Universe Definition and 1000 Threads

I can't find any values of acceleration or rate of change of acceleration of the expansion of the universe when I looked it up and I need these values for a theory I'm working on that could supersede dark energy and show the universe is closed even if everything accelorating away from us and...

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...

Is it correct to say: Under the Many Worlds Interpretation: If we rerun the universe repeatedly from the same state S(0), it winds down the same way each time, which is determinism; Under the Copenhagen Interpretation: If we rerun the universe repeatedly from the same state S(0), it may well...

In the detection of Cosmic Microwave Background Radiation I understand there's still some low frequency radio astronomy left to do before hitting the 'opaque universe' boundary where/when light could not propagate. What, if anything, can we still hope see?

I am confused about the shape of the de Sitter universe. The Misner-Thorne-Wheeler says it can be regarded as the submanifold given by ##-x_1^2 + x_2^2 + x_3^2 +x_4^2 + x_5^2 = k## of a flat space with lineelement ##\mbox{d}s^2 = -\mbox{d}x_1^2 + \mbox{d}x_2^2 + \mbox{d}x_3^2 +\mbox{d}x_4^2 +...

Was wondering if acceleration seems to be a more fundamental property/quantity in the universe as compared to velocity or distance because acceleration can be defined in more absolute terms in a frame depending on the forces acting inside that reference frame. Considering a very simple example...

Are we sure that in the next billion years we (whatever “we” are then) won’t change the dreary future of the universe to make it closed because we then, as now, will find it more beautiful? I ask because Frank Tipler’s, The Physics of Immortality, is premised on a closed universe. It would be a...

As I listen to lectures trying to explain the universe based on the laws of physics, it makes no sense and it can be all over the place with holes and gaps. Dark this or a catastrophe here.So the physical laws seem to be lacking when it comes to explaining the nature of reality. We can use these...

The cosmological principle holds that at large enough scales, the universe is homogeneous and isotropic (i.e. symmetrical). But, there is meaningful evidence from astronomy observations of anisotropy at the largest observable scales in the universe, which a new preprint (discussed below) sets...

In so far I know eternal inflation can only constraint the minimum duration that inflation happened to produce the properties of the universe we can experimentally test (a minimum of some 60 e-folds, or in time, at least some 10^-30 seconds), but as eternal inflation is generically eternal to...

a) ##P_f=\frac{nRT_f}{V_f}=\frac{nR\frac{T_i}{2}}{2V_0}=\frac{1}{4}\frac{nRT_i}{V_0}=\frac{1}{4}P_i## b) ##Q=\Delta U=nC_V \Delta T=n\frac{5}{2}R(-\frac{T_i}{2})=-\frac{5}{4}nRT_i=-\frac{5}{4}P_i V_0## (##L=0## since the gas expands in a vacuum;Now, (a) and (b) are both correct but not (c), for...

I have heard of and read about the big bang theory for the start of our universe. Are there other competing theories that I should look at. Thanks Chiefly

When I discuss about the Big Bang, the expansion of the Universe and the fact that on average every galaxy is receeding from us, I get "oh, so then we are at the 'center' of the Universe." I know that's not the case, we are not in a special place, etc. But, is this a proven fact, or is just a...

I was cleaning out my email archive after being notified I was out of storage. I decided to keep these. Enjoy. (original at: https://dingo.care2.com/cards/flash/5409/galaxy.swf) This one would be impossible on a phone and slow on a laptop, use a fullsize computer if available. I couldn't...

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...

I found this question in a textbook, not sure if this question has been asked before. Not sure if the author just wanted to make the reader think or he had anything specific in mind that he wanted the readers to understand. Most of the people immediately conclude that the speed of light doesn't...

I wonder how physicists who support this theory explain the increase of entropy?

[Moderator's note: Spin off from previous thread due to topic/level change.] Although the initial state of the universe is subject to much speculation, it seems to have started as all (or almost all) energy with particles being created out of photon-photon interactions.

So I am a fan of astronomy, cosmology and astrophysics from a FAR. The math is way beyond my abilities but I like to just sometimes read and think about the very small parts I am able to comprehend. So my hats off to all of you that are able to fully enjoy this stuff, you're very lucky...

If the universe were to re-collapse, the Hubble constant would increase over time. Since the age of the universe is the inverse of the Hubble constant, the age of the universe will decrease. Does that mean time reversal?

Consider a celestial object distance d1 away from an observer, receding at v1=H0d1, Fig.a. And the same object at some time later, now at distance d2 and receding at higher speed v2=H0d2, Fig.b -- 1) where does the accelerating force come from? 2) the resulting universe-size vs time...

It seems to me as the space between galaxies expands it reduces the gravitational attraction between them. Assuming that this is happening between the majority of the galaxies in our Universe it would cause the acceleration of expansion by a factor of 1/r2.

Hi and thank you for answering questions on complex subjects like this. I’m a non-scientist who is jealous of your brain capacity! Recently, in a discussion with a chemist, he said the particles appearing from nothing might have been the initial cause of the universe. I’m familiar with the...

Using Jorrie’s calculator we can get the following T vs. t graph up to z = 20,000: In a log-log plot the above curve is represented by the red solid line in the figure shown below: The dashed line shows results for the small a limit using the equation T = 2.725 K / a. Here are some...

I have noticed that many PF participants seem to favor a cosmological model that our universe is infinite (and flat), but some (like myself) favor a finite universe. I have become curious about the subjective probability distribution, so I am hoping that many PF participants will post their...

If galaxies are now expanding away faster than light, then how would we know if this has now started to slow, stop and or reverse ? Is there a chance we may never know and the universe collapse won't be seen coming ?

Because if stars are radiating light in every direction, shouldn't there be light all over? And we only see the light/photons that our eyes are receiving? Though there may be dark spots from where crests meet troughs? Or am I completely wrong and missing something?

Our planet (Earth) has a wide variety of different elements, which at some time had to be generated (presumably in stars) and distributed through space to new forming planets (like ours), when the stars exploded. What is the minimal amount of time needed for this to happen? It has often been...

I wish this forum allowed indentation using tabs. Some of these questions are dependent on the answers to others. I have used outline numbers to indicate nested questions. I have BA in physics and mathematics, 40+ years ago. I was thinking (always a dangerous thing). I had some questions. I...

In a globally hyperbolic spacetime you have a global time ##t: M \longrightarrow \mathbf{R}## which foliates it into Cauchy surfaces ##\Sigma_t## and the metric can be written ##\mathrm{d}s^2 = -N^2 \mathrm{d}t^2 + h_{ij}(\mathrm{d}x^i + N^i \mathrm{d}t)(\mathrm{d}x^j + N^j \mathrm{d}t)##. By...

Hi again. I'm still off work and struggling to learn some physics. I'm searching for discussion about the possibility of a universe changing its curvature as it evolves. I'm still new here and so I'd be grateful for advice either about: (i) How to search for past discussions, or...

In an isotropic universe, every observer sees themself as being at the center. But consider 3 observers, A, B, and C who are 5 billion light years apart and all lined-up in a straight line with B at the center. B knows this to be true because A is in one direction and C is in exactly the...

How can we know the age of the universe, with time dilation due to the mass and velocity of matter which condensed after the big bang. If time passes at different speeds depending where it is perceived, does this not make the age of the universe somewhat moot.

I feel a bit troubled by the use of "our observed universe" as a concept of reason concerning the nature of universe. For example, the following is part of the answer of Halc to my previous question: [ Rotation around an attractor only works given the causal reach of the mass, so nothing outside...

It seems that all celestial systems are in different plates, e.g. solar system, milky way galaxy, clusters of galaxies...although different systems might have their own plates, it seems to turn out that small systems are always on the plate of a bigger system...so is the whole universe on a big...

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...

If the universe is either infinite, or it repeats, then I would assume that it would be possible for my atoms to come together again at some incomprehensibly long amount of time after my death. If this were the case, would my consciousness that I am currently experiencing now ever exist again...

Well, okay, I should say: what does Newtonian gravitation look like in a ##2+1## dimensional Newtonian universe? Consider a flat Earth, i.e. a region ##\mathcal{E} = \{ (x,y): x^2 + y^2 \leq R \}## with mass density ##\rho##, then for ##r > R## a natural guess for the gravitational field seem...

Hi, Never done much cosmology but reading around I have found several sources saying the early universe (pre roughly 300,000 years) the early universe was full of gamma radiation. Since the universe has expanded this gamma radiation has been red shifted into the microwave region. Other sources...

That is, is there a angular circle size which is approximately in all directions statistically the same with respect to its CBR temperature variation contents (after an adjustment for the veocity of the Earth in some specific direction)? If the answer to the above question is "Yes", is this...

It is mathematically proven that any intrinsically curved manifold can be derived from embedding in a flat space of sufficient number of dimensions. I heard somewhere, but lost that reference, that for our universe 10 flat dimensions are needed in the most general case.. May I ask - Is the...

https://www.physicsforums.com/threads/is-the-universe-a-closed-system.620503/ Hello. I was looking back at this thread from 2012 and, to be honest, I'm a bit confused. Quoting Drakkith... To our knowledge it is. At minimum you could count the observable universe as a closed system because...

The Canadian physicist, Lee Smolin proposed in the 90's that laws of nature may evolve and change over time. He begun proposing that only the constants of nature would evolve, while the most fundamental laws would remain the same, but recently he has change a little bit his ideas and has...

Suppose we're in a closed and non-expanding universe. You shine a beam of light in one direction such that it circumnavigates the universe and returns to its starting point from the other side. Now you put a detector at this starting point. Would this be a one way measurement of the speed of...

Hello all, I recently did a thought experiment and thought, "what if the universe as a whole is spinning?" This could solve the dark energy problem, as if the universe was spinning, there would be outward pull, and therefore keep the universe expanding. And, I don't know if this means anything...

If we take a spherical distribution of matter wherein gravitational force and cosmological-constant force are equal upon an object on its surface, then does the time that it took for that volume to grow to the size wherein the two forces are equal match the time it took for the universe to start...

Again sorry, it is in Spanish

Hi, if possible I would like to ask a question to experts in this area, it is correct to say that: In ancient times, ranging from around 12,000 BC. until 400 BC, the universe was more compact, and consequently the distances between solar systems were smaller? From what I have been able to...

Is there a reason why we can't see what's on the other side of these Voids in Space? Are we to Believe it is Void to the End of our view of the universe?