Jump to content

Banner.jpg.b89429c566825f6ab32bcafbada449c9.jpg

Explaining expansion.


Recommended Posts

As an astronomy provider I'm often asked to discuss the expansion of the universe. 'But what does space expand into?' is the common objection and I usually reply that it doesn't expand into anything. I think the reason that this question comes up is that newcomers wrongly think of the universe as a sphere with an edge that must move outwards into 'something' as it expands. If this idea is deep rooted, as it usually is, it is hard to dislodge.

So lying in bed this morning I had a different idea. Next time I'm asked I'm going to try saying, 'It expands into the new space it has just created.' I can then go on to say that this new space is not created outside the universe (which has no outside) but inside it. Each cubic metre of space creates a new bit of space. Space grows.

We'll see if this proves easier to digest.

Note that this post is not about cosmology, it is only about explaining cosmology. When I was teaching English Lit I found that if only you could hit on the right phrase with which to do so you could explain the trickiest of ideas. With the wrong phrase you go round and round and get nowhere.

Olly

Link to comment
Share on other sites

  • Replies 48
  • Created
  • Last Reply

That sounds like getting into a real problem area.

I wouldn't "make up" something as it could be a problem. Imagine another person with you that has some knowledge and you are in real trouble and you do not want to apper to just make things up. You are in a business and that also implies other aspects.

I can imagine saying that at the Cambridge nights and having half the cosmology department hearing it. I would get metaphorically torn to shreds.

The other thing is do not assume that the person asking does not already have some idea.

Link to comment
Share on other sites

I like it Olly.

I think a lot of the problem is also caused by the fact that whenever a TV programme demonstrates the Big Bang, they always have some spectacular explosion or other....not helpful.

The other thing that gets to me, and you see this in all of the explanations of the Big Bang and aftermath, is the.....'x seconds after the Big Bang, the universe was the size of a pea' or some similar statement.

This is simply wrong.....the universe has always been the size of the universe...it's in the meaning of the word. There is nothing external to measure it against, so such statements are meaningless. All you can say, is that over time, the relationship between the bits of the universe has changed....either they've all mysteriously got smaller and are contracting, or the space between them is expanding ( no idea how you'd be able to tell the difference...perhaps someone here could explain). As far as we are aware, it's the latter.

Link to comment
Share on other sites

This is a hard one to explain. I sometime try the following tactic: Suggest that distances (in space and time) can only be measured relative to "events" (all very relativistic), or more properly the relative positions of particles. Then suggest that we can only measure the size of space relative to the particles furthest apart. Those particles that have travelled furthest since the big bang define the size of the universe.

This notion does not sit well with inflation, in which space itself expands.

The key problem is, our brains never evolved to deal with problems outside the meso-scale environment we are aware of, in which 3 dimensions of flat, euclidean space and one of time seem to fit nicely.

Link to comment
Share on other sites

Thanks Michael....I see what you mean with regards to that explanation not sitting well with inflation. It sounds like there's still a heck of a lot to figure out!

Link to comment
Share on other sites

That sounds like getting into a real problem area.

I wouldn't "make up" something as it could be a problem. Imagine another person with you that has some knowledge and you are in real trouble and you do not want to apper to just make things up. You are in a business and that also implies other aspects.

I can imagine saying that at the Cambridge nights and having half the cosmology department hearing it. I would get metaphorically torn to shreds.

The other thing is do not assume that the person asking does not already have some idea.

We all agree that the universe is expanding. I'm not making that up.

People have a real difficulty with this, largely because they are fixated an an explosion-like visualization as Rob descirbes. I'm not making that up either.

So my problem is to find a way of expressing the expansion of the universe in such a way as to steer people away from the 'explosion' visualization which is a misleading blind alley. Unlike anything else the universe cannot be seen or conceived of from the outside. However, most of our linguistic models have evolved to describe things which can be observed and envisioned from the outside. I thnk this is the heart of the problem.

I realize that semantically 'expanding into the space it has just created' is not logical. It isn't meant to be. It is meant to be tongue in cheek and delivered with a smile. The idea is to suggest that space begets space. Is this not so? While I might be deluding myself I hope that I have at least some grasp of cosmology having taken UCLAN's one year course. But how would you explain the expansion of the Universe to beginners? I'm all ears, quite genuinely. I'm not interested in being told I'll be torm apart but I'm very interested in being told why.

Olly

Link to comment
Share on other sites

My understanding of it is that the universe is expanding into what was nothingness before. Ultimately, the universe as we know it is the ever-expanding mass of galaxies, nebulae etc created in the Big Bang but it's still surrounded by an immeasurable void. It has to be immeasurable as, to measure anything, we have to have some sort of frame of reference such as time to use. As there's nothing, not even a single photon there who's progression or energy can be measured, it remains nothingness. The known universe simply expands into this as fast as the energy it produces can propagate into it. Of course, my understanding of this could be deeply flawed as I'm no cosmologist and even if it's not, I'm sure it's just an over-simplified view.

Link to comment
Share on other sites

It's a toughy but when the most given explanation in media or writing for the layman is that of the balloon or the loaf of rising raisin bread then maybe that is the simplest approach.

It certainly makes something easy enough to visualise at the least.

Link to comment
Share on other sites

My understanding of it is that the universe is expanding into what was nothingness before. Ultimately, the universe as we know it is the ever-expanding mass of galaxies, nebulae etc created in the Big Bang but it's still surrounded by an immeasurable void. It has to be immeasurable as, to measure anything, we have to have some sort of frame of reference such as time to use. As there's nothing, not even a single photon there who's progression or energy can be measured, it remains nothingness. The known universe simply expands into this as fast as the energy it produces can propagate into it. Of course, my understanding of this could be deeply flawed as I'm no cosmologist and even if it's not, I'm sure it's just an over-simplified view.

In the BB theory I'm sure that there is no immeasurable external void. Such a void would be the universe. I think this is the 'explosion misconception' in action. It imagines the universe expanding into a void and this is, in my understanding, not at all what the BB envisages.

Olly

Link to comment
Share on other sites

This is clearly a very intelligent conversation so I'm going to apologise in advance if I seem a bit daft but I've always been confused about something involving expansion. It's this. We can see objects 13 billion light years away right? Well that means that light started it's journey 13 billion years ago right? Easy. But the thing that confuses me is that 13 billion years ago, even if the universe expanded at the speed of light, it would've only been about 750000 light years across.... So how do we see it 13 billion light years away?

Link to comment
Share on other sites

This is clearly a very intelligent conversation so I'm going to apologise in advance if I seem a bit daft but I've always been confused about something involving expansion. It's this. We can see objects 13 billion light years away right? Well that means that light started it's journey 13 billion years ago right? Easy. But the thing that confuses me is that 13 billion years ago, even if the universe expanded at the speed of light, it would've only been about 750000 light years across.... So how do we see it 13 billion light years away?

First of all, inflation takes care of the increased size. Don't be worried if you do not understand inflation, nobody really does. The other problem is that you have to think in terms of  relativity. An object speeding away from us in a north-celestial direction at the speed of light for 13 billion years is 13 billion light years away from us. A similar object going in the opposite direction is also 13 billion light years away. Seen from these objects, they are each 13 billion light years away from each other. Confusing, isn't it?

Link to comment
Share on other sites

While we're on the subject... (long and rambling - Sorry!):

I sense "scientists" often see such things as (clearly!) "understood" and move on to the calculations... greater things etc. <G> But I can see no objection to use of vivid analogy and simplification in explanations to the general audience. Such devices (and ones far more controversial!) are regular features of TV science these days. :p

Even then, a clever kid (usually 10-12 y.o.? lol) might ask whether his / her atoms are expanding too... To which the answer is no - The atomic (nuclear) force dominates the expansion locally. Galaxies maintain their size, 'cos they are "gravitationally bound" etc. 

Yet this still seems a tad unsatisfactory. Others will know "Andromeda" (M31) is approaching us - Destined to collide with our galaxy - Apparently due to gravitational attraction? (Until this morning, I imagined this as initial random momentum). Then I saw: "We are gravitationally bound - Thus space does not expand between us"! There's an thought? As the crow flies? (See later!) ;)

[Ourselves & M31]... [M81 & M82] - Known as gravitationally bound, have no expansion internally. BUT space can freely expand between these systems in [square] brackets? To my mind implying gravity "switching off" at some distance? Or maybe just that acceleration of the expanding universe becomes comparable to (exceeds) gravitational acceleration - at some distance.

But what of space outside the shortest distance between bound entities - Recalling the curved field field patterns between magnets. Does this expand or not? Does it transition smoothly? I'm happy with "jumps" on the quantum scale, but prefer *smoothness* on a cosmological scale. As often in Astronomy, we may have no "experiment" to check intermediate distances?  :o

So maybe (see para one) "This doesn't matter" - Let's ALL just get on with our lives!  :D

P.S. Perhaps the above is spurious or naive (incomprehensible! lol). Opinion does seem to diverge (even among scientists!) re. distance and local effects. I guess I'm ever trying to fathom out just what constitutes a closed (gravitational?) *system*?  Binary transitions or effects becoming too small (smaller than some value?) to be significant etc. etc.  :)   

Link to comment
Share on other sites

This is clearly a very intelligent conversation so I'm going to apologise in advance if I seem a bit daft but I've always been confused about something involving expansion. It's this. We can see objects 13 billion light years away right?

Well that means that light started it's journey 13 billion years ago right? Easy. But the thing that confuses me is that 13 billion years ago, even if the universe expanded at the speed of light, it would've only been about 750000 light years across.... So how do we see it 13 billion light years away?

Not so fast! Let's take these one at once.

We can see objects 13 billion light years away right?

We can. But we can see objects much further away than that as well. Bear with me. The next point reveals why.

Well that means that light started it's journey 13 billion years ago right?

Wrong. This would be the case in a non-expanding universe.  A light flight time of 13 billion years would give a distance of 13 billion light years in a static universe. But in an expanding one the source of the light is being driven away during the flight by the expansion of the universe, so a flight time of 13 billion LY means that, at the end of the flight, the source object is far further away than 13 billion LY. If I'm not mistaken (and I might easily be mistaken!) deciding on how to work out how much further is rather difficult and ivolves problems of simultenaity but here I flounder. (If not before!)

If I'm up a gum tree the professionals will doubtless be good enough to say so, I hope.

Olly

Link to comment
Share on other sites

Do we know for certain that there is nothing out side this universe and that it has no bounderies.

where does the multiverse fit into all this.

Theres also the question of Dark Matter and Dark Energy, how does that fit into the eqation.

Avtar

Link to comment
Share on other sites

I thought WMAP had confirmed that the Universe is flat with a 0.4% margin of error.

If it is flat then this suggests that it is infinite in extent and we can only see a finite volume because it has a finite age.

Link to comment
Share on other sites

So there is no "nothingness" beyond the bounds of the Universe because there are no spatial bounds?

Ok, so there is no "nothingness". How much less than "nothing" is something that doesn't exist? i.e: the  nonexistent "nothingness"  is nothing then?

It's the "nothing" that bothers me, or actually the lack of "nothing"!!

Link to comment
Share on other sites

That's what makes your head grind to a halt thinking about these things. We have evolved in a finite environment, and cannot grasp the concepts that current theories of the Big Bang and a universe that has no spatial boundaries require. Our very language is bound up in finite concepts. We simply aren't wired up for it, but we have developed a very good method of describing and testing these things called mathematics.

Link to comment
Share on other sites

This is a complex subject. I am some body who knows a little about this as an amateur cosmologist and reading a bit on the subject, but by know means a expert.

The way expansion was explained to me was as analogy to blowing up a balloon, the matter in the universe is on the surface of the balloon, as the surface stretches so do the images on the balloon, no new matter is created. Thus the fabric of space time is expanding and all matter (which is just energy particles bouncing about) that exist in it is stretched.

Thus at any point in the universe if you look in any direction the space time is stretching and the matter in it is stretching. On a micro scale it would take millions of years to measure the effect on any object here on earth.

But on a macro scale of galaxies this movement is seen as stretching of the light towards the red spectrum.

So there is no center to the expansion, which is a weird thought, my atoms are all slowly moving a way from each other as I expand as part of the expansion kicked off by the big bang.

Space time is stretchy stuff and gravity is the effect that matter has as it stretches.

So on the surface of the earth space time is more stretched than up in space where satellites are, thus a clock ticks faster on a satellite, they have to adjust the clocks on GPS SATs to make sure they say at the same rate as earth.

Interesting stuff.

Link to comment
Share on other sites

Do we know for certain that there is nothing out side this universe and that it has no bounderies.

where does the multiverse fit into all this.

Theres also the question of Dark Matter and Dark Energy, how does that fit into the eqation.

Avtar

I don't think we know anything for certain.

'Where does the multiverse fit into all this?' I don't think it does fit into it, it fits outside it, but this is 'outside' in an unfamiliar sense. In our normal use of the word 'outside' we mean outside a finite and bounded space like your house. Your garden is outside your house. That's fine. The garden is outside the house but it is still within the same set of dimensions (three spacial ones plus time) as your house.

When someone talks about other universes outside our BB universe they don't mean 'outside ' in this sense at all, I don't think. They mean outside it in the sense that it does not occupy the same set of dimensions - or at least is not constrained by the same set. It may involve our dimensions plus others, maybe.

I suppose we can get a vague sense of how this might be by thinking about the past or the future. Where is the living Dodo? It certainly isn't here and we have no access to it but we have good reason to think that it was here once. It is 'outside of here' if you like, in a different sense of 'outside.' You have to take this idea a step further when thinking about multiverses.

Olly

Link to comment
Share on other sites

This is a complex subject. I am some body who knows a little about this as an amateur cosmologist and reading a bit on the subject, but by know means a expert.

The way expansion was explained to me was as analogy to blowing up a balloon, the matter in the universe is on the surface of the balloon, as the surface stretches so do the images on the balloon, no new matter is created. Thus the fabric of space time is expanding and all matter (which is just energy particles bouncing about) that exist in it is stretched.

Thus at any point in the universe if you look in any direction the space time is stretching and the matter in it is stretching. On a micro scale it would take millions of years to measure the effect on any object here on earth.

But on a macro scale of galaxies this movement is seen as stretching of the light towards the red spectrum.

So there is no center to the expansion, which is a weird thought, my atoms are all slowly moving a way from each other as I expand as part of the expansion kicked off by the big bang.

Space time is stretchy stuff and gravity is the effect that matter has as it stretches.

So on the surface of the earth space time is more stretched than up in space where satellites are, thus a clock ticks faster on a satellite, they have to adjust the clocks on GPS SATs to make sure they say at the same rate as earth.

Interesting stuff.

The 'surface of a balloon' analogy is a classic and, of course inspired. However, I've found it only patchily successful with beginners because not everyone can rid their imagination of the balloon's spherical self. The analogy only works for people who can successfully remove from their imagination anything but its surface. Not everyone can do that.

A variant which I like has the galaxies represented by coins stuck onto the surface. While the balloon's surface expands, the galaxies, which are gravitationally bound, don't. The fact that they are 'stuck where they are' with glue also shows that their increasing separation has nothing to do with proper motion. This helps to explain the difference between a Doppler and a Cosmological redhift.

Olly

Link to comment
Share on other sites

According to the prevailing scientific model of the Universe, known as the Big Bang, the Universe expanded from an extremely hot, dense phase called the Planck epoch, in which all the matter and energy of the observable universe was concentrated. Since the Planck epoch, the Universe has been expanding to its present form, possibly with a brief period (less than 10−32 seconds) of cosmic inflation. Several independent experimental measurements support this theoretical expansion and, more generally, the Big Bang theory. The universe is composed of ordinary matter (5%) including atoms, stars, and galaxies, dark matter (25%) which is a hypothetical particle that has not yet been detected, and dark energy (70%), which is a kind of energy density that seemingly exists even in completely empty space. Recent observations indicate that this expansion is accelerating because of dark energy, and that most of the matter in the Universe may be in a form which cannot be detected by present instruments, called dark matter. The common use of the "dark matter" and "dark energy" placeholder names for the unknown entities purported to account for about 95% of the mass-energy density of the Universe demonstrates the present observational and conceptual shortcomings and uncertainties concerning the nature and ultimate fate of the Universe.

Link to comment
Share on other sites

It's the "nothing" that bothers me, or actually the lack of "nothing"!!

Language is a perverse beast. By default the existence of 'Nothing' defines it as a 'Something'. From a language aspect.

The coins on a balloon analogy stopped working for me when external pressure was applied to one side of the balloon.

The coins at the pressure point moved closer together, whilst on the opposite side they moved further apart.

Continuing the point pressure did not seem to bring the outlying coins any closer to the pressure point but continued to spread them.

So should we be looking for the 'imploding' situation of the coins becoming closer together on the high pressure site?

Link to comment
Share on other sites

Language is a perverse beast. By default the existence of 'Nothing' defines it as a 'Something'. From a language aspect.

The coins on a balloon analogy stopped working for me when external pressure was applied to one side of the balloon.

The coins at the pressure point moved closer together, whilst on the opposite side they moved further apart.

Continuing the point pressure did not seem to bring the outlying coins any closer to the pressure point but continued to spread them.

So should we be looking for the 'imploding' situation of the coins becoming closer together on the high pressure site?

I don't understand your objection. In the balloon analogy as I understand it only the surface of the balloon exists. It doesn't have sides so no pressure can be brought to bear on them.

Or are you talking about distortions to the surface? And, if so, what is their real world equivalent? I'm probably missing something!

Olly

Link to comment
Share on other sites

I like the surface of a sphere analogy. It seems a perfect way to describe a finite but unbound surface. For 2 dimensional beings living on that surface there is no "other side".

We can project their universe into our 3 dimensions and can introduce all kinds of mayhem for the inhabitants. If we distort the surface by depressing the surface at some point it will cause local stretching.

The locals will have no comprehension of what is causing the distortion. They can't measure anything beyond forward/back and left/right. But on investigation they will be aware that a region of their universe doesn't behave like the rest of it. Their observations will show that distances are and straight lines are distorted.

Their mathematicians will theorise on what might be happening. They can comprehend a 3rd dimension outside of their universe and how something in that dimension might distort their universe.

For the natives their two dimension are all that there is. No such thing as up or down. When their universe gets distorted in the 3rd dimension it isn't moving into a void. There is no void!

There, how does that sound?

Just transfer that into our 3 dimensions and then wonder why we can't understand what's going on in any 4th dimension!!

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.