# Contents of the U

by bobie
Tags: contents
PF Gold
P: 533
Matter in U is extimated 1050Kg, what is the radius/volume of the U where this amount of matter is distributed?
 The present overall density of the Universe is very low, roughly 9.9 × 10−30 grams per cubic centimetre.
What is the density of matter: 9.9*10-30*4/100?
Can we get that volume dividing 1050 by this figure?

 a(t) is the linear scale of the universe (so a(t) increases as time t increases), then the volume that a given amount of matter occupies is proportional to a(t)3, and thus matter densities scales as 1/a(t)3. Radiation is made of photons, and photon density also scales as 1/a(t)3, but the expansion of the universe also scales the wavelength by another factor of a(t), so radiation energy density scales as 1/a(t)4.
Is that factor in addition to cosmological redshift or is it already included in z?
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Thanks ∞
P: 12,425
Please provide a reference for your quotations - where did you get these numbers from.
That same reference should tell you where they got these numbers from and what they mean by "the U" ... (I'm guessing you mean U=Universe here).

 Quote by bobie Matter in U is extimated 1050Kg, what is the radius/volume of the U where this amount of matter is distributed?
According to: Paul Davies in The Goldilocks Enigma, the mass of ordinary matter in the Observable Universe is commonly quoted as 1050tonnes or 1053kg ... which you can look up: it's about a 46.6 billion ly radius.

 What is the density of matter 9.9*10-30*4/100
density: 9.9*10-30*4/100 = 36.6x10-32(g/cm3)

Off: http://curious.astro.cornell.edu/que...php?number=342
A common figure for the density of matter in the observable universe is 3x10-30g/cm3 ... which has the same order of magnitude as the figure you started with. This what you mean?

A proton masses about 10-24g so you can figure out how much volume of empty space must surround a single proton in order to get the same average density as the observable universe.

 Can we get that volume dividing 1050 by this figure?
Probably not.
Now you know the radius, you can do the calculation and see.

 Is that factor in addition to cosmological redshift or is it already included in z?
Which factor?
PF Gold
P: 533
 Quote by Simon Bridge Which factor? Please be specific.
 a(t) is the linear scale of the universe (so a(t) increases as time t increases), then the volume that a given amount of matter occupies is proportional to a(t)3, and thus matter densities scales as 1/a(t)3. Radiation is made of photons, and photon density also scales as 1/a(t)3, but the expansion of the universe also scales the wavelength by another factor of a(t), so radiation energy density scales as 1/a(t)4.
The extra factor bolded in the quote reduces the density of energy. I suppose it is related to cosmological redshift z (+1), which is proportional to the linear increase in space, when space is obviously always increasing by volume . So how does this extra factor work on z?

Is the content of matter/ energy now exactly the same as at the time of BB?
when did dark matter/energy originate?

Thanks Simon

 Homework Sci Advisor HW Helper Thanks ∞ P: 12,425 Contents of the U a(t) scales the wavelength - i.e. it is part of the redshift.
PF Gold
P: 533
 Quote by Simon Bridge a(t) scales the wavelength - i.e. it is part of the redshift.
Do you mean part of the redshift, is it not just THE redshift?
Do you know if the energy/matter content is the same now as at BB?
If it is the same, was dark matter/energy present at BB?
What is the difference between dark matter and black holes?
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Thanks ∞
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 Do you mean part of the redshift, is it not just THE redshift?
... $$a(t)=\frac{1}{1+z}$$

 Do you know if the energy/matter content is the same now as at BB?
No I don't - nobody does.
But it is the usual bet.

 If it is the same, was dark matter/energy present at BB?
Yes.

 What is the difference between dark matter and black holes?
Black holes are a specific formt hat matter takes, it is unknown what form or forms constitute dark matter.
Certainly some dark matter may be from black holes, but not all.

Much of these answers can be looked up easily, i.e.:
http://en.wikipedia.org/wiki/Scale_factor_(cosmology)
http://en.wikipedia.org/wiki/Dark_matter

The way to get the best use of these forums is to do your own research first - look up your questions esewhere - and when you get stuck you can ask specific questions.
PF Gold
P: 533
 Quote by Simon Bridge when you get stuck you can ask specific questions.
The values are somewhat different, the total amount should then be 1.45*1053/4(.8)*100 kg = 3*1054kg

I could find nowhere if total energy is exactly the same as at BB
These where preliminary questionsjust to see if I got it right.

My question is:
if the energy content of the visible U is the same as at BB then, presumably, that applies to any part of U.
If the energy of the U is always the same, then, if it collapses once, it should always collapse, since the conditions will never change.
But this is questioned by both the theorists of BB and the theorists of Big Bounce, that admit that this might be the last Bounce, can you tell me where to find an annswer?
Or probably I should ask this in the Cosmology forum
Astronomy
PF Gold
P: 23,084
 Quote by bobie ... then if it collapses once, it should always collapse, since the conditions will never change. ...
This is a misconception, Bobie. The famous deSitter space of around 1917 already has only one bounce in the sense that it contracts, rebounds once, and then expands forever. It is a classical solution to the classical 1915 Einstein GR equation. Indeed it was one of the first solutions to the GR equation that was ever discovered.*

"Conditions" are obviously different because in the collapsing phase it is collapsing and in the expanding phase it is expanding.

Can you give us a link to where you got the idea that if it collapses once it must always collapse again? This is simply untrue and I am curious to see an online source that gave you that misconception. Maybe I can show you that it does not apply, or that you have misinterpreted what you read. It is a strange idea.

I hope others, such as Simon Bridge!, will also respond. Maybe they can guess what is confusing you and help you.

*The story is interesting. the deSitter solution was discovered by two people at about the same time, Willem de Sitter in Holland, and Tullio Levi-Civita in Italy, in 1917 shortly after the GR equation was first published. But for most of the the next 80 years (until 1998) it was believed by most cosmologists that a key physical constant in Nature was zero. This constant must be positive in order for the deSitter solution to be viable. So for some 80 years the solution was treated by most people as a kind of mathematical curiosity, beautifully simple and interesting mathematically but not applicable in a practical way.

The in 1998 it was learned, I guess to the surprise of many people, that this natural constant in the Einstein equation was NOT zero, after all, but indeed was positive (as the deSitter solution requires) and could be measured by astronomers. So this became an admittedly very simplified but in a rough sense more useful picture. In its broad outlines (despite being very simple) the deSitter solution to GR can serve as a useful diagram.
PF Gold
P: 533
 Quote by marcus "Conditions" are obviously different because in the collapsing phase it is collapsing and in the expanding phase it is expanding.
Hi marcus, I didn't read that, I watched the interview to Ashtekar and he does not exclude that as a possibility, talks about eternal inflation, probably I got it wrong, as I linked that to the original Indian theory of the expanding egg.
He says that BB was not a singularity but a Bounce, staritng from an expanded U,
.. but that implies that U has expanded , can you imagine that as a starting point of a theory?

Conditions in the collapsing are obviously different , but if it is a cycle the conditions of the whole cycle are the same.
 whether U will recollapse has to do with the energy content of U to day
I thought that only a difference in critical mass could produce a change that would change the result, and asked here
But if the contents of the U remain the same, what makes the second expansion different?
I read somewhere else: since now Ω is almost exactly =1 then the expansion will go on forever, do I remember right? (I read a lot of articles lately and I cannot retrace that)
If so, doesn't Ω vary similarly in the first , second or third bounce? radius of curvature varies with expansion. Same applies to density or other parameters. What really matter is energy content.

btw, congrats for being world-wide credited!
Astronomy
PF Gold
P: 23,084
 Quote by bobie ... I thought that only a difference in critical mass could produce a change that would change the result, ...
Hi Bobie, I think you mean critical DENSITY. Density is average mass (or energy) per unit volume.

You may have in mind the "pre-1998 mistake". Before 1998 the public was often told a kind of oversimplified "catechism" to the effect that universes were of three kinds "closed, flat, or open" depending on whether their average density (not their mass!) was greater than, equal to, or less than a certain "critical density". This is rubbish and it seems to have taken a long time for it to be cleaned out of the public's mind.

Universes can be SPATIALLY closed---e.g. like sphere surface except 3D instead of 2D---and their density can be greater than critical, and yet they can expand forever.

And there is no reason a universe cannot have been contracting forever, and then rebound, and then expand forever. This was already conceptualized and solved mathematically by Willem and Tullio in 1917. It is one of the easiest solutions to find, of our law of gravity (the 1915 GR equations). The universe does not have to start as an "egg" or something like a "point". It can begin as an infinite contracting volume of space, or in more precise language, it can have no beginning. It can always have been contracting, until it rebounds, and then keep on expanding for the rest of eternity.

If someone is bound by the biological prejudice that everything including the universe must have a temporal beginning as an egg-like point then we need to come up with some kind of meditative exercise to overcome this preconception. Nature can have more imagination (have faith in Her ). She can have at least as much imagination as a Dutchman and an Italian in 1917

BTW thanks for the congratulations! It was nice of them. Ashtekar and Agullo both really impress me, the way they come across, and the interviewer does a great job too!!!
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Thanks ∞
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 I hope others, such as Simon Bridge!, will also respond.
... you seem to be doing well enough... ;)

Should this be "beyond the standard model"?
In the context of "astronomy and astrophysics" and PF rules and guidlines, we are stuck with standard BB approaches. I don't think there is any astronomy that would tell us conditions before the expansion.

As for the question:
 My question is: if the energy content of the visible U is the same as at BB then, presumably, that applies to any part of U. If the energy of the U is always the same, then, if it collapses once, it should always collapse, since the conditions will never change.
Conditions certainly change - and there is a stage in all these models where current physics does not work so there is no telling what may happen in between collapse and expansion. Maybe the Universe bounces into a different shape each time?

 But this is questioned by both the theorists of BB and the theorists of Big Bounce, that admit that this might be the last Bounce, can you tell me where to find an answer?
... nobody knows where to find the answer: wait and see?

 Or probably I should ask this in the Cosmology forum
... probably - because you are asking beyond the standard model.

We can't see before the last compressed state so what happens before is highly speculative.
The trick, as usual, is to try to find models that are falsifiable.
I understand there are a lot of people working on quite clever approaches to that.
PF Gold
P: 533
 Quote by marcus Hi Bobie, I think you mean critical DENSITY. Density is average mass (or energy) per unit volume. ..Nature can have more imagination (have faith in Her
No I meant that: critical mass.
You know that in many posts I have professed my trust in Nature's fantasy, but its basic tenet is coherence. I do not know much about LGC, but, correct me if I am wrong,
aren't these people trying to adapt Gr-Friedman model to a completily different conception of the world, which affects its very fabric/foundations: gravity, discrete space (based on Planck) etc.?

They are dodging the problem of the energy that caused BB, is that so? they are supposing the energy comes from a previous collapse of an expanded U, ignoring the fact that it must have expanded ?.
But, BB had already dodged the problem inventing a space that bloats and pushes matter away.
It is arguable if this is possible or makes sense at all, but anyway the issue here is that in this way you are surreptitiously creating ex-nihilo a huge amount of gravitational PE, which remains dormant as long as U keeps expanding, but when you envisage a collapse you are cashing in, and the content of energy of U cannot stay the same and next bang will produce more matter , until you reach critical mass, the ball will not bounce but get shattered.

If the basic unit of space is plancks length, does it make sense to talk of G at 10 units?
They suppose G is inverted at that scale, do they specify at what distance that inversion takes place?, assuming they did, whatever the boundary, if you accumulate energy from negative PE don't you lose everything you gained when you cross that boundary and must fight against an equal and opposite force?

Is this all nonsense?
Thanks
Astronomy
PF Gold
P: 23,084
 Quote by bobie ... they are supposing the energy comes from a previous collapse of an expanded U, ignoring the fact that it must have expanded ?. ...
I was talking about one of the earliest (1917) classic solutions of GR. It is a form of universe discovered by Willem de Sitter and Tullio Levi-Civita. Two of the founding father of relativity. De Sitter universe is an established mainstream topic. In that case the universe is eternal and has a single bounce.

There is no reason to insist that the contracting phase, prior to the bounce, "must have expanded" at some earlier time.

I am not proposing that you or anyone believe in this or that particular model. I was hoping you might acknowledge that eternity is also consistent. Is also a logical possibility.

I think in the public there may be an obsession with the thought of U having expanded from some imaginary "egg" and having had a "beginning". We should probably try to get away from that magic egg mythology and there are various mental pathways for doing that.

If you want to ask about LQG, a good place would be the BtSM forum. Loop cosmology also has an eternal single-bounce version of its model---it is probably the most studied version. But here I was only talking about the de Sitter universe picture, not quantum cosmology.
Astronomy
PF Gold
P: 23,084
 Quote by bobie ... He says that BB was not a singularity but a Bounce, starting from an expanded U, ...
If you mean that Ashtekar implied a bounce must start from a U that has previously expanded, then your statement seems unlikely to be true. I don't remember him saying anything of the sort. Maybe you can tell me what minute and I can go and see for myself.

Ashtekar was one of the people who studied the eternal single bounce version most extensively.

One does not need to assume that the prior, contracting phase initially expanded from anything.

I just don't think he said what you seem to be claiming he said.

But that is really material for BtSM forum.

In this context probably we should simply refer to the well-established CLASSICAL picture. It's an oversimplification, but it's an example of the eternal single bounce universe that is hardly "beyond" anybody's standard model
PF Gold
P: 533
 Quote by marcus If you mean that Ashtekar implied a bounce must start from a U that has previously expanded, then your statement seems unlikely to be true. I don't remember him saying anything of the sort. Maybe you can tell me what minute and I can go and see for myself.
They do not say that explicitly, they dodge the problem, as usual, sweeping it under the carpet, starting at T0 with an already expanded U. I only exposed that:
 Quote by bobie they are supposing the energy comes from a previous collapse of an expanded U, ignoring the fact that it must have expanded ?.
Agullo says :
(17:06) "U started in the past being infinitely big" then contracted..and quantum effects avoided a singularity..and produced a bounce". infinitely big is a U that has expanded infinitely or "the past" begins absurdly at the moment of contraction
(16:42)" ..the reason whether U will recollapse or not has to do with the energy content of U now"
well, the energy content now is known yet he doesn't know what that implies
(16:22)"..if U were to recollapse then the end of that collapse is not like a .. singularity..in GR)
I was simply drawing the logical conclusions of their statements:

..next collapse is determined by content of U now, which (you didn't refute that*) is hugely greater* than at last BB/bounce. They can't say if there will be a next collapse, but should that happen, it would produce a new bounce....
*That criticism was aimed at the SM and is the issue of this thread, so if you wish we can discuss it here

I suppose you are not prepared to discuss criticism to LQC as you ignored the other considerations, but, should you be interested, I am prepared to discuss them in the BtSM forum.
Thanks again for your friendly attention.
Astronomy
PF Gold
P: 23,084
AFAICS nothing absurd about an eternal universe. Newton's universe was eternal. I think that has been the norm over the centuries since Newton. To take a specific case, de Sitter and Levi-Civita were, as I said, founding fathers of Relativity and their 1917 solution of the 1915 GR equation does not have a beginning. It is eternal. As it happens the first phase has distances contracting and then later distances are expanding. So speaking informally you could say it "starts" by contracting (although there is no moment of beginning) because the contracting phase comes before the expanding phase.

Einstein's original solution to his GR equation was also eternal (had no beginning). Apparently he didn't think that was absurd either. As I said, it has been the norm.

You obviously think differently, but you don't give any reasons why an eternal universe absurd. You just imply that it's absurd not to have a point-like "beginning" of some sort: Existence then, I guess by its very nature, has to have expanded from something compact. It sounds like a prejudice or preconception. That could in fact be what is absurd, but I don't know about that. I'm having to repeat myself an awful lot. Have the sense that it's not getting through to you. May have to give up trying.

 "the past" begins absurdly at the moment of contraction
You misinterpret. That is not what anyone was saying. Not de Sitter, or Levi-Civita, or Einstein, or Ashtekar, in the conceptions I referred to---in them there is no "moment of beginning" either contractive or expansive (or in Einstein's original concept, steady).
 Mentor P: 16,947 This thread is drifting into unsupported attacks on mainstream theories and is therefore closed.

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