Cosmology Definition and 948 Threads

From the paper, https://arxiv.org/abs/astro-ph/0305015, on page 3, How did the author arrived with equation (10)? By using the radiation density and (8) defined in the previous paragraph, ## ρ_r = \frac{3}{4}Ts ~## (radiation density) I think the author got it wrong (## ρ_r = \frac{4}{3}Ts ~##)...

Its been formalized for ~15 yrs now by Steinhardt & Turok; Wiki sez it has problems, but will not elaborate. My concern is that despite their denial, their version of CC is built on branes, which are of course a Very speculative basis, since it originates in M-theory. Worse, S&T do not seem tb...

recently this paper Quantum Gravity in the Sky: Interplay between fundamental theory and observations Abhay Ashtekar, Brajesh Gupt (Submitted on 15 Aug 2016 (v1), last revised 12 Nov 2016 (this version, v2)) Observational missions have provided us with a reliable model of the evolution of the...

Why is the power spectrum defined as ##P(k) = \frac{k^3}{2π^2} |w_k|^2 ## where ##w_k## is the mode function? Cosmology books and papers just states that it is defined that way but there are no details on why.

Hi, I am an undergraduate physics student. I am interested in pursuing a PHD in Cosmology. I still don't know which part, but for now I was curious about the computational aspect. Do you know some online course or book that treat well the subject?

There's something that I'd like to know and haven't been able to find in any book: 1.- Is the number of second order phase transition university classes known? 2.- Do we know quantitatively know the behavior of all such second order phase transition university classes? 3.- Do we know the...

In the past few months, there appear some observational results supporting the linear coasting models for the universe. http://www.nature.com/articles/srep35596 http://iopscience.iop.org/article/10.3847/0004-637X/828/1/35/meta This brings the coasting models again to the centre-stage of...

I apologize if I'm vague in how I present this discussion. As I am actively developing, I do want to try and keep certain specifics more private. At the same time, I want to encourage a vibrant discussion with a free range of ideas. You have an open ability to interpret what I'm asking, so feel...

Can anyone in the cosmology community direct me to hard research data that specifically demonstrates: there is no detectable redshift difference in stars that are the same distance from the Earth but in all different directions. This is of course related to 'does the universe have a preferred...

I came to understand that the space between galaxies are expanding and not that they are speeding from each other and as space expands the gravitationally bound systems remain in their own relative position as the effect of gravity is more compared to the expansion of space. But now consider two...

Based on this lecture notes http://www.helsinki.fi/~hkurkisu/CosPer.pdf For a given coordinate system in the background spacetime, there are many possible coordinate systems in the perturbed spacetime, all close to each other, that we could use. As indicated in figure 2, the coordinate system...

How does Hubble parameter and scale factor's derivative differ geometrically? I am reading S. Caroll's GR book. But I cannot get the full representation of these two parameters. On the book, it says How can \dot{H} and \ddot{a} be opposite of each other on the same instance if both are...

I have been reading the TASI Lectures on Inflation by William Kinney, (https://arxiv.org/pdf/0902.1529v2.pdf). I came across the mode function eq (128) (which obeys a generalization of the Klein-Gordon equation to an expanding spacetime), as I read through until eq (163), I know that it is the...

Hi everyone, I want to derive the Friedmann equations from Einstein Field Equations. However, I have a problem that stems from the energy-momentum tensor. I am also trying to keep track of ## c^2 ## terms. FRW Metric: $$ ds^2= -c^2dt^2 + a^2(t) \left( {\frac{dr^2}{1-kr^2} + r^2 d\theta^2 + r^2...

I have been thinking at this for a couple of days now: why are stars grouped in the massive collections that we call galaxies? I can assume that in the very early Universe, matter was grouped in these areas, that matter interacted thus resulting in the formation of stars. Then, the...

I have been wondering lately if the research output in brane cosmology has decreased over the last couple of years. When it first started, there were a lot of people working in the field. Any thoughts?

I just want to know what is the most logical next step after finishing the book Introduction to Modern Cosmology by Liddle? Some of the books that Liddle cited are Advanced undergraduate: Introduction to Cosmology by Matt Roos Gravitation and Cosmology by Steven Weinberg (Undergrad!?)...

Let us consider the FRW metric for flat space expressed in terms of conformal time ##\eta## and cartesian spatial co-ordinates ##x,y,z##: $$ds^2=a^2(\eta)\{d\eta^2-dx^2-dy^2-dz^2\}.$$ As in the standard FRW co-ordinate system one can see that if two observers are separated by a constant...

I'm in a deep discussion with a friend. He says that the Standard Model of particle physics is actually known by Standard Model of cosmology and that both are the same and that the SM of particle physics is in the Minkousky geometry... I disagree about this, I do think that the SM of particle...

I'm currently writing a paper on the Multiverse. I do have a theory of my own and have made the outline and started working on it. However, the language seems to unscientific, in the sense, its not very objective and crisp. How do i improve the quality of language and content of the paper. Any...

I think gravitational radiation mapping, but this will not happen until we have space based observations, until then we have to rely on LIGO which is in its infancy.

Hi everyone, I got the basic ideas quintessence (minimally coupled) and derived the KG equation for scalar field: $$ \ddot{\phi} + 3 H \dot{\phi} + \frac{\partial V(\phi)}{\partial \phi} = 0 $$ where $$H=\frac{\dot{a}}{a}$$ and $\phi$ is the scalar field. There are various models depending...

I'd really appreciate if someone could recommend me a (some) good textbook(s) about cosmology. My mathematical background knowledge is: - Usual stuff. - Differential geometry: Nash and Sen book. Nakahara book - Topology (algebraic and differential): Nash and Sen book. Nakahara book. - Group...

Hello, If anyone could help me with the prerequitisetes for an undergraduate General Relativity and Cosmology course I will enroll into, It will be much appreciated. The syllabus is the following(sorry for the rough translation): -Review of Special Relativity -Spacetime in GR -Geodesics...

I suppose that my questions are pretty basic, but I've been trying to find out the answers and not succeeded. 1.- Do cosmologists really think that the vacuum state suddenly changed in the early Universe? If so, would it be like a phase transition? If so, first or second orther? 2.- Does the...

I have been reading through Mukhanov's book "Physical Foundations of Cosmology" and have reached the section where he discusses the process of reheating. In it he mentions that the decay of the inflaton into bosonic states can be "Bose enhanced", i.e. that if ##n## previously created particles...

From Introduction to Cosmology by Matt Roos, he wanted to derive the Hubble parameter in terms of the scale factor. From the Friedmann's equation, ##\frac{R'^2 + kc^2}{R^2} = \frac{8πG}{3}ρ## The density parameter is ##~Ω(a) = \frac{8πG}{3H_o^2}ρ(a)~## and let ##~Ω_k = \frac{-kc^2}{H_o^2}##...

Authors: George F. R. Ellis, Roy Maartens, Malcolm A. H. MacCallum Title: Relativistic Cosmology Google Books Link: https://books.google.com/books?id=FPRFi3L0h9kC&printsec=frontcover#v=onepage&q&f=false

What are the prerequisites to read this book? In the book he stated that there are no GR and QFT knowledge assumed but some people said that it is not true. Can anyone comment on this book? Thanks.

Hello, Is the book General Relativity by Hobson, Lasenby and Efstathiou good in the sense that is intuitive on the mathematics AND physics that it presents? Please vote and if you could also comment your opinion it would be much appreciated. Thanks!

When referring to the absolute magnitude ,for example, in the B filter of a distant galaxy(considered point source) what are we talking about? Are we talking about the observed redshifted flux density(luminosity density) hypothetically seen from 10 pc away (1) ; or are we talking about the...

Based on my understanding, Hubble sphere is the region that contains GALAXIES receding subluminally while GALAXIES outside are receding superluminally (obviously on the border, GALAXIES recede at the speed of light). Particle horizon is the limit of what we have seen so far since the (big...

Hi, I'm currently in my last year of undergraduate degree, I have a basic knowledge of GR (A. Zee) but I have a very limited knowledge of cosmology, I did not cover the cosmology portion of Zee's book as it is too superficial and I don't want to waste my time reading that. I think my time will...

The Pi just posted a bunch of videos from their conference on time http://pirsa.org A lot of the big names in cosmology were there: Sean Carroll, Lee Smolin, Abhay Ashtekar , Andy Albrecht, George Ellis etc I have to say though I'm a little surprised at some of the questions. How is it i (...

And how we use it to calculate distances from stars to the earth.Thank you

I'm interested in all discussions but particularly those concerning the areas of QFT and cosmology. I've done much studying on the concept of quantum gravity over the past couple years and I want to learn more :)

multiple_authors submitted a new PF Insights post Where Did the Big Bang Happen? Continue reading the Original PF Insights Post.

I just have a quick question about a concept I think I haven't fully grasped from my cosmology course. Why does non-relativistic energy have an equation of state with w=0? Also, is the concept of pressure different in general relativity than in thermodynamics or statistical mechanics?

Hello Everyone! My name's Christina and I work at the Everyman Theatre in Liverpool... We have a show coming up in the main space at the beginning of July that's going to be performed by YEP (Young Everyman and Playhouse)... and hopefully it's going to be a love story about all things Physics-y...

Could you recommend me some good textbooks on cosmology, please? I'm looking for something that covers both general relativity topics and, which is more important for me, particles astrophysics stuff. I like the structure of Bergström&Goobar's book but it's a little bit sketchy. Something with...

It's said that information is never destroyed ( I know there is some controversy here regarding black hole information loss but let's put that aside for the moment). But what about created? I information a conserved quantity for the universe as a whole? Is there a consensus on this issue? It...

Hi, Etherington't reciprocity theorem states that distances measured by angular separation and by luminosity differ. My question is which one (if any of them) is the actual distance. I can understand they might differ in an expanding universe, but there's still a physical distance in such one...

I'm reading Cosmology by Harrison and in Chapter 21 he discussed the horizon riddle, I understood the problem that he posed but his solution was confusing. "Consider two widely separated observers, A (for Albert) and B (for Bertha). We suppose they can see each other. Each has a horizon such...

A recent discussion at the Expanding from and eventually to a singularity thread has been both interesting and informative, and it has shown me very clearly that I would benefit from a few good books, rather than the piecemeal approach I've been taking to understanding singularities and the Big...

Good day to you all I just started studying cosmology recently using several book and especially using TASI Lectures on Inflation(https://arxiv.org/abs/0907.5424) On page 25 the fine tuning problem of the flatness is introduced with conditions as |Ω(BBN)-1|≤ ο(10-16) |Ω(GUT)-1|≤ ο(10-55)...

Galaxies that are greater than a distance of c/H metres from Earth have recessional velocities exceeding the speed of light and begin to fade. Thus, theoretical astronomers 3 trillion years in the future will see only the Milky Way in the night sky. What is the reason for this fading of galaxies?

So CMBR points to a flat universe, and this seems to be the generally accepted model. But in a flat universe is expansion not supposed to slow exponentially, stopping after an infinite time? How does this fit with the observation that distant type Ia supernovae show the universe's expansion to...

Homework Statement I am just looking through some old notes I have from for cosmology, and there's something cropped up that i can't seem to figure out: Say I have two (or more) values for H_o each with errors such as: H_{o_1}=70^{+a+b}_{-c-d} and H_{o_2}=69^{+e+f}_{-g-h} How would I go...

Would general relativity (axiom: spacetime is continuous) still work if we changed the axiom to: spacetime is discrete (but each individual chunk of it is so incredibly small* relative to a quark or gluon that it makes no difference)? *e.g. 3D's of Planck length, 1D of Planck time

Hi, I don't know where to begin, please forgive me for asking this if it's been answered before. I'm confused, I don't know what to study, can you help me out? I'm fascinated by the Universe, my only regret in life is that I don't live in space far away from humans. I've always liked...