Debunking the Big Bang Theory: Colliding Branes as a Possible Alternative

[marcus]

Good you emphasize on the observable universe vs actual extent. Anyway. Do you know how many Planck volume can fit in say the hydrogen atom up to the electron orbital? You really think that if the observable universe energy were contained in the hydrogen atom. The Planck volume would merely hold a few micrograms. This would make the Planck scale unimaginably small. I wonder if your analogy is valid (ignoring quantum gravity and HUP).

Or for a radius of 40 Billion light years, how many meters or miles across would be the Planck length? Any ideas?

Answer to blue question is yes. Actually much less than a few micrograms. A hydrogen atom is very big. To get PLANCK density you must compress observable down to something like the size of a proton, the nucleus of the hydrogen atom.

This is around 100 thousand times smaller than the atom, if I remember right.

 

[marcus]

BTW Glyde, Brian Powell said something useful a few posts back.

Science is a dispassionate pursuit that does not benefit from preconceived notions of what "exceeds the imagination". It is an incorrect picture to imagine Planckian energy densities somehow confined to a small region, fighting against this confinement with outward pressure. It is true that both density and pressure determine the gravitational properties of the stress-energy, and, when you place near-Planckian energy densities into the Friedmann equation, you get a perfectly well-behaved cosmological solution.

Sure, such high densities have not been tested in the lab. But we understand the equation of state of radiation, and we have lots of observational evidence that supports the Friedmann model. Extrapolation of these physical theories into untested regimes (as long as the theory is appropriate to these regimes) is a perfectly reasonable and substantiated practice.

I have to go. I want to remark on something in Brian's post. Back later.

OK I'm back. BTW the conventional ideas of the radii of proton and observable U differ by a factor of 10⁴¹. My feeling is that this is not so surprising. I wouldn't say a proton is very SMALL compared with the photons of highest energy light. Indeed a proton is kind of mediumsized as things go. But the observable U is very large. So between medium size and large there are these 41 powers of ten.

If you want to find out some about Planck scale----energy, volume, density, mass, length etc.----Wikipedia is not too bad.
http://en.wikipedia.org/wiki/Planck_units

The linear size associated with the proton is 10^-15 meters. You can read what the Plancklength is and see what their ratio is. The corresponding volumes would be related by the cube of that ratio, whatever you find it to be.

Ive got some other stuff to attend to, but see what you can find out meanwhile on your own.

 

[stglyde]

Answer to blue question is yes. Actually much less than a few micrograms. A hydrogen atom is very big. To get PLANCK density you must compress observable down to something like the size of a proton, the nucleus of the hydrogen atom.

This is around 100 thousand times smaller than the atom, if I remember right.

Energy units are electrovolts.. for example.. the possible mass of Higgs boson to be 125 TeV. I wonder what is the electron volts of the energy of the entire "observable" universe. Now it's still unbelievable that when you put it in the diameter of the nucleus. The Planck volume is almost negligible. But then energy doesn't have pauli exclusion so I wonder why a certain volume should hold certain energy or in what sense is the statement that the energy of the observable universe is put into the size of a hydrogen nucleus and the Planck volume holds only a few microgram since energy doesn't have spatial extension. Anyone else can explain? Thanks (this paragraph is ignoring quantum gravity and HUP for sake of discussion of energy Tev of the universe and containment of it in a limited spatial extension like in a hydrogen nucleus).

 

[Chronos]

I believe the most recent estimates on higgs mass are around 125 GeV.

 

[ThomasT]

... I wonder why a certain volume should hold certain energy ...

That would seem to me to depend on what's being talked about (ie., a certain volume of ... what?), and what it's assumed (or observed) to be doing.

... or in what sense is the statement that the energy of the observable universe is put into the size of a hydrogen nucleus ...

Why would anybody want to suppose that the energy of the observable universe was ever the "size of a hydrogen nucleus"?

Regarding the title question of the thread, and your OP, the standard 'big bang' theory accounts (in a necessarily limited way) for the evolution of our universe from a point (the 'big bang') beyond which (ie., further into the past) backward extrapolation from current observation and theory is impossible.

Whether it's 100% true is a question/consideration that can't be answered. But it makes sense and it's internally consistent (afaik), and (also afaik) it doesn't say or imply that the observable universe was ever the size of a hydrogen nucleus, or the head of a pin, or whatever (also just afaik).

Kudos to bapowell and marcus for their usual informative replies, and to Astronuc for putting the (sophisticated) cosmological speculation into (imo) the proper perspective.

Would I bet my life on the standard big bang theory being 100% true? No. But it certainly makes sense insofar as it's relevant. If somebody comes up with a better account, I feel sure that they'll publish it.

 

[marcus]

Glyde what you said about radiation energy (light, UV, gamma etc) not having Pauli exclusion is very important. The shorter the wavelength the more energy, too. There is no limit to how many photons fit in the box AND the smaller they are the more energy each one carries.

Can't you estimate how many Planck volumes are in a proton volume?

Planck length is about 10^-35 meters and proton scale is about 10^-15 meters. So cube their ratio.

Proton volume is about 10⁶⁰ times Planck volume. Like I said, the proton is kind of medium sized.

The standard metric unit of energy is a JOULE. It is the amount you expend if you lift a (kilogram) book about 10 centimeters off the table. It is the amount of thud you hear when you drop it back onto table.

The energy density of the U, with all matter converted to the common currency of light, is 0.22 nanojoules per cubic meter. What would density be if the volume of the observable were replaced by the proton volume but with same amount of energy. You know the linear size ratio (I gave it earlier).

I hope you get the hang of doing some simple physics arithmetic on your own soon if you have not already. Go ahead risk making mistakes. If you do someone will probably catch them, no harm done.

 

[stglyde]

Markus, I'll ponder on the above. But I need to know something now and the reason why I wrote this thread. Remember Lorentz when he tried to explain the reason for the null result of the Michelson-Morley Experiment. He claimed length can contract and time can shorten when something moving in the ether, that's why the MMX produced null result and all experiments up to the present can't distinguish between Lorentz Ether Theory and Special Relativity because they both use the Lorentz Transformation (which was invented by Lorentz before Einstein discovered SR). I think you are pretty familiar with LET. Now let's not debate about LET. What I want to know is whether the Big Bang is compatible with Lorentz Ether or how the Big Bang can give birth to Lorentz Ether. If no experiment can distinguish LET and SR. Can LET perhaps be refuted by strong evidence of the Big Bang.. assuming Big Bang and Lorentz Aether is totally incompatible. If not. Any papers or models of how the Big Bang can give rise or give birth to the Lorentz Ether?

 

[marcus]

Markus, I'll ponder on the above. But I need to know something now and the reason why I wrote this thread. Remember Lorentz when he tried to explain the reason for the null result of the Michelson-Morley Experiment. He claimed length can contract and time can shorten when something moving in the ether, that's why the MMX produced null result and all experiments up to the present can't distinguish between Lorentz Ether Theory and Special Relativity because they both use the Lorentz Transformation (which was invented by Lorentz before Einstein discovered SR). I think you are pretty familiar with LET. Now let's not debate about LET. What I want to know is whether the Big Bang is compatible with Lorentz Ether or how the Big Bang can give birth to Lorentz Ether. If no experiment can distinguish LET and SR. Can LET perhaps be refuted by strong evidence of the Big Bang.. assuming Big Bang and Lorentz Aether is totally incompatible. If not. Any papers or models of how the Big Bang can give rise or give birth to the Lorentz Ether?

Glyde, this is something of a new line of questioning. I will try to carry it along so we don't forget it. Maybe someone else will respond in a useful way, who knows more and has thought more about it than I have.

But before I try to understand the new I want to finish the discussion of Planck energy density and how to picture it.

People have different conjectures about the start of expansion---the actual start, that very moment, is not covered by classical GR and standard cosmology. So people are working on various models and they typically do involve densities around Planck.

One very concrete and definite model of the start of expansion is the (LQC) BOUNCE and when they study different cases and either solve the equations or run the computer simulations with various inputs it typically happens that the bounce occurs when the density is 41% of Planck.

In effect we always face the need to picture Planck energy density, with whichever model.
If you iike to picture stuff in your mind, as many do. The simplest is to think of that density of LIGHT filling the universe. Imagine that ordinary matter boiled away into light already at lower density. Nothing that occupies any space is left. Only photons. You know from LASERS that you can put as many photons as you want on top of each other like sardines without limit.

The typical photon in the mix has wavelength equal Planck length. (the smaller the more energetic, the hotter the light). That is wavelength equal to 10^-35 meter. So within the space of a proton sized 10^-15 meter he has room for a lot of ripples. A proton is a huge space for these photons because, being such hot and energetic light their wavelength is very small. And they have no Pauli territoriality, they welcome each other's company.

So let's put some numbers. The energy equivalent of 22 micrograms (i.e. Planck mass) is 1.9 billion joules or in round numbers 2 billion joules. It's like the energy equiv of a tank of gas. It is the Planck energy unit.
So at Planck density, or at 41% of Planck energy or whatever, everything is pure energy and we can picture the U filled with very hot bright light with about a billion joules in each Planck volume. Or two billion, if we are imagining Planck density instead of 41% of it.

...

...estimate how many Planck volumes are in a proton volume?

Planck length is about 10^-35 meters and proton scale is about 10^-15 meters. So cube their ratio.

Proton volume is about 10⁶⁰ times Planck volume...
The standard metric unit of energy is a JOULE. It is the amount you expend if you lift a (kilogram) book about 10 centimeters off the table. It is the amount of thud you hear when you drop it back onto table.

The energy density of the U, with all matter converted to the common currency of light, is 0.22 nanojoules per cubic meter...

So a proton volume has 10⁶⁰ Planck volumes each of which contains a two billion joules of light. So it contains 2x10⁶⁹ joules.

Lets compare that with (the energy equivalent of) the observable universe. The Hubble distance is about 13.8 billion light years. If you type that into google you get
1.3 x 10²⁶ meters. Radius of the observable is about 3.3 times Hubble distance so say 4x 10²⁶ meters. I think that makes the observable volume about 2.5 x 10⁸⁰ cubic meters. You might check that with a calculator.
So what do we get if each cubic meter of today's universe has on average the matter equivalent of 0.22 nanojoules? I get around 5 x 10⁷⁰ joules.

This has been quick and sloppy, it is not good enough to quote in another post. anyone who cares to do so could improve the accuracy and the result might change by up to an order of magnitude. But it gives the right idea. If you think of the proton volume as a room containing jillions of photons of very high temperature short wavelength light, and you imagine that the energy density is Planck,

then that proton volume contains about the same as the energy equivalent of the observable universe volume. This is admittedly kind of clunky. Anyone who wants is welcome to make it neater and more precise.

 

[Tanelorn]

Marcus, Re Proton energy ratios, that is very interesting that they are so close. It can't be any more than a mathemetical curiosity can it? Also how can all the protons in the observable universe have similar values of energy to just one proton?

 

[marcus]

Hi T., I did not see your post when I was typing this. Not sure how to respond to your question though. What I wanted to say was it's important to stress a point that Brian Powell made earlier. Not to think of the UNIVERSE as the size of a proton at the start of expansion.
We were talking about the observable and what is destined to expand to form the part that is currently observable to us.

The whole universe (at expansion start) is presumably quite a lot bigger than a proton!
And energy is all over the place. You cannot carve out and mentally isolate a little protonsized pocket of it and say "this energy in this pocket is what is going to make our observable universe".

You could only do that if everything held still and remained nicely compartmentalized. But it is all over the place.

So all this mental exercise with the proton volume does for us is give us practice imagining the ENERGY DENSITY we think the U had, as a whole, at the start of expansion. At that moment the "observable" part was not well defined and was all over the place without clear boundaries, so it had as yet no meaning. So the exercise is simply about visualizing a density.

There is no meaningful entity that had, at that time, "the size of a proton". So one cannot say that what is now our observable U started out as something the size of a proton. It is not so neat and compartmentalized.

The actual energy is all over the place and we can't lay claim to some definite chunk of it as ours. But we can talk about the density. In each protonsize volume there was at that time AFAIK enough to be equivalent to what now is in our observable region. That gives an idea how much is in any given volume at the time expansion started.

The whole universe should be pictured as (probably much) much larger than a proton. All of that same density. And of course boundaryless. No edge and no outside.

Particular bounce cosmology models would give you different estimates of the exact density (like LQG fairly consistently says 41% of Planck) but whatever the different models they tend to say something that is within an order of magnitude or so of Planck density. So that is what the mental exercise is really about imagining.
===========
Tanelorn AFAIK the proton is just an arbitrary choice to establish a visual image of a scale of size and volume. AFAIK it primarily comes up in cosmo popularizations where they have to give the reader something to visualize, like an atom, or the nucleus of an atom, or in this case the nucleus of a hydrogen atom (i.e. proton). To establish an idea of scale.

So it's probably just completely arbitrary. And as you know from the point of view of the three little quarks inside, a proton is just this big empty space with three flies buzzing around in it. The flies think that THEY are fundamental, not the big room they are buzzing around in. So the proton is here just a nice way to imagine a convenient amount of nearly empty space that has a definite size.

That's what I think anyway. Could be wrong.

 

[Tanelorn]

Hi Marcus, I just reread and saw that you said each cubic meter contains the same energy as a proton and not the whole observable universe. Thats what happens when you go speed reading!

Anyways seasons greetings to you and all at PF!

 

[Notwen7]

The standard big bang model purports that the big bang happened everywhere at once, not at a single point. This is an important misconception to straighten out. The colliding branes theory is a proposal that seeks to explain the physical mechanism for the big bang itself. Therefore, it is not in opposition to the standard big bang model; rather, it seeks to extend it. The colliding branes theory is still only hypothesis. The colliding branes theory has the rather unfortunate name of "ekpyrosis" in case you wish to read more about it.

Can you explain how it happened at multiple points at once? Also how do scientists arrive at these hypotheses?

 

[Tanelorn]

Notwen you are being misled by a bad name. I dislike Big Bang the name a great deal for that reason.
It is not an explosion from a point.
Inflation from a state of very high density and high temperature takes place everywhere at the same time.
The universe was almost infinite then and much much bigger now!

 

[Notwen7]

Notwen you are being misled by a bad name. I dislike Big Bang the name a great deal for that reason.
It is not an explosion from a point.
Inflation from a state of very high density and high temperature takes place everywhere at the same time.
The universe was almost infinite then and much much bigger now!

Indeed, I am. So in a sense it is almost like the dew point of a liquid? I'm imagining the small little vapor bubbles forming simultaneously being similar to the release of energy and matter from many places of the universe?

 

[Drakkith]

Indeed, I am. So in a sense it is almost like the dew point of a liquid? I'm imagining the small little vapor bubbles forming simultaneously being similar to the release of energy and matter from many places of the universe?

No. Imagine being a raisin inside a muffin in the oven. As the muffin starts to rise, all the other raisins seem to get further away from you as the muffin bakes. The muffin is like space and the raisins are like galaxies. The big difference is that a muffin occupies a finite volume. The universe is thought not to. Imagine being inside an infinitely large muffin mix. The raisins 10 miles from you would be receding from you at a much higher rate than the raisins next to you. This is all the "Big Bang" is. It is simply that the universe was once in a much denser state than we are now, similar to how the muffin mix is much thicker before cooked. No explosion, no bang.

 

[stglyde]

No. Imagine being a raisin inside a muffin in the oven. As the muffin starts to rise, all the other raisins seem to get further away from you as the muffin bakes. The muffin is like space and the raisins are like galaxies. The big difference is that a muffin occupies a finite volume. The universe is thought not to. Imagine being inside an infinitely large muffin mix. The raisins 10 miles from you would be receding from you at a much higher rate than the raisins next to you. This is all the "Big Bang" is. It is simply that the universe was once in a much denser state than we are now, similar to how the muffin mix is much thicker before cooked. No explosion, no bang.

How about spacetime.. isn't the fact that redshift occurs is because space is being expanded.. so can't we say spacetime was getting bigger from the initial core or Big Bang gave birth to spacetime? If not.. and if spacetime already exists. Can it also support the observation that redshift occurs and even the microwave background radiation becomes 3 degrees kelvin because the wavelength got expanded too. Can they do it without space being expanded?

 

[Drakkith]

How about spacetime.. isn't the fact that redshift occurs is because space is being expanded.. so can't we say spacetime was getting bigger from the initial core or Big Bang gave birth to spacetime? If not.. and if spacetime already exists.

You can say whatever you want. The theory that the Big Bang was conceived from says nothing about this, it only says that the universe was once in a very dense state.

Can it also support the observation that redshift occurs and even the microwave background radiation becomes 3 degrees kelvin because the wavelength got expanded too. Can they do it without space being expanded?

Can what support this?

 

[stglyde]

You can say whatever you want. The theory that the Big Bang was conceived from says nothing about this, it only says that the universe was once in a very dense state.



Can what support this?

Isn't it that microwave background radiation 3 Kelvin temperature was a result of the wavelength being expanded due to spacetime being expanded from the Big Bang? So logic says spacetime was once smaller perhaps the same size as whatever bang...

 

[Cosmo Novice]

Isn't it that microwave background radiation 3 Kelvin temperature was a result of the wavelength being expanded due to spacetime being expanded from the Big Bang? So logic says spacetime was once smaller perhaps the same size as whatever bang...

This is completely moot if the U is infinite. If it is infinite now then it always infinite.

 

[stglyde]

This is completely moot if the U is infinite. If it is infinite now then it always infinite.

Is this your own speculation as a novice or did you hear it elsewhere? Can other non-novice confirm if it's true?

 

[bapowell]

Isn't it that microwave background radiation 3 Kelvin temperature was a result of the wavelength being expanded due to spacetime being expanded from the Big Bang? So logic says spacetime was once smaller perhaps the same size as whatever bang...

Your wording isn't very clear, so I'm not certain of your question. The photons that make up the cosmic microwave background have been redshifting with the expansion of the universe since they were "created" at decoupling.

 

[Cosmo Novice]

Is this your own speculation as a novice or did you hear it elsewhere? Can other non-novice confirm if it's true?

This is a mathematical conjecture; if something is infinite now then it was always infinite. This is not to say that infinities cannot grow. This is more to do with number theory than cosmology but this is what I have deduced from a lot of independent research into the matter.

I may be incorrect and open to any challenges/corrections to my statement.

If the Universe is infinite, it must have always been infinite; the reason being; if you could pause expansion right now and move quickly across the Universe, you would cross an infinite amount of Universe without ever encountering the same patch of Universe twice. Now as we know the Universe is just in a less dense state, if you could have traveled across it 10 billion years ago it would still have been infinite just also a lot more dense. Its very logical if you think about it. This is of course assuming a spatially flat and infinite U.

 

[Calimero]

...if you could pause expansion right now and move quickly across the Universe, you would cross an infinite amount of Universe without ever encountering the same patch of Universe twice.

Not quite. You would encounter same patch infinite amount of times. Read this, if you have time and will, interesting stuff!

 

[ThomasT]

So, wrt recent replies, it seems to me that it isn't, and can't be, known whether our universe is infinite or not. It also seems to me that this is irrelevant to the OP question of whether the mainstream big bang theory is true or not. There's no way to know. Period. So, might as well close the thread.

But then there is that interesting thing about evidence for the expansion of the universe. And I have to ask myself: why would an infinite universe appear to be expanding?

 

[nazarbaz]

Cantor proved that we could always conceive of bigger infinities...
But what do we mean by infinity in this particular case ? What does it imply both on the ontological (perhaps I must say ontical) and phlosophical levels ? Causality still holds ?
It's dubious...

 

[bapowell]

And I have to ask myself: why would an infinite universe appear to be expanding?

Can you imagine an infinite rubber sheet being stretched in all directions?

 

[marcus]

... if something is infinite now then it was always infinite. ..
If the Universe is infinite, it must have always been infinite; the reason being; if you could pause expansion right now and move quickly across the Universe, you would cross an infinite amount of Universe without ever encountering the same patch of Universe twice. Now as we know the Universe is just in a less dense state, if you could have traveled across it 10 billion years ago it would still have been infinite just also a lot more dense. Its very logical if you think about it. This is of course assuming a spatially flat and infinite U.

I agree Cosmo N. That certainly makes sense and is in line with conventional wisdom of the pros. Callimero's objection does not apply since even Tegmark would not say that you would cross the identical patch, only SIMILAR patches which by law of probability would come up.

Thomas, nobody said that you can't EVER decide between the finite and infinite versions of the model. They've been working on that. In a math science the aim is to find the simplest best fit model---the most reliable understanding of the world. You never establish absolute truth---at each step you only get the best we can do so far.

In that sense, if the U is spatially finite like, say, a 3D hypersphere with a very slight positive curvature then it is quite likely we will find that out with a few more missions like WMAP and Planck. Or with the help of the Webb telescope (if it doesn't get cut from NASA budget.) They have a 95% confidence interval for the largescale curvature and it is slightly lopsided on the positive side---which s suggestive of finite but not conclusive. The confidence interval contains zero and some possible range on both the pos and the neg side.

the idea is to get more data to narrow down the confidence interval and see what emerges. If they narrow it down and the new tighter range does NOT incude zero that would say we are in a 3D space analogous to the 2D balloon surface but one higher dimension, i.e. finite volume.

So, wrt recent replies, it seems to me that it isn't, and can't be, known whether our universe is infinite or not. It also seems to me that this is irrelevant to the OP question of whether the mainstream big bang theory is true or not. There's no way to know. Period. So, might as well close the thread.

I see your point. Nothing in science is said to be 100% true. We don't know that there were dinosaurs because space aliens could have put fake bones in the rocks to fool us. But it is extremely improbable.
But we can, in a practical sense, say that the expansion cosmology model is true. Because supported by vast amount of interlocking data of many different kinds. As more data comes in it keeps confirming that the model is reliable. So I would say not LITERALLY 100% but as good as you can reasonably ask for.

And of course there is a lot of research working on Big Bang 2.0, the next upgrade. The "singularity" or failure point in version 1.0 will be replaced by something more physical, maybe extending back in time to before when the old model fails. The theory is evolving but the main features remain unchanged.

But then there is that interesting thing about evidence for the expansion of the universe. And I have to ask myself: why would an infinite universe appear to be expanding?

Why not? What we see is distances expanding according to a pattern that is one of the solutions of the GR equation. The pattern of expanding distances is pretty much the same whether we are in a finite or an infinite volume of space. Either way, stuff gets farther apart without there being motion in the ordinary sense of going somewhere.

An infinite space can appear to be expanding to the people who live in it just in the same way that a finite space can appear to be expanding to the people who live in it. Either way, there is no "outside" to space---so you could get confused if you try to picture looking at it from outside. Focus on the experience of the creatures living in the changing geometry.

Ha! I see that Brian Powell just said this all much more concisely. In one sentence, while I was mulling it over.

 

[Calimero]

I agree Cosmo N. That certainly makes sense and is in line with conventional wisdom of the pros. Callimero's objection does not apply since even Tegmark would not say that you would cross the identical patch, only SIMILAR patches which by law of probability would come up.

I don't know marcus, not much of probability guy myself, but it appears to me that any finite set can be found in infinite set infinite amount of times. That would mean IDENTICAL configuration down to the last atom for any finite volume, if laws of physics are same everywhere.

 

[marcus]

I don't know marcus, not much of probability guy myself, but it appears to me that any finite set can be found in infinite set infinite amount of times. That would mean IDENTICAL configuration down to the last atom for any finite volume, if laws of physics are same everywhere.

You might be right! Assuming geometry is discrete too, I guess. There have to be the same atoms and electromagnetic waves and it all has to be in the same POSITION, as here. I haven't thought about it. It seems to extremely improbable to find an exact duplicate. But if geometry can assume only a finite set of configurations then I guess the probability of an exact duplicate could be non-vanishing.

this is the type of thing that causes me to psychologically feel more comfortable with spatially finite universe models. but since infinite has not been ruled out so far, i want to keep an open mind about it.

 

[stglyde]

I agree Cosmo N. That certainly makes sense and is in line with conventional wisdom of the pros. Callimero's objection does not apply since even Tegmark would not say that you would cross the identical patch, only SIMILAR patches which by law of probability would come up.

Thomas, nobody said that you can't EVER decide between the finite and infinite versions of the model. They've been working on that. In a math science the aim is to find the simplest best fit model---the most reliable understanding of the world. You never establish absolute truth---at each step you only get the best we can do so far.

In that sense, if the U is spatially finite like, say, a 3D hypersphere with a very slight positive curvature then it is quite likely we will find that out with a few more missions like WMAP and Planck. Or with the help of the Webb telescope (if it doesn't get cut from NASA budget.) They have a 95% confidence interval for the largescale curvature and it is slightly lopsided on the positive side---which s suggestive of finite but not conclusive. The confidence interval contains zero and some possible range on both the pos and the neg side.

the idea is to get more data to narrow down the confidence interval and see what emerges. If they narrow it down and the new tighter range does NOT incude zero that would say we are in a 3D space analogous to the 2D balloon surface but one higher dimension, i.e. finite volume.


I see your point. Nothing in science is said to be 100% true. We don't know that there were dinosaurs because space aliens could have put fake bones in the rocks to fool us. But it is extremely improbable.
But we can, in a practical sense, say that the expansion cosmology model is true. Because supported by vast amount of interlocking data of many different kinds. As more data comes in it keeps confirming that the model is reliable. So I would say not LITERALLY 100% but as good as you can reasonably ask for.

And of course there is a lot of research working on Big Bang 2.0, the next upgrade. The "singularity" or failure point in version 1.0 will be replaced by something more physical, maybe extending back in time to before when the old model fails. The theory is evolving but the main features remain unchanged.




Why not? What we see is distances expanding according to a pattern that is one of the solutions of the GR equation. The pattern of expanding distances is pretty much the same whether we are in a finite or an infinite volume of space. Either way, stuff gets farther apart without there being motion in the ordinary sense of going somewhere.

An infinite space can appear to be expanding to the people who live in it just in the same way that a finite space can appear to be expanding to the people who live in it. Either way, there is no "outside" to space---so you could get confused if you try to picture looking at it from outside. Focus on the experience of the creatures living in the changing geometry.

Ha! I see that Brian Powell just said this all much more concisely. In one sentence, while I was mulling it over.

In the infinite space before inflation.. the whole of the universe can be crossed many times in 1 second (although I know that everything was in pure energy and the 4 forces haven't suffered symmetry breaking yet but just for sake of illustration). While after inflation and billions of years afterwards, the whole of the universe can be crossed in many aeons instead of 1 second. Agree with this? But then, you guys won't agree with the terms "Big Bang gave birth to spacetime" because you think spacetime may already exist although very tiny. So you guys just agree on "Big Bang expanded the very tiny spacetime, that is all we know. We can never know if Big Bang also act as spacetime seed much like sperm-egg (space-time) being fertilized?", right guys?

 

[stglyde]

Glyde, this is something of a new line of questioning. I will try to carry it along so we don't forget it. Maybe someone else will respond in a useful way, who knows more and has thought more about it than I have.

But before I try to understand the new I want to finish the discussion of Planck energy density and how to picture it.

People have different conjectures about the start of expansion---the actual start, that very moment, is not covered by classical GR and standard cosmology. So people are working on various models and they typically do involve densities around Planck.

One very concrete and definite model of the start of expansion is the (LQC) BOUNCE and when they study different cases and either solve the equations or run the computer simulations with various inputs it typically happens that the bounce occurs when the density is 41% of Planck.

In effect we always face the need to picture Planck energy density, with whichever model.
If you iike to picture stuff in your mind, as many do. The simplest is to think of that density of LIGHT filling the universe. Imagine that ordinary matter boiled away into light already at lower density. Nothing that occupies any space is left. Only photons. You know from LASERS that you can put as many photons as you want on top of each other like sardines without limit.

The typical photon in the mix has wavelength equal Planck length. (the smaller the more energetic, the hotter the light). That is wavelength equal to 10^-35 meter. So within the space of a proton sized 10^-15 meter he has room for a lot of ripples. A proton is a huge space for these photons because, being such hot and energetic light their wavelength is very small. And they have no Pauli territoriality, they welcome each other's company.

So let's put some numbers. The energy equivalent of 22 micrograms (i.e. Planck mass) is 1.9 billion joules or in round numbers 2 billion joules. It's like the energy equiv of a tank of gas. It is the Planck energy unit.
So at Planck density, or at 41% of Planck energy or whatever, everything is pure energy and we can picture the U filled with very hot bright light with about a billion joules in each Planck volume. Or two billion, if we are imagining Planck density instead of 41% of it.


So a proton volume has 10⁶⁰ Planck volumes each of which contains a two billion joules of light. So it contains 2x10⁶⁹ joules.

Let's go back to this. So a proton volume has 10⁶⁰ Planck volumes each. Since we don't deal with such numbers in daily life. We don't know how small it is. Supposed the proton volume is the size of a sphere 1 light years across. How many inches or meters is the Planck volume equivalent? In our daily life. We only deal at most with 10^2, not 10^60. So can't imagine it. But maybe we can calculate. For a proton volume to become 1 light year volume. The Planck volume (expanded) becomes 1 light years of volume divided by 10⁶⁰? Any math genius can share what is the equivalent miles of the Planck length for the proton length to be 1 light year? Thanks.

Lets compare that with (the energy equivalent of) the observable universe. The Hubble distance is about 13.8 billion light years. If you type that into google you get
1.3 x 10²⁶ meters. Radius of the observable is about 3.3 times Hubble distance so say 4x 10²⁶ meters. I think that makes the observable volume about 2.5 x 10⁸⁰ cubic meters. You might check that with a calculator.
So what do we get if each cubic meter of today's universe has on average the matter equivalent of 0.22 nanojoules? I get around 5 x 10⁷⁰ joules.

So the whole universe of 5 x 10⁷⁰ joules can be put in the volume of about 2 proton that can contain 2x10⁶⁹ joules each? Amazing. But there are some Big Bang book that mentions 5 x 10⁷⁰ joules can be put in the Planck volume which they assume is the singalarity? How can this be?

So let's put some numbers. The energy equivalent of 22 micrograms (i.e. Planck mass) is 1.9 billion joules or in round numbers 2 billion joules. It's like the energy equiv of a tank of gas. It is the Planck energy unit.
So at Planck density, or at 41% of Planck energy or whatever, everything is pure energy and we can picture the U filled with very hot bright light with about a billion joules in each Planck volume. Or two billion, if we are imagining Planck density instead of 41% of it.

Maybe it's related to this. How come the Planck mass is equivalent to the energy of 22 micrograms or 1.9 billion joules or in round numbers 2 billion joules and not 5 x 10⁷⁰ joules. Is it possible to put 5 x 10⁷⁰ joules in the Planck volume? Why impossible?

Thanks a lot.

This has been quick and sloppy, it is not good enough to quote in another post. anyone who cares to do so could improve the accuracy and the result might change by up to an order of magnitude. But it gives the right idea. If you think of the proton volume as a room containing jillions of photons of very high temperature short wavelength light, and you imagine that the energy density is Planck,

then that proton volume contains about the same as the energy equivalent of the observable universe volume. This is admittedly kind of clunky. Anyone who wants is welcome to make it neater and more precise.

 

[stglyde]

Let's go back to this. So a proton volume has 10⁶⁰ Planck volumes each. Since we don't deal with such numbers in daily life. We don't know how small it is. Supposed the proton volume is the size of a sphere 1 light years across. How many inches or meters is the Planck volume equivalent? In our daily life. We only deal at most with 10^2, not 10^60. So can't imagine it. But maybe we can calculate. For a proton volume to become 1 light year volume. The Planck volume (expanded) becomes 1 light years of volume divided by 10⁶⁰? Any math genius can share what is the equivalent miles of the Planck length for the proton length to be 1 light year? Thanks.

I'd share some answers i got.

1. There are 20 orders of magnitude between the hydrogen nucleus and the Planck length (~10^-15 m and ~10^-35 m respectively). Since a light year is ~10^16 m, your answer would be about one-tenth of a millimeter.

or

2. Planck length is so small if we were able to magnify an atom to the size of the known universe,the Planck length would be about 100 feet.

Planck length is incredibly so tiny and yet we worry a lot about Planck scale physics. Gee.

 

[marcus]

...
Planck length is incredibly so tiny and yet we worry a lot about Planck scale physics. Gee.

Yes! maybe you are beginning to see why! It's the only way to picture really really high temperature and density. I woudn't say worry but it certainly is interesting.

A space full of radiation at Planck temperature you can think of as being full of photons with incredibly short wavelength. The shorter wavelength a photon has the higher energy it carries.

If you have a box full of nothing but light and you heat it up to Planck temperature then it is full to the extreme of energy density with photons of the extreme short (Planck length) wavelength.

This is how to understand the universe at LQG bounce or at start of expansion. So hot that nothing can exist but radiation. And a photon in that conditions delivers an incredible 2 billion joules.

In cosmic rays we occasionally see particles (not photons though) with energy of 1 joule or 10 joules. That is already an amazing amount of wallop for a single particle to deliver.

So we are talking about photons HUNDREDS OF MILLIONS TIMES MORE POWERFUL than what we can detect in cosmic ray

And as you heat the box of light, and the photons get more and more powerful, the photons get SMALLER AND SMALLER and there get to be MORE AND MORE of them. Higher temperature means smaller photons, more energetic photons, and higher density.

It is easy to see how energy can get so dense because the photons don't mind crowding (they don't take up any space) and they are in any case Planck wavelength small(which you have learned is very small).

No ordinary matter would be existing, too hot for it. Just pure energy of radiation. So that's one way to think of it.

And of course as we have seen radiation at Planck density has enough energy to make all the galaxies we can see, all the observable, out of just the energy in the volume of a hydrogen atom. We've calculated that about the Planck energy density. It is seriously extreme.

I can see why an intelligent person beginning to grasp Planck scale energy density would be impressed and say "Gee".

Merry christmas by the way.

 

[ThomasT]

This is just a note to say Happy Holidays to all for whom such salutations are relevant, and to thank you guys and gals for helping ignorant laypersons such as myself to understand our universe in some way approximating the way that the pros do.

This turned out to be a really interesting and informative thread for me. Thanks to marcus and bapowell for replies to my somewhat cynical take on things.

It remains for me to review and understand marcus' "back of an envelope" calculations. My general, layperson, thinking on this might have been a bit off.

What I do understand of the standard 'big bang' cosmology does make sense to me.

Of course, I'm just a novice wrt this stuff. It's fascinating. I 'gravitated' to PF because I desperately wanted to understand Bell's theorem. Which I think I now understand. But it took about eight (8) years. (Ok, I'm a slow learner.)

This cosmology stuff promises to be even more difficult.

Anyway, your efforts are appreciated.

 

[stglyde]

Markus,

In one of the quantum gravity books. It is mentioned that there are 4 roads to quantum gravity:

1. quantising General Relativity
2. quantising a different classical theory, while still having general relativity emerge as a low- energy (large-distance) limit.
3. having general relativity emerge as a low-energy limit of a quantum theory that is not a quantization of a classical theory
4. having both general relativity and quantum theory emerge from a theory very different from both

You have listings of many Quantum gravity models in your earlier post. Are they part of the above or are they new additions? How do you sort categorize each based on the above classifications?

I'm interested in 4. What models have you come across that is about both general relativity and quantum theory emerge from a theory very different from both, and which of them is your favorite, and why?

Thanks a lot for your help.