Expansion Definition and 1000 Threads

I tried substituting x=cos2theta but it was of no use.I thought many ways but i could not make 0/0 form.So please help.

In an isothermal process, for an expanding gas ##\Delta U_{sys}=0## and ##Q=-W## but then, How can we evaluate ##Q_{surr} ##? It should be ##Q_{surr}=-Q_{sys}##, but I don't know how to show it in equations. If I try to get the result through the principles: ##\Delta U_{sys}=-\Delta U...

What happens to the fabric of spacetime during the expansion of the universe? Does it stretch or expand? If it does not stretch or expand, does new spacetime form to "fill the gap" as such? Hypethotically speaking, I have two celestial objects separated by a gap 1 mile wide. Due to the...

I realize that my understanding of this matter and it's vast number of related concepts is rudimentary and incomplete. Kindly forgive my ignorance and try to explain your thoughts in layman's terms. Thank you for your time.

I'm not an expert in this matter, and at best only aware of some superficial facts and a layman's understanding of them. So please forgive me for any ignorant mistakes in my thoughts, and kindly point them out to me. Going by the Lambda-CDM model, the expansion of the Universe will eventually...

Is the likelihood (or unlikelihood) of a Big Rip changed by the new findings of faster universe expansion (https://www.sciencedaily.com/releases/2019/04/190425104128.htm). In https://www.physicsforums.com/threads/will-the-big-rip-basically-be-the-big-bang-for-the-next-universe.971039/, phinds...

For the reversible expansion of an ideal gas the heat flowing out of the surroundings and into the system is equal to the work done by the system. Since both system and surroundings have the same constant temperature the entropy increase of the system is equal to the entropy decrease of the...

While doing some calculations I came across some terms which are ##\frac{(v•\vec \nabla)v}{a(t)}## and ##\vec \nabla•[\rho(1+\delta)v]## where all quantities have spatial dependence other than "a" which has only time dependence , the first term here is canceled and the the second term is...

Hi! Obviously my understanding is at a pretty elementary level, but nevertheless I'm puzzled by the reaction of astronomers and astrophysicists to the discrepancy in the measurements of the expansion of the early universe based on the Cosmic Microwave Background, and the measurements of its...

Above is my attempt at a solution, however, this did not yield the correct answer. Any help is greatly appreciated!

Hi, I'm currently working through a tensor product example for a two qubit system. For the expression: $$ \rho_A = \sum_{J=0}^{1}\langle J | \Psi \rangle \langle \Psi | J \rangle $$ Which has been defined as from going to a global state to a local state. Here $$ |\Psi \rangle = |\Psi^+...

I found the first 4 terms of the series: ½-(1/16)x^2+(1/64)x^4-(7/1536)x^6. I cannot however simplify this to a sum. the 7 in the numerator of the last term of the above expansion is the sticking point.

The subject of the thread summarizes my question: Could the accelerating expansion be explained by a "spinning" universe?

If the galaxies we see are as they were a few million years ago, then how do we know that the universe is expanding since we don't know their current velocities? Hubble's graph shows a linearly proportional relationship between velocity and distance but the fact that the further a galaxy is, the...

Hello, I am working through Spivak for self study and sharpening my math skills. I have become stuck on an exercise. What I need to show is the following: $$ (a + b) \sum_{j = 0}^{n} \binom nj a^{n-j}b^{j} = \sum_{j = 0}^{n + 1} \binom{n+1}{j} a^{n-j + 1}b^{j} $$ My attempt, starting from...

Hi all, I'm having a problem understanding a step in an arxiv paper (https://arxiv.org/pdf/0808.3566.pdf) and would like a bit of help. In equation (29) the authors have $$R = \frac{\sigma}{\sqrt{\pi}} \int dk \ e^{-(k - k_0)^2 \sigma^2} \ \Big( \frac{ k - \kappa}{ k+ \kappa} \Big)^2$$ where...

I tried following: $$ \Delta l = \alpha l_0 \Delta T $$ $$ (\Delta l)^2 l_0 = \alpha l_0^2 \Delta T \Delta l $$ $$ \Delta A l_0 = \alpha A_0 \Delta T $$ $$ \Delta A = \frac{ \alpha A_0 \Delta T }{ l_0 } $$ If we remember that: $$ \Delta l = \alpha l_0 \Delta T $$ So we have $$ \Delta A = \frac{...

The universe will expand exponentially when dark energy completely dominates the energy density of the universe. To clarify, does that make the present expansion 'quasi-exponential'?

This is my first post and I need to preface my question by saying I have no physics background, so I'm genuinely asking for help in understanding. A thought occurred to me about the continuing expansion and acceleration of the universe and I'm asking for your help in understanding where my...

There are galaxies that are so far away that metric expansion causes them to have a co-moving recessional velocity that exceeds the speed of light. However, those galaxies are also so far away that the time it took the light to reach us was itself billions of years in the passage of its journey...

From the friedmann equation H2=8πGρ/3-k/a2, 1=ρ/(3H2/8πG)-k/a2H2 1=ρ/ρc-k/adot2 adot=+-√[k/(ρ/ρc-1)] It is therefore if expansion/contraction is taking place , Positively curved space will have ρ>ρc And negatively curved will have ρ<ρc Is it the case??

I read in Wikipedia that "For supernovae at redshift less than around 0.1, or light travel time less than 10 percent of the age of the universe, this gives a nearly linear distance–redshift relation due to Hubble's law. At larger distances, since the expansion rate of the universe has changed...

Homework Statement Taken from a Ring theory class: Let ##A## be the ring of decimal number:$$ A= \{ n10^k|n \in Z, k \in Z \}$$ Show that ##x \in A## if and only if it's decimal expansion is finite. Here we assume that ##x \in R##. Homework Equations I have no relevant equations really...

From friedmann equation And For a flat universe with k=0 and ρ=ρc ,da/dt becomes undefined and d2a/dt2 becomes 0 But for the present time we know that our universe is flat and expanding with a acceleration (q -ve) , Therefore is it here something I am missing??

Hi Folks, I've read that by the time mankind is ready for intergalactic travel, our nearest galactic neighbour will be too far to even travel to. But I've also read that Andromeda is on a collision course with the Milky Way. Which is correct? And if the universe is expanding and everything is...

https://phys.org/news/2019-01-galaxies-physics-cosmic-expansion.html can dark energy vary over the history of the universe?

Homework Statement [/B] Oceans cover 2/3 of the earth’s surface, with an average depth of 3.7 km. The average surface temperture is 17◦ C. Taking this temperature as representative of the entire ocean, and knowing that the coefficient of volume expansion for water at this temperature is β = 1.7...

I once read somewhere a long time ago (and so I am not sure if this is true, let me know if it isn't) that the universe doesn't have a center of expansion. I get it. Neither does a typical balloon. But a balloon has an axis about which it is symmetrically expanding. Obviously that is not the...

Homework Statement Im hoping that this experimental setup is common enough in 1st year physics that someone could at least point me in right direction because I may be misunderstanding how the arm and pointer connect together. I need to come up with an equation that relates how far the metal...

For ##m>T## we can write $$n\propto T^{3/2}\exp[{-(m-\mu)/T}].\tag{2}$$ For ##T>m## we can write ##n\propto T^{3}## where ##n## is number density of particles with mass m. We can derive this relationship by using ##n\propto a^{-3}## and we also know that ##a\propto T^{-1}##. Is there a...

Homework Statement A steel ring with a hole having area of 3.990 cm2 is to be placed on an aluminum rod with crosssectional area of 4.000 cm2. Both rod and ring are initially at a temperature of 35.0°C. At what common temperature can the steel ring be slipped onto one end of the aluminum rod...

Homework Statement Hi I am looking at the attached as part of the derivation and am stuck on how we go from 18 to 19 [/B]Homework Equations Above below The Attempt at a Solution [/B] I'm pretty stuck. Lambda is small and not sin so can't see why one would expand out sine in a Taylor...

Homework Statement My doubts are on c) Homework Equations $$< H > = \int \Psi^* \hat H \Psi dx = \frac{2}{a} \int_{0}^{a} sin (x\frac{\pi}{a}) \hat H sin (x\frac{\pi}{a}) dx$$ The Attempt at a Solution I understand that mathematically the following equation yields (which is the right...

If expansion is moving objects away from each other in space, does that imply an interface of some sort between space and mass, a connection, or friction? If so, how is it that mass can drift through space and not eventually come to a stop? If moving space exerts a force on mass, why wouldn't...

If considering the free expansion of an ideal gas adibatically such that the final volume is double of the initial volume. Since dU=0 for free expansion ,which implies Ti=Tf for ideal gas...(1) From adiabatic relation , TiViγ-1=TfVfγ-1, to satisfy (1) ,γ-1=0 and for ideal gas Cp-Cv=nR or...

I wanted to ask whether it might be feasible and advantageous to use an expansion medium other that water to go from liquid to vapor to turn a turbine to generate electicity in a reactor. Because of water's very high specific heat and heat of vaporiaztion, would it be possible find another...

Good day all! I have the following exercise One kilo-mole of air expands adiabatically from an initial state p1=1 bar, T1=340 K to a final state V2= 2V1. Compute the temperature of the final state, for the following cases: Joule expansion (gas free expansion with W12=0) My problem is with the...

As stated in the question--is there any evidence of a non zero rate of change of the acceleration of the expansion of the universe. What is the evidence for it and is it negative or positive? Also if there is what would it take to determine if there was a jounce?

Hi, this is just a question I've had for years and have not been able to figure out because of conflicting information. I have read that space is expanding and that it's speeding up, but also have heard that in about 5 billion years our galaxy and Andromeda will collide. How is this possible...

A friend of mine and I have been discussing how to apply the first law of thermodynamics to analyze the quasi static expansion of an ideal gas in a cylinder featuring a piston having both mass and friction (with the cylinder). We have identified two different systems that can be used in the...

The metric of a 3-D positively curved space is dr2+ Sk(r)2(dθ2+sin2θdΦ2). Now if this space expands with a scale factor a(t) from r to r'. Whether the change in the radial component be a(t)dr and angular component be Sk(r')dθ and Sk(r')sinθdΦ since the change due to expansion is already...

Hope nobody finds these couple of questions obnoxious. As the expansion of space-time carries apart the galaxies and the stars which make them up, does it also increase the separation of the planets within the planetary systems of those stars? Furthermore, does it also carry apart all the atoms...

Homework Statement Given a problem where air expands adiabatically with a given initial temperature, initial pressure, final pressure, and gamma (ratio of specific heats) , how would one go about solving for the final temperature? I'm just eager to get a general sense of direction. Homework...

Homework Statement Homework Equations ΔL=LαΔT σ=EΔL/L The Attempt at a Solution For part a, I used the coefficient of linear expansion for copper and the change in temperature to find the change in length (0.068%). I thought part b had the same answer. The two are attached, and I would...

I am writing a document on the Joule-Thomson effect. But in my research for it I have come across something that I am not completely clear on. In my document I explained the free expansion of an ideal gas and am trying to transition/segue into the Joule-Thomson effect. However now I am having...

From the outset let me assert that I am not putting into question the expansion of space. Redshifts and all that. But there is an example which one often reads in popular astronomy articles which appears at first glance to be faulty, although I suspect it is my reasoning that is faulty, and I...

So I know that we believe that most galaxies are moving with a high velocity away from us. We also know that the galaxies are accelerating away from us (not only that, but they are jerking away from us as well, as in the value of the acceleration away from us is increasing), and we use dark...

First of all, sorry for my naive question here which likely doesn't make sense. The universe is currently expanding with galaxies receding from each other at increasingly faster pace. Is it possible that the universe is a closed hypersphere system, where galaxies are actually not actively...

In plasma physics we have what is known as plasma dispersion function. There are two approximation under which this function can be expanded: when the argument is less than 1, we can use power series expansion and when the argument is greater than 1 we can have asymptotic expression. My...

Imagine that for some reason the current slow expansion of the universe is going to reverse, and the universe is going to collapse back to the pre-inflation state. [Let's make this an assumption without worrying about the mechnism] So we have a very dilute and cold distribution of energy, since...