Hello, everyone.

After months of battling to come out of my damned (atypical) clinical depression’s Event Horizon, I finally find myself far enough from the black hole… far enough to feel like coming back here to ask questions and pick your brains again

I would love to devote this thread to discuss and gather in one place some basic information of the most current and relevant cosmological theories about our origin, and how they measure against each other.

I’ve been reading and doing some research about cosmology lately. But the more I read, the more confused I get.

I think I have a decent layman-level understanding of the Big Bang (thanks to places like this, TalkOrigins, etc).
I am familiar with Witten’s M-Theory and its colliding Branes causing the BB.
I’ve watched all of Krauss’ lectures of his Universe from Nothing theory.
I’m aware of a Cycling Universe hypothesis, confused about Alan Guth’s inflation theory (I thought his “pockets” came right after the initial bang, but I read somewhere that it BYPASSES the BB and it goes directly from a previous state to a big “bounce”…?).
I’ve heard something about “repulsive gravity” when reaching the Planck Scale, and this being what instantiated the BB (directly opposing the idea of the universe emerging from quantum fluctuations) —
See? I probably already mixed up something in there!!!
I also read somewhere, dramatically stated, that the Big Bang may be soon be forgotten and replaced, presumably by one of these new concepts.

The more reading I do, the more confused I get, because I get snippets of information from different theories, SOME of which, apparently, make the Big Bang redundant or unnecessary to explain how our universe(s?) came into existence.
I don't know how these theories relate to each other, or which ones are good solid theories and which are mere unsubstantiated hypotheses.

It is my understanding that, traditionally, it didn’t make sense to even talk about a “before” the BB, because space-time emerged at T=0… but now?
I also saw one of those BBC videos, where they almost mock Lawrence Krauss’s concept of a Universe from Nothing and his virtual particles. Is this concept still relevant? I LOVE it! I would be sad to have to let it go…


So, in general, I would like to understand and obtain an overview of:
-- What are all these theories? (or are they still hypotheses?)
-- Which are the most relevant at the moment?
-- What are their general features?
-- Who are their main proponents?

-- Is the traditional Big Bang still valid? (the more you go into the past, the universe was smaller, hotter and denser).



More specifically, I’d love to know more about them in therms of:

- Their explanatory power.
I understand that, for a new theory to be taken seriously, it must give a better explanation of whatever theory it intends to replace, it must agree with observation and reality, it needs to make new, testable predictions, and be falsifiable.
Once we identify these theories, which of them are really theories, and which are still hypotheses?

How do they explain what seems to be smoking-gun evidence in favor of the BB, like the CMBR, the accelerated expansion of the universe, etc??
Please correct me if I’m wrong, but it is STILL A FACT that, in the past, the universe was hotter, denser and smaller, yes?
So how do those other theories account for all that? And at what point do they meet? How far into what we call the BB do they go? Which of them directly replace or bypass the BB?


- Their relevance and plausibility:
I suppose we should start with the least speculative ones. Are they Observable? Testable? Falsifiable? What would we need, in terms of technology, to be able to test them?

How do M-Theory and Krauss’s Universe from Nothing come into the picture? Are they still relevant?

Have they figured a way to unite Relativity with Quantum physics? Or did they bypass this as well?

What features make these newer theories a better explanation?




- Their problems and challenges and yet-to-explain issues
I’m not sure how we can test some of those concepts… It seems that we’re peeling an onion, and the more we peel, the less of a chance of ever figuring out our origin we see…

Aren’t these theories pushing our limits of testing/speculation too far away? At what point are they considered philosophy?

As far as I know, we haven’t discovered the Graviton yet. I know gravity is the weakest force, but how can these other theories be better if they don’t take it into account yet? Or do they? How far are we into our current understanding of gravity? Does it really flow into other dimensions, thus its comparative weakness to the other forces? If so, which theory does this belong to? What is preventing us from figuring this out?

How do they account for Dark Matter and Dark Energy? Or do they give us new concepts or better clues that render them unnecessary?

I’ve also heard of a Big Crunch.
What could make the universe overcome acceleration, then slow down, and then shrink back into a singularity and start all over again? Gravity seems too weak to be able to catch up to Dark Energy, so if it’s not gravity, what would bring everything back together?

I read that the concept of a singularity signals to our own ignorance due to a lack of proper understanding. Do you agree with this? Have we finally gotten rid of the darned singularity?

Sorry about asking so many questions, but I'm THAT confused... I realise this is an extremely vast topic.

But I don’t think I will make more progress until I get a clearer view of which theories are the newest ones, how they relate, and how their features measure against reality and observation and things we thought we already knew.
So any comments, book recommendations, or threads or papers that discuss all this will be really appreciated.

Thank you so much for any insight!

Hello. How much are you paying, per hour? j/k and bookmarking.

I know... Sorry about my flood of questions.

Please feel free to comment only on a question or two, not the whole thing.

Perhaps something as simple as a diagram showing the branching of the main theories would be so helpful, especially when it shows how they evolved.

First of all, the Big Bang theory was only intended to explain the behaviour of the universe once it was instantiated. What happened before this was never the remit of Big Bang theory. The idea of space-time coming into existence upon said instantiation, was in part a consequence of physicists not being in a position to develop ideas about what happened before, because the standard cosmological model at the time involved a singularity. Singularities constitute a sort of mathematical no-go area - they are, in effect, locations within a function domain for which the function in question ceases to be well-behaved, or worse still, ceases even to be defined. If you can't even define a function at a given point in the function domain, you can't really say much about how that function behaves at that point. The Big Bang singularity represented, in effect, a point where the functions representing the behaviour of physical systems cease to be defined. It was therefore thought that no progress beyond that point could be made.

This view changed as a result of several developments. The one I'm best acquainted with is braneworld cosmology, which proposes that the universe was instantiated as a result of two branes colliding. This model requires that spacetime has always existed, and that as a result, the "t=0" of the Big Bang theory merely delineates the moment at which the instantiating collision occurred from the standpoint of the instantiated universe entity. Since that entity didn't exist beforehand, a "t<0" from the standpoint of that entity makes no sense, but does make sense from the standpoint of other entities that did exist beforehand.

What makes braneworld collision particularly worth pursuing further, is that [1] it doesn't involve singularities (or, more rigorously, doesn't involve non-regularisable singularities - a quick look at complex analysis, and how singularities are treated therein, will explain how singularities can be regularised, provided they are of the correct type), and [2] provides a potential empirical test of the theory, Braneworld collisions, if they are genuinely responsible for instantiating universe-type entities, leave within those universe-type entities evidence of their having taken place, in the form of a particular spectrum of primordial gravitational waves. If that spectrum is observed (hence the expense being spent on LIGO etc), then this is an indication that braneworld cosmology is something more than mere speculation.

Good explanation Cali and I also think braneworld a plausible approach tho I tell you plausible at this scale is a stretch.

So.... Our universe is a brane-child?

Thank you very much for your knowledgeable answer, Calilasseia

Calilasseia wrote:First of all, the Big Bang theory was only intended to explain the behaviour of the universe once it was instantiated. What happened before this was never the remit of Big Bang theory.

Got it. So in this respect, would you approve of an analogy to abiogenesis and evolution? (the BB analogous to the latter in that respect).

Calilasseia wrote:
The idea of space-time coming into existence upon said instantiation, was in part a consequence of physicists not being in a position to develop ideas about what happened before, because the standard cosmological model at the time involved a singularity. Singularities constitute a sort of mathematical no-go area - they are, in effect, locations within a function domain for which the function in question ceases to be well-behaved, or worse still, ceases even to be defined. If you can't even define a function at a given point in the function domain, you can't really say much about how that function behaves at that point. The Big Bang singularity represented, in effect, a point where the functions representing the behaviour of physical systems cease to be defined. It was therefore thought that no progress beyond that point could be made.

Thank you for your more rigorous explanation of what a singularity is.
I was only familiar with the more layman version of it, that it represents a point of infinite density that is infinitely small.
Are these infinities what you referred to when you wrote about functions "ceasing to be defined"?
By "well behaved", do you mean that they can be accurately described mathematically?
So is this the heart of the trouble scientists have been having for decades, that they can't happily "marry" the required General Relativity and quantum mechanics to describe singularities?

Calilasseia wrote:
This view changed as a result of several developments. The one I'm best acquainted with is braneworld cosmology, which proposes that the universe was instantiated as a result of two branes colliding. This model requires that spacetime has always existed, and that as a result, the "t=0" of the Big Bang theory merely delineates the moment at which the instantiating collision occurred from the standpoint of the instantiated universe entity. Since that entity didn't exist beforehand, a "t<0" from the standpoint of that entity makes no sense, but does make sense from the standpoint of other entities that did exist beforehand.

Is this braneworld cosmology related to Witten's M-Theory in any way?

Are these branes described in the mathematics? Do they come directly out of them?
If this is so, how does a brane look in its native language mathematics?
I wouldn't understand it, but I would LOVE to see how a brane is represented mathematically anyway. If it's not too much trouble, perhaps you could post an example when you have time? I'm really curious about it.

Calilasseia wrote:
What makes braneworld collision particularly worth pursuing further, is that [1] it doesn't involve singularities (or, more rigorously, doesn't involve non-regularisable singularities - a quick look at complex analysis, and how singularities are treated therein, will explain how singularities can be regularised, provided they are of the correct type), and [2] provides a potential empirical test of the theory, Braneworld collisions, if they are genuinely responsible for instantiating universe-type entities, leave within those universe-type entities evidence of their having taken place, in the form of a particular spectrum of primordial gravitational waves. If that spectrum is observed (hence the expense being spent on LIGO etc), then this is an indication that braneworld cosmology is something more than mere speculation.

Wow. That does sound promising!

I had read this Krauss article about it when they confirmed they found the gravitational waves.
http://www.nytimes.com/2016/02/14/opini ... egion&_r=0

I am truly astounded about what scientists can do and measure these days! The accuracy of the equipment!

So then I suppose the "signature", the spectrum of gravitational waves from colliding branes would necessarily be much more obvious and easy to detect than the ones they saw with the colliding black holes, yes?

So, when the time comes and we find them, what would be the next step to test this theory further? What else would be convincing of braneworld cosmology?

Just for speculation, what else could gravitational waves be an indicative of? What else, besides mega-collisions, could create them?

Thank you again for your time and your knowledge, Calilasseia

I think you need to relieve Cali of this burden ..there is lots of good info on the web and open source courses

http://ocw.mit.edu/courses/physics/8-82 ... fall-2008/

http://www.openculture.com/physics_free_courses

and



have fun

Macdoc wrote:I think you need to relieve Cali of this burden ..there is lots of good info on the web and open source courses

Oh, no!
I didn't mean to come off as a burden giver.

Anyone who wishes to participate in the thread can do so freely, with NO pressure or expectations on my part. I know you guys are busy, and I understand this is on a time and desire-permiting basis only.

So please, no burden or pressure meant for Cali or anyone else. Answer as much or as little as you want.

Thank you for the links.. I will watch them tonight.
I have seen a few of professor Susskind's lectures before. They are fascinating. I try to follow as much as I can, but it's too advanced for me. I'm a composer

Macdoc wrote:I think you need to relieve Cali of this burden ..there is lots of good info on the web and open source courses

http://ocw.mit.edu/courses/physics/8-82 ... fall-2008/

http://www.openculture.com/physics_free_courses

and



have fun

Love watching Leonard Susskind's lectures.

Thanks for starting this thread. Bookmarked!

I'm thinking Cali's brane-cyclic cosmology is what Steinhardt-Turok proposed, or theirs is a version? This is a good video for their ideas and why they distrust inflation:



Also:




And I haven't seen this one, on why Turok-Steinhardt's theory fails *** See info below.***


***Edit: Addendum&Warning--OK, this last one, Barr is a genuine, good physicist, but he appears tainted by religion. I will have to look into a lot more, but be wary of this video!

Another edit, wiki lists one of Barr's publications as "Stephen M. Barr, (2011) Science and Religion: The Myth of Conflict (Explanations). Catholic Truth Society" So, I don't want to bother watching it now, I'll leave it there in case anyone has anything to comment on it, but there's way too many other things to do with my time

This is a good, concise Steinhardt ripping into inflation. Even has Ed Witten directing the Q&A at the end, the single Q&A.

Thank you for all those videos. They're extremely informative. They've answered some of my questions.

I'm not sure how I landed here, but I found this thread that also addresses my questions.
It's a great thread, and I was nicely surprised to recognize a familiar and great poster, hackenslash (he participated heavily there).
http://www.theleagueofreason.co.uk/view ... 78d2611f91

I hope he can come back here soon after his "vacation"!

Today's Royal Astronomical Society meeting seems anything but ordinary...
The last (of 3) talks sounds particularly intriguing and I wonder if/when it'll get streamed somewhere soon:

...
2016 GERALD WHITROW LECTURE
Dr Neil Turok (Perimeter Institute, Canada)
Universe

A spate of new observations are providing powerful clues about the laws of fundamental physics and the cosmos. The implications are revolutionary: the universe is astonishingly simple on the largest and the smallest observable scales, with great complexity in between. These findings contrast sharply with expectations from popular twentieth century paradigms including inflation, supersymmetry and string theory, which led many to take seriously the idea of a wild and unpredictable "multiverse" on large scales. Key "predictions" derived from that picture have been recently falsified, posing observational challenges to the paradigm which compound its many logical problems. In this talk I will discuss a new, and in my view more promising, approach to understanding the quantum nature and integrity of the universe.

The PRL paper on which this talk is based is found here in the arxiv.

Abstract

We study quantum cosmology with conformal matter comprising a perfect radiation fluid and a number of conformally coupled scalar fields. Focusing initially on the collective coordinates (minisuperspace) associated with homogeneous, isotropic backgrounds, we are able to perform the quantum gravity path integral exactly. The evolution describes a “perfect bounce”, in which the Universe passes smoothly through the singularity. We extend the analysis to spatially flat, anisotropic universes, treated exactly, and to generic inhomogeneous, anisotropic perturbations treated at linear and nonlinear order. This picture provides a natural, unitary description of quantum mechanical evolution across a cosmological bounce. We provide evidence for a semiclassical description in which all fields pass “around” the cosmological singularity along complex classical paths.

That does sound interesting.

Quantum mechanics of a cosmological bounce.—For homogeneous, isotropic cosmologies, one can choose a Weyl gauge in which the metric is static and the scalars (φ, χ⃗ ) encode all of the dynamics. While the metric is nonsingular in this gauge, the theory is still problematic because the effective Planck mass, given by the coeffi- cient of R, can vanish, so that gravity becomes strongly coupled. Our strategy is to first identify this singularity in the quantum propagator, and then understand how to analytically continue around it. Our key assumption, which we shall test in various calculations, is that there are no singularities obstructing such a continuation. We set D = 4 unless otherwise stated.

D doesn't get mentioned again but, from earlier stuff, isn't this the (spatial)dimension count? If not, what is it, please? Otherwise, this reads like further continuation of the earlier Steinhardt & Turok stuff. I've no promble with that but I'd like an independent check...

ETA of course D gets mentioned again

(restoring the dimension D).

Phew!1 Carry on...

I e-mailed the Royal Astronomical Society to ask if the Turok talk would be posted at their youtube account...

...
Apologies for not replying sooner. I'm pleased to say that we will be publishing this talk in the near future. I will let you know when it's available.

Kindest regards,

Steven Pryer
IT and Information Manager

So, I'll post the link as soon as...

I think this was delved into on RDF (I believe Hack was involved there, too).

A lot of it was over my head but I still found it jolly interesting.

My response at the time was "My Brane hurts..."

(Sorry)

Yes, the original Steinhardt & Turok stuff did the rounds there but Neil Turok is now looking elsewhere than stringy physics to help resolve issues with the bang-phase of the cycle. This new method proposes by-passing/avoiding the singularity by judicious use of conformal time, analytic continuation and other clever logics... Looking forward to the talk though...