[Bogie_smiles (OP)]

What are they saying about Quantum Gravity?

The topic of this thread and its content is intended to be kept appropriate for the hard science sub-forum, Cosmology. I will utilize the “edit” feature of the NS software to revise my posts and analytical notes as study into the topic and member comments make that appropriate.

The question, what are they saying about Quantum Gravity, is intended to be a learning experience for me, and hopefully the content will not be over the heads of us layman science enthusiasts. But let’s consider the thread open to both a general discussion on the topic, and to technical content as long as they are accompanied by comments to explain them in terms a layman can hope to understand.

Here is a Wiki link, and the opening paragraph to start things off:
https://en.wikipedia.org/wiki/Quantum_gravity

“Quantum gravity (QG) is a field of theoretical physics that seeks to describe gravity according to the principles of quantum mechanics, and where quantum effects cannot be ignored,^[1] such as near compact astrophysical objects where the effects of gravity are strong.”

You see right off that this is a topic that involves theoretical physics, so it may require some technical analysis. Being an interested party, I may offer my personal analysis of and comments about linked material, but my comments are strictly layman level and are not intended to be relied on as known fact, but instead they are intended for discussion and to encourage opposing comments, corrections, and elaborations.

“The current understanding of gravity is based on Albert Einstein's general theory of relativity, which is formulated within the framework of classical physics. On the other hand, the other three fundamental forces of physics are described within the framework of quantum mechanics and quantum field theory, radically different formalisms for describing physical phenomena.^[2] It is sometimes argued that a quantum mechanical description of gravity is necessary on the grounds that one cannot consistently couple a classical system to a quantum one.^{[3][4]:11–12}”

Analytical note: As stated in the Wiki, QG research seeks to describe gravity in environments where quantum effects cannot be ignored, and that means where the effect (force) of gravity is strongest, and were the curvature of space must be the greatest if general relativity is the answer. But GR isn’t the answer in those environments because of known quantum effects that are not yet compatible with GR, referring to the postulates of Quantum Mechanics and the particles of the Standard Model of Particle Physics. The standard model assigns the effect of gravity to the missing graviton particle. So the Standard Model and the Graviton particle will be topics that have to be better understood and reconciled in the QG discussion.

“While a quantum theory of gravity may be needed to reconcile general relativity with the principles of quantum mechanics, difficulties arise when applying the usual prescriptions of quantum field theory to the force of gravity via graviton bosons.^[5] The problem is that the theory one gets in this way is not renormalizable and therefore cannot be used to make meaningful physical predictions. As a result, theorists have taken up more radical approaches to the problem of quantum gravity, the most popular approaches being string theory and loop quantum gravity.^[6] Although some quantum gravity theories, such as string theory, try to unify gravity with the other fundamental forces, others, such as loop quantum gravity, make no such attempt; instead, they make an effort to quantize the gravitational field while it is kept separate from the other forces.”

You can see we are getting into some pretty technical material already, but I don’t think it will hurt to follow the Wiki links and get familiar with the terms and topics.

“Strictly speaking, the aim of quantum gravity is only to describe the quantum behavior of the gravitational field and should not be confused with the objective of unifying all fundamental interactions into a single mathematical framework. A theory of quantum gravity that is also a grand unification of all known interactions is sometimes referred to as The Theory of Everything (TOE). While any substantial improvement into the present understanding of gravity would aid further work towards unification, the study of quantum gravity is a field in its own right with various branches having different approaches to unification.”

That paragraph is helpful, and limits the scope of the QG topic, but still the scope is challenging. I am going back over the Wiki and its links some more, so any analysis I offer is subject to reanalysis as we go, but if you are on top of the topic, feel free to jump in anytime.

“One of the difficulties of formulating a quantum gravity theory is that quantum gravitational effects only appear at length scales near the Planck scale, around 10⁻³⁵ meter, a scale far smaller, and equivalently far larger in energy, than those currently accessible by high energy particle accelerators. Therefore physicists lack experimental data which could distinguish between the competing theories which have been proposed.^[7][8]”


That is a problem when it comes to keeping the content here in line with NakedScientist guidelines, because there isn’t yet much that is known-science when it comes to QG. I will try to keep my comments and analysis within expected parameters, and ask the any participants do the same. Let’s learn together.

[Bogie_smiles (OP)]

Reading through the Wiki again reveals some terms that present hurdles to comfortably moving forward at the layman level, so let’s address them as they come up. The following two terms seem daunting to me as a layman, but it is necessary to at least look at what they mean and how they are used in the discussion of quantum gravity. I don’t hope to become literate right away, but better familiarity with the terms is progress for me:


https://en.wikipedia.org/wiki/Renormalization


Renormalization is a collection of techniques in quantum field theory, the statistical mechanics of fields, and the theory of self-similar geometric structures, that are used to treat infinities arising in calculated quantities by altering values of quantities to compensate for effects of their self-interactions. However, even if it were the case that no infinities arise in loop diagrams in quantum field theory, it can be shown that renormalization of mass and fields appearing in the original Lagrangian is necessary.


https://en.wikipedia.org/wiki/Lagrangian_(field_theory)


Lagrangian field theory is a formalism in classical field theory. It is the field-theoretic analogue of Lagrangian mechanics. Lagrangian mechanics is used for discrete particles each with a finite number of degrees of freedom. Lagrangian field theory applies to continua and fields, which have an infinite number of degrees of freedom.


If anyone wants to give us some tips on how to best understand these terms, please jump in. I have added them to my study list :list gets pretty long, fast, lol:


However difficult I find it to understand those two terms, it helps to recognize how they apply to the search for QG. They indicate that there is effort in two directions on the part of researchers, meaning from both the classical standpoint and the field theories standpoint. The difference in those disciplines is what makes the work toward formulating a quantum gravity theory so complicated.


These two terms represent ways that researchers deal with the complications. One complication noted in the QG Wiki above is the fact that the effects of gravity in the most energy dense environments (near very massive objects and at extreme relative velocities, where the Planck units come into play: https://en.wikipedia.org/wiki/Planck_units) cannot yet be accurately measured. Another complication is introduced when infinities enter the mathematics, as in infinite space, time and energy.


Analytical note: The use of the terms Renormalization and Lagrangian can begin to make sense from the context that various complications exist, and that the work to describe the quantum behavior of the gravitational field may unavoidably include a new approach all together. That might require “a theory of quantum gravity that is also a grand unification of all known interactions”, known as "The Theory of Everything (TOE)”. That would put the course of the research on the idea mentioned in the OP Wiki, that the study of quantum gravity is a field in its own right with various branches having different approaches to unification.”

[guest45734]

Do you wish to restrict your thread to Quantum Gravity only ?

Your first wiki link points to the fundamental difference between relativity and quantum gravity. Both view space time differently. A clear understanding of what space is and how it works is required to understand gravity. Both might be telling the truth but not the whole truth. As this paper describes Quantum theory and the structure of space-time https://arxiv.org/pdf/1707.01012.pdf space time and gravity can be regarded emergent. 

As a separate issue the theoretical Graviton has never been detected but is fundamental to many of the approaches looking at quantum gravity, this theoretical virtual particle/force carrier might not exist. Avoiding maths, how does said force carrier get out of a blackhole, it cant unless it behaves like hawking radiation, with entangled pairs. In which case is it a graviton or just a random quantum fluctuation in space, with no specific properties, other than it is entangled to its partner particle momentarily.

[Bill S]

how does said force carrier get out of a blackhole,

I think it doesn't need to.  There's a thread, somewhere, about gravity "escaping" from a BH. I don't have time to look for it, or, unfortunately, to do more than skim this thread, but perhaps someone else could find it more quickly than I.
 

[Bogie_smiles (OP)]

Do you wish to restrict your thread to Quantum Gravity only ?

Yes, however, as you can see in my comment in the OP, the topic relates to a broad range of physics and cosmology, so if the thread branches out, that is fine. However, I don’t want to go off in every possible direction, but rather take an orderly approach by trying to cover material presented before branching off. I'm going through the OP Wiki and the links in it to address them, and will be expanding the content of the thread as time passes.

Your first wiki link points to the fundamental difference between relativity and quantum gravity. Both view space time differently. A clear understanding of what space is and how it works is required to understand gravity.

That sounds good, buy may not be so easy. There is no generally accepted clear understanding of what space is, and the solution to quantum gravity will have its own description of what space is, just like the main theories have their own sets of postulates and axioms. Not everyone will agree on the definition of space, but it will have to be addressed.

Both might be telling the truth but not the whole truth. As this paper describes Quantum theory and the structure of space-time https://arxiv.org/pdf/1707.01012.pdf space time and gravity can be regarded emergent.

It will take readers a lot of time and effort to absorb that link. That is why I made the following comment in the OP: “But let’s consider the thread open to both a general discussion on the topic, and to technical content as long as they are accompanied by comments to explain them in terms a layman can hope to understand.” Can you boil your link down to a brief executive summary?

As a separate issue the theoretical Graviton has never been detected but is fundamental to many of the approaches looking at quantum gravity, this theoretical virtual particle/force carrier might not exist. Avoiding maths, how does said force carrier get out of a blackhole, it cant unless it behaves like hawking radiation, with entangled pairs. In which case is it a graviton or just a random quantum fluctuation in space, with no specific properties, other than it is entangled to its partner particle momentarily.

I can see you are quite interested in the topic, and I encourage you to post and say what you think is appropriate. I may or may not be able to keep up with you, and if not, I will have to go at my own pace, so bear with me.

[guest45734]

That sounds good, buy may not be so easy. There is no generally accepted clear understanding of what space is, and the solution to quantum gravity will have its own description of what space is, just like the main theories have their own sets of postulates and axioms. Not everyone will agree on the definition of space, but it will have to be addressed.

How about this as a starting point https://en.wikipedia.org/wiki/AdS/CFT_correspondence

It will take readers a lot of time and effort to absorb that link. That is why I made the following comment in the OP: “But let’s consider the thread open to both a general discussion on the topic, and to technical content as long as they are accompanied by comments to explain them in terms a layman can hope to understand.” Can you boil your link down to a brief executive summary?

Items discussed in the first wiki link are expanded on and discussed in the paper. It explains both quantum theory, and space-time, are an emergent phenomena, all the current theories attempting to explain gravity are only partly correct, but all have merit. A new direction may be required to explain gravity.

Theories originating in string theory such as the holographic universe and my favourite at the moment Verlindes emergent/entropic gravity allow all those intriguing things such as entanglement/wormholes/extra dimensions and dont need dark matter to explain the universe. Verlindes approach gives a theoretical basis for MOND theory which is basically curve fit developed a few years ago to explain the movement of the outer parts of galaxies without introducing arbitrary amounts of dark matter.

[guest45734]

There's a thread, somewhere, about gravity "escaping" from a BH.

I could not find it, perhaps it doesnt exist anymore like the graviton maybe

[Bogie_smiles (OP)]

That sounds good, buy may not be so easy. There is no generally accepted clear understanding of what space is, and the solution to quantum gravity will have its own description of what space is, just like the main theories have their own sets of postulates and axioms. Not everyone will agree on the definition of space, but it will have to be addressed.

How about this as a starting point https://en.wikipedia.org/wiki/AdS/CFT_correspondence

That seems to be appropriate material, and I will add it to my list of things I need to review and try to grasp. I’ll refer back to it when I get there, but go ahead and explain QG from your perspective too, though I will lag behind.

It will take readers a lot of time and effort to absorb that link. That is why I made the following comment in the OP: “But let’s consider the thread open to both a general discussion on the topic, and to technical content as long as they are accompanied by comments to explain them in terms a layman can hope to understand.” Can you boil your link down to a brief executive summary?

Items discussed in the first wiki link are expanded on and discussed in the paper. It explains both quantum theory, and space-time, are an emergent phenomena, all the current theories attempting to explain gravity are only partly correct, but all have merit. A new direction may be required to explain gravity.

Thank you for that explanation.

Theories originating in string theory such as the holographic universe and my favourite at the moment Verlindes emergent/entropic gravity allow all those intriguing things such as entanglement/wormholes/extra dimensions and dont need dark matter to explain the universe. Verlindes approach gives a theoretical basis for MOND theory which is basically curve fit developed a few years ago to explain the movement of the outer parts of galaxies without introducing arbitrary amounts of dark matter.

Yes, and I have given my thoughts on related topics like the meaning of nothingness, and of universe that I plan to use in my analytical notes as the thread unfolds. Feel free to go where you want though, and I’ll add my comments where I feel the material is consistent with stepping through the topic of QG at a pace where I can absorb the material.


Please look at the OP where I comment about keeping the content in line with The Naked Scientists guidelines, and try to use disclaimers when you post material that the other members or guests might take for fact instead of speculation or hypothesis.

[guest45734]

How about this as a starting point https://en.wikipedia.org/wiki/AdS/CFT_correspondence

That seems to be appropriate material

You might want to look at de sitter space first, not all theories need anti de sitter space and are simpler without it 

[guest45734]

I have given my thoughts on related topics like the meaning of nothingness

interesting!!!

[Bogie_smiles (OP)]

How about this as a starting point https://en.wikipedia.org/wiki/AdS/CFT_correspondence

That seems to be appropriate material

You might want to look at de sitter space first, not all theories need anti de sitter space and are simpler without it 

I will, and thanks. My list of related research grows.

https://en.wikipedia.org/wiki/De_Sitter_space

In mathematics and physics, a de Sitter space is the analog in Minkowski space, or spacetime, of a sphere in ordinary Euclidean space. The n-dimensional de Sitter space, denoted dSn, is the Lorentzian manifold analog of an n-sphere (with its canonical Riemannian metric); it is maximally symmetric, has constant positive curvature, and is simply connected for n at least 3. De Sitter space and anti-de Sitter space are named after Willem de Sitter (1872–1934), professor of astronomy at Leiden University and director of the Leiden Observatory. Willem de Sitter and Albert Einstein worked closely together in Leiden in the 1920s on the spacetime structure of our universe.
In the language of general relativity, de Sitter space is the maximally symmetric vacuum solution of Einstein's field equations with a positive cosmological constant (corresponding to a positive vacuum energy density and negative pressure). When n = 4 (3 space dimensions plus time), it is a cosmological model for the physical universe; see de Sitter universe.
De Sitter space[1][2] was also discovered, independently, and about the same time, by Tullio Levi-Civita.[3]
More recently it has been considered as the setting for special relativity rather than using Minkowski space, since a group contraction reduces the isometry group of de Sitter space to the Poincaré group, allowing a unification of the spacetimetranslation subgroup and Lorentz transformation subgroup of the Poincaré group into a simple group rather than a semi-simple group. This alternative formulation of special relativity is called de Sitter relativity.

[Bogie_smiles (OP)]

I have given my thoughts on related topics like the meaning of nothingness

interesting!!!

People don't all agree on this definition so let me know how you would modify it.

Nothingness; No space, no time, no energy, and no potential for any space, time or energy.

[Bogie_smiles (OP)]

Reply #12

 
As I work through the items on my list so far, the OP Wiki made reference to approaches to solve quantum gravity, and said “… theorists have taken up more radical approaches to the problem of quantum gravity, the most popular approaches being string theory and loop quantum gravity.^[6] Although some quantum gravity theories, such as string theory, try to unify gravity with the other fundamental forces, others, such as loop quantum gravity, make no such attempt; instead, they make an effort to quantize the gravitational field while it is kept separate from the other.” This approach sounds the most consistent with much of my layman level research over the years, which I have been updating out in the New Theories sub-forum.

It may be disappointing to Dead Cat that I choose to analyze the Loop Quantum Gravity approach first, because he likes the off-shoots from String Theory. I will leave string theory for Dead Cat if he wants to follow that particular “popular” approach.

I’m going to start with LQG. The LQG Wiki below says, in reference to the versions of LQG, “They all share the basic physical assumptions and the mathematical description of quantum space. Research follows two directions: the more traditional canonical loop quantum gravity, and the newer covariant loop quantum gravity, called spin foam theory.”

I have heard it said for years that string theory isn’t as promising as it seemed to be in it’s early years and some have said I should look more closely at LQG. That is strictly my layman opinion and I am open to string theory if Dead Cat or someone else wants to pursue and promote it. Side note: I do remember following discussions about string theory with Alphanumeric, a member in another science forum (who, btw, got his Ph.D. in something like Theoretical Physics and Mathematics, with the intention of working in the field of String Theory).

https://en.wikipedia.org/wiki/Loop_quantum_gravity


Loop quantum gravity (LQG) is a theory of quantum gravity, merging quantum mechanics and general relativity, making it a possible candidate for a theory of everything. Its goal is to unify gravity in a common theoretical framework with the other three fundamental forces of nature, beginning with relativity and adding quantum features. It competes with string theory that begins with quantum field theory and adds gravity.

From the point of view of Albert Einstein's theory, all attempts to treat gravity as another quantum force equal in importance to electromagnetism and the nuclear forces have failed. According to Einstein, gravity is not a force – it is a property of space-time itself. Loop quantum gravity is an attempt to develop a quantum theory of gravity based directly on Einstein's geometric formulation.

To do this, in LQG theory space and time are quantized, analogously to the way quantities like energy and momentum are quantized in quantum mechanics. The theory gives a physical picture of spacetime where space and time are granular and discrete directly because of quantization just like photons in the quantum theory of electromagnetism and the discrete energy levels of atoms. Distance exists with a minimum.

Space's structure prefers an extremely fine fabric or network woven of finite loops. These networks of loops are called spin networks. The evolution of a spin network, or spin foam, has a scale on the order of a Planck length, approximately 10⁻³⁵ metres, and smaller scales do not exist. Consequently, not just matter, but space itself, prefers an atomic structure.

The vast areas of research developed in several directions that involves about 30 research groups worldwide.^[1] They all share the basic physical assumptions and the mathematical description of quantum space. Research follows two directions: the more traditional canonical loop quantum gravity, and the newer covariant loop quantum gravity, called spin foam theory.

Physical consequences of the theory proceed in several directions. The most well-developed applies to cosmology, called loop quantum cosmology (LQC), the study of the early universe and the physics of the Big Bang. Its greatest consequence sees the evolution of the universe continuing beyond the Big Bang called the Big Bounce.



To be continued ...

[yor_on]

As far as I know 'renormalization' means you putting the cutoffs where you find it appropriate, looking at statistics and practicality of use. That means that you impose 'realistic standards' on the mathematics you chose to work with, ignoring results that doesn't fit with what you would expect.
=

And I have to admit that I'm still partial to Einsteins description in where 'gravity' isn't a 'force'. Think of being in a geodesic for seeing that way of looking at it.

[Bogie_smiles (OP)]

As far as I know 'renormalization' means you putting the cutoffs where you find it appropriate, looking at statistics and practicality of use. That means that you impose 'realistic standards' on the mathematics you you chose to work with, ignoring results that doesn't fit with what you would expect.
=

That is very helpful. Seems to be an acknowledgement that if the mathematical expression of the theory describes a model that is internally consistent, and consistent with scientific/measurable observations, it merits a place at the table. However, if it doesn’t hold up in extreme energy densities, or known but as yet unmeasurable events in some generally accepted/known environments, it can’t be considered a precise solution.

And I have to admit that I'm still partial to Einsteins description in where 'gravity' isn't a 'force'. Think of being in a geodesic for seeing that way of looking at it.

I can put myself there too, if I consider that I am a macro object existing in a geodesic that works well to describe the relative motion of macro objects.

On the other hand, that same geodesic cannot easily be mathematically described for quantum level objects. If I consider that I am a macro level object composed of micro (quantum) level energy increments where sub-atomic particles are interacting at Planck scale/close quarters, then macro level geodesics are not detailed enough to predict quantum level motion.

The quantum level action exists in those local environments that have energy density differentials at close quarters. Relative motion in those environments would be minutely influencing the relative motion of macro objects. If so, geodesics would be adequate mathematics at the macro level in environments we can easily observe and measure, but insufficient for predicting motion at the quantum level (at Planck scale distances) in extreme energy density environments, like around blackholes or deep within massive objects, i.e., in cases of extreme relative acceleration.

[yor_on]

Well, quantum level objects are a tricky thing to define. A electron superimposed f.ex,(superpositions) or take the experiment in where they actually 'photographed' electrons proving them to 'exist' :) Seems to me that most of the things that defines us macroscopically is a direct result of time passing, It's the outcome that defines us, and the experiment. You can turn it around and state that the experiment will define the outcome too. So there are consistent laws but? What actually goes on down there is another question.
=

And no, unless you're using decoherence to define it quantum objects are what we all are made of, so we all follow 'geodesics' no matter what scale we define it from. Using decoherence you might be able to formulate it differently though.

Actually, that is a preoccupation of mind for me, because to me decoherence is not only about the scale(s) of a universe. To me it question dimensions.

[jeffreyH]

For renormalization you need statistical mechanics. Here is one of Leonard Susskind's lectures.

[Bogie_smiles (OP)]

Reply #17

Well, quantum level objects are a tricky thing to define. A electron superimposed f.ex,(superpositions) or take the experiment in where they actually 'photographed' electrons proving them to 'exist' Seems to me that most of the things that defines us macroscopically is a direct result of time passing, It's the outcome that defines us, and the experiment. You can turn it around and state that the experiment will define the outcome too. So there are consistent laws but? What actually goes on down there is another question.
=

And no, unless you're using decoherence to define it quantum objects are what we all are made of, so we all follow 'geodesics' no matter what scale we define it from. Using decoherence you might be able to formulate it differently though.

Actually, that is a preoccupation of mind for me, because to me decoherence is not only about the scale(s) of a universe. To me it question dimensions.

Fair enough. Coherence is fundamental to a stable quantum system, but measurement seems to entail decoherence. It also seems that interference at the quantum level causes decoherence, such as when wave patterns are interrupted by the presence of particles or objects. Let’s look a the Wiki:

https://en.wikipedia.org/wiki/Quantum_decoherence

“Quantum decoherence is the loss of quantum coherence. In quantum mechanics, particles such as electrons are described by a wavefunction, a mathematical description of the quantum state of a system; the probabilistic nature of the wavefunction gives rise to various quantum effects. As long as there exists a definite phase relation between different states, the system is said to be coherent. This coherence is a fundamental property of quantum mechanics, and is necessary for the functioning of quantum computers. However, when a quantum system is not perfectly isolated, but in contact with its surroundings, coherence decays with time, a process called quantum decoherence. As a result of this process, the relevant quantum behaviour is lost.

Decoherence was first introduced in 1970 by the German physicist H. Dieter Zeh[1] and has been a subject of active research since the 1980s.[2] Decoherence has been developed into a complete framework which is said to solve the measurement problem.[3]
Decoherence can be viewed as the loss of information from a system into the environment (often modeled as a heat bath),[4] since every system is loosely coupled with the energetic state of its surroundings. Viewed in isolation, the system's dynamics are non-unitary (although the combined system plus environment evolves in a unitary fashion).[5] Thus the dynamics of the system alone are irreversible. As with any coupling, entanglements are generated between the system and environment. These have the effect of sharing quantum information with—or transferring it to—the surroundings.

Decoherence has been used to understand the collapse of the wavefunction in quantum mechanics. Decoherence does not generate actual wave function collapse. It only provides an explanation for the observation of wave function collapse, as the quantum nature of the system "leaks" into the environment. That is, components of the wavefunction are decoupled from a coherent system, and acquire phases from their immediate surroundings. A total superposition of the global or universal wavefunction still exists (and remains coherent at the global level), but its ultimate fate remains an interpretational issue. Specifically, decoherence does not attempt to explain the measurement problem. Rather, decoherence provides an explanation for the transition of the system to a mixture of states that seem to correspond to those states observers perceive. Moreover, our observation tells us that this mixture looks like a proper quantum ensemble in a measurement situation, as we observe that measurements lead to the "realization" of precisely one state in the "ensemble".

Decoherence represents a challenge for the practical realization of quantum computers, since such machines are expected to rely heavily on the undisturbed evolution of quantum coherences. Simply put, they require that coherent states be preserved and that decoherence is managed, in order to actually perform quantum computation.”

JeffreyH's link to Susskind's lecture is also an important acknowledgement. When working with QM, statistics, data, and randomness are important tools/distinctions between the macro and quantum realms:

For renormalization you need statistical mechanics. Here is one of Leonard Susskind's lectures.
(See the YouTube link in his post above)

[Bogie_smiles (OP)]

Reply #18

To follow up from reply #12 where I stated that I would follow the quantum gravity topic down the Loop Quantum Gravity path, as opposed to the String Theory path.

The LQG wiki points out that Loop Quantum Cosmology (LQC) covers the study of the early universe, the physics of the Big Bang, and then on beyond the Big Bang. The Wiki quoted in Reply #12 then refers to a model called the Big Bounce, which should be looked at here in passing as we discuss the broad topic of Quantum Gravity:

https://en.wikipedia.org/wiki/Big_Bounce
The Big Bounce is a hypothetical cosmological model for the origin of the known universe. It was originally suggested as a phase of the cyclic model or oscillatory universe interpretation of the Big Bang, where the first cosmological event was the result of the collapse of a previous universe. It receded from serious consideration in the early 1980s after inflation theory emerged as a solution to the horizon problem, which had arisen from advances in observations revealing the large-scale structure of the universe. In the early 2000s, inflation was found by some theorists to be problematic and unfalsifiable in that its various parameters could be adjusted to fit any observations, so that the properties of the observable universe are a matter of chance. Alternative pictures including a Big Bounce may provide a predictive and falsifiable possible solution to the horizon problem, and are under active investigation as of 2017.
 

Analytical note: The big bounce is not thought to be as credible as other options as we follow the path of Loop Quantum Cosmology, as pointed out in the Big Bounce wiki, since serious consideration has turned to a big bang with inflation. Further, inflation theory is thought to be too unpredictable and leaves too much to a matter of chance instead of to theoretical physics, and that brings us to the areas of active investigation today.


To be continued …

[Bogie_smiles (OP)]

Reply #19

You will note that the Loop Quantum Gravity Wiki link in reply #12 refers to physical consequences of LQG that proceed to the well-developed Loop Quantum Cosmology (LQC) which we look at here:



https://en.wikipedia.org/wiki/Loop_quantum_cosmology (LQC).

Loop quantum cosmology (LQC) is a finite, symmetry-reduced model of loop quantum gravity (LQG) that predicts a "quantum bridge" between contracting and expanding cosmological branches.

The distinguishing feature of LQC is the prominent role played by the quantum geometry effects of loop quantum gravity (LQG). In particular, quantum geometry creates a brand new repulsive force which is totally negligible at low space-time curvature but rises very rapidly in the Planck regime, overwhelming the classical gravitational attraction and thereby resolving singularities of general relativity. Once singularities are resolved, the conceptual paradigm of cosmology changes and one has to revisit many of the standard issues—e.g., the "horizon problem"—from a new perspective.

Since LQG is based on a specific quantum theory of Riemannian geometry,[1][2] geometric observables display a fundamental discreteness that play a key role in quantum dynamics: While predictions of LQC are very close to those of quantum geometrodynamics (QGD) away from the Planck regime, there is a dramatic difference once densities and curvatures enter the Planck scale. In LQC the big bang is replaced by a quantum bounce.

Study of LQC has led to many successes, including the emergence of a possible mechanism for cosmic inflation, resolution of gravitational singularities, as well as the development of effective semi-classical Hamiltonians.

This subfield originated in 1999 by Martin Bojowald, and further developed in particular by Abhay Ashtekar and Jerzy Lewandowski, as well as Tomasz Pawłowski and Parampreet Singh, et al. In late 2012 LQC represents a very active field in physics, with about three hundred papers on the subject published in the literature. There has also recently been work by Carlo Rovelli, et al. on relating LQC to the spinfoam-based spinfoam cosmology.

However, the results obtained in LQC are subject to the usual restriction that a truncated classical theory, then quantized, might not display the true behaviour of the full theory due to artificial suppression of degrees of freedom that might have large quantum fluctuations in the full theory. It has been argued that singularity avoidance in LQC are by mechanisms only available in these restrictive models and that singularity avoidance in the full theory can still be obtained but by a more subtle feature of LQG.[3][4]

Due to the quantum geometry, the big bang is replaced by a big bounce without any assumptions on the matter content or any fine tuning. An important feature of loop quantum cosmology is the effective space-time description of the underlying quantum evolution.[5] The effective dynamics approach has been extensively used in loop quantum cosmology to describe physics at the Planck scale and the very early universe. Rigorous numerical simulations have confirmed the validity of the effective dynamics, which provides an excellent approximation to the full loop quantum dynamics.[5] It has been shown that only when the states have very large quantum fluctuations at late times, which means that they do not lead to macroscopic universes as described by general relativity, that the effective dynamics has departures from the quantum dynamics near bounce and the subsequent evolution. In such a case, the effective dynamics overestimates the density at the bounce, but still captures the qualitative aspects extremely well.[5]
[End of Wiki quote]

We are at a pretty current point in regard to the question “What are they saying about Quantum Gravity” as we follow the Loop Quantum Cosmology (LQC) path. A layman has had to be able to deal with a lot of technical material posted above to stay with what they are saying about quantum gravity along the LQG path.

The status of research is expressed in some detail in a 2011 paper by several researchers mention in the above LQC wiki; Abhay Ashtekar and Parampreet Singh. Jointly they put out a paper called “Loop Quantum Cosmology, a status report” which I insert here:
https://arxiv.org/abs/1108.0893

Loop Quantum Cosmology, a status report:
The goal of this article is to provide an overview of the current state of the art in loop quantum cosmology for three sets of audiences: young researchers interested in entering this area; the quantum gravity community in general; and, cosmologists who wish to apply loop quantum cosmology to probe modifications in the standard paradigm of the early universe. An effort has been made to streamline the material so that, as described at the end of section I, each of these communities can read only the sections they are most interested in, without a loss of continuity.

That summary links to the full 138 page PDF:
https://arxiv.org/pdf/1108.0893v2.pdf

I’m in the process of reading the pdf now, and I’ll be back to let you know if I think that having you read the entire PDF is the best path for the thread to take.