What is Cosmology: Definition and 918 Discussions

Cosmology (from Greek κόσμος, kosmos "world" and -λογία, -logia "study of") is a branch of astronomy concerned with the studies of the origin and evolution of the universe, from the Big Bang to today and on into the future. It is the scientific study of the origin, evolution, and eventual fate of the universe. Physical cosmology is the scientific study of the universe's origin, its large-scale structures and dynamics, and its ultimate fate, as well as the laws of science that govern these areas.The term cosmology was first used in English in 1656 in Thomas Blount's Glossographia, and in 1731 taken up in Latin by German philosopher Christian Wolff, in Cosmologia Generalis.Religious or mythological cosmology is a body of beliefs based on mythological, religious, and esoteric literature and traditions of creation myths and eschatology.
Physical cosmology is studied by scientists, such as astronomers and physicists, as well as philosophers, such as metaphysicians, philosophers of physics, and philosophers of space and time. Because of this shared scope with philosophy, theories in physical cosmology may include both scientific and non-scientific propositions, and may depend upon assumptions that cannot be tested. Cosmology differs from astronomy in that the former is concerned with the Universe as a whole while the latter deals with individual celestial objects. Modern physical cosmology is dominated by the Big Bang theory, which attempts to bring together observational astronomy and particle physics; more specifically, a standard parameterization of the Big Bang with dark matter and dark energy, known as the Lambda-CDM model.
Theoretical astrophysicist David N. Spergel has described cosmology as a "historical science" because "when we look out in space, we look back in time" due to the finite nature of the speed of light.

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  1. Andy_K

    Cosmology Review of Hidden In Plain Sight Series

    Has anyone read the 7-book series http://amzn.to/2lwgn66 by Andrew Thomas? Just wondering what you think of his conjectures / speculations at the final sections of each book, i.e. on the link between relativity and quantum mechanics, equation of the universe, etc... I like that he...
  2. W

    I Exact expression for the tensor-to-scalar ratio

    Based on the vast cosmology texts, I have seen that the expression for the tensor-to-scalar ratio ##r## in cold inflation is, ##r=16\epsilon_H = -16\frac{\dot H}{H^2}\qquad## where, ##~~~\epsilon_H = -\frac{\dot H}{H^2}## ##H## is the Hubble parameter, and ##\epsilon_H## is the Hubble slow...
  3. Arman777

    Studying Learn Cosmology: Beginners Guide to Friedmann Equations

    I want to learn cosmolgy very much,Like from the begining,Proper distance comoving distance how can we measure them then friedmann equations etc. I know leonard susskind lectures but he is describing too general,I want more detail. Is there any other good video lectures...(I couldn't find) a...
  4. M

    I Future Cosmology knowledge

    OK, let's imagine that in a few billion years time, when our local group of galaxies have merged, our Sun has reached and surpassed it's giant red phase stage and has become a White Dwarf, and we as a species are extinct. At the same time the distant galaxies will have moved beyond our...
  5. S

    I Solve Nonlinear DE: Friedmann Eqns for H 0-10^7

    From cosmology, the friedmann equations are given by, ##H^2 = (\frac{\dot a}{a})^2 = \frac{8\pi G}{3} \rho \, , \quad \frac{\ddot a}{a} = -\frac{4\pi G}{3}(\rho+3p) \, , \quad## where ##\rho = \frac{1}{2}(\dot \phi^2 + \phi^2)## and ##p = \frac{1}{2}(\dot \phi^2 - \phi^2)## To get ##\dot H##...
  6. Arman777

    I Time & Cosmology: Observing Past Quantities & Effects on Equations

    I know that If we look further we seethe past...So we are measuring some quantities like a mass of a far galaxy or dark matter mass of a galaxy...All in general does this mean we are observing past quantities ? These time dependent quantities can change with time so How this affect our...
  7. T

    Help Ted Decide on a Career Path: Seeking Advice from Experienced Professionals

    Hi guys I'm ted. I'm posting this thread because I'm not sure what career to follow and I could really use some help. First of all let me give you some info for me.I'm16-17 y.o and going to high school. At 15 I came 2nd to my countrie's astrophysics Olympiad.Read apostol's calculus and since...
  8. S

    I Friedmann Equation Analysis, expansion of the universe?

    Friedmann's Eq can be viewed here https://ned.ipac.caltech.edu/level5/March08/Frieman/Equations/paper1x.gif What I don't get is that all the texts/analyses of Friedmann's equation say that if the right hand side is negative it means that the universe will expand reach a critical point and then...
  9. smodak

    Particle The Standard Model in a Nutshell - Goldberg

    Just found this new book on amazon. Looks interesting. I purchased. https://www.amazon.com/dp/0691167591/?tag=pfamazon01-20 The TOC, Preface, and intro could be found here http://press.princeton.edu/titles/11050.html I wish they included a sample chapter as well (the book in not cheap), but I...
  10. Jorrie

    I Evolution of the Energy Density Parameters

    I am busy with an effort to show how the energy density parameters evolve over time in an update of the LightCone7 calculator. See the posts on the thread Steps on the way to Lightcone cosmological calculator. As part of this effort, I ran into some difficulties with deciding how to find and...
  11. S

    I Rescaling the equation of motion of inflation

    From the equation of motion of inflation, $$\frac{d^2\phi}{dt^2} + 3H\frac{d\phi}{dt} + \frac{dV}{d\phi} = 0$$ Example: ##V= \frac{1}{2}m^2\phi^2## $$\frac{d^2\phi}{dt^2} + 3H\frac{d\phi}{dt} + m^2\phi = 0$$ If I want to make the DE dimensionless then I let ##~t = \frac{1}{H_o} \tilde t~## and...
  12. Bolhuso

    I Where did the Dark Energy (or similar mechanism) come from?

    Hi all! I'm new to this forum and, as a matter of fact I only recently rediscovered my interest for physics. I am starting to catch up, yet I find that Wikipedia sometimes falls sort for some basic questions, and I was hopping that this community could gently help me understand some concepts...
  13. A

    I Expanding universe and the Boltzmann brain problem

    Hello I have been reading Sean Carroll's book "From eternity to here" where he mentioned the concept of functioning brains emerging from random fluctuations on a quantum level due to the expansion of universe. They have been called Boltzmann brains https://en.wikipedia.org/wiki/Boltzmann_brain...
  14. S

    I Different forms of energy density in inflation

    From the second Friedmann equation, $$H^2 = \frac{1}{3M_p^2} \rho \quad (k=0, flat)$$ In warm inflation, radiation is present all the way therefore not requiring proper reheating process, so $$\rho = \rho_\phi + \rho_r \, ; \quad \rho_\phi = inflaton, \, \rho_r = radiation$$ But, $$\rho =...
  15. S

    I How Can I Graph Hubble Term vs. Inflaton Using Mathematica?

    From cosmology, ##H^2 = \frac{ρ}{3M_p^2} = \frac{1}{3M_p^2}(½\dot φ^2 + ½m^2φ^2)## Suppose ##V(φ) = ½m^2φ^2## where ##ρ## = density ##M_p## = Planck mass I want to graph ##H## vs. ##φ## but there is a ##\dot φ## and I know this is a differential equation, can somebody help me what to do here?
  16. C

    I Cosmological electroweak phase transition

    There is a question that has been bugging and puzzling me for a long time and I wondered if somebody could help me to figure out what the answer is or where I am not thinking straight. Let us assume that the standard model of particle physics is more "right" than "wrong" and there is no such a...
  17. C

    I Scaling and the renormalization (sub)group

    I am aware of only two fields where the renormalization (sub)group ideas can be systematically and unambiguously applied: particle physics and equilibrium critical behaviour. 1.- Are there any others? 2.- What are these ideas used for in fluid mechanics? 3.- When cosmologists speak about...
  18. S

    A Gravitational waves in warm inflation

    https://arxiv.org/pdf/astro-ph/0006077v2.pdf page 5. Tensor Perturbations "Tensor perturbations do not couple strongly to the thermal background and so gravitational waves are only generated by quantum fluctuations, as in standard supercooled inflation". Why? Tensor perturbations are created...
  19. Jules Winnfield

    B Galaxy Clusters, Masses and Radius

    This is a request for a good reference. I'm doing a report on dark matter and am wondering if there's any recent references that collects a bunch of galaxy clusters, their dark masses and the radius of the cluster. Preferably in a table that I can shred or download but I'll copy if I have to...
  20. G

    A Flatness Problem Differential Equation

    Hello to everybody, Since 3 days, I've been trying to obtain Eq. (173) from https://ned.ipac.caltech.edu/level5/Sept03/Trodden/Trodden5.html I know I need to derivate with respect to a, but it is being impossible to obtain the final answer. Can anybody help me with a detailed derivation...
  21. S

    I How Did the Author Derive Equation (10) in the Context of Radiation Density?

    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 ~##)...
  22. J

    A What is the Current Status of Cyclic Cosmology and its Relation to M-Theory?

    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...
  23. K

    I How do cosmologists view loop quantum cosmology?

    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...
  24. S

    I Why is the Primordial Power Spectrum Defined as P(k) = (k^3)/(2π^2)|w_k|^2?

    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.
  25. O

    Computational Cosmology: Undergrad Physics Student Guide

    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?
  26. C

    I Phase transitions university classes

    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...
  27. M

    I Realistic coasting cosmology from the Milne model

    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...
  28. Tesagk

    Writing: Input Wanted The role of cosmology on a world....

    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...
  29. Jim Hasty

    A Redshift data on stars the same distance from earth

    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...
  30. R

    B Gravitationally bound system in an expanding space

    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...
  31. F

    I Gauge transformation in cosmological perturbation

    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...
  32. Elnur Hajiyev

    A Hubble parameter vs Scale factor's derivative

    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...
  33. F

    I How to Understand the Primordial Power Spectrum in TASI Lectures on Inflation?

    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...
  34. D

    I Energy-momentum tensor and Friedmann Equations

    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...
  35. T

    I Why are stars grouped in galaxies?

    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...
  36. S

    A Exploring the Decline of Brane Cosmology Research: A Reflection

    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?
  37. S

    Cosmology Book after Modern Cosmology by Liddle

    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!?)...
  38. J

    I Should Einstein's Field Equations be modified for cosmology?

    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...
  39. A

    I Is the SM of particles physics and Cosmology SM

    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...
  40. S

    Other Writing a Paper on the Multiverse: Tips for Quality Language & Content

    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...
  41. wolram

    B What is the next step in cosmology

    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.
  42. D

    A Exact solutions of quintessence models of dark energy

    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...
  43. C

    Recommended Cosmology Textbooks for Mathematical and Physics Backgrounds

    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...
  44. J

    Courses Prerequisite to General Relativity/Cosmology Undergad course

    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...
  45. C

    A Do cosmologists take spontaneous symmetry breaking seriously?

    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...
  46. D

    I Bose enhancement in early universe cosmology

    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...
  47. F

    I Hubble parameter in terms of the scale factor

    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}##...
  48. ShayanJ

    Cosmology Relativistic Cosmology by Ellis, Maartens and MacCallum

    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
  49. F

    Cosmology Physical Foundations of Cosmology by Viatcheslav Mukhanov

    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.
  50. J

    Relativity General Relativity by Hobson, Lasenby and Efstathiou

    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!
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