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. S

    I Dark Energy contribution to plasma temperature in galaxy clusters?

    I have a question about this work called "Dark energy and key physical parameters of clusters of galaxies"*There, towards the end, the authors talk about the isothermal velocities and tempreature parameters of the gas and particles circulating between galaxies in clusters. In particular they...
  2. Mahsum

    How to Approach a Term Project on Dark Matter Density Distribution?

    Hello, I have asked a similar question before, but this time I want to ask it a bit differently to be understood better. I am a physics student at a university and this semester I am going to make a term project about Dark Matter Density Distribution by using rotation curves. However, I am a...
  3. T

    A Save Time with CLASS Code: Neat Trick for Background Quantity Evolution

    Is there a neat way to "not" run the internal Boltzmann solver (for perturbations) in CLASS code and rather just solve for the background quantities? This way I can save the time otherwise spent in evaluating the perturbations and transfer functions. I am only interested in the time evolution of...
  4. S

    I Tidal effects of galaxies orbiting one another with dark energy?

    I recently found a question in a physics discussion site [1] about whether there was a "distance" between two galaxies where both the gravitational force and the influence of dark energy would be balanced. The answers and comments seem to indicate that there is indeed such a "radius" around a...
  5. O

    I What are P03 procedures or guidelines?

    Hi. I'm not sure if this should be in Cosmology or Astronomy - a moderator can feel free to move it if required. Also the prefix (study level) may be an I or an A, you can change that, just thought I'd get a bigger audience with an I. I'm trying to read this paper: "The Inner Structure...
  6. S

    I Dark energy contributing to, or modifying, mass estimates?

    I have found some papers (like this one: https://www.aanda.org/articles/aa/full_html/2009/45/aa12762-09/aa12762-09.html) which say that dark energy increases the potential energy in a system of a quasi-stationary gravitationally bound many-body system. It also says that because of this, the...
  7. F

    A Poisson noise on ##a_{\ell m}## complex number: real or complex?

    1) In a cosmology context, when I add a centered Poisson noise on ##a_{\ell m}## and I take the definition of a ##C_{\ell}## this way : ##C_{\ell}=\dfrac{1}{2\ell+1} \sum_{m=-\ell}^{+\ell} \left(a_{\ell m}+\bar{a}_{\ell m}^{p}\right)\left(a_{\ell m}+\bar{a}_{\ell m}^{p}\right)^* ## Is Poisson...
  8. S

    I Same state as in the Big Bang in a collapsing universe?

    Suppose the universe were to eventually collapse in a Big Crunch [1]. How closely could the universe's final moments resemble those at the beginning of the universe? Could the universe return to its original state exactly in some kind of "Big Crunch" or "Big Bounce" model? [1]...
  9. A

    I What do the authors of the paper mean here exactly by path integral?

    Hi, I am new here so apologies if i am not using the right subforum. I don't have a physics background so i am not very technical but i do have a little bit of understanding. I was reading this paper by hawking/hertog and came across something that ended up confusing me. Here is it: "Pre-big...
  10. A

    I Why Is a Flat Universe Infinite?

    I cannot find an answer online. Help please.
  11. D

    What is the Connection Between Biology and Cosmology?

    More of a biology person but always had an interest in cosmology as well.
  12. S

    I DeSitter cosmological horizon stability?

    If the universe keeps expanding at an accelerated rate (given by the cosmological constant) then the universe would approach a DeSitter spacetime where there would be a cosmological horizon that would radiate just as the event horizon of a black hole radiates Hawking radiation I thought that...
  13. S

    I Could entropy be reversed eventually in the far future?

    In the far future there will be most likely a point where a maximal state of entropy will be reached in the universe and after the last black hole evaporates there could be no more structures and no more work could be done. According to the Poincaré recurrence theorem for a closed universe...
  14. M

    A Vavryčuk's conformal FLRW metric as an alternative to dark energy and dark matter

    Václav Vavryčuk has written two articles on replacing the standard FLRW metric in cosmology with what he calls a "conformal FLRW metric", which he claims explains astrophysical and cosmological phenomena traditionally attributed to dark energy and dark matter/MOND, such as the dimming of type 1a...
  15. pinball1970

    I New research puts age of the Universe at 26.7 billion years

    The article https://phys.org/news/2023-07-age-universe-billion-years-previously.html#:~:text=42-,New%20research%20puts%20age%20of%20universe%20at%2026.7%20billion%20years,as%20old%20as%20previously%20believed&text=Our%20universe%20could%20be%20twice,%22impossible%20early%20galaxy%20problem.%22...
  16. M

    A Evolution of radio sources mimics a non-expanding universe

    new article by Pengfei Li: Distance Duality Test: The Evolution of Radio Sources Mimics a Nonexpanding Universe
  17. J

    Unending Education: Exploring Physics, Astrohysics, and Cosmology

    I never stop learning physics, especially astrohysics and cosmology.
  18. J

    B History of Cosmology: 1915-1929 | Einstein, Hubble & GR

    Hi, I've recently developed an interest for the history of the development of cosmology and find it very interesting. The key events I have been reading up on are: 1915 - Einstein's theory of General Relativity was published. 1923 - Hubble discovered a Cepheid variable in the Andromeda...
  19. S

    I Topological phase transitions for the whole Universe...?

    Physicist Grigory Volovik has put forward some ideas about the universe undergoing a topological phase transition (especially in the early stages of the universe). He published a book called "*The Universe in a Helium Droplet*" where he explained his ideas. You can find a brief discussion here...
  20. Alpha2021

    A Is M_{Pl} the Planck mass or the reduced Planck mass?

    What is the value of M_{Pl} used in the Planck (CMB) collaboration's observation papers, such as the one referenced in this link: https://arxiv.org/pdf/1807.06211.pdf. Specifically, I am wondering if it refers to the Planck mass or the reduced Planck mass?
  21. S

    I Consequences of the absence of global symmetries...?

    I found some interesting discussions in this site (e.g: https://www.physicsforums.com/threads/smolin-lessons-from-einsteins-discovery.849464/; https://www.physicsforums.com/threads/relatismo-to-the-max.83885/) which are related to Lee Smolin's ideas that laws are not immutable and can therefore...
  22. S

    I No symmetries in the Universe at the Big Bang...?

    I apologize in advance if this is a stupid question but... According to some scenarios about the beginning of the universe (namely cosmological inflation), in layman's terms, everything was born out of a quantum fluctuation which caused a violent expansion. In this case, since an expanding...
  23. ergospherical

    Linearisation of continuity equation (cosmology)

    After expanding to first order in ##\epsilon## and subtracting off the unperturbed equation, I get\begin{align*} \frac{\partial \delta \rho}{\partial t} + 3H \delta \rho + \frac{\bar{\rho}}{a} \nabla \cdot \delta \mathbf{v}=0 \end{align*}I'm not sure how to deal with the ##3H \delta \rho## term...
  24. R

    A What does the TE CMB spectrum reveal?

    Does the polarization spectrum TE measured by the Planck and WMAP satellites show evidence for superhorizon fluctuations at low multipoles and are these evidence for pre-bigbang inflation?
  25. R

    I Estimating Hot & Cold Spots in CMB Sky Maps

    Is it possible to estimate the number of hot and cold spots of average diameter of 1° in a full-sky map of the CMB?
  26. S

    I Solutions that break the Lorentz invariance...?

    I was reading a discussion where some physicists participated* where the topic of Lorentz invariance violations occurring in cosmology is mentioned. There, they mention that we can imagine a Lorentz-violating solution to the cosmological equations. What do they mean by that? Can anyone specify...
  27. S

    I Inhomogeneities and topological defects in cosmology...

    I have heard that some types of inhomogeneties and topological defects (like cosmic strings) in cosmology have been proposed to be able to break fundamental symmetries of nature such as the Poincaré, Lorentz, diffeomorphism CPT, spatial/time translational...etc symmetries... However, I have not...
  28. H

    I Dark Energy Strength in Great Voids of Galaxies

    Assuming dark energy is fairly, uniformly distributed through out the cosmos, how strong is it, or how much energy is associate with it, out in the deepest, emptiest voids in space? I'm specificlaly refering to the great voids in between the great walls of galaxy clusters. I'm making the...
  29. S

    I What does it mean that symmetries do not hold globally?

    Perhaps this is a stupid question but, if Lorentz symmetry and time translational symmetry are not global in an expanding universe, wouldn't that mean that is possible that other Hubble spheres outside our observable universe could have other symmetries or an absence of the Lorentz symmetry? I...
  30. M

    I Expansion of the Universe and the cosmological principle

    Wikipedia states the following in their article about the expansion of the universe: If the cosmological principle was discovered to be false in our universe, i.e. our universe was discovered to be inhomogeneous or anisotropic or both on very large scales and the FLRW metric does not hold for...
  31. Bandersnatch

    A Black hole mass coupled to expansion -- astrophysical source of dark energy?

    Observational evidence for cosmological coupling of black holes and its implications for an astrophysical source of dark energy Comments?
  32. G

    Programs Should I major in Physics & Astronomy or Physics & Math?

    I'm currently in my first year of a combined major in Physics and Astronomy. I'm hoping to study theoretical cosmology, and get a PhD in physics. I've been told it would be wise to get a double major in Physics and Math if I wish to study theory, but would it also be wise to get a combined major...
  33. V

    A Cosmological Density Perturbation vs Homogeneity: Questions Answered

    When arriving at the standard model of cosmology, i.e. the exapnding universe, we assume based on experirmental data that the cosmos is homogenous on large enough scales. But when we go back in time, when the galaxies are beginning to form, we note that because of the growth of density...
  34. Al-Layth

    I Should I study astronomy first or cosmology first?

    tldr summary
  35. P

    I Is Penrose's Cyclic Cosmology Model Only Applicable on a Local Scale?

    A recent thread asked about Penrose's proposal on cyclic cosmology. It was closed due to lack of any remotely acceptable sourcing, even after prompting. Much of the original professional publication on this is not available on arxiv. However the following includes a summary of conformal cyclic...
  36. S

    I Hubble flow kinetic energy into other types of energy?

    Spacetime expands at an accelerated rate and the particles with movement associated to this expansion are coupled to the Hubble flow. In many papers that I've read, objects coupled to the Hubble flow are treated as if they have some velocity and kinetic energy associated with it.However, can...
  37. F

    A Expression of Shot noise when expanding ##a_{\ell m}## coefficients

    I would like to arrive at the following expression for the quantity ##o_{\ell}## ( with "DM" for Dark Matter ): ##o_{\ell}=b_{s p}^2 C_{\ell}^{D M}+B_{s p}## with Poisson noise ##B_{s p}=\frac{1}{\bar{n}}(\bar{n}## being the average number of galaxies observed). the index "sp" is for spectro...
  38. harutsuki

    I think I ended up in the wrong Laboratory? (Theoretical Cosmology)

    TL;DR Summary: I dont enjoy my lab subject. I am currently in grad school (not US) and in the theoretical Cosmology laboratory, however I feel like I ended up in the wrong side. I did undergrad research project in astrophysical data simulation and thought that this is what I want to do in...
  39. S

    Are there online cosmology book clubs for studying together?

    Wondering if there are any online “book clubs” that people read cosmology textbooks together?
  40. S

    I Conservation of energy in quasar outflows?

    I found this article* about the behavior of quasar outflows in cosmology and how they can create a magnetic field. In section 2.1.4., the authors say that when a quasar produces a "wave" or an outflow, the material will be emitted with energy coming from both the quasar itself and the Hubble...
  41. S

    I Avoiding heat death in an accelerated expanding universe?

    https://en.m.wikipedia.org/wiki/Heat_death_of_the_universe Is the heat death of the universe completely unavoidable in an universe with an accelerated expansion dominated by dark energy like ours? Or can there be any way to avoid it according to current knowledge, observations and experiments...
  42. Sciencemaster

    I Calculating Spacetime Around Multiple Objects

    In describing the spacetime around a massive, spherical object, one would use the Schwarzschild Metric. What metric would instead be used to describe the spacetime around multiple massive bodies? Say, for example, you want to calculate the Gravitational Time Dilation experienced by a rocket ship...
  43. S

    I Deviations of conservation laws in cosmological evolution?

    If energy is "not conserved" in General Relativity (or at least, it is difficult to define it) in the context of an expanding accelerating spacetime (like it happens in our Universe), are there any observations of deviations from the strict conservation laws in the evolution and formation of...
  44. G

    I Data Showing Dark Matter Is Not Cold Neutrinos?

    How do we know that cold neutrinos do not make up 100% or a large percentage of the dark matter content in the universe? In my mind, the only way to prove that dark matter is not simply cold neutrinos would be to measure the density of cold neutrinos in the universe and then calculate the...
  45. S

    I Energy from quantum systems in an expanding universe?

    I found a paper (https://arxiv.org/pdf/astro-ph/0411299.pdf) which talks about quantum systems emitting energy due to spacetime expansion. Is this true or only a hypothesis?
  46. cosmologyscience

    Exploring the Universe: Galaxy Formation, Redshift, and Cosmology

    Hello, I'm an independent researcher in cosmology and working on topics like cosmological redshift, galaxy formation and galaxy cluster dynamics. Came across the forum searching for details about the solar system orientation in the galaxy rotation disk. Very inspired and enthusiastic about the...
  47. S

    I Multiple (Non-Interacting) Universes?

    Before anything, I would like to clarify that I am aware that this is speculative physics more than established mainstream physics (and Smoot is not claiming that his ideas are true). However I think that it is interesting to discuss these models even if they are not yet proven to be right. I...
  48. A

    A How to Plot the Light Path in a Space-Time Diagram Using Python?

    I am trying to replicate the space time plot (the 2nd plot with Proper distance vs Time) as in this thread: space-time I wrote everything in python using the astropy cosmology package. Everything went smooth, but I am stuck at plotting the light path on the 'purple path', as per the above...
  49. Onyx

    B Sign of Expansion Scalar in Expanding FLRW Universe

    Considering the FLWR metric in cartesian coordinates: ##ds^2=-dt^2+a^2(t)(dx^2+dy^2+dz^2)## With ##a(t)=t##, the trace of the extrinsic curvature tensor is ##-3t##. But why is it negative if it's describing an expanding universe, not a contracting one?
  50. R

    On the origin and the evolution of information in the Universe

    [Mentor Note -- thread moved from the schoolwork forums to the technical forums] Homework Statement:: Tentative Note and summary on the origin and the evolution of information in the universe. Relevant Equations:: none As a teacher of physics I got many questions asked by my students when...
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