# Expanding Universe

The question of what the universe is expanding into seems to come up all the time and the answer is always the same--here's an example from one of the many FAQs around the internet on the subject that all say the same thing:

"If the universe is expanding, what is it expanding into?"

The universe is expanding---we can see this by observing that galaxies, on average, are moving away from each other. It's natural to ask the next question, "What is the universe expanding into?" It turns out that this question doesn't even make sense. What, after all, is the universe? The meaning we use in cosmology for this word is, "Everything there is." But if this is so, then there is nothing else for the universe to expand into, since it is everything there is.

How can it be expanding if there is nothing for it to expand into? As suggested in another FAQ answer, scientists think that the universe may be infinite. If it is, then the problem is resolved. Consider viewing the expansion backwards in time. The current universe is infinite. At half this size, it was half of infinity, but any mathematician can tell you that half of infinity is equal to infinity. At half that size, it was one quarter the present infinity, and yet still equal to infinity. And so on. The universe has clearly contracted going backwards, and yet at each moment it was infinite.

What if the universe turns out to be finite? Let us make an analogy. A finite universe will curve back on itself, like a sphere or a torus (a doughnut shape). Imagine a balloon in the shape of a sphere, with a grid of dots drawn on it. As the balloon is blown up, it will expand into the third dimension, and all these dots will move away from each other. Now remove the third dimension. Believe it or not, it is mathematically possible to describe the expansion of the sphere's surface (otherwise known as a 2-sphere) without using a third dimension to do so. Similarly, the universe would be curved into a 3-sphere. A 3-sphere is an object of finite volume (just as the 2-sphere is an object of finite surface area), and finite circumference. If you could travel in a 3-sphere in a straight line for long enough, you would return to where you started. The 3-sphere expands, but remains an object solely contained by its original three dimensions, like the 2-sphere expanded without using a third dimension.

To read more about how a 2-sphere can exist without the third dimension, check out this FAQ answer!
In other words, the universe isn't expanding into anything because it is everything. And that's ok.

But I don't understand how that's reconciled with inflationary cosmology, which if I haven't misunderstood anything predicts that our visible universe is most certainly not "everything" and is in fact only a tiny piece of a tiny bubble/pocket (of which there are unimaginable numbers) in a much larger (probably infinite) Universe. So the expansion of the visible universe is a local characteristic of our particular bubble--in fact, there's even an article at space.com today in which Mario Livio discusses the possibility that the value of dark energy varies from pocket universe to pocket universe (and I assume he's talking about the type of pockets that arise in inflation).

Is that last bit correct? That the expansion we observe regards only our little piece of the Universe and not the entire Universe itself?

Anyway, my main question: if the expansion of spacetime is just the expansion of the tiny amount inside our little bubble of Universe then isn't the generic "the universe isn't expanding into anything because it is everything" flat out incorrect? Isn't the expansion of the universe then similar to a bubble in my can of Sprite expanding (in that there clearly is an outside of the bubble to expand into)? Or am I missing something?

I guess I just don't know how the accelerating expansion of "the universe" meshes with the superuniverse that inflation seems to say we're in.

Related Astronomy and Astrophysics News on Phys.org
Inflation didn't say that we were in a superuniverse. Inflation simply mentioned that at one point just after the big bang the expansion of our universe was so huge that it exceeded the speed of light. Due to this there is now an area which we cannot observe (outside of our observable universe) because light hasn't reached that part yet. As far as we know so far, and there is lot of different ideas on this, there is only one universe... our own. Just because there is an area that we cannot see it doesnt mean that it isnt part of our universe. The expansion now isn't expanding into anything... the space itself is stretching so that if you hooked a ruler from one galaxy to another and left it for a bit. Once you pulled it back the distance on the measure wouldn't have moved but the the distance between each cm measure would be larger ie. as space stretched, the gaps between each measure on the ruler also stretched, however, the galaxy's didnt actually do any moving of their own. Therefore our universe can expand at an accelerated rate now because at one point there was inflation, this led to a slowing of expansion, and now it is speeding back up again. Some scientists say that this is how it will be forever... a cycle of accelerated and decelrated expansion. Who knows one day expansion may even cease and we will be on our way to a big crunch. Hope this helps... anymore questions on this topic just reply and i will try and sort them out for you. Regards K_

Please see my other post then.

people with questions about inflation theory, or BBT at all, should question it altogether.

Do you think that the Shapiro effect can better explain galactic redshifts than the Doppler Effect, or maybe at least some combination of the two?

That way when a galaxy has z>1 nobody needs to start thinking about FTL expansion.

so, is this website smoking crack?

http://www.geocities.com/newastronomy/Index.htm

marcus
Gold Member
Dearly Missed
europium said:
Inflation didn't say that we were in a superuniverse. Inflation simply mentioned that at one point just after the big bang the expansion of our universe was so huge that it exceeded the speed of light. Due to this there is now an area which we cannot observe (outside of our observable universe) because light hasn't reached that part yet. As far as we know so far, and there is lot of different ideas on this, there is only one universe... our own. Just because there is an area that we cannot see it doesnt mean that it isnt part of our universe. The expansion now isn't expanding into anything... the space itself is stretching...
thanks. I want to confirm the picture you give, europium, which agrees fairly closely with the standard cosmology picture
all that superuniverse stuff about bubbles
strikes me as highly speculative
unsupported by evidence or even by clear rational argument

for a working cosmologist there is, I believe, one universe with one set of
fundamental physical constants
and it is expanding (not "into" anything which would be kind of absurd, but just expanding)

the distance between (a generic pair of) galaxies is increasing
and a long tape measure would show that increase (if you could stretch one
out that far). I assume the tape measure would not stretch since it is held together by iinteratomic forces that form molecular bonds. the atoms forming the tape measure dont get bigger, the lattice spacing does not get bigger, tape measures and galaxies dont get bigger------galaxies simply get farther apart.

the distance between a typical pair of galaxies is expanding FTL
(the expansion speed is proportional to distance and a randomly chosen pair of galaxies will, with high probability, be separated by more than the hubble distance)

basically I'm just agreeing with europium's post, though we may differ
on a few details

""the distance between a typical pair of galaxies is expanding FTL
(the expansion speed is proportional to distance and a randomly chosen pair of galaxies will, with high probability, be separated by more than the hubble distance)""
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andromeda and the milky way are typical galaxies
they are moving toward one another

if a random far-flung galaxy is flying away from us at FTL speed, how do we see it?
are we seeing the light that left it BEFORE it crossed the light barrier (wrt us)?

marcus
Gold Member
Dearly Missed
shrumeo said:
""the distance between a typical pair of galaxies is expanding FTL
(the expansion speed is proportional to distance and a randomly chosen pair of galaxies will, with high probability, be separated by more than the hubble distance)""
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andromeda and the milky way are typical galaxies
they are moving toward one another

if a random far-flung galaxy is flying away from us at FTL speed, how do we see it?
are we seeing the light that left it BEFORE it crossed the light barrier (wrt us)?
Most of the galaxies which are observable (to us now) are at redshift z greater than 2
and so they were receding from us FTL at the time they emitted the light which we are now receiving

and they still are at present receding FTL from us

look at a basic cosmology tutorial like Ned Wright or Lineweaver
or the remedial paper on FTL recession speeds by Tamara Davis, called
"Expanding Confusion"

Or even better, go to Morgan's cosmology calculator and put in z = 2
and it will tell the recessionspeed both now and when the light was emitted.
http://www.earth.uni.edu/~morgan/ajjar/Cosmology/cosmos.html [Broken]

Shrumeo, when you use Morgan's calculator, put Omege = 0.27
and Lambda = 0.73 and H = 71

this is for the observed features of the U, namely flat with dark energy 0.73 and hubble parameter 71

then just type in z = 2 and click on "calculate"

the calculator program was published in "Sky and Telescope"
but Siobahn Morgan, who teaches astro courses, kindly put it on line.
Ned wright, who teaches cosmology at UCLA, has a similar one.
they are great tools for learning cosmology!

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marcus
Gold Member
Dearly Missed
shrumeo said:
if a random far-flung galaxy is flying away from us at FTL speed, how do we see it?
are we seeing the light that left it BEFORE it crossed the light barrier (wrt us)?
If a farflung galaxy is flying away from us at FTL speed
and, say, was flying away from us at FTL when it emitted the light
which we are now receiving, then (you ask)
"how do we see it?"

We see it by the light which it emitted traveling to us (and overcoming the expansion of space, in the process).

You ask are we seeing it from before. I think you mean from before it started receding FTL. this is a strange question because maybe it was always receding FTL.
I think by "light barrier" you mean the Hubble radius
It helps to understand that this is not a barrier and that it is always
changing. altho not a physical barrier it can be a useful distance scale.

the hubble parameter H has been decreasing rapidly and the
hubble distance c/H
has therefore been increasing
so the hubble sphere (with that radius) has been expanding
at an incredible rate

Morgan's calculator gives values of H in the past. Much bigger than present 71. For z = 2, H is something like 200.

so consider a photon that is emitted towards us from some galaxy outside the hubble sphere and begins traveling towards us. at first it is "swept back" and the distance to it increases steadily
but then the expanding hubble sphere engulfs it and now it is traveling
in space that is NOT receding FTL
and it is traveling at c, thru that space, so it now makes headway towards us and eventually gets here.

Lots of ways cosmologists have of thinkingabout this, the fish swimming in a current of water analogy etc etc. Always somewhat intuitive.

Tamara Davis thesis goes into this some, also her remedial article
"Expanding Confusion" which deals with the confusions people have about the expansion of space

Chronos
Gold Member
z>1 is not greater than lightspeed last time i checked. z has a minimum limit of -1 but not a maximum.

i just go by what i hear. you can dismiss what i say if you wish based on that.
i read in one place that z>1 means light speed. sorry if i was misinformed.

Ned Wright looks like he REALLY wants inflationary cosmology to be right. He defends it like a creationist.

the redshift only makes it look like FTL speed if you ASSUME that it's only due to expanding space and/or the doppler effect. we really don't know why FOR SURE there is a redshift vs. distance relationship. start woking under a different paradigm and all the data could be made to fit that one too, at least better than it fits BBT

i think i even read that when someone went and redid what Hubble did, they got different results than Hubble. The relationship between redshift and distance wasn't linear for them, but quadratic. forgive me, i can't remember where I read it but it was online somehwhere. So the redshift, for them, seemed to be proportional to the square of the distance. Again, I wish I could find that again, sorry.

then old Alton Harp keeps finding SEEMINGLY physically linked objects with drastically different redshifts. i haven't read enough about it. who's to say what's connected to what 10 b ly away

((yeah, up in my earlier post i said something about the shapiro effect. somehow, i was again misinformed. supposedly the effect is not on the energy of the light but on the velocity of light (???) which of course, is always c in all refence frames, so the shapiro effect doesn't even make sense to me. ))
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no one has found any dark matter, BBT falls apart without dark matter
no one has found any dark energy, BBT falls apart without dark energy
BBT (despite what folks say) relies on a CURVED universe, not the flat one found
BBT relies on isotropy of matter over large scales, it's not there nearly as much as it should be
BBT relies on large fluctuations in the CMB, not the dinky ones found
BBT predicts CBR of 5-50K, 1920's calc's said that space should be filled with ~3K microwaves, which it is
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i can say that monkeys fly out of my butt, than make a calculator that will tell you the number of monkeys that would fly out of my butt whether they are baboons or macacs
that does not mean monkeys fly out my butt.

Chronos
Gold Member
the z thing is about spectral line placement relative to a 'motionless' reference frame. -1 z means it is a zero wave length [not possible mathematically]. the plus side is how many multiples of the 'normal' wavelength. the plus multiple, using relativity rules is, however, mathematically unlimited.

i do agree with your objection to dark matter [i have a problem with 'invisible' properties of the universe]. removing 'dark matter' also removes most needs for 'dark energy' to explain this universe.

The fundamental question - what is the universe expanding into? - is obviated by the metric - we do not have to consider the Hubble sphere as embedded into a 4 dimensional space - rather the location of all points is contained within a 3 dimensional spatial section. Its easy to see with a two sphere - if the present universe is represented as the surface of a balloon, it is finite - and there is nothing other than the surface - now add air to the balloon - the surface area is larger, but it is still finite - it hasnot expanded into anything - rather, what there is has become larger in the sense that the points are further apart - but it is still a two dimensional surface that contains the entire universe - all of the points can be measured with respect to each other by geodesics laid out on the surface - you do not need to consider the radial dimension (it doesn't really exist because there is no third dimension). Now ratchet up mathematically to one more dimension, and you have the Robertson-Walker metric that describes our three dimensional space in terms of three dimensions.

Chronos
Gold Member
this is pretty much the same as saying the universe is self referential. cahill makes the same claim. but so did lorentz. the real question remains whether there is an absolute reference frame. i think there is. acceleration, under gr, insists there must be a lorentzian type self referential frame of reference. a body that has inertial resistance to motion must subscribe to this principle.

Nereid
Staff Emeritus
Gold Member
shrumeo said:
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no one has found any dark matter, BBT falls apart without dark matter
no one has found any dark energy, BBT falls apart without dark energy
BBT (despite what folks say) relies on a CURVED universe, not the flat one found
BBT relies on isotropy of matter over large scales, it's not there nearly as much as it should be
BBT relies on large fluctuations in the CMB, not the dinky ones found
BBT predicts CBR of 5-50K, 1920's calc's said that space should be filled with ~3K microwaves, which it is
-----------------------------------------------------------------------------

i can say that monkeys fly out of my butt, than make a calculator that will tell you the number of monkeys that would fly out of my butt whether they are baboons or macacs
that does not mean monkeys fly out my butt.
These are all very good questions.

Dark matter has an awful lot more going for it that just an expanding universe/cosmological models; there's X-ray observations of clusters (assume an equilibrium IGM, the only way you can account for the X-rays is a whole lot of non-baryonic mass), gravitational lensing (both weak and strong; of still more distant galaxies), rotation curves, galaxy motions in clusters (including the Great Attractor), ... Sure, we're far from dotting all the i's and crossing all the t's, but the only alternative that comes even remotely close is MOND, and that's all at sea wrt lensing (among other things). The fact that estimates of the amount of DM, from different sets of observations, are all close also suggests it's real (for those interested, this last statement can be re-cast in more rigourous, formal statistical terms).

IMHO, a lot more work is needed around dark energy; leave it a decade or two and see what the next few TB of data says.

The other objections (isotropy, large CMBR fluctuations, 5-50K) are new to me - do you have some references please?
the redshift only makes it look like FTL speed if you ASSUME that it's only due to expanding space and/or the doppler effect. we really don't know why FOR SURE there is a redshift vs. distance relationship. start woking under a different paradigm and all the data could be made to fit that one too, at least better than it fits BBT
Such as?
i think i even read that when someone went and redid what Hubble did, they got different results than Hubble. The relationship between redshift and distance wasn't linear for them, but quadratic. forgive me, i can't remember where I read it but it was online somehwhere. So the redshift, for them, seemed to be proportional to the square of the distance. Again, I wish I could find that again, sorry.
If you look at the original Hubble paper, you have to wonder how it got past peer review! The error bars on the data have shrunk an awful lot in the past ~80 years, and the distance scale of the universe (essentially, the present value of Ho) is far better, and more accurately, determined; Hubble was out by quite a lot.
then old Alton Harp keeps finding SEEMINGLY physically linked objects with drastically different redshifts. i haven't read enough about it. who's to say what's connected to what 10 b ly away

((yeah, up in my earlier post i said something about the shapiro effect. somehow, i was again misinformed. supposedly the effect is not on the energy of the light but on the velocity of light (???) which of course, is always c in all refence frames, so the shapiro effect doesn't even make sense to me. ))
I have yet to get back to Arp et al (and turbo-1's excellent posts); stay tuned!

Nereid said:
The other objections (isotropy, large CMBR fluctuations, 5-50K) are new to me - do you have some references please?Such as?
hey, i just know what i have read, and i've only read other physicists objections to IBBT posted on the web (and those I listed were the few that I remember)

http://www.science-frontiers.com/sf017/sf017p04.htm
http://nowscape.com/big-ban2.htm

This may all be a great big pile of crackpot bullsh*t. I wouldn't know, I'm a complete ignoramous, since I do not work in the field. Could be why I keep coming back to this forum. I realize the "science-fronteirs" website look like silliness.

This one ( http://nzphoto.tripod.com/3dastro/ [Broken] ) seems to straddle the fence and take a devil's advocate approach to the discussion.

I couldn't find any pages listing the objections I stated, I don't think.

I would try to Sci-Finder some papers, but I honestly wouldn't know what to search for.

Could "The Great Attractor" simply be a center of gravity, in the middle of a large "orbit system" within or surrounding the observable universe? Or is that totally inconsistent with what's seen?

How the heck did they conclude, based on the WMAP, that the "age" of the universe is 13.7 bya +/- 1% ?? This is based on a particular model.

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Red shifts

It is accepted among all cosmologist that the universe is expanding due to the Doppler red shifts of some galaxies that seem to point to the universe expanding from a common point (center). This with the evidence of the cosmic background radiation points to the VISIBLE universe being an expanding spatial bubble. Red shifts can also show contraction such as those seen on the event horizion of a black hole so one may ask as to whether we are deceiving ourselves. The question of what the universe is expanding into may seem basic to some but it is profound all the same. The idea of the universe expanding into nothing is almost absurd since the thought of "nothingness" is ugly to both scientist and philosophers.
To me, the idea that the universe keeps getting bigger and bigger in one direction and smaller and smaller in the other points to the microscopic and macroscopic being one and the same. As far as we know, infinities are just a concept. Stephan Hawkins once said that the universe is probably finitie but unbounded. Asking what the universe is expanding into is like asking what is north of the north pole.

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shrumeo said:
Could "The Great Attractor" simply be a center of gravity, in the middle of a large "orbit system" within or surrounding the observable universe? Or is that totally inconsistent with what's seen?

How the heck did they conclude, based on the WMAP, that the "age" of the universe is 13.7 bya +/- 1% ?? This is based on a particular model.
The Great Attractor (GA) is a local anomaly in the CMB. While the intrinsic WMAP dipole shows that we are moving with velocity towards this "Great Attractor" we find that the gravitational acceleration we experience from observable galaxies and galaxy clusters cannot reconcile the magnitude of our velocity towards the GA. Now the problem with the GA is that it hasnt been observed simply because it is located behind the plane of the Milky Way. However, Galaxy cluster surveys in the X-ray (ROSAT stuff) have managed to spy an overdensity in a region pointing magically in the direction of the WMAP dipole. I think someone already mentioned the usefulness of DM in X-ray cluster mass estimation.

I think the 13.7 bya was based on the Omega=0.7, Matter=0.3 model. Don't take that for gospel because it could easily have been calculated with respect to the older but popular Einstein-deSitter model (Matter=1). But yeah the age as you've quoted was calculated by a certain model.

hellfire
shrumeo said:
How the heck did they conclude, based on the WMAP, that the "age" of the universe is 13.7 bya +/- 1% ?? This is based on a particular model.
The density perturbations in the universe after inflation came into the particle horizon an started oscillating due to the radiation pressure. Since the matter-radiation plasma before recombination was freezed during recombination (with decoupling of matter and radiation), some of these oscillations are still visible in the CMB anisotropies. These are called "peaks". The first one corresponds to the perturbation entering the horizon at recombination, the second one to the half length and so on.

The power of the peaks is determined by the radiation density, the baryon density and the cold dark matter density. How each parameter influences the peaks is a very interesting subject, but above my level. To have a cosmological model to predict the age of the universe also the curvature, the density of the dark energy and the Hubble parameter are needed.

AFAIK, there is no possibility to infer about dark energy from the acoustic peaks and one is forced to take other information within the CMB data: the secondary anisotropies (CMB fotons perturbed after recombination). To estimate dark energy the following are be used: the Sunyaev-Zeldovich effect (scattering of electrons in the intra-cluster medium with CMB photons), which, together with X-ray observations, may be used to extract distance-redshift relations from far clusters, and also the intergrated Sachs-Wolfe effect (gravitational blue- and red-shift), which may give information about the cosmological evolution of gravitational potentials.

How the Hubble parameter is estimated making use of the CMB data is not clear for me. I think there is some kind of degeneracy so that different values of curvature and hubble parameter fit the data.

Regards.