Rather than screw up a good thread I'm gonna pull out the above quote and ask a question.

I understand that the big bang is not an explosion per se but there is an expansion from a singularity which would be a singular point in space and so all the momentum is outward away from that point but why would that outward momentum ever combine as the above quote indicates to create motion other than pure radial motion?

I understand that if all the momentum of the universe were added up it would all sum to a zero momentum value but my question above should not be affected by this observation, I would think.

When I see galaxies spiralling I conclude that two opposing momentum masses met and their interaction resulted in rotational motion but why did the pure radial motion become somehow opposed?

Hum,
it is perhaps better just to say that there was no momentum given to objects during the big bang implosion, and that the total momentum is zero.

The galaxies we see are not moving through space, they are moving with space.
It is space that is expanding (ie, think cherries in a cake as it is baked )

BTW, the spirals we see are due to shock/density waves `rotating` and don't reflect the true motion of the stars.

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`Irony` actually does mean `metal like`...

This is a good question, and a partial answer comes when you notice that the spiral galaxies swirl in all different directions. This means that globally there is no "net swirl", so the large-scale motion is just radial (and is best not viewed as motion at all). Nevertheless, on smaller scales, instead of no swirls, you tend to get swirls in opposite directions. This is just how things work. Have you ever watched the bubbles come off the hand of someone swimming? They tend to set up opposing swirls, rather than just going in a straight line. Or smoke coming off a train leaving the station, same thing-- opposite swirls. Swirling motion tends to self-create in dynamical systems, it doesn't need to be there at the start. In baseball, the most difficult of all pitches is the knuckleball, because it is hard to throw a ball with very little spin.

Sorry, Ken, but in each of your examples there are two, or more, bodies colliding to produce swirling motion and so we have a driver and a driven.

The hand drives the water, the smoke drives into the stationary air, the hand drives the ball (hopefully without imparting spin).

In the big bang description we have pure radial motion outward and separating - there are no collisions to set up rotational motion.

When two photons collide they produce matter/anti-matter but there is a collision of opposing momentums involved.

They are over-simpified examples, to be sure. The real mechanism one must analyze in swirling spiral galaxies is that of gravitational instability. Once you get matter to compress, you have a source of gravity, that can induce bulk motions into a flow. That is like the hand pulling the water. That introduces a shear between matter falling toward the "attractor" and less affected matter, and that means you have local angular momentum. Granted, globally it will add to zero, but not locally. So when that local region undergoes its own gravitational contraction, voila, you have a spinning galaxy. Pretty much any time you have dynamics, you'll have pockets of angular momentum develop, and when you contract those pockets, you get a pretty obvious spin. That's why I say it is the lack of spin that is hard to achieve, in general, even though my examples are not to be taken too literally.

But there are gravitational instabilities, and that is the source of nonradial flow. Don't get me wrong, your question is quite a good one, but simulations of gravitational instabilities achieve spiral galaxies without "putting in" anything special other than Newton's laws.

But, nowhere is it said that everything has the exact same speed. If one object moves (radially) past another object, gravitational interaction will impart a transverse component to their trajectories.

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Just one example of how 'non-radial' motions can arise.

At some time in the history of the universe, there were decays of short-lived particles. In each of those decays, more than one particle was emitted. To conserve momentum, the decay products exited in at least two directions. These directions were random, wrt the 'radial motion', so almost all of them produced particles moving in the 'transverse' direction. From then on, elastic collisions randomised the direction of motion of the particles in the (local) region of the universe where the decay originally took place.

So, how early in the history of the universe were there decays which produced at least two particles? Very likely well before the regime that we can probe using the highest energy regime we know today, from particle physics (Fermilab, CERN, and all that).

I have never heard of this "shock/density wave" description could you elaborate upon this?

It's true, the spiral arms are where stars form, but stars form from gas, so the spiral arms are really defined by what gas does, not what stars do. Gas can have "density waves", which move according to their own rules. For example, the gas is in Keplerian orbit, so any feature carried in the gas itself would get all wrapped around due to the fact that the inner orbits take much less time than the outer ones. Spiral arms don't do that, because they are their own features that rule the gas, not features that are carried around by the gas.

Kaptain K, I am thinking of earlier when the universe was pure electromagnetic radiation, all moving at the same speed: c.

I have thought those same thoughts, Nereid, but, from what I have heard, in order for EM radiation to produce particles the two photons must collide (exactly opposing direction/momentum??) which with all the EM radiation moving radially outward and spreading then I just do not see such collisions occurring.

Ken G may be on to something in regard to gravitational instabilities but then I think "how long will two parallel beams of light travel before their mutual gravitation causes them to merge?" or in other words: if the gravity between two lasers is barely detectable then a slight gravitational instability would be even less, I would think.

That is not correct, and it is the root of your misconception. The singularity is not a "singular point in space", by any conception. It is the entire universe, all of it, which is singular at the bang. There is no radial expansion from a point in space, because all of the points in space are singular, simultaneously.

Consider for a moment, the 2D surface of an expanding sphere, and how it would appear to a 2D observer on that sphere, as an analogy to the expansion of our universe. As the sphere expands, its radius grows in length, and its surface area increases. A 2D observer on that surface cannot physically see the radius of the sphere, because it exists in 3 dimensions, and the observer is rigorously constrained to 2 and only 2 physical dimensions. The observer can only see the 2D surface of the sphere. Now, ask the question: Which point on the surface of the sphere is the center of expansion? Obviously, the answer is "none", because the center of expansion is hidden away in the invisible & inaccesible 3rd dimension. The 2D observer can see that their 2D universe is physically expanding, growing in surface area. They can see that the radius of curvature of the surface is increasing, because they can measure it (by seeing the changes in a triangle constructed on the surface, for instance, at least in principle). But they see the entire surface expand in every direction, and they do not see any expansion from any conceivable point on their surface, neither physically, nor mathematically.

The case is the same for our universe, except that our surface is 4D (spacetime). We are no more able to see expansion from any point in that 4D surface, than are the observers on the 2D surface. They cannot physically access the 3rd dimension, where the expansion is "really" happening. Likewise, we cannot physically access the 5th (or higher) dimension, where the expansion is "really" happening. But in both cases, we can access those dimensions mathematically. In the colliding branes idea in string cosmology, for instance, our 4D spacetime is treated as a surface ("brane") embedded in a 5D "bulk" mathematical space.

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The point of philosophy is to start with something so simple as not to seem worth stating, and to end with something so paradoxical that no one will believe it. -- Bertrand Russell

Not so, there's very little question that gravitational instabilities are the fundamental cause
of structure in the universe. They would also be the source of shear motions, and ultimately, spinning galaxies. But there also has to be a "seed" mechanism of some sort, to give the instabilities a head start. One idea is that "inflation" of quantum mechanical fluctuations provided that seed. Dark matter is also needed to give the fluctuations the gravitational punch needed. Attractions between photons play no significant role.

This is exactly correct, So "SOMEHOW" 'space' was warped (in addition to the 'dent/well' caused by the baryonic matter) into a spiral structure surrounding a Massive Black hole, right out in the 'middle of nowhere'.

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RussT
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Everything is, as it should be, otherwise, it wouldn't be!

Bogie and Nereid are having a discussion in ATM about this very thing and he keeps explaining how 'space' had to already be here, and she keeps asking for him to show how his EEP's can be shown to exist at Planck time of the Big Bang.

So, my questions are these, IF our part of the universe, the part we can see since the 'last scattering', started out Planck size (where the unification of GR and QFT Must take place in the BB model), and with Inflation, expanded exponetially to the size of a grapefruit, and now our universe is 13.7 billion years old, expanding radially for that long, then...
1. why do we see most if not all of the expansion in 'Many' Voids?
2. wouldn't all the spheres of 13.7 billion years, that are outside our sphere, need a grapefruit at their center, for 'space' to expand everywhere at once, in the beginning?

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RussT
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Everything is, as it should be, otherwise, it wouldn't be!

Perhaps a (perhaps over-simplified) review of some of the key features of the current cosmological models might be in order.

Tim Thompson has pointed out some of the misunderstandings (again), one of which is this 'singularity'. Without a working quantum theory of gravity, the nature and evolution of the universe in the Planck regime cannot be studied - QM and GR's mutual incompatibility just gives nonsense.

Between the first Planck second (~10^-44 s) and the highest energy regime we can say something about, based on particle physics, is a very long time (comparatively speaking; measured in Planck seconds, for example). That this was an era dominated by radiation seems OK (leaving aside inflation), but 'dominated by' is not the same as 'there is only photons'. As the universe cooled, and the strong, the weak, and EM forces 'froze', lots of particles, of many different kinds, would have been created - plenty of opportunity for 'non-radial' motion.

But to a more interesting question - what were the initial seeds, which later became galaxies? As Ken G has pointed out, this is not well-constrained, but quantum fluctuations do seem to fit at least one bill.

Finally, was the early universe hot? And what do we mean by 'hot' anyway?

When we refer to these time demarcations and even up to the first 3 minutes whose time are we using? All the mass of the universe is pressent within the spherical radius of a proton and so time dilation would be pretty severe and actually going backwards from normal time - so how can we say what is happening in the first second, unless we are subconsciously refering to "earth" time?

The entire universe was compressed into a sphere the size of a proton and although that does not represent a true mathematical point it is a point for all practical purposes, in my opinion.

I have thought in terms of the expanding balloon analogy but my mind keeps going back to the momentum argument because if the universe was as it is now, only very compressed, then who is to say when it came into being: at the size of a proton or at its current state?

What the universe was doing at that point has nothing to do with your question-- the nonradial motions come from gravitational instablilities long after that point. Some questions are about origin while others are about evolution. The former currently lacks any describable theory, while the latter is the Big Bang and includes the explanation of spiralling galaxies.

"FRW spacetimes come fully equipped with a specially distinguished time coordinate (called the comoving or cosmological time)" (source, my bold).

At what time, in a non-inflationary universe, was "[a]ll the mass of the universe [..] pressent within the spherical radius of a proton"?Let's take the size of the proton as a fermi (10^-15m). That's ~10²⁰ times as big as the Planck length, or much the same ratio as a millimetre to the distance to Alpha Centuri. I guess that you and physicists have a very different conception of 'for all practical purposes', when it comes to the early history of the universe, in cosmological models.And that's a very good question!

What folk who study this question from a scientific perspective ('cosmologists', 'physicists') try to do is use the best theories they have, today, to model the universe.

The best theories we have today are General Relativity (GR) and the quantum theories of the strong, weak, and electromagnetic forces; these last three are wrapped up in a neat package called the Standard Model (of particle physics). If you use just these two (GR and the Standard Model), you can 'run the tape backwards' to an earlier state of the universe, and do so consistently*. However, you cannot go back further in time, without extrapolating from (or extending) the Standard Model. For example, the neutrinos are massless in the Standard Model, but we know from good observations and experiments that at least one kind/flavour of neutrino is not massless.

How far does this take you? Back to the time when the universe had a temperature approximately the same as that of the most energetic collisions in our particle accelerators. That's a very great deal later than a Planck second!

*'consistency' here means 'internally consistent', and 'consisent with all relevant observational and experimental results'.

As we all know matter has momentum and even electromagnetic radiation has momentum which was all expanding/moving radially outward so I wonder: if gravity possesses momentum, as does EM radiation, then how could it deviate from the momentum of the source - the EM radiation?

So much for using popular literature descriptions of the early universe because some of the ones I have read refer to the universe inflating from the size of a proton to the size of a grapefruit during that period - this inflation represents a radial enlargement along all 3 spatial axes.

That's the symmetry-breaking character of instability.


There's nothing really wrong with that picture, except that is suggests there's nothing outside the "grapefruit". If that we true, we would see effects due to the presence of an "edge".

As Tim said, "There is no radial expansion from a point in space, because all of the points in space are singular, simultaneously." You seem to be considering a grapefruit and imagining that it is expanding from its center point. Not so with the early universe. As an analogy, the expansion is occurring from every atom in the grapefruit, as if every atom is the "center".

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Everyone is entitled to his own opinion, but not his own facts.

When I think of EM radiation I conceptualize it as having 100% of its momentum pointed in the direction of its travel and it therefore has zero restmass (since EM radiation is the fundemental component of the universe - there is no smaller unit for it to divide into).

When I think of rest mass I conceptualize it as having zero percent directional momentum: two photons collide to form a particle - the velocity effects of the two opposing momentums cancel each other out to form a rest mass. If a third photon strikes this rest mass then it imparts a velocity vector to the rest mass which, of course, is momentum.

So here we have a photon with all its energy channeled into one direction - if a graviton were to have momentum and transfer its momentum in order to cause "gravity" then somehow a graviton must convert linear radial momentum into tangential momentum. But even if it did then the amount of momentum is only equal to, at most, one photon's worth of momentum. So we would have one errant photon in the bazillion photon flow - it can not trigger a chain reaction of increasing momentum conversions to ultimately form spiral galaxies.

This imagery of space expanding around each component (atom) of the universe should also apply to each photon (since the two: matter and EM radiation; are synonymous) and so each photon is separated from its neighbor --- which results in a lack of interaction. Without interaction there is no universe because the photons must interact in order to create matter.

Radiation in the universe tends to be highly isotropic-- it is not favoring any particular "radial" direction. Each photon, when observed as such, has a direction, but its wave function prior to observation need not, and even more to the point, there will be plenty of photons moving in all directions. None of this is where the swirling comes from.
Noboby thinks photons are responsible for large or small-scale structure in the universe, or for the swirling of galaxies. The leading candidate is gravity from dark matter. Again, all you need is some tiny spatial inhomogeneity to serve as a seed for gravitational instability, which can than give you tangential momentum aplenty.

Photons coming from a distant star travel in a radial pattern away from the emitter. The shape of the emitter has influence on the actual pattern of emission but ultimately it is an electron/atom that emits and the resulting emission is a function of the atom and the photon that excited the atom.

"Nobody" must also think that the universe was, at one time, in an incredibly hot state where even matter did not exist - the universe transitioned from photons to matter to galaxies - and in each stage the components had momentum.

That's not the "radial" you are talking about here, and it is also a totally insignificant amount of light. We are talking about the factors that shape large and small scale structures in the universe, like what makes galaxies.

I am well aware of this, for some reason I am unable to communicate to you that this is not relevant to how galaxies form. I will tell you one more time what forms galaxies (have you googled "Jeans mass"?): gravitational instabilities in the matter that is around when galaxies form-- i.e., dark matter. This is the sole explanation that comes remotely close to explaining current observations.

I understand about the gravitational abnomolies of matter that create galaxies - I am exploring one step closer: how light, which has momentum, can combine to form matter if all the momentum is radially outward and separating?

I only went on to describe the spiral galaxies to illustrate the great momentum conflict that must have occurred to create these structures.

As far as the inflation/expansion goes it would only accentuate the separating effect rather than squelch it.

You, Ken, want to focus on the matter phase but I want to focus on the photon phase.

As we approach the conditions 10^-44 seconds after the beginning, things get somewhat speculative. However, during the first 20 or 30 microseconds after the universe came into existence, accelerator experiments indicate there was a quark gluon plasma, which is a form of matter. As it expands and cools, it goes through a number of significant symmetry-breaking phases, including baryogenesis and nucleogenesis, neither of which, to my understanding, involves matter being created from photons as you seem to describe.

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Everyone is entitled to his own opinion, but not his own facts.

At those energy levels, I am not sure photons actually exist as such. If I remember right, until nucleogenisis, EM is still tied up as electroweak, so it may be a different particle

So that answers my question because if matter came about instantly then the entire universe was created "as is", from a momentum standpoint ... but in a highly compressed state.