# Probably a simple question

Hi forumers!

Right, trying to get my head around the principle that if the universe is "everything", what is it expanding into?

It's not expanding into anything. Analogies to day-to-day world break down here. Perhaps think of 'expanding' itself differently---i.e. just, 'getting bigger.'

FlexGunship
Gold Member
Hi forumers!

Right, trying to get my head around the principle that if the universe is "everything", what is it expanding into?

The universe is best expressed as the series of dimension on which events occur. The current total number of known dimensions is four. You can think of them as height, width, depth, and time; or x, y, z, and t.

Presently, the location of any event can be described using only those four dimensions (of course, some hypotheses purport many more). Let us assume, for the purposes of this discussion, that the entire universe could be represented on a single piece of graph paper.

There is a known distance between the lines on the graph paper. However, if the paper were elastic (i.e. stretchy), we could increase the area in between the lines. This is a property of the paper, and NOT of the region in which it expands into.

In the same way, an expanding universe does not expand INTO something, simply, the distance between the lines gets longer. The truly interesting thing is that, from our perspective on Earth, it seems that everything around us is expanding evenly. We would only expect to see this from the center of the "graph paper."

Does this mean the Earth is at the center of the universe? Hardly; it means that there is a boundary, beyond which, the universe is expanding faster than the speed of light. Since nothing can propagate itself faster than the speed of light, it means we see a homogeneous mixture of the universe (since the universe was once a homogeneous mixture itself) until a very specific point after which we see nothing at all.

So, to summarize, the universe isn't expanding into anything, the area between any given points is simply getting larger.

Many thanks.

So proportions given to the universe are the "dust cloud" thrown out from the big bang, and not inclusive of the void space?

Does this imply that the void space is truly infinite?

Chronos
Gold Member
Probably not. Space without intervening matter is - meaningless.

Probably not. Space without intervening matter is - meaningless.
Ok, my doubt is, as per Science will that void space is still considered as space but not belonging to Universe?

In other words are Universe and void space different? If so then we have two things, void space and known Universe. But will it not make the definition of Universe invalid? (Because, when we say Universe it should include everything even the void space also, I believe)

I have this question for many years in my mind but explanation given by people that "space without matter is meaningless" is not convincing to me at all.

Probably not. Space without intervening matter is - meaningless.

Meaningless or just not quantifiable/measurable? For me there is a big difference for the sake of perception.

Ok, my doubt is, as per Science will that void space is still considered as space but not belonging to Universe?

In other words are Universe and void space different? If so then we have two things, void space and known Universe. But will it not make the definition of Universe invalid? (Because, when we say Universe it should include everything even the void space also, I believe)

I have this question for many years in my mind but explanation given by people that "space without matter is meaningless" is not convincing to me at all.

There's no void space required. Space-time expands, but it doesn't have to expand into anything, since that would just be more space-time by definition.

However just because a "hyper-space" isn't required, that doesn't mean it doesn't exist. Many modern physics & cosmology theorisings require a hyper-space, but presently we don't have any physical evidence for it.

Chronos
Gold Member
Meaningless or just not quantifiable/measurable? For me there is a big difference for the sake of perception.
Agreed, not quantifiable/measurable = meaningless.

Gold Member
I think a question like this is similar to questions such as "what is consciousness" or "what was before the beginning of the known universe". Anyone who might have an answer to one of these questions would probably have an answer to all of them. If you want to ponder what is outside the known universe then you have to think beyond your ordinary way of thinking. You might have to ponder what a universe might be like without the dimension of time or the dimensions of space. You might imagine what does it mean to be a singularity or what are strings made of (if they exist). In other words, you can't package such a question into terms used in the ordinary world of reality as we know it. That doesn't mean you should stop thinking about it, as who knows what clever and interesting ideas you could come up with. My point is, to think about a question like this you definitely have to think outside the box.

Chronos
Gold Member
It is pointless to ponder what is outside your known universe, as well as pointless to project your own mind beyond its cognitive limitations.

Gold Member
It is pointless to ponder what is outside your known universe, as well as pointless to project your own mind beyond its cognitive limitations.

This is just wrong. Sorry. The progress of science depends heavily on pondering what is beyond one's current comprehension. Do you think relativity would have come about if Einstein hadn't considered the possibility that time is variable in nature even though at the time such a thought was about as unreasonable as one could possibly imagine? You are simply wrong here.

The present opinion is the undetectable universe is to the detectable universe what our detectable universe is to an atom. Try wrapping your head around that one without grinding synaptic gears.

The truly interesting thing is that, from our perspective on Earth, it seems that everything around us is expanding evenly. We would only expect to see this from the center of the "graph paper."

This is not necessarily true. If everything is expanding at the same rate, then no matter which area you are looking from, everything would appear to be expanding away from you at the same rate.
The only catch would be that you could not be at the edge of the universe because then it would only expand in directions facing towards the interior while (i imagine) it would appear that you traveling away from the universe into the abyss.

Correct me if I am wrong. I have not gone to school on the subject but I spend a lot of time on these forums because astronomy is a fascinating subject to me and am always looking to expand my knowledge.

FlexGunship
Gold Member
This is not necessarily true. If everything is expanding at the same rate, then no matter which area you are looking from, everything would appear to be expanding away from you at the same rate.
The only catch would be that you could not be at the edge of the universe because then it would only expand in directions facing towards the interior while (i imagine) it would appear that you traveling away from the universe into the abyss.

Correct me if I am wrong. I have not gone to school on the subject but I spend a lot of time on these forums because astronomy is a fascinating subject to me and am always looking to expand my knowledge.

Well, allow me to carefully and considerately correct you then.

Imagine you are at the intersection of a two-by-two grid. If every square is expanding uniformly (maybe, one unit/second), then the edges of the grid recede from you at equal rates.

However, if you add an row of squares to the top (now 2x3) but remain in your current position (not the center), then that row is moving away, firstly at one unit/second from the original two-by-two grid, but now this row is expanding also! So the farthest edge is now moving away from you at two units/second!

Allow me to try to diagram it using some simply arrow (so I don't have to link a bunch of pictures).

Let's imagine that arrows expand at the rate of "one hyphen/per line." That is to say, that every type I type a new line, every arrow will expand by one hyphen. We can imagine that each arrow is a "lightyear" or some other unit of distance in the universe. Let's start with a symmetrical example (where we are at the center of the arrows) by imagining that we are located in a univser that has five bodies: the '#', the '$', the '%', the '*', and us. We are located at the big 'U'. #<- <-$<- U ->% -> ->*
#<-- <-- $<-- U -->% --> -->* #<--- <---$<--- U --->% ---> --->*
#<---- <---- $<---- U ---->% ----> ---->* The distance between "U" (which stands for Us!) and the '$' or the '%' increases by "one hyphen/per line." Since they are an equal distance from us on both sides, they appear to be moving away at the same rate.

The distance between us and the '#' or the '*' increases by "three hyphens/per line." Since they are an equal distance from us on both sides, they appear to be moving away at the same rate. Notice that the distance between the '*' and the '%' is increasing by "two hyphens/per line. This is also the rate at which the '%' and the '$' a re moving away from each other. So, if we were in the the place of the '%', we wouldn't see the edges moving away at the same rate. The '#' would move away at an amazing "four hyphens/per line"!! So we can conclude, that things seem to recede equally on all sides only if you are in the middle of the expanding medium. (The wild card here, is that if something moves away from you at faster than the speed of light because of aggregated expansion, then you don't get to see it.) Last edited: FlexGunship Gold Member The present opinion is the undetectable universe is to the detectable universe what our detectable universe is to an atom. Try wrapping your head around that one without grinding synaptic gears. I think you mean "your present opinion." I don't think I've heard this espoused as anything other than a fun musing. Well, allow me to carefully and considerately correct you then. Imagine you are at the intersection of a two-by-two grid. If every square is expanding uniformly (maybe, one unit/second), then the edges of the grid recede from you at equal rates. However, if you add an row of squares to the top (now 2x3) but remain in your current position (not the center), then that row is moving away, firstly at one unit/second from the original two-by-two grid, but now this row is expanding also! So the farthest edge is now moving away from you at two units/second! Allow me to try to diagram it using some simply arrow (so I don't have to link a bunch of pictures). Let's imagine that arrows expand at the rate of "one hyphen/per line." That is to say, that every type I type a new line, every arrow will expand by one hyphen. We can imagine that each arrow is a "lightyear" or some other unit of distance in the universe. Let's start with a symmetrical example (where we are at the center of the arrows) by imagining that we are located in a univser that has five bodies: the '#', the '$', the '%', the '*', and us. We are located at the big 'U'.

#<- <- $<- U ->% -> ->* #<-- <--$<-- U -->% --> -->*
#<--- <--- $<--- U --->% ---> --->* #<---- <----$<---- U ---->% ----> ---->*

The distance between "U" (which stands for Us!) and the '$' or the '%' increases by "one hyphen/per line." Since they are an equal distance from us on both sides, they appear to be moving away at the same rate. The distance between us and the '#' or the '*' increases by "three hyphens/per line." Since they are an equal distance from us on both sides, they appear to be moving away at the same rate. Notice that the distance between the '*' and the '%' is increasing by "two hyphens/per line. This is also the rate at which the '%' and the '$' a re moving away from each other.

So, if we were in the the place of the '%', we wouldn't see the edges moving away at the same rate. The '#' would move away at an amazing "four hyphens/per line"!! So we can conclude, that things seem to recede equally on all sides only if you are in the middle of the expanding medium.

(The wild card here, is that if something moves away from you at faster than the speed of light because of aggregated expansion, then you don't get to see it.)

Thank you for helping me understand. I think I have a good idea on the concept now. I do have one question though. If each line has the distance between arrows increased by one hyphen per line, could you not state the rate of expansion is one hyphen per arrow per line?
Say each line represents a second. Each arrow represents a light year (as you said).

#<- <- $<- U ->% -> ->* #<-- <--$<-- U -->% --> -->*
#<--- <--- $<--- U --->% ---> --->* #<---- <----$<---- U ---->% ----> ---->*

The distance between "U" and the '$' or the '%' increases by "one hyphen per arrow per line." Since they are an equal distance from us on both sides (1 arrow), they appear to be moving away at the same rate. The distance between us and the '#' or the '*' is 3 light years, which means 3 additional hyphens per second.. Now the distance between the '*' and the '%' is increasing at the same rate(1 hyphen per light year per second). 2 light years distance = 2 hyphens per second) Now, if we were in the the place of the '%', we would see the same rate of change (1 hyphen per light year per second) the distance between the % and the # are 4 light years and therefore you add 4 additional hyphens per second. Does that make any sense what so ever or am I just very lost. FlexGunship Gold Member I do have one question though. If each line has the distance between arrows increased by one hyphen per line, could you not state the rate of expansion is one hyphen per arrow per line? Yes. Definitely. The unit is "length/length/time." Since the length/length is actually a unitless ratio and inverse seconds is a frequency (in units of Hz), the rate of expansion of the universe can be expressed as a frequency (for a given ratio). Does that make any sense what so ever or am I just very lost. I think you've got it. Yes. Definitely. The unit is "length/length/time." Since the length/length is actually a unitless ratio and inverse seconds is a frequency (in units of Hz), the rate of expansion of the universe can be expressed as a frequency (for a given ratio). I think you've got it. So if I understand correctly; If you measure the rate of change as 1 hyphen/second, then you must be at the center to observe equal expansion but if you measure the rate of change as (1 hyphen x light year)/second then your position could be anywhere and you would still observe equal expansion? FlexGunship Gold Member So if I understand correctly; If you measure the rate of change as 1 hyphen/second, then you must be at the center to observe equal expansion but if you measure the rate of change as (1 hyphen x light year)/second then your position could be anywhere and you would still observe equal expansion? (hyphen/lightyear)/second (no 'x'). Otherwise, yes. And what we see from our point of view, is that the universe is uniformly expanding on all sides all the way to the edge of the visible universe. (hyphen/lightyear)/second (no 'x'). Otherwise, yes. And what we see from our point of view, is that the universe is uniformly expanding on all sides all the way to the edge of the visible universe. #<- <-$<- U ->% -> ->*
#<-- <-- $<-- U -->% --> -->* #<--- <---$<--- U --->% ---> --->*
#<---- <---- $<---- U ---->% ----> ---->* The distance between "U" and the '$' or the '%' increases by "one hyphen per arrow per line." Since they are an equal distance from us on both sides (1 arrow), they appear to be moving away at the same rate.

The distance between us and the '#' or the '*' is 3 light years, which means 3 additional hyphens per second.. Now the distance between the '*' and the '%' is increasing at the same rate(1 hyphen per light year per second). 2 light years distance = 2 hyphens per second)

Now, if we were in the the place of the '%', we would see the same rate of change (1 hyphen per light year per second) the distance between the % and the # are 4 light years and therefore you add 4 additional hyphens per second.

Does that make any sense what so ever or am I just very lost.

Why is it (hyphen/lightyear)/second? If you are adding an additional hyphen for each light year are you not multiplying 1 hyphen by the amount of light years between objects? 1/3 =/= 3 additional hyphens. where 1*3 does.

FlexGunship
Gold Member
Why is it (hyphen/lightyear)/second? If you are adding an additional hyphen for each light year are you not multiplying 1 hyphen by the amount of light years between objects? 1/3 =/= 3 additional hyphens. where 1*3 does.

It's a unit not the solved quantity.

Once you have [(hyphen/lightyear)/second] you MULTIPLY by lightyears to find out how many "hyphens/second" you are adding (see note (1)). Then multiply by the number of seconds to get the total number of hyphens (see note (2)).

(1) (h/l)/s*l = h/s
(2) h/s * s = h

It's a unit not the solved quantity.

Once you have [(hyphen/lightyear)/second] you MULTIPLY by lightyears to find out how many "hyphens/second" you are adding (see note (1)). Then multiply by the number of seconds to get the total number of hyphens (see note (2)).

(1) (h/l)/s*l = h/s
(2) h/s * s = h

Alright thank you. this makes a lot of sense now. The way I was doing it, the unit analysis didn't work because you ended up with hyphen-lightyears/s but the answer(number) was the same but you really just wanted hyphens per second.

(hyphen/lightyear)/second (no 'x'). Otherwise, yes.

And what we see from our point of view, is that the universe is uniformly expanding on all sides all the way to the edge of the visible universe.

So if we measure the rate of change by d/s then we are either at the center or most likely in the senario you mentions previously;

Does this mean the Earth is at the center of the universe? Hardly; it means that there is a boundary, beyond which, the universe is expanding faster than the speed of light. Since nothing can propagate itself faster than the speed of light, it means we see a homogeneous mixture of the universe (since the universe was once a homogeneous mixture itself) until a very specific point after which we see nothing at all.

but if we measure the rate of change by (d / ly)/s we could be anywhere? (which doesn't exclude the outcome of the (d/s) scenario?

Do we have a rate which the universe is expanding? Is it in ly/s or (_ ly / 1 ly)/s

FlexGunship
Gold Member
#<- <-$<- u ->%-> ->* #<-- <--$<-- u -->%--> -->*
#<--- <---$<--- u --->%---> --->* #<---- <----$<---- u ---->%----> ---->*

Recreated here with uni-width font and center alignment for additional clarity.

FlexGunship
Gold Member
Do we have a rate which the universe is expanding? Is it in ly/s or (_ ly / 1 ly)/s

It is known as Hubble's Constant or H0. And the units are a bit different than we've been discussing. (Km/s)/MPc. Which can be re-written as (km/MPc/s) which is length/length/time as previously discussed.

Km - kilometers
s - seconds
MPc - megaparsecs

H0 is given as 70.8 ± 4.0 (km/s)/Mpc in the most current literature (http://wmap.gsfc.nasa.gov/universe/uni_expansion.html).

Since both MPc and Km are units of distance, it's still possible to simplify the unit to a frequency (1/s or Hz). Try this link; it should work (http://www.google.com/search?q=Goog...&aql=&oq=&gs_rfai=&pbx=1&fp=214da5a6d7ecf55e").

That means that every megaparsec of space is growing by about 70km per second.

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