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 Receding movement of a stopped object in an accelerating universe?

    Imagine we attach an imaginary cosmological scale rope to an object that is very far away from us. Before attaching the string, the object would be receding from us due to spacetime expansion. After attaching it, tension would form in the string and we would eventually stop the object. After...
  2. S

    I Cosmic strings increasing internal energy?

    I was reading an article by Edward Harrison, which tackles the problems of conservation of energy at cosmological scales. At some point (point 2.4) he cites several article, including one by Rees and Gott, which he says indicates that the internal energy of a comoving volume (e.g. a cosmic...
  3. S

    I Many Worlds as Many Histories?

    I was reading this paper from George Smoot (https://arxiv.org/abs/1003.5952) where he assumes the holographic principle as true and conjectures that our universe would be encoded on the "surface" of an apparent horizon as the weighted average of all possible histories. In that way, there would...
  4. F

    A Relation between a_{\ell m} noise and Poisson noise with C_{\ell}

    We assume two galaxy population, ##\mathrm{A}## and ##\mathrm{B}##; the corresponding maps have the following ##a_{\ell m}## : ## \begin{aligned} &a_{\ell m}^{A}=b_{A} a_{\ell m}^{M}+a_{\ell m}^{p A} \\ &a_{\ell m}^{B}=b_{B} a_{\ell m}^{M}+a_{\ell m}^{p B} \end{aligned} ## Here, ##b_{A}## and...
  5. SH2372 General Relativity - Lecture 9

    SH2372 General Relativity - Lecture 9

    0:00 Gravitational lensing 23:19 Cosmology 33:00 Robertson-Walker spacetime 38:20 FLRW universe 1:01:35 Big Bang
    • Orodruin
    • Media item
    • Big bang Cosmology Gravitational lensing
    • Comments: 0
    • Category: Relativity
  6. A

    Looking for an up-to-date cosmology textbook

    I know I should not study cosmology from books that are too dated. Can you recommend an up-to-date cosmology book? I am studying GR from Carroll's book. Do I need to study astrophysics first or other topics?
  7. Tertius

    I Co-Moving Coordinates & Lapse Function N(t) in ADM Decomposition

    In the ADM decomposition, like in the construction of the FRW metric, the coordinates are defined to be co-moving, so we know $$d\tau = dt$$ (i.e. the lapse function is normalized away) Starting from a five-dimensional embedded hyperboloid (as in carroll pg. 324) ## -u^2 + x^2 + y^2 + z^2 + w^2...
  8. Adwit

    From General Relativity to Dark Energy

    If we insert the values from (2.9), (2.10), (2.11) into (2.5) & (2.6) how can we get (2.13) & (2.14) ?? I need to see the calculations step by step.
  9. S

    I If the Universe had a different size, would physics change?

    I found an article by James Bjorken (https://arxiv.org/abs/hep-th/0210202) which argues that universes with different size would have different physics (like different Standard Model parameters). When applying this reasoning to our own universe, Is this pure conjecture? Or is there some truth...
  10. SherLOCKed

    I Will the upcoming experiment PICO measure kSZ temperature anisotropy?

    I know for sure PICO will be measuring polarization anisotropies with high fidelity. In addition, the PICO science paper shows that it will make full-sky Compton-y maps but the plots are mostly limited to l=1000. Will PICO be able to measure kSZ temperature anisotropy at l=3000?
  11. Arman777

    A Understanding distances in Cosmology

    I am trying to understand this graph but I am confused about the distance definitions. So there's an object located at a comoving distance ##r##. The proper distance of the object at ##z'## can be written as $$d(z{'}) = \frac{1}{1+z{'}}\int_0^{z{'}} \frac{dz{'}}{H(z{'})}$$ In this case...
  12. BiaCarvalho

    A Can Representing Ideas be Possible?

    but I could not develop, is it really possible to represent this way?
  13. M

    I Formula for redshift of photon decoupling

    I have come across an old formula from my notes and I have no reference for it but it is using truncated digits in its formula to calculate the redshift for decoupling. The formula is nearly as accurate as the observed data from Planck 2018. So I would like to figure out the derivation of those...
  14. M

    Math Math needed for researching Cosmology?

    Greetings everyone, I'm looking for ideas about what areas of math I should do to prepare for a PhD in Cosmology. (So far I have an undergrad math & stats degree). I personally love pure math and view it as a series of puzzles; but somehow cosmology sounds more "applied". Anyway there are so...
  15. M

    Preparing for a PhD in Cosmology: What Math Should I Focus On?

    Greetings everyone, It's lovely to be here and I decided to join because I thought I might be able to get ideas about what math I should do to prepare for a PhD in Cosmology. (All I have so far is an undergrad math & stats degree). I mean I love pure math and view it as a series of puzzles...
  16. K

    I What does the equation dΩ32 = dr2 + r2dΩ22 mean in terms of flat space?

    A 1-sphere exists in 2D space. It is a circle in flat space. A 2-sphere is a 1-sphere embedded in 3D space. Its surface is non flat and 2 dimensional. A 3-sphere is a 2-sphere embedded in 4D space. Its surface is non flat and 3 dimensional. What does this last sentence mean in...
  17. Soony143

    How Do Physics Forums Assist in Solving Complex Research Queries?

    It is very amazing help8ng and encouraging forum. I am a research student of MS PHYSICS at COMSATS UNIVERSITY ISLAMABAD. I hope this firum will be much helpful to help me finding some tedious numerical solutions of a research paper.
  18. Buzz Bloom

    I Observations fit poorly with the Standard Model of Cosmology

    Reference: https://scitechdaily.com/breaking-cosmology-too-many-disk-galaxies-a-significant-discrepancy-between-prediction-and-reality/ The following are quotes from the reference. 1. The Standard Model of Cosmology describes how the universe came into being according to the view of most...
  19. SherLOCKed

    How is the Power Spectrum of Matter Density Field Defined?

    The definition of power spectrum of matter density field is given by eq(1). I have also seen definitions of power spectra given by eq(2) . Does this mean (2\pi^3) has been absorbed in the correlation function? $$P_{xx}(k)=(2\pi^3)\delta(k-k^\prime)<x(k)x(k^\prime)>$$ .. (1)...
  20. robphy

    I Ta-Pei Cheng's "Relativity & Cosmology" Course on YouTube

    I was browsing YouTube (again) and stumbled upon this set of lectures by Ta-Pei Cheng. (I've seen the cover of the textbooks... but I'm not familiar with the author-lecturer or the details of the texts... but this looks interesting.) http://www.umsl.edu/~chengt/...
  21. katatosh

    Major in Physics or Physics/Astronomy for Grad School Cosmology?

    Hello all! I have been taking an academic break from school for certain mental health reasons that I won't waste your time talking about. Now, I am only a freshman in college. My school's physics department currently offers 2 majors: Physics and Physics/Astronomy. Obviously, I have lots of time...
  22. CyclicAvatar

    PhD in condensed matter theory or theoretical cosmology

    Hi all, I just graduated from my master's program in theoretical physics. I did 60% of the coursework in high energy physics and rest in condensed matter theory plus a few experimental physics courses. I did my master's thesis in what can be called as theoretical cosmology, studying particle...
  23. I

    Converting an expression of a particular k-mode to the spatial domain

    $$n_\vec{k} = \omega a^2(\vec{k})\tag{1}$$ One way is to write the inverse Fourier transforms of the terms above. So, eqn (1) becomes $$\int\mathrm{d}^3x\ n(\vec{x})e^{-i\vec{k}\cdot\vec{x}} = \omega \int\mathrm{d}^3x^\prime\ a(\vec{x^\prime})e^{-i\vec{k}\cdot\vec{x^\prime}}...
  24. gregatphysics

    I Do all geodesics begin and end at the big bang?

    So. It was late night, the limpid sky a near cloudless darkness, somewhat lightened by the waxing moon. I being somewhat stoned and looking at the stars and constellations as I sometimes do, and it came to seem to me, in my imagination, that In the spaces between the stars, I was observing (in...
  25. ergospherical

    Astrophysics Cosmology and Astrophysics through problems: Padmanabhan

    I’ve really been struggling with this book since I started it a couple of days ago. It’s essentially a problem book designed to teach you about astrophysics, structure formation, radiation, fluid dynamics, etc. through a curated set of questions. Some of the questions take a ridiculously long...
  26. F

    A Calculating the variance of integrated Poisson noise on a defined quantity

    It is in cosmology context but actually, but it is also a mathematics/statistical issue. From spherical harmonics with Legendre deccomposition, I have the following definition of the standard deviation of a ##C_\ell## noised with a Poisson Noise ##N_p## : ## \begin{equation}...
  27. E

    I Cosmology: Compton Drag Explained

    What is the Compton Drag in cosmology?
  28. MathematicalPhysicist

    A Holographic Cosmology lecture of Leonard Susskind

    In the following lectures: Prof. Susskind writes the terms ##\exp(3Ht_n)## and ##\exp(3ht_n)##. If I understand correctly he computes ##R(t)=R_0 \exp[\int^{t_n}H(t')dt']## where ##R(t)## is the scale factor of the Universe and ##H(t')## is Hubble's parameter. What I still don't understand is...
  29. F

    A Photometric Galaxy Clustering Error and Poisson Noise

    The error on photometric galaxy clustering under the form of covariance which is actually a standard deviation expression for a fixed multipole ##\ell## : ## \sigma_{C, i j}^{A B}(\ell)=\Delta C_{i j}^{A B}(\ell)=\sqrt{\frac{2}{(2 \ell+1) f_{\mathrm{sky}} \Delta \ell}}\left[C_{i j}^{A...
  30. M

    I So I wrote a book on cosmology....

    I have been studying cosmology, quantum field theory, and inflation all by myself, taking notes, solving the equations, and when I write down my work, it looks like a book. I think of self-publishing it; I believe it could be useful to others interested in the field. However, I am only a...
  31. 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!
  32. 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...
  33. 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...
  34. 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...
  35. 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...
  36. 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 ...
  37. 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?
  38. 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...
  39. 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]...
  40. 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...
  41. 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...
  42. 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...
  43. 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...
  44. 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...
  45. 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...
  46. 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.
  47. 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...
  48. 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...
  49. L

    How Do You Solve H(T) at 1 MeV in 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...
  50. 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...
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