Cosmology Definition and 918 Threads

From this link https://en.wikipedia.org/wiki/Baryon_acoustic_oscillations#Measured_observables_of_dark_energy , I can't get this relation : ##c\Delta z = H(z)\Delta \chi\quad\quad(1)## with ##z## redshift, ##H(z)## Hubble constant at redshift = ##z## and ##\chi## radial coordinates. One...

I found an article written by physicist George Ellis that confused me a little. http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.498.4569&rep=rep1&type=pdf At some part, he says: 3.2 Non-uniqueness: Possibilities There is non-uniqueness at both steps. Stating “all that is possible...

I've been reading up on inflation, and have arrived at the so-called slow roll conditions $$\epsilon =-\frac{\dot{H}}{H^{2}}\ll 1\; ,\qquad\eta =-\frac{\ddot{\phi}}{H\dot{\phi}}\ll 1$$ I have to admit, I'm having trouble understanding a couple of points. First, how does ##\epsilon\ll 1##...

I am making this thread to discuss interesting open questions in Gravity and Cosmology with people working in these or even relevant area. Anyone who is working in these (and relevant) areas, please identify yourself here and we will begin and hopefully people will catch on and contribute.

Hi, I'm currently trying to wrap my head around how the Ly-α forest works. This is what I have so far; A distant quasar produces these Ly-α photons, which occur when a hydrogen's electron drops from n=2 to n=1, and has a wavelength of around 121nm (depending on the exact transition). This...

Hi to whoever reads this, Some back ground info: I am a current undergrad student halfway through their degree, and I'm starting to think about what I'd like to do after I receive my diploma. I know I want to continue studying, and possibly move to a different country to do that. I know I can...

I have completed post grad in Physics and in the transition phase between post grad and Ph.D. I am interested in cosmology and also completed a post grad course on Astrophysics and Cosmology. Can anyone enlighten me about what are the current debates and active topics of research going on in...

Take note of today's Obied-Ooguri-Spodyneiko-Vafa 18. Curious situation: After it had influenced much the development of the string theory community’s attention and almost all of its public media perception since 2003, after what must be thousands of followup publications, the argument of KKLT...

If the uncertainty in the age of the Universe is ##\Delta t## then the Uncertainty Principle implies that it has an uncertainty in its energy ##\Delta E## given by $$\Delta E \ \Delta t \sim h.\tag{1}$$ If this energy fluctuation excites the zero-point electromagnetic field of the vacuum then a...

There is an assumption in cosmology say that there is another universe Composed of antiparticles? I mean that the atom composed of positron rather than an electron, anti-proton rather than a proton and anti-neutron rather than a neutron.

Hello all, Can you tell me what is the best book to study QFT when you are thinking to follow a PhD in cosmology (Dark energy, scalar fields, extension to GR, string theory).

The Dynamical Casimir Effect is the production of real photons from the vacuum in a system where one has moving mirrors (see https://www.technologyreview.com/s/424111/first-observation-of-the-dynamical-casimir-effect/). The frequency of the photons is related to the ratio of the velocity v of...

I was just curious because if space can expand faster than light, doesn't that mean there will be a lot of space that we just can't see? Do objects just vanish because we can't see them? For instance, if a hypothetical alien lived in MACS0647-JD galaxy which is 13.3 billion light years away...

we say everytime a couple of matter-antimatter particles get born near the edge of a black hole one of them falls into it and the other one escapes. And we everytime mention that the antimatter particle kind of eats a bit of black holes mass out...and by time the black hole gets smaller and...

According to Source: They discover a mega cluster of 14 galaxies that originated only 1400 years after the Big Bang This is the most active place in this universe

The question relates to the deterministic views of people like Descartes or Pierre de Laplace, the infamous "an omniscient intelligence who could know precisely the position and momentum of every single particle in the universe would be able to predict the future with total accuracy, past...

I've always loved physics, since my first science fiction novel in 1971. I won a scholarship to do a Math-Physics double honours program at University of Waterloo in 1977, but was immediately seduced into computer science by the promise of being paid big bucks to play with computers all day...

Hey! I was reading some script and when it comes to the cosmological redshift, it says, that only relativistic particles are affected by cosmological redshift. This does feel quite natural, however, I haven't been able to come up with an explanation that shows it with proper physics and...

<< Mentor Note -- after a very long Mentor discussion, we acknowledge that this paper, while potentially controversial, has been published in a reputable peer-reviewed journal. We believe that a discussion of this paper can be useful and constructive. Thanks >> This paper of mine was...

Hi! I just started high school and have developed a passion for cosmology and astrophysics. I would like to know if I really need to be very good at physics and mathematics (and if there is more that I need to be good at) and where do i find a job (spaces like N.A.S.A., SpaceX etc). Thanks for help!

Is it possible for me to do cosmology and quantum mechanics as a joint degree at uni? If so where and does it cost extra? I’ve been wondering for a whole and I’m not sure where to look.

If the universe was in an energy eigenstate then d<A>/dt = 0 for any dynamic variable A. Stuff moves which implies that the Universe isn't in an eigenstate. What factors drive the energy spread?

NASA decided to stop *WFIRST and concentrate on Mars project (send humans to Mars). What do you think about it? In my opinion, since a lot of ventures have begun space developments such as SpaceX and this proves that rockets make money, NASA should tackle WFIRST. Generally, we cannot earn...

Homework Statement Show that ##4\pi G(\dot \phi)^2=\epsilon a^2 H^2## Homework Equations Over dots mean derivative with respect to ##\eta##. $$\frac{1}{a}\frac{d}{d\eta}=\frac{d}{dt}$$ $$H=\frac{\dot a}{a^2}$$ $$\epsilon=\frac{-\dot H}{aH^2}$$ $$(\frac{\dot...

Homework Statement The general goal of the problem is to derive some useful identities involving the slow-roll parameters during inflation. For part a show that: $$\frac {d} {d\eta} (\frac {1} {aH})= \epsilon - 1$$ Homework Equations $$\epsilon \equiv \frac {d} {dt} (\frac {1} {H})= \frac...

According to [Dark Energy and the Accelerating Universe](https://ned.ipac.caltech.edu/level5/March08/Frieman/Frieman5.html) quantum field theory says that the energy density of the vacuum, ##\rho_{vac}##, should be given by $$\rho_{vac}=\frac{1}{2}\sum_{\rm...

So recently I saw a video with Richard Gott explaining a self-creating universe. There's three quick questions I'm confused about that I did not understand very well from the video: 1. How does a closed time-like curve allow the universe to self-create? Wouldn't the universe just go around the...

I find it strange that astrophysics is not a pre-requisite to study cosmology in my university, students are only required to have taken general relativity. Is this a trend? How important would you say is astrophysics to learn cosmology?

Hi, I'm a new member. I joined this forum because I'm interested with astrophysics and cosmology. I'm actually in the field of the Humanities, currently taking up my Master's in Art History. I like physics because I think it is where art and science beautifully intersect. I started to see the...

Hi, I am quite confused about followed question, I think scientist think the last scattering surface was dense plasma at the temperature of 3000K. If the today's universe much cooler and less dense then "the last scattering surface" how can anyone says entropy increased by time? Isn't universe...

Hello everyone.. Myself Kaustov from NIT Trichy, pursuing BTech. in production technology. Very much interested to be a part of physics forum. I have knowledge on Theoretical Astrophysics and Cosmology and also Particle physics. Very much interested to be part of all the discussions and to help...

Lets say I have a new Idea and maybe it has scientific value maybe not, that's not the topic now. I want to put this idea out there and I've already talked myself into trying to publish rather than posting. Maybe I would post, it depends on how it gets rejected. I'm sure not many like me try...

I've been looking at one of Max Tegmark's articles about his 'Mathematical Universe' hypothesis, here on arXiv. As a preliminary, note that Tegmark's framework has four 'levels' of multiverses, with each level being an infinite collection of multiverses at the level below it. The second or...

I was wondering if I could get suggestions for good general-background book on cosmology and the development/evolution of the universe. This is for myself, and I'll leave any more specifics to the topics open as I'd be look into whatever suggestions I receive. I'm a biophysicist, and back in...

Hello. I think I don't understand very well the Paul Steinhardt's cyclic model of Universe(s). According to Paul Steinhardt, 2 universes get closer. Then, there's the big bounce, which products effects like a big bang. If 2 universes get closer, they have a (relative) speed ( speed is the...

Hi! With the re-release of the textbook "Gravitation" by Misner, Thorne and Wheeler, I was wondering if it is worth buying and if it's outdated. Upon checking the older version at the library, I found that the explanations and visualization techniques in the sections on differential(Riemannian)...

This is a question that I have tried to pose several times without any success but, anyway, I would like to try again for the very last time. Asume for a moment that EW-SSB (electroweak spontaneous symmetry breaking) actually happened in our early universe. Imagine that our Standard Model of...

while deriving the friedmann equation using Newtonian Mechanics the 2nd term of the r.h.s is coming to be 2^U/(r^2*a^2) where U is a constt,but it is replaced by -kc^2/(r^2*a^2)?

I know that according to Einstein's theory of relativity, space-time is like a fabric which can be pliable. Gravity is the shape, or the warping of that fabric. In this analogy, what would dark energy (the unknown form of energy that is causing the universe to expand) be?

Hi all! I am currently trying to figure out a possible original research question that I can pursue in cosmology. I don't have ready access to a particle accelerator or anything like that so I would assume that it would have to be theoretical. I am sorry to ask such a broad question, but I don't...

I have a question about the gravitational time dilation explained in Appendix B of the book "Cosmology" written by S. Weinberg. Why can the author say "In the negative gravitational potential at the surface of a star clocks therefore tick more slowly than in interstellar space, or in the much...

In obtaining the Press Schechter function, the variance on mass scale is differentiated with respect to mass. The variance on a mass scale is a function of wavenumber k through the Power spectrum and transfer function. To eliminate k dependence, some value of k must be inserted into the...

Hello, I don't know if this is the right place to post this topic, I could not figure out the right one. I have recently finished my Masters in Condensed Matter. Now I want to follow a PhD where I can work/research on the dynamics of the Universe especially on dark energy, modified gravity...

Hi everyone, My name is Zack and I'm new to the forum. I recently changed careers from audio engineering to astronomy and am queuing up to attend the University of Oregon in about a year. I'm a big fan of all the historic great astronomers, scientists and others who have contributed to the...

suppose, for example, Pre-inflationary universe in loop quantum cosmology Tao Zhu, Anzhong Wang, Gerald Cleaver, Klaus Kirsten, Qin Sheng (Submitted on 22 May 2017 (v1), last revised 4 Jun 2017 (this version, v2)) The evolutions of the flat FLRW universe and its linear perturbations are studied...

I was wondering if we are only aware of the observable universe, and the actual universe could be infinite, what do we mean when we say the age of the universe is 14 billion years? Are we talking about the age of only the observable universe or the whole universe?

Hello, I am a bit confused on the relation between the Hawking effect(radiation) and the Unruh effect. What I understood with my little knowledge is that the Hawking temperature is the temperature that is emitted at the event horizon of a black hole as measured by an observer at infinite spatial...

I've been reading a lot of "Modern" Cosmology by Scott Dodelson and Introduction to Particle Cosmology by A. D. Dolgov and Cosimo Bambi. I like how rigorous and through Dodelson is but in 2017 it is quite dated. I love how modern and up to date Introduction to Particle Cosmology is but I wish it...

I'm a physics Ph.D student who just complete his first year of classes. I struggled a lot and found this year to be academically the hardest of my life. However in the end I did get excellent grades in all of my core classes. I would really like to take it easy next semester and only take 1...

Why do galaxy and galaxy clusters look so colorful?