Expanding Definition and 625 Threads

Is our known universe expanding? Hypothetically we should we be seeing a light year further every year as the light hits us from distant stars. However once we take into account the expansion of the universe, are we really seeing any further? I am confused by the while topic of light and being...

Hello all, at the beginning, let me say that I haven't studied General relativity yet, I might have some knowledge at popsci level only... So I apologize if the question looks stupid... At cosmological scale we observe that universe is expanding. If I understood well, this includes only the...

given all the great minds of physics, i still have trouble with this one. "the uverse is infinitely big" and "the uverse is expanding" my understanding is you can't have both, because to define expansion you need measurable differential on a boundary, thus expansion by definition infers...

Hi everyone, I was thinking of a new theory in physics regarding dark energy, black holes and the accelerating expanding universe. What if most of the matter created by the Big Bang was pushed at the edge of the new created space/time like explosions, the matter is at the edge of the explosion...

Homework Statement I'm given that the energy of a particle in a rectangular box is the following: E =\frac{\hbar \pi^2}{2m}(\frac{n_x^2}{L_x^2}+\frac{n_y^2}{L_y^2}+\frac{n_z^2}{L_z^2}) I'm to show that if the length of the box is increased adiabatically and quasistatically from L_x to 8L_x...

I can not find a paper on this, but is it possible that the universe is expanding faster than expected? https://www.sciencedaily.com/releases/2017/01/170126132624.htm Because galaxies do not create perfectly spherical distortions in the fabric of space and the lensing galaxies and quasars are...

Hello I have been reading Sean Carroll's book "From eternity to here" where he mentioned the concept of functioning brains emerging from random fluctuations on a quantum level due to the expansion of universe. They have been called Boltzmann brains https://en.wikipedia.org/wiki/Boltzmann_brain...

Hello, A quantum free particle (no forces acting on it) is usually introduced as a plane wave. A plane wave has a very specific momentum and energy but the function is not normalizable and the particle has a change to be everywhere in space. Mathematically, this plane wave cannot be...

Hi, I remember seeing a few months ago, at a lecture about statistical signal processing, something which looked similar to commutation relations, only with a gaussian, instead of a delta function. Basically, it looked like this: $$\left[\phi(x),\phi(y)\right] = ie^{-\alpha(x-y)^2}$$ This...

http://www.ox.ac.uk/news/science-blog/universe-expanding-accelerating-rate-%E2%80%93-or-it Now, a team of scientists led by Professor Subir Sarkar of Oxford University's Department of Physics has cast doubt on this standard cosmological concept. Making use of a vastly increased data set – a...

I came to understand that the space between galaxies are expanding and not that they are speeding from each other and as space expands the gravitationally bound systems remain in their own relative position as the effect of gravity is more compared to the expansion of space. But now consider two...

Homework Statement Expand the function f(z)=1/z(z-2) in a Laurent series valid for the annual region 0<|z-3|<1 Homework Equations I know 1/z(z+1) = 0.5(1/(z-2)) - 0.5(1/z) Taylor for 0.5(1/(z-2)) is : ∑(((-1)k/2) * (z-3)k) (k is from 0 to ∞)For the second 0.5(1/z) the answer is a...

Hi PF! The other day in class my professor mentioned something about expanding linear independent vectors, but he did not elaborate. From what I understand, if ##x_1,...,x_k## are linearly independent vectors in ##V##, where ##dimV=n>k##, how would you extend ##x_1,...x_k## to a basis ##\{...

As I understand it, the Universe is red-shifted (emission spectra) from any point of reference looking outwards. The Universe is expanding, but is matter being created at the same rate ? Does this mean that the density of matter in space is decreasing ? (density = mass / volume). What does this...

The expansion of the universe is in a state of perpetual acceleration as evidenced by the cosmological redshift. But is there a jerk to this acceleration? Is the acceleration of the universe's expansion itself speeding up, staying the same, or slowing down?

Here they use information gathered form Cepheids, are these parallax measurements accurate? From science daily. Date: June 2, 2016 Source: Space Telescope Science Institute (STScI) Summary: Hubble Space Telescope astronomers have discovered that the universe is expanding 5-9%...

if the uverse is expanding and known to be accelerating, at what rate is this acceleration? even at small # over the billions of years, would it not be at speed of light already, and if so, how would observable mass be traveling at the speed of light?

According to the paper they used 300 SNe Ia along with Cepheid variables, as a basis for determining cosmological distances. With the exception of 19 SNe Ia, which were calibrated using nearby Cepheids, the remaining 281 SNe Ia were based purely on their light curve to determine their type...

I've spent quite a bit of time researching cosmology on here and various other sources (NOT pop science) and although I feel I have a much better understanding in general, there is one specific thing that I just can't get my head around. Is the space between galaxies expanding or is it just the...

I know this thread, about why the Universe can't expand inward, is fairly old; but I stumbled across it today and there was something mentioned here that sparked a question I feel like people here would be qualified to answer. What was mentioned, was that a singularity is a point at which our...

Hi I have two novice question If the universe is expanding and it is all moving apart, how is it that galaxies are colliding? If our galaxy and all others are moving away from some singularity central point. And as we see other galaxies moving away from us at a speed relative to there distance...

Homework Statement Consider the position vector of a mass m at height h above the Earth's surface to be \underline{r}=(R+h)\underline{e}_z where R is the radius of the Earth. Make a Tylor expansion in h/R <<1 of the gravitational field...

Is the universe now expanding? In that case, the Earth should be spinning away from the Sun right? I've heard that the moon is leaving the Earth 4cm-5cm every year If we are spinning away from the Sun, average temp. on the Earth must be decreasing Is there any scientific estimation on how...

A rubber ball is bouncing on a flat surface. Every time it bounces, it loses energy, while the sound of the ball hitting the surface accelerates (because the intervals of the ball impacting the surface become shorter with each bounce). The ball seems to be accelerating when, in fact, it is...

As a layman, I have come to understand basic expansion (meaning without dark energy) as objects which are not locally gravitationally bound are moving away from each other due to some inertia created from an initial period of inflation. This rate of recession is the Hubble constant and it...

I'm working on a pretty simple ODE but am getting really confused about one little bit. It's of this form: y'' + y = t So, in solving the complementary solution I use the characteristic method to find: λ2 + 1 = 0 Hence λ = ± i Therefore: yc = Aei t + Be-i t This expands to: yc = Acos(t) +...

Expanding universe or contracting matter? this may look very weird question, but what if instead of that the universe is expanding, all matter is contracting as a function of its (proper) time? Δs' = Δs_0 /F(t) The contraction of matter would effect on the length unit what we use. I am...

OK I might be stupid and english isn't my native language, so I'm sorry in advance. So: The red shift is bigger, the further away a galaxy is (they move away faster), and the closer it is, the shift is smaller and it goes to blue for Andromeda (towards). Everyone knows that. BUT When you...

Homework Statement Homework Equations p.q+p.r The Attempt at a Solution I've expanded p.(q+r) to give p.q+p.r. The magnitude of p is 3, and since ABE is an equilateral triangle, the magnitude of q is also 3, right? So then p.q=9, but the answer scheme states that p.q=4.5. I'm still pretty...

Well I'm just a high school student but I'm very passionate about Astronomy. I usually ask myself that into WHAT is our Universe is expanding if the expanison theory is true ! I'll be waiting for responses..

Hi guys. I have a question regarding the expansion of space. Alright, so I know that space expands proportionally to the amount of distance. However, I'm not sure why this occurs. Can someone inform me on that? Additionally, I know any individual placed anywhere in the universe would observe...

If the Universe were not expanding uniformly (i.e., at the same rate in all places), then would different places would see a different Hubble law than we do? And if If the extragalactic distance scale changed, would the Hubble constant change?

I like physics involving the cosmos. I was wondering what theories are out there that explain how the observable universe is expanding at an increased rate rather then slowing down due to gravity or other variables. I know the dark energy theory that basically states there's a energy that...

Hello Everyone, Back when Einstein was formulating General Relativity his equations just could not predict a static universe. I have read that they actually predicted an expanding Universe. Later Friedmann derived an equation from GR that would explain how an Expanding Universe would evolve...

I have read that as the universe expands that the "fabric" of spacetime expands. Does this mean that the metric basis for a given space is expanding? Also, does this mean the distances between the atoms in the lattice of matter is also increasing proportional to the expansion of the cosmos?

Hi. I was recently researching the Hubble Sphere - a concept that is within cosmology - and someone claimed something interesting. The individual contended that the universe has been decelerating since the big bang, and that the so called "acceleration" in the expansion of the universe is space...

Homework Statement I'm trying to understand a proof for Mersenne number theorem, but I'm having trouble understanding the expansion from 2ab-1 into (2a-1)((2a)b-1+(2a)b-2+...+2a+1)). Homework Equations Not sure The Attempt at a Solution I don't know where to start really and I can't seem to...

I can't be the only person to ever have this thought so I am hoping one of you star gazers can tell me what I am missing. I understand Hubble's theory and the idea of space itself expanding. My question is this. If we know the universe is expanding because of the redshift and the farther away we...

After learning a bit from the thread "The Dark Sky Ahead", I have tried to think through the dynamics of gravitational interactions in a simplified scenario. Unfortunately the GR math of the problem is over my head. My attempt below, using a simpler approach, leaves me quite confused. If my...

I know this question is quite odd as we're still researching on this, but is the universe speeding up, aka, accelerating? Or is the speed constant? Or is it slowing down? Thoughts? (PS - I don't know what the prefixes B I A means, I have chosen a random A, please consider if it's wrong, am...

As the universe expands, space itself gets "stretched" and objects drift apart, like dots on elastic surfaces when force is applied. So one meter billions of years ago is two meters today, but does it necessarily mean light takes twice as long to travel this "new meter"?

Hi PF members, I have a question about electromagnetic radiation propagating in an expanding universe. We know that because of space expansion, light from a distant source gets redshifted, its wavelength becoming longer and its amplitude flatter. If we move far enough from the source, we...

I'm new to this impressive forum and have a question that may have been addressed a thousand times, but here goes. A FLRW metric is happy with a time-varying scale factor a(t) and zero curvature parameter k and could care less about density. The combination of a FLRW metric and the Einstein...

Hello, I have problem with understanding of expanding of formula in this video (at 30min 05s), after few second he neglects term (r/R)^2). For me there is missing factor 2 at second term (in "j" component). Similarly as here at 24min 10s. If somebody knows what's the matter there...please...

Homework Statement ## y^({9}) + y''' = 6 ## Homework EquationsThe Attempt at a Solution ## y^({9}) + y''' = 6 ## ## r^9 + r^3 = r^{3}(r^{6}+1)=0 ## ## r = 0, m = 3 ## ## r^6 + 1 = 0 = e^{(i(\pi + 2k\pi)} ## ## r = -1 = e^{i(\frac {\pi +2k\pi} {6})} ## ## k = 0 , r = e^{i(\frac {\pi}...

Can we really say that the universe is accelerating its expansion because we find that galaxies further away have more redshift? We know that we are looking back in time when we look at the stars, and when we look at the galaxies furthest away from us, we are seeing them as they were a long time...

I just came across this in a textbook: ## (\partial_{\mu}\phi)^2 = (\partial_{\mu}\phi)(\partial^{\mu}\phi) ## Can someone explain why this makes sense? Thanks.

Is ther a term for the space that the expanding universe has not reached yet. Is it called a void or is it some other term. Also if you could link some articles about this space the universe hasnt reached yet I would love to read about it. Thank you all.

Homework Statement ## L (v^2 + 2 \pmb{v} \cdot \pmb{ \epsilon } ~ + \pmb{ \epsilon} ^2)##, where ## \pmb{\epsilon}## is infinitesimal and ##\pmb{v}## is a constant vector (## v^2 ## here means ## \pmb{v} \cdot \pmb{v} ## ), must be expanded in terms of powers of ## \pmb{\epsilon} ## to give...

Hello, I've worked through most of Carroll's appendix on the non-coordinate basis. I see and agree how the spin connection and tetrad one-forms are useful while calculating. However as an example he sets out to apply the formalism to a spatially flat, expanding universe. ds^2 = -dt^2...