The currently detectable interval of black hole mergers is on the order of seconds or fractions of seconds. So each black hole merger, no matter how long the end to end process, is only detectable by LIGO for this tiny interval of time. How often are such intervals thought to happen in the observable unverse?
24119
My question is a more detailed version of the one found , which elicited some good information but the question was never really answered.   From table 4 in a  we see the estimated rate of BH-BH mergers ranged from $10^{-4}$ to $0.3 \rm \,Mpc^{-3} Myr^{-1}$. This corresponds to $10^{-10}$ to $3\times10^{-7} \rm\,Mpc^{-3} yr^{-1}$. They write:      For BH–BH inspirals, horizon distances of ...2187 Mpc are assumed. These distances correspond to a choice of ... 10M for BH mass.    In the left panel of figure 4 found in  there is a plot of BH mass vs horizon distance. For 2015-2016 sensitivity levels and 10M mass, the horizon distance is 300 Mpc. So we need to multiply the above numbers by $300^3=2.7\times10^{7}$. This gives a range of $2.7\times10^{-3}$ to $8.1$ detectable mergers per year, given the prior assumptions that a typical signal would come from black holes with ~10 solar masses. Also, in the  we find:      We present the analysis of 16 days of coincident observations between the two LIGO detectors from September 12 to October 20, 2015.    Therefore the duration of the experiment was $16/365$ years. This gives an expected rate ranging from $1.18\times10^{-4}$ to $3.55\times10^{-1}$ mergers during the experiment. As far as I can tell, this can be taken as an upper bound, since not all of these events will really be detected (due to non-optimal location, coincident noise, etc).   Are there other factors to take into account?
23380
Possible Duplicate:          Is black hole formation observable for a distant observer in finite amount of time? Specifically, let's take uniform non rotating distribution of 10 solar masses within solar radius, assume no outward pressure, and calculate its gravitational collapse. I assume distant observer would never witness complete formation of black hole with event horizon corresponding to entire 10 solar masses. What would be observable in finite amount of time, then? For example, would there be a black hole with smaller radius, while the rest of mass is still falling into its event horizon?
70107
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 $V(matter)=0$, $E=very big$?
109053
When people quote the  no reference seems  to be made to the frequency, presumably this is about the current state of detectors. Or are the frequencies detected the same as the resonant frequencies of the bars used? What range of frequencies are  gravitational waves expected to have?
21255
Yes, I am aware of observations of "massive objects", for example near the center of the galaxy. But is there any evidence that the object in question is a black hole, with all its commonly attributed properties? Or are black holes just an unconfirmed artifact of general relativity, i.e. something that is an abstract mathematical concept and not a real object?  EDIT: That is not an answer to my question. I did not ask about means of detecting them, I am aware of those means. I'm interested if there's anything suggesting that the properties commonly attributed to black holes, other than large mass, have anything to do with reality, or are completely abstract and deduced from math.
9916
Excuse me if the question is naïve. I am not a physicist.  Let's imagine formation of a black hole from a homogeneous collapsing star. At certain moment when enough of matter is inside of a small enough spherical area the event horizon starts to exist. When does the singularity start to exist and why?  The matter below the event horizon still continues to collapse towards the centre (e.g. in the reference frame of the falling matter). What are the necessary conditions for the singularity to appear?  I have found two similar questions but the answers did not really explain this problem to me:
1175
When speed is the path traveled in a given time and the path is constant, as it is for $c$, why can't light escape a black hole?  It may take a long time to happen but shouldn't there be some light escaping every so often?  I'm guessing that because time is infinite inside a black hole, that this would be one possible reason but wouldn't that mean that we would require infinite mass?  What is contradicting with measuring black holes in solar masses, what means they don't contain infinite mass.  So how can this be?
84978
I try to understand two principles formulated by Leonard Susskind in his book The Black Hole War:  1, To any observer who remains outside a black hole, the stretched horizon appears to be a hot layer of horizon-atoms that absorb, scramble, and eventually emit (in the form of Hawking radiation) every bit of information that falls onto the black hole.  2, To a freely falling observer, the horizon appears to be absolutely empty space. [...]     Now, in reaction to (an unanswered)  by Nathaniel, let us suppose that a distant observer A sees (according to the principle #1) Hawking radiation from a black hole (with Planck spectrum and say measurable temperature). Now suppose there is a freely falling gas somewhere between the event horizon and the observer. According to the principle #2, to the gas (considered an observer B) the horizon is empty space so there is no light coming from it to absorp.  Would the observer A detect absorption lines in the black hole spectrum? How does it change with distance of the gas (or the observer) from the black hole? Will the distant observer see the lines if he is free falling?
82528
The classical explanation of a black hole says that if you get to close, you reach a point - the event horizon radius - from which you cannot escape even travelling at the speed of light. Then they normally talk about spaghetti.  But here's a thought experiment. What if I have a BH with event horizon radius R such that the gravitational gradient at the event horizon is far too weak for creating pasta. I build a Ring with radius R+x around the BH. Then I lower a pole of length x+d from my ring towards the BH, so that the tip passes beyond the event horizon.  Now what happens when I try to pull the pole back?
22195
I think we all heard general statements like "once big enough star burns out there is nothing to prevent the gravitational collapse ending in a black hole". But I can't remember even seeing the process described precisely.  Here's the deal: at first there is a nice object, a star. Stars can be nicely modeled by general relativity, nuclear physics and statistical physics combined and very much is known about these models and it can be observed whether they agree with things like light and neutrino fluxes, surface temperature and probably also lot of other stuff I know nothing about.  After the collapse we are left with another nice object, a black hole. We know that .  The question is: what happens in-between? More precisely, between the time when all of the nuclear material has been burned out (and if possible ignore effects like reheating of the star after big enough compression) and the time where there is nothing more than just a black hole.        Give a description of what happens during the collapse?   How does the star "lose its hair"?   Can the actual collapse be solved analytically?   At what point is singularity created?      Update: I don't want to know what an outside observer will see. Instead, I'd like to find out what an individual part of the dead star will "feel" when a black hole is about to form near it. In other words, I want a complete solution (ideally analytical, but numerical would be also completely fine)  Feel free to assume anything that makes your life easier. Spherical symmetry is definitely fine. Also, if for any reason the questions don't make sense (like Cauchy problem is ill-defined in the presence of the singularity) feel free to interpret them in a way that make them sensible (e.g. assume that black hole is built from D-branes).    Also, I have a feeling that what I intended as a simple question at first ended up being pretty complex. If you think it should be split into smaller (and therefore more manageable and answerable) parts, let me know.
31538
I understand that time is relative for all but as I understand it, time flows at a slower rate for objects that are either moving faster or objects that are near larger masses than for those that are slower or further from mass.  So, the illustrative example I always see is that if I were to leave earth and fly around at near light speed for a while or go into orbit around a black hole the time I experience would be substantially shorter than for those I left behind at home on earth and I'd come back to find that I've only aged however long I felt I was gone by my own clock but that on Earth substantially more time would have elapsed.  Following through on this model, the stars in orbit around the black hole at the center of the Milky Way are aging much much slower (relative to us), right?  So does it not follow that the center of the galaxy is some appreciable (I have no idea how to go about putting this in an equation so will avoid guessing at the difference) amount "younger" than the stuff further away from the center?   If this is not true, could someone please explain why not, and if it is true, can someone please point me to where I can calculate the age of the center of the galaxy :-)  And to be clear... what I'm asking is... if there was an atomic clock that appeared at the center of the galaxy when the center was first formed, and we brought it through a worm hole to earth today - how much time would have elapsed on that clock vs. the age we recon the galaxy currently is? (13.2 billion years)
1703
I was wondering if it is possible for a single photon to form a black hole if it has a small enough wavelength.If so, what would this wavelength be? I came across this question because I am reading about loop quantum gravity, and have heard that it is 'untestable'. I know that the loops making up spacetime in loop quantum gravity are thought to have a size in the order of $10^{-35}$m, and I was wondering whether making a photon with a wavelength small enough to investigate this would form a black hole.  I am aware of the two posts here, but neither seem to really answer my question because they refer to several photons:  and .  Thank you.
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)
39310
Since 90 % of matter is what we cannot see, why can't it be black-holes from early on? Is is possible to figure out that there are no black holes in the line of sight of various stars/galaxies we observe?
2349
I'm a big fan of the podcast Astronomy Cast and a while back I was listening to a Q&amp;A episode they did. A listener sent in a question that I found fascinating and have been wondering about ever since.   From the show :      Arunus Gidgowdusk from Lithuania asks: "If you took a one kilogram mass and accelerated it close to the speed of light would it form into a black hole? Would it stay a black hole if you then decreased the speed?"   Dr. Gay, an astrophysicist and one of the hosts, explained that she'd asked a number of her colleagues and that none of them could provide a satisfactory answer. I asked her more recently on Facebook if anyone had come forward with one and she said they had not.  So I thought maybe this would be a good place to ask.
38911
Let's say a black hole of mass $M$ and a very compact lump of anti-matter () also of mass $M$ are traveling toward each other. What does an outside observer see when they meet?  Will they blow themselves apart in a  matter/anti-matter reaction? Or will their masses combine, never quite meeting in the infinite time dilation at the event horizon?
76615
I am by no means an expert in this field, however something puzzles me about the speed of light and the relativity of time and space (space-time).  Is is universally acknowledged that the speed of light (299,792,458 m/s) is the universal speed limit, and that nothing can travel faster than light? That is a measurement based on a man-made interpretation of time (hours, minutes and seconds etc. are man made...there is nothing natural dictating how long a second should be).   For instance, according to Einstein, time and space bend around the physical matter of the universe, so for example, time near or on the surface of a "super massive black hole" should be drastically slower, relative to that of earth. Lets say for example that for every second that passes on the black hole, 10 seconds pass on earth, so essentially time on the surface of the black hole is 10 times slower than the time on earth.  Given the example above, is the speed of light at the surface of the black hole still 299,792,458 m/s, or is it 299,792,458,0 m/s?