Observable Universe and speed of light

Panrich

As a simplistic soul, can someone help me get my head around this explanation about the size of the universe taken from space.com?

http://www.space.com/24073-how-big-is-the-universe.html

The observable universe

Astronomers have measured the age of the universe to be approximately 13.8 billion years old. Because of the connection between distance and the speed of light, this means they can look at a region of space that lies 13.8 billion light-years away. Like a ship in the empty ocean, astronomers on Earth can turn their telescopes to peer 13.8 billion light-years in every direction, which puts Earth inside of an observable sphere with a radius of 13.8 billion light-years. The word "observable" is key; the sphere limits what scientists can see but not what is there.

But though the sphere appears almost 28 billion light-years in diameter, it is far larger. Scientists know that the universe is expanding. Thus, while scientists might see a spot that lay 13.8 billion light-years from Earth at the time of the Big Bang, the universe has continued to expand over its lifetime. Today, that same spot is 46 billion light-years away, making the diameter of the observable universe a sphere around 92 billion light-years.



My simple question relates to two points side by side just after the big bang that are accelerating away from each other in opposite directions (forming the outer boundaries of the universe). At the speed of light, they can be no further than 28 billion light years apart now. In order to be 92 billion light years apart, they would have travelled faster than the speed of light.

ThunderCat

Panrich wrote: »

As a simplistic soul, can someone help me get my head around this explanation about the size of the universe taken from space.com?

http://www.space.com/24073-how-big-is-the-universe.html

The observable universe

Astronomers have measured the age of the universe to be approximately 13.8 billion years old. Because of the connection between distance and the speed of light, this means they can look at a region of space that lies 13.8 billion light-years away. Like a ship in the empty ocean, astronomers on Earth can turn their telescopes to peer 13.8 billion light-years in every direction, which puts Earth inside of an observable sphere with a radius of 13.8 billion light-years. The word "observable" is key; the sphere limits what scientists can see but not what is there.

But though the sphere appears almost 28 billion light-years in diameter, it is far larger. Scientists know that the universe is expanding. Thus, while scientists might see a spot that lay 13.8 billion light-years from Earth at the time of the Big Bang, the universe has continued to expand over its lifetime. Today, that same spot is 46 billion light-years away, making the diameter of the observable universe a sphere around 92 billion light-years.



My simple question relates to two points side by side just after the big bang that are accelerating away from each other in opposite directions (forming the outer boundaries of the universe). At the speed of light, they can be no further than 28 billion light years apart now. In order to be 92 billion light years apart, they would have travelled faster than the speed of light.

It's true that within the universe nothing can exceed the speed of light but the universe itself is capable of expanding quicker than that. Take the few seconds after the big bang, the universe expanded to a vast size and did so at a rate that was way beyond the speed of light. Think of the universe as a balloon. The speed of light is the limit for all things within the balloon, but the balloon itself is not bound to this limit and can expand at a faster rate. Hopefully someone else here might give you a better description but that's my understanding of it.

animaal

Sorry to be the "I don't know what a tracker mortgage is" guy, but I really don't understand...

If we see something that's 13 billion light years away, it means the light from it has taken 13 billion years to get to us. But 13 billion years ago, when the light was emitted by (or reflected off) that "thing", it was much closer to us (because the universe was so much smaller) - so how come the light is only getting to us now?

Obviously I just don't get it, but I'd appreciate if somebody could tell me where I'm going wrong.

Zubeneschamali

The Universe is expanding. Let's pretend that it's expanding by 1% per year. Then a thing one light year away would seem to be receding at 1% of light speed. A thing 100 light years away would be receding at 100% of light speed. And anything further away would be invisible, receding faster than light.

Remember: nothing is actually moving, certainly nothing is moving faster than light. Space itself, all of it, is expanding.

The actual rate of expansion is much less than 1%, so we can see 14 billion light years, not just 100.

Myrddin

Isn't the period where the Universe expanded faster than light referred to as the inflation period?

GerB40

A wise man once said "If you think you understand quantum mechanics, you don't understand quantum mechanics".
The line between theoretical and factual is ever changing so what we "know" now is often dubious at best.

EoghanIRL

Usually when you try and accelerate objects close to the speed of light their mass will begin to increase , making light speed almost impossible for ordinary objects . Also the energy required to accelerate an object to the speed of light would increase as the mass increases .
As already pointed out however the speed of light doesn't apply to the universe itself as a body .

AugustusMinimus

GerB40 wrote: »

A wise man once said "If you think you understand quantum mechanics, you don't understand quantum mechanics".
The line between theoretical and factual is ever changing so what we "know" now is often dubious at best.

What the OP is asking about has nothing to do with quantum mechanics and has all to do with relativity and the like.

Must say, what I find interesting about this subject is that we really don't have much of an idea just how big the universe is. We can only tell how big the observable universe is.

Added to that, objects which are past the cosmic horizon also don't exert a gravitational pull on us as gravitation also travels at the speed of light.

EoghanIRL

Anyone have a simple answer as to why the the cosmic speed limit doesn't apply to the universe .

Zubeneschamali

EoghanIRL wrote: »

Anyone have a simple answer as to why the the cosmic speed limit doesn't apply to the universe .

Because nothing is moving faster than light. Each little bit of space is expanding at .00000007% per year (or whatever). Unmeasurable at normal scale, but adding up over 14 billion light years.

ps200306

Panrich wrote: »

My simple question relates to two points side by side just after the big bang that are accelerating away from each other in opposite directions (forming the outer boundaries of the universe). At the speed of light, they can be no further than 28 billion light years apart now. In order to be 92 billion light years apart, they would have travelled faster than the speed of light.

animaal wrote: »

Sorry to be the "I don't know what a tracker mortgage is" guy, but I really don't understand...

If we see something that's 13 billion light years away, it means the light from it has taken 13 billion years to get to us. But 13 billion years ago, when the light was emitted by (or reflected off) that "thing", it was much closer to us (because the universe was so much smaller) - so how come the light is only getting to us now?

Obviously I just don't get it, but I'd appreciate if somebody could tell me where I'm going wrong.

Let's consider a few scenarios.

Suppose the universe is static and unchanging. Then there are no horizons because light has had an infinitely long time to reach us. However, if it is infinite in extent we have Olber's paradox and if it is finite it is unstable to gravitational collapse -- Einstein knew a static universe was problematic even before Edwin Hubble discovered the cosmological red shift.

Post Hubble, we know that the universe is expanding. Let's take a simple rubber band model of the expanding universe:



The pins in the rubber band are galaxies in space and we label them A, B, and C from left to right. Now pull the two ends of the rubber band away from each other. The galaxies move away from each other, and because the rubber stretches evenly along its length, the recession speed of any two points depends on how far apart they are: galaxies A and C recede from each other at twice the speed that either recedes from galaxy B.

Ok, suppose the picture above represents the scenario today,and that we're in galaxy A. It's possible that galaxy C is receding from us at greater than light speed, and galaxy B at sub-light speed. Assuming a constant rate of expansion, light emitted today from galaxy B must become visible to us sometime, whereas light emitted from galaxy C will never be seen. Strangely, this doesn't mean we can't see galaxy C. In the past it was closer to us, and the recession rate (which remember depends on distance) might have been lower than light speed. The distance at which the current recession speed becomes the speed of light is called the Hubble distance, and the volume it encloses is called the Hubble volume. Whether more and more galaxies are entering the Hubble volume as time goes on, or galaxies already within the Hubble volume are leaving it, depends on the expansion rate. We'll come back to this question. I am hesitant to give this link about the Hubble distance, since it claims the Hubble distance defines the edge of the visible universe, which is completely wrong, but it gives some useful introduction.

When we look at something a light year away, we see the light emitted from it a year ago. We say that the lookback time for this object is one year. Since the universe is about 13.7 billion years old, the maximum lookback time is 13.7 billion years. Light that reaches us from then has been travelling for 13.7 billion years. But it has travelled far further than 13.7 Gly. (The abbreviation Gly is often used for a billion light years). The space it has traversed has been expanding while it travelled. So to calculate the actual distance travelled we would have to chop up the path into tiny little segments and sum them all. The space traversed in the last year hasn't expanded all that much, so it only counts for about one light year. But the space traversed five billion years ago or ten billion years ago has expanded a lot since then. We have to do a bit of integral calculus along the whole path to get the overall distance. It works out that the furthest away objects we can see are at 46 Gly.

This is the present radius of the observable universe. The Hubble distance is only about 14-15 billion light years. So we can actually currently see lots of stuff that is now receding at greater than the speed of light. In that sense, the Hubble distance does not define our current cosmological horizon, which is the so-called particle horizon at 46 Gly.

Here we must correct another inaccuracy. The OP's link mentioned that "scientists might see a spot that lay 13.8 billion light-years from Earth at the time of the Big Bang". Well clearly that can't be right -- at the time of the Big Bang there was no earth, but even the point that the earth would eventually occupy wasn't 13.8 Gly from anywhere, since the universe itself was much smaller than that. I don't know if that's just a horrendous misunderstanding in the article, or whether they are talking about co-moving distance. Co-moving distance is defined so that objects moving apart with the Hubble flow (i.e. with the expansion of space) are at a constant co-moving distance. Their proper distance (i.e. the distance you would measure if you could instantaneously lay out measuring sticks along the path to them) varies with the expansion of space. The co-moving distance is defined to be equal to the proper distance today. To use our earlier rubber band analogy, we'll mark the pink backing paper in the picture with the locations of the pins in the rubber band today. The marks in the backing paper show the co-moving distance, which never changes, while the proper distance changes as the rubber band expands. Today the co-moving and proper distances to a galaxy at the current particle horizon are both 46 Gly. The co-moving distance to that galaxy has always been 46 Gly, whereas the proper distance was much smaller in the past and will be greater in future.

So now we come back to the question of what's going to happen to our cosmological horizon. We can see galaxies beyond the Hubble distance, so the implication is that they might eventually disappear, since the light leaving them now can never reach us. Except, that might not be true, because the expansion of the universe might have slowed down since the light left those distant objects. The Hubble distance expands as time goes by, since the maximum lookback time increases as the universe ages. Will this bring more galaxies into view as the expansion of the Hubble volume overtakes the expansion of the universe, or will galaxies leave the Hubble volume if the universe is expanding fast enough?

Paradoxically, the answer appears to be "both". This is because the expansion rate appears to have been changing over time. The expansion started very fast, but slowed because of the mutual gravitational attraction of the galaxies. During this phase the universe is said to have been matter dominated. However, about two billion years ago, something seems to have changed. Space itself seems to have an energy that acts like a repulsive force. Whereas the gravitational attraction between the galaxies gets weaker as the universe expands, the dark energy increases as the quantity of space increases. This means the rate of expansion has started to accelerate!

So now we have this weird situation -- when we look back to more than two billion years ago we are looking back to a time when the rate of expansion was decreasing, and new galaxies are thus appearing for the first time over our horizon. But in the future, the accelerated expansion will dominate and galaxies will start to disappear again. So there is a limit beyond the current particle horizon, called the cosmological event horizon, which is the furthest we will ever see at any time in the future. It is at a co-moving distance of about 60 Gly, although by the time it actually comes into view its proper distance will be much greater. After that time our observable universe will become emptier and emptier, until eventually there are no galaxies visible outside our local cluster in the sky.

LeinsterDub

AugustusMinimus wrote: »

Added to that, objects which are past the cosmic horizon also don't exert a gravitational pull on us as gravitation also travels at the speed of light.

This seems wrong , can anyone else confirm this?

Indricotherium

I suppose I think of it as two things traveling away from each other at near the speed of light, the distance between them is increasing at almost double the speed of light.

Maximus Alexander

Indricotherium wrote: »

I suppose I think of it as two things traveling away from each other at near the speed of light, the distance between them is increasing at almost double the speed of light.

Edit: Actually never mind, on re-reading you're implying a third frame of reference.

Zubeneschamali

ps200306 wrote: »

The pins in the rubber band are galaxies in space and we label them A, B, and C from left to right. Now pull the two ends of the rubber band away from each other. The galaxies move away from each other, and because the rubber stretches evenly along its length, the recession speed of any two points depends on how far apart they are: galaxies A and C recede from each other at twice the speed that either recedes from galaxy B.

Just to add one note: when we look into space, we see it expanding uniformly in all directions. This might suggest that we are at a special point, like pin B at the centre of the rubber band. But in fact, anyone anywhere in space would see the same thing, because the "rubber band" is effectively infinitely long and has no centre.

mickmackey1

Indricotherium wrote: »

I suppose I think of it as two things traveling away from each other at near the speed of light, the distance between them is increasing at almost double the speed of light.

As speed increases, distance contracts. So although the distance between the two objects will increase, it can never do so at a rate faster than the speed of light (excluding the expansion of space which is separate from the context of this question).

ps200306

AugustusMinimus wrote: »

Added to that, objects which are past the cosmic horizon also don't exert a gravitational pull on us as gravitation also travels at the speed of light.

LeinsterDub wrote: »

This seems wrong , can anyone else confirm this?

It's correct. Gravity propagates at the speed of light. If the sun somehow instantly disappeared, we wouldn't notice for eight minutes, since both its light and its gravitational influence take eight minutes to traverse the 93 million miles between us.

So it's true that if the universe is expanding such that two points are receding from each other at greater than the speed of light, they can have no mutual gravitational attraction.

This gives rise to the so-called horizon problem -- if such regions of the universe have no radiative or gravitational connection, how come the universe is homogeneous and isotropic, i.e. looks the same on a large scale in all directions, since there is no way for things to have "smoothed out" since the Big Bang? That's the problem addressed by the theory of Cosmic Inflation.

ps200306

Indricotherium wrote: »

I suppose I think of it as two things traveling away from each other at near the speed of light, the distance between them is increasing at almost double the speed of light.

mickmackey1 wrote: »

As speed increases, distance contracts. So although the distance between the two objects will increase, it can never do so at a rate faster than the speed of light (excluding the expansion of space which is separate from the context of this question).

This isn't quite right. You have to say what frame of reference you are talking about. If the two objects are spaceships and you're on one of them, you will never see the other spaceship recede from you at more than the speed of light. However, if you are an observer at rest with respect to the frame of reference in which the spaceships move in opposite directions at the speed of light, you will indeed see each recede at the speed of light and the distance between them increase at double the speed of light. This does not violate Special Relativity because no single object is seen to move at superluminal speed.

When we're talking about the expansion of space, somewhat different rules apply, as you say. Some scientists are opposed to talking about recessional velocities at all, even though that is the way Hubble expressed it. The Hubble Law says that for a given separation in space, expansion will carry two objects apart from each other at a certain speed. Currently the speed is a little over 20 km per second for every Mly (million light year) separation.

But another way to view it is that the two objects are at rest in space, and the scale factor of space itself is changing. An analogy used is that of raisins in baking bread. The raisins are not moving with respect to the dough, but the dough expands and carries them apart. This may seem like a pointless distinction but it is important. It defines a special type of rest frame in which the dough isn't moving. All of the raisins can be said to be at rest in this frame even though they are ostensibly moving apart from each other.

The equivalent in the real world is the frame in which the Cosmic Microwave Background has no dipole. If we were moving in space, the stars and galaxies (and the CMB) in front of us would be blue shifted and those behind us red shifted -- a dipole. If there is no dipole we know we are at rest. Observers everywhere at rest with respect to the Hubble flow see no dipole, so are at rest with respect to this special cosmic frame of reference, even though they all see each other red shifted (as Zubeneschamali says in #15), and therefore apparently moving. Another term that is used is that they are fundamental observers.

Another thing worth pointing out is that we never directly see any co-moving speeds higher than the speed of light. Remember that when we refer to the proper distance to an object we are talking about a purely hypothetical situation in which we could instantaneously lay out rulers along the path to the object, seeing each interval along the path expanded as it is now, i.e. at today's cosmic scale factor. We can't do this in practice, in fact we can't directly measure any cosmic distances or speeds.

One of the few tools we have at our disposal is red shift measurement. But, as mentioned before, the red shift is accumulated along the path of the light to us during the (potentially) billions of years of travel time with space expanding all the time. It is only by mathematically integrating along the path that we infer recession speeds and changes in distance higher than the speed of light.

If we just look at the light itself and infer a Doppler speed from the red shift, we never see anything above light speed. (The theoretical maximum would be the last scattering surface, the source of the CMB, at about red shift = 1000, inferring a speed of 99.8% c).

Myrddin

^^ fascinating stuff. I really love astrophysics but am not blessed with a brain that works well with complex mathematics, so have to visualise everything in my head. Still love it though

ps200306

I'm not too hot on the mathematics myself. I also have to try to visualise things. It gets hard when you are trying to picture signals that propagate through a space that is itself expanding over time. Anyone's who's looked at Special Relativity probably has the concept of a light cone. Three dimensional space at a point in time is represented by a two dimensional surface, leaving a third axis to represent time. An event is represented by a point in space and time, and its light cone spreads outward into the future. Any other events that it could influence are in its future light cone. Any events that could influence it are in its past light cone. Two events occurring at the same time can only interact in the future when their light cones intersect. The slope of the edge of the light cone is determined by the speed of light. The distance a light speed signal has travelled at any future time is determined by the radius of the light cone at that time.



Imagine the past light cone of an observer in an expanding space. In the recent past, the light cone spreads out like the one above. Events that happened one light year away become visible after a year. But in the distant past, things are different. Nine billion years ago, our Hubble limit was less than six billion light years away, yet events from it are only reaching us now. The light from them had to traverse through expanding space. Today the point from which they were emitted is over 15 Gly distant. It makes sense that the travel time of 9 billion years, multiplied by the speed of light, is somewhere between the distance at time of emission (6 Gly) and the current distance (15 Gly). If we go back further in time, the distance at emission is less, and the current distance is greater, because there has been more expansion. Here's a picture of the past light cone from one of my text books:



The vertical axis shows the lookback time (or, in reverse, the age of the universe). The horizontal axis shows the distance at time of emission of a light signal reaching us now. The teardrop-shaped light cone is marked on the left hand side with distances to which the point of emission has receded today. The right hand side is marked with the value of the red shift we see for such a light signal.

We can make the light cone above look more like the conventional one. We have to change the horizontal axis to show the distance of the point of emission today. The vertical time axis will show conformal time, that is, the time that would be required for a light signal to reach us from the point of emission today if space wasn't expanding. This gives us a straight edge to the light cone. The left hand side of the cone is marked with distances at time of emission -- you can see that these increase to a maximum at 5.67 Gly and then reduce again:



I'm not embarrassed to admit I pored over these for days trying to get my head around what they were showing.

(Pics from Relativity, Gravitation and Cosmology by R.J.A. Lambourne, Cambridge University Press, (c) The Open University 2010 )

i57dwun4yb1pt8

AugustusMinimus wrote: »

What the OP is asking about has nothing to do with quantum mechanics and has all to do with relativity and the like.

Must say, what I find interesting about this subject is that we really don't have much of an idea just how big the universe is. We can only tell how big the observable universe is.

Added to that, objects which are past the cosmic horizon also don't exert a gravitational pull on us as gravitation also travels at the speed of light.

Gravity moves at the speed of light ? Has never been proven
if it was it would be BIG news

ps200306

DaDumTish wrote: »

Gravity moves at the speed of light ? Has never been proven
if it was it would be BIG news

We've good reason to suspect it does, though. Unless Newtonian gravity is right (which we know for sure isn't the case) and General Relativity is wrong, the "speed of gravity" must be finite. Orbital decays of pulsar binary systems are consistent with gravitational energy being carried away at some finite speed. If GR is right, that speed is the speed of light. If it's wrong, it's some other finite speed. GR has passed many other tests which increase our confidence that it's right.

mickmackey1

Indeed so, even something as basic as communication with spacecraft in the Solar System involves calculations which imply that gravity 'travels' at light speed.

ps200306

mickmackey1 wrote: »

Indeed so, even something as basic as communication with spacecraft in the Solar System involves calculations which imply that gravity 'travels' at light speed.

How so?

mickmackey1

ps200306 wrote: »

How so?

Because if gravity traveled at any other speed than that of light, the trajectories that are calculated for spacecraft would be wrong since they include gravitational influences that change as the spacecraft, Sun and planets change their positions.

(Admittedly I copied that from here)

animaal

ps200306 wrote: »

...

Wow, thanks, I think that takes the award for most helpful explanation of a complex topic ever on Boards. Especially the "teardrop" diagram. I hadn't realised how quickly the universe was expanding early on. It explains a lot.

If I understand it, the following strange things are true:

If event A took place 3 Gly from us 3.5 Gyr ago
and event B took place 3 Gly from us 12.7 Gyr ago
then the light from both is reaching us at (approx) the same time, right now. I.e. we can see both events.
But we can't see an event that happened 1 Gly from us 5 Gyr ago.

ps200306

mickmackey1 wrote: »

Because if gravity traveled at any other speed than that of light, the trajectories that are calculated for spacecraft would be wrong since they include gravitational influences that change as the spacecraft, Sun and planets change their positions.

(Admittedly I copied that from here)

I see your reference is written by an apparently bona fide "Doctor". But at the risk of appearing cocky, I remain to be convinced. First, because this fascinating article talks about how, although General Relativity was taken into account for calculating MESSENGER trajectories near the sun, it's generally not required for calculations in deep space. I'm pretty sure plain old Newtonian dynamics has been used for spacecraft trajectories in the past (except in those cases where there were special requirements, e.g. testing GR itself, or very precise timing requirements like GPS). Another interesting case is the famous Pioneer anomaly. In that case, anomalous accelerations in the two Pioneer spacecraft, although measured to exquisite precisions of billionths of a metre/sec² were unable to distinguish between GR and competing gravitational theories.

The other point is, I plain don't understand the language. Newtonian gravity has to take into account that the sun and planets change their positions. How is this a special case for GR? The force vector to the gravitating body is continually changing direction. I can see how a difference in the speed of light and the "speed of gravity" would cause the vector to point in a slightly different direction to where the the gravitating body appears to be. But he doesn't give any evidence that this is significant to the calculation of trajectories, and other references I've read suggest otherwise.

First to admit, though, I am coming from a position of ignorance.

ps200306

animaal wrote: »

Wow, thanks, I think that takes the award for most helpful explanation of a complex topic ever on Boards. Especially the "teardrop" diagram. I hadn't realised how quickly the universe was expanding early on. It explains a lot.

If I understand it, the following strange things are true:

If event A took place 3 Gly from us 3.5 Gyr ago
and event B took place 3 Gly from us 12.7 Gyr ago
then the light from both is reaching us at (approx) the same time, right now. I.e. we can see both events.
But we can't see an event that happened 1 Gly from us 5 Gyr ago.

There are some anomalous things on the diagram alright, but I'm not sure if I'm seeing that one. (No guarantee I'm reading it right, of course). If we're talking about distances at time of emission, then I read those off the horizontal axis.

So to take your example, for 3.5 Gyr ago, I look down the vertical axis (on it's right hand side scale of lookback time). Then I go horizontally out to the light cone line. For 3.5 Gyr of lookback time, I hit the light cone at about red shift z=0.3 on the right, or co-moving distance 4 Gly on the left. From either point I go vertically down to the bottom horizontal axis and read a distance at time of emission of 3 Gly, agreeing with your reading.

12.7 Gyr ago is harder to do accurately because of the rapid expansion, but I can still agree with your reading of 3 Gly distance at time of emission.

But for 5 Gyr ago I am reading a distance at time of emission of an event on the light cone of approximately 4 Gly. All events closer to us than that for that lookback time should have been observed already, no?

mickmackey1

ps200306 wrote: »

The other point is, I plain don't understand the language. Newtonian gravity has to take into account that the sun and planets change their positions. How is this a special case for GR? The force vector to the gravitating body is continually changing direction. I can see how a difference in the speed of light and the "speed of gravity" would cause the vector to point in a slightly different direction to where the the gravitating body appears to be. But he doesn't give any evidence that this is significant to the calculation of trajectories, and other references I've read suggest otherwise.

Must say I was a bit nervous about posting the link, as some of the language he uses to deal with other questions seems a bit, shall we say, 'populist'.

We're probably safer sticking with your pulsar example, while acknowledging that experimental verification remains elusive.

But no physicist would suggest that, if the Sun suddenly disappeared and its gravity was reduced to zero, we would feel the effects instantaneously; rather we would have to wait 8 minutes, as is the case with light.

animaal

ps200306 wrote: »

But for 5 Gyr ago I am reading a distance at time of emission of an event on the light cone of approximately 4 Gly. All events closer to us than that for that lookback time should have been observed already, no?

Yes, that's a clearer way of saying what I was thinking - any point within the area of the teardrop is actually on the surface of a smaller teardrop - one that represents light visible to us at a past time.

Justin1982

Hopefully this explanation helps guys.

There was a big bang most likely, there may have been a rapid inflationary period (pinch of salt), the universe existed for something like 300,000 years (again more salt for easier digestion). But all that doesn't matter.

Why?

Well because during that 300,000 years after big bang light could not propagate they way we think of it today. Universe was too dense. So once it expanded enough then light could propagate in its normal sense. Again this light started propagating 300,000 years after the big bang. So one of the photons of light that started propagating freely traveled through the universe unimpeded for billions and billions and billions of years. Just travelling and travelling and travelling for about 13.798±0.037 billion years until right just now it hit you on the head.

Personally that's the way I like to think of the age of the universe and the cosmic horizon. All the stuff before the cosmic horizon cannot be seen visually because light was basically not around at that stage. And what went on before it is kind of irrelevant when talking about the age of the observable universe.

ps200306

Justin1982 wrote: »

Hopefully this explanation helps guys.

There was a big bang most likely, there may have been a rapid inflationary period (pinch of salt), the universe existed for something like 300,000 years (again more salt for easier digestion). But all that doesn't matter.

Why?

Well because during that 300,000 years after big bang light could not propagate they way we think of it today. Universe was too dense. So once it expanded enough then light could propagate in its normal sense. Again this light started propagating 300,000 years after the big bang. So one of the photons of light that started propagating freely traveled through the universe unimpeded for billions and billions and billions of years. Just travelling and travelling and travelling for about 13.798±0.037 billion years until right just now it hit you on the head.

Personally that's the way I like to think of the age of the universe and the cosmic horizon. All the stuff before the cosmic horizon cannot be seen visually because light was basically not around at that stage. And what went on before it is kind of irrelevant when talking about the age of the observable universe.

The 300,000 years is just a rounding error though when we're talking about a 13.7 Gyr age for the universe. Lopping 300 Ky off that puts the recombination era (when photons first started travelling) at 13.6997 Gyr ago (except the 300 Ky is actually a hundred times smaller than the uncertainty in the age, so that level of precision is unwarranted). Things like cosmic inflation do have an observational implication, albeit indirect. Regions at the cosmic horizon that are separated by (IIRC) 7.5 degrees on our sky aren't causally connected, so we need something like inflation to explain the homogeneity and isotropy of the universe.

i57dwun4yb1pt8

so how does gravity leave a black hole , if it cannot travel faster than light ?

it must be instant , or much faster than light .

ps200306

DaDumTish wrote: »

so how does gravity leave a black hole , if it cannot travel faster than light ?

it must be instant , or much faster than light .

A couple of ways to look at this ... one answer is "same as light leaves a black hole at the speed of light". Ok, light can't escape a black hole but it can escape a point arbitrarily close to the event horizon. To be sure, it will be increasingly gravitationally red shifted with proximity to the event horizon, but it nevertheless propagates at the speed of light from the point of view of all observers outside the black hole.

Another point is that gravity doesn't "leave the black hole" in the sense of something flowing out of it. Gravity is the curvature of space around the black hole. The gravity well formed long before the black hole, under the influence of whatever object was the black hole's progenitor. As the black hole formed, the gravity well got increasingly steep.

Imagine you're standing on the surface of the progenitor star, with a pal beside you. The surface of the star falls away as the collapse occurs and your pal goes with it. (We'll ignore that it's the core, not the surface, of a star that collapses). You have the presence of mind to have turned on your super-powerful hover boots, so manage to resist the star's surface gravity. (The force on you doesn't change as the black hole forms, as both the mass below you and your distance from the centre stay the same).

You are in radio contact with your less fortunate pal as he disappears down the drain. He somehow manages to avoid spaghettification and measures the increasing force with his gravitometer all the way down, radioing the results back to you. It's digital communication in the form of little bursts of wavepackets. His radio signal is increasingly redshifted as he falls, so it takes longer and longer for a sequence of packets to arrive. Initially your pal is transmitting a reading every microsecond, and you see that the gravitational force is rapidly soaring. Later, there are larger and larger gaps between readings. They stretch to seconds, then minutes, hours, and days. Eventually, although the signals keep coming, they take years to read, then thousands and millions of years.

Now, this is completely at odds with your pal's experience. From the start of collapse to the formation of the event horizon takes seconds at most. His fall rate is limited only by the speed of light as measured by himself. We don't know what happens as the event horizon forms beside him, but we know it's all over in a flash. You and your pal's regions of the universe see things in each other's vicinities happening at completely different rates!

This is true not just for what we generally think of as "informational processes" (such as your pal's gravity readings) but for all of physics. The universe at your location doesn't know that the black hole has formed. The gravitational well -- i.e. the curvature of space in your vicinity -- persists because it doesn't "know" it's not supposed to. It doesn't need anything to leave the black hole, in fact it depends on that not happening! There are no naked singularities.