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...
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)
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...
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} &...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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)
-...
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)...
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...
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"...
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...
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...
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...
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...
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...
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)...
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...
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...
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...
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...
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...
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...
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'...
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...
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...
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...
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...
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...
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...
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...
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...
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}##...
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...
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...
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...
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...
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?
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...