# Position of Earth in the Universe

1. Jul 16, 2006

### Born2Perform

After Hubble studies all are according about space expansion;

I heard many times expert people on this forum conclude that the earth must be quite near the center of it, in order that our view is isotropic and 10/11 galaxies are moving away from us.

I think this is a non-sense: think for a second what would be the situation if we would be about at the edge of the universe: the same. galaxies near the center would appear to us moving with growing speed in time.
We cannot say we are accelerating in order that space itself is expanding and we are firm, so simmetry principle is still valid..

my english is disastrous but...isn't right that Earth could be at almost every position in universe?

2. Jul 16, 2006

### Garth

Yes, the universe is thought to be homogeneous, the CMB is certainly isotropic to a high degree of accuracy (to within one part in 105), so the Earth could be almost anywhere within it and the view on the largest scales would be pretty much the same as from here.

The universe does not have an 'edge' nor 'centre', just as the Earth's surface does not have an edge or centre.

Garth

3. Jul 16, 2006

### SpaceTiger

Staff Emeritus
Either those people weren't experts or you misunderstood what they were saying. It has been said several times that the earth is at the center of the observable universe, but the observable universe is just all that we can see, not all that there is. It should be no surprise that we're at the center of a region defined by all that we can see.

4. Jul 21, 2006

### Chaos' lil bro Order

lol@ST funny last sentence.

5. Jul 21, 2006

### VikingF

If the universe is infinite, then asking where Earth is, is like asking where on the x-axis we'll find x=3.

6. Jul 22, 2006

### Jenny

Homogenous universe:
To tie in expansion:
Have you heard of the rubber band 2D analogy of expansion?
Imagine that we rre a dot on a rubber band and there are numerous dots on the rubber band with similar spacing between each dot. the rubber band is expanding, we would see every dot moving away from us and further dots moving faster (because it's the space between EACH dots getting larger AT THE SAME RATE, but from one DOT, the further the dot we are looking at, the faster it is moving away.)
This analogy applies to the universe, except the rubber band is a 1D figure expanding in 3D space. We can have a 2D figure expanding in 3D space if we imagine a rubber balloon expanding- it similarly has equally spaced dots on it)

What Garth said is right: "The universe does not have an 'edge' nor 'centre', just as the Earth's surface does not have an edge or centre."
The Earth's surface, ie. 2D space, doesn't have an edge or centre, if you think that the centre is the core, well, that's in 3D space...(I just said that because I think it's important to realise that the analogy of the Earth is GREAT except when we try to imagine a universe with it's 3D space... it's not existing in an extra dimension, so hard to visualise!.. in fact, impossible- or is it? hehe, somebody tell me)

7. Jul 22, 2006

### Jorrie

Always useful to remind people that we are at the center of our observable universe just like a sailor on the open sea is at the center of his observable ocean. There is no edge there, just a horizon.

8. Jul 22, 2006

### oldman

I fear that when folk (Space Tiger and yourself in this case) imply like this that there is something beyond the observable universe they are elevating an assumption that we can never verify to the status of an accepted fact.

This takes us into the territory of space-operas that invoke galactic empires, bug-eyed monsters, black holes and time warps whenever they are set beyond the now-familiar territory of the solar system; we've always found it convenient to relegate unexplained mysteries to far-away places, or to the distant past or future, where they entertain us but can't threaten us.

Extrapolation beyond the red horizon/edge? of the observable universe is rather reminiscent of the old practice of talking up demons, deities, spirits, sea-serpents, and dragons, or the new habit of speculating about bubble universes.

Isn't the standard model, with its inflationary scenario, speculative enough already?

9. Jul 22, 2006

### marcus

Hi oldman,

I want to make the point that in the LambdaCDM model there is indeed a "cosmological horizon"-----a kind of limit to how much the observable universe can, in principle, encompass even if one waits 100s of billions of years.

but that is not what one means by "observable universe". Observable means the galaxies and stuff that is ALREADY observable----with light and particles we are already receiving. That is what they conventionally mean by it, and THAT is steadily enlarging. As time goes on we get light from more and more galaxies.

For me it is not reminiscent. I am not interested in the "bubbles" of eternal inflation or all that Susskind stuff. But the gradual enlargement of the up-till-now observable universe is for me a humdrum straightforward assumption----nothing like "multiverses" and bubble speculation.

you express your idea in an entertaining and persuasive way, but I think it involves an error (see red)

I will associate myself with Jorrie and SpaceTiger in accepting the assumption that there is plenty of universe beyond the edge of what we have so far observed, and we will eventually be able to observe it.

call this a prediction or an assumption, as you chose. it is in principle verifiable and it is totally unimaginative (not like your "dragons")

I assume that the edge of what we have (up to the present) observed steadily moves outwards from us and, if one is very patient (extremely patient ) one can OBSERVE it to do so.

I assume the region from which light and neutrinos etc. have so far reached us (conventionally called the observable universe) is continually enlarged so as to contain more and more galaxies

Note that I am not talking about the Albertesque expansion of space but the increase in the number of galaxies which we are in principle able to observe. the simple arrival of more data.

I assume this because it is the simplest. One does not have to invent anything. The least complicated cosmological model fitting the data predicts that (very long term ) we will notice that more and more galaxies are in the observable universe.

To put it in concrete operational terms (so that you can "swing your cat"), the simplest model predicts that some day the redshift of the last scattering (the CMB) will be not 1100, but 1200. and then the temperature of the CMB will be not 2.75 kelvin, but more like 2.50 kelvin.
And within that enlarged sphere, enclosed by a more distant last scattering surface, there will be more galaxies---for our decendents or replacements to count and study, if Life in that future time is still curious about the world.

Assuming as I do is, imo, NOT SPECULATIVE. One takes the simplest model that fits----one checks it as well as one can against all available data---this enlargment is what it predicts----ONE CAN IN PRINCIPLE VERIFY although it will take a long time. And for this enlargment NOT to happen would be very surprising and awkward. One would have to rebuild the model probably in some ugly way, to accomodate the idea that the enlargement of what can so far be observed should abruptly halt---with no more galaxies coming in over the horizon as it shifts outward.
============

BTW just as a footnote. the CMB was originally only some 3000 kelvin. there is lots of hotter stuff in galaxies that one can see (like Xray, gamma).
so at some time in future I suppose people will "see" galaxies at 1100 but brighter and clearer than they now see CMB at 1100---because hotter.
and then neutrinos do not redshift, so that's another means to see deeper

10. Jul 22, 2006

### marcus

oldman,
maybe you would like this

http://arxiv.org/astro-ph/0605709 [Broken]

using the current WMAP data, and what they regard as the simplest cosmological model(s), they calculat that

THE REST OF THE UNIVERSE MUST BE AT LEAST 10 TIMES THE VOLUME of the observable universe.

Of course it could be infinitely larger, or it could be finite but 1000s of times larger. But they use the observations of flatness to conclude that the real volume is AT LEAST tenfold larger than the volume of the observable universe.

Last edited by a moderator: May 2, 2017
11. Jul 23, 2006

### Jenny

sorry to pick out such a small part of your discussion.. but I'm a little confused

Redshift used in that sense is the lenghtening of all wavelengths. However, neutrinos are particles without wave properties (right?) so obviously they don't redshift in that sense. However, if neutrinos from a source that is emitting neutrinos for 3 seconds, from a region of space where there is a large cosmological redshift (ie. very far away) the observer here on earth would see an affect due to large 'redshift' or distance. The source would be seen to be emitting neutrinos for longer than 3 seconds. This is the slowing down of time in the past. (you can look it up online probably for an explination)
What I don't get Marcus, is why does neutrinos not redshifting mean that we can look deeper with it?
{i'm not arguing that neutrinos don't offer a method of observing an earlier universe (theoretically, but not practically at the moment) than electromagnetic radiation. But the reason for that isn't because it doesn't redshift in the conventional way is it? It's because the universe became transparent to neutrinos earlier than electromagnetic radiation. "In general, the weaker the interaction of a particle with matter and radiation, the further back in time it can come from without having been scattered or blocked."
Ref: Swarup, A 2006, “Ghostly particles carry imprint of early cosmos”, New Scientist, 10 April 2006, p18}

12. Jul 23, 2006

### oldman

Thanks, Marcus. Yes, it was indeed an error to say "never", and you are right about things changing. I can only plead oversimplification of what has for some time struck me as a quite complicated situation -- see below.

Consider horizons:

First, there is a boundary which limits our experience, which I’ll call
the Horizon. By this dictionary definition, we can only speculate about happenings concerning objects (that I'll call events) beyond this boundary, at least until the situation changes as you describe.

Second, there is a working horizon, beyond which our instruments are not yet sophisticated enough to penetrate. Our working horizon varies
with the kinds of objects studied and the wavelength of radiation they
emit, and may be changed by invention. At infrared, optical and X-ray
wavelengths it now lies among remote galaxies with active centres, whose light is strongly reddened. At longer wavelengths the working horizon is the cosmic microwave background which presently serves as our Horizon.

In cosmology there are also two kinds of intangible boundaries that have the potential to be our Horizon. These boundaries depend on the nature and history of the universe, and especially on general relativity.

A “big” (as compact as you like but perhaps infinite, with a finite scale factor) universe, which somehow suddenly “began”, contains ourparticle horizon, beyond which light has not had time to travel to us since the universe began. The particle horizon defines the limit of communication and hence the maximum size of isothermal regions. Our working horizon and our Horizon cannot be further away than the particle horizon. In such a “big” universe, as time passes, the particle horizon dilates away from us at the speed of light, bringing previously unobservable events into our ken as time passes.

That is why I shouldn't have said "never". But there is more:

In a “big” enough universe, which is expanding rapidly enough, there
will be a boundary beyond which events expand away from us at
"speeds" faster than light (allowed by general relativistic expansion).
This is our event horizon. Its size depend on the rate of expansion. In the fullness of time the particle horizon may, as it dilates, reach the
event horizon. It cannot dilate further, since the event horizon
may then limit our experience and become our Horizon. One expects light from near the event horizon to be reddened by general relativistic expansion near light speeds.

Observation shows: (1) that our working horizon is red, from which
we conclude that it is close to our event horizon and (2) that on opposite
sides of the sky our red working horizon looks the same. This is to be
expected iff the distance to the particle horizon for all observers is the
same (Copernican principle) and about twice the distance to both our
working horizon and the nearby event horizon.

But the distance to the particle horizon cannot be greater than that to the event horizon.

This unsatisfactory situation is known as the "horizon problem of the
standard model".

The inflationary scenario resolves the difficulty by manipulating the
position of the event horizon with a changing rate of expansion. With
suitably chosen parameters (the number of e-fold expansions, the
duration of inflation and the rate of ordinary expansion before and
after inflation) inflation temporarily reduces the size of the event
horizon, which, as it shrinks, gathers up the particle horizon, as it were, and moves it back into a part of the universe which has already reached thermal equilibrium.

As inflation ends the event horizon gets biggers as the expansion rate slows, but leaves behind in the early universe a relic in the form of a particle horizon around only part of an isothermal region. During the subsequent ordinary expansion this particle horizon dilates and sweeps over some, but not all, of the events in the expanding isothermal relic of the primeval universe.

The net result is that our present horizon can come to enclose only part of the pre-inflation isothermal universe. The horizon problem is thus resolved, or should I say finessed?

My trouble is really with the inflationary scenario, that involves speculation about a primordial universe now beyond our Horizon. But I suppose I'm being too old-fashioned.

13. Jul 23, 2006

### SpaceTiger

Staff Emeritus
There are many astronomers (usually observers) who would agree that we should only discuss the observable universe. However, I'm not one of them, and I have several reasons.

First is the inflationary scenario, which has been and will continue to be tested observationally. Although it does not require a universe of size bigger than we observe, it does allow for it just as naturally as a universe that is smaller. Observations of the microwave background rule out a universe of size much smaller than our particle horizon, so the majority of what remains of parameter space leaves much that we don't see.

Second is the Copernican principle. The size of the observable universe has grown many times over since the end of inflation. The cosmic microwave background, at z~1100, contains some 40,000 observable regions that could not observe one another at the time of decoupling. If the universe had a topological scale around the size of our particle horizon, that would suggest that we are somehow special and the observable universe has stopped growing just in time for our appearance. Traditionallly, astronomers have tried to avoid theories that put humans in a special place because nature is, for the most part, not sensitive to our existence. Rather, such theories usually arise as a result of our hubris.

Finally, although these other parts of the universe are not currently observable, they may have been in causal contact with our part of the universe prior to inflation. As such, they should not be neglected from a theory of our origins.

I'm not saying that we're sure the universe is larger than we can observe, I just think it's important to keep it in the discussion. Like marcus, I think your comparison to dragons, demons, etc. is unwarranted.

Last edited: Jul 23, 2006
14. Jul 23, 2006

### marcus

good point Jenny! what I said was careless.
I agree with what you said that I bolded.

it might be that their not losing energy as space expands will ALSO contribute to the usefulness of neutrinos as a probe into early universe, but I believe their primary advantage is what you said.

===============
incidentally what i was talking about was not really the present situation of observational cosmology but the far distant future when galaxies are seen at z = 1100, instead of just 6 or 7 like now:
the problem I imagine facing observational astronomy way in the future is that ordinary visible light will become feeble long wavelength stuff offering poor resolution----microwaves don't give such a great image. so astronomers may find themselves relying more on light that was originally emitted as Xray or gamma----or even (as I meant to say) not even on light at all, but particles like neutrinos.

I didnt mean to suggest that right NOW the primary attraction of neutrinos is that they don't redshift, I meant that in a hypothetical future context where ordinary light from the most distant galaxies has been redshifted to near uselessness

Last edited: Jul 23, 2006
15. Jul 24, 2006

### Chaos' lil bro Order

Great post ST, I thoroughly enjoyed it. Does inflation theory have observational evidence, or is it a theory born out of necessity used to solve the horizon problem? I suspect it doesn't show in the WMAP because WMAP surveyed the 370,000 year old CMB and inflation occured in the first fractions of second.

16. Jul 24, 2006

### Jorrie

Thanks for a great post marcus! One thing that puzzles me in what you said in the quoted portion above: I understand that as time goes on, we can see further in light-travel-time. But, due to the accelerating lambdaCDM expansion, will some of those present farthest galaxies not move out of our then (larger) observable universe?

17. Jul 24, 2006

### oldman

I'm persuaded by most of what you say in your reasonable post.

I wasn't aware that the horizon problem is so exacerbated by the uniformity (I presume) of the cosmic microwave background. Thanks for pointing this out. It certainly emphasizes the need for a solution to this difficulty, like inflation.

You also said: "If the universe had a topological scale around the size of our particle horizon, that would suggest that we are somehow special and the observable universe has stopped growing just in time for our appearance" . I'm not clear what you mean by "topological scale". Do you mean the separation of places where the universe might connect on to itself, as if it were closed or multiply connected in some fashion?

I thought one of the less appealing features of inflation was that it did away with the Copernican Principle, in that it solved the horizon problem by assigning to us a special location somewhere in the central regions of an relic isothermal patch of the primeval chaos. You seem to be saying the opposite; namely that an appealing feature of inflation is that it supports the CP. I am muddled by how this can be.

Last edited: Jul 24, 2006
18. Jul 24, 2006

### SpaceTiger

Staff Emeritus
That's right.

The first two points are separate. I'm saying that the Copernican Principle can be used to support the idea that the universe is larger than our particle horizon if the observable universe has grown many times over since the epoch of recombination, or earlier (after the end of inflation in the standard model). This is not the same as saying that the Copernican Principle can be used to support inflation over other theories of the early universe.

19. Jul 24, 2006

### SpaceTiger

Staff Emeritus
Last edited by a moderator: Apr 22, 2017
20. Jul 24, 2006

### SpaceTiger

Staff Emeritus
You're right, in fact, but to be part of our observable universe, we only need to be able to see them at some point in their history. If the universe accelerates to the point where these galaxies are expanding away from us at faster than the speed of light, then their evolution after that time will never be observable to us. However, there will still be light traveling to us from these galaxies that was emitted earlier in their history. As time approaches infinity, we'll see the light that was emitted from them just before they began receding at faster than the speed of light. Of course, this light will be redshifted to the point of being unobservable, but in theory, it will be there.

21. Jul 24, 2006

### oldman

Thanks for this reference, Marcus.

I see that Scott and Ziblin also calculate that the "apparent particle horizon" is about 3.5 times as far as the Hubble radius. But the Hubble radius is where the recession velocity extrapolates to c and is (close to?)the radius of the event horizon. Now whichever cosmological horizon is closest to us, is our actual Horizon -- the one that in fact limits our experience ---see my post #12 in this forum. There can only be one experience-limiting boundary.

Ipso facto, we can have no way of telling if what is beyond the Horizon of this epoch is more universe, or shoals of sea serpents. We may believe in a bigger universe, and indeed it is a lot more reasonable than sea serpents. But we can't be sure, and must categorise such extrapolations as speculation.

I maintain that in a speculative subject like cosmology one should clearly distinguish between such activities and things we can hope to someday disprove or verify.

Last edited by a moderator: May 2, 2017
22. Jul 24, 2006

### SpaceTiger

Staff Emeritus
The particle horizon is the experience-limiting boundary. The Hubble radius is not an actual horizon (just a scale distance) and the event horizon is the maximum comoving distance that can be causally influenced by us in the future.

How can we know what will and won't be someday verifiable. I think it's best to leave the theoretical possibilities open and not try to force our own limitations on the universe itself.

23. Jul 24, 2006

### marcus

Hi oldman, I would guess from your post that you believe that if a galaxy is NOW receding faster than c, then no matter how long we wait we will never receive the light that it is emitting in our direction NOW.

If you believe this, please affirm. I happen to disagree and will try to persuade you to my viewpoint. (Many people seem to believe this.)

Many people also seem to believe that it would be impossible for us to be receiving light NOW which was emitted by a galaxy back THEN at a time when the the galaxy was receding from us at speed greater than c.

Please say if you think this. As I understand it, this is a misconception based on misunderstanding the physics. You may also possibly have this (in-my-view) misconception----it is a translation back in time of the situation I described earlier. If so, I will try to convince you otherwise.

I will illustrate this with an hypothetical example of a galaxy whose light we are receiving now, which light was emitted when the recession speed of the galaxy was 2c.

Last edited: Jul 24, 2006
24. Jul 24, 2006

### marcus

yes, in case anybody has the misconception which I imputed to oldman (not actually knowing if he believes it himself or not)

here is an example.

take this calculator
http://www.earth.uni.edu/~morgan/ajjar/Cosmology/cosmos.html [Broken]

put in omega (matter) = .27
lambda (dark energy) = .73
present hubble parameter = 71
redshift z = 4

then you will see that any galaxy we observe with redshift z
was receding from us at speed 2.03 c (over twice the speed of light)
at the moment when it emitted the light which is arriving here today.

and any galaxy with redshift MORE than 4, you can easily check, was receding from us at MORE than twice c, when it emitted the light which we are now receiving from it.

the essential reason is that over the course of history the hubble parameter has decreased, and so the hubble radius has increased (reaching out and encompassing light which might have thought it had no chance to reach us ). in earlier times the hubble radius has grown extremely fast.

the dynamic shrinking of the hubble parameter is governed by a differential equation called the Friedmann equation---key to a lot of standard cosmology physics. the calculator here is based on the standard (Friedmann) model----all it does is "solve the differential equation" for you.

Last edited by a moderator: May 2, 2017
25. Jul 24, 2006

### SpaceTiger

Staff Emeritus
If he did believe this, then he would be correct, at least in the asymptotic $\Lambda CDM$. The expansion is now accelerating, so the Hubble sphere is getting closer. In order to see galaxies (as they are now) that are now receding at greater than c, the expansion would have to decelerate in the future. This is possible, but is not in the standard model.

Either way, though, the Hubble radius is still not the same as the particle horizon and does not define the size of the observable universe.

Last edited: Jul 26, 2006