Experimentally, the universe keeps expanding. I do not understand well what is meant by expanding universe.   If we assume an inertial reference frame for which space is defined and if in this reference frame we define a proper length as the distance between 2 atoms in the $O_2$ molecule(both atoms belong to the same inertial reference frame). My question is :   Since the universe is expanding, is that distance going to increase with universe expansion or could that distance be used to measure that universe has indeed expanded?   I feel there is some picture that I have got wrong.
2038
I have looked at other questions on this site (e.g. "why does space expansion affect matter") but can't find the answer I am looking for.  So here is my question: One often hears talk of space expanding when we talk about the speed of galaxies relative to ours. Why, if space is expanding, does matter not also expand? If a circle is drawn on balloon (2d plane), and the balloon expands, then the circle also expands. If matter is an object with 3 spatial dimensions, then when those 3 dimensions expand, so should the object.  If that was the case, we wouldn't see the universe as expanding at all, because we would be expanding (spatially) with it.  I have a few potential answers for this, which raise their own problems:   Fundamental particles are 'point sized' objects. They cannot expand because they do not have spatial dimension to begin with. The problem with this is that while the particles would not expand, the space between them would, leading to a point where the 3 non-gravity forces would no longer hold matter together due to distance Fundamental particles are curled up in additional dimensions a la string theory. These dimensions are not expanding. Same problems as 1, with the added problem of being a bit unsatisfying. The answer seems to be (from Marek in the previous question) that the gravitational force is so much weaker than the other forces that large (macro) objects move apart, but small (micro) objects stay together. However, this simple explanation seems to imply that expansion of space is a 'force' that can be overcome by a greater one. That doesn't sound right to me.
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Added: 5 times down vote for now! Down voter is this religion or physics, please try to explain your decision.  I'm confused about this.  In physics we know for a , but I think that there is a contradiction in this term. The  are the phenomena that  contradicts the vacuum existence because, according to them, the vacuum isn't the empty space.  In vacuum the creation of particle-antiparticle pairs is allowed for small times and this is also proven in practice.  From the other side, the relativistic theory says about space-time that interactions like gravitation bend the vacuum (empty space). There seems to be a contradiction to me. If vacuum is an empty space then we can't bend it, because we can't bend something nonexistent. In other words: we can't bend 'nothing'.  Can empty space really exist in physics?  EDIT1: quote: Luboš Motl     "By definition, the space without   energy is the space whose total value   of energy is equal to 0."   But this space is nonexistent, so it is abstract and should exist only in our mind... Why is so? My second statement of this post says that in physics we cant bend nothing because such a bending is only thinking and not physics!  Another possibility is that space is unknown kind of energy, but this is contradiction in modern physics!  EDIT2:  Can any physics believe that nothing exists? By mathematical logic no! And mathematic is elementary tool in physics. Anything other than that is religion!  Nothing (empty space without energy) is only a logical state!  EDIT3: quote: Roy Simpson:     The General Relativity Vacuum is a   space-time model region without   matter.   and Luboš Motl says: "By definition, the space without energy is the space whose total value of energy is equal to 0."  Agree...   But this is only mathematical Euclidean space + time so this is only a mathematic and not physics! With other words: this is only a method of mathematical mapping. But, in real (not theoretic) physics we can't mapping empty things. Empty is only a logical state!  EDIT4:   Roy Simpsons argumentation seems to me acceptable.   quote: Roy Simpson:     Einstein struggled with this too, and   the problem has come to be known as   the "Hole argument" within GR. You   have to decide whether you are just   interested in GR's vacuum (empty   space) or the full physical vacuum   which includes quantum aspects as   well.   Thanks
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Is the spacetime continuous or discrete? Or better, is the 4-dimensional spacetime of general-relativity discrete or continuous? What if we consider additional dimensions like string theory hypothesizes? Are those compact additional dimensions discrete or continuous?   Are there experimental evidences of continuity/discreteness?  When particles move inside space do they occupy spacetime by little chunks? What would imply if spacetime is discrete on continuous theories?  I've found little information on the web and books.  Probably my question is ill-posed and I apologize for this.
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I have read that the night sky should have been bright because every spot should end up pointing to a star in the infinite universe but this is not the case because the universe is expanding. I am thinking if the universe cannot be expanding with speed faster than light then the dark night sky is also due to the universe is relatively young and when it gets older the night sky will eventually end up bright.   Is this correct or am I missing something?  Will the speed of universe expansion will make the sky bright but the red shift make it invisible to our eyes ?  Also if it's correct, does this mean that cosmologists do detect new stars appearing from nowhere from time to time? Does this really happen?
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What are the mechanics of time dilation and length contraction? Going beyond the mathematical equations involving light and the "speed limit of the universe", what is observed is merely a phenomenon and not a true explanation of why time dilates or length contracts.  It has been proven to work out, but do we know why?  Is it something that happens at a subatomic level?
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It would seem that LIGO measures wibbles in the metric (not manifold) of spacetime:    It would seem that the expansion of the universe is an expansion of the metric of spacetime:    Imagine for a moment that LIGO is not an interferometer. (So, it just plain times speed changes, rather than using phase shift of orthogonal directions.)  If the ends of one of the arms was indeed receding away from each other, at a speed consistent with the expansion of the universe, is the sensitivity of real-world LIGO of the needed sensitivity of a machine which could measure that shift?  On other words: ideally I'd like to know these two meters:  (A) The left arm of LIGO is about 4km. It was stretched/shrunk (a few times) for roughly .01 seconds by the gravitational wave.  How many meters was it stretched/shrunk in .01 seconds?  (B) Assuming the same abstract LIGO arm was affectable and affected by the expansion of the Universe. How many meters is it stretch/shrunk every .01 seconds?    Note - of course, an interferometer is an ingenious device to measure extremely small changes in speed - assuming the changes are orthogonal. Of course, an interferometer, per se, can't at all measure the expansion of the universe since that is uniform in both directions. The sense of my question is, can something that measures distance changes > as accurately as &lt; the LIGO does, measure the expansion of the universe?  How big or small is the ongoing expansion of the universe compared to the wibble from the black hole system in question?
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What is the exact shape of the universe? I know of the balloon analogy, and the bread with raisins in it. These clarify some points, like how the universe can have no centre, and how it can expand equally everywhere in all directions. But they also raise some questions, like if you are on the surface of a balloon and travel in 1 direction, you will eventually return to your starting point, is it possible our universe have this feature?  If it has, or had, would this be a symmetry of sorts ($\psi(x)=\psi(x+R)$), and as such have a conserved quantity associated with it (by Noether)? Assuming "small curled up dimensions" wouldn't these dimensions have this type of symmetry, what are the associated conserved quantities? Is it known exactly what the geometrical shape of the universe is? (on a large scale) (I am not talking about only the observable universe). How does one define the "size" of a dimension, is this scale only applicable to curled up ones? Is it possible to describe to a layman the shape of the universe without resorting to inept analogies?
31447
just after the Big Bang and before inflation the currently observable universe was the size of a coin. One millionth of a second later the universe was the size of the Solar System, which is an expansion much much faster than speed of light. Can space expand with unlimited speed?
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There is something I don't understand about the  $H$, as it seems to clump two concepts together that I can't quite unify in my head. On the one side, we have  $$V = D H$$  which means that for a given distance $D$, there is a certain amount of new space created over time - and $H$ is simply the factor that makes this relationship work. So, for example, say we have two points 1 Mpc apart, this would mean they recede at about 70 km/s away from each other  (given our current approximation of $H$).  Now the thing I can't wrap my head around is that   $$T =\frac{1}{ H}$$  is also the age of the universe. Contrary to claims made on, say, Wikipedia this means that $H$ cannot possibly have been a constant throughout the past 13 billion years, because mathematically $1/H$ means that $H$ must be continually shrinking as the universe ages.  So if $H$ did start out as some huge value and is now shrinking over time, doesn't this mean the expansion of the universe is slowing down? Because if $H$ is shrinking, I'll get a lower value of $V$ today than I'll get tomorrow. Shouldn't the notation then be more like  $$V = DH(t)$$  So which one is it? If $1/H$ is simply the solution for $D=0$, how can we use it as the expansion-velocity-per-unit-of-distance at the same time? What's worse, how can literature say $H$ has probably been more or less constant forever and simultaneously assert that $1/H$ is the current age of the universe? What am I missing?
116784
Physicists have estimated the universe to be flat and infinite, but I can't seem to understand what this actually means. First of all, what do they mean when they say that the universe is infinite,  do they mean that space is unbounded? Or do they mean that the space is currently bounded and is expanding forever (so the bounds are getting further apart) ? And if space is unbounded, how does it expand, how can infinity get larger?  Finally a flat universe could also be bounded if for example it has a toric shape which has zero curvature, could this be a possible shape of the universe?    What also confuses me is that, since the beginning of time, the universe had a very small size, but how did it become infinitely large in a finite amount of time?
71963
Regarding this thread:    Why is the idea that the total energy in the universe "zero" so popular (re: Laurence Krauss) and why is the flatness of the universe used to back this up when, according to that post, an open universe would not conserve energy so the total energy of the universe cannot be "zero", can it? What's the energy being defined as zero and why is that energy used to predict things about the universe when "the" energy (as the term is used in that post) is not zero?  Additionally, I found one very good explanation here for this,    This seems to indicate that in flat space (the sort of space used in these zero energy universe theories) the mass can NOT be positive thus contradicting the notion that mass is positive and gravity negative and the whole thing winds up being zero. Did I interpret that correctly?  The MO question is cross-posted to Physics.SE here,
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From watching cosmology lectures, it seems that the space between galaxies is expanding at an accelerating rate, my question is since it is the space that is (acceleratingly  expanding), the special relativity does not apply in this case? in other words since it is not anything that is accelerating with relative distance, would at some point galaxies that are far enough from each other separate faster than speed of light?  Please note, I am not asking if the galaxies would move faster than speed of light, but whether the rate of expansion of space be faster than speed of light so in effect each galaxy would be as if it is behind an event horizon where even light can not escape?    If yes, is there a name for such an event horizon that is cause by expanding space outside rather than the conventional black hole event horizon caused by stretching of space within the event horizon?   How would one be able to differentiate between a black hole even horizon and a galactic even horizon caused by accelerating expansion of space outside? (not sure if the concepts of inside and outside are still meaningful in such cases)
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Could someone judge my (stoner) hypothesis that the speed of light has changed over time -- i.e. as the universe has expanded in volume light has slowed down, perhaps going so far as back to the big bang when it was infinitely fast and there was no time because everything happened at once etc. Thinking that the speed at which information can propagate through the universe is linked to the size of it seems intuitive to me. My question -- is there an easy disproof of this? Would Einstein have to be wrong? Does it violate anything supposedly more fundamental such as quantum or string theories? Do any current experiments invalidate it? If not can you show me in any case why you think its unlikely.   Edit 8/24  I'm accepting Mark M.'s answer but will post this here because there is a character limit on comments  @Mark M thanks, good answer but as someone who has only read some popular physics, and should leave this to the experts, I'm still muddled in my personal theory. I don't see why you should need two units to measure the speed of light. The thing I have a hard time wrapping my head around is the relation of time and distance. They seem like they could fundamentally be the same thing. If you say time is measured fundamentally by the vibration of so and so quantum object in space... why can't we just measure that vibration distance as the constant... I'll repeat myself to try to be clear... there is a certain minimum distance that particles have to go to interact with each other... if it wasn't vibrating there wouldn't be time,its what creates the illusion of time...so instead of talking about speed or c as distance/time.... can't we simply talk about that distance a quantum object vibrates....I'll lead up to my point.....perhaps there can need be only one constant here, and that is the physical size of the universe. a tiny metal tuning fork doesn't appear to be vibrating at all but if you blew it up to the size of the empire state building the metal rods would move from window to window. perhaps as our universe expanded in size the length of that minimum vibration (perhaps infinite at point zero) would have expanded, and therefore created the illusion of time and the speed of light, which, as the universe expands, will continue slowing down. perhaps we are like a big balloon and we have been blown up and all the fields/particles-without-size are vibrating more and more in that space.. am i missing something obvious here?
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Conceivably it expands with the speed of light. I do not know, but curious, if there is an answer. At what velocity, does our universe expand?
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Ok, from astronomical observations we can tell that the observable matter is separating - so rewind the clock about 13.7 billion years and it was all at a single point.  However, how do we distinguish between the following two options:     Universe is expanding   Matter distribution is increasing into infinite void   Clarification 1: (My notion of) The traditional notion is that all time/space/matter was created at the instant of the big bang.  I.e. BB was inital conditions of: $t=0$, $V=0$, $E=very big$  People say "the universe is expanding", rather than "observable matter is separating".  Why is this?  How do we know that the big bang event wasn't started by all matter condensed at a single point within a larger (otherwise empty) universe?  How do we know that BB wasn't: $t=0$, $V(universe)&gt;0$ but $V(matter)=0$, $E=very big$?
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Lets say that there are only two bodies in the universe, 65 kg each. Other than that the universe is completely empty, no neutrons, no photons, no dark energy/matter, not even neutrinos (that is to make things less complicated. If the loss of other things leads to something like the universe exploding like a bubble at the speed of light or something, you can change these parameters. I'm mainly concerned about gravity here). Those two bodies are placed apart from each other at the distance of the observable universe. Will they start moving into each other? Will they collide? (Optional question: If so, with how much speed will they collide?)
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Imagine yourself and anything you're able to perceive to be drawn onto a rubber band. Any means of determining distances would be affected by the stretching/contracting of the rubber band. The relation between any two lengths would remain exactly the same because every length is increased/decreased by the same factor as the band stretches/contracts. According to my understanding, that is what the dilation/compression of space is like for us and why it should not lead to any change in distance.  This also applies to relational measurements of length as they are performed, for example, by devices like LIGO. Although the deformation (e.g. gravitational waves) propagates only in one of two directions of measurement, the relation between both lengths should not be affected because none of the individual lengths should be affected.  Accordingly, anything that travels the four kilometers in LIGO should take the exact same amount of time, no matter whether space is currently stretched or compressed. The speed of light being constant should be irrelevant in this this case.  If gravitational waves stretch space itself, how can they have an effect on the interference of two laser beams? According to my understanding, compression or dilation of space itself (as opposed to something in space) should not affect the distance between two points.  Disclaimer: Obviously, the above is not true. It's just how I thought is was. Hopefully, it helps to see where my understanding went awry.  ————  In this question (  ), the problem I had has actually been solved. It comes down to the fact that atoms (because of the non-gravitational forces holding them together) are in fact not affected by spacetime deformations. This makes these deformations detectable.