Universe Definition and 1000 Threads

As I am watching the current season of How the Universe Works, I am a bit confused when listening to physicists talking about energy as if it was a thing. The way I understand it, energy doesn't exist by itself, but is rather the result of interactions involving matter. Energy is released...

Hello. I'm trying to pin down something Stephen Hawking was referring to in his book, A Brief History of Time. During the next few years I developed new mathematical techniques to remove this and other technical conditions from the theorems that proved that singularities must occur. The final...

If you had a universe that only contained a single photon - would it be valid to call it a universe? If so, what properties would it have, like time, no of dimensions etc? And if you added a second photon,could you say that you then have a universe with 2 photons in it?

i don't really understand why S of the universe must be always positive,i know that only reversible process have constant entropy but why real proceses always increase S in the universe? sorry for bad english I am not from USA or UK

From the friedmann equation H2=8πGρ/3-k/a2, 1=ρ/(3H2/8πG)-k/a2H2 1=ρ/ρc-k/adot2 adot=+-√[k/(ρ/ρc-1)] It is therefore if expansion/contraction is taking place , Positively curved space will have ρ>ρc And negatively curved will have ρ<ρc Is it the case??

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?

Is the problem with interpretations of quantum mechanics that one can describe the universe consisting of "stuff" that exists ontologically in different but conflicting ways, so that the universe can really only be described informationally instead of ontologically? That is: does the universe...

From the energy equation E=m0c2/√(1-v2/c2) for non-relativistic gas molecules (v<<c) ,E reduces to m0c2...(1) From ideal gas law PV=nRT P=nRT/V P=nkBNAT/V P=(nNA)kBT/V P=(nNAm0)kBT/v P=mtotalkBT/vm0 P=(mtotal/V)kBT/m0 P=ρkBT/m0 (If n moles of a gas is taken in volume V at temp T and volume V,m0...

From friedmann equation And For a flat universe with k=0 and ρ=ρc ,da/dt becomes undefined and d2a/dt2 becomes 0 But for the present time we know that our universe is flat and expanding with a acceleration (q -ve) , Therefore is it here something I am missing??

Did hydrogen reach some maximum level in the early universe before fusion began creation of helium and thus decreasing the amount of hydrogen, or do fission reactions maintain some kind of balance? If the universe is constantly expanding, the density of remaining hydrogen would seem to decrease...

Can the universe be treated as a thermodynamic isolated system? And why?

By how many times is the size of the observable universe larger than expected because of inflation?

I had a question about Schrodinger's cat that extends to the universe. First, I'm sure everyone knows the Schrodinger's cat set up so I won't repeat it. I will just ask, how can the cat be dead or alive prior to measurement? This measurement would be either atoms in the radioactive substance...

Hi, I have become very interested in cosmology recently and I had a question about the possible shapes of the Universe. I understand the critical density plays a role in the shape. This is what I know (or think I know). At the critical density of energy and matter, the universe is flat. If it...

I once read somewhere a long time ago (and so I am not sure if this is true, let me know if it isn't) that the universe doesn't have a center of expansion. I get it. Neither does a typical balloon. But a balloon has an axis about which it is symmetrically expanding. Obviously that is not the...

Its stated that empty universe should have a hyperbolic geometry (Milne Universe) but I don't understand how its possible. $$H^2=\frac {8\pi G\epsilon} {3c^2}-\frac {\kappa c^2} {R^2a^2(t)}$$For an empty universe when we set ##\epsilon=0## we get $$H^2=\frac {-\kappa c^2} {a^2(t)}$$...

What is the mass of all photons emitted in the universe since the Big Bang?

Hello. I have some questions regarding Alan Guth's book on the Inflationary Universe. The following, from pages 185 & 186, has piqued my interest. I have reproduced Figure 10.6, above. Please read on. My questions follow. Please also take note that I can only understand these matters at a...

"Our universe could be the mirror image of an antimatter universe extending backwards in time before the Big Bang. So claim physicists in Canada," https://physicsworld.com/a/our-universe-has-antimatter-partner-on-the-other-side-of-the-big-bang-say-physicists/

Let us suppose we have a metric in the form of, $$ds^2=-c^2dt^2+[(a^2(t)+b(r)e^{-lt})(dr^2+r^2d\Omega^2)]$$ Where scale factor is defined as ##(a^2(t)+b(r)e^{-lt})## Is this metric describes homogeneity and isotropy or not ? I think it cannot since there's an ##r## dependence, and there are...

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...

As stated in the question--is there any evidence of a non zero rate of change of the acceleration of the expansion of the universe. What is the evidence for it and is it negative or positive? Also if there is what would it take to determine if there was a jounce?

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

Homework Statement 2.Relevant equations[/B]The Attempt at a Solution How does a reversible process in the universe imply the entropy doesn't increase? I understand that the change of entropy in a closed reversible cycle is 0 in the system, but I don't get why a not closed reversible process...

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?

This is the provocative question posed by Sabine Hossenfelder in her article in Forbes, covering the ongoing debate of inflation critics like Paul Steinhardt (once one of the theory founders) and scientists who develop models of inflation. Link to her article...

I do not understand how (in few bilion years) Milky Way and Andromeda will collide? If the universe is expanding and these objects which are far away have bigger speeds than those who are closer, than Milky Way and Andromeda will just be far away every moment... (?) Could someone help? :-)

{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...

Please correct me understanding if it is wrong. I know that the oldest light we can observe is 13.8 billion light years away. However, we know that space undergoes inflation, and as a result, there is a maximum observable radius of light that could possibly be seen from any point, regardless...

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...

Homework Statement I have the following questions as homeworks and I would like to get help. Here's some informations given to help us to answer : Photometry : U=11.60 B=11.16 V=10.20 Redshift : z= 0.00780 Central velocity dispersion : ##\sigma_{v}## = 210 km/s Introduction : 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 am searching sources, which would express purely (without any other expression on how the data was obtained, etc.) elementary origins of the universe. As we would have certain artistic visualisation on how our Mars rover would operate on landing on mars, are there sources which give data on...

There is a Baez essay about the vaccum energy density, where he says: So did they do what I might naively think of doing, namely just plotting the redshift data over a few years? Or is it a more subtle method that directly measures the time derivative of the expansion? And if I understand...

My understanding is that the universe seems quite likely to be flat, and therefore infinite. Following an infinitely large object back in time to the big bang, it would never become finite. (However many times you divide infinity by 2, it is still infinity.) We tend to picture the big bang as...

Is dark energy the reason for the expansion of the universe?

Please clarify. I have read that according to NASA the universe's expansion rate is 74.3 km/s +/- 2.1 at Mega parsec. At a distance of one Mega parsec, space is moving 74 km/s away from us. At 2 Mega parsec it is moving away at 148 km/s. Do I understand this correctly? But this is velocity, it...

Note to the Quantum Physics moderator and physicists reading this: this is a cross-posting of a thread from the Probability Forum I'm just going to put a link to it here because I'd also like to get a physicist's perspective on the matter, the thread is about a book we are discussing but you...

Hey i was here for my airsoft problemes but while I am waiting... What is containing the universe? is nothingness somehting? Or is it simply infinite?how is there an infinity of things? and if life is just a chemical raction from matter how with a spark it suddenly has will, would it mean...

So, basically, multiverse make sense because we can't imagine our universe standing in nothing(or being everything that just expands itself). If we take this principle, we should have a multi-multiverse, a multi-multi-multiverse and so on. (When the chain ends?). Is it more evidence to support...

So I have sort of a conceptual question about the big bang and gravity. Imagine yourself in a universe, in which existed about the number of particles/energy in a 3X3 metre room at any given moment. This universe has the same laws of physics, constants and is identical in every way to our...

I apologize if this has been asked before, but I have never seen a satisfactory or straightforward answer. When we say the universe is expanding, do we mean it is "growing" or "stretching"? In other words, if you considered a sheet of graph paper expanding, are we adding more squares or...

Could the universe have been created by the collision of two older universes with the same dimensions? Could the expansion we observe be caused by these two universes still merging like boubbles coming together and the resulting increase of gas makes the film grow larger?

i got a bit lost in the responses to my last question so I am guessing this one is really going to be beyond me. Assumptions I have used for my questions are: · Speed of light = 299792.458 km/s · Hubble constant = 71 km/s/Mpc (I know about the tension of H0 being 68 and 73 but...

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...

When I think about it, I see galaxies getting farther away with a constant acceleration. An acceleration that must be caused by a force ( pinned Dark Energy ). I lack in knowledge of the subject but yet I wonder what is the value of the acceleration in the expansion of the universe. Just like in...

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

I have come across a questions which reads "Why does even photon travel?". After reading special and general theory of relativity, this one bugs me all over. For a particle moving closer to speed of light, clock slows down and the space around it contracts. I see a photon travel and I see things...

Why is the accelerating expansion of the universe not explained simply by the perspective of acceleration caused by a constant (non accelerating) universal expansion? Let's take two particles (X,Y) in a 1d expanding universe where it expands universally by 1 unit per per unit time. O represents...