Universe Definition and 1000 Threads

do we know for a fact that the gravitational constant has always been the same since the dawn of the universe? I feel like gravitational forces should slowly be decreasing as the universe ages (meaning that assuming mass of Earth doesn't change, we should weigh a bit less in a billion years)...

Now two points here. (1) Most Cyclic Universe theories I've heard require the universe to reverse course and fall back into a Big Crunch to recycle again. Now that Dark Energy has been discovered, the chances of a Big Crunch have gone away. (2) Some theories suggest that the universe started...

for the matter+lambda universe why is Ω[Λ,o]=1-Ω[m,0]?

Hi all I read a question on this subject from some time ago but was not satisfied with the clarity of the answer and in the light of recent experimental results I wish to clarify my understanding of this ancient scientific question. What is the cause of Inertia? So is inertia simply the total...

Are there any indications on how large the universe might be? Not the observable universe, but the universe which came into being 13.8 billion years ago and of which our observable universe is a part?

It is said that the imbalance of matter versus antimatter in the present universe implies CP violations at very high energy. It seems to me that it most directly implies baryon number nonconservation: If we assume (and I'm not exactly sure why this is a necessary assumption) that immediately...

The article Spontaneous creation of the universe from nothing (Dongshan He, Dongfeng Gao, and Qing-yu Cai) published by the American Physical Society discusses a mathematically proof that the universe could be spontaneously created from nothing using the Wheeler-DeWitt equation (pictured below)...

Hi all, Yet another question: if the universe is finite, then linear momentum should be quantized (I assume in a similar manner to an infinite potential well since there are boundary conditions). My question is, then, if one computes a value for ##\Delta p## (momentum variance), is the variance...

My question has multiple parts, does the following paper: https://arxiv.org/abs/1405.0298 Disprove the possibility of creation of another universe through quantum fluctuations if heat death occurs? If it does could another universe still be created through quantum tunneling? Can quantum...

Does it?

Greetings! I'm new here and I think about this place as soon as I see what the statement asks. Homework Statement Considering the volumetric density ρv=(e-2r/r2), figure the total charge (ℚ) of the universe. Homework Equations [/B] ρv=ΔQ/ΔV -> (ΔQ ∝ ΔV) ℚ=∫v ρv dxdydz The Attempt at a...

I've seen numerous descriptions of inflationary phase a-la "The universe was supercooled from about 10^27 down to 10^22 kelvins". However, I do not understand what is the basis for such estimation. We do not even know with any certainty how long the inflationary phase lasted. For one, "eternal...

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...

This may sound like a noob question, even more if it's asked by a undergraduate physics student, but here it's: How do we know about the existence of 100 billion galaxies out there (even that it's an average value)? I mean, how do we know about these exact number? Why not, say, 200 billion or...

Is there any method/model for determining simultaneity of events farther apart than light can ever travel? I'm guessing not. Would it be wrong to say that the non-observable universe exists only in our past (or future)?

I am reading The Essential Cosmic Perspective and there says "We cannot observe light coming from anything more then 14 billion-light-years away". ( In my opinion this statement is wrong cause observable universe diameter is 46.5 billion light years. I guess authors meant something else, or...

One thing that has always puzzled me is that the farther we look out into the universe we see that galaxies are moving away from us faster than galaxies that are closer to us, but also the farther we look out into space the farther back in time we are observing. I have trouble understanding...

If the universe was all created simultaneously in the Big Bang then why is it that all matter does not move at the same rate in relation to all surrounding matter away from the singularity causing all matter to basically form a massive increasingly expanding sphere with some significant width...

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...

Is our concept of scale, of micro and macro, of quantum and cosmic, relative to our perception as humans? If General Relativity and Quantum Physics both yield truth, then perhaps it is an error in human cognition that seems to perceive a disconnect between the two. The universe is just the...

I was wondering if we are only aware of the observable universe, and the actual universe could be infinite, what do we mean when we say the age of the universe is 14 billion years? Are we talking about the age of only the observable universe or the whole universe?

I just watched a presentation about the acceleration of the Universe by Perlmutter. At this moment in the video he points the correct line for the scale factor of the Universe. Aaaaand that lines starts around 15 billion years ago, not 14 billion. So I thought maybe the latest estimations of...

Is there any comparison between the characteristics and the state of the QGP in collider experiments and in the early universe? Can one compare properties like temperature, pressure, specific heat, viscosity etc. using standard or non-equilibrium thermodynamics? Can one compare the...

Could anyone please guide me how to do these problems?

My creationist grandpa says that we are at the center of the universe and that other galaxies are in spherical shells around us and claim that this means we are in a special place in the universe. Basically he believes in the "quantized redshifts" thing you may or may not have heard of, which...

This question on quora discusses whether an infinite universe would allow for a repetition of patterns and arrangements of matter. The top answer gives a very convincing argument as to why this would be impossible because, he reasons, that spacetime is causally connected. However if the universe...

Say that they are not entangled. Assume classical quantum laws still hold but that there are only two particles in this universe with nothing else. How would one of the particles measure the other? Both particle's location and momentum are nonexistent until a measurement occurs by one on the...

Since in the beginning of the universe all matter was concentrated in a ridiculously small space, how didn't the whole universe just ended up as a huge black hole right after the gravitational force appeared? Only solution I can think of is that space itself was expanding faster than matter...

bapowell submitted a new PF Insights post A Poor Man's CMB Primer. Part 5: Quantum Seeds Continue reading the Original PF Insights Post.

Yes I know I’ll/we’ll never fully understand the universe, but I want to have a better understanding of it than I do now. So what thing(s) should I study? I’ve been reading about things such as general relativity, black holes, subatomic particles and the Big Bang, but what else should I be...

Consider two bodies A &B are moving apart with a velocity V due to the expansion of space. According to an observer in A the body B is moving away and an observer in B feels the body A is moving apart. Can some one answer in which body the time dilates and why?. ( I am specifying once again that...

The ubiquity of the big bang holds no matter how big the universe is or even whether it is finite or infinite in size. How are we so confident that the part of the universe that is unobservable also falls under the Big Bang model?

In the early stages of the universe, before the formation of the first particles, did mass exist? If yes, what had mass? If no, can one say that the early universe was massless?

I've been trying to understand how we know that the observable universe is flat, and I'm having difficulty finding any sources that explain exactly how the calculations were done. On this WMAP website (https://map.gsfc.nasa.gov/mission/sgoals_parameters_geom.html), it says: "A central feature of...

Since time and distance are relative, it would seem that the universe's age (and shape, too) depends on the measurer's frame of reference? Like while for us the universe is 14 billion years old, to an observer in a black hole it might be only a few seconds old (and small). What frame of...

True or False? A clear answer would clear up years of confusion. Thank you. Edit: I mean observable universe, obviously. I always thought the Big Bang model somehow also accounts for the part of the universe beyond the observable limit, since it is always advertised as the beginning of "the"...

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...

Einstein said, "The distinction between past, present and future is only an illusion, even if a stubborn one." That's my problem--that illusion is so vivid and stubborn. Paul Davies said in his book "About Time": There is only one rational conclusion to draw from the relative nature of...

It's here so the dark matter "being" can feed on dark energy and grow up. When they achieve a more advanced state they move on to create their own universes, where their offspring can grow up. So far they've ignored life around the "hot spots", the climate there is too uncomfortable for them...

I am trying to understand how the beginning of the universe unfolded. I've tried to read up on it, but some specific questions never seem to be addressed. What is the stuff that is in the beginning of the universe? I commonly read that it's just "energy," but what is energy in this context...

I know that early oscillating models of the universe fail due to the second law of thermodynamics. One thing that I am unclear about is since as far as i know the laws of physics break down in a singularity can the second law of thermodynamics break down also? When I see comments to the...

Dear community, I stumbled upon this ecology article (https://www.witpress.com/elibrary/dne/4/2/402, page 4) and have some confusion about a statement in there: "Before further unpacking the formal defnition of entropy, one would be justifed in asking why not simply choose (1 – p) instead of...

I can not claim that I fully understand my own question, but I feel it is worth sharing. Suppose there is a physical system composed of MxN degree of freedom where M is the number of the states and N the number of particles. When a measurement is undertaken, the physical system collapses to...

Hello! It is my first question on this site! So I have some problems: 1. Is it possible to get something from absolute nothing? 2. I saw theory about universe from "nothing" but was it really absolute nothing? Is that possible? I know about quantum fluctuations and virtual particles but it is...

It is been said that the universe is expanding but the question is,where will the universe end? Are we the only universe?

Neutrinos were flavor eigenstates at the time of their decoupling from baryonic matter. Since they were not pure mass eigenstates, how do you take this fact into account if you try to study how they evolved as the universe expanded? Could we determine if the heaviest neutrino could be non...

I often find that in books or lectures discussing the arrow of time and entropy and trying to explain how we have such complexity that the explanation is that when the universe was in it's early stage and was just a relatively small ball of apparently high entropy gas that it was in fact very...

Let me preface this post by saying that I only have a very cursory understanding of general relativity. I happen to know that if we assume the cosmological principle, then the hypersurface ##\Sigma_t## of the spacetime manifold ##M##, for any positive ##t##, is either a 3-sphere, a...

If I've understood correct, every particle in the universe exists in a waveform, which I've understood to mean propability where the particle does exist? Then which I've understood is, that the wave transforms into a particle only when observed, is this correct? Then if it is correct, how...

By using particle physics natural units with ##\hbar=c=1## so that Planck's length ##l_P=(8\pi G)^{1/2}## we can express Einstein's field equations as $$G_{\mu\nu}=l_P^2\ T_{\mu\nu},$$ where ##G_{\mu\nu}## has dimension ##[\hbox{proper length}]^{-2}##, ##l_P## has dimension ##[\hbox{proper...