Expanding Definition and 637 Threads

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...

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...

When I ask Google how to expand space, I get suggestions on removing junk or questions on what it is expanding in to. I am going to try asking here and I know I must phrase the issue properly. It is agreed that many scientists claim that space expanded around the time the universe began. I...

If we have a function ##f(x+\Delta x)## where ##\Delta x << x##, is it valid to approximate this as: $$f(x + \Delta x) \approx f(x) + f'(x)\Delta x$$ even if ##\Delta x## is not necessarily small? If not, what is the valid expansion to first order?

I have been playing around with Taylor expansion to see if I can get anything out but nothing is jumping out at me. So any hints, suggestions and preferably explanations would be greatly appreciated as I’ve spent so so long messing around with it and I need to move on... But as always, thank you

Assume that we have a flat FRW metric expressed in conformal time ##\eta## so that the line element is $$ds^2=a^2(\eta)(d\eta^2-dx^2-dy^2-dz^2)\tag{1}$$ where ##a=1## at the present time ##\eta=0## and the speed of light ##c=1##. This metric has the following non-zero Christoffel symbols...

Summary: How do we know that the universe is expanding? Am I correct in my understanding that we believe the universe is expanding due to the objects we can see, i.e. galaxy's moving away from us, and that the further away from us they are, the faster they apear to be receding away from us...

Now...exactly when and how do CMB photons lose their 3000˚ K creation energy to become 2.7˚ K?

i have read that it is generally thought to be a consequence of the big bang (so, matter in motion) + something (dark energy) making that matter accelerate. why is the big bang needed in it? can't you just have acceleration? by the way, i am not sure what is intended by 'prefix' beside the...

where does 32 (red) come from ? nevermind looks its (-8)(4)=-32 but will probable have more ? on this example

Observation shows that the Universe is homogeneous (and isotropic) at the large scale, while one expects to see inhomogeneity (increasing density at greater distances) on the past light cone due to expansion. This seems inconsistent. Am I misunderstanding something here?

What is the rate of expansion of the universe and what is it's acceleration rate of expansion? Exactly? I recall it's about 67km/s at 1 mega parsecs? But then what's the acceleration rate? This exact information I cannot find. If the universe is expanding, and this expanding is accelerating...

Is this how you would draw the Minkowski space diagram with space expanding, (without acceleration or black holes) ?

Has anyone read the book of this author? What is your opinion about this topic? https://www.google.de/search?q=tired+light+lyndon+ashmore&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjbo_exj5vfAhXK2qQKHdHsD6AQ_AUIDygC&biw=1280&bih=623

Instead of expanding, could the universe be rotating? Everything rotates. Atoms, solar systems, galaxys. Could it be that we just can't see enough of the universe to see it's rotation?

{Reference: Wikipedia's Friedmann-Lemaitre-Robertson-Walker (FLRW) Metric article)} The FLRW (1935) mathematical model of the universe is the one most used by cosmologists. It is differentiable, which means it is based on sound, consistent, mathematical formulations. (The Lambda-CDM model is...

I've a general question. I'm self-studying classical mechanics using various means one of which is Leonard Susskind's Theoretical Minimum lecture series. I'm on Lecture 7 and we are doing Liouville's Theorem. My understanding of it so far is that in phase space as something expands in, say, the...

If the mass of the electron has been changing during the evolution of the universe, then the orbits of the electrons would also change, which will shift the light spectrum of each atom. Could this explain red shift of far galaxies, and the shift is not because the universe is expanding? Henry

I have read that conservation of energy is not a meaningful concept in an expanding universe cosmology. See here http://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/ However I have also heard the if the net energy os the universe is zero then it can have a vacuum genesis...

What is the source of the energy needed by the ever expanding space-time? It could not be baryonic matter, since it accounts for only 4%. It could not be radiation, since it accounts for only 0.008%. Is it dark matter being converted? Is it quantum fluctuations? Kurt Ludwig

Equation 9.2.25 defines the inner product of two vectors in terms of their components in the same basis. In equation 9.2.32, the basis of ## |V \rangle## is not given. ## |1 \rangle ## and ## |2 \rangle ## themselves form basis vectors. Then how can one calculate ## \langle 1| V \rangle ## ? Do...

How does the expansion of space cause light to lose energy?

This idea has always bugged me: If we are looking at the past when observing the redshift of far away galaxies (ex: 10 billions light years).. then how can we tell that the universe at the present time is still expanding at same rate, deaccelerated, or stopped expanding?

I would argue that if you are computing the work done by an expanding gas in a frictionless piston, in an irreversible expansion where the inner pressure is greater at all times than the (constant) outer pressure, that you should integrate the internal pressure over volume change, not multiply...

The Dynamical Casimir Effect is the production of real photons from the vacuum in a system where one has moving mirrors (see https://www.technologyreview.com/s/424111/first-observation-of-the-dynamical-casimir-effect/). The frequency of the photons is related to the ratio of the velocity v of...

The minimal necessary math to describe the potential path of an electron expands over time. Second by second, the space necessary to describe the possible positions of the electron expands. . Does that imply expansion of physical space? And does that shift the explanation of the expansion of...

This question has been bothering me for decades: Imagine a point source in space that emits one photon per second. Would the photon expand in a globe in all directions until it strikes an object or would the photon shoot off in a random direction? Suppose you have one target ten meters away...

<< Mentor Note -- after a very long Mentor discussion, we acknowledge that this paper, while potentially controversial, has been published in a reputable peer-reviewed journal. We believe that a discussion of this paper can be useful and constructive. Thanks >> This paper of mine was...

Regarding the expansion of space... In one book I've read the diameter of the universe is more than 100B light years across, even though the age of the universe is only roughly 14B years old. This is due to accelerated expansion of space. In another book it says that space is expanding in a...

According to what I've been taught, the distance-related redshift seen by Edwin Hubble is an artifact of an expanding Universe. That is, as light travels through space, space itself expands, redshifting the light (matter is embedded in space and does not itself expand but is instead carried...

The universe seems to be expanding since the farther away an object is, the faster it is moving. However, because of the finite speed of light, the farther away we look in distance, the further back in time we look. Does that mean that galaxies were moving faster in the past and are now slowing...

This looks very promising. It doesn't work for all stroke patients, but so far about half of the patients who went through the imaging procedure did have brain tissue conditions that indicated they would still benefit from removing the clot causing the ischemic stroke. That extended time...

How can the universe keep on expanding if it's infinite? Expanding metal, like a cube of aluminium, has a surface area which forms a border for the matter contained inside. So the universe must have a border for the matter it contains.

Homework Statement [/B] Hi I am looking at this action: Under the transformation ## \phi \to \phi e^{i \epsilon} ## Homework Equations [/B] So a conserved current is found by, promoting the parameter describing the transformation- ##\epsilon## say- to depend on ##x## since we know that...

Greg Bernhardt submitted a new PF Insights post Coordinate Dependent Statements in an Expanding Universe Continue reading the Original PF Insights Post.

Is it expanding from the edges? Or from the center?

If me and my pet duck, who weighs next to nothing, were placed far away from any gravity source and in such a way that our mutual attraction was negligible, would I see my duck accelerate away from me? If so, how fast?

If space time (the universe) is infinitely expanding what happens when it rips? Have we observed Virtual particles in an area and counted them? Does the expansion of space time affect the amount of Virtual particles in a particular area? Is it theoretically possible to expand space time to a...

Normally when one is asked what is the universe expanding into, the answer is its not expanding into anything. Space itself can expand. However In the context of eternal inflation, our universe is a bubble in a sea of inflating space. And hence assuming eternal inflation is right, does the...

Imagine that I have a straight, statically-charged, cylinder-shaped tube with arbitrary (ideally infinite) extent. The charge is distributed evenly over the tube such that the field inside the tube is zero. For convenience, let's line up the tube centered along the x-axis such that the...

Suppose an object is moving away from you. Is there a way to determine, experimentally or otherwise, whether it is moving through space or whether it is sitting still but appears moving because the space between you and the object is expanding? Galaxies that are sufficiently far away from us...

In a dark energy dominated universe, it seems that all the particles get away from each other and that the final state will be one with one or zero particles per horizon. This sounds very intuitive, but it is based on classical physics and GR. Particles have wavefunctions and this is whar...

hello there, I am totally into all astrophysics. so I want to know, how fast is the known universe at the most outer edge moving? or in other words, how fast is the most outer edge expanding away from earth. - and what's the rate of acceleration? I mean even if, as I understand it, the space...

Homework Statement (c) A standard aluminium ingot is 746 mm in length at 20°C. When the ingots reach a temperature of 500°C. Calculate:(ii) the induced force in an ingot assuming it has become jammed in the furnace and is NOT free to expand y =...

It seems like the following sentence implies that 'dark energy' *(assuming that is the cause of the acceleration) is infinite: "the universe appears to be expanding at an increasing rate,[1] so that the velocity at which a distant galaxy is receding from the observer is continuously increasing...

Hello! Can someone explain to me, in real analysis, what is the difference in expanding a function as a Taylor series around 2 different point. So we have ##f(x)=\sum c_k (z-z_1)^k = \sum d_k (z-z_2)^k## and as ##k \to \infty## the series equals f in both cases, but why would one choose a point...

I'm looking for a way to determine the velocity a projectile can reach from expanding gasses (explosion or deflagration). I'm sure there are a lot of variables in here, but for reference, we can work with the 12 pounder Napoleonic cannon. The powder burning is 2.5 pounds of black powder made up...

As a follow-on to this thread, which in turn followed this closed thread, I'm starting a new thread on a related real-world problem. Say I have a 2-liter plastic soda bottle filled partway with water, and pressurized. It has been sitting inverted (with the opening pointed down, and sealed) for...

I know that the universe is expanding since the beginning of time or at least the beginning of the universe it self. So I was wondering if the universe has always expanded evenly (some parts getting bigger while other grow not as fast) and if it doing so now?? The example in my head is like...

Homework Statement expand f(z)=\frac{1}{z(z-1)} in a laurent series valid for the given annular domain. |z|> 3 Homework EquationsThe Attempt at a Solution first I do partial fractions to get \frac{-1}{3z} +\frac{1}{3(z-3)} then in the second fraction I factor out a z in the denominator...