Aterna's question---the unexpected cosmic horizon

[Martin]

Aterna posted this interesting question

http://www.scienceforums.net/forum/showthread.php?p=431829#post431829

BTW welcome! (she's a new member at SFN)

 

The way I interpret it is you get to travel at a steady speed of c, in a straight line. And the question is, where do you get to? Assuming you can leave earth and travel in a straight line indefinitely (you are immortal--infinite lifetime--so there's no time limitation.)

 

the answer depends on the standard cosmology model, currently supported by vast amounts of data and a consensus of working astronomers, but models do change so that's something to remember.

 

the answer is a bit surprising----traveling at the speed of light for essentially forever (hundreds of billions of years, way more than the current expansion age) you can just barely get to a galaxy that we can now see, at redshift z = 1.7.

 

That isn't even especially far, compared with what we are currently looking at. We see galaxies out at z = 6 and z = 7----way way farther than z = 1.7.

In effect MOST of the galaxies we can see and study and catalog are out of range to an immortal Aterna traveling at the speed of light.

 

And we could in principle make a list of all the galaxies she could reach, so she could pick her target. The list would consist of all galaxies with a redshift less than or equal to 1.7---approximately.

===================

 

Check out Ned Wright's calculator---it embodies the standard LambdaCDM cosmology model. See what presentday distance corresponds to redshift 1.7.

Is everybody familiar with Wright's calculator and able to use it to convert z to lightyears?

http://www.astro.ucla.edu/~wright/CosmoCalc.html

If you don't have the link handy, just google Ned Wright and take the first hit. It's his cosmology website.

 

You can use Wright's calculator to make a rough calculation that will show you where this number 1.7 is coming from.

 

If there is any interest in this thread, I will explain that, or someone else will. It is not especially hard. The distance we are talking about is called the cosmological horizon and the issue is how to get a rough and ready estimate of it. The underlying cause of the horizon is dark energy i.e. the positive Lambda constant.

========================another version of the question===================================================

Before getting into that, there is another version of Aterna's question which is suppose instead of traveling at a realistic speed, or something reasonable like the speed of light, suppose you travel at a billion c.

so in one year you go one billion LY.

 

Just suppose. And say you leave earth and travel in the straightest possible line. How far can you get before you are back at earth?

 

The most recent bunch of cosmological data that we have is the WMAP 5th year data that came out this year

Komatsu et al (WMAP 5th year data, cosmology implications)

http://arxiv.org/abs/0803.0547

 

According to them you would have to travel AT LEAST 650 billion lightyears before you completed the circumference and reached earth again. This is a 95% confidence level.

 

 

See Figure 2 on page 4 and convert the units to lightyears. They give a lowerbound for the radius of curvature.

( 23 *(1/0.72)*3.26 billion lightyears = 104 billion lightyears )

A 'best fit' estimate derived from Ned Wright's January paper was 130 billion lightyears, not far from their lower bound.

So space might have infinite volume but it also might very well have finite. And if it has finite volume then they are telling us the length of the longest possible straight line is AT LEAST 2 pi times 104 billion lightyears-----in other words about 650 billion LY, a lowerbound estimate for the present circumference.

 

Aterna's question was what if you leave earth and travel in the straightest possible line at a constant speed. Where do you get? What's it like there?

Well if the constant speed you travel is c, then at best you get to other galaxies which can be rather like ours but just happens to be around redshift 1.7 or about 15.5 billion LY from us at present.

 

But if you do the same thing but travel at a billion times the speed of light then you can go at least 650 years before you are back in your old neighborhood.

 

It is important to go fast, as I indicated, so that the universe will not have a chance to expand significantly while you are en route.

Edited September 2, 2008 by Martin
multiple post merged

[Dr. Jekyll]

Yes, I find this interesting.

 

The galaxis (well "any space") at the cosmic horizon travels away from us at the speed of light, and the galaxies beyond at an even greater speed. How does relativity theory handle these velocities, that arise due to the expansion of space?

 

If we can observe galaxies beyond the cosmic horizon, we must be observing them as they were just before they exited the horizon, or?

 

Is the cosmic horizon something like the event horizon for a black hole?

 

Do all these questions have an answer? If not, why?

[Martin]

Hi Dr. J,

I am glad you find these things interesting. Have a look at the cosmology SciAm article in my sig---the princeton.edu link. It will explain some of what you are asking about. I will give some short, possibly puzzling answers---but in the SciAm article you can find more extensive explanations that will serve you better than what I can type in a few minutes.

 

The galaxis (well "any space") at the cosmic horizon travels away from us at the speed of light, and the galaxies beyond at an even greater speed. How does relativity theory handle these velocities, that arise due to the expansion of space?

 

Special Relativity and General Relativity are two different theories discovered at different times, by the same person. SR only deals with ordinary motion, the kind that gets you somewhere. The expansion of spatial distances does not get anybody anywhere. The SR speed limit does not apply to the rates of expansion of distance (called recession speed).

 

All the speed limit of SR really guarantees is that nothing can catch up to and pass a photon. I can be in galaxy A and you can be in galaxy B and the distance between us can be increasing at speed of light---and yet you aren't getting closer to anything, you aren't traveling anywhere, you aren't racing photons. Photons still pass you by as if you are sitting still, and (from a SR point of view) you are sitting still. Relativity (einst. general relativity) allows for this, in fact it requires superluminal expansion in a universe like ours. So there is no problem with relatvity handling it. It insists and depends on it.

 

Have a look at the SciAm article. Most of our SFN posters have already, except new arrivals.

 

You have a misconception about the cosmic horizon. You say it is where the galaxies are that are receding at the speed of light. That is not true, at the horizon they are receding a good bit faster. What you mean is galaxies at the HUBBLE RADIUS. that is 13.8 billion LY. Galaxies at that distance are receeding at c.

We easily get light signals from them, and indeed stuff further out.

 

Most of the galaxies that astronomers are now observing were beyond the hubble distance, and were receding faster than light, at the time they emitted the light that we are now receiving from them. It is interesting how this works and the SciAm article explains it clearly with pictures. The key is that the Hubble radius changes with time.

 

If we can observe galaxies beyond the cosmic horizon, we must be observing them as they were just before they exited the horizon, or?

 

Please explain what you mean by the cosmic horizon. The standard definition puts it at round 15.5 billion LY. The galaxies that are receding at exactly c are currently at 13.8 billion LY. The most distant stuff that we regularly observe, the crud that radiated the cosmic microwave background, is currently at a distance of about 45 billion LY (that is more or less as far as we can see, the most distant stuff that we are getting lightwaves from at present). Right now I don't see the sense in what you say. When you have read the SciAm article you will understand how we are seeing stuff which was receding several times faster than light when it emitted the light and which still is receding faster than light. It is neat how this happens. Check it out.

 

Is the cosmic horizon something like the event horizon for a black hole?

yes there is a kind of limited similarity!

but one has to be cautious about pushing the analogy too far. the differences are rather more important than the element of likeness.

Edited September 3, 2008 by Martin

[Dr. Jekyll]

I only got a BA in physics and it was around 13yrs ago since I studied physics, at that point the accelerated expansion of universe wasn't even known. I know that the expansion of space does not contradict the typical "information being send faster than light," in relativity.

 

I just thought that it is possible to set up the typical thought experiment "traveling train on a rail that emits light" and also include expansion of space. I figure that the expansion of space does not play any role, and we still get the same SR formulas. But I havn't sat down and calculated on it.

 

Yes, I mixed up Hubble radius with cosmic horizon. I meant Hubble radius.

 

If we get signals from galaxies outside the Hubble radius, then if we traveled at the speed of light (hypothetically) we could go there? The converse is true, since those galaxies sends information to us at the speed of light. Maybe I misunderstood your post where you say one could only reach z=1.7 galaxies when traveling at the speed of light, I got the impression that those galaxies are at the Hubble radius.

 

I imagined that the Hubble radius would be kinda like the event horizon. When a galaxy exits the Hubble radius, we would only observe photons emitted at that boundary. Forever and ever, similar to what you as an outside observer would see when an object hits the event horizon.

 

I will check out the SciAm article, I figure it clears things up!

[Martin]

I will check out the SciAm article,...!

 

Good. Besides myself, quite a few other posters here at SFN have recommended it. It has cleared things up for a number of people. If it doesn't work for you, ask more questions.

 

If you got a physics BA back in 1995 and still remember some, then you are way ahead. You are probably comfortable with using a scientific/engineering calculator and with some differential equations.

 

the core of standard cosmology is the Friedmann equation that determines the Hubble parameter--and lets you see how H(t) evolves with time. H(t) is proportional to the square root of density (careful! density including dark energy as well as matter.)

 

So H(t) is constantly declining and the Hubble radius, which is c over H(t), is always increasing. If you are still handy with a calculator, you could try calculating the Hubble radius for yourself---c/H(t) for t=present.

H(present) is estimated to be around 71 km/s per Megaparsec.

 

the speed of light is about 300,000 km/s

so does it make sense to you that the current Hubble radius is

300,000/71 Megaparsecs?

 

google will tell you a parsec is 3.26 lightyears, so it is just an easy units conversion and you will have the Hubble radius in lightyears

 

 

If we get signals from galaxies outside the Hubble radius, then if we traveled at the speed of light (hypothetically) we could go there?...

 

most of the galaxies we are able to observe are outside the Hubble radius, we currently observe stuff that is currently at a distance of 45 billion LY from us.

 

the current Hubble radius is only 13.8 billion LY. You need to read the SciAm article. People get hung up on misconceptions. The Hubble radius is just a changing distance scale---should never be pictured as analogous to a black hole event horizon.

 

I imagined that the Hubble radius would be kinda like the event horizon. When a galaxy exits the Hubble radius, we would only observe photons emitted at that boundary. Forever and ever, similar to what you as an outside observer would see when an object hits the event horizon.

 

No. it doesn't work like that. things currently outside the Hubble radius (as long as they are not too far outside) can even today send us messages and they will eventually get here. A rocket ship out at that distance could easily cruise over the line while continuing to send us messages which we would receive.

 

the black hole event horizon business is an imperfect and misleading analogy. there are different kinds of horizons. the hubble radisus is not the same as the cosmological horizon, the cosmological horizon is not the same as the particle horizon (another important horizon in cosmology), none of these horizons are like a black hole event horizon in any simple way.

 

As a former physics major in college you know not to trust verbal similiarity, and to use equations and your calculator to help you think.

==================

Edited September 5, 2008 by Martin

[Dr. Jekyll]

I read the SciAm article, I have seen it before but didn't pay much attention to the Hubbe distance-part .

 

It is 100% clear that light from galaxies beyond the Hubble distance can reach us, if the Hubble constant decreases. But, I thought that the Hubble constant increases with time, since the expansion of the universe is accelerating. I mean, that for a distance [math]d[/math] we have

[math]v(t)=H(t)d[/math]

and the expansion of the universe is accelerating giving

[math]v'(t)>0 \Rightarrow v'(t)=H'(t)d>0 \Rightarrow H'(t)>0[/math].

I guess I misunderstood something?

 

Physics wasn't my major, math was!

Edited September 16, 2008 by Dr. Jekyll

[Martin]

But, I thought that the Hubble constant increases with time, since the expansion of the universe is accelerating.

 

No, that is a common mistake. The scalefactor a(t) increases, and accelerates. But the Hubble parameter is a'(t)/a(t) so it can decrease even in the accelerating expansion case. People often get confused about that, your math should help you figure it out. A fraction can decrease even if both the numerator and denominator are increasing.

 

Hubble parameter does not increase with time. That's wrong. It decreases with time. I can tell you the limit that it will approach, in the standard model. If you want.

[dazed]

Hello folks. This is my first post here but have been reading for a while. I've followed most of what has been said ( my degree is in Bio back in 1974) but have three questions.

 

1. If space is expanding why does the space between the local stars not increase? Is the gravity of objects so far apart able to counteract the expansion??

 

2. If the observable universe is approx 45 Billion LY, how much larger is the TOTAL universe? I think that someone on this thread said that we couldn't know. Shouldn't it be able to be calculated from the CBR??

 

3. If our galaxy and the Andromeda galaxy have different Hubble radii, does it follow that if someone leaves earth at C and travels say 100 years would their Hubble radius be different? Or would they have to be traveling faster than C??

 

Thanks to anyone who can clear this up for me.

 

Interesting place !!

[Martin]

Hello folks. This is my first post here but have been reading for a while. I've followed most of what has been said ( my degree is in Bio back in 1974) but have three questions.

 

1. If space is expanding why does the space between the local stars not increase? Is the gravity of objects so far apart able to counteract the expansion??

 

Local structures are gravitationally bound. Our galaxy is not expected to drift apart. Clusters of galaxies (like our own group) are expected to be stable or even to clump together some more. The expansion of distances affects larger scale stuff, or where there is no effective force binding things together.

 

2. If the observable universe is approx 45 Billion LY, how much larger is the TOTAL universe? I think that someone on this thread said that we couldn't know. Shouldn't it be able to be calculated from the CBR??

 

It might be infinite. The best we can do is put a lower bound on the size. A recent series of WMAP reports had a figure of what they thought the smallest could be. It amounted to a circumference of a bit over 600 billion lightyears. Say 700 billion, in round numbers. they put some sigma on it, I forget what the confidence level was. I think it was conservative.

 

the idea is if it is finite then it is most likely the 3D analog of the 2D surface of a sphere (but no inside or outside of the sphere, just the sphere) and one way to express the size is by the circumference.

 

Ned Wright, who has the famous cosmo website and teaches at UCLA, earlier gave a figure that amounts to a circumference of somewhat over 800 billion lightyears, that was his "best estimate" assuming that it is finite. We always have to keep in mind the possibility that space is infinite.

 

3. If our galaxy and the Andromeda galaxy have different Hubble radii, ...

Wait, how come different values of the Hubble radius?

 

The Hubble radius is defined to be C times the reciprocal of the Hubble parameter, which is assumed to be currently the same for everybody in the universe.

 

If you got a degree in Bio 30 years ago then you can easily memorize the Hubble parameter----it is 71 km/s per megaparsec.

 

I strongly urge you to calculate the RECPROCAL of the Hubble parameter using google calculator. Just go to google and put this in the box and press return:

 

1/(71 km/s per megaparsec)

 

Google will actually calculate what it is for you, then you can multiply by the speed of light and that will give the Hubble radius

====================

 

Maybe you are confused about what the Hubble radius is! The Hubble radius, or Hubble distance, is the distance that is exactly large enough to be increasing at the speed of light. Hubble Law says that the rate distances increase is proportional to how long they are.

 

If a distance is twice the Hubble distance then it will currently be increasing at twice C. If a distance is half the Hubble distance (aka Hubble radius) then it will currently be increasing at half the speed of light.

 

Distances (outside of bound structures like galaxies and clusters) increase by a fixed small percentage each year. So larger distances increase proportionately more than smaller ones.

 

You sound as if you didn't understand what the Hubble radius is. I'm curious to know. What did you think it was. (it is easy to get scientific jargon wrong)

Edited October 5, 2008 by Martin

[dazed]

Martin: Thanks for the answers.

 

You are right I was using the term incorrectly.

 

According the the Princeton article the edge of the observable universe is like an event horizon. It follows that the Andromeda Galaxy would have a different event horizon so would be able to see light from objects that we could not see.

 

That makes my question:

 

If someone leaves the earth and travels in one direction at C would the event horizon they can see be different from what we can see from here or would thay have to travel at multiples of C??

 

Still trying to wrap my mind around some of this stuff.

 

My thought is that you would have to be at multiples of C or you would just be maintaining the current event horizon.

[Martin]

...

 

If someone leaves the earth and travels in one direction at C would the event horizon they can see be different from what we can see from here or would thay have to travel at multiples of C??

...

 

If someone leaves the earth and travels for a while at some speed and then stops and looks around, then the matter they can observe (their observable universe) is different from what someone back on earth would at that same moment be able to observe.

 

your observable universe depends on your location.

 

the distance from you to the horizon is going to be essentially the same for all contemporary observers, wherever you are. but what objects are included within that range of observation will be different.

 

================

 

I want to assume that the someone you imagine travels for a while at some speed (it can be 1/10 c, or 1/2 c, or whatever) and then stops and is again approximately at rest with respect to the CMB. Because someone who tries observing on the fly would have so much of the light Doppler shifted. If he tries to observe while he is traveling at very near C, then the light from behind will be so radically redshifted as to be undetectable and much of the light from ahead will be Xray wavelengths liable to damage his eyeball. It's not that he would be getting light from objects different from those a stationary observer at the same location would see, but that the wavelengths and skymap would be woefully distorted.

 

I don't even want to consider someone traveling actually faster than C. It doesnt make sense to me.

 

Distances to many observed galaxies are increasing faster than C, but they are not traveling, they are essentially sitting still relative to the CMB and the expansion process (the socalled Hubble flow). This is old wellknown stuff. If you haven't already done so, google Ned Wright balloon analogy.

Edited October 6, 2008 by Martin

[JohnB]

Local structures are gravitationally bound. Our galaxy is not expected to drift apart. Clusters of galaxies (like our own group) are expected to be stable or even to clump together some more. The expansion of distances affects larger scale stuff, or where there is no effective force binding things together.

Just to make sure I've got this right. *Pulls out the trusty rubber sheet.*

 

If we had two objects on the sheet, then as space expands, they move apart. Provided they are far enough apart for no effective force to act between them. However, if they are close enough together for an effective force to act, it would be like joining the objects with a piece of string. They would stay roughly the same distance apart while the rubber sheet moved under them. Is that right?

 

Secondary question. If the "other" object is staying the same distance from us, yet the space between us is expanding, would it appear to be moving toward us?

 

I would say "No" because it is staying the same distance away rather than moving closer. But. I would say "Yes" because if it didn't move toward us relative to it's local bit of space (from our POV), then it would get further away. Or am I mixing frames of reference?

[Royston]

Not sure if this has been asked before, (sorry more questions) but is it possible for two galaxies to cancel out any net force between them, i.e there is some critical distance and providing they are of the right mass, local gravitational effects cancel the expansion due to dark energy (exactly) so the two galaxies are essentially statically bound to each other. They would have to be isolated, or not disturbed by any surrounding gravitational effects from other galaxies.

 

EDIT: Hmm, I think I'm possibly being a bit dumb here. Is the universe too dynamic for this to happen e.g the galaxies masses would have to stay constant. Or such an effect would be fleeting.

Edited October 9, 2008 by Snail

[Martin]

... is it possible for two galaxies to cancel out any net force between them, i.e there is some critical distance and providing they are of the right mass, local gravitational effects cancel the expansion due to dark energy (exactly) so the two galaxies are essentially statically bound to each other. They would have to be isolated, or not disturbed by any surrounding gravitational effects from other galaxies.

 

Snail, I believe this is the correct intuitive idea. In effect that there must be borderline cases of clusters of galaxies which are just barely holding together.

 

I don't speak as an expert. I am a retired mathematician who has gotten interested in physics and can function only as an interested bystander. You probably remember that for a few years (like roughly 2001-2005) people used to be investigating these scenarios where the dark energy was not constant but would perhaps increase. Then things that were once bound systems would begin to drift apart. It would be the borderline cases that would be the first to begin to disperse.

 

Happily, the astronomers got in some more data that seemed to confirm that the cosmological constant was, indeed, constant. We would not be seeing an increase like that envisaged. The researchers now seem to have lost interest in discussing those scenarios. But you never know, some new data could bring the idea back to life.

 

In any case if, as people seem to think, the Lambda constant is in fact constant, then I'm pretty sure that there would be, as you say, borderline cases of systems that are just barely viable.

 

BTW our local group of galaxies, some dozen plus smaller clusters IIRC, is expected in the long run to probably merge (starting with the merger of Milky with Andromeda) but the more distant neighbors are expected to drift off. We aren't part of any borderline stable systems as far as I know. But looking ahead like that is necessarily kind of speculative. the best paper I've seen on it is by Lawrence Krauss, if anyone wants a link let me know.

 

Just to make sure I've got this right. *Pulls out the trusty rubber sheet.*

 

If we had two objects on the sheet, then as space expands, they move apart. Provided they are far enough apart for no effective force to act between them. However, if they are close enough together for an effective force to act, it would be like joining the objects with a piece of string. They would stay roughly the same distance apart while the rubber sheet moved under them. Is that right?

 

Secondary question. If the "other" object is staying the same distance from us, yet the space between us is expanding, would it appear to be moving toward us?

 

I would say "No" because it is staying the same distance away rather than moving closer. But. I would say "Yes" because if it didn't move toward us relative to it's local bit of space (from our POV), then it would get further away. Or am I mixing frames of reference?

 

John, offhand I would say "No". But I must confess (it may seem odd to you) that I haven't ever thought about this! I won't be able to reply immediately. Maybe someone else (D_H, Swansont, several others perhaps) will respond before I get back to it. One comment is that the size of these hypothetical motions between those pairs of neighbors near enough to be bound gravitationally tends to be small:

 

We are talking about Hubble Law expansion, a pattern of increasing distances between objects which are stationary wrt CMB.

The percentage rate of increase is 1/140 percent per million years.

So small distances, even if they were increasing, would doing so at such a small rate that you might not notice.

 

Here are a few general remarks, not answering your question but just sharing concepts. I don't consider the balloon analogy or the related rubber sheet as a physical model. I visualize using the balloon analogy because it helps me see how things are moving.

You might like to google "Ned Wright balloon analogy" and watch his animations---he teaches the graduate level cosmo courses at UCLA. he's a good teacher and dedicated to outreach too. The animations show how galaxies move and also how traveling photons of light move.

But being able to picture how things move, how distances change, doesn't necessarily give an intuition about the underlying physics.

 

Another comment, not really relevant to your question but I'll make it anyway. For me, the all-important thing is the CMB. there is this bedrock idea of being stationary with respect to the matter in the early universe---which you know you are if your CMB has no Doppler hotspot/coldspot direction. And there is the related basic notion of a stationary observer, with radar distances measured in simultaneous short segments along a chain stationary observers.

 

There is also the idea of what cosmologists call "cosmological time" or universe time, as measured by stationary observers*. The present moment consists of stationary observers for whom the CMB temperature is 2.725 kelvin. The Hubble law, which cosmologists use throughout, is formulated using an idea of absolute simultaneity, a present moment. The idea of a stationary observer is an idealization---in principle they would have to be located out in the space between galaxies so that their clocks are not affected by being down in some local gravity potential. the idea is really only approximate, but still useful.

 

In the balloon analogy, the stationary points are those which stay at the same latitude and longitude on the balloon. The galaxies are always painted on or glued on so they are stationary, which is fairly realistic because in real life the galaxies move so slowly that they are effectively at rest.

 

Have to think about this later, have some realworld stuff to deal with.

Edited October 9, 2008 by Martin

[Royston]

Snail, I believe this is the correct intuitive idea. In effect that there must be borderline cases of clusters of galaxies which are just barely holding together.

 

I don't speak as an expert. I am a retired mathematician who has gotten interested in physics and can function only as an interested bystander. You probably remember that for a few years (like roughly 2001-2005) people used to be investigating these scenarios where the dark energy was not constant but would perhaps increase. Then things that were once bound systems would begin to drift apart. It would be the borderline cases that would be the first to begin to disperse.

 

Happily, the astronomers got in some more data that seemed to confirm that the cosmological constant was, indeed, constant. We would not be seeing an increase like that envisaged. The researchers now seem to have lost interest in discussing those scenarios. But you never know, some new data could bring the idea back to life.

 

In any case if, as people seem to think, the Lambda constant is in fact constant, then I'm pretty sure that there would be, as you say, borderline cases of systems that are just barely viable.

 

BTW our local group of galaxies, some dozen plus smaller clusters IIRC, is expected in the long run to probably merge (starting with the merger of Milky with Andromeda) but the more distant neighbors are expected to drift off. We aren't part of any borderline stable systems as far as I know. But looking ahead like that is necessarily kind of speculative. the best paper I've seen on it is by Lawrence Krauss, if anyone wants a link let me know.

 

Thanks Martin, I must say I'm quite surprised, as this followed my line of thinking to an extent. We have two constants, Lambda and the gravitational constant, I then thought of the Andromeda / Milky Way situation but wondering if they were pulled away from each other (to a critical distance), and finally I thought of the Milikan oil drop experiment, but applying it to astronomical distances, masses et.c (obviously the drop is static due to the electrostatic force and gravity...but the principle still applies.)

 

Of course the drop experiment is under controlled lab conditions, there are no collisions, supernovas, black holes forming et.c et.c, hence my afterthought that such conditions would be fleeting (in astronomical terms)...but that's certainly given me something to mull over.

 

I'd be interested to read the paper by Lawrence Krauss, despite it's probably beyond my scope...but any information I can grab from it would be useful.

Edited October 9, 2008 by Snail
typo

[Martin]

I'd be interested to read the paper by Lawrence Krauss, despite it's probably beyond my scope...but any information I can grab from it would be useful.

 

I think the Krauss article is good background knowledge, and largely accessible reading, although it does not specifically address your exact question.

It looks ahead to what the universe will look like some 50-100 billion years from now, if there are creatures trying to study it.

 

It takes into account the affect of accelerated expansion. The projection is based on the standard LambdaCDM model, the mainstream model most people use.

 

So he and his co-author take the trouble to imagine in a fair amont of detail what things will look like and what information will be available to scientists then based on observations they can make at the time.

 

The surprising thing is that the universe will look static to them! Because there won't be any more galaxies visible to them so they can measure the redshift! They will still be living in an expanding LCDM universe but they wont see any evidence of expansion. I am oversimplifying a bit and my figure of 50 billion is probably wrong, but that is the rough idea.

 

http://arxiv.org/abs/0704.0221

The Return of a Static Universe and the End of Cosmology

Lawrence M. Krauss (1,2), Robert J. Scherrer (2) ((1) Case Western Reserve University, (2) Vanderbilt University)

5th prize 2007 Gravity Research Foundation Essay Competition, to appear, GRG October 2007

(Submitted on 2 Apr 2007)

 

"We demonstrate that as we extrapolate the current LambdaCDM universe forward in time, all evidence of the Hubble expansion will disappear, so that observers in our 'island universe' will be fundamentally incapable of determining the true nature of the universe, including the existence of the highly dominant vacuum energy, the existence of the CMB, and the primordial origin of light elements. With these pillars of the modern Big Bang gone, this epoch will mark the end of cosmology and the return of a static universe..."

Edited October 9, 2008 by Martin

[Boise State]

great thread, this is mind boggling. My question is who or what started it all meaning our infinite universe

[npts2020]

great thread, this is mind boggling. My question is who or what started it all meaning our infinite universe

 

If the universe is infinite it has no beginning by definition.

[Martin]

If the universe is infinite it has no beginning by definition...

 

It might make for better conversation if you would rephrase your post as question(s):

 

Do we know whether the universe is infinite or not?

Supposing the universe is infinite, does that imply that past time is infinite?

[npts2020]

It might make for better conversation if you would rephrase your post as question(s):

 

Do we know whether the universe is infinite or not?

Supposing the universe is infinite, does that imply that past time is infinite?

 

 

Well, if time is part of the universe and the universe is infinite, as postulated by Boise State, why does that not imply that time is also infinite or does infinity only go in one direction? IMO infinity only exists as a mathematical construct like a circle or a line, any sufficiently large number to be incomprehensible to the human mind will seem like infinity. I do not suppose that the debate over the size of the universe has been settled but it seems certain to me that it could be much larger than the part we can see.

[Martin]

If the universe is infinite it has no beginning by definition...

 

Well, if ...the universe is infinite... why does that not imply that time is also infinite or does infinity only go in one direction?

I think you just got it. An infinite length of 3/4 inch copper pipe is infinite length. But not infinite diameter. When you say something is infinite it has to be clear which spec or specs are infinite. If it isnt clear, either you have to say or the person listening has to ask.

 

Spatial infinite has never been assumed to be time-infinite.

Space infinite has never ruled out having a beginning.

 

I personally am skeptical about whether time terminates or extends back before the big bang. It might or it very well might not. But I would never say that there is no beginning by definition. It is something that has to be settled by more astronomical observation and more testing of theoretical models. At this point nobody who knows what they are talking about will toss off an answer as if it were obvious.

 

 

...I do not suppose that the debate over the size of the universe has been settled but it seems certain to me that it could be much larger than the part we can see.

 

Yes, you have that part right. It almost certainly is quite a bit larger than the observable chunk.

The latest CMB data indicate that it is either spatial infinite or else, if it is spatial finite, the circumference is over 650 billion lightyears with 95 percent confidence.

 

That is, if you had fantasy hypergoggles that allowed you to see matter infinitely far away (without waiting for the light to travel from that matter to you) then in that 650 billion LY circumference universe the farthest matter you could see would be 325 billion LY away.

 

In a 2D balloon analogy that would be the point on the opposite side of the balloon.

 

Right now, with ordinary light and no hypergoggles, the most distant matter that we are getting light from, and therefore can see, is 46 billion lightyears from us. And we don't see it the way it is now, because the light coming from it left home a long time ago (incidentally when that matter was considerably closer)

 

So that gives an idea. In real life we can see 46, and we know with 95 percent certainty that the universe is big enough that with hypergoggles instant vision we COULD see 325. that is seven times farther than 46.

 

So the universe might or might not be spatial infinite---but if it is spatial finite thenwe are pretty sure that it is AT LEAST SEVEN TIMES bigger than we can see.

 

In order to chat, I need questions about the universe. See if there are some things that you don't already think you know and you want to find out about. Otherwise you can continue making assertions and arguments but I may not necessarily reply.

Edited October 15, 2008 by Martin

[npts2020]

Martin: Thanks for the clarification, I don't believe that we have any serious disagreement on this. It is just my opinion that when you define the universe as infinite with no further elaboration, that includes all components of said universe. Now, if you wish to say that time is not part of the universe and exists completely on its own (possible but must be defined as such when defining an "infinite" universe), that is an entirely separate discussion.

[Aterna]

I know I'm late, but wow, you actually made a thread on my question.

 

I was actually hesitant to post that question as I thought nobody would care and it wasn't worth thinking about, me not being very good at science and all and thinking it was a stupid question. Also I must say, reading what you had to say about it in this thread confused me very much and I'm not sure I understand everything. But I'll give it a few more reads and see what I can comprehend. Thanks.

[Martin]

Well you're back!!! Long time no see

 

We got along fine without you, but it is nice you checked back on it.

 

I thought your question was a good one because very graphic. It turns out there are specific galaxies that you can point to that are right on the borderline, and some just a little farther that you could never get to. I found some photographs of them, forgot where the link is now.

 

I know I'm late, but wow, you actually made a thread on my question.

...

 

Here is the gist. You may not understand but that is OK, it depends on how much you have studied this kind of thing.

 

the most obvious distance-related feature of a galaxy is its REDSHIFT. The symbol used is z. The convention is that z+1 is the ratio by which wavelengths of its light have been stretched out. There are characteristic color stripes in starlight and it is easy to see when the wavelengths have been stretched out and measure how much.

 

If a galaxy's light has z = 1.7 that means the stretch factor is 2.7 and that all the wavelengths are longer by a factor of 2.7. there are machines to do this and get the number.

 

ANY GALAXY WITH z = 1.7 is borderline too far to get to! If you leave here today and travel towards it at speed of light you never get there even if you travel for a trillion billion years. Why? because the sucker is receding away too fast. It is right on the border, so I am not 100 percent sure which side of the border it is. For sure a z = 1.8 galaxy you couldn't get to.

 

It's very tantalizing. We can see lots of galaxies with z > 1.7. And we see lots that are even much farther, with like z = 2, or 3, or 4 (these are much much farther). We even see galaxies with redshift z = 7. And even farther.

 

Most of what we see and take photographs of we could never reach, leaving now and traveling at the speed of light.

 

It is because of accelerating expansion. I think it's interesting that so much of what we see is stuff we could never get to. For some reason it reminds me of women. Well anyway, that is the story.

 

Galaxies with redshift less than 1.7, like those with z = 1.6, you CAN get to if you leave today and travel at the speed of light. There are lots of those too, the more nearby ones, so we shouldn't feel so bad.

 

And maybe we should be thinking about sending messages rather than traveling because it is messages that go at the speed of light, while realworld travelers go a good deal slower.

 

Now that is enough. Ask questions if you are curious about any aspect of this. Someone is likely to answer.

Edited October 18, 2008 by Martin

[Aterna]

Ahh, that makes much more sense now that you explained it like that.

 

So, basically, if I were to shoot out the earth right now going at the speed of light I'd never reach those galaxies because they are moving away even faster than me (due to the universe's expansion???). If that's true than, how about going way faster than the speed they're receding? We'd have to eventually reach it.

 

And that's kinda where I was trying to get to with my original question. It was meant to sound something like: if I could right now leave the earth and go in a straight like (this is not a matter of reaching other galaxies) at a speed faster than anything else and eventually reach the "edge" what would I find?

 

I know that the universe is shaped in such a way that you'd be back where you started, but I guess it's because I can't picture it that way that I ask this question. Realizing that the universe is shaped that way, I always think of a circle (which is neverending and you always end up where you started). So I picture myself in the middle of the circle, going toward the edge in a straight line, OUT the circle. I feel really dumb because I'm probably picturing the wrong thing here. I just don't understand how you could possible end up on earth again.