Cosmological anthropocentrism

[Tez]

Ok I'm not a cosmologist, but I'm surrounded by the critters, and theres some fascinating stuff going on that you certainly aren't going to read about in the papers - so I'm going to try to explain it here. The basic problem is that we seem to live at an extremely special epoch of the universe's devolpment - and for those of us averse to anthropocentric explanations of things this is rather disturbing.

You may have noticed that in the last few years there has been a steadily more compelling array of evidence that (a) the universe is exactly flat and (b) about 70% of the energy density is provided by so called "dark" energy, while 30% is mass energy - about 5% of this latter is actually visible matter, the rest is dark matter.

Now the relative contribution of these energy densities changes as the universe expands - at early times dark energy is negligible and the universe is matter dominated, while at later times the universe is dark energy dominated. [Since dark energy is repulsive - i.e. the vacuum exapnds away from any point, it means we're all eventually going to be stretched out flat as pancakes. The amount of dark energy is about 6kev per cubic centimetre. ]

What has physicists all in a tizz, is that we seem to live right in the transition point from matter dominated to dark energy dominated. The transition is very sharp - if you check out Figure 1 on page 5 of the following pdf file: Dark energy and the preposterous universe you'll get some idea (also worth reading section 1.3 just under the figure).

The reaction to this has been varied. Steven Weinberg apparently exploded the first time he first saw that data, and when he finally calmed down he started arguing there must be a vast ensemble of universes, and intelligent life can only evolve in those in which this cosmolgical coincidence has occurred. (You can read some his anthropic arguments in astro-ph/0005265).

Personally I find Weinberg's argument unsatisfying - seems to me that I can imagine many universes with no dark energy at all that are still conducive to intelligent life. In fact until a few years ago I believed I lived in one! However Weinberg isn't known for throwing whacky ideas around, and he certainly has a deeper appreciation of the problem than me.

Unfortunately I cant really tell you much more than that, because I dont really know more. However given this is a skeptics science board i thought some of you might find this conundrum interesting...

The final thing I'm going to do is copy paste in a section of an article thats being written by P. Pinto and T. Tyson that I'm looking over for them - I think its going to be in Sci. American or something similar... It explains how gravity can be repulsive:


How can gravity be repulsive? In Newtonian gravity, the gravitational force exerted by an element of a massive medium is proportional to its mass density. Since this density is always positive, the force never changes sign and classical gravity is always attractive. Any relativistic generalization of the gravitational force has not only to involve the energy density (instead of the mass density) but also the momentum density (as energy and momentum can be transformed into each other by changing the reference frame). Within Einstein’s framework of General Relativity, the gravitational force exerted by an element of an isotropic medium is proportional to the sum of its energy density and three times its local pressure (which measures the momentum flow).
A medium can have a negative pressure: a common example is a rubber ball that is forced to expand beyond its equilibrium radius. If this negative pressure is large enough (greater in magnitude than a third of the energy density), it can thus produce a repulsive gravitational force! In particular, vacuum energy where the pressure is equal and opposite to the energy density (Einstein’s “cosmological constant” is an example) will produce such repulsive force. It such vacuum energy is dominant, it would generate an accelerated expansion of the universe. Another important example is the case of a particle field that is highly out of equilibrium. This is the mechanism believed to have produced the inflation in the early universe.

 

[a_unique_person]

Wheew, I wish I had waited till after reading this to hit the Jim Beam. (It is Friday night, after all).

1. Hadn't ever heard the universe is flat. You don't mean literally, but there are seem pretty high galaxies out there.

2. why would it be flat? i thought that was just a model?

3. was there something about one of those early satellites going faster than expected as it left the solar system.

 

[Soapy Sam]

-for a certain value of "flat".

Take a boat on a calm sea. Everything around you not only looks flat, it really is pretty flat. The curvature is not discernable to the eye, until you see something vanish over the horizon.

I think the same applies to the curvature of the universe. Even if we assume it's spherical (if that sentence has any actual meaning), the degree of curvature of such a large sphere would be unimaginably small. To any realistic extent, it's flat. Just stay away from the edge and you'll be OK.


As for the Anthropic Principle. Hey we live in a universe where cigarette butts are possible. Does this mean it evolved solely for the benefit of cigarette butts?

 

[Keneke]

Could this theory of repulsive gravity be truly used to create anti-grav devices in the future? Cause, you know, the woowoos have already latched onto this theory like flies to flypaper, and I want to make sure before I go spreading this paper around.

 

[Brian the Snail]

Thanks for posting a link to the article. It was very interesting

Originally posted by Tez

What has physicists all in a tizz, is that we seem to live right in the transition point from matter dominated to dark energy dominated. The transition is very sharp - if you check out Figure 1 on page 5 of the following pdf file: Dark energy and the preposterous universe you'll get some idea (also worth reading section 1.3 just under the figure).

Personally, I can't see what all the fuss is about. The x-axis of Fig. 1 is on a log scale, so of course the peak appears much sharper than it actually is. It looks as through the gradient of /omega is significant between around log(a)=-1 and log(a)=0.5, which corresponds to a=0.1 to a=3.2 (where I'm assuming that it's base 10). This corresponds to a huge range of times, between (if my calculations are correct, always an iffy assumption ) about 500 million years to about 70 billion years since the Big Bang. So I don't see how we are "living in a special epoch" at all!

Unless I'm missing something important...

 

[Tez]

You're right Brian - that figure isnt very convincing as it stands. I think what people are worrying about is not just that figure (which shows the first derivative of Omega_Lambda) but also the fact that we live very near the crossover point - where the two actual values of Omega_Matter and Omega_Lambda are about the same. This wont be true for very long (relatively speaking!)

In the attached figure (courtesy of Tony Tyson) we live on the far right of the diagram, the two relevant curves are the solid white (Omega_Matter) and the solid yellow (Omega_Lambda). They have just crossed and are rapidly going up/down to 1 and 0 respectively.

Form what I can gather the puzzle is also tied up with the fact that this particular behaviour is a function of the 6kev/cc value I mentioned earlier - and there seems to be nothing "natural" about this energy scale...

 

[sorgoth]

Ok.......I don't quite get the negative gravity thing.


Is there a more...explaining definition?

 

[UnrepentantSinner]

On a much more mundane note, a universe where 99.9999% of it is hostile to human life, and a home world where about 78% of it is hostile and another 10% inhospitable to human life hardly constitutes an athropocentric cosmos.

So let's be careful out there...

 

[Oso]

The basic problem is that we seem to live at an extremely special epoch of the universe's development - and for those of us averse to anthropocentric explanations of things this is rather disturbing.

First, thanks for the pdf and the preview. Very exciting.

I understand your concern, but I'm with UnrepentantSinner. In fact it points out the very reason I'm less concerned. Despite the fact that nobody wanted it, Einstein called it "my greatest blunder", we've had no choice but to add a 5th force! The consequences of this may be that in the short term some will attribute some anthropocentric spin to it, but the very fact that we now have a fifth force should encourage you to believe that such a perception will have little detrimental effect.

After over 4 billion years we've only reached the ability to have this discussion in the last 4 years! That doesn't make me become concerned with some anthropic principle, what it makes me do is wake every single day and say... YES!

Well gotta go, time for the Lakers Rockets game.
YES!

 

[jj]

Ok, now I am out of my field here...

A couple of things come to mind.

The first is a simple question, is there anything special that this era would mean for ratio of hydrogen to "metals' (I'm using the astronomical term for metal, not the "conduction electron" term), or to the distribution of "metals" in terms of elements or isotopes.

Ditto for formation of stars? Would this have any influence on the size or type of star?

The final question is something I don't expect an answer for... Does this rather singular (modulo the log presentation, etc, which does bunch things up a bit, yes?) time simply constitute an observation of something on the order of "it's always like this, and there's something we haven't digested and internalized here yet".

Yes, I know the last one is a whopper

 

[Tez]

Ok, now I am out of my field here...

A couple of things come to mind.

The first is a simple question, is there anything special that this era would mean for ratio of hydrogen to "metals' (I'm using the astronomical term for metal, not the "conduction electron" term), or to the distribution of "metals" in terms of elements or isotopes.

Ditto for formation of stars? Would this have any influence on the size or type of star?

The final question is something I don't expect an answer for... Does this rather singular (modulo the log presentation, etc, which does bunch things up a bit, yes?) time simply constitute an observation of something on the order of "it's always like this, and there's something we haven't digested and internalized here yet".

Yes, I know the last one is a whopper

Re the first two points on element abundences etc, AFAIK there's no significant effect from the dark energy, since its only really beginning to have an effect now, and thise things were primarily fixed in the first coupla GYrs. In the sense that it'll cause an accelerating expansion (and thus dilution) from now on, I presume there will be an effect.

 

[Tez]

On a much more mundane note, a universe where 99.9999% of it is hostile to human life, and a home world where about 78% of it is hostile and another 10% inhospitable to human life hardly constitutes an athropocentric cosmos.

So let's be careful out there...

Agreed - I think the issue is more one of "coincidental timing" than true anthropocentrism. An analogy I'm trying to think up: if we observed now that the sun was in the middle of a sharp phase transition (extending 50 million years or so) from 1 type of star to another, then we might conjecture that it was no accident that complex life evolved around this point. Of course it still could be...

 

[Whyatt]

This is actually something i know all about (kinda).

This is my third year of studying cosmology so here a few points before i've gotta go home:

Soapy sam
-for a certain value of "flat".

Take a boat on a calm sea. Everything around you not only looks flat, it really is pretty flat. The curvature is not discernable to the eye, until you see something vanish over the horizon.

I think the same applies to the curvature of the universe. Even if we assume it's spherical (if that sentence has any actual meaning), the degree of curvature of such a large sphere would be unimaginably small. To any realistic extent, it's flat. Just stay away from the edge and you'll be OK.


As for the Anthropic Principle. Hey we live in a universe where cigarette butts are possible. Does this mean it evolved solely for the benefit of cigarette butts?

No offence but your fundamentally wrong here:

The world has three spacial dimensions, imagine a cube with x,y and z axis drawn on it. Thats our universe.

But we can't comprehend a higher dimensional shape than this so replace it with a flat square piece of card. Let that represent our 3 dimenional world.

Now the standard model of cosmology is that this form could be different shapes: represent (and due to) a constant called "k."

k can hold three values 1,0, and -1.

k=1: the universe is closed and will collapse in on itself:
Imagine the flat card is wrapped around a sphere. There is no curvature in the universe to anyone inside it. But from a imaginary higher dimesional viewpoint, the universe looks spherical.

k=-1: the universe is the shape of a hyperbolic parabala: (This is the shape of the popular crisp/chip called "pringles") Same as above but the universe will expand at a faster and faster rate.


k=0: The universe is just like our initial piece of card. And the universe's constants are exactly right to be between the two alternatives.

Although k=0 seemed the nicest it seemed unlikely as it would require a constant that could possible change during inflation to tend to 1, And would require an accuracy of 1 in 10^29!!!!

(thats i with 29 zeros after it)

Thats about right but i've been rushed.

 

[arcticpenguin]

The current musings about dark matter and dark energy are so speculative that I'm certainly not going to change my worldview to align with them.

Remember a couple years ago, when the apparent age of the universe was less than the age of the oldest known stars? "We couldn't possibly be off by a billion years" said the star guys. "We couldn't possibly be off by a billion years" said the Big Bang guys. Well, guess what?

Never forget the assumptions your current theory is riding on.

 

[Tez]

The current musings about dark matter and dark energy are so speculative that I'm certainly not going to change my worldview to align with them.

The musings on their origins are certianly speculative, the observed emperical effects on the universe's dynamics are not...

 

[garys_2k]

This is actually something i know all about (kinda).

This is my third year of studying cosmology so here a few points before i've gotta go home:



No offence but your fundamentally wrong here:

The world has three spacial dimensions, imagine a cube with x,y and z axis drawn on it. Thats our universe.

But we can't comprehend a higher dimensional shape than this so replace it with a flat square piece of card. Let that represent our 3 dimenional world.

Now the standard model of cosmology is that this form could be different shapes: represent (and due to) a constant called "k."

k can hold three values 1,0, and -1.

k=1: the universe is closed and will collapse in on itself:
Imagine the flat card is wrapped around a sphere. There is no curvature in the universe to anyone inside it. But from a imaginary higher dimesional viewpoint, the universe looks spherical.

k=-1: the universe is the shape of a hyperbolic parabala: (This is the shape of the popular crisp/chip called "pringles") Same as above but the universe will expand at a faster and faster rate.


k=0: The universe is just like our initial piece of card. And the universe's constants are exactly right to be between the two alternatives.

Although k=0 seemed the nicest it seemed unlikely as it would require a constant that could possible change during inflation to tend to 1, And would require an accuracy of 1 in 10^29!!!!

(thats i with 29 zeros after it)

Thats about right but i've been rushed.

OK, but I am confused about those parts of your explanation of k=-1 and k=0. All the news is saying that we're in a "flat universe," but in the same articles they say we're expanding at an ever-accelerating rate. I'd thought the "flat" argument was that the expansion would slow, asymptotically, toward zero, but never really quite make it. IOW, there's JUST not enough gravity to bring things to a halt and make them back up into a big crunch, but that we would decelerate nonetheless.

Is this a different kind of "flatness" from what I'd heard about? I thought the never-stopping model was the saddle-shaped one.

 

[Brian the Snail]

OK, but I am confused about those parts of your explanation of k=-1 and k=0. All the news is saying that we're in a "flat universe," but in the same articles they say we're expanding at an ever-accelerating rate. I'd thought the "flat" argument was that the expansion would slow, asymptotically, toward zero, but never really quite make it. IOW, there's JUST not enough gravity to bring things to a halt and make them back up into a big crunch, but that we would decelerate nonetheless.

Is this a different kind of "flatness" from what I'd heard about? I thought the never-stopping model was the saddle-shaped one.

I think the confusion arises from the fact that the different models that Whyatt talked about are for a universe without a cosmological constant, and is therefore interacting only through gravity. So for example a "flat" universe in this case would have a mass density exactly at a "critical value" such that it would expand asymptotically towards a fixed size, as you say.

However, in the last few years we've learnt that there is a cosmological constant, in the form of dark vacuum energy, which counteracts gravity by pushing things apart. This causes the acceleration of the expansion that is observed now. This is despite that the fact that the geometry of the universe has been observed to be flat (through the Cosmic Microwave Background, I seem to recall).

Although k=0 seemed the nicest it seemed unlikely as it would require a constant that could possible change during inflation to tend to 1, And would require an accuracy of 1 in 10^29!!!!

I thought that the whole point of inflation was that it was supposed to have pushed k towards 0 (a flat universe)

BTW Thanks Tez for posting the Powerpoint figure- it was much clearer than the other one.

 

[Tez]

Originally posted by jj Ok, now I am out of my
field here...

A couple of things come to mind.

The first is a simple question, is there anything special that
this era would mean for ratio of hydrogen to "metals' (I'm using
the astronomical term for metal, not the "conduction electron"
term), or to the distribution of "metals" in terms of elements or
isotopes.

Ditto for formation of stars? Would this have any influence on
the size or type of star?

Hey jj, seems this question you asked (so long ago I'm sure you've
lost the associated brain cells) was actually heading down possibly the right track.
Basically the "present era" is one of "structure formation" - when
stuff coalesced into grumpy skeptics, and it seems that we may not
have taken this clumping into account properly, and therefore
assert that there is dark energy.

Heres the story in a little more detail.

At lunch the other day I asked an observational cosmologist (the
above mentioned Tony Tyson who gave me the plot) what he thinks is
the most interesting paper he's seen recently, and he said this
one:

http://arxiv.org/abs/astro-ph/0311257

Basically the author asserts: "..there appears to be a dark energy
component because the observations are fitted to a model that does
not take into account the impact of inhomogeneities on the
expansion rate.."

Paraphrasing the author a little further: "In the usual approach
(assuming a homogenous and isotropic universe) one first averages
the metric and the stress energy tensors, and then feeds these
into the Einstein equations. Unless the homogeneity is perfect at
all scales, one should first feed the inhomogenous versions into
the equations, then do any averaging."

Although only a first order perturbation calculation, he
definitely seems to get things the right order of magnitude.

In particular, this gives a natural explanation of the
claimed-to-be-most-puzzling feature of dark energy - the
"coincidence problem", i.e. why is the dark energy taking over so
recently. In this model inhomogeneities become important at the
onset of structure formation - which is about now, and now is
about the only time that things have settled enough to produce
intelligent life..

 

[Wrath of the Swarm]

How is this a problem? The Anthropic Principle seems more than sufficient to explain this - and that's considering that intelligence can only arise in conditions similar to these. It could just as easily be a case of Adams' Puddle.

 

[Tez]

How is this a problem? The Anthropic Principle seems more than sufficient to explain this - and that's considering that intelligence can only arise in conditions similar to these. It could just as easily be a case of Adams' Puddle.

What do you mean by the Anthropic Principle? (i.e. weak or strong form etc...)