no centre to the universe?

[JamesF]

no understand what he was saying but if it does not match present observations why is it still used?

It does match present observations. Everything still appears to be moving away from everything else. That's all it concerns itself with.

But look, if it bothers you that much, just forget about the whole balloon thing. it's only an analogy. It's not real and it's not meant to represent what's real. Those people who can understand how the analogy works by demonstrating how something can expand with no centre in two dimensions can keep using it, and you'll just have to find another way to get your head around the concept. It's no big deal.

James

[sologuitarist61]

ok but it dosent match observations so what is the point of it as i said poor analogy

and because it dosnt match observations maybe there is a centre to the universe because hubble was wrong with is analogy

Your argument is not valid either and if you cannot see what is trying to be explained to you then I am afraid there is little hope for this thread moving forward to any useful conclusion.

In the universe everything moves away from everything else (with the exception of local circumstances). In some areas the movement may be slower and in other areas faster, but the overall effect is the expansion that others have tried to explain, is the same everywhere (the expansion that is, not the rate).

As to whether there is an actual centre to the universe then that is something else entirely, and if there is, it hasn't been found yet and may not even be see-able. However, from your point of view you could surmise that the Earth is at the centre, as everything appears to me expanding away from us (Andromeda excluded), but although that appears to be the case, it is a false assumption. The 2D explanation makes perfect sense if you give it a chance and is the best explanation that makes sense of the universe as we see it with our present day knowledge.

[sonic190178]

the balloon analogy doesnt hold if take mega persec distances and the speed of light in to it

so i will try to explain better for you

picture one of the dots as the milkyway and as the balloon expands the dots near to the milkyway dot would appear to expand faster than the dots furthest away from the milkyway dot which is the opposite to the currant observations of the universe in which the galaxies aer expanding at 74kmps mega parsec

so a galaxy 100 mega parsec away is traveling away from us at 7400kmps

and for the balloon analogy the expandion is the same rate all over but if the balloon was a lot bigger like universe size with the speed at which light travels the dots near to us would appear to be travaling faster than the ones furthest away

[ollypenrice]

Yes, the fact that the Hubble Constant turns out not to be a constant does not trouble the balloon analogy at all (or so it seems to me). A few billion years ago the balloon was expanding more slowly than it is now.

There is no conflict with observation. I also think that all cosmologists, including die-hard Steady Staters, use the balloon analogy.

Olly

[ollypenrice]

the balloon analogy doesnt hold if take mega persec distances and the speed of light in to it

so i will try to explain better for you

picture one of the dots as the milkyway and as the balloon expands the dots near to the milkyway dot would appear to expand faster than the dots furthest away from the milkyway dot

No, the dots close to the Milky way dot would move away from the Milky Way dot more slowly than the dots further away. Say the rubber surface is expanding at a rate of one mm per minute per squqre centimetre. A dot a centimetre away recedes from the Milky Way dot at 1mm per minute. A dot two centimetres away recedes at two mm per minute, etc. This is what we see with galaxies, local gravitational bonds excepted.

which is the opposite to the currant observations of the universe in which the galaxies aer expanding at 74kmps mega parsec

so a galaxy 100 mega parsec away is traveling away from us at 7400kmps

and for the balloon analogy the expand at the same rate all over

Again, no. Any single square centimetre of balloon expands at the same rate as any other. The expansion of each square centimetre accumulates, so the more square centimetres between you and a given dot, the faster that dot moves away from you. I now think I can see where your error lies. If I'm wrong do please forgive me. You are thinking of the expanding balloon as if it were a conveyor belt but it isn't like that. Two objects on a conveyor belt stay the same distance apart because the conveyor belt is moving, but it isn't expanding.

Olly (red just used for clarity, not for shouting!!!)

Oh, just two further clarifications; when we are talking about the balloon and the relative moving apart of dots it is vital to remember that we are measuring movement only around the surface of the balloon. We are not allowed to take shortcuts through it in the analogy. Also the fact that the speed limit of the universe is the speed of light does not stop galaxies from moving apart at more than lightspeed. That is because they are not moving in the normal sense. They are moving apart because the space between them is expanding.

[JamesF]

You're trying to endow the analogy with details it was never intended to have. The conceptually difficult bit about the expansion is that everything expands away from everything else no matter were you view it from. That's seriously hard to get your head around in 3D. So the 2D analogy was thought up. It demonstrates in 2D how things can expand away from everything else without a centre. THAT IS THE ONLY THING IT DOES. It does not consider differential expansion rates, dark matter black holes, gravity or anything else whatsoever AND IT WAS NOT INTENDED TO DO SO. It is purely and simply a means of allowing people who can't get a handle on the 3D version to grasp the principle.

This is my last posting on the matter because I feel like I'm kicking a puppy. If you don't understand it at this point., take yourself to the library assuming you still have one, take a load of books out on cosmology, and do a lot of reading.

James

[sonic190178]

also if you do the maths 13.7 billion year old universe

and the galaxies are expanding at 74km per sec mega parsec

a mega parsec is 3.3 millon light years

13.7 billion / 3.3 millon = 4151 mega parsec

4151 * 74 kmps = 307212 kmps faster than the speed of light

speed of light 300000 kmps

= galaxies furthest away from us expanding faster than the speed of light

[Moonshane]

my understanding of big bang theory, albeit slightly (read very) limited, is that the universe was contained within a single point of origin (a singularity) and therefore everything began from the same point which was effectively 'everywhere' (although there was effectively no such thing as space so everywhere is not really an appropriate expression). therefore everything is moving away from everything else and there is no such thing as a centre of the universe. the problem is, understanding what a singularity is, or creating a suitable analogy for the 'expansion' of the universe is extremely difficult in our (read my) rather primitive ape minds.

[sonic190178]

the balloon analogy doesnt hold if take mega persec distances and the speed of light in to it

so i will try to explain better for you

picture one of the dots as the milkyway and as the balloon expands the dots near to the milkyway dot would appear to expand faster than the dots furthest away from the milkyway dot

No, the dots close to the Milky way dot would move away from the Milky Way dot more slowly than the dots further away. Say the rubber surface is expanding at a rate of one mm per minute per squqre centimetre. A dot a centimetre away recedes from the Milky Way dot at 1mm per minute. A dot two centimetres away recedes at two mm per minute, etc. This is what we see with galaxies, local gravitational bonds excepted.

which is the opposite to the currant observations of the universe in which the galaxies aer expanding at 74kmps mega parsec

so a galaxy 100 mega parsec away is traveling away from us at 7400kmps

and for the balloon analogy the expand at the same rate all over

Again, no. Any single square centimetre of balloon expands at the same rate as any other. The expansion of each square centimetre accumulates, so the more square centimetres between you and a given dot, the faster that dot moves away from you. I now think I can see where your error lies. If I'm wrong do please forgive me. You are thinking of the expanding balloon as if it were a conveyor belt but it isn't like that. Two objects on a conveyor belt stay the same distance apart because the conveyor belt is moving, but it isn't expanding.

Olly (red just used for clarity, not for shouting!!!)

Oh, just two further clarifications; when we are talking about the balloon and the relative moving apart of dots it is vital to remember that we are measuring movement only around the surface of the balloon. We are not allowed to take shortcuts through it in the analogy. Also the fact that the speed limit of the universe is the speed of light does not stop galaxies from moving apart at more than lightspeed. That is because they are not moving in the normal sense. They are moving apart because the space between them is expanding.

what you have said works on the small scale but not on the scale of the universe with the speed of light taken in to account so if you class 1cm as a megaparsec than take the time light travels at it would not appear as you have said

[ollypenrice]

also if you do the maths 13.7 billion year old universe

and the galaxies are expanding at 74km per sec mega parsec

a mega parsec is 3.3 millon light years

13.7 billion / 3.3 millon = 4151 mega parsec

4151 * 74 kmps = 307212 kmps faster than the speed of light

speed of light 300000 kmps

= galaxies furthest away from us expanding faster than the speed of light

You have to be careful with the maths here because it works like compound interest. During the light travel time the expansion continues so a galaxy that sent out light which has been in flight for 13.7 billion years before reaching us is no longer 13.7 billion LY away, it is far further away than that.

And it is perfectly true that galaxies can be moving from us at more than lightspeed. See my previous post. They are not receding because they are 'moving' but because space is expanding. The galaxies consider themselves to be keeping more or less still - which in everyday terms they are. There are two kinds of redshift. Doppler redshift is caused by normal movement. Cosmological redshift is caused by expansion of space.

The galaxies which are moving away at more than lightspeed are not part of the 'observable universe' and, so far as we know for now, never will be.

Olly

[Folkert]

The balloon is meant 4D, and the location where it started.... t = 0..... So it started here, just 13.4 billion years ago....

[sonic190178]

so there s ia test that can be made the light that hubble telescope captured of the furthest galaxy if the light disapears from us in a next 50 ,100 ,1000 years then the universe is expanding faster than the speed of light

but if we start to get more galaxies still further out then something is wrong with the currant model

meself i think we are looking at it the wrong way around the universe is slowing down

and if space is expanding faster than light at the edge of the universe then it has no mass so why would this cause the galaxies to expand

but is space truely empty ?

i always thought the it not possable to make a true vaccum?

and thats where the 3d 2d comes in to it if you think of space as being 2d then it possable but not in a 3d space

[SamAndrew]

Trying not to get started in this one as it's too complicated to discuss on a forum,

space isn't expanding faster than the speed of light at the edge of the observable universe, it's all expanding at the same rate ?

[ollypenrice]

Trying not to get started in this one as it's too complicated to discuss on a forum,

space isn't expanding faster than the speed of light at the edge of the observable universe, it's all expanding at the same rate ?

This is an astronomy forum: we can discuss astronomy! Really the point is very simple: you have a small cube of space expanding quite slowly. If that 'quite slowly' is about 70 kilometres per second per megaparesec then that is pretty slow. A second is not very long, a kilometre in astronomy is scarcely worth measuring, but a megaparsec is bogglingly enormous. So the expansion of the universe is very very slow per small cubic unit. However, there are an awful lot of these small cubic units in a universe so their expansion adds up. This is the key point and the one that is causing the confusion. Let's look at this sentence;

space isn't expanding faster than the speed of light at the edge of the observable universe, it's all expanding at the same rate ?

1) We assume that space is expanding at the same rate at any given time. For the moment we don't have much choice. While this is unsatisfactory it has nothing to do with this thread.

2) How do we measure expansion? Imagine a wall made of bricks. For some reason, at a given moment, each bit of mortar between the bricks expands by a millimetre in a second. Two adjacent bricks move apart by a mm in that second. Two bricks a thousand bricks apart move apart by a thousand mm in a second. The difference in speed of separation between adjacent bricks is a thousand times less than the speed of separation between bricks a thousand bricks apart. But he expansion of the mortar was the same everywhere.

Olly

[sologuitarist61]

2) How do we measure expansion? Imagine a wall made of bricks. For some reason, at a given moment, each bit of mortar between the bricks expands by a millimetre in a second. Two adjacent bricks move apart by a mm in that second. Two bricks a thousand bricks apart move apart by a thousand mm in a second. The difference in speed of separation between adjacent bricks is a thousand times less than the speed of separation between bricks a thousand bricks apart. But he expansion of the mortar was the same everywhere.

Olly

Great analogy Olly, never thought of it quite like that before - good one - I shall remember that

[SamAndrew]

There's nothing wrong with discussing it, it would just take me too long to respond to all the points in Sonic's last post

But yes that was my understaning of expansion, I just wasn't totally confident in stating that the rate of expansion was uniform across the universe at any given time. Thanks for confirming that for me

[ollypenrice]

Remember that this is only an assumption and one which EInstein was obliged to make. Perlmutter etc have already refined this by looking at changes in the rate opf expansion over time. I dare say that, one day, searching for non-uniformity in the expansion in different parts of space may become possible and potentially productive. As far as I know we no way of doing this but it may be going on already. We need a professional to come in. What astronomers can and do do, though, is search for large scale local gravitational effects causing galaxies to 'beat against the current' of the Hubble Flow. This research throws up phenomena like 'The Great Attractor' etc.

Wonderful stuff.

Olly

[Scott]

really interesting thread here although i must confess i'm a bit lost. Can anyone point me towards some reading with regards to the expansion of the universe bearing in mind i've only got a year 10 education and that ended 35 yrs ago. I'm not completely thick but i'm no brain surgeon either.The phrase "willing to learn" spring to mind. be gentle with me, i break easily

Scott

[George Jones]

Can anyone point me towards some reading with regards to the expansion of the universe

I'm not sure, but you might try

http://www.ras.org.uk/publications/other-publications/2028-cosmology

[DirkSteele]

This link is a nice visual to help everyone understand why every point in the universe would come to think that it is the centre.

http://www.exploratorium.edu/hubble/tools/center.html

[cfpendock]

Just a couple of observations on this extremely interesting topic. First, I think that it is unlikely that the universe exists in 3d. At least 4d and probably much more. Secondly, the simple arithmetic for calculating speed and distances doesn't apply when the space you are trying to measure distances in is warped, not just by gravity but by time.

Naturally I cannot possibly justify the above comments.

[George Jones]

Yes, the fact that the Hubble Constant turns out not to be a constant does not trouble the balloon analogy at all (or so it seems to me). A few billion years ago the balloon was expanding more slowly than it is now. There is no conflict with observation.

Right. At any instant in time, the Hubble constant is the same throughout space, but at different times, the Hubble constant is different. In the recent past:

1) the Hubble constant was greater than it is now;

2) the rate of expansion of the universe was less than it is now.

I'm not going to get into this *apparent* contradiction in this post; maybe in a later post.

also if you do the maths 13.7 billion year old universe

and the galaxies are expanding at 74km per sec mega parsec

a mega parsec is 3.3 millon light years

13.7 billion / 3.3 millon = 4151 mega parsec

4151 * 74 kmps = 307212 kmps faster than the speed of light

speed of light 300000 kmps

= galaxies furthest away from us expanding faster than the speed of light

Things are actually worse than this. The value you used for the Hubble onstant is (close to) its value at the present times (i.e., not) in the past, but the present distance to the edge of the observable universe is 14400 megaparsecs = 46900 billion light-years. Running through your calculation with this number, and with the Hubble constant taken to be 70.4 km/(sec Mpc) leads to a recessional speed of about 3.4 times the speed of light for objects at the edge of the observable universe.

Secondly, the simple arithmetic for calculating speed and distances doesn't apply when the space you are trying to measure distances in is warped, not just by gravity but by time.

Exactly. Because of spacetime curvature between "over there" and "over here", it is difficult to define the speed of an object "over there" with respect to us "over here" in a way that respects all of our everyday experiences with speed. This leads to a first explanation for the possibility of recessional speeds greater than the speed of light.

Special relativity prohibits speeds greater than the speed of light. Cosmology, however, is governed by the curved spacetime of general relativity, to which special relativity is a good *local* approximation. Consequently, we will never see anything moving faster than the speed of light in our local neighbourhood. Stuff at the edge of the universe is not in our local neighbourhood, and thus is not governed by the laws of special relativity.

Alternate (more technical) explanation for recessional speeds greater than the speed of light.

speed = distance/time, so if different definitions of distance and time are available, we can have have differing definitions of speed. The definitons of distance and time used in cosmology lead to cosmological recessional velocities that correspond not to velocity in special relativity, but to something different called rapidity (sometimes called the "velocity parameter"). In special relativity, there is a relationship between velocity and rapidity, which, for some reason is not used in cosmology. If this relationship were uswed in cosmology than a recession rapidy of 3.4 corresponds to a recessional speed of 0.998 times the speed of light.

Very technical note;

velocity = tanh( rapidity ) = tanh(3.4) = 0.998

Here, tanh is the hyperbolic tangent.

If anyone wants me to expand on this technical alternative explanation, I will.

[Scott]

I'm not sure, but you might try http://www.ras.org.uk/publications/other-publications/2028-cosmology

George, thanks for this link. very interesting read although i think i'll be going over it a few times to get it to sink in