Cosmology Definition and 948 Threads

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. Meruem

    B What is the black that we see in the night sky?

    Summary: Can black means many thing? The black we see in night sky has to mean many things First of all, from point of views of human eyes then the photons can't reach our eyes so it appears black. Secondly, the objects appear smaller as we go farther from that object. So, at some point object...
  2. PeterDonis

    A Implications of Symmetry and Pressure in Friedmann Cosmology

    The thread title is the title of a recently published paper: https://iopscience.iop.org/article/10.3847/1538-4357/ab32da The paper claims to resolve an ambiguity in "cosmological backreaction" models, which are models that take into account spatial inhomogeneity to derive correction terms to...
  3. H

    If you could only study one thing in cosmology what would it be?

    Summary: What would you study If for some reason you were required to limit your area of study in cosmology to one topic, what would it be?
  4. Arman777

    Courses Is it necessary to learn GR and SR to study cosmology?

    I know this question sounds stupid but somehow i need confirmation I guess. Is it necessary to learn GR and SR to study cosmology ?
  5. M

    A Anisotropic Universe and Friedmann Equations

    The Friedman Equations is based on the cosmological principle, which states that the universe at sufficiently large scale is homogeneous and isotropic. But what if, as an hypothesis, the universe was anisotropic and the clustering of masses are aligned to an arbitrary axis (axial pole), how...
  6. J

    I Does G effectively change in an expanding metric?

    Assume that we have a flat FRW metric expressed in conformal time ##\eta## so that the line element is $$ds^2=a^2(\eta)(d\eta^2-dx^2-dy^2-dz^2)\tag{1}$$ where ##a=1## at the present time ##\eta=0## and the speed of light ##c=1##. This metric has the following non-zero Christoffel symbols...
  7. S

    A Is Seth Lloyd's cosmological model background independent?

    I have been interested in Seth Lloyd's cosmological model (which proposes that the universe is a some kind of quantum computer or at least similar to it: https://en.wikipedia.org/wiki/Programming_the_Universe, https://arxiv.org/abs/quant-ph/0501135) since long ago. I was wondering if his...
  8. SJay16

    B What’s more “important": GR or QM?

    Let’s say to the average Astronomer conducting research; generalizing the research to “astronomy” ; which would be more useful for the “average” Astronomer on a day to day basis: Quantum Mechanics or General Relativity? Obviously most are, but which so more? Which would be referred to more?
  9. SJay16

    B How well should Astronomers know General Relativity?

    I know that it would vary depending on the type of research a specific astronomer would be doing ( Astrophysics/Cosmology research versus an Astronomer researching exoplanets ) ; but in your opinion, “how much” or “how well” should an Astronomer with a graduate degree in Astronomy know General...
  10. Arman777

    I Exploring the Cosmos through Coding to Cosmology Projects

    For Project I need to find something related to the Cosmology and Coding (combined) any ideas ? Btw it should not be so simple such as Ned's Cosmology calculator.
  11. Bandersnatch

    I Why is this Hubble plot linear for Omega=2 closed universe?

    On Ned Wright's pages one can find this graph: plotting some supernova data against different expansion models. The main thing here that gives me a pause is the linear relationship for the closed universe with ##\Omega##=2 (red line). There doesn't seem to be any weird scaling involved. What is...
  12. koulbichok

    I Gaussian probability distribution of formation PBH

    Hello. If we consider PBH formation from collapse of large density perturbation in the early Universe, a mass PBH depends on density contrast as And δ must be larger then . Also we have β — an abundance of black holes, it's the ratio of the PBH energy density to the total energy density, this...
  13. koulbichok

    A Abundance and mass spectrum of PBH

    Hello. I'm trying to study primordial black holes for my work, but I still can't understand what are abundance and mass spectrum? Could you explain? Thanks in advance.
  14. J

    I Does the Friedmann vacuum equation have a linear solution?

    Does the Friedmann vacuum equation have a linear solution rather than an exponential one? Using natural units one can write Friedmann's equation for the vacuum as $$ \begin{eqnarray*} \left(\frac{\dot a}{a}\right)^2 &=& \frac{8\pi G}{3}\rho_{vac}\\\tag{1} &=& L^2 \left(\frac{\rho_0}{L^4}\right)...
  15. Dark Energy, Cosmology part 2: Crash Course Astronomy

    Dark Energy, Cosmology part 2: Crash Course Astronomy

    The majority of the universe is made up of a currently mysterious entity that pervades space: dark energy. We don’t know exactly what it is, but we do know that dark energy accelerates the expansion of space.
    • madscientist404i
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    • Astronomy Cosmology
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  16. Cerenkov

    B The singularities of gravitational collapse and cosmology

    Hello. I'd just like to check a some points concerning the two kinds of singularities that Penrose and Hawking describe in this paper. https://royalsocietypublishing.org/doi/pdf/10.1098/rspa.1970.0021 The Singularities of Gravitational Collapse and Cosmology. 1. According to the Cosmic...
  17. C

    Observing galaxies: area of sky would I need to survey

    Homework Statement Given that there are 10-2 Ellipticals per Mpc3 and my garden telescope can reach to 14 mag. How large an area of sky would I need to survey to find 100 Elliptical galaxies ? (assume the typical absolute magnitude for an Elliptical galaxy is -21 mag).Homework Equations...
  18. Jozefina Gramatikova

    Cosmology deceleration parameter

    Homework Statement Homework Equations modified Freidmann: The Attempt at a Solution for part b) I am not sure what to do as I am not sure what they mean by and
  19. J

    A Cosmological fluctuations (Weinberg's cosmology, p. 284)

    Hi everyone, I'm unable to understand how to derive Formula (6.3.11) in Weinberg's cosmology book. It's a relation between time-related derivation (d/dt) and RW-scale-factor-related derivation (d/dy, where y = a(t)/aEQ, a(t) is the RW scale factor in the metric and the EQ subscript denotes the...
  20. R

    Can random, unguided processes produce a rational brain?

    I am fascinated by Einstein’s quote that the most unbelievable aspect of the universe was that it was intelligible. So my question is does anyone know whether it is so unlikely as to be absurd to suppose that random unguided processes could produce a rational brain in man in as little as 3...
  21. Jozefina Gramatikova

    Cosmology: luminosity and magnitude

    Homework Statement Homework Equations [/B]The Attempt at a Solution (a) (b) Could you tell me if what I am doing is correct? In part (b) they want M as a function of L so I assumed we have to get rid of the l. Do I have to use l(m=0) = 1.52 x 10 ^(-8) ?[/B]
  22. A

    Other Looking for a cosmology problem for pursuing my PhD

    Hi I am now about to pursue my PhD in cosmology and my guide has told me to find a problem which can be solved (which will be my research topic) regarding dark energy or general relativity. I find it very difficult to choose a topic . Now it would be very beneficial if this community people...
  23. JD_PM

    What's book would you recommend on Cosmology?

    I have searched and came across: Introduction to Cosmology by Barbara Ryden https://www.amazon.com/dp/0805389121/?tag=pfamazon01-20 Modern Cosmology by Dodelson https://www.amazon.com/dp/0122191412/?tag=pfamazon01-20 What would you recommend? This is for a Master's course where basic...
  24. Arman777

    I What are the latest thoughts about the timescape cosmology

    I came across a theory called "timescape" cosmology. If I understand the theory correctly it assumes that the universe is not homogenenous due to the large voids and gravitationally bounded system. Also time would be different for the observers on these two different locations. 1- Is CMBR and...
  25. A

    B Comoving distance in cosmology

    For calculating the proper distance in cosmology why is the proper time between two points (galaxy) dΓ is taken 0?? e.g dΓ2=dt2-a(t)2dr2 Taking dΓ=0 and ∫dr=∫dt/a(t)
  26. FourEyedRaven

    Relativity Raychaudhuri - General Relativity, Astrophysics, & Cosmology

    Hello. The book "General Relativity, Astrophysics, and Cosmology", by Raychaudhuri, et al., looks like a good compact exposition of GR, astrophysics and cosmology, which is exactly the kind of book I'm looking for. What do you think of this book? Thank you in advance.
  27. J

    I Footnote on page 151 of Weinberg's Cosmology book

    Hi all, In this footnote, it is mentioned that Eq. 3.1.7, giving the pressure p(T) of a particle, can be derived from the law of conservation of energy (Eq. 3.1.4) Tdp(T)/dT = ρ(T)+p(T) and a previous definition (Eq. 3.1.6) of the energy density ρ(T) based on Fermi-Dirac or Bose-Einstein...
  28. A

    B Why are ds and dω taken as zero in calculating proper distance in cosmology?

    While studying the proper distance in cosmology I came across the thing as The FRW metric ds2=c2dt2-a(t)2[dr2+Sk(r)2dω2] And Sk(r)=Rsin(r/R) Where a(t) is the scale factor and dω2= dθ2+sin2θdΦ2, While calculating the proper distance at the time of emmission of light the term ds and dω are...
  29. S

    Dodelson Cosmology 4.8 Temperature of Nonrelativistic Matter

    Homework Statement Show that the temperature of non-relativistic matter scales as ##a^{-2}## in the absence of interactions. Start from the zero-order part of Eq. (4.68) and assume a form ##f_{dm} \propto e^{-E/T}=e^{-p^2/2mT}##. Note that his argument does not apply to electrons and protons...
  30. F

    I Infer the distance of a closest neighbor galaxy from correlation

    I am trying to estimate the distance of closest galaxy neighbor knowing the expression of number of neighbors into a volume ##\text{d}V##, the mean density ##n_\text{gal}## and the correlation function, i.e with this expression : ##\text{d}N=n_{\text{gal}}\,\text{d}V\,(1+\xi(r))## with...
  31. 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...
  32. Chromatic_Universe

    A Equilibrium recombination temperature for Helium

    How to get to the expression for equilibrium recombination temperature for Helium?
  33. 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...
  34. 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...
  35. 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 -...
  36. Tree Universe

    B Can Black Holes Be Annihilated?

    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...
  37. 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...
  38. 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...
  39. 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?
  40. 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: $$...
  41. 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...
  42. 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)...
  43. 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...
  44. 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...
  45. 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...
  46. 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...
  47. T

    How Does the Cosmological Constant Change with the Expansion of the Universe?

    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...
  48. T

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

    Hello What is Cosmology what does it do
  49. 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...
  50. 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)...
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