# What is Expansion: Definition and 1000 Discussions

Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions.Temperature is a monotonic function of the average molecular kinetic energy of a substance. When a substance is heated, molecules begin to vibrate and move more, usually creating more distance between themselves. Substances which contract with increasing temperature are unusual, and only occur within limited temperature ranges (see examples below). The relative expansion (also called strain) divided by the change in temperature is called the material's coefficient of linear thermal expansion and generally varies with temperature. As energy in particles increases, they start moving faster and faster weakening the intermolecular forces between them, therefore expanding the substance.

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1. ### B Pressure on creation of the Universe

I had a question today as a part of my studies which was 1) If space is ever expanding what is the pressure on the creating space (universe) upon creation and 2) at what rate might you assume the quantum field is also being created and is it being affected by this pressure of creation. This is...
2. ### I Can the expansion of the Universe reduce the energy of a photon to 0?

Is it theoretically possible for the expansion of the universe to red-shift the energy of a photon all the way to zero? If so, what happens to the photon? Or does the photon's energy only approach zero as an asymptote?
3. ### I Questions about the accelerating Hubble expansion

I don't understand the discovery of "accelerating expansion" in 1998 (hence the postulate of dark energy, etc..), because Hubble's old law already described exponentially accelerating expansion in 1929, right?
4. ### I Inflationary vs deflationary phase of Big Bang

Hi I'm a medical doctor, this is just a hobby my uncle got me interested in as a kid. If the universe is acceleration and galaxies are moving further apart, we assume the universe as a whole is expanding as a result. It's also projected to my understanding that eventually the universe will...
5. ### Stress in cylinders due to thermal expansion

I have two concentric thick cylinders in close tolerance (currently 50 micron gap), with a thin layer of glue between them. Internal cylinder is made from steel and external cylinder is made from ceramic (so the thermal expansion coefficient is not the same) The assembly is headed form inside...
6. ### Find the sum of the coefficients in the expansion ##(1+x)^n##

##(1+x)^n=1+C_1x+C_2x^2+C_3x^3...+C_nx^n## Let ##x=1##, hence ##2^n=1+C_1+C_2+C_3...+C_n## which is equal to the sum of the coefficients. I originally thought the sum of the coefficients would be ##2^n-1## since the very first term ##1## is just a number and has no variable. But apparently...
7. ### I Relationship Between Expansion and Curvature in the Universe

Orodruin said in another thread that "the spacetime of a spatially flat universe generally has curvature". Is the expansion (or contraction) of a spatially flat universe the manifestation of the curvature of its spacetime? If so, does the expansion correspond to a positive or negative curvature?
8. ### I Spacetime expansion - time dimension expansion

I am trying to find some sources about time dimension expansion during spacetime expansion. Why do We assume that only spacial dimensions expand? I only found a lot of weird arguments. Btw, temporal dimension expansion does not mean that second was shorter in the past. Similarly to spacial...
9. ### B Where does the light energy lost due to expansion go?

Sorry for my questions as an amateur interested in physics: If light changes its frequency during a long time of travel in space (vacuum?) for all observers (redshift) and therefore its energy decreases (E=hf), what "entity" absorbs this energy? I suppose the answer will be that the wavelength...
10. ### B Can the "creation" of space in the Universe affect the speed of light?

Can the expansion/creation of space in the universe affect the speed of light over long distances? (dividing the space traveled (including expanded space) by the travel time). If the Hubble constant is valid throughout the universe, it follows that from a certain Megaparsec of distance, galaxies...
11. ### A Gravitational Instability and accelerated explanation of Universe?

Ref: Principles of Physical Cosmology by P.J.E. Peoples, Princeton Univ. Press This question is likely fully answered somewhere, but I cant find it. Hubble obtained (1926) the mean mass density of the Universe as 10^-31 g/c.c. The gravitational stability equation gives (McCrea, Milne (1934))...
12. ### Engineering Thermal expansion of oil overflowing from a tank

Just wanted to know if I was barking up the wrong bush or not with my working out if anyone could help?
13. ### I Understanding Space-Time Expansion: Debunking the Myth of Non-Physical Space

How can space expand when space is not a physical thing? I’ve heard some say that is not expanding but rather it is getting less dense, which to me implies the same thing.
14. ### I What was expansion speed at the time of the CMB?

To be clear, I'm looking for the speed at which points in space were moving apart from one another in the universe as it existed 370000 years after the big bang, not the Hubble parameter (expansion rate) derived from our current distance within our current time. Thanks in advance!
15. ### B How did expansion take 380,000 years to let light travel freely?

Here's as far as I've gotten in the research (please correct any errors I'm making). The stages of high density had included these periods: • light at high energy was in a cycle of forming into matter, forming back into light, and so on • until things cooled enough for nucleons to form •...
16. ### I Result of this integral in large Lambda limit

$$\int \text{d}^4 q \, \frac{1}{(q^2 + m^2)\left(1+\frac{q^2}{\Lambda^2}\right)^2} =2 \pi^2\left(\frac{\Lambda^2}{2}-m^2 \log \frac{\Lambda}{m} \right) + o(\Lambda^0)$$ How to get this result? The notation ##o(\Lambda^0)## means all terms constant in Lambda, which we ignore because we are...
17. ### I Is the expansion of the universe affecting time or just space?

I read this old thread: https://www.physicsforums.com/threads/what-happens-to-time-as-space-is-expanding.1001016/ And I am confused by the responses. When I consider time dimension expansion, I perceive it the same way as space dimension expansion. The expansion doesn't change the unit size...
18. ### B Expanding Universe -- What is it expanding into?

I believe in the Big Bang but I struggle with something. If the Big Bang happened and universe is expanding there has to be something (or really nothing) there for the universe to be expanding into. Any thoughts?
19. ### Is Thermal Expansion Fatigue a Concern for Aluminum Alloy Motorcycle Frames?

Hello, assuming to keep an aluminum alloy motorcycle frame not in use, still, will the daily temperature variations cause thermal fatigue due to the continuous expansions and retractions or is the force developed by a few degrees of variations not sufficient to create microstructural dislocations?

34. ### A Free expansion of an ideal gas and changes in entropy

For a freely expanding ideal gas(irreversible transformation), the change in entropy is the same as in a reversible transformation with the same initial and final states. I don't quite understand why this is true, since Clausius' theorm only has this corrolary when the two transformations are...
35. ### I Can there be some kind of photon emission caused by space expansion?

Are there any kind of observed and experimentally verified processes or mechanisms where photon emission occurs and which are directly cause by spacetime expansion in some way?
36. ### A Understanding Free Expansion of Gas: Velocity Distribution Function Evolution

Hey all. I'm trying to understand the evolution of the velocity distribution function of a gas undergoing free expansion. I know that at t=0, the velocity distribution function is given by ##u(t=0, x)=Ax^{1/2}##, where ##A## is a known constant. From the momentum equation, I have: ##du/dt+u...
37. ### I Exploring Work Done in Quasi-Static & Non Quasi-Static Expansion

I'm wondering what's the difference between work done on quasi-static and non quasi-static expansion. In a quasi-static process, the gas inside the system must do a work to "extend". However, in a non quasi-static process, where the gas inside the system doesn't move fast enough to "push" the...
38. ### Thermal expansion of air bubbles in plastic

I am interested in the potential for air bubbles in a plastic structure to expand with heat, and put pressure on the plastic surrounding the air bubble. In this case the plastic structure is formed by melting a thermoplastic powder. In between the grains of powder are voids with air in them. As...
39. ### B Distance of Stars / Size of the Universe calculation

Hi, I am a new user, This question is bothering me for a long time and now with all the Webb telescope hype I need to ask: It sounds very logical to think that a star that is a billion light years away is seen as it was a billion years ago because the light took 1 billion years to get here...
40. ### 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?
41. ### B Expansion of Space Itself, rather than due to an Explosion like the Big Bang?

Please explain how we know that it is space itself that is expanding rather than an expansion due to an explosion such as the big bang. In either case wouldn’t every object appear to be moving away from us? How do we determine that space/ time is expanding? Tex
42. ### I Relationship between geometry and expansion rate

As I understand it, the flatness problem of Bob Dicke, says a flat universe in unstable and so has to be set very precisely in the early universe to give us the flat universe we see today. Is this the same problem as saying the expansion rate had to be finely tuned and if so how are the two...
43. ### B Bound Systems in an Expanding Universe: How Can Galaxies Merge?

As the universe expands all objects are moving away from us, so how is it posible that the Andromeda galaxy is on a path to merge with our galaxy?
44. ### I Mining the Universe's expansion?

There is an article written by astrophysicist Edward Harrison [1] which defends that energy could be extracted from attaching an imaginary cosmologically long string to a receding object from us in an expanding universe. He says that the energy extracted is potentially limited (in decelerating...
45. ### B Spatial Expansion -- Coincidence?

I was trying to follow how Hubble constant supposedly works. If it is about 70 km/sec/Mpc, then the the expansion at 13.8 Bly distance is about the speed of light. That, if I got it right, seems too convenient don't you think. I know it's not the same start to finish. Thanks if you can correct...
46. ### A Non-minimally coupled inflation — expansion

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...
47. ### A Series expansion of ##(1-cx)^{1/x}##

I am trying to understand the series expansion of $$(1-cx)^{1/x}$$ The wolframalpha seems to solve the problem by using taylor series for ## x\rightarrow 0## and Puiseux series for ##x\rightarrow \infty##. Any ideas how can I calculate them ...
48. ### SpaceX says 5G expansion would make Starlink 'unusable' for most Americans

Is this something to pay attention to, or just some big companies posturing against each other? SpaceX says 5G expansion would make Starlink 'unusable' for most Americans
49. F

### Link between Z-transform and Taylor series expansion

Hello, I am reading a course on signal processing involving the Z-transform, and I just read something that leaves me confused. Let ##F(z)## be the given Z-transform of a numerical function ##f[n]## (discrete amplitudes, discrete variable), which has a positive semi-finite support and finite...
50. ### I Expansion of Space: Questions on How the Universe Works

I first apologize if this has been posted more than once; sorry. There is a lack of clarity at my end as to whether I am logged on here? Say we have two galaxies, the Milky Way (MW) and galaxy A separated along a straight line 20 x the radius of the Universe visible to us. If I understand...