# What is Expanding: Definition and 635 Discussions

The expanding Earth or growing Earth hypothesis asserts that the position and relative movement of continents is at least partially due to the volume of Earth increasing. Conversely, geophysical global cooling was the hypothesis that various features could be explained by Earth contracting.
Although it was suggested historically, since the recognition of plate tectonics in the 1970s, scientific consensus has rejected any significant expansion or contraction of Earth.

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1. ### B Expansion of the Universe

Einstein's equations in general relativity indicate that the universe is expanding. Einstein himself believed that the universe should be stable so he introduced a correction to his equations which made for a static universe. He later admitted that was his biggest blunder. Hubble proved by...
2. ### I Mass in an expanding or static spherical distribution of matter

In a spherical distribution of matter - such as with clusters of galaxies - how to calculate how much mass there should be for it to not expand with the expanding universe - in other word, for it to be a bound, static system?
3. ### I Why didn't the Big Bang form a Black Hole?

(pop science) It seems like the mass in the universe at the point of origin was way higher and the size way smaller than required to form a black hole, so why didn't our universe just sit forever as a single black hole? From a figure for the mass of the universe you could calculate the event...
4. ### Heat exchanged in Expanding and cooling gas

For part(b), My solution is, ##\Delta E_{int} = Q - W = \frac{3}{2}(P_fV_f - P_iV_i)## ##Q = W + \frac{3}{2}(P_fV_f - P_iV_i)## ##Q = 4000 + \frac{3}{2}((1 \times 10^6)(6 \times 10^{-3}) - (3 \times 10^6)(2 \times 10^{-3})## ##Q = 4000 J## However, according to the solution b. ##−4000 J## Can...
5. ### Volume-pressure equation for an expanding ideal gas

There is no atmosphere pressure. My work : pA=k(x-x0) => pA=(k/A)(V-V0) But this should be false beccause I want to use W=∫PdV to find work done by the gas but my final anwer is wrong ... Please guide me where my mistake is if you have enough time. Thanks.
6. ### A Expression of Shot noise when expanding ##a_{\ell m}## coefficients

I would like to arrive at the following expression for the quantity ##o_{\ell}## ( with "DM" for Dark Matter ): ##o_{\ell}=b_{s p}^2 C_{\ell}^{D M}+B_{s p}## with Poisson noise ##B_{s p}=\frac{1}{\bar{n}}(\bar{n}## being the average number of galaxies observed). the index "sp" is for spectro...
7. ### I Can gravitational waves gain energy in an expanding FRW spacetime?

I was reading this paper (*Green's functions for gravitational waves in FRW spacetimes:* [https://arxiv.org/abs/gr-qc/9309025](https://arxiv.org/abs/gr-qc/9309025)) and I had a specific question about one statement in the paper that I would like to ask: At page 6, the author says that...
8. ### I Avoiding heat death in an accelerated expanding universe?

https://en.m.wikipedia.org/wiki/Heat_death_of_the_universe Is the heat death of the universe completely unavoidable in an universe with an accelerated expansion dominated by dark energy like ours? Or can there be any way to avoid it according to current knowledge, observations and experiments...
9. ### I Methods for energy harvesting in expanding universe?

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...
10. ### I Energy from quantum systems in an expanding universe?

I found a paper (https://arxiv.org/pdf/astro-ph/0411299.pdf) which talks about quantum systems emitting energy due to spacetime expansion. Is this true or only a hypothesis?
11. ### I Particle production in an expanding universe?

I was reading this interesting article [1] which talks about particle production in an expanding universe. Usually this process is proposed to have occurred in the early universe, when the expansion was in the inflationary phase and it was so powerful that matter was created in particle...
12. ### B Sign of Expansion Scalar in Expanding FLRW Universe

Considering the FLWR metric in cartesian coordinates: ##ds^2=-dt^2+a^2(t)(dx^2+dy^2+dz^2)## With ##a(t)=t##, the trace of the extrinsic curvature tensor is ##-3t##. But why is it negative if it's describing an expanding universe, not a contracting one?
13. ### I Matter Gaining Energy from Expanding Spacetime?

Sean Carroll has an article (https://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/) where he explains that matter can gain energy from spacetime expansion. At the end of the article, he says: In general relativity spacetime can give energy to matter, or absorb it from...

23. ### I Energy conservation in an expanding universe

The total amount of energy is still a conserved quantity, even in an expanding universe based on a positive and constant energy density, and even under the rapid exponential expansion during inflation, total amount of energy is conserved. For how this works, see this lecture by Alan Guth, the...
24. ### B Can you fall below the horizon on an expanding balloon?

This scenario came up (as a dream) in a sci-fi novel (Robert J. Sawyer's The Terminal Man). Two people of height h, initially within sight of each other, at d distance apart , on a huge balloon of radius r. As the balloon expands, they might or might not lose sight of each other over the...
25. ### Expanding explosion from an Asteroid

An asteroid of mass M explodes into a spherical homogenous cloud in free space. Due to energy received by the explosion, the cloud expands and the expansion is spherically symmetric. At an instant, when the radius of the cloud is R, all of its particles on the surface are observed receding...
26. ### B What is the Universe expanding into?

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...
27. ### B Exploring the Possibility of Expanding Atoms

Please understand that I am a professional car detailer, not an astrophysicist. Hence the question. If space is expanding, and atoms contain space. Would that mean that the atoms themselves would also be expanding? Making not only the universe expand but also everything contained in the universe...
28. ### Chemistry Expanding and Naming of Abbreviated Chemical Formulas

I noted that the number of carbons in the main chain is 7 (5 carcons and the outer two are attached to an outside 'CH3' grouping .. and the bonds are all single so we have a Heptane. We have the main chain diagram structure as follows .. -C-C-C-C-C-C-C- .. If we then number these carbons...
29. ### B 2D Model of the Universe as an expanding ball

Can a simplified 2D model of our universe be an expanding ball? Where the surface of the ball is the 2D universe time is the vector normal of the ball measured in imaginary number i. So light will move at 45 degree to any vector normal. The expanding ball gets bigger because time is causing it...
30. ### MHB Expanding Brackets: Math Help for 2nd Term Maths

Hi there, I am currently doing an Ext Math 1 subject and haven't really come across any issues when needing to expand brackets, however, have come across the below equation I can't quite figure out... Any help would be greatly appreciated! $$(p-q+r^2)(3-(p^2+q^2))$$ The double brackets as...
31. ### Potential energy increase of an expanding stellar object

How much energy is required to double the radius of a uniformly dense stellar object? Express the answer in terms of mass and the radius of the object.
32. ### B What happens to time as space is expanding?

I have one question I hope someone here can answer for me. Relativity theory tells us that space and time are sort of the same thing, as a spacetime. So when space is expanding, what happens to time? I find it hard to believe that time is somehow unaffected by the expansion of space, so while...
33. ### MHB Simplifying and expanding expressions

I have a test on simplifying and expanding expressions, could someone help me with this. I don't understand the formula and the way you do it
34. ### Hyperons, pions and an expanding universe

Again sorry, it is in Spanish
35. ### MHB Expanding and simplifying brackets

Hello, I have been trying to solve the top line equation to get the result (the bottom line). I am searching for a clue (the steps) on how to obtain those four brackets as a result.
36. ### Thermodynamics: work done on the atmosphere by an expanding gas

V = V(n,P) PV = nc dV = (-cn/P²)dP + (c/P)dn dW = (cn/P)dP - (c)dn This was how i tried to attack the problem, but it will end with log and probably it is wrong, any tips to fix it?
37. ### B Speed of Light & Expanding Universe: Does Hubble's Law Violate Rule?

Hubbles law states the rate of recession of galaxies increases proportionally with distance, and the cosmological horizon is where distant galaxies recede away at the speed of light. Does this not violate the rule of faster than light speed travel?
38. ### I How Does The Cosmic Microwave Background Show That The Universe is Expanding?

I've been searching for weeks and still with the doubt. I just know scientist look the content of the CMB and with general relativity calculates the expansion rate today that is 73 km/s/Mpc, but nowhere does it say how exactly. What does the contents of the universe have to do with the...
39. ### B If the Universe keeps expanding this way, it gets torn apart

The expansion of the universe is accelerating. So the big rip is the best bet for how it ends isn't it? This fellow seems to think so: I guess the heat death is not the most likely, unless new evidence shows otherwise.
40. ### B Expanding Universe: How Does It Conserve Energy?

I read somewhere that expanding universes create more energy as they expand, and I was thinking over time there would be a considerable amount of energy created due to this expansion. Even with a very small cosmological constant the energies created over time would probably dwarf anything that...
41. ### I Jeans Instability in an Expanding Universe

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...
42. ### Expanding Brackets with Partial Derivatives

Hi, I just need some (hopefully) quick calculus help. I have the following: ##(y\frac {\partial } {\partial z}(z\frac{\partial f} {\partial x}))## After it is expanded this is the solution: ##(yz\frac {\partial^2 f} {\partial z \partial x} + y\frac{\partial f} {\partial x} \frac{\partial z}...
43. ### I Does the expanding Universe follow Lorentz contraction?

As object separate with a receding velocity proportional to the distance, it would seem appropriate to think that objects and space itself, which are located at a distance sufficiently far away (and beyond) to were recession velocities are large enough that Lorentz length contraction effects...
44. ### B Dilating or expanding a closed ball in Riemannian geometry

Hello. If a closed ball is expanding in time would we say it's expanding or dilating in Riemannian geometry? better saying is I don't know which is which? and what is the function that explains the changes of coordinates of an arbitrary point on the sphere of the ball?
45. ### I Transformation matrix for an expanding space

Hello. I am confused with this matter that how should we write the transformation matrix for an expanding space. consider a spacetime that is expading with a constant rate of a. now normally we scale the coordinates for expansion which makes the transformation matrix like this: \begin{pmatrix}...
46. E

### Expanding a bracket of derivatives

I am wondering why the two methods below give different answers. If I multiply z through the second bracket I get $$(\frac{d}{dx} +x)(-\frac{dz}{dx} + xz)$$which, on expansion, yields -\frac{d}{dx}\frac{dz}{dx} -x\frac{dz}{dx} + \frac{d(xz)}{dx} + x^{2}z = -\frac{d^{2}z}{dx^{2}} + x^{2}z +...
47. ### I How do we know space is expanding?

I understand that there are many forums and topics written on the fact that space is expanding. But I've looked and looked, and couldn't find an answer to my specific question. How do we know space is expanding, rather than galaxies just regularly moving apart? How do we know galaxies are...
48. ### I Can Lagrangian mechanics be applied to motion in an expanding universe

Summary: Since L = T - V, and T equals the kinetic energy (KE) of a particle whose trajectory is to be calculated, how is KE defined since some of its motion will be due to the expanding universe? My understanding may well be wrong, but it is the following. if a particle is stationary at...
49. ### I Is this explanation of "space itself expanding" correct?

Hi, I always used to struggle with the concept of "space itself expanding" (didn't Einstein say that the aether did not exist?) until one day an idea struck me that seemed to explain it all, without any need for complicated math. After talking about it with others, I got some great feedback...
50. ### Expanding an action about a background field

If we expand the action ##S[\phi]## about ##\varphi(x)##, ## S[\phi] = S[\varphi] + \int d^d x \frac{\delta S[\varphi]}{\delta \varphi (x)} f(x) + \frac{1}{2!}\int d^dx_1 d^d x_2 \frac{\delta^2 S[\varphi]}{\delta \varphi(x_1) \delta \varphi(x_2)} f(x_1) f(x_2) + \mathcal{O}(f^3)## But...