Mainstream cosmology basics and sources

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Mainstream cosmology basics and sources

Postby Marshall on April 28th, 2007, 5:24 pm 

Everyone is free to believe or disbelieve whatever model cosmology they like. We normally don't see people here urge one version over another. But be aware that there is currently a rough consensus among working professionals on a kind of standard model cosmology. If you want to discuss cosmology with other people it is a good idea to understand the basics of mainstream cosmology (even if you just want to disagree with it! :-) )

So I am hoping that everyone who posts regularly here at SCF's Space and Astronomy will already be familiar with, or will pick up, the basics of mainstream cosmology. And I also hope that experienced people will share expertise and helpful source links. If you have some online cosmology material you think is especially useful, please add it to this thread.

To begin:
In mainstream cosmology, the spacetime metric used is called the FRW metric (Friedmann Robertson Walker).
The scalefactor in the metric is written a(t) or R(t) and is governed by the Friedmann equations.
With the proper choice of parameters, plugged into the equations of the model, one gets versions of what is called the LCDM ( "consensus" )cosmology.
The letters stand for Lambda Cold Dark Matter.
In this thread we should among other things clarify the meaning of these terms.

In recent years (since 1998 and moreso recently) mainstream cosmology has become an observational science in the sense that the main activity of professional cosmologists is DATA-FITTING----that is they try to adjust the parameters of the LCDM model so as to get the best fit to the data.
There are 4 or 5 independent bodies of data: CMB, galaxy surveys, gammaray bursts, supernovae surveys...

The current best fit values of the chief parameters are probably familiar to you.
0.73 for Lambda, or the dark energy fraction
0.23 for the dark matter fraction
0.04 for ordinary matter (or one can combine both types of matter and use the figure 0.27)
71 for the present value of the Hubble parameter.
These are the very same numbers that you use when you employ one of the online cosmology calculators to convert redshift.

Some links.
Ned Wright's cosmology tutorial
http://www.astro.ucla.edu/~wright/cosmolog.htm

Ned Wright's cosmology FAQ
http://www.astro.ucla.edu/~wright/cosmology_faq.html

Ned Wright's most basic cosmology calculator
http://www.astro.ucla.edu/~wright/CosmoCalc.html
(he has links to some more advanced or specialized calculators)

Morgan's calculator
http://www.uni.edu/morgans/ajjar/Cosmology/cosmos.html

Murphy's coordinate conversion tool
http://fuse.pha.jhu.edu/support/tools/eqtogal.html

Lineweaver and Davis' Scientific American article Misconceptions about the big bang March 2005.
AS LONG AS THIS PRINCETON LINK WORKS IT IS BETTER THAN THE OTHERS

http://www.astro.princeton.edu/~aes/AST105/Readings/misconceptionsBigBang.pdf

The article used to be available at the SciAm website but I've noticed that what they have there has been dwindling----some very educational graphics have been eliminated---maybe because storage is scarce.
Here are the links to the same article at the SciAm website. But these links have been going dead or else the GRAPHICS that you used to get have been disappearing. So these SciAm links may not be as good as the Princeton one

http://www.sciam.com/article.cfm?chanID=sa006&colID=1&articleID=0009F0CA-C523-1213-852383414B7F0147

The Lineweaver Davis article had some very useful SIDEBARS giving pictorial diagrams with a question together with right and wrong answers explained. For easier access, here are links to individual sidebars.

http://www.sciam.com/media/inline/0009F0CA-C523-1213-852383414B7F0147_p39.gif
What kind of explosion was the big bang?

http://www.sciam.com/media/inline/0009F0CA-C523-1213-852383414B7F0147_p40.gif
Can galaxies recede faster than light?

http://www.sciam.com/media/inline/0009F0CA-C523-1213-852383414B7F0147_p42.gif
Can we see galaxies receding faster than light?

http://www.sciam.com/media/inline/0009F0CA-C523-1213-852383414B7F0147_p43.gif
Why is there a cosmic redshift?

http://www.sciam.com/media/inline/0009F0CA-C523-1213-852383414B7F0147_p44.gif
How large is the observable universe?

http://www.sciam.com/media/inline/0009F0CA-C523-1213-852383414B7F0147_p45.gif
Do objects inside the universe expand, too?
Last edited by Marshall on December 20th, 2007, 6:45 pm, edited 3 times in total.
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Expansion?

Postby PeSla on April 28th, 2007, 6:11 pm 

Marshall, this post a most excellent idea :-)

I hesitate to post here as I have become so deep into metaphysics. but what is the scientific or mathematical reasoning behind the last of the listed links I clicked on at random? I mean the wrong thing is that if the universe expands the things in it expand also -but how would we know then if it shrunk or expanded as everything is relatively the same unitary size.

And is it right to say as the universe expands the things in it stay the same in clumping? I mean what if the universe is really staying the same and the things in it shrinking. This may not be a trivial question when we think that matter in the local gallaxy is coming closer together while the galaxies are moving further apart? These too being dark matter questions where those theories might apply. As in the link on the SCF homepage by TommyHoi, that diamonds could be forever, if there will be always protons in an ever expanding universe what then? Can we imagine each proton approaching infinitely far from each other? Does this concept not just push aside such questions a the so called end and beginnings of cosmology beyond what is observational, perhaps until we reach then?
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Re: Expansion?

Postby Marshall on April 28th, 2007, 6:52 pm 

PeSla wrote:Marshall, this post a most excellent idea :-)


Thanks!

but what is the scientific or mathematical reasoning behind the last of the listed links I clicked on at random? I mean the wrong thing is that if the universe expands the things in it expand also -but how would we know then if it shrunk or expanded as everything is relatively the same unitary size.
...


if everything expanded in lockstep including our standards of length we would not SEE the evidence of expansion
but we do see evidence

your scenario would not fit the observed data. that is why it can be discarded, PeSla.
=============

there are fringe speculations where the speed of light changes, or the rate of atomic clocks.
But in this thread we stick to pretty mainstream notions. Our standards of length are the distance light travels in a specific period of time. Clocks are clocks, metal rods are metal rods. All these things stay the same. why not?, they are objects. The UNITS of distance do not change.

the distances that change are principally those between widely separated objects each of which is roughly stationary with respect to the CMB.

by contrast, neighboring objects are apt to be BOUND together in some fashion, by gravity e.g. in orbit around each other or in a cluster, by molecular forces, or by the metallic bonds between atoms in a steel rod. In the standard picture, distance between bound objects does not increase
==============

I know of no intuitive or commonsense reason why distances between widely separated objects NOT bound together by any force should NOT increase.
That is only our terrestrial experience because distances we are used to are between places on the rigid surface of the planet, analogous to marks on a metal rod. But I see no reason why our expectation should extend to points not joined by orbit dynamics or mechanical linkage. So it doesn't surprise me that these distances are increasing. Why shouldn't they increase, or decrease for that matter?

and then the simplest solutions of the Einstein equation PREDICTS that they should either do one or the other, so it is even less surprising.

that is my take on it. you have yours. we all make our peace with the observed geometry of the universe in one way or another.
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Postby Marshall on April 28th, 2007, 7:13 pm 

In the first post I mentioned that the branch of astronomy called cosmology had become an OBSERVATIONAL SCIENCE
One of the main things they do is get more and more accurate estimates of the fundamental parameters that go into the consensus LCDM model.

The LCDM can be either spatially flat and infinite, depending on what parameters you put in.
Or it can be spatially finite with some overall average positive curvature, if you put in other values of the basic numbers. I won't enumerate the more exotic possibilites.

Just to get an example of what cosmology is like these days, here is a fairly characteristic and quite recent paper. Don't get bogged down in the technicalities. Just sample it to get an idea of the flavor

http://arxiv.org/abs/astro-ph/0701584
Constraints on Dark Energy from Supernovae, Gamma Ray Bursts, Acoustic Oscillations, Nucleosynthesis and Large Scale Structure and the Hubble constant
Edward L. Wright

The point is he has several different large bodies of data: Supernovae, GRB, the CMB, element abundance, galaxy surveys (the history of galaxy and cluster formation)...
And he wants to estimate two parameters: the dark energy fraction (commonly assumed to be around 0.73) and the dark energy equation of state (often taken to be w = -1).
so he goes about fitting these things to the data, and in the process he gets "best fit" values and errorbars for a bunch of other parameters as well.

these parameters, in the mainstream cosmology context, determine the shape of the universe, its inferred past and the rough outlines of its future
small differences can sometimes have a significant effect so it seems reasonable, at least to professional cosmologists, to try to get the most accurate estimates with the narrowest margin of uncertainty that one can.

I just wanted to illustrate that with the example of a recent paper.
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a mainstream picture of the universe's past and future

Postby Marshall on April 28th, 2007, 7:24 pm 

This simple picture
http://nedwww.ipac.caltech.edu/level5/M ... igure1.jpg

is about the closest thing I know to a mainstream diagram of LCDM cosmology.

It appears, among other places, in a 2003 journal article by Charles Lineweaver. If you want to see it in context, here are some links:
The PDF version http://arxiv.org/abs/astro-ph/0305179
The Cal Tech HTML version http://nedwww.ipac.caltech.edu/level5/March03/Lineweaver/frames.html
The key figure http://nedwww.ipac.caltech.edu/level5/March03/Lineweaver/Figures/figure1.jpg
The same figure in context with surrounding text and caption
http://nedwww.ipac.caltech.edu/level5/March03/Lineweaver/Lineweaver2.html

The part of the figure to concentrate on is the top strip. The other two srips plot the same diagram on different time and distance scales (the way scientists sometimes use a LOGARITHMIC plot or some other device as a mathematical convenience.) They let you study details but the scales are nonlinear distorted.

When I print this out on my printer it fills up over half the page, so it is nice and large and legible. Take a look and if you have any questions about interpreting, ask---maybe somebody will clarify a point or two.
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simple Google calculations to avoid having to look things up

Postby Marshall on May 26th, 2007, 11:42 am 

in astronomy they use several distance scales, e.g. sometimes LIGHTYEARS (ly, Gly ...) and sometimes PARSECS (pc, kpc, Mpc...)

so how many lightyears to a parsec?

put "pc/c" in the window and press search, you get

1 Parsec / the speed of light = 3.26163626 years

that means it takes 3.26 years for light to go one parsec-----and so a parsec equals 3.26 lightyears.
==============

in astronomy you are always using the fact that the recession speed increases by 71 km/s for every Megaparsec (Mpc) you go out.
that is, the Hubble parameter H at present equals 71 km/s per Mpc

Eventually everybody ends up memorizing 71 km/(s*Mpc)

the HUBBLE RADIUS or Hubble distance is how far away the objects are which are receding at exactly the speed of light and you might want to know that in lightyears. Instead of looking it up, you can make Google calculate it just by calculating the reciprocal 1/H

put s*Mpc/(71 km) in the window and press search, you get

1 (s * Mpc) / (71 km) = 1.37720275 × 1010 years

that is 13.77 billion years
so the distance we are looking for is 13.77 billion lightyears.
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Postby BorisOfTerreHaute on May 26th, 2007, 12:05 pm 

Marshall,

Wonderful post.

Great links and great visuals illustrating just the kinds of things I'm currently struggling with.

Thanks
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Postby BorisOfTerreHaute on May 26th, 2007, 12:07 pm 

What is the origin of the parsec? And why do we use both parsec and lightyear?
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Postby Marshall on May 26th, 2007, 2:19 pm 

BorisOfTerreHaute wrote:What is the origin of the parsec? And why do we use both parsec and lightyear?


http://en.wikipedia.org/wiki/Parallax_of_one_arc_second

parallax second

Astronomy is one of the oldest sciences, Greeks and before them Babylonians etc.
Astronomers tend to be deeproot conservatives when it comes to units and terminology.

A SECOND is an ancient unit of angle which they still use (it is 1/60 of a MINUTE which is 1/60 of a DEGREE) and also called "arc second" to avoid confusion with the unit of time by the same name

PARALLAX is the slight angle that things seem to shift when you look first thru your right eye and then thru your left
actually by convention the parallax angle is half the angle it jumps, but you get the idea.

to see the parallax jump you have to blink back and forth, shut the left (look with the right) then quickly shut the right (look with the left) then back again.

hold your thumb out, against a distant background of trees (more distant things dont jump so noticeably so the distant background is like a fixed reference)
then when you alternate eyes you see your thumb jump back and forth against the fixed background of trees

if you know the distance between your eyes, you can use the parallax angle (and simple trig) to compute the distance to your thumb

=============
As soon as the distance to the sun was measured and people knew the diameter of the earth orbit
then they could use THE ORBIT DIAMETER LIKE THE DISTANCE BETWEEN YOUR EYES and the very distant background of stars could be like the background of trees and a nearby star could be like your thumb, so they could measure distance to nearby stars by the angle they JUMPED when you changed from one side of the earth's orbit to the other side 6 months later.

The first human to measure the distance to a star was the great BESSEL in around 1830-1840

He found the distance by
1. knowing already the diameter of our orbit
2. measuring the JUMP (parallax) angle of the star against the background, measuring it in SECONDS of angle which means 1/3600 of a degree.
3, using simple trig.
=============
After that, being traditionalists, astronomers HAD to define their first extrasolar distance unit as THE DISTANCE TO A STAR WITH EXACTLY ONE ARC SECOND OF PARALLAX.

The standard distance from earth to sun is called the AU or Astronomical Unit
and one parsec = 206,265 AU

how do we know? google calculator. ask yourself how many arc seconds are in a RADIAN of angle
well there are pi radians in 180 degrees and 180 degrees is 180*60*60 arcseconds so put this into the window

"180*60*60/pi" and press search

you get out 206,265 or more precisely (since google leaves you to do the rounding off)

(180 * 60 * 60) / pi = 206 264.806
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Postby Lincoln on May 26th, 2007, 3:12 pm 

Nice job Marshall.
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Postby BorisOfTerreHaute on May 26th, 2007, 8:03 pm 

Amazing. But wouldn't it be easy to just stop using parsec as a unit and just stick to LY?
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Postby Marshall on May 26th, 2007, 8:37 pm 

Thanks Lincoln!
Boris, yes it would be easier for everybody if astronomers reformed their units in various ways, but they are tradiitionalists, partly (as I mentioned) because of their centuries-long history, and they don't take easily to change.
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Postby BorisOfTerreHaute on May 26th, 2007, 8:48 pm 

Yes, I do understand that. I just wanted to make doubly sure that I was not missing a weird technical reason for keeping it.
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Postby Lincoln on May 27th, 2007, 7:40 am 

There is a crucial reason to keep the parsec, especially this weekend, marking the 30th year of insiders raising one eyebrow when hearing of a spaceship making the Kessel run in under 12 parsecs....
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Postby Marshall on May 27th, 2007, 10:08 am 

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Postby BorisOfTerreHaute on May 27th, 2007, 9:00 pm 

LOL. Thanks. :-)
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Re: Mainstream cosmology basics and sources

Postby xcthulhu on November 11th, 2009, 5:40 am 

Marshall wrote:The largescale structure looks wispy like cobwebs. There is a good talk by a Nobel laureate explaining how that comes about. Google TED Smoot.
TED is an organization that sponsors lectures for the public.
George Smoot is one of the world's top cosmologists, and gives a great slide presentation with short computer generated movies of how the cobwebs condense out of nearly uniform matter distribution.


You better understand if you see the visuals of it happening.
Matter starts out nearly uniform with very small almost imperceptible random fluctuations in density, left over from a much hotter and denser time. Space is expanding and matter is cooling, being cooled by the expansion.

Then the matter begins to fall together.


Here is the Smoot talk that Marshall was mentioning:

http://www.ted.com/talks/george_smoot_o ... verse.html
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Re: Mainstream cosmology basics and sources

Postby Tomyhoi on October 20th, 2010, 11:11 pm 

An interesting Find: A fairly new Researcher in cosmology. Looks promising.


Introductory Overview of Modern Cosmology
Burin Gumjudpai†
Institute of Cosmology and Gravitation
University of Portsmouth
Portsmouth PO1 2EG, United Kingdom
†e-mail: burin.gumjudpai@port.ac.uk




http://arxiv.org/PS_cache/astro-ph/pdf/ ... 5063v2.pdf
http://www.tptp.in.th/content/history
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Re: Mainstream cosmology basics and sources

Postby Marshall on November 17th, 2011, 3:42 pm 

I posted some nice quotes from Einstein earlier and the source link went dead. So I found a new source link and will repost with some introductory comment. It's possible that geometry is real without it being the geometry of anything substantial. Could there just be a web of geometric relations among events, without there being any thing or "fabric" corresponding to "space"?

In GR the gravitational field is the geometry abstracted from whatever continuum may have been used to define it on. One takes an equivalence class of metrics---in effect throwing away any particular manifold that might have been used to define the metric on. The familiar idea of general covariance is invoked.

The idea of space not having a real physical existence goes back at least to these 1915 quotes of Einstein. This is from a book called The Genesis of General Relativity edited by Jürgen Renn (Springer 2007). The author is Michel Janssen, he provides details on the sources. This doesn't really matter, the quotes are widely known and this is just one place they appear.

https://netfiles.umn.edu/users/janss011/home%20page/Besso-memo.pdf

==quote page 827, footnote 117==

117 In the introduction of the paper on the perihelion motion presented on 18 November 1915, Einstein wrote about the assumption of general covariance “by which time and space are robbed of the last trace of objective reality” (“durch welche Zeit und Raum der letzten Spur objektiver Realität beraubt werden,” Einstein 1915b, 831).

In a letter to Schlick, he again wrote about general covariance that
thereby time and space lose the last vestige of physical reality” (“Dadurch verlieren Zeit & Raum den letzter Rest von physikalischer Realität.” Einstein to Moritz Schlick, 14 December 1915 [CPAE 8, Doc. 165]).

==endquote==
Einstein was not always right or consistent. But this is a widespread idea. Even though geometric relations among events exist, space may not. Perhaps the matter fields are not defined on a physical manifold or continuum but instead are defined on the gravitational field itself. And that is a very puzzling, not fully satisfying idea.

But maybe it is the best we have so far, until the deeper degrees of freedom (from which both geometry and matter arise) are determined.
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Re: Mainstream cosmology basics and sources

Postby Carcass Drip on January 6th, 2012, 8:16 am 

Marshall wrote:Everyone is free to believe or disbelieve whatever model cosmology they like.


So you have given devine permission for people to excercise their basic right to believe whatever they like?

Gee - lucky you cleared that up

For a moment there I was afraid that you were going to prohibit people from independent cerebral thought

Phew!!!!

That was a close one folks!
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Re: Mainstream cosmology basics and sources

Postby Marshall on January 8th, 2012, 3:09 pm 

Carcass Drip wrote:
Marshall wrote:Everyone is free to believe or disbelieve whatever model cosmology they like.


So you have given devine permission for people to excercise their basic right to believe whatever they like?
...


No. Just stating the obvious. No permission, divine or otherwise, is needed. No more off-topic comments please. Don't want to clutter thread with discussion unrelated to cosmo basics.
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Re: Mainstream cosmology basics and sources

Postby Marshall on February 29th, 2012, 6:06 pm 

The Standard Solar Model data might come in handy
http://www.ap.stmarys.ca/~guenther/evolution/ssm1998.html

This gives percentages of H, He, and heavier elements. Derived by a computer model of fusion burning in core starting with pre-star gascloud abundances. Provides basis for estimating the remaining lifetime of the sun, should questions come up concerning that.
==============

The total entropy of the observable universe as of today might be of interest to someone.
http://arxiv.org/abs/0909.3983 This was published in Astrophysical Journal in 2010.
One of the co-authors is Charley Lineweaver.
Here's a conference presentation writeup based on it, some nice color visuals.
http://www.mso.anu.edu.au/~charley/pape ... arisv2.pdf

==============

FWIW here's a source that helps keep track of who's going where in GR-QC academia
http://sites.google.com/site/grqcrumourmill/
http://sites.google.com/site/grqcrumourmill/home
sample moves in 2012:
Ed Wilson-Ewing/ Marseille -> LSU
Hanno Sahlmann/ Pohang -> Erlangen
Renate Loll/ Utrecht -> Nijmegen
Marc Geiller/ Paris -> Penn State
Thomas Cailleteau/ Grenoble -> Penn State
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Re: Mainstream cosmology basics and sources

Postby Marshall on May 24th, 2013, 7:24 pm 

As a way of making the background of ancient light realer
http://www.astronomy.ohio-state.edu/~dh ... notes4.pdf
a cubic meter contains 413 million CMB photons

If you have a ruler that has centimeters and millimeters marked on it, a typical wavelength is 1 mm.
That is the peak wavelength of the power curve on a wavelength scale.
Plotted on a frequency scale the power peaks at around 1.8 mm.

If you are at cosmic rest--that is not moving relative to this soup of ancient light that fills all space--then the brightness is the same in all directions to within A THOUSANDTH OF ONE PERCENT. That the "temperature" of the light is the same in all directions to within one part in 100,000.

You can tell you are moving relative to the ancient cloud of matter and the light it emitted, if you see a doppler hotspot. Motion towards a source raises the intensity of the light.

We are still IN the cloud of ancient matter except that by gravity it has gradually coallesced into clumps and fallen into lumps with their random individual motions.

So there is this criterion of stillness: cosmic rest.

And that is where the idea of COSMIC TIME comes from. It is the scale of time as perceived by observers AT COSMIC REST.

Any model of the cosmos has to have some definite timescale, and cosmic time (also called by other names) is what the standard LCDM model runs on. It is based on the Friedman equation (with a certain choice of constants) and that is in turn based on the GR equation. But that's probably not the most important thing to understand at first. The main thing, I think is to grasp the basic ideas of:
cosmic rest
cosmic time
proper distance (distance at a particular instant of cosmic time)
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