Iggy

So you're perfectly happy telling everyone that the universe must be eternal and infinitely old based on nothing more than your gut feeling while faulting Krauss for basing his conclusion on all the available scientific evidence? When you base your conclusions on wish fulfillment then it probably does seem very strange what Krauss is doing. He is basing his conclusions on evidence. It's just the way scientists think. It's a fundamentally different approach.

Let me see if I understand... I'll quote what Krauss said and bold the key point: You're accusing Krauss of hubris for this analysis? I think you may have missed his point.

"currently current" is usually called the present instant. "currently viewed" is usually called the past light cone. Both are included on a spacetime diagram. You need only learn to make one to understand how a complete understanding includes both. If I agree that modeling spacetime gives a person godlike powers would you give it a try?

Not exactly. The amount of redshift is not only a function of what the galaxy was doing when it was emitted, but what space has done between the galaxy and us between emission and observation. Observations have to be made at several distances in order to model the universe. The observables (things like redshift and luminosity) made at one particular distance (and equivalently from one time in the past) can't be used to solve the matter and dark energy density which is what one needs to know to order to model the past, present, and future of the universe. closer observations are more representative of recent behavior. Something that is true if expansion is constant can't very well indicate deceleration. If you can't picture it I can offer an analogy. Imagine a long string of rubber covered with ants. Each ant is programmed to walk to the nearest ant larger than itself and it can see roughly 5 inches to accomplish that task. The string is 100 meters long and doubles in size every 5 minutes or so. You'll end up with clusters of ants accelerating away from each other. The greater the distance between clusters the greater the velocity between them. Our past light cone and the present instant aren't separate perspectives. They are different elements of a complete understanding that is in no way godlike. Signal delay is not the mysterious problem it apparently seems to you. Cosmology is quite lucky to be able to peer into the past. That just sounds like rhetoric to me.

With general relativity the attractive force of gravity falls off with the square of the distance and the repulsive term (the cosmological constant) increases proportionally with distance. The result is that things near you will want to be accelerated toward you and things further away will want to be accelerated away. The simple answer is that the equations of gravity determine the gravitational interaction. If you had a string of evenly spaced galaxies you would get clumps that collapse around the most massive parts and end up with a string of clumps of galaxies where each cluster accelerates away from the others. Like Spyman points out, the universe is going through something like that process now where small collections of matter are fighting to collapse and the larger collection which they make up are fighting to expand. The result is a bit like ripping cotton candy to pieces. Tension falls off with the square of distance. The force working against tension increases linearly with distance. When forces are not in balance you have acceleration. Over smaller distances tension is the stronger force and the acceleration is toward collapse. Over larger distances tension loses and the acceleration is toward expansion. The further away things are the faster they're moving away from us. The large scale structure isn't being held together. The small scale structure will all collapse in on itself much like the Andromeda galaxy is doing with the Milky Way.

Yes. If one assumes that the universe follows the cosmological principle then one can use the Friedmann model. The model assumes it. It is a postulate of the model. This is the fact: I'm not sure you exactly know what you're disagreeing with. It was the above quote.

Ok. You disagree with this statement: It is a proven scientific fact. Does that matter? I could quote some supporting material if it does... I guess you're free to disagree. Why do you think reality follows the principle of a non-evolving universe? Is it because you like that principle, or because you prefer to imagine reality following the principle? I mean... if your beliefs about reality are shaped by the way you prefer to imagine it even when all the evidence and facts disagree then it would hardly do any good for me to point out evidence and facts. You would just say "I disagree".

Objects appear more developed tomorrow than today if that's what you mean. Depends what you mean by sensible. Observability becomes a practical problem. yes time dilated. yes. right yes It is proportional if you express the age of the universe in conformal time and the distance in comoving distance. Infinitely far in the future the comoving distance to the CMB will be about 60 billion lightyears (one third larger *in comoving distance* than it is now). In other words, infinitely far in the future there will be 1.3 times as many galaxies as there are now between us and the CMB. Of course, both those galaxies and the CMB will be redshifted beyond observability exactly like things are when they fall into a black hole. The outer edge of the universe? The distance to the CMB approaches a comoving distance of 60 Glyrs as time approaches infinity. If you watch the mass that is currently emitting CMBR over the next billion years you'll see it develop into galaxies. Making sense of the diagrams on this page would be well worth your time.

That interpretation would be inconsistent with general relativity, so you'd have to disbelieve GR too. The usual interpretation is,

I'm sorry, Pantheory. I see you responded to my post again, but I really can't reply to the content seriously. You implied that you were a theoretical physicist while not knowing how the deceleration parameter worked. You thought the Hubble constant implied constant expansion over time. Your last post represents redshift as the sole indicator of expansion while referring to your "technical paper"... I'm sorry, but I would be incapable of responding as if we're having some kind of debate.

Nonsense! Another 999 billion years of argument and I'm sure I'll have you convinced with time to spare. Then... you deny all of cosmology, no? You disbelieve the big bang, for example? There is too much evidence that the observable universe was extremely hot and dense about 14 billion years ago to be seriously scientifically disputed. 'There is nothing new under the sun' seems like a predisposed impulse people have. If it weren't for those meddling scientists the philosophers would have gotten away with it too!

Icarus, I believe you would benefit from reading the following links. I'll quote a part of each, If you start with static coordinates and assume that galaxies are moving through space you will get different formula from starting with comoving coordinates and assuming that space is expanding. They are different formula because the variables mean different things in each. It isn't an error, it is just a different way of looking at things. Comoving coordinates are a much easier, more exacting, and more natural way of solving expansion.

It was the Prometheus/Pandora story, but I don't remember which rendition. Every Greek myth has 10 ancient authors and 10 versions after all... it may have been the Theogony because I know I've read it, but I really don't remember. It shouldn't surprise you though. Prometheus' brother gave each species a gift (speed, fangs, etc.) but ran out when he got to humans so the story has Prometheus making us upright and giving us fire to set us apart making us first among animals and whatnot.

It sounds like that was a reply to md65536's post. No, I wouldn't have said that. For me to feel the sun's rays means I'm inside the sun. Hearing my words in your head means your name is Iggy. My dog has been alive for 6 years so he has visited Alpha Centauri. I think I've learned better than to try and convince you otherwise. Again you say in different words that you have large questions in your mind and the universe is surprising, complicated, and difficult to model, and therefore the definitive predictions of well supported science are wrong and the definitive predictions of your gut feeling are right. I'm sure there's nothing more to be said. The answer to the opening post is that Krauss is correctly conveying the predictions of cosmology. It is a straightforward solution of the Friedmann equation using the parameters of standard cosmology.

well, I think the vector would be the same size as all the others on the red circle. Maybe this would be a good way of looking at it, So that all of the red lines are the same size, the ones where the tangent is the line of expansion too. It's important also that the lengths are proportional, so for example AB/AD is equal to AC/AE and AF/AG since velocity from the center needs to be proportional to distance. I'm pretty sure the standard wisdom is that this checks out. [edited for having proportional lengths transposed]

That's right. I do recall now the previous conversation we had regarding Hubble's law where you asserted something rather telling. Let me see... Ok... so, I'm sorry to say it is impossible to know the basics of relativistic cosmology while mischaracterizing it such. Had I remembered where you were coming from I wouldn't have pressed you for details about how it was in error. I should just have said, You are entitled to your opinion.

I believe this simply can be explained by errors in the Hubble formula. What can be explained? The model's prediction can be explained? In no sense does the prediction of a model lack an explanation. I can't understand what you're on about. Nice of you to correct cosmologists. What is the correct formula for angular diameter distance?

In the present (Lambda-CDM) model the angular size increases beyond z=1.5. The wikipedia quote I gave is correct, any cosmology calculator that includes angular diameter would show. and if the James Webb space telescope doesn't disprove everything we've learned about expansion there will always be things that remain unobserved. Bias does live in the margins of knowledge after all.

Very distant galaxies should appear large in the current model.

I'm not amused. Of course that's what it shows. edit: My apologies if you weren't trying to mess with me with that. Yes, the left image shows everything receding from the blue dot. The velocity at which it recedes is proportional to its distance from the blue dot. We are meant to consider how this appears to the black dot -- in particular, would things appear isotropic to the black dot if its observations are limited to the red circle. The answer is yes and it is shown on the right. If the black dot considers itself at rest then the four stars will be moving directly away from the dot at equal speeds. In other words, if you subtract the vector of the black dot on the left from each vector you get the vectors on the right.

I don't exactly follow. If you shoot a cannonball north and another cannonball northeast the distance between the cannonballs increases over time. Anyone who remains directly between the cannonballs will indeed say that they are both receding. I found a better reference for what I was pointing toward by the way,

I'm familiar with that story. I don't recall it ending with the creation of humans to keep the river in its banks, or the creation of humans at all. She gives birth to 8 gods to make Enki well. I think you might be confusing it with the Babylonian creation myth where Abzu was trapped in canals. This is what people typically refer to when they reference the Sumerian human creation myth: enki-ninhursag myth. The story you're talking about is directly above the highlighted portion. The next story, the one below, is the human creation myth. The Hebrew and Greek creation myths have humans being created as leaders of the animals, but the Sumerian myth is quite different in that people were created essentially as slave labor for the gods. G.S. Kirk said in The Nature of Greek Myths, 1974,

I was talking about this. I didn't see your topic. In the sentence I wrote t is the time of the observation, d is the distance between the observation and the supernova, and c is the speed of light. There was no second variable for time and there was no equals sign in the sentence I wrote because I wrote "the supernova happened at" in their place. [math]t_S = t_O - \frac{d}{c} [/math] ts is the time of the supernova, to is the time of observation. Are you trying to make this sensible to you, or are you trying to make it appear less sensible to others? The equation I gave ignored expansion. The galaxy was closer at the time of the supernova than it is at the time of the observation. If you want to calculate how much closer find the redshift (z = 0.045) and solve: [math]d_S = \frac{d_O}{1+z} [/math] If the galaxy is 600 million lightyears when it is observed, like you said, then it was 574 million lightyears when it was emitted (at the time of the supernova). This is no more exciting than calculating the distance of a car diving away from you. If you really want to learn this stuff I'm afraid internet postings aren't a good way to pick it up, and believe it or not, you aren't going to poke a hole (to borrow MD's analogy) in standard cosmology by asking elementary questions. Do you really want me to explain the function of c in the solutions of general relativity? Your confusion doesn't make the model any less reliable. Right. You can't understand it. You can't picture it. So, you doubt anyone else can. You must realize how absurd that sounds.

No, I think it is pretty well known that expansion would appear isotropic even if we weren't at the center and without the metric expansion of space. Unfortunately, the best link I could find is talking about white holes, but it gets the idea across, A collapsing sphere of dust isn't something that requires the expansion of space and it would appear isotropic to one of the dust particles that had a limited field of view (couldn't see the edge), so I don't think you can say that "physicists and astronomers had to find a way to explain this and as this couldn't be explained by dynamics, a new concept that "space expands" was introduced." I couldn't really comment on the rest of your conclusions, I'm sorry, I mostly just skimmed your post.

To which Sumerian myth are you referring? In the story I know humans were created to relieve the lesser gods of their work... farming, dredging canals, plowing... those kinds of things.