Universe Definition and 1000 Threads

The Universe is constantly expanding, and from the observation of Hubble, super novas, the figure turns out to be a acceleration. The theory that's most accepted seems to be dark energy at work. As the story goes, before big bang, there was nothing, maybe not even time or space, so how does...

I have very little understanding of the Big Bang, but it seems like it would require a finite universe even though there seems to be a scientific consensus that an infinite universe is a strong possibility. How are these ideas compatible? If space started expanding from a small point at a finite...

Space is expanding, which separates objects from each other in space. Won't this add potential energy? Doesn't this violate the law of conservation of energy? But if it does not, how can space be added without adding potential energy?

I understand that Gott derived a formula for calculating the size of the observable universe, and the value of the diameter based on current obsrvations is 93 Gly. Can someone please show the mathematical derivation of Gott's formula, or give a reference to a source which shows this derivation?

How the scientists are determining that a galaxy exist & many galaxies also there. How they find the universe? without seeing with our eyes, a human being can never ever judge the things. but in the existence of other galaxies & universe how they came to conclusion by the telescopes & advanced...

Dear Friends! Probably more than a decade back understanding tells us : If the average matter density of the universe happens to be smaller than a critical value,then the galaxies will never come to a halt and turn back(Open Universe).Which will imply cold and lonely death. If on the...

If I imagine an interplanetary fight between two early races (the universe is about 8 billion years old) and one of them invents a super weapon to extinct the other. The weapon is a grey goo weapon, a swarm of billions of machines, each with the ability to bend light around it to cloak itself...

How does the universe so conspire such that ΔS≥0, entropy increases?

I saw a video by a physics writer who said that the beginning of the universe was like digging a hole: the hole and the pile of dirt cancel each other out. This of course means that the ground, so to speak, before the digging was zero. But where is the "hole" once the universe began? I asked...

Homework Statement Can we consider the universe to have a uniformly charged distribution? If so, shouldn't the field at any point in space be zero? Since the universe is infinite, will it be symmetrical about any point, field should be zero right? Why is this not true?[/B]2. The attempt at a...

I know that space is expanding, so the further away you go from my location, the faster space is expanding, asymptomatically approaching the speed of light. I also know that as relative velocities approach the speed of light, the length of space contracts. From this I come up with a limit for...

What are we going to do about it? arXiv:1508.00631 [pdf, other] The fate of a Universe driven by a linear potential Ricardo Z. Ferreira, Pedro P. Avelino Comments: 6 pages, 3 figures Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); General Relativity and Quantum Cosmology...

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...

Forgive my ignorance?.. If we can see 13.8-ish billion light years away how can the universe be the same age? Matter cannot travel at the speed of light, so how are we as far away (in light years) as the universe is old?

This is probably a silly question, but it's summertime and I can't ask my professor. If the universe turned out to be closed (even in light of WMAP's findings), would it be possible to consider the entire universe a closed system?

Dear PF Forum Can I ask something quoted from a closed thread? In a closed thread: https://www.physicsforums.com/threads/predicted-heat-death-of-the-universe.824652/ This quote was presented by a Brian Cox. A scientist? And if he is, I'd like to ask this question. Is it technically possible...

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.

In this documentary they discussed some research experiments which concluded that the universe is infinite. I didn't really understand it. Can someone explain how we know that the universe is infinite? Wouldn't this also mean that the universe was infinite at the big bang?

I watched a BBC documentary that said that the observable universe is about 46 billion light years in size. How can this be if the age of the universe is 13.7 billion years (and nothing travels faster than the speed of light)?

So as time goes on there are more and more interactions in the world. That means that there are more wavefunction collapses going on. Some think that there is a universal wavefunction that is guiding all the individual wavefunctions. So I have to wonder if all these smaller, individual...

I accept that what I write below will be unpopular and argued with vehemently, however I think it should be considered with an open mind and recognised for the sense that it has. Originally the concept of an expanding universe (and therefore by extrapolation the concept of the Big Bang that...

From what I understand, our Hubble's sphere is just relative to Earth and has a diameter of 93 billion light years putting the edge of the observable universe at 46-47 billion light years away. So every object in space will essentially have it's own Hubble's sphere and objects near the edge of...

Predicted Heat Death of the Universe In his thought provoking documentary “Wonder of the universe” ( ) Professor Brian Cox explains the thinking behind current predictions for the “Heat death of the universe”. He explains the predictions through the effect of the second law of thermodynamics...

Hi all, I was recently watching one of Susskind's 'Theoretical Minimum' lectures in which he says that the entropy of the universe may be measured via the number of observable photons, and that somehow these quantities (photon number and total entropy) are somehow linked. Could anybody with...

In wikipedia says Physical baryon density: ##Ω_bh^2=0.02230±0.00014## and Physical dark matter density:##Ω_ch^2=0.1188±0.0010## Matter density:##Ω_m=0.3089±0.0062## so If we collect baryonic matter density and dark matter density we...

Is the U curved or are there new physics in play here. arXiv:1507.04684 (cross-list from gr-qc) [pdf, other] Consistency of non-flat $Λ$CDM model with the new result from BOSS Suresh Kumar Comments: 5 pages, 1 figure Subjects: General Relativity and Quantum Cosmology (gr-qc); Cosmology and...

Is Dark matter homogenenius in Universe ? I don't think so but I don't know any idea about it. Thank you

Is every combination possible based off of uncertainty? When the big bang happens in another universe and one electron moves slightly differently because of uncertainty it changes everything. Is that why there are "infinite" number of universes? When people say infinite do they really mean...

If you were to extend Born's Rule to sum or integrate over all points in space-time, would you necessarily get only either zero or one? Otherwise put, is it true that "anything that can happen will happen"?

Hi all. I am curious as to why quark stars have not dominated by now, given their inability to decay as they are a mass of fundamental particles. What prevents this?

I am trying to understand some things but I seem to be confused a little... I was watching a video and it said when you look with hubbles telescope you see the past universe but I seem to have trouble understanding this, does that mean we could see the future if we knew which direction to point...

Hello, I think it was Kurt Gödel who asked this while taking about time. The age of the universe and its size, is whatever it is for us because we are moving at a specific speed relative to light? If that is true, then a consciousness living on a planet moving at triple the speed of Earth...

An interesting discussion at this conference here: http://physics.princeton.edu/cmb50/videos/20150612_session6_2.mp4 David Spergel said the next project should be to look for something called f and L or is is FNL? Can someone explain what this is and what its significance is? What did you guys...

I read in a book that there is no center of the universe because for example, you put dots around a balloon, and when you put air into the ballon, the dots will separate from each other, and each dot will see themselves as the center of the balloon, depending on the dots' perspective. That only...

This paper; http://arxiv.org/abs/1506.08573, First law of thermodynamics for dynamical apparent horizons and the entropy of Friedmann universes, offers a themodynamic evaluation of the apparent horizon of the observable universe. While not for the mathematically faint of heart, it is interesting...

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...

Hey all, I was reading a webpage which I found rather confusing: http://www.dailygalaxy.com/my_weblog/2015/06/todays-galaxy-insight-the-lonely-galaxies.html The problem I found with this is that the guy (who is a famous theoretical physicist) is stating that because the objects will be traveling...

I understand the concept of galaxies moving away from us at faster than the speed of light so that they lie outside of a "cosmic horizon" which we cannot see past. This would limit our observations to a "visible" universe which may be far smaller than what the "actual" universe may be. Alan...

I stumbled across this app for iPhone which may appeal to those favoring the many-worlds interpretation of QM: http://aerfish.com/UniverseSplitter/index.html I thought it was cool that an app can actually run a quantum experiment each time you use it. I saw mention of it on a Sean Carroll...

The Size and Age of Our Universe If the universe is 14 billion years old, (roughly) then we can only see out in any direction for 14 billion light years. Thus any stars beyond 14 billion light years are invisible to us because their light hasn’t had time to reach us. If we cannot find an edge...

I understand that the force of gravity prevents galaxies from expanding, as space increases. However, I question if universal expansion separates photons (electromagnetism), as they are traveling along parallel paths (Would the normal distance between them increase over time)? Thank you.

The entire observable universe is contained rather graciously in this wonderful picture, for your admiration.

Is it true that the universe itself expands faster than c = 3 × 10^8 m/s at some places / areas / points. I have heard it but wasn't able to confirm. Because outside of the the universe, there ain't anything as far as we understand and hence the universe should not have to follow any law as...

Hello! My name is Brenna Bruce and I am 15 years old. This past year I have discovered my thirst for knowledge about the universe. How it works and what makes it work (to my understanding is explained by mostly physics) are concepts I want to be able to grasp. My question for anyone reading this...

Hello All :) I've heard it proposed that a reason for the fine tuned nature of our universe in a way to allow the existence of complexity and life could be that the universe itself is the product of evolution in a multi-verse. I don't want to debate around the anthropological principle as I am...

If we suppose universe is infinite than there will be no parallel universes.So I know that If ##Ω_k##→Universe will be infinite.Then there will be no parallel universes cause there's one universe. I am confused.Is that mean the parallel universe idea is wrong ?

I was searching a cosmology calculator and In one calculator I saw this equation. ##1=Ω_k+Ω_m+Ω_Λ## is that true true for all type universe's (open,flat,closed) ? and is here ##Ω_k={-k\over H^2_0 /a^2_0}## isn't it. (And If you can give me an article about this issue I will be...

Hello, I just recently found out that one could find the Friedman's equation in Newton's approximation (without GR) by assuming that the universe in homogeneous and isotropic simply by using F=ma and the conservation of energy. On can then find that the scale factor goes as t^2/3, as expected...

apologies if this has been asked before: I'm trying to understand the expansion of the universe and i was wondering. . . . as the universe expands, are new points of space (or is it spacetime) being created? if the answer is 'yes', what are the ramifications of this? my understanding...

I am looking for reliable information about the functional dependence of the diameter ##d(t)## of the visible universe on the time ##t## since the big bang singularity, based on the different hypotheses currently deemed competitive.