The accelerating expanding universe

Good reply (as is Oni's) - my only concern is this bit: No, unfortunately it isn't. Clearly it is not possible to measure the acceleration in real-time given the time-scales this acceleration is acting over. And the local motion of any particular object (galaxy, quasar, cluster) will totally drown out the acceleration. What you need to do is look at large groups of objects of a range of red-shifts and infer the acceleration statistically. Even now I'm not sure of the range of possible rates of increase of expansion, and the most sensible statement is still probably that:the null hypothesis of "no rate of increase of expansion" can be discarded in favour of the alternative hypothesis of "there is a positive rate of increase of expansion" at a very high degree of confidence.Edited by cavediver, : No reason given.

Not much Except that our coarse estimates of the acceleration show it to be fairly constant over the periods examined. If dark energy really is a result of a true Cosmological Constant, then it is constant. If it is the result of a dynamical field (quintessence), then it could well vary with time and may decrease, or even increase (remember the Big-Rip?) But observation suggests that any change is slow on the scale of the few billion years covering the earliest observations to present day.

Sure:1) No - this is a bit mixed up, and rather Newtonian.If we stick with the Newtonian picture, we can describe the old non-accelerating Big-Bang cosmology: the galaxies are flying away from each other following some initial "push". Their mutual gravitational attraction is slowing them down, but how much slowing is a function of how many galaxies there are per volume.There are two principle scenarios: a) the galaxies are sufficiently far removed from each other and moving suifficiently quickly that although they will always continue to slow, they will never stop moving away. This is the Open Universe; and b) the galaxies will eventually slow, halt, and start to move towards each other under their mutual gravitational attraction. This is the Closed Universe. There is a c), the Flat Universe, which is the limiting case between a) and b), where the galaxies will continue to expand, but "only just".Now we add to this the "dark energy", the on-going "push" that is causing the galaxies to move apart ever more rapidly. This is sufficient to ensure that the galaxies will never halt. But the on-going push is an active force independent of both the original expansion push of the Big Bang, and the gravitational attraction of the galaxies.2) Inflation occured very soon after the Big Bang, within the first second of the Universe. The expansion of the Universe is unrelated to inflation - inflation was an extra boost on that inflation - but it is possible that Inflation and the acceleration of the expansion are related, as these are very similar in nature. We call the field responsible for driving Inflation the Inflaton field, and the field for driving the acceleration is Dark Energy. It is just about possible that these could be the same field, behaving with very different energy scales at different times in the Universe.

Just another field - almost certainly scalar - with appropriate density and pressure. There is a problem with both the name Dark Energy, and the way it is presented, in that the suggestion is that we have absolutely no clue as to the nature of this mysterious substance. The truth is that we are "baffled" by Dark Energy in much the way that marine biologists are "baffled" by newly-found deep-sea creatures. They exceed expectations in weirdness and defy some previously held thoughts - but they don't need us to re-write physics, only a bit of biology. Dark Energy certainly extends our view of the Standard Model of particle physics, but it's par for the course for those of us in theoretical fundemental physics research.

Oh dear - I thought it was actually the best (most understandable) bit And knots are a velocity, not a distance - nautical miles are abbreviated to nautical miles

An EM field can be described as being made up of virtual particles (photons, electrons, positrons) - virtual particles don't exist as such, they are simply fourier modes that make up the larger field configuration. But just as fourier modes of a sound signal are easier to deal with than the complex sound signal itself, so too are virtual partciles easier to calculate with than the full field configuration.

I am sorry - I was very busy around Christrmas and by the time I had checked back with EvC your question had been buried. Good questions.Obsevation strongly suggests that objects at cosmological distances (say >20Mlyrs) are moving away from us, with velocity proportional to distance.The first conclusion could quite reasonably be that the reality is exactly as it appears - these cosmological objects are indeed simply moving away from us. This leads to the problem that we then appear to be located at the centre of the Universe. Why should we be located in such a special location within the Universe?Now, this would make complete sense within the paradigms of certain religions. However, it doesn't really stand up to scrutiny. The centre of this "exploding" Universe is not *us*, not the Earth, not the Sun, not even the Milky Way galaxy in whose outer reaches we reside. The "centre" is located somewhere within our Local Group of galaxies, of the order of 5MLyrs away from us, in the middle of empty inter-galactic space. That doesn't sound like the religiously inspired anthropocentric view espoused...So, the alternative is that we are nowhere special in the Universe, and our observations of a receding Universe are universal: this is what you would see at any other point in the Universe. This leads us to the ideas of isotropy and homogeneity: the Universe looks the same in all directions and looks the same from all points in the Universe. The assumption that this is true is called the Cosmological Principle.It is obviously false on sub-cosmological scales: the Solar System, Milky Way, and Local Group all fail to show isotropy and homogeneity. But as we go further out into the scale of SuperClusters we start to see the Universe becoming more regular. And as we reach the limits of our observation, we see the Cosmological Microwave Background Radiation (CMBR) revealing isotropy to an extreme level.Assuming the Cosmological Principle is true, that would imply that everything on cosmological scales is moving away from everything else. It also implies that there can be no "edge" to the Universe. The only obvious way we can arrange this is if the Universe is infinite. But now, every location in the Universe regards itself the centre of this expansion, with itself unmoving. To rationalise this, we regard the space itself as expanding, and the cosmological objects as unmoving. With this step, we can then further imagine a finite Universe that wraps itself up into a (hyper)sphere, where the expansion is caused by the expansion of the sphere.So, this is the conclusion we reach from observation. What about from theory? In 1915, Einstein published his General Theory of Relativity. This taught us about space-time, showing how it is much more than just the empty space within which "real" objects reside: it is an entity itself with very real properties and it can curve, warp, expand and contract. I'm sure you are aware of how well tested GR is - in fact, it is one of the two most well-verified scientific theories ever discovered! (in terms of the accuracy of its predictions confirmed by obseravtion)When we use GR to calculate the properties of space-time when we are looking at a smooth distribution of matter which is both isotropic and homogeneous, we find there are essentially two answers: space-time is infinite, and is either expanding or contracting; or space-time is finite and wrapped into a (hyper)sphere, and is either expanding or contracting. In all these cases, the matter is not moving, but the space istelf is expanding (or contracting).So we have arrived at the same answer, both by arguing the case from observation, and by considering the theoretical calculations. Hopefully this is sufficeint to convince you that it is indeed the space that is expanding.What does it mean for "space" to expand? Well, that is another question...Edited by cavediver, : tidying up

Hmmm, is your name Fred Hoyle, and is it 1948?Or are you just 63 years late??? This is precisely Fred's old argument against the Big bang comsology, and why he desperately tried to get the Steady State cosmology to work. He strongly believed that the Cosmological Principle should be applied to space-time, not just space, as you suggest. However, this is forgetting one essential concept: space and time, no matter how intrinsically linked, are NOT the same. Time appears in the metric of space-time with the opposite sign to space - we say that space-time has (3,1) or (1,3) signature; it is Riemannian or pseudo-Lorentzian, not Lorentzian; it has an indefinte metric, not a definite metric; etc.So, we do not necessarily expect to see the Cosmological Principle apply in the time direction, and this is exactly what we see when we look at solutions to Einstein's field equations of General Relativity, especially those of the Friedmann, Lemaitre, Roberston, Walker kind that give rise to the Big Bang cosmologies.Edited by cavediver, : No reason given.

I'm sorry, you are confused. The FLRW (for short) solutions to to the field equations of General Relativity are how they known. This is a reference to the solutions, not to the physicists who lie behind those names. Yes, if I did mean something as vague as that, it would be somewhat a rephrasing of your vague statement in your initial post. But of course, that is nothing close to what I mean. Now you see, this is what happens when you pick up the ball and run. First, make sure you've picked up the right ball... Oh, I don't think I would ever want us to agree The problem is that you speak with such confidence yet your words betray a very weak understanding of the subject matter. It makes for a somewhat wasted conversation. For example, to not understand my references to time appearing with opposite sign to space is to not understand General Relativity at its most basic level.The simple facts are that observation strongly suggests that the Universe is spatially isotropic and homogeneous, yet temporally differentiated. This very isotropy and homogeneity results in a "natural" frame of rest, and asscoiated "universal" time that does in some way resemble the fixed time of Newtonian space-time. However, this is simply the time associated with the cosmological co-moving frame, and is no more the "real" time than any other time defined by individual observers moving in their own frames.This is basic undergrad cosmology and General Relativity, and is soundly backed up by observation. Edited by cavediver, : No reason given.

No, he is just an idiot Please see here for utter inanity and inability to understand the simplist of space-time concepts: Nothing Can Move in Spacetime! By Definition!There has to be some illness that gives rise to "I can't possibly see how this makes sense. Thus, everyone else is wrong" Edited by cavediver, : No reason given.

What is "my" theory?And where is your evidence that Einstein did not like this theory, and found it absurd? Again, I think you have the wrong person. To which blunder are you referring?Possibly, I'm guessing, you are referring to his use of the Cosmological Constant in creating the Einstein Static Universe solution. That was no blunder in my book, no matter how much he berated himself for missing the opportunity to predict an expanding or collapsing Universe. No matter how distasteful he found the presensce of the constant term in the Einstein Equation, it is necessary for completeness without some further mechanism to facilitate its removal. And you criticise me for (alleged) appeals to authority Ah, you seem to be confusing Special Relativity for General Relativity. In an FLRW cosmology, there is no frame of reference that contains the Big Bang in its future light cone - for very obvious reasons when you consider the relevant space-time geometry. Like-wise, there is no frame of reference that conatins the Big Crunch in its past light cone. The Big Rip, however, should it exist... yes, that certainly lies in the past of some frames. So?

Yes, this is a common crank perspective - they have just about managed to understand the basics of SR and so are completely thrown by GR when it seems to contradict many of the "truths" they have learnt; and thus they conclude that GR, one of the most successfully tested theories known to science, must necessarily be wrong simply because of their own incomprehension. Don't you just love them Exactly.

There is no inconsistency and no contradiction. SR simply describes the local (tangent space) physics, and GR describes the global physics. In other words, GR restricted to the local environment, reduces to SR. Expansion (and inflation) of the Universe is necessarily a global feature and thus SR is not applicable. Locally, the speed of light is never violated, and that is all that matters. Again, your incomprehension is simply your own - not that of the rest of us.Edited by cavediver, : No reason given.

Much of Alfred's ramblings are confused and meaningless, but this does actually mean something The Copernican Mediocrity Principle is essentially the first version of what we now call the Cosmological Principle: there is nothing special about our spatial location in the Universe. Alfred, much like Hoyle many many years ago, feels that this should apply also to our temporal position, i.e. there is nothing special about our location in time. It is a perfectly acceptable and reasonable philosophical stance except that it is highly contradicted by observation and theory. Because Alfred feels that his philosophical position should hold more weight than the entire world-wide body of professional astronomers and comsologists, he believes that our observations have been misinterpreted and our theory of space-time defective.However, Alfred's above post is quite correct in that nothing in SG's post suggests that General Relativity should give rise to a situation where there Copernican Principle does not apply in the time direction. The confusion arises because General Relativity is not a theory of the Universe - it is the mathematics that generates theories of space-times - a meta-theory. When we use GR to construct the space-time around a black hole, we see perfect time symmetry. We only see the lack of time symmetry when we ask GR to produce a theory for the space-time of the Universe. Alfred doesn't like this. Tough

Hmmm, that's a big question, and it's difficult to know where to jump in. I suppose the simple answer is that the Universe is not translation symmetric in the time direction. In other words, it looks different at different times. The stars that existed before the Sun were of different composition, and the generation before that were even more different. As you go further back in time, the Universe appears denser and hotter. If you look back close to the 14 billion years, the Universe suddenly becomes opaque.Consequently, we can't say that we could be at any time in the Universe, because the Universe wasn't suitable for our existence at those prior times. It took nearly 10 billion years to have the heavy elements in adequate abundance for us to coming into being. We are thus forced into considering abiogenesis. In a Universe that perpetually looks like the Universe today, life could have always existed!