Does this explain what generates gravity?

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#5,821
After rejecting his original idea of a static Universe, Einstein favoured the idea of a Universe locked in a perpetual cycle of Big Bang followed by Big Crunch.

So you are in good company, TNT! 😎

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Crucially, in an Oscillating Universe, time is endless and beginningless, so there is no need to consider there to have been a beginning of time!
 
#5,822
Why do people get so interested in all this Big Crunch stuff? It really seems not to be at all likely. It's all just expanding. It doesn't care what Einstein thought. The Universe is as we find it.



Reluctantly continuing Galu's segue into the Weyl Tensor, Penrose's Conformal Cyclic Cosmology and the nature of the Cosmos, I have had a delve into General Relativity currently beyond my ability.

Here goes, and this is doubtless full of mistakes, because I don't know a lot about Tensors beyond them treating SpaceTime as a sort of fluid.

Penrose's current wheeze is to consider a Universe that consists just of Gravitational Waves and no Matter. Even the Protons, Electrons, Black Holes and Dark Matter have evaporated. The Photons have redshifted into nothingness, I suppose.

This is not our Universe by my reckoning, nor will it ever be. No-one has ever seen a Proton decay.

He is looking at a a particular solution to the fiercely difficult Einstein Field Equations. Pure Weyl Tensor. Which is pure Wave Theory of Gravitational Waves, or maybe Gravitons in a bulk or classical sense.

Decomposition of the Ricci Tensor.png


OK. This is just breaking up a Matrix into elements. S is a Scalar, and might be the acceleration of Space. W is the Weyl Tensor, which is Traceless, meaning the sum of the main diagonal elements in a 4X4 is zero,

What is the PLAN?

Traceless Weyl Tensor.png


I suspect he is thinking that with a Symmetry comes a Conservation Law, At which point I am lost. But he is thinking he has discovered something about Cosmology in our Universe, but I can't see how.

I suppose this is like separating electromagnetism into electrostatics and Maxwell's Wave theory. Or separating DC from AC in electrics.

Sabine has the advantage of us in understanding the Einstein Field Equations, so is more qualified to comment, But I think everybody is groping in the dark here.


More down to Earth is a nice Star Map site I have found, which shows the current Night Sky when you go to it, and you can click on objects for more information.

https://in-the-sky.org/skymap.php

I was hoping to get a shot of the two big star clusters in Hercules last night, but the weather let me down. But a useful site for identifying things of interest on a proper computer.
 
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#5,824
system7 said:
The Photons have redshifted into nothingness, I suppose.
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Redshift is only an observation effect, not a physical state - right!? Photons are all there - all matter has become photons and universe has become totally dark as if there is noting to reflect against, "light" can't be seen anymore even if it is omnipresent. Then all photons are sucked back to one single point where it gets "quite" hot&dense... and 3,2,1 ... boom.

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#5,825
TNT, I have NO IDEA what is supposed to be going on in "Conformal Cyclic Cosmology". I am merely trying to grasp if there is any interesting things to learn about the Einstein Field Equations here.

There is only one mention of Photons in the Penrose paper I read "The Physics of Conformal Cyclic Cosmology March 31 2025", but I thought an expanding Universe breaks Conservation of Energy?

https://arxiv.org/abs/2503.24263

According to conformal cyclic cosmology (CCC), the currently conventional description of the entire history of the universe (but without an initial inflationary phase) provides but one cosmic aeon of an unending sequence of such aeons, where the future conformal infinity of each aeon joins essentially smoothly to the conformally stretched big bang of the next, across a spacelike 3-surface, referred to as a crossover 3-surface. Whereas in previous accounts of CCC a detailed description of the physics of crossover had been somewhat problematic, a novel idea is introduced here to show how crossover takes place naturally during a temporal period of the universe that is dominated by gravitational waves referred to here as a gravitational wave epoch (GWE). Accordingly, the geometry at the crossover surface is conformally smooth, except at a discrete set of points, referred to as Hawking points, each representing the final Hawking evaporation of the dominant black hole of a galactic cluster in the earlier aeon. It is shown here (using 2-spinor and twistor techniques) that there is a mass-energy conservation law that holds across the crossover surface, showing that the rise of temperature within such Hawking spots should be effectively determined by the total mass of the pre-crossover galactic cluster involved. This rise of temperature on the CMB map within Hawking spots is found to be in quantitative agreement with the masses of the largest galactic clusters observed in our own aeon what suggests that the physics in the previous aeon was, at least in the gravitational sector, similar to ours. A second observational feature, the actual angular diameter of the Hawking spots seen in our CMB, which is about twice what should have been expected, is associated to the presence of GWE just after the crossover and before the start of the usual cosmological epochs.
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Penrose may be saying that Black Holes survive to leave an imprint on the next Aeon, maybe rings or such, but this is just a lot of mathematics. I am lost really.

To reiterate:


https://backreaction.blogspot.com/2022/02/will-big-bang-repeat.html

You sort it out. I have more practical projects to do.
 
#5,827
system7 said:
Penrose may be saying that Black Holes survive to leave an imprint on the next Aeon
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'Hawking Points' are bright areas in the CMB that Penrose argues are the results of Hawking radiation from supermassive black holes in a previous aeon.

In response to Penrose's claims of the existence of Hawking points, Stephen Hawking said he could see his own initials in the CMB! 😊

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However, there has since been strong observational evidence of numerous previously unobserved anomalous circular spots, of significantly raised temperature, in the cosmic microwave background: https://academic.oup.com/mnras/article/495/3/3403/5838759
 
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#5,829
I doubt, no, I'm certain (almost) that noting can survive a BB - its such an insane reset - everything has become plasma/gas... and the run for a (short) "while" in the dryer - how could any trace of the "previous" survive that. It just simply can't. No information is transfered from universe n to n+1. Period.

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