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. TheHeraclitus

    I Can someone explain Quasi-steady-state cosmology to me?

    I know it says Universe is eternal, how does that work with gravity? If all the stars and galaxies had infinite time to attract each other why are they still apart? I guess this is a naïve question because no one talks about it but I can't figure it out. Thank you for answers!
  2. DracoMorpheus

    Seeking a nerd well-versed in Cosmology and Dungeons & Dragons

    Summary:: A Standard Model for Fantasy Role-Playing? Greetings fellow nerds. I am an amateur when it comes to cosmology. I know little in terms of cosmology beyond what I've seen on documentaries, of which I have watched dozens, if not hundreds. I've read Hyperspace by Michio Kaku and Black...
  3. M

    I ##\Lambda##CDM: Evolution of the population of galaxies

    The number of visible galaxies is to vanish in the long-term future with the ##\Lambda##CDM model (book and papers of L. Krauss et al). I am interested in a quantitative study on how the population of galaxies decreases; I could not find one so I do it myself: The scale factor...
  4. shahbaznihal

    A Galaxy statistics calculation in Saslaw's book

    I am trying to follow a calculation from the book of William C. Saslaw, The Distribution of the Galaxies: Gravitational Clustering in Cosmology. The calculation is shown on the pages following page 122 in chapter 14 where the author talks about the Correlation function. I am able to reproduce...
  5. F

    A Expression of the mean and variance of spectroscopic Shot Noise

    Hello, I would like to know the right expression for the expression of variance of Shot noise in spectroscopic probe. Sometimes, I saw ##\sigma_{SN,sp}^{2} = 1/n_{sp}## with ##n_{sp}## the average density of galaxies, whereas my tutor tells me that ##\sigma_{SN,sp}^{2} = 1/n_{sp}^{2}## , so I...
  6. F

    A Fourier transform and Cosmic variance - a few precisions

    I cite an original report of a colleague : 1) I can't manage to proove that the statistical error is formulated like : ##\dfrac{\sigma (P (k))}{P(k)} = \sqrt{\dfrac {2}{N_{k} -1}}_{\text{with}} N_{k} \approx 4\pi \left(\dfrac{k}{dk}\right)^{2}## and why it is considered like a relative error ...
  7. S

    Nobel laureates who may consider the multiverse hypothesis?

    Apart from Frank Wilczek, Kip Thorne, and perhaps Gerard 't Hooft, are there any more (living) Nobel laureates in physics who are open to the possibility that multiple universes exist?
  8. E

    Other Should I give up string theory due to its job prospects?

    I'm currently a fresh grad student in theoretical physics, and I'm still deciding to choose which research group to join. My current understanding (maybe I'm wrong) is the PhD theme pretty much determines the topic for future post-doc research so I kinda need to choose very carefully. I'm...
  9. T

    B A question about which Cosmology theories are mainstream and how?

    I was reading recently about string theory recently which postulates that there are 11 dimensions representing all possibilities for everything. This is an interesting theory, although very difficult to prove and therefore somewhat speculative in my opinion. [Ed.: personal speculation removed]...
  10. JD_PM

    Universe with matter and radiation | Cosmology

    a) For a flat universe ##(k=0)##, so ##(1)## simplifies to ##\dot a^2 = \frac{C_r}{a^2}##. The solution to this first order, separable ODE (given the I.C. ##a(0) = 0##) is \begin{equation*} a(t) = \left( 4 C_r\right)^{1/4} t^{1/2} \tag{2} \end{equation*} We switch to conformal time by...
  11. T

    Question on Cosmology Solution for Tired Light Hypothesis

    Good evening, I have a question on a cosmology problem I have solved from Barbara Ryden’s Introduction to Cosmology 2nd Edition. I believe I have answered the question correctly, resulting in the following linear redshift relation when using separation by variables and some algebra manipulation...
  12. F

    A Combining 2 probes in order to increase Figure of Merit

    This post is slightly different from a previous post sent to mathematical forum : this is because I talk about here the MATLAB function "eig" with 2 arguments but this concerns actually the combination between 2 biased tracers in Cosmology context. I am looking for a common basis of...
  13. S

    I Are there other renowned physicists who believe in evolving laws of physics?

    The Canadian physicist, Lee Smolin proposed in the 90's that laws of nature may evolve and change over time. He begun proposing that only the constants of nature would evolve, while the most fundamental laws would remain the same, but recently he has change a little bit his ideas and has...
  14. S

    I Brane cosmology without string theory?

    There are several models of brane cosmology (https://en.wikipedia.org/wiki/Brane_cosmology) and several physicists working in this field (e.g Lisa Randall and Raman Sundrum), but as you will notice, apparently they are all directly related to string theory. This has several consequences, for...
  15. lomidrevo

    I Are masers orbiting black holes a reliable standard ruler in cosmology?

    One of the distance measures in cosmology is angular diameter distance, that can be used to determine a distance to objects whose actual (spatial) size is known, i.e. standard rulers. Beside baryonic acoustic oscillations, do we know other objects (or maybe I should better say structures) that...
  16. sahilmm15

    Astronomy Books on astronomy and cosmology at the beginner level

    I am curious about space. How it works, what does it contain, all different sorts of things. Any suggestions on what books to read for a beginner so that this curiosity do not fade away with time.
  17. I

    I What does Causality really mean?

    In the wikipedia article of the observable universe I have read the following: "Both popular and professional research articles in cosmology often use the term "universe" to mean "observable universe". This can be justified on the grounds that we can never know anything by direct...
  18. S

    I Wheeler's Pre-geometry model and multiple universes?

    John A Wheeler proposed that the universe emerged from a primordial chaos (some sort of pre-geometry or pre-space) from where all physics emerged in the universe. I thought this could be interesting in the context of multiple universes. Therefore, has any well renowned physicist considered...
  19. L

    Solve H(T) at 1 MeV: Cosmology

    Here's the problem: It is more common to define the “effective number of relativistic degrees of freedom” by writing the total radiation energy as ρR = ργ + ρν + ρe± = (π^2/30) g∗*T^4 , where g∗ = 2 + 7/8(6 + 4) = 43/4 . (1.52) With this, the expansion rate during the radiation era is given by...
  20. S

    I Is there any inflationary model without a fundamental theory?

    Cosmological inflationary models are general models in the sense that they could be applied to a variety of fundamental theories. Most physicists working in inflation assume that there is only one (but yet unknown) fundamental theory which through inflation would produce multiple regions or...
  21. E

    Redshift & Total Energy Density

    1. If today vacuum and matter contribute 71 % and 29 % to the total energy density of the universe, at what redshift z were they contributing equally? 2. If today vacuum, matter, and radiation contribute 71 %, 29 %, and 0.01% to the total energy density of the universe, at what redshift z were...
  22. AHSAN MUJTABA

    Cosmology: Hubble's law and Redshift

    I tried to attempt this by taking z=dv/c but I saw that relation Liddle's book on Cosmology and I really don't know where this came from. Secondly, I will need some series ( maybe Bionomial) in terms of z from which I can show small z. I think it's simple but here it's not clicking to me.
  23. AHSAN MUJTABA

    I Cosmology Comoving coordinates and observers

    I just want to visualize the math, any help would be appreciated. TIY
  24. haushofer

    A BGV-theorem and Penrose's Conformal Cyclic Cosmology

    Dear all, Some time ago I stumbled upon the famous BGV-theorem, - https://en.wikipedia.org/wiki/Borde–Guth–Vilenkin_theorem - https://arxiv.org/abs/gr-qc/0110012 which states that on spacetimes which have, on average, a positive Hubble constant, one can find timelike geodesics which cannot be...
  25. S

    A High energy symmetry breaking and laws of physics?

    In some models of the beginning of the universe, like for example in chaotic inflation, space would stop expanding in some points, creating Hubble volumes that could experience different spontaneous symmetry breaking, which would result in different properties, such as different physical...
  26. andrew0310

    Physics What is it like to do theoretical physics research?

    Hello everybody, I am an undergraduate student studying computer science and I love it, especially Artificial Intelligence. I'm a curious person, so there are a lot of things that I find intriguing. But I sometimes wonder if a physics career would be more interesting, not because I don't like...
  27. S

    I Bootstrap model and different laws?

    Physicist Geoffrey Chew proposed the concept of bootstrap (related to S-matrix theory) where he denied that fundamental laws of nature existed at all, as it is indicated in a writing in his memory written by one of his collaborators ([https://www.fritjofcapra.net/in-memoriam-geoffrey-chew/])...
  28. Buzz Bloom

    I Contradictory Cosmological Values: Crisis for Cosmology?

    These two independent sources have cosmological values that seriously contradict each other. The article "Planck evidence for a closed Universe and a possible crisis for cosmology" https://www.nature.com/articles/s41550-019-0906-9 arXiv:1911.02087v1 [astro-ph.CO] 5 Nov 2019 gives as the...
  29. S

    Binnig's fractal evolution applied to multiple universes?

    Gerd Binnig, Nobel laureate in physics in 1986, proposed in his article "The fractal structure of evolution" [1] that everything in the universe, including its laws, had changed and became what we have got today through a process which mixes some concepts from darwinian evolution and fractal...
  30. ohwilleke

    I Exploring ΛCDM Model: Cosmology Intertwined in 4 Pre-Prints

    In a series of four pre-prints a group of cosmology researchers has laid out an agenda for the next decade or so of research that is worth considering in connection with a Letter of Interest for the Snowmass 2021 conference (which may or may not end up being conducted in person in 2021) (the...
  31. thegroundhog

    Unanswered Questions in Popular Science: Exploring the Gaps in Quantum Physics

    I read a lot of popular science books, especially on quantum physics. My favourite authors are Brian Cox, Sean Carroll, Jim Al-Khalili, Marcus du Sautoy and Brian Greene. I have joined this forum as the more I read the more questions I have that are not covered in detail by the above writers...
  32. SDSS releases largest 3D map of the universe ever created

    SDSS releases largest 3D map of the universe ever created

    Scientists from the Sloan Digital Sky Survey (SDSS) have released a comprehensive analysis of the largest three-dimensional map of the universe ever created....
    • fresh_42
    • Media item
    • cosmology
    • Comments: 2
    • Category: Astronomy and Cosmology
  33. Arman777

    I Exploring Negative Energy Density and its Impact on Universe Models

    There are some universe models where ##\Lambda < 0##. In this case, the energy density of the dark-energy becomes negative. At this point, does it make sense to talk about "negative dark energy density"? Or is it possible to think of this energy as curvature on space-time? Such that, ##\Lambda <...
  34. Arman777

    I Meaning of the inertial energy density

    I am reading an article, which talks about graduated dark energy (gDE) model. In this model, it's assumed that the inertial mass density exhibits power-law dependence to its energy density $$\rho_{inert} = \gamma\rho_0(\frac{\rho}{\rho_0})^{\lambda}$$ Where ##\gamma## and ##\lambda## are real...
  35. A

    What are the Top Topics in Biology, Astrophysics, and Cosmology?

    hi! I'm not completely sure what I'm doing but my neighbor sent me this website so i can learn more about what I'm interested in. mostly, I'm interesting in learning about biology, astrophysics/cosmology, and the like. I'm super excited :p
  36. U

    I If the Universe is closed, is it thereby self-contained as well?

    https://www.google.com/amp/s/phys.org/news/2019-11-universe-rethink-cosmos.ampWhat do the results of the closed universe study tell us in terms of past cosmic sequences, if it is indeed the proper description of the universe? Would it entail a self-contained universe? By self-contained I mean...
  37. Mary Conrads Sanburn

    I Einstein's View Of Cosmology

    Einstein's conversion from a static to an expanding universe Albert Einstein accepted the modern cosmological view that the universe is expanding long after his contemporaries SPRINGER Until 1931, physicist Albert Einstein believed that the universe was static. An urban legend attributes this...
  38. Arman777

    A N-body simulation with varying neutrino mass

    I am trying to find a picture of the N-body simulations that shows the LSS. Particularly I am looking for different neutrino masses without the CDM. For instance pictures likes this But with more varying/different neutrino masses. I am looking for articles
  39. W

    I Conservation of energy in cosmology

    Models like Vilenkin's tunnelling from nothing model described here: https://www.sciencedirect.com/science/article/abs/pii/0370269382908668 claim the universe came from "nothing". It is claimed this doesn't violate any conservation laws because the negative energy of gravity and the positive...
  40. Arman777

    I Curvature of Space in the Context of Cosmology

    Recently I asked a question about the curvature of the universe. https://www.physicsforums.com/threads/constant-curvature-and-about-its-meaning.977841/ In that context I want to ask something else. Is this curvature (##\kappa##) different than the Gaussian Curvature ? Like it seems that we...
  41. RUTA

    Insights Dark Energy Part 2: LCDM Cosmology

    Continue reading...
  42. I

    I Big Bang singularity: can we say anything about its size?

    The Big Bang is sometimes described as started from a singularity, which I have interpreted as meaning that its characteristics are undefineḍ. If that is the case, can we not even say that the Universe as a singularity was still infinite? Otherwise we are implying that the singularity was...
  43. K

    I Are Basics of Cosmology Difficult to Grasp?

    Is Cosmology in a basic level more difficult to understand than the other areas of General Relativity? By "basic level" I mean, in the way the subjects are presented for example, in Weinberg's textbook: Gravitation and Cosmology, Principals and Applications of General Relativity .
  44. RUTA

    Insights Dark Energy Part 1: Einstein-deSitter Cosmology

    Continue reading...
  45. Arman777

    I Comoving Reference Frame in Cosmology

    In cosmology, the preferred reference frame is the comoving rest frame. I am trying to understand why we are using this frame and not another one. The only reason I could come with is this. Since the expansion of the universe is homogeneous and isotropic it seems our best choice is using the...
  46. W

    I Understanding inflationary cosmology

    There is a prediction I have often hear about regarding inflationary cosmology but I am having trouble grasping what it really means. I am wondering if anyone could give a layman explanation fo the following : https://arxiv.org/pdf/1312.7619.pdf "Modes of a given (comoving) wavelength should...
  47. Arman777

    I Cosmology: Inertial Reference Frames & Misconceptions

    I was reading an article about the misconceptions in cosmological horizons and I wanted to clarify an idea. https://arxiv.org/abs/astro-ph/0310808 In cosmological perspective, is there a global inertial reference frame (maybe CMB) ? Or all inertial frames are defined locally and there is no...
  48. FourEyedRaven

    Cosmology Exploring Cosmology through Challenging Problems: A Graduate-Level Workbook

    Hi. Are there good cosmology problem books at the graduate level?
  49. J

    I Is the Brans-Dicke Vacuum Cosmology Theory Still Relevant?

    Has anyone heard of papers describing Brans-Dicke vacuum cosmology solutions? I'd be interested to know if the following calculations make sense. I assumed a constant mass density vacuum with equation of state ##p=-\rho##. I then plugged this equation of state into the Brans-Dicke equations...
  50. P

    Schools How would you rate UC Davis' and U.H. at Manoa's Cosmology programs?

    Not when compared to each other but compared to other programs in the country... Also, I'm looking for a comparison in terms of future career prospects in Astronomy and quality of PhDs produced... Thanks!
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