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. F

    I Total number density of galaxies and problematic expression

    Hello, I am asked to give the formal expression of the total number density of galaxies and explain why is this expression problematic in practice? From what I saw from my research and into my lectures, I have found the follwing relation which gives the number of galaxies ##N## with mass ##(m...
  2. Chromatic_Universe

    A Equilibrium recombination temperature for Helium

    How to get to the expression for equilibrium recombination temperature for Helium?
  3. J

    I Matter density in Weinberg's Cosmology book

    Hi everyone, On Page 72 of S. Weinberg's Cosmology book, it's mentioned, just after Equation 1.9.16, that, for the universe matter density ρ(r) to be an analytic function near the origin (spherical symmetry), it has to be given near r = 0 by a power series of r2. I'm not a math wizard, so can...
  4. Hugh de Launay

    I Is the increment of the expanding universe part of cosmology

    {Reference: Wikipedia's Friedmann-Lemaitre-Robertson-Walker (FLRW) Metric article)} The FLRW (1935) mathematical model of the universe is the one most used by cosmologists. It is differentiable, which means it is based on sound, consistent, mathematical formulations. (The Lambda-CDM model is...
  5. Tree Universe

    A Model of the Universe/Multiverse

    Hi Everyone, One of Hawking's last publications studied the model of universe/multiverse and conjectured that the exit from eternal inflation is "finite and reasonably smooth" - in other words, the inflated boundaries of multiverses could be finite and smooth. Here is the paper link -...
  6. Tree Universe

    B How to annihilate black holes?

    Hi Everyone, I'm interested in how to annihilate black holes (they are like blenders separating all the things apart and agglomerate like "tumors/cancers" in the universe). However, I'm new to this field and need some clarifications on some of the basic concepts like "annihilation" and...
  7. Rafid Mahbub

    A Exploring parameter space of inflation models

    I have been studying primordial black hole formation through inflation for a while and I was curious to know how the parameters in an inflation model are determined such that they are consistent with CMB constraints. In my literature reviews, there are quite a few models that exhibit an...
  8. S

    Dodelson Cosmology 6.8 Inflation Klein Gordon Equation

    Homework Statement Show that Eq. (6.33) follows from Eq. (6.32) by changing variables from t to ##\eta##. Homework Equations (6.32) $$\frac{d^2\phi^{(0)}}{dt^2}+3H\frac{d\phi^{(0)}}{dt}+V'=0$$ (6.33) $$\ddot{\phi^{(0)}}+2aH\dot{\phi}^{(0)}+a^2V'=0$$ The Attempt at a Solution So...
  9. R

    B Cosmic Expansion: Benefits of Hubble Law over Stretching Space(-Time)

    Why should you prefer the view that cosmological expansion is due to distant galaxies moving apart according to the Hubble Law instead of cosmic space stretching, like stretching of space(-time) in the vicinity of black holes?
  10. Rafid Mahbub

    A Numerical solution of the Mukhanov-Sasaki equation

    Hi, I am trying to figure out how to solve the Mukhanov equation numerically in Mathematica, but have some problems dealing with it. In terms of the number of efolds, the Fourier modes satisfy the following ODE in terms of the Hubble slow roll parameters: $$...
  11. T

    I Question on the Derivation of Temperature/Scale Factor Relation

    In 'Introduction to Cosmology' by Barbara Ryden, there is an argument made using the first law of thermodynamics to derive the relation T(t) ∝ a(t)-1 on pages 29 and 30. MENTOR NOTE: removed copyrighted material. I've been able to work out all the omitted details up to 2.37, which gives the...
  12. N

    I Communicating about the LHC and Cosmology in a creative way?

    Hello everybody, I'm a undergraduate student in Physics at my first year, and we got a project in our course: Astrophysics and Relativity. The topic is basically about how we can use the experiments at LHC in astrophysics/cosmology, (Which of course is trying understand the early universe etc)...
  13. J

    I Particle creation, energy density and the Compton wavelength

    The Compton wavelength of a particle is given by $$\lambda=\frac{h}{mc}.$$ One can construct an expression for the energy density ##\rho## of a particle of mass ##m## given by $$\rho = \frac{mc^2}{\lambda^3}=\frac{m^4 c^5}{h^3}.$$ What is the physical significance of the mass scale ##m## in the...
  14. T

    How is the horizon length related to the power law spectrum

    Homework Statement For a power spectrum density fluctuations ##P(k) \propto k^n##, I need to find the scaling (with respect to ##a##) of the horizon wavenumber ##\frac{2\pi}{\chi_H}## in a matter dominated universe in terms of ##n##. ##\chi_H(a)## is the evolving particle horizon, in a flat...
  15. Ebanflo

    I Is any attention being given to Conformal Cyclic Cosmology?

    Conformal Cyclic Cosmology, or CCC, is a hypothesis put forward by Roger Penrose in the early 2000s. My understanding of physics is lacking so my explanation will not be that clear, but I will summarize it here. Essentially, the existence of a previous spacetime, or "aeon," is postulated. This...
  16. alejandromeira

    B Exploring Olbers' Paradox in Matt Roots' Introduction to Cosmology

    I'm beginning to study the Matt Roots book Introduction to Cosmology and in the section 1.3 Olbers' Paradox he writes: "If the surface area of an average star is A, then its brightness is B=L/A. The sun may be taken to be such an average star, mainly because we know it so well. The number of...
  17. T

    Cosmology Fine Tuning Problem

    Homework Statement Suppose (incorrectly) that H scales as temperature squared all the way back until the time when the temperature of the universe was 1019 GeV/kB (i.e., suppose the universe was radiation dominated all the way back to the Planck time). Also suppose that today the dark energy...
  18. T

    I What is the purpose of Cosmology and where can I find information about it?

    Hello What is Cosmology what does it do
  19. A

    Physics Starting my PhD in cosmology -- question about my career prospects

    Hello all,Next year, January I will start my PhD in Cosmology. Salary is fair and I don't have to pay the tuition fee. I always loved the idea of doing a thesis about Cosmology. My master's degree was about computational physics where I had to learn how to program in C/C++, Fortran, Matlab...
  20. Thiwanka Jayasiri

    A Solving WIMP & Dark Matter Equation

    I'm trying to figure out how to arrive at the final expression, as given in the lecture notes. I tried to work this out by myself but getting different figure and units. I've considered the followings, normalize the equation from its values e.g. spins and x parameter, as g~100, x~10(m/t~10)...
  21. astroman707

    What math is used the most in cosmology/astrophysics?

    Considering one is taking all the required math courses for a typical physics degree, what math electives are most crucial to the field of cosmology/theoretical astrophysics? Also, is it true that mathematical modeling courses(discrete and dynamical modeling across physics) are more important to...
  22. F

    I Trying to reproduce curves with angle of CMB anisotropies

    I am looking for a way to get, by a simple numerical computation, the 3 curves on the following figure: For this, I don't know what considering as abcissa (comoving distance ?, i.e ##D_{comoving} = R(t)r## with ##R(t)## scale factor and ##r## the coordinate which appears into FLRW...
  23. F

    I BAO : Relation between redshift, Hubble constant and radial

    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...
  24. S

    A Multiverse theory with impossible universes?

    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...
  25. D

    I Troubles understanding slow-roll conditions in Inflation

    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##...
  26. shahbaznihal

    A Discuss Open Problems in Gravity & Cosmology | Join the Conversation

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  27. E

    I Understanding the Ly-α Forest: A Guide for EnSlavingBlair

    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...
  28. A

    Best Astrophysics Masters Programs in Canada: Recommendations and Opinions

    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...
  29. Avi Nandi

    A What are the frontiers of cosmology research?

    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...
  30. Urs Schreiber

    A No metastable type IIB de Sitter vacua

    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...
  31. J

    I Does the Uncertainty Principle imply a linear cosmology?

    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...
  32. Malek

    B Are Antiparticles the Key to Understanding the Universe?

    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.
  33. A

    Quantum Quantum field theory and cosmology

    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).
  34. J

    I Dynamical Casmir Effect in expanding space?

    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...
  35. platosuniverse

    B Doesn't there have to be more than one observable universe?

    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...
  36. Pouya Pourkarim

    I Why is Hawking Radiation Unequal for Matter & Antimatter?

    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...
  37. Maha odeh

    A This is the most active place in the universe

    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
  38. G

    B Is quantum physics "retro-deterministic"?

    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...
  39. M

    Is Quantum-Geometry Dynamics the Key to Solving Modern Physics Conundrums?

    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...
  40. binbagsss

    GR: conformally flat S-T cosmology

    Homework Statement Homework Equations look up, look down, your pants fell down The Attempt at a Solution [/B] Time-like killing vector is associated with energy. ## \frac{d}{ds} (\frac{\mu^2\dot{t}}{R^2})=0## Let me denote this conserved quantity by the constant...
  41. R

    I Why are non-relativistic particles not redshifted?

    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...
  42. E

    I Observational evidence against expanding universe in MNRAS

    << 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...
  43. Mlesnita Daniel

    B What do I need to become an Astrophysicist?

    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!
  44. V

    Programs Cosmology and quantum mechanics as a joint degree?

    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.
  45. Robert Shaw

    What's the energy-spread of the quantum Universe state?

    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?
  46. BlackholeGirl

    NASA [NASA] Cosmological projects instead of space exploration

    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...
  47. T

    Modern Cosmology Dodelson Problem 6.12b (Inflation)

    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...
  48. T

    Modern Cosmology by Dodelson: Problem 6.12 a

    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...
  49. J

    I Should the energy density of the vacuum be zero?

    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...
  50. relatively-uncertain

    I How do CTCs allow a universe to self-create?

    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...
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