# How much of Einstein Online have you read?

## Which of these Einstein Online cosmology articles have you read?

18.2%

27.3%

27.3%

18.2%

18.2%

9.1%

18.2%

27.3%
9. ### 9. None read (at least so far)

54.5%
1. Oct 23, 2008

### marcus

Einstein Online is an amazingly informative website that the AEI (Albert Einstein Institute) started in 2006 and is still adding to.

They have a section on cosmology that is the best introduction I know of not needing a lot of math. Indeed it is nearly math-free but still manages to be up-to-date and accurate, which is quite an accomplishment.

To get there, google "einstein online spotlights cosmology".

They have other sections that spotlight other topics as well, like relativity, black holes, quantum theory,...

The section that spotlights cosmology has 8 articles they call basics, plus some extras that go beyond those 8 basics.

I'm wondering how many other PF members besides myself have been reading Einstein Online, particularly the cosmology basics.

If you too have visited the site, which of the 8 cosmology basics have you read?

If you have read one or more you want especially to comment on or recommend, please post the name(s) or link(s)!

Which of these have you read? (The poll allows multiple answers.) If none just check option 9.

1. The shape of space
2. Cosmic sound: curvature and the cosmic background radiation
3. A tale of two big bangs
4. Big bang nucleosynthesis
5. Equilibrium and change
6. Elements of the past
7. The mathematical universe
8. Of gravitational waves and spherical chickens
9. None read (at least so far)

2. Oct 23, 2008

### vincentm

To be honest, i've never heard of Einstein online, weird... i'm usually in the know of these things..

3. Oct 23, 2008

### marcus

It's new! Two years is not a long time. How many years was Ned Wright's cosmology tutorial online before everybody knew about it and was using it?

I know what you mean though. I felt the same way when I discovered it, where has this been?!. I think it was a PF member Atyy who pointed it out.

4. Oct 23, 2008

### vincentm

Got a link?

5. Oct 24, 2008

### mysearch

Hi Marcus,
Initially, I hadn’t read any of them, but have now quickly looked through the basic set. Therefore, I am not sure I should have ticked yes in the poll or not:uhh:

Equally, I wasn’t sure whether you wanted any feedback in this thread. If not, just delete this post, but thought it might be useful to log some initial reactions to each of the basic articles:

The shape of space:
“Interesting possibilities for large-scale geometry, and the mathematically inclined reader will react with slight regret to the announcement that, going by astronomical data, the most boring among these possibilities is realized: The geometry of our universe at cosmic scales appears to be that of ordinary high school geometry, corresponding to a universe with a density equal to the critical value.”
My commiserations to the “mathematical inclined”, but I for one prefer the simplification this would bring. Maybe all the chapters dedicated to this complexity, which is counter intuitive to most people might now be moved to an appendix just joking

Cosmic sound - curvature and the microwave background radiation
While I found the introduction to acoustic oscillation very useful, I not sure I understood the sound horizon:

At the close of those 400,000 years, the largest possible coherent oscillations had a spatial extent of 230,000 lightyears (or 70,000 parsec). There was simply no time for more: With the speed of sound at 60% light speed and a time of roughly 400,000 years, the largest regions in which coherent oscillations could develop had a spatial extent of 0.6·400,000 = 240,000 lightyears; with the more precise value of 380,000 years for the cosmic time when atoms formed, the result is 230,000 lightyears. This upper limit is called the "sound horizon".​
Understand the 0.6c*400,000 years implication, but wouldn’t the propagation speed be a function of density, which was undergoing rapid change in the first 400,000 years? Equally, wouldn’t the scalefactor apply to the sound horizon, i.e. would the wavelength of the acoustic oscillations expand?

A tale of two big bangs
To be honest, I found the discussion of two big bangs somewhat confusing as I thought it was to be some introduction of two perspectives of the standard model. Whereas, the main issue seem to relate to that fact that physics cannot describe any event earlier than Planck Time, i.e. about 1E-43second. However, after this the expansion of space seems to be discussed on the basis of the thermodynamics of a physically expanding universe, i.e. density is predicated on volume expansion. Not sure that this is adequate preparation for the debates that seem to be linked with the issue of space expansion

Big Bang Nucleosynthesis: Cooking up the first light elements
Useful summary.

Equilibrium and Change: The physics behind Big Bang Nucleosynthesis
Certainly highlights some of the improbable coincidences required to arrive at our present universe. No wonder anthropic principles are often discussed.

Elements of the past: Big Bang Nucleosynthesis and observation
Again, useful technical detail.

The mathematical universe
OK as an introduction to this very interesting topic, however, some specific example would have been really useful to help people on their way, i.e.
"Whenever the values of these quantities are known at some particular time (the "initial values"), the equations make it possible to determine them at all later times."​

Of gravitational waves and spherical chickens
Not sure I would classify this one as basic! Sometimes it is not easy for people new to this topic to judge what is accepted fact and what is unverified theory.

P.S. BTW - thanks for the confirmation of my density questions.

6. Oct 24, 2008

### marcus

Yes! I keep this one in my sig:
http://www.einstein-online.info/en/spotlights/cosmology/index.html
Anyone who likes the site is welcome to spread knowledge of it the same way, by simply including it in their signature. I think it's a great site, a cut about anything I've seen for cosmo outreach except Ned Wright's site which is well-known already.

Mysearch, thanks so much for your response on the poll AND for your comments on each of the various cosmo basics sections.
Yes definitely to your implied question, I was hoping people would tick off what they read after they had had a chance to look at and sample the site.

Several interesting questions here. The speed of sound in a given gas depends mainly on the temperature, not the density. It also depends on the makeup of the gas, but for a given gas it varies with temperature. He is trying to just sketch the ideas, so he skips over a lot. You caught a place where a footnote or appendix with more explanation for technically inclined readers would help.

during those 400,000 years the temperature would have declined from very very hot to only moderately hot at the end like 3000 kelvin (surface temp of a reddish star). And the speed of sound in the gas would have declined from very very fast down to something pretty moderate like just a few times faster than sound in air. (it goes as the square root but that's a technicality)

So when he says 0.6 c, he is doing a quick and dirty average over the whole cooling history. He is not dotting every aye and crossing every tee. It is a casual outreach writing style but I think it works. You are quite right about the acoustic oscillations being elongated by the increase in distances! We can see those relatively small oscillations as the mottling of the CMB sky map, big as constellations plastered all over the sky. They have been enlarged way much. You know those pretty red and blue mottled oval CMB maps. Am I responding to your actual question? If not ask again.

Last edited: Oct 24, 2008
7. Oct 27, 2008

### Chronos

I'm not convinced 400,000 years is a reliable figure. Time may have been a volatile property in the very early universe.

8. Oct 28, 2008

### TalonD

Read the Shape of Space so far. open, closed or flat, very elementary and familiar so far. But great for a beginner who knows little. Not that I'm any expert. I have a few questions if this is the appropriate place to ask them. First of all the author's name is Marcus, is that you? or another Marcus? A question of critical density, density of what?, is it density or mass that matters? not sure I am clear on the difference. and is it matter or energy or both? they both have mass. And Iv'e been told that both mass and density (two sepperate things?) have an effect on the curvature of space? Also wondering about the quantum vacum energy, is it's existence proven? does it have mass? does it have an effect on the critical density ?

9. Oct 28, 2008

### marcus

Different Marcus. Good place to ask questions related to any of the EO cosmo basics.

We're talking about the Friedmann model---used all the time in cosmology. I usually put the critical density in terms of an energy density. That takes everything together, ordinary matter, dark matter, radiation, dark energy.
but you have a choice, you can arranged the units so you are talking about mass density---they are equivalent in the usual E=mc^2 way.
At present the critical density is about 0.8 joule per cubic kilometer.
You could convert that to grams per cubic meter if you want.

Both mass and momentum enter into the righthand side of the Einstein equation. They could have been talking about that. Putting things on a per unit volume basis, you could translate the RHS into terms of energy density and pressure. Both energy density and pressure do incluence curvature. But except in certain special cases, pressure is negligible. In the main Friedmann equation there is no pressure term---there is pressure in a secondary Friedmann equation but the main equation doesn't have it. In real life matter and radiation are so dispersed---pressure doesn't count.

I don't really understand your question. They could have been talking about momentum or pressure as separate from mass and density. If not, I can't say. mass density is just mass per unit volume, so not really very different.

To me the existence of something, to make things add us, is darn near certain. But it isn't proven what it is. Call it what you want. Dark energy? Yes it does contribute to the total.
So when you want to compare actual density with critical density you definitely add in dark.

Critical density is a calculated level that the actual density must meet, approximately, if space is going to appear approximately flat. That is triangles add up to 180 degrees etc.

I expect this is all covered in the EO page.

Glad you asked, and hope you read some more of their cosmology pages.

Last edited: Oct 28, 2008
10. Oct 29, 2008

### TalonD

I mistakenly clicked on the poll only for the shape of space, I'll be reading all of it but cant go back and select more on the poll. To revisit my previous question, it was pressure I was thinking of rather than density as contributing to the curvature. Just a minor brain flatulation there. :P

11. Oct 29, 2008

### marcus

What you are talking about regarding pressure is really interesting. It is another way that Einstein's gravity departs from Newton's.
It isn't just the mass of material in the sun that creates the pull experienced by the earth. If you break it down that way, you can say the pressure in the core of the sun actually makes its own small separate contribution.

I think everyone is aware that the present theory does not offer a mechanism to explain why energy/momentum (or density and pressure on a per volume basis) bends geometry. That is something that a more fundamental quantum gravity or quantum geometry will have to tackle. But it does give an accurate formula for the effects of density and pressure, and to me it seems curious that pressure should contribute.

You can see why the effect is apt to be small by doing dimensional analysis. If you convert mass density to equivalent energy density (joules per cubic meter) then you see it is dimensionally the same as pressure (newtons per square meter, a.k.a. pascals). And you have to make that conversion (mass density to energy density, multiplying by c^2) in order to compare the two on the same terms. But that involves favoring the mass density by multiplying in this huge c^2 factor. So in normal everyday situations the (mass-equiv.) energy density is going to dwarf the pressure.

12. Oct 30, 2008

### mysearch

Hi Marcus,
Given that this poll/thread seems to be about some of the basic principles of the LCDM model, I wondered if you would mind me extending your previous discussion concerning pressure and density. By way of reference, it is linked to some of the issues raised in the following thread: https://www.physicsforums.com/showthread.php?t=267808

Modelling the energy density of matter, CDM, radiation and dark energy in accordance to the Friedmann equation set seems to suggest that the relative energy density of dark energy falls away in comparison to matter, CDM and radiation to almost zero by around the time of decoupling, i.e. +380,000 years. I have attached a diagram as a general reference below. I understand this is caused by the following sensitivity of these components to expansion:

Radiation…………….1/a^4
Matter & CDM………1/a^3
Curvature…………….1/a^2
Dark energy………….1/a^0

However, if you push the model that seems to determine these figures back even further, it then suggests that radiation becomes the dominant energy density. So my basic question is:

What causes the universe to expand in its earliest epoch?

The positive expansion effect of dark energy seems to be gone. Matter and CDM are also negligible in comparison to radiation, which only ever cause a gravitational slowdown of expansion.

Does this only leave the energy density and pressure of radiation to explain expansion?

In later epoch, it seems that the pressure of radiation does not cause any expansion because the expansion of space under dark energy creates no potential gradients that would cause a net flow of photons. On the other hand, photon pressure inside a star does prevent the collapse of a star under gravity, at least, while fusion is still creating photons. One other issue that is puzzling me concerning the nature of ‘expansion’ within the general big bang concept relates to the fact that the big bang is said not to be described as an explosion, but rather the uniform expansion of space.

If so, is it correct to assume that the expansion of the space could never have continued linked to any classical ideas about inertia and momentum?

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13. Oct 30, 2008

### marcus

Einstein classic GR has been tested and shown to be a good description of dynamic geometry---in every way people can think of testing it. Friedmann eqns derive from it. And according to GR and the Friedmann model in particular, once distances have started expanding they continue----although the increase can speed up if there is negative pressure and slow down if there is positive pressure or too much energy density. If you accept GR, you accept that changes in geometry tend to continue of their own accord.

The classic Einst. and Friedm. eqns do not explain or give any mechanistic analogy. They describe how geometry changes, describe a process. Various tests/observations bear witness to their correctness. I realize this isn't fully satisfactory. A lot of people would like to understand how this works at a deeper level.

BTW radiation pressure would tend to slow down expansion, since it is a positive pressure.

Two questions are left unanswered at present. WHY does GR do such a good job at explaining how geometry evolves? WHAT STARTED the expansion originally, since all GR can tell us is how it continues, once it has begun, and what things may make it gradually slow down or speed up.

Might be good if you would start a thread with this question as title
"what caused the universe to start expanding?" or words to that effect, like "what started expansion in early universe?"
I personally might not want to answer, because I don't know. But other people might jump in and provide answers for you. If I thought of anything I would happily contribute it as well. The main thing is it would bring the question out in the open. You might not actually get an answer. Quite possibly there is no answer at the present moment in history.

The present thread is an Einstein Online thread, so to keep reasonably on topic we should relate your question to the EO cosmo materials.

Can you find anything at EO about the root cause of increase in distances? If you can find something at EO to hang the discussion on, that we can all read---some quote, or page and paragraph reference---that would be helpful.

A lot of current reasearch is about nonsingular cosmology models, many of which have a bounce. The bounce is what initiates this pattern of expansion of distances.
There are also inflation scenarios, depending on an imagined inflaton field which briefly accelerates expansion and then abruptly decays. Maybe someone who believes in some specific inflation scenario would say that is what initiated the pattern of expansion.

I can't answer your deepest question. Expansion just is. GR, as a theory of how geometry evolves, says that once started it will proceed of its own accord. I can't answer why GR is a correct theory. I can't say how the expansion got started. But since GR appears correct as far as it goes (i.e. away from singularities where it fails) the expansion (even tho it has no apparent mechanism causing it) cannot shut off abruptly. GR is a theory describing the continuous evolution of geometry which does not explain its own correctness. You test it however you can, and if it passes, you use it.

I suspect that the next couple of decades will see the appearance of a more fundamental theory that does not have the particular singularities now bothering us (although it may have others, theories typically have some limits to their applicability) and which will be more thoroughly explanatory----will describe in deeper detail how matter interacts with geometry and what underlies both space and matter.

Last edited: Oct 30, 2008
14. Nov 13, 2008

### marcus

Einstein-online has a clear, down-to-earth discussion of the Big Bang which addresses several things that commonly confuse people.
Their page on it is called A Tale of Two Big Bangs.

It explains a lot of things, but what particularly comes to mind at the moment is how they handle the big bang singularity.

They describe how astronomers use the singularity as a convenient time-marker---the place where the classical model they used for many decades broke down and wouldn't go back any farther in time. It is a good temporal reference point that everybody uses.

But astronomers normally do not imagine that such a singularity occurred in nature. It is simply the place where the vintage-1915 General Relativity model blows up. That's not the only model and at this stage we simply don't know the correct way to continue extrapolating back in time.

Einstein-online puts it this way:
"...Whether or not there really was a big bang singularity is a totally different question. Most cosmologists would be very surprised if it turned out that our universe really did have an infinitely dense, infinitely hot, infinitely curved beginning. Commonly, the fact that a model predicts infinite values for some physical quantity indicates that the model is too simple and fails to include some crucial aspect of the real world. In fact, we already know what the usual cosmological models fail to include: At ultra-high densities,..."

In other words, the signs are that vintage-1915 General Relativity, and the vintage-1922 Friedmann model of cosmology that was derived from it, and which has now been used for over 80 years, are too simple. The fact that they predict infinite density at a certain point as you go back in time is commonly taken to indicate that they fail to include some crucial aspect of the real world.

Last edited: Nov 13, 2008
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