Cosmology Definition and 917 Threads

I am an independent researcher with a deep passion for exploring the frontiers of theoretical physics, cosmology, computational modeling, AI, and advanced structural engineering. My work is driven by curiosity about the nature of reality and the hidden principles that govern it. I focus on...

https://en.wikipedia.org/wiki/Recombination_(cosmology) Was a matter density right after the decoupling low enough to consider the vacuum as the actual vacuum, and not the medium through which the light propagates with the speed lower than ##({\epsilon_0\mu_0})^{-1/2}##? I'm asking this in...

This is pretty theoretical, so I don't know whether it would better belong in the "other physics" section. As I understand it, a pair of pants situation of topology change where one universe splits in two is described by a global wick-rotated riemannian metric so as to avoid the causality...

So DESI 3 year study results were revealed and when combined with other data from other sources it revealed a 3-sigma significant results that dark energy evolves with time, more specifically that it has been decreasing in the last 11 billion years or so...

TL;DR Summary: Looking for cosmology books with manuals Any such book available? The only one that I could find is by James Rich. It's an okayish book but any other alternatives? Ryden is also one but it's not advanced/rigorous enough for me.

HI, everyone, I am Ram from Chennai, India. I have very recently started research on topics in Cosmology, Thermodynamics and Relativity. I have no formal credentials or academic affiliations. I am here to learn more about my areas of interest from experts and also to seek critiques of my work.

Although the speed of light is a 'cosmic speed limit', and is always constant, this seems to be a paradox. Firstly, the units we use to measure it relate to the Earth. How can we measure a metre or a second without the Earth as a reference point? And if we do, what meaning does this have for the...

I live in Ukraine, Kharkiv, and I attempt to forget about russian missiles by physics- cosmology in part- Interpretation of Hubbl 's law

I had been thinking the area I wanted to specialize in was theoretical astroparticle physics. After digging deeper into the exact roles of various sub-fields, I realized what I actually want to do lies in theoretical cosmology. Essentially, I want to study dark energy, or dark matter as a...

I know that this is speculative and that discussing theoretical models with no evidence supporting them is probably pointless, but I'd like to ask a question about Coleman's baby universe model to understand it better and clarify some questions I have about it: In this article...

Dear all, I have a rather basic question about an equation in David Tong's lecture notes on cosmology; see http://www.damtp.cam.ac.uk/user/tong/cosmo.html My question is about eqn. 1.19 (page 14), in which the cosmological redshift is derived. It's not about the physics, but about some basic...

With experts , like Roger Penrose now questioning the Big Bang Theory, I too am curious about it and other Cosmology issues.

Inviting comments as well

I would like to calculate the principal and Gaussian curvature of the spatial part of the Friedmann-Robertson-Walker (FRW) metric; specifically, the negative Gaussian curvature ##k=-1##. The FRW metric is, \begin{equation*} ds^2 = -dt^2 + R(t)^2 \left( \frac{dr^2}{1-k r^2} + r^2 d\Omega^2...

I would like to ask some questions about an interesting paper that was written back in the late 90's (https://arxiv.org/abs/astro-ph/9701131) 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...

I am a retired high school and college teacher doing independent study in physics. I am interested in cosmology and information mechanics, a unified theory that derives gravity, quantum mechanics and more from an elegant new conceptual foundation in which the information represented by physical...

I am working on a new theory of a potential alternative explanation for observed dark matter phenomena. Should be able to share soon.

Ever since i was a child I loved staring out at the night sky. I sat in the library going between comics and cosmology. Inventiveness was born from the two. Looking back, had my interest been supported i may well have turned out a physicist. Alas, there is always time. In the meanwhile, I’ll...

I have been an avid amateur cosmologist for 40-some-odd years. I was hooked reading "The Collapsing Universe" by Isaac Asimov. I've been diddling about with it ever since. I learned relativity by reading "Discovering Relativity for Yourself" by Sam Lilley. And I've been playing with it ever...

Hello! I am a creator who loves the fascinating world of science. I enjoy exploring science from a unique angle, which often leads to new and creative ideas. While some of these ideas can be speculative and challenging to prove, I make sure they align with scientific principles and logical...

This book was recently offered (and sold) in the Buy, Sell, Trade, Giveaway Book Marketplace section of the Forum. That inspired me to pull out my copy (1988 "Updated Edition" from Basic Books). I read this probably 30 years ago. My question is, how much of this story is still considered valid...

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)...

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.

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...

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...

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...

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...

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...

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.

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...

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...

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...

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...

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...

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...

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...

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?

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!

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.

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...

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...

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...

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...

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...

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.

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...

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...

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...

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...