The infinite space of the Universe

Ok You are wrong - they are talking about space being finite - in the manner that Catholic Scientist has described to you. Travel in any direction in the Universe, and without deviating from your straight path, you will eventually return to where you started. This is almost certainly practically impossible, because the expansion of the Universe will outpace your attempt to reach your starting point. If the expansion should slow and halt, then it may be possible. The actual topology of the Universe that wraps it into a finite shape is not totally certain - in all likelihood it is spherical (as in the topology of the three-dimensional spherical surface of a four-dimensional ball) ALternatively, it could have the topology of a torus, or something more complex. It is possible that future detailed examination of the CMBR will reveal clues to the actual topology, should the Universe be spatially finite (still an open question) .

Fortunately NASA is not THE authority on reality The Universe looks flat because of inflation - these measurements are far more an indication of the extent of inflation, than a determination of the open/flat/closed status of the Universe.Take a beach ball, and look at a small patch to see how curved it is. Now blow the ball up to the size of the Milky Way. How curved does your small patch look now? Say you can look as far as 100 miles away across the surface - can you determine whether this surface is flat or part of a ball?

Yes, exactly. The trouble is the CMBR (actually the surface of last scattering) is not that far away in terms of the size of the Universe, so the triangle used is just not that large. In the same way that we can use a triangle on our beach ball, but if our triangle is only 100 miles per side, and the ball happens to be the size of the Milky way, we are going to conclude that our patch of balloon is flat. This is very simple.Edited by cavediver, : No reason given.

Yes, except the Universe is far flatter than the Earth. GR says that the matter/energy content of the Universe curves space-time - the natural motion of an object following this curvature is what we perceive as a "force" pulling the object towards mass, and we call this "force" gravity.

We don't, yet. It may be finite, it may be infinite. What you call the material content is just another aspect of space-time, and is as finite or infinite as the space. No, it is not a hypothesis. It is a (eventually) verifiable prediction of General Relativity, should the density of the Universe be above a critical value. There is no boundary - if the Universe is finite it is topologically compactified into a hyper-sphere, or possibly some other more complex topological object (e.g. hyper-torus) Again, there are no *last*galaxies. All of space is filled with matter, whether the Universe is finite or infinite.

Yes ** NO **Common misconception. "Gravity" is a perceived effect of the curved space-time. It has nothing to do with the generation of the curvature itself. That is a completely local effect: the curvature at a point in space-time is a function of the energy density at that point and the immediately surrounding space-time curvature. Yes, although the 'expanding faster than the speed of light' is a very imprecise concept and doesn't actually have to be true to still prevent someone from returning to their starting point. No - this is still finite. Infinite means infinite. extra distance generated per unit distance per unit time - so say an extra 10 metres for every 10^8 metres every year. However, we often swap this round to give km per second per Mega-Parsec, which ends up giving the cumulative recession velocity (km/sec) per unit (mega-parsec) distance. So at 10 km/sec/Mpc, an object 10 MPc away will be receeding at a velocity of 100km/sec. This recession velocity can exceed the speed of light becasue it is not a true velocity - it is simply a measure of how much distance is being generated between two objects (galaxies). There is no actual local movement of either galaxy, although observations of each from the other certainly show recessional movement.

In the old cosmology, this is exactly what happens... so what? Given that the Universe is around 14 billion yesrs old and strill expanding, even if it were to recollapse, it would take >14 billion years to do so. Why is this an issue?In modern cosmology we recognise an accelerative component to the expansion, suggesting that even if the Universe is finite, it will not recollapse.

Clearly, you do not Especially if you think weightless means in the absense of a gravitational field. Do you really think that the Shuttle, at an altitude of just a couple hundred miles, is outside the Earth's gravitational field?Escape velocity has a very strict definition, and it is the velocity required of an object to reach infinite distance from a gravitating body - precisely because the gravitational field does potentially reach to infinity.

Ah yes - whenever I used to get stuck in my quantum gravity research, I would reach for Chambers or the OED and low-and-behold, it would hold all the answers. There must be some really clever guys compiling those dictionaries. I wonder who they are...

And philosophers would have exactly what to do with this? Those of us who work in space-time physics do not consult philosophers, in much the same way as we do not consult dictionaries. Both will often consult us to ask us what WE think (metaphorically of course in the case of most dictionaries.) We have a fairly good handle on the nature of space-time. What underlies space-time is the current work in progress. Questions of the 'nothingness' of space, of space being merely a place to 'be', and of how space can 'expand' merely reveal the ignorance of the questioner and their unfamiliarity with the subject matter.Edited by cavediver, : No reason given.

Ah, it's not a good start, is it? You refer to a short *layman* description of curvature cosmology at the NASA site. I give you the correct understanding behind this, and you reply as above. So you are wasting my time already. Why would I want to waste any more on you?You ask 'what is absolute vacuum in physical terms?' - I have to reply matrix style that you need to understand your question before you can understand the answer. What do you mean by 'physical terms'? What do you mean by 'absolute'? And what do you even mean by 'vacuum'?Thank you for attempting to give me a hint. I can assure you, I don't really need one. And your sugggestion of 'empty 3d space' as a possible, if incorrect, answer is a bit bizarre.Ok, for those with an actual interest in 'an' answer, the problem is what fields are we considering for this vacuum? If we're just talking about the 'basic' linear matter/force fields (quantum electrodynamics- i.e. photons & electrons) with a flat background metric (space-time), then we have the simple quantum vacuum state of the fields. There are no real particles in the vacuum state, but the fields are certainly not zero valued as is demonstrated by the Casimir Effect (by reducing the field vacumm fluctuations below that of the true vacuum).This simple picture grows vastly more complicated when we hit the non-linear fields of Quantum ChromoDynamics and gravitation - their vacuum structures form global non-trivial backgrounds.

Yes, it is another non-linear set of fields. This is where vacuum studies get very interesting as you're into the world of spontaneous symmetry breaking and the Higgs mechanism (and associated Higgs boson of course) In its most symmetrical state, what we think of as the electrodynamic fields (photons, electron, etc) are wrapped up into the eletcroweak fields. But this is not the lowest energy state. The true vacuum of the electroweak fields breaks the symmetry and we get the weak force and electrodynamics effectively splitting apart. Yes, though replace 'vacuum' with 'space-time' and you have the basic idea. Well, a theory of everything at the very least has to unify the four 'force' fields and all the related matter fields into one master field. Ok, ready to go deep?Reality as we see it is simply a multi-layer of fields. There is some pre-existing topology, so for example lets assume these fields are wrapped into a sphere (surface of a sphere), but its more like a stretchy empty bean-bag in that there is no concept of shape, geometry or distance - jut global topology. One field gives rise to a concept of 'distance'; at each point in the field, its values give the distance between neighbouring points at that location. Over the entire surface, this gives rise to the global shape, curvature and large scale distance. This is the gravitational or metric field. Its local values are given as a function of the values of the other fields at that point and the neighbouring values of its own field. Local fluctuations in this field give rise to gravitational waves, and at the qunatum scale, gravitons. The other fields are the ones we discussed above. This is the totality of reality. What we think of matter, space, vacuum, particles, people, stars, voids, curved space-time, etc, are all just aspects of this layer of fields. Edited by cavediver, : No reason given.

I didn't actually specify a dimension of the reality before, but I guess we all assumed it was 4 dimensional (or 3 + time, depending on how you were picturing it.)In a string/brane/M context, the dimension would be 10 + time, and our reality would exist as a 4d sub-slice of this higher-d space. So the effective fields that we see in 4d are only part of the real 11d fields. The bulk is the whole 11d space. Your quote is explaining that the effective 4d gravity is decomposed from the 11d in a slightly different way to the other effectuve 4d forces.

No, not at all. SuperGravity was our original attempt at an 11d theory of everything, which was essentially just a Grand Unification of all of the fields, including gravity. Now we suspect that there is something deeper than a 'simple' unification that gives rise to the picture of fields that I have described - string theory/M-theory being possibly behind this.

No, but not finding it might well do so Yes, there are many possibilities, from supersymmetric partners of the standard model particles (photinos, winos, zinos, selectrons, squarks, etc) to min black holes - lots of exciting stuff