Light travels at a constant velocity, and the Universe is expanding. Assuming a uniform universal expansion of 60km/sec/megaparsec (it simplifies the math), this means the farthest we can see into space is approx. 5000 megaparsecs (approx. 16,300 million light years) in any direction. Any farther and the expansion exceeds the speed of light, and thus, the light from sources beyond this point can never reach us. This is called (I think) the "visual horizon" (VH from here on). Here on Earth this means we can ignore anything outside the VH. It will never affect us, because no energy can ever reach us from beyond it. Nothing travels faster than light speed. It is like we are surrounded by a black hole. Let's say there is an object right on the edge of the VH. For arguments sake we'll say it is just 6km from the VH (we can barely see this thing it's light is redshifted so much but I digress Please Register or Log in to view the hidden image!). Will it disappear over the VH? My intuition says it will, but I am suspicious, as much of cutting edge science laughs in the face of layman's intuition. However, let me finish on the pot Please Register or Log in to view the hidden image! Assuming it can, what if the VH is a black hole? Could this explain gamma ray burts as being objects releasing massive amounts of energy as they heat up and fall into it? How much energy would a black hole eating a quasar release for example? Also, could it be this is where the missing mass needed to make Omega equal to 1 be hiding? I think I heard recently that we can account for only 30% of the mass needed to reach unity? What if the other 70% of the mass is in a black hole and the whole universe is bent around it? I apologise in advance if my layman's level of understanding makes my questions inappropriate or ill conceived, but I am prone to these kind of leaps in my ignorance of the finer details. Please Register or Log in to view the hidden image! But if you don't ask no questions you don't get no answers. Oh, and one more question. Please Register or Log in to view the hidden image! When we see objects with redshifts of 5 and higher, they are receding from us at appreciable fractions of c. Do we see them in real time or do we see them in slow motion as relativity would seem to imply? kind regards
Just "horizon" will be fine. No, because the horizon is itself moving at light speed. The black hole is moving at a bit less than light speed. The horizon wins. Are you hypothesizing that we're INSIDE a black hole, and we're seeing matter falling into it from the outside? If so, there are more disagreements between such a theory and experiment than are tolerable even for conjecture. As of yet, no one really knows what GRB's are, but the most likely candidate is merging black holes. Well, scientists only concern themselves with the observable universe, because a) it's all we can see and b) it's all that can ever affect us. We measure omega in our observable universe. We measure Hubble's constant in our observable universe. We measure the rate of change of the scale factor in our observable universe. In short, there's no way the missing mass could be outside and still have an effect on the density of space inside. Indeed. The redshift itself is an example of seeing them moving in "slow motion." - Warren
Hmm, this is what I was suspicious of. Let me run my reasoning by you and, if you don't mind, point out where it is going askew. The horizon is a result of the constant speed of light and the universal expansion. An object 6km from this horizon is moving along with the expansion at near c relative to us. One second later space has expanded. The object, moving at near light speed, is nearly a light second farther away. The horizon, however, has not moved, because it is a result of the constant speed of light and the universal expansion, and they haven't changed from the last second. The object is farther away than the horizon. But you say the horizon is moving at light speed. Please Register or Log in to view the hidden image! No, the reverse. That we are outside the black hole but that the Universe is "wrapped" around it, which is why there is a horizon in every direction we look. Basically, I guess I'm asking could the gravity of this black hole (with 70% of the mass of the Universe predicted by the resulting Omega from Guth's Inflation) be powerful enough to curve the entire universe around it? I did wonder if GRBs had a distinct signature and that if my statement was void on that basis alone (which is why I added my disclaimer). I feel that's a bit of an over-generalisation (mainly because I am currently reading how string theory deals with dimensions near the planck length and we can't see there but we can argue that another day Please Register or Log in to view the hidden image!), but I understand what you're saying in the current context. Right, I think I understand what you're saying. Even if there is something outside the horizon it doesn't matter as it has no physical affect on anything we see inside the horizon. Does that mean we can treat the space inside the horizon as a closed system? Now that is interesting, because I thought this would be the case, and maybe it has also given me a clue as to why that object won't go over the horizon. Because no massive body can travel at light speed we will never see it go over the horizon. It will simply get slower and slower and dimmer and dimmer. But this give me pause for another (possibly ill-founded) question: if we will never see it go over the horizion, then is this analogous to objects falling over the event horizion of a black hole? I'm sure I've read that from outside the hole, we would see it get slower and slower and dimmer and dimmer. But, we know black holes eat stuff and get bigger. kind regards
Hey Paul, Your logic goes bad right there. The horizon is a result of the finite speed of light and the finite age of the universe. It has nothing in particular to do with the expansion -- even if the universe weren't expanding, the horizon would still exist. That's an interesting gedanken there. However, the light horizon and a black hole event horizon have little to do with each other. I'll explain more specifically in a bit. Well, there's nothing about the Plank length that necessarily means we can't see inside it. The Planck length is just the natural unit of length in the natural unit system. There need not be anything particularly amazing about it, but there may be, and it is exciting to think about the possibilities. On the other hand, I can say outright that no object whose light has not yet reached us can affect us. Right. The light horizon would look like a black hole event horizon in the limit as the distance goes to infinity -- since at infinity, the object would have an velocity exactly that of the speed of light, and we'd see it frozen in time and redshifted to zero frequency. But the observable universe is not infinite, so we don't see things that way, and never will. This is why the black hole horizon and the light horizon are different. Your questions impress me! You should really consider becoming a physicist -- the enthusiasm you display in every question, from the mundane details of integration to the cutting edge of cosmological theory, is impressive. No one really knows right now exactly how black holes deal with information. We do know that (mathematically, at least) black holes hide information from the outside world -- the only three quantities you can learn about a black hole are its mass, charge, and angular momentum. This means that all the characteristics of the mass that falls into them (such as spin, etc.) just seems to get destroyed. This may be evidence that we have the physics all wrong right now. It is true that if you throw a tennis ball into a black hole, you'll never actually see it cross the horizon. You'll see it slow further and further down until it finally seems to freeze on the horizon. But then... if you never see the ball cross the horizon, how can you say it's inside the horizon? How can you measure the black hole's mass to have increased by the tennis ball's mass if you can still see the tennis ball on the horizon? I am probably not qualified to go into any more of the details of the black hole system, because I am not well versed on the requisite math and might lead you astray... - Warren
The appearance of a horizon and a black bole are similar because the of the properties of light. The edge of the horizon is really not moving, rather the space that is between us and it is expanding. It is really a question of `information`. If (FOR EXAMPLE) you were to travel to the Horizon ,instantly, half of the universe would `disappear` and a new portion would appear. The visible size would be limited by the time for information/light to travel from the centre to the edge. It is just an illusion that you are looking back in time as you look deeper into space.
Hi Warren Please Register or Log in to view the hidden image! I've split my reply in two as when I tried to post it all at once the system didn't like it for some reason (I had to type it all again too but nevermind....). Bottom end first. Please Register or Log in to view the hidden image! Thank you very much. That's very kind of you. It's encouraging to know that all the time I have spent familiarising myself with the more popular concepts of science (and it is mainly due to the respective authors' abilities to convey complex math as visual ideas and analogies that I have got to here) hasn't been for nought. Please Register or Log in to view the hidden image! It's only in recent months that I have begun to have reservations over black holes and their implications. I wonder what string theory will make of them, or if it might even rule them out. By that I mean remove the singularity. It's probably just a coincidence but I find it interesting that it shows only three properties, and space is 3 dimensional for example. In any case, I agree. I feel there is room for doubt on the current picture of them. You anticipated my next question. Please Register or Log in to view the hidden image! Not only that but no matter how high above the event horizon you are when you throw the ball, you will never see it cross. Fair enough. Although it would be interesting to hear the ideas of someone who is more educated on the subject as a whole than myself. So, if you don't mind let me throw this thought at you, and if you want to reserve comment then that's fine. I am falling towards a black hole as I get closer to it what will happen to the event horizon? Relativity states that no matter what my velocity I will always see the speed of light as a constant. Any measurement I make outside the black hole will reveal the speed of light to be constant. The black hole's event horizon is the line at which the gravity becomes so strong that light cannot escape. The escape velocity is equal to c. This means that wherever I am outside the black hole the horizon will always exist because it marks a line where a mass would be accelerated to c, and that cannot happen according to relativity. My conjecture: No matter how fast I accelerate I can never reach the event horizon in the same way that no matter how fast I accelerate I will always see light moving at c. In effect, the event horizion is moving away from me at c. I have another reason to think that I would never reach that horizon also. According to Stephen Hawking, black holes "evaporate". They have a temperature which is inversely proportional to their mass. The bigger they are the less they evaporate and vice versa. I can't remember the numbers but let's say a 3-4 solar mass hole takes 10<sup>60</sup> years. I'm in my spaceship again Please Register or Log in to view the hidden image!. I am hovering above the event horizon of a black hole. I am sending you (back on earth) a picture of the clock on my spaceship. When you look at the picture you find that I am experiencing a time dilation that equates to one second on my ship being equal to 10<sup>60</sup> earth years (ok, I understand that I couldn't do this but particles can because they will be reaching dilations in excess of this prior to reaching the horizon). I hang around the hole for 1 year as measured by me and then accelerate away from the horizon. 1 year has passed on my clock. From your perspective (even though you are long dead) billions upon billions of years have passed and that black hole evaporated long ago. ie. From your perspective I hung around there hardly moving whilst the hole got smaller and smaller eventually evaporating completely and then I flew off. Imagine this for a particle. My conjecture: Black holes evaporate before you reach the horizon. As usual, my disclaimer: I am a layman so if I have made some bad logical errors, or just plain misread the situation, then please feel free to correct me. kind regards
part two: Fair point. However, does that mean that the expansion and the finite constant of light won't create an horizon when the universe reaches a sufficient age? I think I'm beginning to grasp it now. In effect, distance is time because it takes time to cover a distance so any light that has reached us must be inside the horizon, whereas the black hole is the result of a massive body warping spacetime. Maybe different causes with equivalent effects would be a better way of seeing it? Hmm, that sort of shatters an illusion I had about it then. I was under impression that in order to probe smaller and smaller distances you need higher and higher energies, and that the energy needed at planck length scales would cause a miniature black hole (don't ask me where I got that idea - probably in a fit of fantasy!). Agreed. This is the context in which I took your statement. kind regards
"I am falling to-wards a black hole as I get closer to it what will happen to the event horizon? " This is different from the Horizon problem... As you fall your speed increases (there is a relativistic effect), but more importantly there is a gravitational effect on all the particles... It`s a question of whether a particle has enough energy to escape the zone.. too little -blackness. As you fall the `edge` stays in the same position, the edge is just the location that defines the upper-limit for particle energy's.
SRY. What was the question? Was it to do with the VH? As i see it, the horizon is not moving. it`s really space that is expanding. There is a misconception the when we look back in time ( by looking deeper into space) we will see the (for example) big-bang singularity. This is of course an illusion the horizon really is (now) a just a bit of space like our own bit of space... All the above is different from a black hole that is really just a deep gravity well... ?
Well, that is what I thought too, but as Chroot said, the horizon (of the Universe) would exist in any case due to the age of the universe being finite. Indeed, many people are under that illusion because they envisage the big bang to be a like a bomb explosion that happened inside the Universe rather than the beginning of space and time itself. Indeed. However, the question you replied to pertained solely to black holes (which is why I split my message in two at that specific point). kind regards
Slinky: Im not entirely sure thats true. Everything about visually observing a black hole depends on your reference frame. For instance, this is highly unlikely, if you could get to a black hole large enough so that when you got close to the horizon that you wouldn't be ripped apart by the tidal gravity, and held steady there for 1 year (right above the horizon), when you came back you would have basically traveled into the future. Now for an OUTSIDE observer who hasnt traveled with you, you appear to freeze and never come back out. The cause of this is known as time dilation. But what is really interesting is you really didnt "travel into the future". Its merely an irreversible illusion created by being in a different reference frame. Its quite amazing actually.... in the coming years we will really discover the true nature of laws that govern the singularities inside black holes. So far all of the things we know now came from some very respected scientists, and to mention a few: Roger Penrose and his singularity theorem. Khalatnikov and Lifshitz Oppenheimer and Snyder's solution to the Einstein field equation. The Reissner-Nordstrom solution which brought forth the idea that a star continually implodes within the horizon of a black hole and eventually "pinches off" our universe and reconnects to another universe or ours somewhere else, only to explode again And some other names like Landau, Belinsky, ect, ect I could go on forver! The true understanding of a singularity or a black hole for that matter will come within the next two decades or so when Quantum Mechanics and General Relativity say "I do" and get the signed, sealed and delivered approval for marriage. Till then, its anybodies guess... Later TPlease Register or Log in to view the hidden image!
Ok, I have sort of addressed this idea in this thread: Black Holes Starve to Death. I hope you don't mind me directing you to it rather than answering you here, but that thread will possibly answer this point you have raised. Almost forever. Please Register or Log in to view the hidden image! I read a particularly intersting book on this Black Holes and Time Warps: Einstein's Outrageous Legacy - Kip S. ThorneHe goes right through the history of black holes. Very interesting, and very informative. It's aimed at the layman but has some math in the notes pertinent to the text. Worth a read for any layman IMO. Hands off my conjectures! Please Register or Log in to view the hidden image! kind regards Paul
Very good book, indeed.Please Register or Log in to view the hidden image! Matter-O-Fact, thats what im reading right now and where I got all of the above from. I love the book, incredible how much history it has along with great examples of complex things. And very well written. Its like a history written by a primary source. Its Priceless. Please Register or Log in to view the hidden image! Later TPlease Register or Log in to view the hidden image!
