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#127
Jan2712, 01:48 AM

Astronomy
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PF Gold
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Yenchin came up with a possibly useful iconic picture of the Einstein field equation (basic GR equation)
http://www.zamandayolculuk.com/cetin...ity_worlds.jpg 


#128
Jan2812, 10:02 PM

Astronomy
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PF Gold
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Emmy Noether's original paper, in English translation:
http://arxiv.org/abs/physics/0503066 http://arxiv.org/pdf/physics/0503066.pdf Section 6 on page 12 discusses the fact that conservation of energy does not hold generally in the curved spacetime of GRsomething pointed out by David Hilbert. The paper first appeared in the Nachrichten der Königliche Gesellschaft der Wissenschaften zu Göttingen (1918) 


#129
Feb2912, 03:58 PM

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PF Gold
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Standard Solar Model, thanks to Phyzguy:
http://www.ap.stmarys.ca/~guenther/e...n/ssm1998.html Gives percentages of H, He, and heavier elements. Derived by a computer model of fusion burning in core starting with prestar gascloud abundances. Provides basis for estimating the remaining lifetime of the sun. ============= The estimated total entropy as of today of the universe within our cosmic event horizon might be of interest to someone. http://arxiv.org/abs/0909.3983 This was published in Astrophysical Journal in 2010. One of the coauthors is Charley Lineweaver. Here's a conference presentation writeup based on it, some nice color visuals. http://www.mso.anu.edu.au/~charley/p...ganParisv2.pdf 


#130
May1012, 10:44 PM

Astronomy
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PF Gold
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When I type this into Google and press space, or equal sign, Google calculates the present critical energy density of the universe.
3c^2(71 km/s per Mpc)^2/(8pi*G) that is because it can interpret "71 km/s per Mpc" which is the present value of the Hubble rate H and because the formula for the critical energy density is ρ_{crit}=3c^{2}H^{2}/8πG It gives the answer in PASCALS but a pascal is the same as a joule per cubic meter. the same unit works for both pressure and energy density. So essentially it tells you the density in question is 0.85 nanojoule per cubic meter. It's gratifying how much the Google calculator recognizes and is able to calculate. It knows things like "mass of earth" "mass of electron" "radius of sun". So you can put a term like "radius of earth" into a formula you want it to calculate, like type in 2pi*radius of earth and you get a bit over 40,000 kilometers. I guess you could say that the Google calculator is a library resource 


#131
Jul2212, 07:10 PM

Astronomy
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PF Gold
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A nice interactive graphic illustrating the various size scales of the universe. I'm not sure if we posted it before. Indications are it's new this year, at least in this version, so probably not.
http://htwins.net/scale2/ New PF member Wakabaloola gave us the link. 


#132
Aug1112, 04:05 PM

Astronomy
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PF Gold
P: 23,121

A nice pedagogical thing. Pairs of masses orbiting each other send out gravity wavesyes we know this. It was the basis of the 1993 Nobel prize to Hulse and Taylor who observed a binary pulsar. But how about an example close to home?
By orbiting the sun, the Earth radiates 196 Watts of power in gravity waves. the frequency of the wave is presumably one cycle per year, very low frequency How do you know? Wikipedia has the formula for the radiated power. Type the following into google search window, which functions as a scientific calculator: 32/5*G^4/c^5*(mass of earth*mass of sun)^2(mass of earth+mass of sun)/(1 AU)^5 It knows what G is and what c is, and what the masses of earth and sun are. So you don't have to look any of that up. When you type that in, or simply paste it in, if you want, and press space or equal sign, it will say 196 watts. thanks to Mfb for this idea. 


#133
Sep712, 11:13 AM

Astronomy
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PF Gold
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A new kind of cosmological calculator. Tabular output instead of oneshot. It reveals patterns: maxima, crossovers, relationships between columns in the table... So it is somewhat better for learning than a oneshot. Perhaps quite a bit better, I don't now yet. We need a place to keep "user manual" type informationwhat you can do with it. At least temporarily until there is a page somewhere online you can link to. For now here is the most usermanualish information I've seen:



#134
Sep3012, 06:41 PM

Astronomy
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PF Gold
P: 23,121

This by Ruth Durrer What do we really know about dark energy?
might be good to have handy as a reference. http://arxiv.org/abs/1103.5331 It explains what is really being measured 


#135
Jan1713, 07:24 AM

P: 68




#136
Jan1713, 03:20 PM

Astronomy
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PF Gold
P: 23,121

This could translate into a cynical attitude when one hears scientists who should know better gushing about "dark energy" to get media attention, or sell books, or get funding. But those are INDIVIDUALS. I would not be cynical about the current cosmic models, they may actually be more solid and justified than you think and if they turn out wrong, well, that is not dishonorable! Theories are meant to be tested and eventually improved or discarded. I would rather be skeptical about the theoriesno disrespect. You might like this recorded interview with Carlo Rovelli: http://edge.org/conversation/aphilosophyofphysics Rovelli is one of today's prominent physicists and is skeptical both of "dark energy" and of "string theory". But derogatory only of the HYPE. He has what I think is quite a reasonable attitude about the cosmological constant Lambda, which people CALL "dark energy" but there is no evidence that it is any kind of energy. It behaves, as far as we can tell, like a simple slight constant curvature. As such it has a natural place in the Einstein equation. For 90 years or so people knew this constant curvature naturally appeared in the GR equation but most people assumed the constant was zero. The GR equation has room in it for TWO geometric constants, Newton's G and Lambda. Lambda is a curvature that for a long time we thought it was zero, and it happens not to be. That's all. To see that viewpoint explained, google "rovelli prejudice". You get a paper by Bianchi and Rovelli called "Why all these prejudices against a constant". Or go here: http://arxiv.org/abs/1002.3966 and click on PDF. Since this is a bibliography thread, it is better to make a separate thread to discuss questions like this. If you want, start one. This thread is mainly for useful links to research papers, videos and other online resources. 


#137
Jan2013, 08:08 AM

P: 151

"I think science is not about data; it's not about the empirical content, about our vision of the world. It's about overcoming our own ideas, and about going beyond common sense continuously. Science is a continuous challenge of common sense, and the core of science is not certainty, it's continuous uncertainty. I would even say the joy of taking what we think, being aware that in everything we think, there are probably still an enormous amount of prejudices and mistakes, and try to learn to look a little bit larger, knowing that there is always a larger point of view that we'll expect in the future." 


#138
Jan2113, 01:02 AM

Astronomy
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PF Gold
P: 23,121

I'm glad you liked it!
I happened to see your responsewas it to the article by Bianchi and Rovelli "Why all these prejudices against a constant?" or to the piece in Edge?because I came here to post this page from Ned Wright's cosmology site: http://www.astro.ucla.edu/~wright/Dltt_is_Dumb.html He thinks light travel time should not be used in press releases as a measure of distance. And he gives an argument in support, and proposes making more use of the redshift number. Proper distance would be another good alternativewhat you would measure by any ordinary means (e.g. radar) if you could somehow stop expansion long enough to measure. 


#139
Feb1313, 01:47 PM

Astronomy
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PF Gold
P: 23,121

Mordred found a recent paper which gives a detailed treatment of the extent of the Habitable Zone under various conditionsdifferent kinds of planetary atmospheres.
http://arxiv.org/abs/1301.6674 


#140
Apr1013, 11:08 AM

Astronomy
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PF Gold
P: 23,121

Introductory cosmology video lectures on Pirsa.
The lecturer is a young guy named Matt Johnson. I watched Lecture 1 and got a good impression of him. He seems fast, alert to questions, and wellorganized. It is blackboard rather than slides, which is normal for these Pirsa introductory lecture series. That doesn't slow him down because he writes quickly and legibly. Click on enlarge, to fill the screen. These series normally run to something on the order of ten lectures. The first three have already been given and are online. Their URLs are: http://pirsa.org/13040062/ http://pirsa.org/13040063/ http://pirsa.org/13040064/ 


#141
Jul314, 06:29 PM

Astronomy
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PF Gold
P: 23,121

Good 43 minute video of probing interviews with Abhay Ashtekar and Ivan Agullo.
https://www.youtube.com/watch?v=IFcQuEw0oY8 Skydive Phil made it. http://www.physicsforums.com/search....archid=4190288 


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