Cosmology Definition and 941 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. phinds

    Completed Steven Weinberg The First Three Minutes

    Here's one that is very easy to find and that I definitely recommend. Steven Weinberg, The First Three Minutes. (paperback) Amazon $31.19, I'm asking $15 with free shipping at media rate ($10 plus shipping if shipping is less than $5)...
  2. M

    B Confused about expansion

    Considering the growing consensus that the universe is infinite, being everything everywhere, how is it possible for infinity to expand? In all my research I find confusion among cosmologists about the important distinction between the part of the universe we cqn see and the whole infinite universe.
  3. M

    Independent researcher

    Hi everyone. I've had an avid interest in several fields of physics all my adult life, now 79. (Long retired as a psychologist.) My fields of interest include cosmology, astrophysics, relativity theory, quantum physics and philosophy of science applied to physics. I look forward to participation...
  4. S

    A Singularity structure being valid for all QFTs?

    I'm trying to understand this paper (https://arxiv.org/abs/1709.02813) in which the authors try to build a wavefunction for the universe without assuming locality and unitarity, so they would be rather emergent from geometrical constructs called "polytopes" and not assumed from the start (they...
  5. S

    I Density fluctuations in a homogeneous linearly expanding universe?

    I would like to ask a question about an interesting paper [1] back from the late 90's There, the authors propose how the universe may evolve from the near future to extremely far time scales Near the end of it (Section VI, D.), they discuss entropy and heat death: They indicate that contrary...
  6. S

    I Could the emission of 2 gravitons in orbits possibly be supressed?

    As bodies orbit each other they emit gravitational waves (presumably by emitting gravitons). There could be configurations of several moons around a planet (for instance) where the quadrupole moment does not change and is invariant. However, there could still be higher order effects, namely...
  7. S

    I Can there be free electrons in interstellar gas?

    If I'm not mistaken, in interstellar gas, there can be clouds of free electrons (not "attached" to any atomic nucleus) But can they stay like that indefinetely? Or will they inevitably end up in atoms? And how are they holding in interstellar gas inside of galaxies? Are they gravitationally...
  8. T

    VERY amateur lover of Cosmology, Astrophysics, and Quantum Mechanics

    Loved Astronomy as a child but strictly personal amateur. My love has evolved to include Cosmology, Astrophysics, and Quantum Mechanics. Although I went up to and through first year of college basics Physics, Calculus, and two years of Chemistry, I have forgotten much of it.
  9. S

    I Could there be new particles after the Universe reaches heat death?

    Apparently, among the options for the fate of the universe, the universe reaching heat death in the extremely far future is the most likely one, after the last black holes would have evaporated (https://en.wikipedia.org/wiki/Heat_death_of_the_universe) But, even if objects in extremely long...
  10. S

    I Question on halos of matter made from massive and stable particles?

    Consider a halo made up from massive and stable particles like neutrinos* (let's not consider protons which, although we don't have any experimental evidence showing that they are unstable and decaying, there are some GUTs proposing theoretical mechanisms where they could decay over extremely...
  11. S

    I Can there be quantum fluctuations without spacetime?

    There is a paper called "On nothing" (https://arxiv.org/abs/1111.0301) which goes on to argue that the universe could not have arisen from a state without spacetime (as some proposals do using quantum fluctuations to explain how the universe was born without spacetime) However, there is a...
  12. S

    I Will all objects in the Universe transform into black holes through quantum tunneling?

    If I understood it correctly, at enormous timescales into the future, it is theoretically expected that eventually stable massive structures (like white/black dwarfs) will suffer quantum tunneling events that would make small pieces of them slowly turn into black holes that would rapidly decay...
  13. S

    I Four Questions on vacuum phase transitions in the Universe...

    I am interested in the topic of vacuum phase transitions in models of the universe. One popular instance of this is a vacuum decay from a metastable vacuum energy level to a "true" one (in which the vacuum would sit at the lowest possible energy level depending on the model) I have 4...
  14. di_a_tonic17

    A Trying to plot a graph of energy densities for radiation and PBHs

    So a bit of context, I've been trying to model a plot of energy density relation for radiation and Primordial BH, but it is not right. Would someone be able to help me out? This is what I have so far. I am assuming the PBHs decay into a Planck relic, and the decay is instantaneous. Does the...
  15. S

    I Could cosmic rays induce a vacuum decay in the future?

    I've been told that very energetic cosmic rays could cause a vacuum phase transition or vacuum decay (and even could cause a true vacuum level to go "uphill" to a false vacuum) due to their high energy levels. I've found some references supporting this claim [1], [2], [3], [4] But also one...
  16. P

    I Does expanding space cause cosmological redshift?

    The cosmological redshift is generally assumed to be due to space expansion. But if spacetime is locally flat Minkowskian then surely photon wavelength should not change?
  17. D

    I Online Christoffel Symbols Calculator

    I would love to hear from you if you have any suggestions, feedback, or criticism. The goal is to build better and more sophisticated software that would push the boundaries of research in astrophysics!
  18. K

    I Cosmology models and QFT

    I know that cosmology usually deals with general relativity but what about QFT, is it modified depending on the cosmological model? Or is it not something that change with the model used.
  19. S

    I Can there be structures made from neutrinos with angular momentum?

    Would it be possible to eventually have structures made from neutrinos somewhere in the universe, as it is indicated in this question (https://physics.stackexchange.com/questions/80390/are-neutrino-stars-theoretically-possible), like halos of neutrino gas surrounding the center of galaxies...
  20. K

    I Questions about paper "How closed is cosmology"

    In the paper, the authors argue that a closed system can exhibit features of an open system one of the being the non-conservation of energy, my questions how can a closed system have non-conservation of energy. (I know in GR, conservation of energy is subtle issue but it doest seem like the...
  21. ergospherical

    A Synchronous to Newtonian gauge

    In the context of cosmology, you can perturb around the FRW background, conventionally:$$g = a^2(\tau)[(1+2A)d\tau^2 - 2B_a dx^a d\tau -(\delta_{ab} + h_{ab}) dx^a dx^b]$$with ##a,b## being spatial indices only (1,2,3). You can do gauge transformations ##\tilde{x} = x + \xi## of the coordinates...
  22. S

    I Supressing black hole evaporation?

    A very massive charged black hole could reach a near-extremal state in the right conditions supressing the rate of emission of Hawking radiation (https://physics.stackexchange.com/questions/490524/evaporation-of-large-charged-black-holes) Meanwhile, the radiation emitted by a black hole can be...
  23. M

    A "The 7 Strangest Coincidences in the Laws of Nature" (S. Hossenfelder)

    I appreciate Sabine Hossenfelder's latest video, and thought I would assemble some references. There's a mix of particle physics and cosmology topics. In general, the coincidences mentioned are mainstream topics in cosmology, whereas the particle physics coincidences are not. 1) Proton/electron...
  24. MrRobotoToo

    I Latest Findings from DESI Collaboration

    Dr. Smethurst summarizes the latest results of the DESI collaboration. What I found especially interesting is that the current value of the Hubble constant that they obtained from baryon acoustic oscillations is in good agreement with the value obtained from Lamda-CDM using Planck data.
  25. S

    I Electrons keeping dynamical quantum fluctuations?

    I was thinking about this paper (https://arxiv.org/abs/1405.0298) where the authors argue that there wouldn't be dynamical quantum fluctuations in a De Sitter space as fluctuations would be static once all perturbative radiation escapes the horizon (in the case that the Universe has a finite...
  26. S

    I Questions about Hawking radiation and extremal black holes...?

    I'm studying if there is some way to avoid black hole evaporation, even if it requires a very special set up of conditions... Theoretically, extremal black holes (both for rotating Kerr and Reissner-Nordström ones) would avoid evaporation as they would not emit Hawking radiation. Since...
  27. H

    First order linearized Euler's equation for a small perturbation

    I'm trying to linearize (first order) the Euler's equation for a small perturbation ##\delta## Starting with ##mna (\frac{\partial}{\partial t} + \frac{\vec{v}}{a} \cdot \nabla ) \vec{u} = - \nabla P - mn \nabla \phi## (1) ##\vec{u} = aH\vec{x(t)} + \vec{v(x,t)}## Where a is the scale factor...
  28. S

    I Vacuum up-tunneling with high-energy events?

    I was reading these papers by Sean Carroll (https://arxiv.org/abs/1405.0298; https://arxiv.org/abs/1505.02780) in which, among other things, he argues against vacuum up-tunneling occurring in the universe. He only acknowledged that it would be possible in the first moments of the universe while...
  29. S

    I Is there an equivalent "redshift" for cosmic rays due to expansion?

    I've found this discussion (https://astronomy.stackexchange.com/questions/43525/is-there-an-equivalent-of-the-red-shift-effect-for-cosmic-rays) where it is said that there is an equivalent redshift of cosmic rays due to the cosmic expansion However, how can this be? Cosmic rays are not EM...
  30. S

    I Is Vacuum Decay from a False to True State Impossible or Just Unlikely?

    I found this paper (https://arxiv.org/pdf/hep-th/0211160.pdf) which argues against the possibility of a decay from a metastable vacuum to a true vacuum state. However, this is the first time I've read this. Is it then impossible that a vacuum decay from a false vacuum may occur (even in...
  31. S

    I Does Kerr's argument against singularities apply to all black holes?

    Roy Kerr has recently written a preprint (https://arxiv.org/abs/2312.00841) in which he strongly argues against the possible existence of singularities inside Black Holes. I've read that his arguments are really powerful and that he is most likely right. But, does it mean that Kerr has...
  32. S

    I Holographic principle in continuous spacetime?

    Can the holographic principle be applied to spacetimes and metrics that are (fundamentally) continuous/smooth? Or only to discrete ones?
  33. S

    I The Evolution of Galactic Scale Cosmic Voids

    According to some papers I've found [1], [2] expanding voids can be found inside clouds of denser materials that can cause them to eventually collapse. I have a question about this: Overdensities generally expand up to a given turnaround radius and then collapse. However, as the elements in the...
  34. S

    I Do the Sachs-Wolfe and Rees-Sciama Effects Influence Matter?

    CMB photons can be affected by the expansion of the universe through the linear integrated Sachs-Wolfe effect (ISW) [1] and the non-linear ISW effect or also called Rees-Sciama effect [1]. In particular, according to the ISW effect, the photons crossing superclusters would leave them having a...
  35. Tommolo

    Online advanced physics and cosmology courses

    Hi! I'm new here! Let me introduce myself! I am Thomas and I have always been interested in science, cosmology and physics. Anyway, I have an equally strong passion for arts and humanities, and after all I decided to form myself as a journalist, which is my daily job. After years and years, I...
  36. S

    I Voids, dark energy and tidal forces...?

    I would like to ask you some questions I have about some interesting work I was reading (https://arxiv.org/abs/1205.4238 & https://academic.oup.com/mnras/article/527/4/11962/7457744) where the authors analysed the effects of dark energy in the shape and evolution of voids Apparently, they...
  37. H

    Equation to give the lookback time as a function of redshift

    Hi, I'm currently reading the introduction to cosmology second edition by Barbara Ryden and at the page 105, the author says we get ##t_0 - t_e = H_0^{-1}[z - (1 + \frac{q_0}{2})z^2]## by inverting ##z = H_0(t_0 - t_e) + (1 + \frac{q_0}{2}H_0^2(t_0 - t_e)^2)##. However, I can't figure out how...
  38. P

    I On Quanta article on the recent tensions of cosmology

    Quanta Magazine recently published "Clashing Cosmic Numbers Challenge Our Best Theory of the Universe" I am no fan of Quanta and I am not a cosmologist but there is certainly the increasing buzz that something is wrong with ##\Lambda##-CDM and cosmological models thanks to the JWST. Is...
  39. M

    A What Is the Scale Length in SPARC Data Analysis?

    im reading through the sparc data and the accompanying scientific paper SPARC: MASS MODELS FOR 175 DISK GALAXIES WITH SPITZER PHOTOMETRY AND ACCURATE ROTATION CURVES right now defining terms is my biggest problem the reading through i come across the scale length' references are pg page, c...
  40. robotkid786

    What are the basics of atomic science and cosmology?

    New to the forum, hoping to study as much physics as I can understand. Atomic science interests me, and so does cosmology - but i'm still so new that I don't know enough maths for the latter, and havent got much knowledge on the former aside from gcse chemistry which was over 10 years ago
  41. Greg Bernhardt

    Astro/Cosmo Award

    Please select up to 3 members who were most impactful in the Astronomy and Cosmology forums in 2023. This is a popular vote. Polls were created by weighing activity and measure of helpfulness. Everyone nominated should feel honored. Many more could be added to this poll, we can never...
  42. S

    I More extended equilibrium configurations due to dark energy?

    Dark Energy puts a constrain on the size of overdensities (like clusters and superclusters of galaxies) and their growth. A higher Dark Energy density would reduce the radius of the zone where matter would be gravitationally bound, because more Dark Energy density would mean that objects would...
  43. F

    A Convention of units for densities in cosmology

    I have a table of densities of galaxies : Expected number density of galaxies for photometric survey per unit area and redshift intervals, ##\mathrm{d} N / \mathrm{d} \Omega \mathrm{d} z\left[\mathrm{sr}^{-1}\right]## and the corresponding density of galaxies per ##\operatorname{arcmin}^2## for...
  44. 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...
  45. 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...
  46. 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...
  47. 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...
  48. 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...
  49. 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...
  50. 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...
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