I've seen several documentaries explaining that the diameter of the universe is currently estimated at over 90 billion light-years. And which that - in the face of the age of the universe being about 13.7 billion years - that doesn't contradict the "speed of light" limit, because in the very beginning "space - not matter - could expand faster than light".  What does it mean? All I can see is that we can actually visualize galaxies 13.5 billion light-years away from us in every direction (Isn't it?). Therefore 13.5 billion years ago they were up to 27 billion light-years apart from one another, having had "only" 0.2 billion years from big bang to reach those positions ... This is not "space" expanding, this is matter composing galaxies having travelled much faster than light! Moreover, they all keep saying that - in peering at galaxies 13.5 billion light-years away from us - we are close to looking at the "beginnings of the universe".  But if the Big Bang was 13.7 billion years ago, we (the matter now building up our planet) were together with them (the matter composing those far away galaxies) just 0.2 billion years before ... How can we see light coming from so far away from us, emitted by objects which were together with us "just a little while" before?
55938
I was reading through the Wikipedia article on  and came across the fact that the most distant Quasar is 29 Billion Light years. This is what the  exactly says      The highest redshift quasar known (as of June 2011[update]) is ULAS-J1120+0641, with a redshift of 7.085, which corresponds to a proper distance of approximately 29 billion light-years from Earth.   Now I come to understand that the  singularity is believed to be around 13.8 Billion years ago.  So how is this possible? Does the presence of such a quasar negate the Big Bang Theory?   I'm not a student of Physics and was reading this out of (whimsical) curiosity. Is there something I'm missing here or the "proper distance" mentioned in the fact is a concept that explains this?  Edit: My Bad! Here's how..  A simple google search led me to  which says the farthest quasar found is 12.9 billion LYs and not 29 billion. So in the end we have just proven that wikipedia needs more moderation.
109098
It would seem that measuring an age of the universe from the big bang requires separating spacetime into a 3D coordinate system and a time track.  I fail to understand why it is appropriate to take the reference of a comoving observer as being stationary to the cosmic microwave background when this seems to be a choice based upon our subsequent detection of that entity.
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?
12198
I'm a layman, but i watched some intereting videos about big bang on youtube[michio kaku, hawking this kind of things, not some crackpots :)]   I described everything on my picture:     So is there any possibility to see that inner side, for example, by creating wormhole, or entering new dimension? Or i get something wrong, on the way, and my model is totally wrong?  And another question. I heard that even if you would fly a ship across the universe you would eventually end in the place where you started, because we live in a 3d space that is defined on a surface of big sphere. But why a big sphere? Universe might be a big mobius strip, and the same principle of ending at start will apply. Is it possible?  Thanks in advance, and please don't bash me for silly questions. Im just curious, but i don't have any degree in physics. Im also not an english native speaker.
117772
The early universe was hot and opaque. Once it cooled enough, protons and electrons were able to form hydrogen atoms. This made the universe transparent, and was known as recombination. We can see the effects of this as the cosmic microwave background radiation.  How precisely can we date the recombination?  tells me that it was 377,000 years after the Big Bang, and that the universe is now 13.z billion years old. A little math tells me that it was 13,699,623,000 years ago. But how precisely can we measure that? Can we say it was 13,699,623,003 years ago? Can I get that in seconds?
43662
Many times I have read statements like, "the age of the universe is 14 billion years" . For example this wikipedia page .   Now, my question is, which observers' are these time intervals? According to whom 14 billion years?
58299
I was reading some earlier posts on this question in stack exchange. These posts give me the impression that the age of the universe is defined in a certain way following prescriptions such as that:   you should be co moving with the galaxy avoid strong gravitational fields choose the longest proper time recorded in all the frames   Defined this way we get the feeling that the age of the universe is not quite objective. Because we will get a different age if we change the definition. To highlight this arbitrariness, let us consider two space-time travellers A and B equipped with rockets and enough fuel who start at the same space point after setting their respective clocks to zero. Let us say that they started long time ago and took very different space-time paths and meet now and compare their clock readings T(A) and T(B). Each one can in principle claim that his clock showed the age of the universe. One may go a bit further and send a large number of travellers and collect all their clock readings at the end and declare the longest reading as the age of the universe. In this case we can say that the traveller who did not fire his rocket is the winner.  The implication in accepting this definition is that the time shown in this clock is the objective or absolute time of the universe while the times recorded in the other clocks are subjective. This way this definition will bring in a preferred reference frame and make the age of the universe an absolute time and not relative. In relativity theories we have not so far invoked preferred reference frames or absolute time concepts. Should the age of the universe be an exception?     I see that my post has some overlap with some earlier posts.However the important difference is that, as agreed by all who responded to my post,the earlier posts did not indicate that the age of the universe could be as arbitrary as the choice of a reference frame and no reference frame has any privileged status compared to the others.If any one wants to use a particular frame for some reason or another we can not question him.But one can not and should not object to the use of a different frame which computes a much younger universe, say an universe only 2000 years old !!! Lo and Behold! 2000 years can not be true as we know that we existed from much earlier days onwards. Hence the interesting question is: what may be a more realistic lower bound for the age of the universe?
86626
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)
43468
If the big bang was the birth of everything, and the big bang was an event in the sense that it had a location and a time (time 0), wouldn't that mean that our universe has a center?  Where was the big bang? What is there now? Are we moving away from the center still? Are these even valid questions?
50359
This is probably an obvious question, but I don't see it answered anywhere.  Imagine we have an object in the Universe that is traveling, relative to the Milkyway at about .99999999999999999999999999c and I realize that's not possible, but imagine.  Speed is relative, so to the object, the Milkyway is flying past it and to us, the object is flying past us.  That's all good and relative.  But the photons that our superfast object encounters are gonzo blue-shifted, and relativity says that's OK too.   But lets say the object is traveling so fast, the photons it encounters are energetic enough to spontaneously undergo pair production.  That seems difficult to explain.  A relatively low energy visible light photon to our perception won't split into an Electron and a Positron, but a very energetic photon can, but it's the same photon just observed by 2 different reference frames?    I understand that observers won't agree on when things happen or on energy but the two observers should agree on events that take place and pair production for one but not the other seems impossible.     Is there a simple layman's way that this can be explained?
50612
So, I'm traveling to another star 100 light years away in my spaceship. This ship has a solar sail pushed by a laser beamed from my home star system, so can achieve a velocity close to c. It's also got a robust parachute to slow down with.  I understand that if I measure light coming from my origin star, it will always still seem to be streaming past me at light speed (but will red shift as my speed increases). Light coming from my destination star also travels past me at light speed, and will become increasingly blue shifted as I gain speed.  I also understand that an observer checking on my speed at my origin or destination will always find it to be less than c.  However, will I perceive that in terms of the time it apparently takes me to reach my destination, my speed was greater than c? In other words, will it seem to take less than 100 years to reach the destination? 10 years on my watch, say. Or 1 year. Or a week?  Ie, as far as I'm concerned, while light keeps zipping past me at light speed, do I continue to accelerate unabated to an arbitrary apparent speed?  If not, how do I notice my continued acceleration being prevented?
15224
The size of our observable universe must have grown over time in the early universe.  Conversely with the accelerated expansion, I have heard that eventually our observable universe will shrink down to our local group of gravitationally bound galaxies.  So then the observable universe must go through a maximum at some point in time.  When is that and what is that maximum size?
120395
Ideally I like a single formula or multiple formulas for different time ranges that would cover the time from the end of inflation through 100+ billion years after the big bang using  the $\Lambda CDM$ Model.  I know that from the end of inflation back to the time of the big bang would be much more speculative, but some wild estimate would be appreciated for that time range also!  The Friedmann–Lemaître–Robertson–Walker metric defines the scale factor, a(t), from the metric: $$-c^2d\tau^2 = -c^2dt^2+a(t)^2d\Sigma^2$$ where $d\Sigma^2$ ranges over the 3 dimensional space of the universe and does not depend on time. Usually the scale of the scale factor is set by defining  $a(t_{now}) = a(13.78 B yr) \equiv 1$
5487
I'm in a freshmen level physics class now, so I don't know much, but something I heard today intrigued me. My TA was talking about how at the research facility he worked at, they were able to accelerate some certain particle to "99.99% the speed of light". I said why not 100%, and I didn't quite understand his explanation, but he said it wasn't possible. This confused me. Since the speed of light is a finite number, why can we go so close to its speed but not quite?  Edit: I read all the answers, and I think I'm sort of understanding it. Another silly question though: If we are getting this particle to 99.99% the speed of light by giving it some sort of finite acceleration, and increasing it more and more, why cant we increase it just a little more? Sorry I know this is a silly question. I totally accept the fact we cant reach 100%, but I'm just trying to break it down. If we've gotten so close by giving it larger and larger acceleration every time, why cant we just supply it with more acceleration? And how much of a difference is there between 99.99% the speed of light, and the speed of light? (I'm not really sure if "difference" is a good word to use, but hopefully you get what I'm asking).
91829
Layman here,  Stumbling through some physics stack posts and started reading the Wikipedia for the . In it, it states     The very earliest universe was so hot, or energetic, that initially no   matter particles existed or could exist except perhaps fleetingly, and   the forces we see around us today were believed to be merged into one   unified force. Space-time itself expanded during an inflationary epoch   due to the immensity of the energies involved. Gradually the immense   energies cooled – still to a temperature inconceivably hot compared to   any we see around us now, but sufficiently to allow forces to   gradually undergo symmetry breaking, a kind of repeated condensation   from one status quo to another, leading finally to the separation of   the strong force from the electroweak force and the first particles.   Where is the "immense energy" going to when it is "cooled"? Is there now no "immense energy"?
93499
I am near the end of the book  by Steven Weinberg. I am reading it in order to get a better picture of the early universe in the Big Bang model. But one thing I am having trouble with and wanted to get some clarity. Namely: Question What is meant by &quot;temperature&quot; if the  in the Big Bang model was mostly radiation, or pre-inflation, something more basic than radiation? Explanation Below I will try to explain why I am getting confused. Please let me know if it needs clarification (but that's also part of the question, I haven't been able to clearly understand temperature in this context yet). Since the  is about the movement of massive particles: $$T   =   {m\overline{v^2}\over 3 k_B}$$ How do you think of temperature when there is no mass, or when there is mostly radiation in the very early universe? Here is where I get confused.. Radiation is moving at the speed of light $c$, and since the speed of light is constant, and since radiation doesn't have mass, it seems like the universe would be &quot;maximally&quot; hot (particles moving at the speed of light). But intuitively that's not true because the temperature formula requires mass. But then if the early universe was mostly radiation, how does it have a temperature? If mass slowly increased to be the majority ~400,000 years after the Big Bang, it seems the temperature would increase rather than decrease. But then taking into account the expansion, it makes sense that even if more matter was forming, the temperature would decrease. But how could it decrease if there was less matter before? To summarize, in trying to pinpoint temperature, I seem to be stuck in cyclic reasoning and am looking for a better way of understanding it. I am working toward learning the purely mathematical form of these theories, but have a ways to go. For now, looking for a description to broadly make sense of &quot;temperature&quot; during the evolution of the few moments after the Big Bang. Examples of how &quot;temperature&quot; is used In the book above, Steven Weinberg talks a lot about the temperature and radiation of the early universe, and how the early universe was dominated by photons/radiation, and ~400,000 years later became dominated by mass-particles when the temperature cooled. Some other quotes from the web include:  In the first moments after the big bang, the universe was extremely hot and dense. As the universe cooled, conditions became just right to give rise to the building blocks of matter – the quarks and electrons of which we are all made. () ...the temperature was so high that the four fundamental forces ... were one fundamental force. () ...the temperature of the universe was still too high to allow quarks to bind together to form hadrons. ()
68184
I understand that the closer something travels to the speed of light, that time will stretch by a factor, and distance will compress by the same factor.  My question is, if something travels in a circle, close to the speed of light, what does the distance of the journey look like to them? They measure that the trip took them 10 minutes. And an outside observer says that the journey took 20 minutes, and the outside observer measured that they did, for example, 1000 laps of a circle circumference 1000 km.  So if the plane had a distance trip counter, what would it read? And if they were looking out of the window, would the circle still look like it had a circumference of 1000km?