Expanding space-time

athon · 19th September 2007, 07:05 PM

I've understood that the observation of an expanding, indeed accelerating expansion, of the universe could be accounted for a stretching of the space/time between objects.

In a discussion with a colleague, he says that it can't be space-time expanding, but a force which operates within it. I therefore can't understand how the universe can be expanding, as it would need to be constructing more space time (or space time is infinite, and we're expanding into it).

Can anybody set me straight? Can space-time expand, or is it stable and do things separate within it?

Athon

GoodGuysEatPie · 19th September 2007, 07:14 PM

Here's a somewhat flawed analogy, but one that might help:

Imagine an ant walking on the surface of a balloon. When you blow up the balloon, it doesn't affect the ant. The thing that would be affected would be, say, the distance between two (or more) ants on the balloon's surface.

When you blow up the balloon, is there any more balloon being "created"? Nope. Still the same amount of balloon there, just stretched. Just think of spacetime (one word) as the "surface" or "fabric" in which "stuff" moves around in. As the balloon expands, ants on the surface just along for the ride, much like galaxies in our own Universe.

The real question, the one that keeps cosmologists up at night, is: what causes the expansion? It could be some kind of energy, but we don't really know at this point. Doesn't mean we won't find out later...

~goodguyseatpie~

fishbob · 19th September 2007, 07:19 PM

Originally Posted by athon

I've understood that the observation of an expanding, indeed accelerating expansion, of the universe could be accounted for a stretching of the space/time between objects.

Great !!!
More room for my stuff.

athon · 19th September 2007, 07:23 PM

Originally Posted by goodguyseatpie

Here's a somewhat flawed analogy, but one that might help:

Imagine an ant walking on the surface of a balloon. When you blow up the balloon, it doesn't affect the ant. The thing that would be affected would be, say, the distance between two (or more) ants on the balloon's surface.

When you blow up the balloon, is there any more balloon being "created"? Nope. Still the same amount of balloon there, just stretched. Just think of spacetime (one word) as the "surface" or "fabric" in which "stuff" moves around in. As the balloon expands, ants on the surface just along for the ride, much like galaxies in our own Universe.

The real question, the one that keeps cosmologists up at night, is: what causes the expansion? It could be some kind of energy, but we don't really know at this point. Doesn't mean we won't find out later...

~goodguyseatpie~

So it is space-time stretching, then? Why don't we get bigger? Why doesn't the impact of space-time on the rules that dictate atomic forces change those rules?

(I hope this makes sense. I'm trying to paraphrase).

Athon

bokonon · 19th September 2007, 07:28 PM

Originally Posted by fishbob

Great !!!
More room for my stuff.

And more time to enjoy it! No wonder people are living longer, and growing taller.

PixyMisa · 19th September 2007, 07:30 PM

We don't get bigger because the four forces (gravity, electromagnetism, weak and nuclear) are stronger than the expansion. Some cosmologists think that if the rate of expansion keeps increasing, there will come a point (called the Big Rip) where the expansion overwhelms the other forces and everything in the Universe larger than a single subatomic particle will disintegrate.

bokonon · 19th September 2007, 07:32 PM

Originally Posted by PixyMisa

We don't get bigger

Speak for yourself.

GoodGuysEatPie · 19th September 2007, 07:34 PM

Originally Posted by PixyMisa

We don't get bigger because the four forces (gravity, electromagnetism, weak and nuclear) are stronger than the expansion. Some cosmologists think that if the rate of expansion keeps increasing, there will come a point (called the Big Rip) where the expansion overwhelms the other forces and everything in the Universe larger than a single subatomic particle will disintegrate.

Originally Posted by athon

So it is space-time stretching, then? Why don't we get bigger? Why doesn't the impact of space-time on the rules that dictate atomic forces change those rules?

(I hope this makes sense. I'm trying to paraphrase).

Athon

Yeah, what PixyMisa said. Spacetime expansion is the overriding motion in the Universe, but only at the very largest of scales. At local scales, like inside galaxies, gravity is stronger, which is why stars aren't flying apart. Electrical and nuclear forces hold smaller things together.

athon · 19th September 2007, 07:44 PM

Originally Posted by goodguyseatpie

Yeah, what PixyMisa said. Spacetime expansion is the overriding motion in the Universe, but only at the very largest of scales. At local scales, like inside galaxies, gravity is stronger, which is why stars aren't flying apart. Electrical and nuclear forces hold smaller things together.

Ok, this follows pretty much what I figured.

Does anybody have a link I can send him? He's got a good grounding in physics (hell, he's writing a book on something to do with wave forms and stuff), but he's a bit of a woo on so many things. He's an ID advocate for one.

I'd love to send him something to back this up.

(include the maths - he's demanding the proof now)

Athon

Puppycow · 19th September 2007, 08:43 PM

Originally Posted by PixyMisa

We don't get bigger because the four forces (gravity, electromagnetism, weak and nuclear) are stronger than the expansion. Some cosmologists think that if the rate of expansion keeps increasing, there will come a point (called the Big Rip) where the expansion overwhelms the other forces and everything in the Universe larger than a single subatomic particle will disintegrate.

Who was it that said "if you think you understand quantum mechanics, you don't understand quantum mechanics"? I'm beginning to wonder if the same thing might be true about cosmology and physics in general.

I recall reading in A Brief History of Time how Hawking thought that in a closed universe scenario that time might begin to run backwards instead of forwards. Even Hawking gets confused.

Yllanes · 20th September 2007, 01:39 AM

Nitpick: spacetime isn't expanding. Space is.

bokonon · 20th September 2007, 01:52 AM

But I have a day more time under my belt today than I did yesterday!

Thabiguy · 20th September 2007, 05:37 AM

Originally Posted by athon

I've understood that the observation of an expanding, indeed accelerating expansion, of the universe could be accounted for a stretching of the space/time between objects.

Yes, at our current understanding, this appears to be the case. You may want to check the article on metric expansion of space on Wikipedia, which tries to explain it in a relatively accessible way.

Originally Posted by athon

In a discussion with a colleague, he says that it can't be space-time expanding, but a force which operates within it. I therefore can't understand how the universe can be expanding, as it would need to be constructing more space time (or space time is infinite, and we're expanding into it).

Space is not something that needs to be, or even can be constructed. It is not material substance like air. There is no "amount" of space. The metric of space is a mathematical construct in our expression of the laws of the universe, not a physical object.

In a way, space is like a coordinate system. It should be apparent that a coordinate system can change properties without anything being created.

Originally Posted by athon

So it is space-time stretching, then? Why don't we get bigger? Why doesn't the impact of space-time on the rules that dictate atomic forces change those rules?

This is a very good question. The answer is that people, planets, or star systems don't get bigger, because they are bound together by various forces - electromagnetic, gravitational etc.

However, this can be misleading, because the actual reasons are not as obvious as it may seem. It is not the case that this is because the binding forces are "stronger" and "overpower" the expansion, or that they are dominant locally, while the expansion of space is dominant on larger scales. Indeed, even if you were so huge that you would reach the intergalactic scale, you would still not expand because of the metric expansion of space, as long as you remained a bound system.

Why do bound systems not expand then, even if the metric of space itself continuously expands within them? How can the universe know whether a system is bound or not, so that it knows whether it should expand or not? The universe does not know; it is the bound system itself that neutralizes its own expansion, by the consequences of being bound.

In a simplified example, if you throw a ball up fast enough, it will become unbound and never return to the Earth. If you throw it with less speed, it will remain bound to the Earth and will eventually slow down, stop, and return. Now, even in expanding space, as long as the ball is bound, it must eventually slow down, stop, and return (otherwise it is not bound, by definition). And it happens that the mathematics of its trajectory in expanding space are such that after travelling out there and back again, it will return to its original orbital distance and velocity and thus remain in a closed orbit that doesn't expand over time, despite the metric expansion of space along its orbit (this is true even for some cases of accelerating expansion, but not for others).

One could oversimplify it even more and say that - very roughly - although the expanding space blows systems apart and gives them outward velocity, it also gives them more inside space to "collapse into" and gain inward velocity, which is what bound systems do and thus remain even, but unbound systems don't do and thus expand.

Originally Posted by athon

(include the maths - he's demanding the proof now)

I don't think there is any proof. This is a part of a mathematical model of our understanding of how universe works. Such a model cannot be proven to be correct. All we can do is verify it against our actual empirical observation and thus evaluate its accuracy.

fuelair · 20th September 2007, 08:15 PM

Originally Posted by athon

So it is space-time stretching, then? Why don't we get bigger? Why doesn't the impact of space-time on the rules that dictate atomic forces change those rules?

(I hope this makes sense. I'm trying to paraphrase).

Athon

Remember the ant. Now, think of the ant as a galaxy that is on/in space time but not part of space time (as noted in post you are responding to). Spacetime expands, the matter in it does not individually get larger, the concentrations of it (galaxies and the matter they contain) move apart.

To see this analogy more accurately, make cutouts of 5 or 10 galaxies and glue them to the surface of a balloon. Blow up the balloon. The galaxies (matter) do not get any bigger, they just move apart. They are not the spacetime fabric, they are passengers on/in it.

NOTE: some people - notably non-science science teachers will do this with students BUT have the students draw the galaxies on the balloon {as if matter were actually spacetime} so the students will think galaxies move away from each other AND get bigger. This is not a good analogy.

GoodGuysEatPie · 21st September 2007, 08:53 AM

Originally Posted by fuelair

NOTE: some people - notably non-science science teachers will do this with students BUT have the students draw the galaxies on the balloon {as if matter were actually spacetime} so the students will think galaxies move away from each other AND get bigger. This is not a good analogy.

Drawing on balloons with a marker or something will definitely lead to more confusion, as you said. Taping buttons to the balloon work much better.

Of course, nowadays, I just use an animation of an expanding balloon when I use this analogy in class.

~goodguyseatpie~

19th September 2007, 07:05 PM	#1
athon Kowalski Join Date: Aug 2001 Location: gone Posts: 9,286	Expanding space-time I've understood that the observation of an expanding, indeed accelerating expansion, of the universe could be accounted for a stretching of the space/time between objects. In a discussion with a colleague, he says that it can't be space-time expanding, but a force which operates within it. I therefore can't understand how the universe can be expanding, as it would need to be constructing more space time (or space time is infinite, and we're expanding into it). Can anybody set me straight? Can space-time expand, or is it stable and do things separate within it? Athon

19th September 2007, 07:19 PM	#3
fishbob Seasonally Disaffected Join Date: Jan 2003 Location: Chilly Undieville Posts: 5,667	Originally Posted by athon I've understood that the observation of an expanding, indeed accelerating expansion, of the universe could be accounted for a stretching of the space/time between objects. Great !!! More room for my stuff.
	__________________ When you believe in things you don't understand, then you suffer . . . " - Stevie Wonder "Stupidity - a callow indifference to facts or data" - Stuart Firestein -neuroscientist. I hate bigots.

19th September 2007, 07:28 PM	#5
bokonon Illuminator Join Date: Aug 2007 Posts: 4,444	Originally Posted by fishbob Great !!! More room for my stuff. And more time to enjoy it! No wonder people are living longer, and growing taller.
bokonon Illuminator Join Date: Aug 2007 Posts: 4,444

19th September 2007, 07:30 PM	#6
PixyMisa Persnickety Insect Join Date: Dec 2002 Location: Sunny Munuvia Posts: 14,914	We don't get bigger because the four forces (gravity, electromagnetism, weak and nuclear) are stronger than the expansion. Some cosmologists think that if the rate of expansion keeps increasing, there will come a point (called the Big Rip) where the expansion overwhelms the other forces and everything in the Universe larger than a single subatomic particle will disintegrate.
	__________________ Free blogs for skeptics... And everyone else. mee.nu What, in the Holy Name of Gzortch, are you people doing?!?!!? - TGHO

19th September 2007, 07:32 PM	#7
bokonon Illuminator Join Date: Aug 2007 Posts: 4,444	Originally Posted by PixyMisa We don't get bigger Speak for yourself.
bokonon Illuminator Join Date: Aug 2007 Posts: 4,444

19th September 2007, 07:14 PM	#2
GoodGuysEatPie Constructive Interference Join Date: Aug 2007 Location: Westchester, New York Posts: 426	Here's a somewhat flawed analogy, but one that might help: Imagine an ant walking on the surface of a balloon. When you blow up the balloon, it doesn't affect the ant. The thing that would be affected would be, say, the distance between two (or more) ants on the balloon's surface. When you blow up the balloon, is there any more balloon being "created"? Nope. Still the same amount of balloon there, just stretched. Just think of spacetime (one word) as the "surface" or "fabric" in which "stuff" moves around in. As the balloon expands, ants on the surface just along for the ride, much like galaxies in our own Universe. The real question, the one that keeps cosmologists up at night, is: what causes the expansion? It could be some kind of energy, but we don't really know at this point. Doesn't mean we won't find out later... ~goodguyseatpie~

19th September 2007, 07:23 PM	#4
athon Kowalski Join Date: Aug 2001 Location: gone Posts: 9,286	Originally Posted by goodguyseatpie Here's a somewhat flawed analogy, but one that might help: Imagine an ant walking on the surface of a balloon. When you blow up the balloon, it doesn't affect the ant. The thing that would be affected would be, say, the distance between two (or more) ants on the balloon's surface. When you blow up the balloon, is there any more balloon being "created"? Nope. Still the same amount of balloon there, just stretched. Just think of spacetime (one word) as the "surface" or "fabric" in which "stuff" moves around in. As the balloon expands, ants on the surface just along for the ride, much like galaxies in our own Universe. The real question, the one that keeps cosmologists up at night, is: what causes the expansion? It could be some kind of energy, but we don't really know at this point. Doesn't mean we won't find out later... ~goodguyseatpie~ So it is space-time stretching, then? Why don't we get bigger? Why doesn't the impact of space-time on the rules that dictate atomic forces change those rules? (I hope this makes sense. I'm trying to paraphrase). Athon

19th September 2007, 07:34 PM	#8
GoodGuysEatPie Constructive Interference Join Date: Aug 2007 Location: Westchester, New York Posts: 426	Originally Posted by PixyMisa We don't get bigger because the four forces (gravity, electromagnetism, weak and nuclear) are stronger than the expansion. Some cosmologists think that if the rate of expansion keeps increasing, there will come a point (called the Big Rip) where the expansion overwhelms the other forces and everything in the Universe larger than a single subatomic particle will disintegrate. Originally Posted by athon So it is space-time stretching, then? Why don't we get bigger? Why doesn't the impact of space-time on the rules that dictate atomic forces change those rules? (I hope this makes sense. I'm trying to paraphrase). Athon Yeah, what PixyMisa said. Spacetime expansion is the overriding motion in the Universe, but only at the very largest of scales. At local scales, like inside galaxies, gravity is stronger, which is why stars aren't flying apart. Electrical and nuclear forces hold smaller things together.

19th September 2007, 07:44 PM	#9
athon Kowalski Join Date: Aug 2001 Location: gone Posts: 9,286	Originally Posted by goodguyseatpie Yeah, what PixyMisa said. Spacetime expansion is the overriding motion in the Universe, but only at the very largest of scales. At local scales, like inside galaxies, gravity is stronger, which is why stars aren't flying apart. Electrical and nuclear forces hold smaller things together. Ok, this follows pretty much what I figured. Does anybody have a link I can send him? He's got a good grounding in physics (hell, he's writing a book on something to do with wave forms and stuff), but he's a bit of a woo on so many things. He's an ID advocate for one. I'd love to send him something to back this up. (include the maths - he's demanding the proof now) Athon
	Last edited by athon; 19th September 2007 at 07:47 PM.

19th September 2007, 08:43 PM	#10
Puppycow Penultimate Amazing Join Date: Jan 2003 Location: Japan Posts: 15,776	Originally Posted by PixyMisa We don't get bigger because the four forces (gravity, electromagnetism, weak and nuclear) are stronger than the expansion. Some cosmologists think that if the rate of expansion keeps increasing, there will come a point (called the Big Rip) where the expansion overwhelms the other forces and everything in the Universe larger than a single subatomic particle will disintegrate. Who was it that said "if you think you understand quantum mechanics, you don't understand quantum mechanics"? I'm beginning to wonder if the same thing might be true about cosmology and physics in general. I recall reading in A Brief History of Time how Hawking thought that in a closed universe scenario that time might begin to run backwards instead of forwards. Even Hawking gets confused.
	__________________ “Some men are born mediocre, some men achieve mediocrity, and some men have mediocrity thrust upon them. With Major Major it had been all three.” ― Joseph Heller, Catch-22

20th September 2007, 01:39 AM	#11
Yllanes Muse Join Date: Sep 2005 Location: Madrid Posts: 826	Nitpick: spacetime isn't expanding. Space is.

20th September 2007, 01:52 AM	#12
bokonon Illuminator Join Date: Aug 2007 Posts: 4,444	But I have a day more time under my belt today than I did yesterday!
bokonon Illuminator Join Date: Aug 2007 Posts: 4,444

20th September 2007, 05:37 AM	#13
Thabiguy Muse Join Date: Feb 2007 Posts: 783	Originally Posted by athon I've understood that the observation of an expanding, indeed accelerating expansion, of the universe could be accounted for a stretching of the space/time between objects. Yes, at our current understanding, this appears to be the case. You may want to check the article on metric expansion of space on Wikipedia, which tries to explain it in a relatively accessible way. Originally Posted by athon In a discussion with a colleague, he says that it can't be space-time expanding, but a force which operates within it. I therefore can't understand how the universe can be expanding, as it would need to be constructing more space time (or space time is infinite, and we're expanding into it). Space is not something that needs to be, or even can be constructed. It is not material substance like air. There is no "amount" of space. The metric of space is a mathematical construct in our expression of the laws of the universe, not a physical object. In a way, space is like a coordinate system. It should be apparent that a coordinate system can change properties without anything being created. Originally Posted by athon So it is space-time stretching, then? Why don't we get bigger? Why doesn't the impact of space-time on the rules that dictate atomic forces change those rules? This is a very good question. The answer is that people, planets, or star systems don't get bigger, because they are bound together by various forces - electromagnetic, gravitational etc. However, this can be misleading, because the actual reasons are not as obvious as it may seem. It is not the case that this is because the binding forces are "stronger" and "overpower" the expansion, or that they are dominant locally, while the expansion of space is dominant on larger scales. Indeed, even if you were so huge that you would reach the intergalactic scale, you would still not expand because of the metric expansion of space, as long as you remained a bound system. Why do bound systems not expand then, even if the metric of space itself continuously expands within them? How can the universe know whether a system is bound or not, so that it knows whether it should expand or not? The universe does not know; it is the bound system itself that neutralizes its own expansion, by the consequences of being bound. In a simplified example, if you throw a ball up fast enough, it will become unbound and never return to the Earth. If you throw it with less speed, it will remain bound to the Earth and will eventually slow down, stop, and return. Now, even in expanding space, as long as the ball is bound, it must eventually slow down, stop, and return (otherwise it is not bound, by definition). And it happens that the mathematics of its trajectory in expanding space are such that after travelling out there and back again, it will return to its original orbital distance and velocity and thus remain in a closed orbit that doesn't expand over time, despite the metric expansion of space along its orbit (this is true even for some cases of accelerating expansion, but not for others). One could oversimplify it even more and say that - very roughly - although the expanding space blows systems apart and gives them outward velocity, it also gives them more inside space to "collapse into" and gain inward velocity, which is what bound systems do and thus remain even, but unbound systems don't do and thus expand. Originally Posted by athon (include the maths - he's demanding the proof now) I don't think there is any proof. This is a part of a mathematical model of our understanding of how universe works. Such a model cannot be proven to be correct. All we can do is verify it against our actual empirical observation and thus evaluate its accuracy.
Thabiguy Muse Join Date: Feb 2007 Posts: 783

20th September 2007, 08:15 PM	#14
fuelair Cythraul Enfys Join Date: May 2006 Posts: 28,959	Originally Posted by athon So it is space-time stretching, then? Why don't we get bigger? Why doesn't the impact of space-time on the rules that dictate atomic forces change those rules? (I hope this makes sense. I'm trying to paraphrase). Athon Remember the ant. Now, think of the ant as a galaxy that is on/in space time but not part of space time (as noted in post you are responding to). Spacetime expands, the matter in it does not individually get larger, the concentrations of it (galaxies and the matter they contain) move apart. To see this analogy more accurately, make cutouts of 5 or 10 galaxies and glue them to the surface of a balloon. Blow up the balloon. The galaxies (matter) do not get any bigger, they just move apart. They are not the spacetime fabric, they are passengers on/in it. NOTE: some people - notably non-science science teachers will do this with students BUT have the students draw the galaxies on the balloon {as if matter were actually spacetime} so the students will think galaxies move away from each other AND get bigger. This is not a good analogy.
fuelair Cythraul Enfys Join Date: May 2006 Posts: 28,959

21st September 2007, 08:53 AM	#15
GoodGuysEatPie Constructive Interference Join Date: Aug 2007 Location: Westchester, New York Posts: 426	Originally Posted by fuelair NOTE: some people - notably non-science science teachers will do this with students BUT have the students draw the galaxies on the balloon {as if matter were actually spacetime} so the students will think galaxies move away from each other AND get bigger. This is not a good analogy. Drawing on balloons with a marker or something will definitely lead to more confusion, as you said. Taping buttons to the balloon work much better. Of course, nowadays, I just use an animation of an expanding balloon when I use this analogy in class. ~goodguyseatpie~