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nutant gene 71
2009-Feb-14, 04:39 PM
Some questions on the 'event horizon' of the observable universe.

Three questions come to mind based on the NewScientist article "Dark Flow: Proof of another universe?" (http://www.newscientist.com/article/mg20126921.900-dark-flow-proof-of-another-universe.html?full=true) which show the 'observable universe' is about 13.7 billion light years distant, though the overall universe is some 45 billion light years (http://www.newscientist.com/articleimages/mg20126921.900/1-dark-flow-proof-of-another-universe.html) away (see map in link), where our visible universe may 'bleed' into another. Or so the article says is possible.

My questions specifically, and this may have been covered on BAUT elsewhere, are concerning that maximum observable distance at the 'edge of the universe' where observation starts to fail, in effect the 'event horizon' where our instrumentation at any wavelength stops working. The questiions are as follows:


1) What is the Doppler-like recessional velocity at that cosmic 'event horizon' of the visible universe?

2) Are the redshifts and luminosity well measured, by Hubble Space Telescope, Chandra et al, at the cosmic 'event horizon' or otherwise documented from historical studies?

3) How much time would need to pass to observe change in redshift and luminosity at the 'event horizon' edge of the visible universe, or disappear entirely over the horizon?


I ask these three questions because the answers to these may provide us with a falsifiable test of current modern astrophysics theory regarding space expansion going back to the origin of the so-called Big Bang. If the time line given the recessional velocities at the edge of the universe shows up as redshift and luminosity changes over a period of years rather than centuries, then it may be possible to train our telescopes (at all wavelengths) on specific targets over time to measure such changes, which I suspect would show as reduced luminosity and increased redshift, or evidence of otherwise fading over the horizon. Also, are these measurements of recessional velocity and changes in luminosity easy to calculate, or does it require sophisticated computing?

Many thanks in advance, on this Valentine's Day. :)

gzhpcu
2009-Feb-14, 07:03 PM
1) What is the Doppler-like recessional velocity at that cosmic 'event horizon' of the visible universe?


Wouldn't that be the speed of light?

Thanks for posting the link to that very interesting article.

speedfreek
2009-Feb-14, 07:22 PM
Hi there!

Firstly, it is important to point out that when the article is referring to what it calls the "cosmic horizon", it is actually talking about the particle horizon (http://en.wikipedia.org/wiki/Particle_horizon). You have referred to it by another name, the cosmic event horizon, which is actually a different thing. These two horizons are both important to your question, however.

The current distance to the particle horizon is 46 billion light-years, if expressed in terms of the region of space where the CMBR we currently detect was emitted from. Those CMBR photons were originally emitted at a distance of only around 42 million light-years away, but at that time the region of space they were in was receding from this region of space at over 58 times the speed of light (Davis & Lineweaver, 2003 - Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe (http://arxiv.org/abs/astro-ph/0310808)) and this is the reason that light which was originally emitted so close to us took so long to reach us.

Those CMBR photons have been travelling for 13.7 billion years and the place they were emitted from (the particle horizon) has receded to around 46 billion light-years away in that time. It was receding at 58 times the speed of light when the CMBR was emitted, and is today still receding at around 3 times the speed of light.

But the cosmological "event" horizon is something different. It represents the distance to an event whose light can never reach us, due to the acceleration of the rate of the expansion of the universe. This horizon is currently around 16 billion light-years away, so if anything happens right now at a distance of over 16 billion light-years, we will never be able to see it here.

The Hubble distance is something else - it is the distance where an object would be receding at the speed of light due to the expansion, and is currently around 14 billion light-years away. We can see objects that were receding at the speed of light when they emitted the light we see. Have a look at that link I posted above, it will explain all this a lot better than I can here.

nutant gene 71
2009-Feb-14, 09:34 PM
Thanks speedfreek, gzhpcu, for your inputs. In reading Davis & Lineweaver, it seems there is a common misconception regarding multiplying Hubble constant time distance when it is very great, so perhaps my first question cannot be answered, if so. I also thought (came to me later) that with relativistic near light c velocities time slows down, so that may impact my second and third question. If time slows (like it does at the black-hole event horizon), then from our observational perspective, any observational changes would likewise slow, or become impossible to measure. Though, I thought to ask these challenging (to me) questions because they could offer a way to falsifiably test for space expansion, though at the moment I am not sure. Other ideas are welcome, of course.

I think this item is perhaps most vexing to me:

But the cosmological "event" horizon is something different. It represents the distance to an event whose light can never reach us, due to the acceleration of the rate of the expansion of the universe. This horizon is currently around 16 billion light-years away, so if anything happens right now at a distance of over 16 billion light-years, we will never be able to see it here.
Can the target galaxy, for example, that is just about to recede past the point of visibility be detected by our instruments as it goes over that 'edge of the universe' observation horizon? And if so, would it appear over time to redshift more deeply, or lose luminosity, before it becomes invisible? Or, in other words, is this a dynamic event which could be measured as the target galaxy approaches the cosmic event horizon, or is it a static event, so it cannot be measured?

Thanks again.

Murphy
2009-Feb-14, 11:25 PM
Wow! 58 Times the Speed of Light!

Id heard before that the Universe could expand faster than the speed of light, but I never thought it would be that much faster, cool.

speedfreek, if I understand you correctly, you said that the speed is now only 3 times c. Does this indicate that the Universe's expansion is slowing down over time? But doesn't that contradict what was discovered a few years ago about the expansion accelerating?

Jeff Root
2009-Feb-15, 01:50 AM
My question is one step past Murphy's. Is the slowing from 58c to 3c
attributed entirely to gravitational deceleration prior to the acceleration?

-- Jeff, in Minneapolis

speedfreek
2009-Feb-15, 03:28 AM
speedfreek, if I understand you correctly, you said that the speed is now only 3 times c. Does this indicate that the Universe's expansion is slowing down over time? But doesn't that contradict what was discovered a few years ago about the expansion accelerating?

The universe is indeed expanding a lot slower today than it was 13.7 billion years ago when the CMBR was emitted, but it is expanding a little faster than it was 5 billion years ago, when the rate of expansion stopped decelerating and started to accelerate. It doesn't contradict the discovery of the acceleration, as the acceleration is included in the calculation of current recession speed of 3 times the speed of light for the particle horizon.

speedfreek
2009-Feb-15, 03:43 AM
My question is one step past Murphy's. Is the slowing from 58c to 3c
attributed entirely to gravitational deceleration prior to the acceleration?

-- Jeff, in Minneapolis

Not entirely, but almost. The majority of that slow-down is due to the initial deceleration, but the final figure is in part due to the acceleration of the expansion and would presumably be a lower figure if the rate of expansion had continued to decelerate. If the rate of expansion had not accelerated, the particle horizon would be closer to us today and its rate of recession would be smaller.

EDIT: On re-reading this I can see it is worded badly. What I mean is that "the slowing from 58c to 3c" is all caused by gravitational deceleration, but the figure of 3c is higher than it would have been without acceleration. So, the slowing from 58c to less than 3c is entirely due to gravitational deceleration prior to the acceleration and the increase of the rate of recession over the past few billion years is due to the acceleration, bringing the figure back up to 3c.

gzhpcu
2009-Feb-15, 05:06 AM
Thanks for the article speedfreak. It clarifies misconceptions I had in this area, such as superluminal expansion of the universe being possible, since it is not an object which is moving, but spacetime which is expanding and the diameter of the universe being larger than the Hubble horizon.

speedfreek
2009-Feb-15, 06:27 AM
Can the target galaxy, for example, that is just about to recede past the point of visibility be detected by our instruments as it goes over that 'edge of the universe' observation horizon? And if so, would it appear over time to redshift more deeply, or lose luminosity, before it becomes invisible? Or, in other words, is this a dynamic event which could be measured as the target galaxy approaches the cosmic event horizon, or is it a static event, so it cannot be measured?

Thanks again.

I would imagine that the light from these objects would be highly redshifted when it reached us, and the last photons would be very far apart. You are correct that the light from the event would be time-dilated too.

One way to think of this situation is to consider the photons in relation to the space they are travelling through. The space around the photons is always "at rest" in relation to those photons, so they always make progress at the speed of light. However, the expansion of the universe is constantly putting more distance, more "space", ahead of those photons.

With any rate of expansion, whether decelerating, constant, or accelerating, you have a distance where an object will apparently recede at light, but as soon as the light leaves that object and moves away from it, it is travelling through space that is apparently receding from us at less than the speed of light, and so it can finally reach us.

With decelerating expansion there is no cosmological event horizon. If the rate is slowing as time goes on, then light will have the space in front of it expanding slower than the space behind it was, and it will make progress towards us, eventually crossing the Hubble distance into space that is receding from us at less than the speed of light.

But with the acceleration of expansion there comes a point where regions of space on the outside of our Hubble sphere are receding from the edge of that horizon so fast that the light moving through them will never reach it. If light cannot reach the edge of our Hubble sphere, it can never progress towards us.

I think it will be many hundreds of billions of years before we finally see light that came from events that are currently just inside our cosmological event horizon and the very last light, the light from that conceptual "edge", would be stretched to a wavelength longer than the radius of the observable universe when it finally reached us. Perhaps the next photon would be tending towards infinitely dim and redshifted, a ghost of an image frozen in time, taking an infinite amount of time to reach us!

astromark
2009-Feb-15, 08:15 AM
Understandably correct as it might be... ' Event Horizon ' does not seem the correct word for this does it ?
I endorse the use of this term for that sphere surrounding a black hole where gravity exceeds C.
At the edge of the visible universe as mater recedes from us is called ' The edge of the visible universe' One day I expect to see images of galaxies disappearing. Its not an event horizon. Its the opposite.


Quote; "But with the acceleration of expansion there comes a point where regions of space on the outside of our Hubble sphere are receding from the edge of that horizon so fast that the light moving through them will never reach it. If light cannot reach the edge of our Hubble sphere, it can never progress toward us." end Quote.

gzhpcu
2009-Feb-15, 10:24 AM
A quote from the OP's first article mentioned:


It is generally thought that our universe began as a tiny patch in some pre-existing space-time forming a bubble which then underwent a burst of exponential expansion. This period of inflation stretched and smoothed our universe, leaving an even distribution of matter and energy.What I find baffling is the concept of a Big Bang in pre-existing spacetime. We speak of the expansion of our universe being due to an expansion of spacetime. So, if our universe, embedded in a pre-existing spacetime (and perhaps a larger universe...) suddenly came into existence (due to a quantum fluctuation, or brane collision...), it starts off as a small area of spacetime which expands enormously.

How does our spacetime expansion manage to expand if it is embedded in a pre-existing spacetime? Does it cause a compression of the pre-existing spacetime? Or send an expansion wave through the pre-existing spacetime?

speedfreek
2009-Feb-15, 12:43 PM
Understandably correct as it might be... ' Event Horizon ' does not seem the correct word for this does it ?
I endorse the use of this term for that sphere surrounding a black hole where gravity exceeds C.
At the edge of the visible universe as mater recedes from us is called ' The edge of the visible universe' One day I expect to see images of galaxies disappearing. Its not an event horizon. Its the opposite.

It is the horizon beyond which we can never see any new events happen. Just as with a black hole, objects on the event horizon would seem to be frozen in time.

Jeff Root
2009-Feb-15, 01:52 PM
A quote from the OP's first article mentioned:

What I find baffling is the concept of a Big Bang in pre-existing
spacetime. We speak of the expansion of our universe being due
to an expansion of spacetime. So, if our universe, embedded in a
pre-existing spacetime (and perhaps a larger universe...) suddenly
came into existence (due to a quantum fluctuation, or brane collision...),
it starts off as a small area of spacetime which expands enormously.

How does our spacetime expansion manage to expand if it is
embedded in a pre-existing spacetime? Does it cause a compression
of the pre-existing spacetime? Or send an expansion wave through
the pre-existing spacetime?
Think "TARDIS".

Really.

-- Jeff, in Minneapolis

gzhpcu
2009-Feb-15, 02:13 PM
Am not that familiar with Doctor Who? (to say, I never saw it...) :confused:

Jeff Root
2009-Feb-15, 04:06 PM
Oh, dear. Well, I never saw Doctor Who until I was about 30 years old.

Actually I think Douglas Adams would be the ideal person to explain this.
Not available at the moment... Someone English, anyway. Lovely accent,
even if they are typing rather than speaking....

Just imagine walking through a door into a room the size of an aircraft
hangar. Go back out the door and walk around the building and it is only
a metre on a side. Space outside the room isn't quite the same as it is
inside the room.

-- Jeff, in Minneapolis

gzhpcu
2009-Feb-15, 04:25 PM
Just imagine walking through a door into a room the size of an aircraft
hangar. Go back out the door and walk around the building and it is only
a metre on a side. Space outside the room isn't quite the same as it is
inside the room.

Except that I can not imagine that without resorting to M-theory and its higher dimensions... Maybe not even then, if we stick to 3 spatial dimensions...

nutant gene 71
2009-Feb-15, 04:59 PM
It is the horizon beyond which we can never see any new events happen. Just as with a black hole, objects on the event horizon would seem to be frozen in time.
Interesting point about objects on the event horizon of black hole seem to be "frozen in time." If this is in fact observable, though dust near galactic center makes observation difficult if not impossible, then it would be a 'test run' for observing similar effect on the universe's observational outer limits. If we can see stars or gas go from appearing to disappearing on the black hole event horizon, then can we see the same effect on the 'edge' of the cosmos? If so, then measuring how this change occurs would give us a better understanding of space expansion on the edge, perhaps?

gzhpcu
2009-Feb-15, 05:33 PM
Interesting point about objects on the event horizon of black hole seem to be "frozen in time." If this is in fact observable, though dust near galactic center makes observation difficult if not impossible, then it would be a 'test run' for observing similar effect on the universe's observational outer limits. If we can see stars or gas go from appearing to disappearing on the black hole event horizon, then can we see the same effect on the 'edge' of the cosmos? If so, then measuring how this change occurs would give us a better understanding of space expansion on the edge, perhaps?
Objects approaching the event horizon of a black hole seem to be "frozen in time" when seen from outside of the black hole, but not for someone inside the black hole looking out. We, in our Big Bang universe, are looking out on the edge of our cosmos.

nutant gene 71
2009-Feb-15, 05:47 PM
Objects approaching the event horizon of a black hole seem to be "frozen in time" when seen from outside of the black hole, but not for someone inside the black hole looking out. We, in our Big Bang universe, are looking out on the edge of our cosmos.
Interesting idea, but I think the difference here is that our observation is from inside a universe expanding, while from inside a black hole (hypothetically) the 'universe' is contracting. :)

gzhpcu
2009-Feb-15, 06:12 PM
Interesting idea, but I think the difference here is that our observation is from inside a universe expanding, while from inside a black hole (hypothetically) the 'universe' is contracting. :)
But, as the black hole gains mass, its event horizon expands... (effect of Hawking Radiation initially too small...):)

P.S. regarding my previous statement, I think I probably misunderstood what Speedfreak meant and was responding to that...

speedfreek
2009-Feb-15, 07:26 PM
If we can see stars or gas go from appearing to disappearing on the black hole event horizon, then can we see the same effect on the 'edge' of the cosmos? If so, then measuring how this change occurs would give us a better understanding of space expansion on the edge, perhaps?

Well I think the theory says that as an object approaches the event horizon of a black hole, a distant observer would see that object slowing down in time such that it would never actually be seen to reach the horizon. The closer it gets to the horizon, the slower it seems to approach it.

The same may be true for galaxies that are receding past our cosmological event horizon - billions of years in the future as we finally see their light coming in, it would be incredibly redshifted and highly time-dilated. If the light emitted from that galaxy when it passes over the horizon will never be able to reach us, then presumably the light emitted just before it crosses the horizon will eventually reach us, but spread out such that the last second of emitted light is received here over a period of many billions of years!

But it is important to remember that all these cosmological horizons are purely conceptual in nature. They are real in that they represent real barriers to our observations, but they aren't real in the sense that anything special is actually happening at our particle horizon, 46 billion light years away.

We assume that there would be galaxies at those coordinates, just as there are galaxies in these parts. To an observer living in a galaxy that is currently 46 billion light years away, our Milky-Way is on their particle horizon, we are living at the edge of their observable universe. To an observer in a galaxy that is currently 16 billion light years away, the Milky-Way as it is today is the last image of our neighbourhood that they will ever be able to see, as we are living right on their cosmological event horizon. And to an observer in a galaxy that is currently 13.7 billion light years away, it is our Milky-Way that is receding from them at the speed of light.

mugaliens
2009-Feb-15, 10:06 PM
Now this is interesting!

"Stranger still, every cluster seems to be rushing toward a small patch of sky between the constellations of Centaurus and Vela."

Sounds to me like a cosmic flush! My only question is, what's the direction of the spin? Clockwise, or counterclockwise...

Seriously, perhaps this is evidence of a gravitational "eddy" as mentioned on another thread. If not an eddy, then perhaps a warp of spacetime along the lines of a Klein bottle of sort through which gravitational energy is focused...

Could these gravitational hotspotes be the source of our "missing matter?"