Cosmolgy Definition and 135 Threads

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  1. F

    A Matter density crude estimates

    Liddle (2015, p.67) writes: "From the crude estimates that a typical galaxy weighs about ##10^{11}M\odot## and that galaxies are typically about a megaparsec apart, we know that the Universe cannot be a long way from the critical density." Was this fact (i.e. that the actual density is likely...
  2. Mistenda

    Together, understanding the universe

    hobbyist, passionate studing of issues such as the nature of gravity, the LamdaCDM model, the dark matter, the dark energy, and the prevalence of ordinary matter over antimatter (baryogenesis)
  3. H

    Solutions for nonrelativistic-matter perturbations

    I'm studying the nonrelativistic-matter perturbations if the expansion of the Universe is driven by a combination of components. I'm currently Following this document (The growth of density perturbations) from Caltech. However, the author doesn't explain how he has found the solutions for the...
  4. chirag1

    A Numerical evolution of Einstein-Boltzmann equations in cosmology

    I'm trying to numerically evolve the Einstein-Boltzmann equations for cold dark matter perturbations using Runge-Kutta method of the fourth order. There are 5 standard equations: $$ \begin{align} \dot{\Theta}_{r,0}+k\Theta_{r,1}&=-\dot{\Phi} \\ \dot{\Theta}_{r,1}+\frac{k}{3}\Theta_{r,0} &...
  5. DanielcoderX

    Seeking insights on careers in theoretical physics and cosmology

    TL;DR Summary: Aspiring physicist seeking insights on job prospects, education paths, and salaries in theoretical physics and cosmology. Share your experiences and advice for informed decision-making. Thanks! Dear members of the forum, I'm currently in Grade 11, seeking insights on...
  6. S

    I Would there be any way to avoid gravitational wave emissions?

    In principle every object orbiting another (e.g. a planet revolving around a star) would emit gravitational waves, relaxing the orbit over time.However, this would not happen if the orbits had a time-invariant and symmetric quadrupole moment. As it is indicated in this question (), it appears...
  7. SherLOCKed

    I Maximizing S/N in Angular Power Spectrum Signals

    The signal-to-noise ratio for angular power spectrum signal Cl under theoretical noise Nl, where Cl and Nl are functions of multipole l, is given as (S/N)^2= \sum (2l+1) (Cl/Nl)^2To increase the S/N we bin the power spectrum signal, if bin width \Delta l, this in principle decreases Nl by a...
  8. A

    A Interpreting SDSS extragalactic data in the era of JWST

    • CERN talk : indico.cern.ch/event/1153372/contributions/5200955/ • Presentation materials : bit.ly/MAGIC23AMayer • CERN MAGIC23 : indico.cern.ch/event/1153372/ Talk Description (Abstract) We present empirical evidence from the Sloan Digital Sky Survey (SDSS), including...
  9. S

    I No Symmetry Spacetimes/Metrics in Theory of Relativity

    In the context of the Theory of Relativity are there any spacetimes or metrics with a complete absence of symmetries? I mean, consider a type of space or metric where no symmetries would hold (at least not exactly, but approximately). A space or metric where the Poincaré invariance (including...
  10. E

    I Research paper and sources investigating Earth's moons

    Hi. I am looking for scientific papers, articles and/or even simulations that investigate how many moons it would be possible for earth to have and maybe modern theories about what these moons can be and look like (i.e what characteristics they have) if they were to exist. I have googled and...
  11. 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...
  12. V

    I Uncertainty Principle in QFT & Early Universe Conditions

    I have a question related to the uncertainty principle in QFT and if it is related to the early universe conditions. Do we still have four-vector momentum and position uncertainty relation in relativistic quantum theory? I have been following the argument related to the early universe and the...
  13. R

    Addressing Misconceptions in Popular Science: A Call for Clear Communication

    I am not a Physicist. I am a retired Social Worker and Public Health Administrator who has taken an interest in Cosmology and Quantum Mechanics/Quantum Field Theory. I am reading as much popular literature in the field as I can as well as watching the excellent presentations on YouTube. I try...
  14. Onyx

    B Solve General Geodesics in FLRW Metric w/ Conformal Coordinates

    Once having converted the FLRW metric from comoving coordinates ##ds^2=-dt^2+a^2(t)(dr^2+r^2d\phi^2)## to "conformal" coordinates ##ds^2=a^2(n)(-dn^2+dr^2+r^2d\phi^2)##, is there a way to facilitate solving for general geodesics that would otherwise be difficult, such as cases with motion in...
  15. S

    I Methods for energy harvesting in expanding universe?

    There has been much discussion about how could we (theoretically) extract energy from the accelerated expansion of the universe. However, the only gedankenexperiment I can found is the "tethered galaxies" one (e.g. https://arxiv.org/abs/astro-ph/0104349). However, has somebody proposed an...
  16. S

    I Internal energy of a comoving volume increasing as space expands?

    I was reading an article by Edward Harrison, which tackles the problems of conservation of energy at cosmological scales. At some part (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...
  17. Safinaz

    A The scale of an extra dimension

    Any help to understand how the authors of this paper Fine Tuning Problem of the Cosmological Constant in a Generalized Randall-Sundrum Model calculted this size of the extra dimension Equ. (3.8) from the scale factor defined by Equ. (3.3) ? Specifically, this paragraph after Equ. (3.8) -...
  18. S

    I How many apparent horizons could the Universe have?

    I was reading a paper written by George Smoot [1], which 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 be one world (or universe)...
  19. svenz706

    A Non-minimally coupled inflation — expansion

    Hello, In the Wikipedia article on "Inflaton" there appears the following formula: ##S=\int d^{4}x \sqrt{-g}[ \frac{1}{2}m^2_{P}R-\frac{1}{2}\partial^\mu\Phi\partial_{ \mu }\Phi-V(\Phi)-\frac{ 1 }{ 2}\xi R \Phi^]## with ##\xi## representing the strength of the interaction between R and...
  20. Tommy White

    Mechanical engineer and scientific researcher.

    Hi, I'm Dutch, not any kind of Quark ;-) (so excuse me for any bad English) and I'm very curious about "how the universe works". Cosmology/astrophysics and modern physics (quantum, relativity). I find it all mighty intersting. Actually all physics and also how it's all developed and "evolved"...
  21. S

    I Varying laws of high energy physics in inflation?

    I am have some questions that have arisen while reading an old but interesting article by Andreas Albrecht [1] that cites an article by Andrei Linde in the 90s on cosmic inflation [2]...Albrecht's paper is related to his ideas on "clock ambiguity" in which he proposes that the laws of physics...
  22. F

    A Need help about a demo with inverse weighted variance average

    I have a problem of understanding in the following demo : In a cosmology context with 2 probes (spectroscopic and photometric), let notice ##a_{\ell m, s p}## the spectroscopic and ##a_{\ell m, p h}## the photometric coefficients of the decomposition in spherical harmonics of the distributions...
  23. 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...
  24. S

    A Branes with any number "n" of dimensions and laws of physics?

    I had a few questions about this paper by Nima Arkani-Hamed, Georgi Dvali and Savas Dimopoulos (https://arxiv.org/abs/hep-th/9907209) which is closely related to the concept of branes (https://en.wikipedia.org/wiki/Brane) 1. The authors of the paper mention that the branes in the model could...
  25. J

    I What is the acceleration of the expansion of the Universe?

    I can't find any values of acceleration or rate of change of acceleration of the expansion of the universe when I looked it up and I need these values for a theory I'm working on that could supersede dark energy and show the universe is closed even if everything accelorating away from us and...
  26. F

    A Cl's : sum into a chi^2 when we have a sum of chi^2

    1) If I take as definition of ##a_{lm}## following a normal distribution with mean equal to zero and ##C_\ell=\langle a_{lm}^2 \rangle=\text{Var}(a_{lm})##, and if I have a sum of ##\chi^2##, can I write the 2 lines below (We use ##\stackrel{d}{=}## to denote equality in distribution)...
  27. Safinaz

    A How to find the value of the baryon asymmetry in this paper

    I'm trying to understand from this paper https://pdg.lbl.gov/2020/reviews/rpp2020-rev-bbang-nucleosynthesis.pdf What is the value of the baryon to photon ratio ##\eta=n_b/n_\gamma## as named in figure 24.1, but I can't get from the figure or the paper how ##\eta## is of order ##10^{-10}## Any...
  28. ohwilleke

    I Is There Evidence to Suggest Our Universe is Anisotropic?

    The cosmological principle holds that at large enough scales, the universe is homogeneous and isotropic (i.e. symmetrical). But, there is meaningful evidence from astronomy observations of anisotropy at the largest observable scales in the universe, which a new preprint (discussed below) sets...
  29. Akshaya dhakal

    B What is the Universe expanding into?

    As we know universe is everything having every matter and energy. From the observation using Hubble's law it was said that the galaxies are moving away from the Earth which means the universe is expanding. If it is everything then what is it expanding into. Is there some outer edge that it can...
  30. Ebi Rogha

    A Block universe and entropy increase

    I wonder how physicists who support this theory explain the increase of entropy?
  31. S

    I Models and theories of laws of physics emerging from chaos?

    Some physicists (like John A Wheeler, Holger B Nielsen or Ilya Prigogine) have proposed that all the laws of physics (including the most fundamental ones) emerged from a primordial chaos (for example, in the case of Wheeler, he proposed that laws of physics emerged from an initial random and...
  32. S

    I Wheeler's Mutability principle and multiple universes?

    Physicist John A Wheeler proposed the "Principle of mutability" which said that it could be the case that the universe would eventually shrink in a "Big Crunch" and the be re-born in another Big Bang. He proposed that the laws of physics (even the considered most fundamental ones) would change...
  33. TheMercury79

    I Hubble relation to Scale Factor

    Imagine a Universe where the Hubble parameter is truly a constant, in both space and time. How much smaller would such a Universe be 14 billion years ago compared to today? Using the Hubble parameter in terms of scale factor: ##H(t) = \frac{\dot{a}}{a}## leads to the differential equation...
  34. S

    I Could fundamental laws change in Dirac's Large Numbers Hypothesis?

    Paul Dirac proposed a hypothesis called "Large Numbers Hypothesis" (https://en.wikipedia.org/wiki/Dirac_large_numbers_hypothesis), where he basically stated that, if he was correct, laws of physics would change with time. But what about fundamental laws and constants? (Not only 'effective'...
  35. S

    Did Paul Dirac believe in multiple universes?

    Prominent physicist Paul Dirac proposed a hypothesis that indicated that constants and laws of physics would evolve with time into different constants and laws of nature (https://en.wikipedia.org/wiki/Dirac_large_numbers_hypothesis) This hypothesis was used by Robert Dicke...
  36. S

    A Does Bohmian Mechanics yield the existence of a multiverse?

    David Deutsch, a theoretical physicist, talks about David Bohm in his book "the Fabric of Reality": "[w]orking out what Bohm’s invisible wave will do requires the same computations as working out what trillions of shadow photons will do. Some parts of the wave describe us, the observers...
  37. S

    A Would Wheeler's oscillatory universe model create different laws?

    According to physicist John Wheeler, the universe is part of a sequence of cycles of Big Bangs and Big Crunches, called cyclic or oscillatory universe model (https://en.m.wikipedia.org/wiki/Multiverse (Mentioned in "Classification schemes") According to this author, in each cycle, the laws of...
  38. S

    Does Lawrence Krauss believe in the anthropic principle?

    Summary: Does Lawrence Krauss believe in the anthropic principle? I have read a lot of texts both indicating that physicist Lawrence Krauss is against the anthropic principle. But I have read several texts indicating that Lawrence Krauss believes that this principle is true and has developed...
  39. S

    I Did Wheeler consider Carter's anthropic multiverse?

    Physicist John Wheeler wrote an article called "Genesis and Observership" where he said near the end: "We have reviewed the evidence out of the big bang tat the universe did come into being and the evidence that not only space but all the structures and laws of physics are mutable. In the...
  40. S

    I Does the Strong Anthropic Principle have any equivalent specific model?

    The anthropic principle is a philosophical consideration that observations of the universe must be compatible with the conscious and sapient life that observes it. There are two main types of anthropic principle: Weak Anthropic Principle (WAP) and Strong Anthropic Principle (SAP) The Strong...
  41. S

    I Are all Cellular Automata models of universes?

    Both Stephen Wolfram and nobel laureate Gerard 't Hooft think that the universe is a Cellular Automata. As far as I know, 't Hooft developped a series of frameworks to build different models of Cellular Automata and Wolfram also proposed a framework where network nodes could produce different...
  42. S

    A Is the Wheeler-DeWitt equation related to Wheeler's It from Bit?

    In this article: https://arxiv.org/pdf/hep-th/0211048.pdf by physicist Andrei Linde, it mentions Wheeler-DeWitt equation and he relates it to Wheeler's Participatory Universe model. Also, I found other sources that seem to establish a connection between Wheeler-DeWitt equation and Wheeler's It...
  43. J

    What is the Wavelength of Emitted Photon in a Cosmological Redshift Scenario?

    Homework Statement Consider a point in the intergalactic medium at some cosmic time ## t_{obs}##, the time of arrival of a photon of wavelength ##λ_{obs}## as seen by a hydrogen atom at that location. The source of this photon a comoving distance ##r## away emitted it at wavelength ##λ_{em}##...
  44. 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...
  45. S

    Can twistor theory be used to describe everything?

    I've been looking at another Penrose's theory/model called twistor theory (https://en.wikipedia.org/wiki/Twistor_theory) that seems very interesting. From what I've read it seems to me that literally everything can be represented by twistors (or, rather, variations of twist theory). The only...
  46. 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...
  47. M

    I What Are the Boundaries of a Flat and Saddle-Shaped Universe?

    Hi, I have become very interested in cosmology recently and I had a question about the possible shapes of the Universe. I understand the critical density plays a role in the shape. This is what I know (or think I know). At the critical density of energy and matter, the universe is flat. If it...
  48. L

    I Harnessing Energy from Black Holes: Possibility or Fantasy?

    Whether it be through Hawking radiation, miniature black holes, or even white holes, is it possible that one day energy could be harnessed from black holes and used on earth?
  49. B

    Gravitationally bound and Hubble time

    Homework Statement Estimate how long a galaxy in the Coma cluster would take to travel from one side of the cluster to the other. Assume that the galaxy moves with a constant speed equal to the cluster’s radial velocity dispersion. How does this compare with the Hubble time, t H ? What can you...
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