Cosmology Definition and 951 Threads

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...

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...

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...

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...

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.

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

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...

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...

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...

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/])...

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...

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...

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...

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...

Scientists from the Sloan Digital Sky Survey (SDSS) have released a comprehensive analysis of the largest three-dimensional map of the universe ever created....

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 <...

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...

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

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...

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...

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

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...

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...

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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...

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 .

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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...

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...

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...

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

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...

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!

I am a physics graduate student, also a physics enthusiast.I prefer Mathematical Physics, and often do some related research.Like everyone of you, I also have a strong interest in physics, and I hope to generate more new ideas and broaden my knowledge through exchanges with you.

I am trying to derive the equation for a case, where we have a dust(zero-pressure) in an expanding universe. There are 4 equations but I think exercising on one of them would be helpful for me. I am trying to derive the equation for a case, where we have a dust(zero-pressure) in an expanding...

Hello, in the context of Forecasts with Fisher's formalism, I make vary cosmological parameters to compute the elements of the Fisher matrix. First, I generate with CAMB code a linear power spectrum. Then, from this, I am computing ##\sigma_{8,\text{linear}}##. Secondly, Before relaunching...

I got a phrase from a book that Stephen Hawking and his daughter Lucy Hawking published in 2011. The book is "George and the Big Bang" which is a science fiction book prepared for children. I read a phrase on that book that interested me. It was: "Perhaps there are many universes, each with...

Does someone know of an official or unofficial errata for the above book of Weinberg?

Listen to this if you have a spare hour, "Cosmology in Crisis". Podcast by LSST project scientists, discussion from Steward Observatory, Univesity of Arizona, Tuscon. https://arizona.hosted.panopto.com/Panopto/Pages/Viewer.aspx?id=bb691636-d47c-4544-84ba-ab4901537be2

As I understand it there have been different attempts to use LQG to make a theory of cosmology. The first one being done by Martin Bojowald and then later one being performed by Ashtekar, Pawloski and Singh. there is a description of what they did that was different but as a non cosmologist I...

According to Nobel laureate Frank Wilczek, the universe emerges from a Grid. This was proposed in his book "The Lightness of Being: Mass, Ether, and the Unification of Forces". He also likes the idea that the universe emerged from a state of "nothingness" (or rather, a quantum vacuum) where...

I have a rudimentary understanding of classical physics and some (lesser) level of relativity and quantum mechanics. All came from undergrad studies before most of you were born. I tried to jump into the quantum world, only to find it necessary to relearn the basics of maxwells equations and...

It has been proposed the Hubble tension can be solved if we assume our galaxy is located in a giant void (such as KBC). I am confused at this point. If we were living in a giant void, we should have measured the Hubble constant lower. Since when the light passes an underdense region it gets...

Let us say that we have a stellar object so its total velocity is defined as $$ v_{tot} = v_{pec} + V_{rec}$$ Where $$V_{rec} = H_0r$$ and $$V(z) = \frac{cz}{1+z}[1+\frac{1}{2}(1-q_0)z - \frac{1}{6}(1-q_0-3q_0^2+j_0)z^2]$$ for small z.So my first question is what is the $z$ value here? Is...

I have heard that one of the of the founders of LQC, Martin Bojowald is now rather critical of it. And sample of his criticism can be found here: https://arxiv.org/pdf/1810.00238.pdf I would be really grateful if someone could explain this in more laymen language . Any thoughts on the issues...

I would be interested in what people think of Fulvio Melia's new cosmological paper in which he argues that the comoving frame is locally inertial only if we have a linearly expanding Universe (or Minkowski spacetime)...

$$1 - \Omega_{tot} = \Omega_κ = \frac{-κc^2}{R_0^2H_0^2} $$ For ##\Omega_κ=-0.0438## we get a some value for ##R_0##. This ##R_0## is the radius of the observable universe right ? Not the universe ?

Summary: Roadmap of most useful math to learn to better understand cosmology I just watched ~20 hours of a serie of videos about cosmology, which stayed at a conceptual level for easier understanding (= minimal math). I understood about 98% of it and now have a decent understanding of...

I am interested in knowing how to calculate the gravitational wave (GW) spectrum with inflation as a source, I have some background in inflation but I am not so familiar about calculating the GW spectrum. I am reading a paper (https://arxiv.org/abs/0804.3249) about it, however, a big part of it...

Michio Kaku and Lawrence Krauss are both well-renowned physicists who propose that the universe (or universes) was generated out of nothing. Krauss, in his book "A Universe from Nothing" argued that the universe was probably created by a primordial "nothingness" with no space and time and...