Cosmology Definition and 951 Threads

If the space between any two objects is expanding at a rate faster than light itself can travel, how is it that we're observing light from things so far away? One would think that the expansion of space affects light too, such that the space between the emitted light and the destination of said...

I'm utterly confused by co-moving distance, transverse comoving distance and proper distance. Is comoving distance = proper distance? Then what is transverse comoving distance? Here's what I know so far: The FRW metric can either be expressed as ds^2 = c^2dt^2 - a^2(t) \left[ \frac{dr^2}{1-kr^2}...

Background: Neutrinos decouple at around 10^10 K (or 1 MeV). This is normally shown as the interaction rate (between neutrinos and electrons) over the Hubble constant: Gamma/H = (T/10^10 K)^3 My problem: I have a function which is dependent on the neutrino-electron interaction. But it does...

Homework Statement [/B] (a) Find the value of A and ##\Omega(\eta)## and plot them. (b) Find ##a_{max}##, lifetime of universe and deceleration parameter ##q_0##. Homework Equations Unsolved problems: Finding lifetime of universe. The Attempt at a Solution Part(a)[/B] FRW equation is...

Hi everyone. Loving the forum, don't know how I haven't stmble upon it already. I'm studying Astronomy at the OU and finding a lot of the maths tough. Especially finding the chapters on Cosmology very difficult. I've seen another post regarding Scale Factor, so hope it's ok posting this too...

PF Insights Blog https://www.physicsforums.com/insights/postindex/ Was the early universe in a disordered state? Why is the radius of the observable universe in light-years greater than its age? Where did the Big Bang happen? Would that be the center of the universe? What is the total...

According to laplace,universe is totally deterministic.(one can tell position of object future position of object if he knows current position and velocity)but Heisenbergs's tells us that there is always uncertainty in position of particle and velocity.but Heisenberg's uncertainty only applies...

If a methane-filled planet or moon like Titan was pulled toward a star, as the methane warmed and became gaseous, would it be possible to ignite it, converting the methane oceans to H2O with a CO2 atmosphere, which could in turn cause photosynthesis to naturally occur? If it's possible, it...

You know friedmann equations derived from kinetic energy and potantial energy conservation.I found these shell model for universe.Here I am curious about something. This shell is like a surface of sphere isn't it.I mean it has only surface and that surface mass is m.And we made our equations...

I've got a homework question that I'm particularly stuck on: Suppose that the halo, assumed spherical, of non-baryonic dark matter surrounding our galaxy has mass ~ 5 x10^12 M solar and radius 0.1 Mpc. What it its average density in Kg m-3? I think that I need to use the formula...

I had a lot of trouble in my early undergraduate career and had received some really bad grades. I retook a lot of courses and after 6 years, I am finally graduating with good marks in my third and fourth year courses and I believe I have an overall good understanding of undergraduate physics. I...

Now that I have pretty much given up on Columbia, but still will have somewhere to go, I will try to make the best of what I have. Since Carnegie Mellon actually earmarks spots for waitlistees, rather than using it to fill up the class when too many No's have been recorded (like WUSTL, which I...

I am currently computer engineering student(second year). Well its not like I don't like computer engineering.but somtimes I think I should have chosen another path.I am pretty good at mathematics.I like to solve maths problems.I am also fascinated about cosmology.I like electronics as well.I...

To date AFAIK Dark matter and Dark energy are mathematical anomalies, What is the evidence for their existence and what is the latest thoughts as to what these hypothetical enteritis are?

I am reading some of "Planck 2013 results. XXII. Constraints on inflation." The paper is full of values for various inflationary parameters under various models, with their confidence intervals. For instance, in Table 5 on page 13, the authors report that — for a model including both running of...

Hello everyone, just a quick question: I have heard that it is impossible to do research in physics by yourself anymore. I heard that "The days of Einstein are gone" and that nowadays in physics, especially string and quantum relativity research, everyone publishes in groups. I feel like this is...

Before I say, I just want to mention that the Danish words for 15 and 18 are femten and atten Atten sounds a bit like the English word "eighteen". So that is where we get our metric prefixes femto- for 10-15 and atto- for 10-18. When you do cosmology the most common quantity, or one of the...

Hi guys, I recently started reading/working through Scott Dodelson's Modern Cosmology in preparation for a Masters course I'm taking next year and one of the exercises has stumped me and (arghhh!) its not one of the solved ones in the back! It is in Chapter 4 (The Boltzmann equations) and is...

The usual calculations for gravitational waves linearize the GR equations around the background solution of flat space time ( g = Minkowski metric matrix ) empty of matter and energy ( T = 0 ) What happens in cosmology, when one must linearize the GR equations about the FRW metric matrix with...

Hello every body. I want to learn about quantum cosmology but I always find lot of difficulties :confused:. I have a very usual background :frown: on Quantum Mechanics and General Relativity. What are the required specific topics of QM or GR that i should learn before I can learn about Quantum...

Homework Statement (a) Show that the equations satisfy FRW equations. (b) Show the metric when ##\eta## is taken as time Homework EquationsThe Attempt at a Solution [/B] The FRW equation is: 3 \left( \frac{\dot a}{a} \right)^2 = 8\pi G \rho Using ##\frac{da}{dt} = \frac{da}{d\eta}...

Is there any book, on cosmology, or anything related (Not too deep) recommended for a high school students interest basis. Thank you.

I am not sure where to find people who may understand the following paper but I figured someone here might be interested and or educated enough to weigh in on this subject. I tried with my local astronomical society and haven't heard anything back from them. This idea is interesting to me but I...

I have always considered cosmologists to be physicists because I noticed that cosmologists usually apply (Correct me if I'm wrong) general relativity. They seem to take approaches similar to those of physicists. Astronomy is the study of celestial objects, while cosmology is the study of the...

Ahoy there. I am twenty years old, and have (as a result of my interest in cosmology and astronomy, and the natural sciences) developed a keen appreciation for mathematics (in physics, primarily). I have always had an interest in the sciences, but lackluster teachers and personal disinterest...

Hello, I am currently studying astrophysics in my undergrad. I am concerned with the fact that I do not know what field I want to explore during grad school yet. I am in my junior year and will probably finish my undergrad in five years due to double majoring in math as well. I realize I have...

What are the important numbers in cosmology that dictates the very fate of the universe? I searched through Google and one site states that they are 13 constants; http://www.popularmechanics.com/flight/g163/13-most-important-numbers-in-the-universe/ What could be the possible consequences if...

Hello, this is my first post. I am a 15 year old from India just finishing my 1st year in high school. One of my friends recommended to me the world science U website - http://www.worldscienceu.com/courses/master_class I watched Max Tegmarks' History of the universe -...

Series of Lectures on Astrophysics and Cosmology This is part 1 of 12 in the series. Click the youtube link to select another video in the series.

...of physics on both quantum and cosmic scales. I am a chemical engineer and avid programmer who has always had an interest in fundamental physics, but this will be my first effort to truly pursue that passion. And I make beer for a living. :)

I understand that the first cosmological computer simulations Did not have DM Only gas dynamics on expanding grids And, that those sims produced no structure of any kind No LSS Only vast expanses of nearly uniform space plasma Devoid of any clumps Essentially reminiscent of the real...

Hi, as an engineer i don't have much formal training in theoretical physics so bare with me. As most of you know, we can only understand 4% of our universe with the current models we have. The rest of the 96% is dark matter and dark energy (23% and 73%, respectively). Could these unexplained...

I just finished reading a book called Three Roads to Quantum Gravity, and in it the author mentioned a few arguments surrounding quantum cosmology. He did not seem too fond of it, but to me (relatively inexperienced) combining the two makes a lot of sense. What are your thoughts on it?

I'm having some trouble understand this correctly, so I was hoping someone could enlighten me a bit :) Okay, so in the early Universe most of the hydrogen and helium was formed, and then kept in equilibrium, and ionized via photons. So we have a plasma of baryonic matter, including dark matter...

I'm currently reading about the Boltzmann equation, used for the early Universe. The equation I end up with, after some simplifications is the following: \begin{equation} a^{-3}\frac{d}{dt}\left(n_1a^3\right) = n_1^{(0)}n_2^{(0)}\langle\sigma v\rangle\left[\frac{n_3 n_4}{n_3^{(0)}n_4^{(0)}} -...

I have read 'comoving observers are a special set of freely-falling observers' . I have the following definitions: Comoving Frame: "defined at a time t is the inertial frame in which the accelerated observer is instantaneously at rest at t=t0. (Thus the term 'comoving frame' actually refers to...

So, I was trying to do a derivation of my own for the FLRW metric, since I couldn't understand the one Wald had. The spatial slice M is a connected Riemannian manifold which is everywhere isotropic. That is, in every point p\in M and unit vectors in v_1,v_2\in T_p\left(M\right) there is an...

Hello guys. I was thinking about alternatives to inflation, especially old ones (such as the hawking-hartle state and imaginary time) and I remebered a theory put foward by Penrose, in which his relatively new CCC is based. Called the Weyl Curvature Hypothesis. No idea of what it is. Could you...

Homework Statement I've been told to calculate the age of the Universe at T = 1 \, \text{MeV}, assuming that a(t=0)=0. Homework Equations Now, I've already calculated the value of H at that temperature, which was around H(1\,\text{MeV}) \approx 0.6 \,\text{s}^{-1}. I've also shown, that in a...

Consider a flat Robertson-Walker metric. When we say that there is a singularity at $$t=0$$ Clearly it is a coordinate dependent statement. So it is a "candidate" singularity. In principle there is "another coordinate system" in which the corresponding metric has no singularity as we...

Homework Statement I've been told to calculate the energy density in the early Universe. It states that it is completely dominated by neutrinos (3 species), photons, electrons, and positrons. Homework EquationsThe Attempt at a Solution Now, I've found an equation to calculate the energy...

I don't really have to look into this just yet but it bothers me. I quite like cosmology and stuff like such. Universal topology and relativity and sum of histories and stuff. Fun! I really would like to study any theoretical physics though, it just strikes my fancy particularly. I figure I'd...

I'm applying for graduate school but am from a basically unknown state school. My stats can be found here: http://www.physicsgre.com/viewtopic.php?f=21&t=5735. I am looking for a low ranked or unranked theoretical cosmology/hep-th school or a similarly ranked observational astronomy program...

I see that we use dimensional analysis involving constants of nature to obtain the Planck length and then apply the uncertainty principle to find the corresponding Planck mass-energy. But the energy and length scales were found by invoking a "particle" interpretation of fundamental entities of...

On the subject of our 'finely turned universe', I have read that the proton-electron mass ratio can not deviate more than 1:1037. In other readings, the allowable deviation was stated as 1% ("If the neutron were very slightly less massive, then it could not decay without energy input. If its...

I have seen in the online Stanford Encyclopedia of Philosophy in the entry on Copenhagen Interpretation of Quantum Mechanics that Niels Bohr had argued that the theory of relativity is not a literal representation of the universe: "Neither does the theory of relativity, Bohr argued, provide us...

http://mathpages.com/rr/s7-01/7-01.htm I am completely unable to follow the following sequence of working between equations 2 and 3. AFAIK the final answer is correct, but the intermediate steps seem to be a "casserole of nonsense". I would appreciate feedback from anyone who can follow this...

Just saw this. "Learn inflationary cosmology from the guy who invented it." Live stream Friday, 10/31, at 2pm EDT (11am PDT) here: http://new.livestream.com/accounts/4044190/AlanGuthLivestream

Homework Statement Given the maximum possible number density of stars in the present universe, assume that they have been radiating light for 10 billion years at a solar rate of ##3.9 \times 10^{26} \ W##. Photons emitted all have a restframe wavelength of 500 nm. Find a crude upper bound for...

Homework Statement (a)[/B] From the continuity equation show that if ##P=\omega \rho## and ##\omega > -1/3##, show that an expanding universe will eventually reach a maximum size. Take ##k=1## (closed universe). (b) Show that if ##\omega <-1##, the energy density ##\rho## will increase as the...