Maybe I didnt have enough coffee yet but just had this thought:

1) Space is defined by energy ( gravity warps time-space ). Without gravity there is no space.
2) The total energy in the universe is a constant! ? ( maybe not ) But first law of thermo dynamics states that energy can not be created.
3) if all of the energy in the universe has always existed. Then space has always been the same.

Lets thing of the big bang. The big bang would have been the biggest gravity well. With the amount of energy / gravity that would have existed at the early times of the big bang, time and space would have been so warped that even that small space would have been warped so drastically that realatively it would be equal to time-space relative to us now.

Alternatively the total amount of energy in the universe grew and continues to grow.

But that seems more like a white hole to me than a big bang.

Another, more mainsteam, alternative is that the mass/energy density of the universe has been continually decreasing. Pretty much the same matter in ever increasing (expanding) space.

Slipping from energy (which would include matter) to gravity, which is an effect of that energy. There's a relevant quote from Einstein (looking it up ...)

[my free translation] "If you take my answer not too seriously but look at it as a kind of joke, then I can explain it for you. Previously, we believed if all things in the world disappeared that space and time would remain. After relativity theory, space and time would also disappear with the things."

In other words, space without "things" (Dinge) in it is meaningless. So far you're not far off the mark.

Locally, anyway.

Here is where I have a disconnect. Why logically would space always be the same? Just because there has been no change in the total energy content? Why not the same amount in a larger volume? Diluted, so so speak.

All the evidence we have seems very much to point to our universe being much larger with a much smaller energy density now than at the time of origin of the CMB, for example, when things were (nearly) so hot as to prevent atoms from forming. And that was hundreds of thousands of years after the big bang.

I think you meant "think", but OK, let's.

Huh? Lost you there after the first sentence, with which I agree in part. At least after matter had formed, within a short time after the big bang, the gravitational force would have been humongous (technical term). Would it have been as intense as a black hole's, say? Probably not, or we wouldn't have a universe, just a big black hole.

Much earlier though, and we run into problems that could only be resolved by a theory of quantum gravity. Unfortunately, though we have some glimmers of what that might be, we aren't there yet.

If space-time is DEFINED by gravity. And space-time can not exist in a vacuum. ( I can see arguments against these two points but I believe them to be relatively accurate) Then all of the universe really can just be one, to quote you, "humungous" gravity well. One in which the measurement of space time is very condensed and time is very slow. How would we be able to determine a true distance or a true time why would our universe be different than universe that conceptually would be smaller but in a proportionatly compressed space-time?

Take for example our universe at the time when it was 1/2 the size it is now. What if the gravitational effect at that point space-time was condensed to 1/2 ( space codensed and time slowed to 1/2 the speed )

what would be the difference? the universe would appear ( actually would be ) twice the size that it actually is. So 2x a universe that was 1/2 the size would be equal to a 1x 1 universe.


Remember that in theory there are also white holes. white holes would have the same gravitational pull as a black hole but actually emit "stuff". Some say that every black hole is also a white hole and that a black hole also emits. So I am not sure that we wouldnt have a universe.

Maybe the universe is just the singularity of a black hole ;-)

What do you think the gravity well would have been like when the entire energy of the universe was the size of a dime? I would think it would be pretty deep. Maybe eternity at that state would be a moment in time. Maybe infinity would be compressed down to the dime itself.

Again ... if energy is constant AND ( an important and ) space time is defined by energy ( gravitational warping ) then I can make some sense of not being able to tell if we are small in a compacted space with slow time or large in a relatively empty endless space.

One more thing ... mathmatecally is there a maximum to how much of a compression of time-space could happen? I would assume that a universe that created black holes would have a gravity well deeper than a black hole. So at what point mathmatically does time stop and space be infinitely compressed?

That question, and the one about our current universe compressed to the size of a dime, are unfortunately outside (or nearly so) the realms of either current quantum mechanics (gravity too intense) and general relativity (particles too close together). So no one knows at present. If you are young enough, perhaps you'll live to the point where we have a theoretical answer to that. Maybe this year, maybe not for decades.

Hence my reference in my last post to quantum gravity theory -- which I hope to live to see.

Quantum theory would seem to put a limit on space and time, but at a very small / short level. Check out Planck time and Planck length. The answer would thus seem to be "no, not infintely."

Doesn't work that way. If the universe were half the linear dimension, the volume would be 1/8 what it is now. The mass density would be eight times as great, but still not enough to have much at all of a global effect on time. The universe right now is rather empty on the average. Eight times that is still pretty empty.

On the other hand, we have no evidence of the actual existence of white holes, so speculation on their properties is currently about as useful as figuring how many angels can dance on a pinhead.

Don't think so. Could we have black holes inside of black holes?

I provided an admittedly not entirely satisfactory answer to that in my latest post.

You mean to say we can't determine the energy density of the space we observe? That has been done, in numerous mutually confirming ways. In general, unless you're in the immediate vicinity of a massive object, the amount of warping of spacetime is pretty much zero.

Look at the very subtle effect that had to be found for light traveling right by the sun, that was the first confirmation of general relativity. Even the sun, big as it is, only makes a tiny ripple.

We're still trying to figure out stuff like what dark matter might be, and why the cosmological constant is not zero (which would be nice, but doesn't jibe with reality as we see it). I'm confident that given the fine minds working on these problems and the ingenious astronomers who keep giving us new and exciting information to work from, that we'll know a lot more towards giving you a complete answer in time to come.

How do you know how much gravity we are ultimately effected by? Maybe there is a larger gravity well that we dont understand? Maybe dark matter, maybe dark energy maybe a huge black hole, maybe the center of the big bang

The point is that our perception of how large space is, is just that our perception of how large space is. Can space appear larger? Can space appear smaller? I dont think that is being argued is it? How much smaller could it appear, how much larger? Could it appear to be 1/2 of what it is now?

The motions of objects we observe are consistent with only local, relatively modest concentrations of energy/matter and generally very weak gravitational fields. Black holes are an exception in the universe. We are not at the center of a black hole. The rest of what you wrote is just wild speculation, and doesn't get us anywhere.

I'm sorry, but I've had my say. Your questions in this block don't make a lot of sense to me, so an attempt on my part to answer them is probably futile.

Maybe someone else cares to pick up on this thread.

So how could you conclude that there couldnt be enough concentration of matter in the universe to shrink or expand the universe signifigantly?

Our earth is a small gravitational well, our sun is a slightly larger gravitational well, the center of our galaxy ( or whatever causes the stars to spin around the center ) is a larger gravitational well. How much gravity are we talking about? From the vacuum of space if we looked at our planet from a system outside of the gravitational effects of the sun or of the galaxy as a whole, how big is the gravity well that effects a rock thrown on the surface of our moon? grant it the strongest percieved force is from the moon itself. But everything on the moon including the rock is being
effected by the earth and everything on the earth and its satelite ( like the moon and the rock on it ) are being effected by the sun and the sun, the earth, the moon the rock are effected by our galaxy. Removing all these gravitational effects. How compressed would time-space be relative to the rock on the moon or to the surface of the earth.

The answers to your questions are in my previous posts in this thread, or can be obtained by some research and study on your part. I can't give you a comprehensive class on the necessary subjects in the space available within this forum. If you have specific questions, try the Q&A forum.

Perhaps you didn't get the message from my most recent post -- I'm done with this thread. Find someone else to speculate with.

I love my espresso machine.

I think I would say gravity takes place in space. Going from that perspective, then space is subject to a gravitational effect from all mass in all directions. On that basis space can be defined by its gravitation signature, i.e. no two places in space would have the same gravitational signature. Space, gravity and energy are inseparable. Though each has its own particular set of characteristics, where you have any one of the three, you have all three .

So I agree with #1 and wanted to offer my perspective to show how I arrive at that agreement with you.

Now as for the particular set of characteristics of gravity, mass is required first of all, right. In order to discuss your idea about gravity I should say that we may differ in that I propose that energy fills all space and when mass is present, mass interacts with the energy of space to cause gravity. So the effect is warped space like you say, but the cause is mass interacting with the energy density of space. Am I out of line with your thinking in this regard? If so, in your ATM idea what causes gravity and gravity wells; how do they work?

Someone urged me to comment here.

Tommac, you're really on a "tear" here at BAUT but I think you should calm down, stop talking, and start reading because basically all your ideas that I've seen so far in your many recent threads are based upon various elementary misunderstandings.

Or am I wrong in assuming you take your own suggestions seriously and really wanted well-informed comments?

If you are indeed serious, then at the very least, IMO you should carefully study http://www.math.ucr.edu/home/baez/physics/ before making more postings. But be aware that reading the FAQ (or popular books) won't even begin to give you the background you would need to study cosmology in a serious way! Indeed, many topics in gtr/cosmology really aren't accessible expect to graduate level students.

[EDIT: after this post appeared, someone other than tommac PM'd me to say in part that he thought I was urging tommac to depart from BAUT forthwith. That's not what I said at all! To repeat: I suggested that tommac take a BAUT break to study the FAQ and other resources before making more posts. Even that suggestion was contingent upon the assumption that he is not just kidding around, as it were. Bogie and others have suggested that BAUT can play a role in incrementally increasing understanding and I don't disagree with that. I hope it is clear that my concern is that youngsters with a serious interest in physics/cosmology start "learning how to learn" as soon as possible.]

You will also need to understand the relation between theory and experiment, between exact analysis and apprxomations, and to have some idea about the relationship between different theories. For example, you once again failed to state what theory you have in mind; in elementary discussions of cosmological models of the universe, generally only the gravitational interaction is relevant and our current default theory of gravitation is gtr, so I'll assume that you want to discuss gtr.

You might have seen any of these stated in popular books, and they all have a kernel of truth as informal descriptions of mathematical arguments put forward at various times and with various levels of sophistication in the research literature. But be aware that to seriously discuss this you'd need to know a lot about boundary conditions in PDES, theory of manifolds, Lorentzian geometry, etc.

At your level, it's probably best to think of it like this: according the Einstein field equations which govern gtr, the distribution of all forms of nongravitational energy and momentum (e.g. bits of fluid, EM fields) is proportional to the Ricci tensor, which you can think of as something like half of the description of the curvature of spacetime, and then via a differential equation the Ricci tensor determines the Weyl tensor, which you can think of as something like the other half of the description of the curvature of spacetime. The curvature tensor and metric tensor (which determines the geometry of spacetime) are closely related in manner which I won't even try to explain even informally (mathematicians understood the relationship long ago, but even graduate gtr textbooks avoid discussing this fundamental topic!). Finally, the geometry of spacetime determines how bits of matter, EM signals, and so on, move in spacetime, so we have a kind of feedback loop.

At the next level, you could think of the EFE as providing consistency conditions ruling out some motions of matter and light in conjunction with some distributions of energy and momentum.

As an example of why you need to know about Ricci and Weyl tensors, in FRW models the Weyl tensor vanishes (these models are "conformally flat") while in vacuum solutions of the EFE the Ricci tensor vanishes. You need to know about the differential equation I mentioned to know how moving mass-energy around can create gravitation and gradually curl up spacetime as we concentrate mass-energy in some "location" in "space" until we have something locally resembling a vacuum solution like the Schwarzschild solution. You also need to know about how spatial hyperslices and congruences of curves relate to elementary ideas of "spatial three-forms" and "timelike vectors" in str.

Your basic assumption doesn't really apply here, but it is not true even in Newtonian physics! You are claiming that constant energy implies static, which is nonsense (think of a highly elliptical orbit in Newtonian gravitation, for example).

I already explained to you why that's not true. The term "gravity well" refers to an isolated massive object, in both Newtonian gravitation and gtr. This is at the opposite extreme from a homogeneous isotropic distribution of matter and momentum as in simple cosmological models. BTW, you need to understand the closest Newtonian analogs of FRW models before you can understand FRW models. You also need a lot of other background which I gather you lack, such as linear algebra, differential equations, a solid grasp of Minkowski geometry and also some background in manifolds and differential geometry.

To repeat: you can't simply assume that "total energy of the universe" makes sense for simple cosmological models, in either Newtonian gravitation or in gtr. In addition, there are at least a half dozen distinct technical issues which you would need to be aware of before you can discuss energy and in particular exchange of energy/momentum between the gravitational field and nongravitational mass-energy and momentum. You'd also need to understand the role played by gravitational radiation, and so on. Much of this material lies beyond typical first year graduate students in physics (and for that matter, lies outside the knowledge of most working physicists). It's pretty subtle stuff and there are many crucial points you would need to appreciate.

I don't mean to single out tommac since he is merely a typical representative of a common phenomenon in BAUT and similar forums. I'd like to add some general comments which apply to many of the threads I've seen in my week at BAUT in the ATM forum and Q&A forum (when cosmology and gtr come up). Also, it seems that the best informed posters at Physics Forums have left, so these comments probably apply to Physics Forums and to other public forums where cosmology and gtr are discussed.

In my week of "slumming" at BAUT I noticed many very naive questions, asked by various people who obviously have not even read the FAQ, and even worse, most of the "answers" are wrong or not even wrong. I'm never sure how many people who read these public forums realize that you cannot learn or have a serious discussion of math/physics at a place like this; almost everyone will need to study graduate level textbooks and make computations and discuss with graduate faculty at a good university in order to learn the basics of gtr/cosmology, and even then there is no guarantee of success!

Something else I've noticed which worries me a bit: many posters recommend Wikipedia articles. Unfortunately, WP is fundamentally unstable and unreliable (it pains me to say so, since I believe in the principle that there should exist a -reliable- free on-line universal encyclopedia). This is particularly true in the case of highly technical subjects like gtr. I know the WP physics articles pretty well and many of them are at times good, but most suffer from imbalance, poor writing (or good writing munged by "edit creep"), inaccuracies (and subtle "sign-changing" vandalism turning correct equations into incorrect equations), or even outright woo promotion. So I would strongly urge anyone with a serious interest in anything to consult the best available published textbooks rather than relying on on-line resources.

Can I suggest that everyone at BAUT who has not studied several graduate level gtr and cosmology textbooks get into the habit of saying "If I understand this correctly" much more often? Because right now I would have to characterize many of the threads at BAUT and other forums as resembling the following:

Poster A: "1 + 3 = blue". Poster B: "No no no, 1 + spanish = rice". Poster A: "no, rice is white". Poster C: "3/0 = Paris".

You get the idea.

And with that I'll leave you to get on with more of the same.

of course thinking in three dimensional terms would help as well

as in a ball and points of connection to all other points in the ball , lines drawn , which crisscross , which crisscross , which crisscross which..........

leads to non-expansion interesting enough

Thank you Chris and to tommac: Well Tommac, we might not appreciate it when a “Chris Hillman” comes along and bursts our bubbles but in reality it also shows a serious well intended gesture from one who has paid the dues. It is rare to see someone take the time to present that side of the picture, the complexities of in-depth science and especially cosmology. It gives us a glimpse of the environment of interacting in a culture where those complexities are developed and moved along.

However, remember that there also has to be a place for us of less rigorous inclinations. BAUT is what we make it. If you want to be able to discuss cosmology in all of its implications in circles where “what ifs” are followed with contingent math that stretches the capabilities of the discipline then BAUT is not your refuge. But if you love to think about physics and the cosmos and want to run your ideas out there then you need to feel free to do that somewhere. I do it here.

This is the where we sort out whether the effort to truly understand the body of knowledge is important to us, or if what we want is to just talk with thinkers who are covering the same ground that the great thinkers had to have covered themselves before they made their impact on science. BAUT can be a basic beginning playing field where answers and sharing takes place from where we are, not from where the great thinkers are. The great thinkers who hang around here are enthusiasts. They are inspired by our desire to understand and they understand our human nature enough to know that we often send up trial balloons with our ideas fully expecting to be shot down, but intending to learn by the experience.

My advice is to participate, ask questions, acknowledge the answers, study, use Wiki if you want, and keep coming back with better ideas as you progress.

Thanks, Bogie!

Just wanted to add by way of emphasis: I agree with almost everything Bogie just said. I trust that careful reading of my previous post will show why this is not inconsistent with what I said there. My comments arise from a pedagogical concern for the welfare of students, and from a desire that all participants in BAUT should understand what they can expect to find here and that they should, as far as possible, get the most out of their participation here, as appropriate to their needs and desires.