. That is to say, it looks the same in all directions, if one goes beyond such local irregularities as the Milky Way, and the Local Group of galaxies. By far the most accurate measurement of the isotropy of the universe, is the faint background of microwave radiation, first discovered in 1965. At the present time at least, the universe is transparant to microwaves, in directions out of the plane of our galaxy. Thus the microwave background, must have propagated to us, from distances of the order of the Hubble radius, or greater. It should therefore give a sensitive measurement, of any anisotropy in the universe.\n\nThe remarkable fact is, that the microwave background is the same in every direction, to a high degree of accuracy. It wasn't until 1982, that differences between different directions were detected, at the level of one part in a thousand, with a dipole pattern on the sky. However, this could be interpreted as a consequence of our galaxy's motion through the universe, which blue shifted the microwave radiation in one direction, and red shifted it in the opposite direction. It need not represent any intrinsic anisotropy in the universe.\n\nIt was not until 1992, that tiny fluctuations on angular scales of 10 degrees, were
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Sorry to Sun of Samsa.  I meant to give you a thumbs up!  The Black Hole Singularity is infinitely small (infinite density and ZERO volume).  The effects of the hole can be miles or light years across!
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Thats a question that physics is still trying to answer.  \nThere are 3 types of theories on how big the universe is and the shape of it.  It is either flat, meaning it will always be this large and continue forever.  There is the closed ellipse type, meaning the universe started from something, but unfortunatly will end after its reaches its max capacity and starts to shrink again.  There is still the hyperbola theory meaning that the universe started at a finite point and is always expanding.
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Scientist have two theories one is that universe does not have a center point and therefore it cannot have a ending or edge. the other theory is that since the universe is so vast that pin pointing the "edge" of the universe can be harder than it looks.
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its actualy String-theory that proposes the existance of many other universes. the theory proposes a so called "Landscape of probabilities" with probabilities of as many as 10 to power of 500 universes, and with that many probabilities it is very likely that some of them will be similar to or just like our own universe.\n\nif you would like to know more about this facsinating subject, i would suggest this book http://www.amazon.com/gp/product/0316155799/104-3626905-5554311?v=glance&n=283155\nits a great and easy to understand book.
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The cosmological constant is a somewhat arbitrary number assigned to some of the relativity equations.  In a very real sense, Einstein put the constant into his equations to "make things come out right." [See source.]\n\nAs a constant proportional to the energy density of the universal vacuum ρvac, it follows no laws...it's just a number with 1/m^2 units.  What makes it interesting is that the shape of the universe is mathematically derived depending on what value is given to the normalized version of the constant.\n\nThere are three possible shapes of the universe: flat like a pancake (G ~ 1), spherical (G > 1), or saddle shaped (G < 1); where G is the normalized cosmological constant.  Not too long ago, the consensus was that the universe was spherical, but now many, if not most, believe the universe is flat. [See source.]
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The idea is that we use a particular type of supernova (type Ia), as a calibrated candle.  By looking at how the supernova dims over time we known intrinsically how bright it was, and thus how far away it is, helping to link the particular redshift distance it is at.\n\nAfter two groups independantly studied these "high-z" events, they both came to the same remarkable conculsion:  By getting measurements of how the universe was behaving long ago (from these high-z events), they see that the universe is actually expanding faster today than it was back then.  The expansion of the universe is accelerating.\n\nThis cause everyone to stop and think - what is causing the expansion?  Couple this result with recent WMAP results about the structure of the universe, and you find that your best models describe the universe today being dominated (from an energy standpoint) by what now goes by the name "dark energy," an energy-like term that drives the acceleration of the universe.\n\nNow where does the cosmological constant fit in?  Well if you put Einstein's cosmological constant back into the equations of universal expansion, you can set a value of it so that the expansion matches what we observe.\n\nLong story short, every type of energy makes the universe behave a ceratin way, at very early times the universe was dominated by radiation, later it was dominated by mass, now it seems that we have eneterd an era that is dominated by dark energy, and a cosmological constant is curenyly a pretty good mathematical solution to this.  What is it "really?"  Don't have a clue, but there is a nobel prize waiting for someone to figure it out.
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a lot more, actually there is a whole "landscape" full of universes, with probability's of more then 10x500 that's 500 decimals, the cosmological constant currently predicts that the universe is not heading to the "big crunch", but rather it will continue expanding unless the cosmological constant changes which is very unlikely to happen.\n\na great book to read on that subject is "the cosmic landscape" by Leonard Susskind, this book is mind boggling.
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Here is the mathematical formula:\n\nThe universe is endless.\n\nSo are the possibilities of having a life-form like humans elsewhere in the universe.\n\nEndless divided by endless equals to unknown
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The number of atoms in the universe is not decreasing...\nThey are spreading out as the Universe expands...
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if the potential gravitational energy from all the mass in the universe exceeds the kinetic energy of all the matter, the universe is considered closed.  so, if the density is high enough, that should do it.  the expansion of the universe would eventually stop, and it would contract back on itself.  this is the definition of a "closed universe" -- one that will eventually stop expanding and then begin to contract.\n\nrecently, though, evidence has been found that the universe is not only expanding, but the rate of expansion is increasing -- a very bizarre conclusion under classical theories.  if this is the case, the universe will never close.  the current most popular theory for addressing this evidence is that of "dark energy".  see the link below for more information on how this conclusion was arrived at, as well as theories involving "dark energy".
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Osmium is the densest matter here on planet earth if I recall correctly from chemistry.  Black holes, white dwarves, neutron stars, the big bang singularity... Are all working theories at this point.  Since we can't simulate them in a lab and can't travel to them to sample them, we are stuck with mathematical testing.  So if you want something that you could pick up (you might need a protective suit), then it is osmium.  Otherwise the highest density occurred about 15 billion years ago at the start of the big bang.
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the universe is finite yet has no boundaries. it may be something like the two dimensional surface of a sphere.  nothing inside or outside the sphere exists for the two dimensional surface.  nothing exists outside the universe in this model.
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Here you go :D This is the Definition  (astrophysics)\n\nA point in space-time at which gravitational forces cause matter to have infinite density and infinitesimal volume, and space and time to become infinitely distorted.
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From all the evidence science has on the subject, the universe began spontaneously as a point source (singularity)of infinite density and energy. At the instant of creation, the temperature of the universe was probably greater than 10e12 Kelvin (1.8e13 Fahrenheit) The best evidence we have today that this is a valid theory (called the "Big Bang") is that all major structures in the universe are rushing away from each other, and the universe today is bathed in the remnants of that initial heat, now cooled down to -454 degrees Fahrenheit.