Confused about the formation of Stuff in an Isotropic-ish Universe

[QuestionMarks]

Confused about the formation of "Stuff" in an Isotropic-ish Universe

First let me state my current understanding so that I can be corrected if not on the right page-- Given "zooming out" far enough, our universe is rather homogenous/isotropic, but not perfectly so. If this is correct, I must admit my confusion and ignorance about the origin of "stuff" in our universe. (a stance perhaps ubiquitous to us all hah)

My confusion is in that, although it makes sense to me that our universe should be slightly heterogeneous so that discrete entities can form (planets, stars, galaxies, etc.), I can't help but viewing any singularity described by the Big Bang as an entirely homogenous event. After all, in such case wouldn't it not make sense to talk about any individual particles or differing instances which would be capable of heterogeneity in that "primeval atom?"

If so, I must wonder how such a homogenous event, which I would imagine should perfectly so and its expansion would also be perfectly even, could give rise to a slightly heterogeneous universe and thus stars, planets, etc.

I've read of initial quantum fluctuations playing a prime role, but I am then confused as I must ask "Quantum fluctuations of what?"---of what parameters within the big bang singularity, of what entities? Perhaps then as well a many-worlds stylized interpretation alleviates the initial question, but I know there are cosmogonies (obviously) that do not support this line of thought and thus somehow reconcile the issues otherwise.

Thanks for the reply, and hopefully I can understand the (likely) seemingly mind-breaking answers hah.

 

[mathman]

The stuff in the early universe was very hot, which means they moved around very fast, so there were small fluctuations in density, leading to bigger fluctuations later, leading to galaxy formation, etc.

 

[marcus]

...I can't help but viewing any singularity described by the Big Bang as an entirely homogenous event.

I think that is your problem. Mainstream cosmology does not describe the start of expansion as a "singularity".
http://www.einstein-online.info/spotlights/big_bangs
As it says in "A Tale of Two Big Bangs" the professional cosmologists would be quite surprised if it turned out there actually was a singularity. Normally it is regarded as a symptom that something is wrong with the theory. Theory needs to be improved so it does not develop a singularity.

So it is a bad place to begin reasoning. Unrealistic assumption.

I think judging from your post that this is what is causing you trouble.

Standard expansion cosmology "Big Bang Theory" does not assume that there ever was an actual singularity. At any stage of expansion which the model describes there is always something that can fluctuate. Some field.

The reason is that the theory is admittedly incomplete. It does not model the precise beginning of expansion.

There are extensions such as Loop Quantum Cosmology which DO model the start of expansion and which also model conditions leading up to it. In these theories there is no singularity (and there is no inscrutable "atom" at the start ) One can model the fluctuations and attempt to make testable predictions. People are working on that. It's hard but it is not a Mystery.

 

[QuestionMarks]

Ah thank you Marcus, I had indeed always presumed a singularity, perhaps in bias for a more philosophically grand idea than an empirical one. That makes the matter seem quite more sensible.

My initial line of thinking does still interest me though, albeit now differently. Within the current consensus of physicists, is there any serious interest (or implications) as to how/why the "initial Big Bang state" (if I might call it that) did not still produce such a homogenous soup (again for lack of wording)? Or is it simply that given our ignorance of that initial state, the quandary is not useful and we stick with the empirical facts? Or is it that there is current known and current knowledge I'm missing?

Pardon if my line of thinking here doesn't make sense or sounds too unfortunately like my original question.

 

[Some Slacker]

Ah thank you Marcus, I had indeed always presumed a singularity, perhaps in bias for a more philosophically grand idea than an empirical one. That makes the matter seem quite more sensible.

My initial line of thinking does still interest me though, albeit now differently. Within the current consensus of physicists, is there any serious interest (or implications) as to how/why the "initial Big Bang state" (if I might call it that) did not still produce such a homogenous soup (again for lack of wording)? Or is it simply that given our ignorance of that initial state, the quandary is not useful and we stick with the empirical facts? Or is it that there is current known and current knowledge I'm missing?

Pardon if my line of thinking here doesn't make sense or sounds too unfortunately like my original question.

The reason is that the theory is admittedly incomplete. It does not model the precise beginning of expansion.

And then to extend further back to your 'initial Big Bang state' there simply isn't a way to talk about this.

From what I gather (this from a non-scientist and a cosmological neophyte) anything before some 380 million(ish) years after the big bang is what the theory is all about, trying to come up with ways to match the CMB data with what came before.

Forgive me if I have overstepped my knowledge, but if I have, if I know anything about the interweb, a correction will be coming forthwith! heh

 

[marcus]

, is there any serious interest (or implications) as to how/why the "initial Big Bang state" (if I might call it that) did not still produce such a homogenous soup (again for lack of wording)?

yes there is intense interest in exactly this problem and many people at work in this area.

I seem to recall that Brian Powell has first hand familiarity with this area of research, having published some related stuff.

The gist is people have various hypothetical models to explain the basic characteristics we see namely near flatness at large scale and approximate isotropy, one of these models is inflation.And whatever hypothetical model you propose must also explain the blotchy ripply speckly DEVIATIONS of all different sizes that we see.

So if you hypothesize an inflation episode well that requires some mechanism like an inflaton field and any quantum field we know about has fluctuations.
Even the familiar electron field can spontaneously burp an electron-positron virtual particle pair. but suppose it does that in a rapidly expanding geometry so that the virtual pair get dragged widely apart before they have a chance to erase each other.

Inflation is popular because if you buy it you get an explanation for the near uniformity but you also get an explanation for the 1/1000 of one percent fluctuations visible in the CMB map of the sky It can be made to predict the SPECTRUM of fluctuations, how many of each angular size. How many blotches that are 1/2 of a degree wide, how many that are 1/4 of a degree wide, and so on.

Natural inflaton field variation is not the only hypothesis competing to explain the statistics of these fluctuations. It is not a solved problem.

So the answer to your question is that Yes there are people quite interested in the visible fluctuations and they have proposed several different models of the very early U, which have some source of variation that purports to give the right amount of fluctuations of various sizes in the sky.

 

[GeorgeDishman]

.. is there any serious interest (or implications) as to how/why the "initial Big Bang state" (if I might call it that) did not still produce such a homogenous soup (again for lack of wording)?

"Quark soup":

http://en.wikipedia.org/wiki/Quark–gluon_plasma

Bear in mind though that even the most homogenous soup exhibits Brownian Motion ;-)

Actually, the universe appears to be more homogenous than expected, the anisotropy in the CMBR is less than was anticipated AIUI.

 

[bapowell]

Yes, QuestionMarks, the origin of initial perturbations is an open question. Prior to the advent of inflation, the initial inhomogeneities -- the seeds of today's large scale structure -- were put in by hand over a homogeneous spacetime (a homogeneous spacetime which itself was of questionable origin.) As Marcus explains nicely, inflation explains both 1) why the universe is flat, homogeneous, isotropic on large scales and 2) why there were small initial seed perturbations on top of this smooth background.

The first point is essentially classical, and has to do with the mechanism of inflation: inflation was an exponential expansion of space that diluted and smoothed out any initial inhomogeneity. After inflation, on the classical level, you are left with a perfectly homoegeneous and isotropic patch of spacetime.

The second point is essentially quantum mechanical: the energy source driving the inflationary expansion is a quantum field. As such, its energy density at a given point in spacetime fluctuates. Inflation requires a sufficiently high energy density to work, so if it happens that in a particular region of the universe this fluctuation results in an energy density too low to sustain it, inflation will end in that region. When inflation ends, the remaining energy density in that region decays to radiation, effectively creating a hot big bang. In sum total, across the initially inflating universe, there are regions like this that stop inflating randomly throughout at different times. Those regions that stop inflating early fill with radiation and proceed to expand according to the standard big bang model; they will therefore be of a lower radiation density than those that stop inflating later. In this way, inflation results in a patch of spacetime comprised of smaller regions of varying densities. These are the initial inhomogeneities that later grow to form large scale structure.

 

[bapowell]

From what I gather (this from a non-scientist and a cosmological neophyte) anything before some 380 million(ish) h

It's 380 thousand years. But the difference is a blink of an eye in cosmological time

 

[Some Slacker]

It's 380 thousand years. But the difference is a blink of an eye in cosmological time

I totally knew that... sigh, my brain and I aren't on speaking terms... as of NOW!

Wish I could add something to this discussion, but I like bapowell's previous post, I had never considered that inflation stopped at different times in different locations, more space magic!

 

[QuestionMarks]

This post has been pretty darn helpful. Thank you all for the clarity of your explanations.

It seems quaint to me that the current theories depend so heavily on those quantum perturbations, though I can't imagine what else, but I still feel so simply because of the foggy possible interpretations of quantum. For instance, in bapowell's explanation, I certainly understand the myriad of possibilities quantum describes (each of those thereby contributing to a situation which would lead to that slight anisotropy), but I find myself wanting to think that those states would still exist in superposition, or at least am still wondering what effectively chose/observed/collapsed/whatever-interpretation-ed them. Unless many-worlds is true and all exist, albeit we only experience one.

Ah questions to more questions...

 

[marcus]

...
It seems quaint to me that the current theories depend so heavily on those quantum perturbations, ... but I find myself wanting to think that those states would still exist in superposition, or at least am still wondering what effectively chose/observed/collapsed/whatever-interpretation-ed them...

I think the idea is that EXPANSION OUTSIDE OF CAUSAL CONTACT froze them.

If a pair of virtual particles is swept so far apart that they are beyond each other's event horizons they can no longer annihilate. Perhaps that's an oversimple toy version of the idea of how rapid expansion freezes things into reality. I'll defer to Brian Powell for better account of it.
=======================

But since you point to the heavy dependence on quantum fluctuations let me mention that one model does not depend so heavily. the Loop cosmology bounce. When GR or Friedmann equation is quantized according to LQG you get a quantum correction term which makes gravity REPEL at very high density.

So a collapsing phase of the universe begins to get close to a critical density and then rebounds. There is a natural episode of faster than exponential expansion (technically "super-inflation") that occurs even without assuming an "inflaton" field. It is just built into the quantum law of gravity/geometry.

If your mind shies away from having all structure be seeded by little frozen-in quantum fluctuations, then you can imagine some inhomogeneity being "inherited" from structure that was imperfectly erased by the collapse preceding the bounce. Still conjectural though Loop seems able to make testable predictions about the CMB spectrum of variation, so there may eventually be tests that put things on a bit firmer basis.

 

[QuestionMarks]

I think the idea is that EXPANSION OUTSIDE OF CAUSAL CONTACT froze them.

If a pair of virtual particles is swept so far apart that they are beyond each other's event horizons they can no longer annihilate. Perhaps that's an oversimple toy version of the idea of how rapid expansion freezes things into reality. I'll defer to Brian Powell for better account of it.

Would that then not require an interpretation of quantum that says some semblance of "some things DO just happen randomly within their possibilities?" Is that indeed consensus? I had some notion of instead "all things happen within their possibilities until/throughout..." (I leave the ellipses there intentionally via ignorance). My later notion would then make me think that all such fluctuations, if even frozen as such you claim (which does make sense to me), would end insignificantly and evenly (unless the former notion which would have different things happening individually).

Marcus, I must say your ease of surveying these "pros" and "cons" (if there isn't a better term) for these big theories is much appreciated. It makes me wish for a nice table of sorts that held up-to-date challenges and accomplishments of these different models. After all, it gets hard for lay interest readers like me to really sort through it all. Heh, for that matter one such as I should admit our reading is limited largely to Barnes&Noble and wikipedia, one which is so dastardly opinionated you often read that these matters are already settled, and wikipedia...well, we all know wiki's faults. I won't speak ill of it though as I love it so.

But I digress.

 

[bapowell]

If a pair of virtual particles is swept so far apart that they are beyond each other's event horizons they can no longer annihilate. Perhaps that's an oversimple toy version of the idea of how rapid expansion freezes things into reality. I'll defer to Brian Powell for better account of it.

Yes, that's quite right Marcus. As the wavelength of the fluctuation grows larger than the horizon, it goes from quantum fluctuation to classical perturbation: the quantum state effectively decoheres. I am not an expert on this classical-to-quantum transition through, and so in response to QuestionMarks, I can't claim that there are no potential interpretational/philosophical issues with this process.

I should also mention that in addition to LQC, string gas cosmology offers an alternative picture of structure formation. I'm not an expert on this either, but the idea is that thermal fluctuations in a string gas in the early universe grow to surpass the horizon during a period in which the Hubble radius shrinks.

 

[marcus]

Komatsu is an expert at reading the CMB sky map and understanding early universe.
He just gave a talk about inferring initial fluctuations at the start of inflation.
The talk has not yet been posted on line but maybe tomorrow check back, if interested.

http://pirsa.org/12050002
New Probes of Initial State of Quantum Fluctuations During Inflation
Speaker(s): Eiichiro Komatsu
Abstract: How did inflation actually happen? Precision measurements of statistical properties of primordial fluctuations generated during inflation offer a direct probe of the physics of inflation. When we calculate statistical properties of primordial fluctuations generated during inflation, we usually assume that the initial state of quantum fluctuations is in a preferred vacuum state called Bunch-Davies vacuum. While there is some motivation for choosing such a state, this is an assumption, and thus needs to be tested by observations. In this talk I will present new probes of initial state of quantum fluctuations during inflation: the 3-point function of the cosmic microwave background anisotropy, the 2-point function of galaxies, and a spectral distortion of the thermal spectrum of the cosmic microwave background.
Date: 22/05/2012 - 11:00 am

It's one of those cases where human are just beginning to find stuff out, it's marvelous for us just to understand 1%. Don't be disappointed if what he has to say is only tentative.
I know him only from papers that he has been lead author of, never saw him give a talk. But regard him highly and hope it will be a good talk.

If anyone is interested in the very early perturbations at start of inflation and watches Komatsu's talk they might want, just for comparison, to glance at this Loop Cosmology paper:
http://arxiv.org/pdf/1204.1288v1.pdf
==sample quote from page 1==

Inflation is a period of quasi-de Sitter expansion and typically the assumption is that slow-roll inflation began with the quantum state describing perturbations in its ‘natural’ vacuum state: the Bunch-Davies vacuum. A priori this is an odd assumption, since it says that the quantum state is tuned to the subsequent (quasi-de Sitter) evolution of the geometry. Essentially this implies that the pre-inflationary dynamics of the universe and the ‘true’ initial state conspired in such a way as to ensure that we arrived at the onset of slow-roll inflation with no particles present (relative to the Bunch-Davies vacuum). The intuition behind this assumption was that even if there were particles present at the onset of inflation, the exponential expansion would rapidly dilute them and hence their consequences can safely be ignored. This intuition misses the important fact that quantum fields in a dynamical space-time experience both spontaneous and simulated creation of particles [2]. The latter effect actually compensates for the exponential growth of the volume in such a way that the particle number density remains approximately constant.​

==endquote==

There's the seed of a controversy here I think, and not purely at theory level. Observation could be involved. Could go either way, it's not even clear what the outcomes could be. Might be interesting.

 

[QuestionMarks]

Sometimes I wish I had instead gone for a physics degree so that I could understand a heavy bit of the lingo posted. Still, perhaps my 50%-ish grasp is sufficient for my means.

And Marcus, I am by no means deterred by our understanding of "1%." That there's still mystery means there is still something to be interested over, and that we might then admit our ignorance to it would only suggest our wisdom.