Expanding Definition and 637 Threads

I know this question had often been asked on PF but I don't get them. What do mean when we say that space is expanding? I mean that space is no physical entity that can expand. Space is just (apparently) "nothingness". Please solve my confusion.

Sorry if this has been answered before, I am very new to this forum. So, if the universe is expanding, how is it possible that Andromeda galaxy, and our Milky Way will collide? Should they not be moving away from each other, given that the distance between them is big enough for space...

Universe is expanding which means the Earth is getting away from the sun. Without the sun, it is probably that all organisms on the Earth would die. Is there any calculation or estimation from scientists that how many years before the Earth is too far away from the sun so that we will all die

Finding a constant in an axpanding univarse Hi, I want to obtain a constant that proportional to total matter density of universe. Let ρ be matter densitiy and ρr be radiation desity. In an expanding universe volume times total density must be constant. It is wrong? let me show it...

A homogeneous and isotropic Universe is described by the FLRW metric: ds^2 = c^2dt^2 + a^2(t)\ d\Sigma^2, where ##a(t)## is the scale factor and ##d\Sigma## is an interval of uniformly curved co-ordinate 3-space which is independent of cosmic time ##t##. If we set ##dt=0## then we find that...

Eric J. Lerner, long time Big Bang opponent, has published a paper that I don't really understand but claiming it indicates the universe is not expanding. Can anyone actually help to point out why or why not his observations should be given consideration? It was Published in Int. Journal of...

I have trouble understanding the following. Lets take 2 galaxies moving away from each other caused by the expanding universe. A spaceship traveling between these 2 galaxies, more than one time, will experience a greater distance for each travel. Therefore my conclusion is, that there must...

For simplicity I assume a flat radial FRW metric (with c=1): ds^2 = -dt^2 + a^2(t)\ dr^2 Now let us consider the path of a light ray, a null geodesic, with ds=0 so that we have: dt = a(t)\ dr Now at the present time t_0 we define a(t_0)=1 so that we have: dt_0 = dr The interval of radial...

Einstein has a thought experiment with two trains which he uses to prove linear motion without acceleration is inertial. Inertial means there is no physical test which will prove which train is moving and which is stationary, no coordinate system is preferred and that coordinate system are...

In 1889 and 1909 Roberto Mantovani published a hypothesis of Earth expansion and continental drift. He assumed that a closed continent covered the entire surface of a smaller Earth. He believed thermal expansion led to volcanic activity, which broke the big continent into smaller continents. The...

Homework Statement Four moles of an ideal gas expands at constant temperature until its pressure is reduced to half of its initial value. What is the change in entropy of the gas? Homework Equations ΔS=Q/T (For constant T) pinitial=p pfinal=.5p Q=W W=pdv nrTln(vf/vi) The Attempt...

Homework Statement A gas initially occupying a volume V undergoes a sudden explosion, causing its volume to triple to 3V. Assuming that the pressure of the environment (atmospheric pressure po) remains constant, find an expression for the work done by the gas. Hint: Please notice that an...

... figure it out 1. if a typical supernova ejects 2 solar masses of matter at a root mean speed of 7000km/s, how much energy is available in the expanding remnant? 2. Homework Equations : unsure 3. The Attempt at a Solution : E = 1/2 mv^2 then divide by the volume...

Since the speed of light is a constant, no matter what location from which a search of the universe is made, matter would appear to be accelerating in the expansion in the same way to the observer. It is understood that about 14 billion years ago the expansion started. From any vantage point...

I've been a bit confused about when/why an expanding gas cools. Here is what I've heard so far: Refrigeration systems work based on using heat that is used merely to break attraction of molecules of liquids at their boiling point (eg, freon released through an expansion valve gets cooler...

Forgive me if this is stupid or has been covered many times in the past but I have a question about dark energy and the expansion of the universe. But first, consider a question posed by Poincare in the eighteenth century. He asked, “If we woke up one morning and everything in the universe had...

Every time I try to read Peskin & Schroeder I run into a brick wall on page 15 (section 2.2) when they quickly derive the Euler-Lagrange Equations in classical field theory. The relevant step is this: \frac{∂L}{∂(∂_{μ}\phi)} δ(∂_{μ}\phi) = -∂_{μ}( \frac{∂L}{∂(∂_{μ}\phi)}) δ(\phi) + ∂_{μ}...

Homework Statement Basically I have the following problem. Given the equation of state \sigma = \frac{b}{T}\frac{L - L_0}{L_0} where b and L_0 are positive constants, calculate the work done over a Carnot cycle by a wire with this equation of state.Homework EquationsI already have the work...

Suppose we have an event horizon H which is a light-like, closed 2-surface. Photons radiated outwards at the horizon along a light-like normal u of the horizon stay on the horizon (this is trivial b/c this statement is nothing else but the definition of the light-like 2-surface). Now suppose we...

Every time I try to read Peskin & Schroeder I run into a brick wall on page 15 (section 2.2) when they quickly derive the Euler-Lagrange Equations in classical field theory. The relevant step is this: \frac{∂L}{∂(∂_{μ}\phi)} δ(∂_{μ}\phi) = -∂_{μ}( \frac{∂L}{∂(∂_{μ}\phi)}) δ(\phi) + ∂_{μ}...

Considering that when we look at distant red-shifted structures of the universe we also look at their respective speed at that respective time of emission, is there any observational evidence that the acceleration is still happening today?

Today in an engineering thermodynamics lecture, the professor gave an example of a gas doing work. We had a cylinder full of helium at a pressure of something like 200kPa absolute and the valve was opened so that the gas would flow out against the atmospheric pressure until the pressures were...

We all know that photons are the particle representation of an EM 2-D wavefront and as the front expands the space between photons increases. We also know that particles of matter can exhibit wave properties. So in an analogous fashion to light, I'm thinking that each of the fundamental...

Hi, I joined this forum because I love learning new things about our universe and I often have questions like the one I'm about to ask. Also I would like to state that I haven't taken any physics classes so terms and advanced talk will be beyond me however, because I am so interested in...

This is a question for those with Shankar in their disposal. When studying from this book, Exercise 5.1.1 puzzled me. Now, it is easy to solve the exercise from 5.1.9. However, how would we expand psi in terms of the energy eigenfunctions,* if we did not have 5.1.9*, but instead started fresh...

Homework Statement Hi I have sometimes seen a function f being written as f \approx f_0 + \varepsilon f_1(x)+ \varepsilon^2 f_2(x) + \ldots where f_0 is an equlibrium value and all higher-order terms are non-equilibrium values (not derivates!). The assumption has always been that...

Space is expanding equally from all points Galaxies are moving away from each other contrary to gravity. However gravity overcomes expansion between nearby galaxies and within the galaxy and the nuclear forces overcome the expansion within matter. What is the formula to determine what the...

Homework Statement How would a circle on a sphere expand as a function of the sphere's radius as the sphere expands? Homework Equations none were provided The Attempt at a Solution \S=4\,\pi \,{R}^{2} A=\pi \,{r}^{2} {\it dA}=2\,\pi \,r{\it dr} {\it dS}=8\,\pi \,R{\it dR}...

Perhaps virtually 100% of physicists out there believe that we live in an 'accelerating universe', but as a person who's been researching the topic for a while, it seems to me the only evidence we have is based around redshift. I know redshift is a good enough argument to convince even...

The figures in http://www.astro.virginia.edu/class/whittle/astr553/Topic16/t16_light_cones.html are very useful in understanding the various world lines in concordant diagrams. Is there any easy way to see how a velocity cone (at the observer's worldline) from a later time than the Big Bang...

Hi I'm having some problems with an task in my class. It's as follows Given the real function f(x)=Xx for X ∈ (0,∞). Then I'm to expand the definition area to [0,∞), f shall still be continuous. What does f(0) have to be? Thanks for any help :)

If we are looking further and further back in time when we look at more and more distant galaxies, then how do we know that they are accelerating (instead of that they were accelerating a long long time ago)??

If space is expanding like balloon and it disappears wrinkles on surface than after sometimes surface will become smooth and expansion will stop and if we further try to expand the universe than surface will tear apart like balloon's surface.Is it true??

Say I need to find Maclaurin Series of ##\sin(\ln |1+x|)## until I reach the forth power. The expansion for ##\sin x## is just ##(x-\frac{x^3}{3!}+...)## For ##\ln |1+x|## it will be ##(x-\frac{x^2}{2}+\frac{x^3}{3}-\frac{x^4}{4}+...)## Then with substitution...

Homework Statement Two moles of an ideal gas are compressed in a cylinder at a constant temperature at 338K until the original pressure is tripled. Calculate the amount of work done. Homework Equations PV=nRT PV/T = c The Attempt at a Solution I don't know how to incorporate the...

Homework Statement Use (the bac-cab rule) to expand this triple product:L = mr x (ω x r) If r is perpendicular to ω, show that you obtain the elementary formula, angular momentum = mvr. (The bold letters are vectors.) Homework Equations A X (B X C) = (A\cdotC)B - (A\cdotB)C...

I am hoping that someone with broad knowledge of the physics literature can tell me when in the past tests were done on light coming from galaxies billions of light years away, tests that over time counted a decreasing number of photons from a single galaxy, or an increase in the wavelength of...

Homework Statement I can't figure this out...if you have a gas in a rigid container, with a membrane separating the gas from a vacuum in the other half of the container, then remove the membrane and allow the gas to expand, is any work done? It's not expanding against force - does that make...

What I think I know: 1. universe is expanding (from red shift observation?) 2. expansion is accelerating; farther galaxies move away at faster rate than nearer galaxies. 3. expansion rate will exceed speed of light 4. observable universe will be a lonely, dark place. Question: 1. how can...

Homework Statement Consider an isolated evacuated chamber. A small adiabatic valve is opened so that the air enters the chamber slowly from the atmosphere. What is the final temperature of the air inside the chamber once it has reached atmospheric pressure? Assume air is an ideal gas with...

Please refer to attached image. Hi, I'm a bit lost here with the first question. Unfortunately the online lecture covering this material isn't available due to their having been made some technical difficulties, and I find our textbook difficult to comprehend! My lecture notes are pretty...

17x10^5 joules are absorbed by an expanding gas that does 8x10^5 joules of work. (a) Find ΔU; (b) Assuming it expands at constant pressure, what's the change in volume? Am I correct to use this: ΔU = Q - W Then after finding ΔU, PΔV = Q - ΔU Or does this make no sense?

how to solve this without expanding the numerator. $\displaystyle \int\frac{(x-1)^3}{x^2}dx$

A hypothesis only by James Leckner Looking at space as a form of energy itself or as another way that energy is converted from one form to another. You would look at space as not the result of a large explosion (per say) but more like the conversion of one form of energy to another or even...

Homework Statement What's the doppler shift (Δf) of the object moving away from us if we measure a wavelength of light λ=1.55μm emitted from it and it's at a distance of 10 megaparsecs? And is the object red or blue shifted? Added information to the problem is for each megaparsec the object...

original equation: ln(0.2048x\frac{5}{2})= -\frac{5}{2} when i just raise this to e i get.. 0.2048T\frac{5}{2}= -\frac{5}{2} and then i can solve for x and get the right answer. but when i expand the logs.. ln(0.2048) + \frac{5}{2}ln(x)= -\frac{5}{2}...

I once asked a physicist to explain what the universe expands into, and he gave me the analogy of raisins in bread dough. That the raisins represents galaxies, galactic clusters, etc...and as the universe (bread dough) expands, the galaxies (raisins) move apart. But this analogy assumes that the...

Hey PF, I recently graduated with a B.A. in math. During my undergrad career I worked on several projects gathering data and developing computation models for them. Now that I have finished my undergrad track I wanted to jump into the workforce before thinking about other options like...

I have been looking around people are talking about the universe expanding. By geometry we are able to figure out that the universe is infinite. The question is, if the universe is infinite than how can be expanding?

Imagine an box with initial dimension x_0 and y_0 with a cavity in the centre with area R^2 . Let's say the box gets stretched in the y-direction but that it's area and the area of the cavity must stay constant. A=x_0y_0-R^2=x(t)y(t)-R^2 Differentiating with respect to time gives...