B Is Dark Energy Causing the Universe's Expansion?

[DaveC426913]

Post #8 disagrees with you on this point

No it doesn't.

To clarify: an "expansion of space" means the volume of space/universe increases from small to large. (At one point, all of space is 10^-20m in diameter, and then later it is 10^-1m in diameter.) That is an expansion of the volume we call space.

Any implication that there is a "thing" that is stretching was unintentional.

 

[PeterDonis]

Post #8 disagrees with you on this point

As @DaveC426913 has clarified, it doesn't. But see below for some caveats on what he said in post #35.

If it is not expansion IN space, and not expansion OF space, then what is it?

The correct technical answer is: a four-dimensional spacetime geometry that contains a family of worldlines, the "comoving" worldlines, with a positive expansion scalar.

That's probably too technical for a "B" level answer, which is why I stuck to just: objects in the universe are moving apart.

an "expansion of space" means the volume of space/universe increases from small to large.

This has to be stated carefully. First, "space" depends on our choice of coordinates; the implicit choice being made in descriptions that use the phrase "expansion of space" is comoving coordinates, i.e., coordinates in which the observers who follow the family of worldines I described above have constant spatial coordinates, and whose time coordinate is equal to proper time for comoving observers.

Second, our current best model of the universe is spatially infinite, so "space" in the above coordinates does not have a well-defined volume.

A better way to state the property of "space" in these coordinates that you are referring to is that the scale factor--the quantity $a(t)$ in the metric that tells you the proper distance between two objects with constant spatial coordinates, as a function of coordinate time--is increasing with time. This does not require any problematic use of the term "volume". It also is just another way of stating what I said before: objects in the universe are moving apart.

 

[StandardsGuy]

No it doesn't.

To clarify: an "expansion of space" means the volume of space/universe increases from small to large. (At one point, all of space is 10^-20m in diameter, and then later it is 10^-1m in diameter.) That is an expansion of the volume we call space.

Any implication that there is a "thing" that is stretching was unintentional.

Wow, I'm flabbergasted. This looks like double talk to me, but I don't want to be confrontational. If volume is a property of space and volume increases, then it seems that space increases. Tell me why it doesn't. Let me ask you one question: Is the universe expanding?

 

[StandardsGuy]

The correct technical answer is: a four-dimensional spacetime geometry that contains a family of worldlines, the "comoving" worldlines, with a positive expansion scalar.

That's probably too technical for a "B" level answer, which is why I stuck to just: objects in the universe are moving apart.
This has to be stated carefully. First, "space" depends on our choice of coordinates; the implicit choice being made in descriptions that use the phrase "expansion of space" is comoving coordinates, i.e., coordinates in which the observers who follow the family of worldines I described above have constant spatial coordinates, and whose time coordinate is equal to proper time for comoving observers.A better way to state the property of "space" in these coordinates that you are referring to is that the scale factor--the quantity $a(t)$ in the metric that tells you the proper distance between two objects with constant spatial coordinates, as a function of coordinate time--is increasing with time... It also is just another way of stating what I said before: objects in the universe are moving apart.

So I take it that "moving apart" is not the same to you as "moving IN space". I'll ask you the same question as Dave: Is the universe expanding (with time)?

 

[PeterDonis]

If volume is a property of space and volume increases, then it seems that space increases.

If you take a balloon and blow it up to a larger volume, does that mean "space increases"?

Hint: there is no definite answer to this question, because it depends on how you define "space".

Is the universe expanding?

I've already answered this, by explaining to you what "the universe is expanding" actually means to cosmologists.

So I take it that "moving apart" is not the same to you as "moving IN space".

"Moving apart" has an invariant meaning (I already gave you the technical definition of what it means). "Moving IN space" does not; it depends on how you choose your coordinates, since "space" depends on that. In physics, things that depend on your choice of coordinates don't have any physical meaning; all of the physical meaning is in invariant quantities, things that don't depend on your choice of coordinates. I explained in post #36 what the relevant invariant quantity is that cosmologists are referring to when they say "the universe is expanding".

In short, you are trying to reason using vague ordinary language, and that doesn't work in physics. You have to learn the precise math that expresses the actual content of our physical models.

 

[DaveC426913]

Wow, I'm flabbergasted. This looks like double talk to me, but I don't want to be confrontational. If volume is a property of space and volume increases, then it seems that space increases. Tell me why it doesn't. Let me ask you one question: Is the universe expanding?

Let's keep context in mind.

My answer was in response to Marcus talking about the Big Bang being an explosion. It is very easy for many people to think of the BB as if it were some sort of explosion, and that it happened in space somewhere - as if there is space for it to explode into.Let's look at your words more carefully:

If volume is a property of space and volume increases, then it seems that space increases.

So, you've used the phrase "volume increases". Does that imply to you that volume could be described some sort of fabric? Volume can stretch, curve and tear?

I don't mean volume of something - like the volume of a balloon. I mean volume - the naked property itself.

Likewise, space is not a thing. Space is nothing more than the distance between given points. There is nothing to stretch curve or tear.

 

[StandardsGuy]

"Moving apart" has an invariant meaning (I already gave you the technical definition of what it means). "Moving IN space" does not; it depends on how you choose your coordinates, since "space" depends on that. In physics, things that depend on your choice of coordinates don't have any physical meaning; all of the physical meaning is in invariant quantities, things that don't depend on your choice of coordinates. I explained in post #36 what the relevant invariant quantity is that cosmologists are referring to when they say "the universe is expanding".

In short, you are trying to reason using vague ordinary language, and that doesn't work in physics. You have to learn the precise math that expresses the actual content of our physical models.

Thank you for your answer and your patience. I re-read post #36. This site keeps showing me that the universe is more complicated than I thought. Can you recommend a book that explains this that I might find in a public library? Or a link?

 

[PeterDonis]

Can you recommend a book that explains this that I might find in a public library? Or a link?

It's somewhat dated now, but I still think Ned Wright's Cosmology Tutorial is a decent place to start:

http://www.astro.ucla.edu/~wright/cosmolog.htm

I would start with "Enter the tutorial" and then, once you've gone through that (it's 4 pages), try the "Frequently Asked Questions".

For the specific invariant thing I talked about, the expansion scalar, unfortunately I don't know of a good detailed treatment online, but this Wikipedia article at least gives a basic overview of what I was referring to:

https://en.wikipedia.org/wiki/Congr...atical_decomposition_of_a_timelike_congruence

Unfortunately I'm not familiar enough with the various cosmology textbooks to be able to recommend a particular one.

 

[Chiclayo guy]

Likewise, space is not a thing. Space is nothing more than the distance between given points. There is nothing to stretch curve or tear.

If there were no 'points', would space exist?

 

[wolram]

From https://www.nap.edu/read/1859/chapter/5#75

Hope this helps, , I think the singularity is unexplained to date.

Cosmologists call the point at the very beginning of time a singularity. Before it, classical gravitational physics can say or prove nothing, leaving all speculation to the metaphysicians. Big Bang theory encompasses a series of events that occurred thereafter, which conform to two continuing constraints: first, the laws of physics, which are believed to be universal, and second, data from observations that are continually probing farther in space, and therefore further back in time toward the event itself. This series of events cosmologists can "date," using either lookback time from the present or cosmic time. When measuring time forwards, singularity (the moment of creation) represents time zero on the cosmic calendar.

 

[timmdeeg]

I think you should better show this as a quote.

 

[PeroK]

If there were no 'points', would space exist?

This sort of question doesn't really make a lot of sense. Spacetime is represented in the theory of GR as a four-dimensional continuum. Spacetime also exists in an experimental sense, that you can do experiments and attribute the concept of space and time to things you are measuring. What the theory does is map the mathematical model of spacetime to these measurable quantities.

 

[timmdeeg]

The correct technical answer is: a four-dimensional spacetime geometry that contains a family of worldlines, the "comoving" worldlines, with a positive expansion scalar.

That's probably too technical for a "B" level answer, which is why I stuck to just: objects in the universe are moving apart.

This has to be stated carefully. First, "space" depends on our choice of coordinates; the implicit choice being made in descriptions that use the phrase "expansion of space" is comoving coordinates, i.e., coordinates in which the observers who follow the family of worldines I described above have constant spatial coordinates, and whose time coordinate is equal to proper time for comoving observers.

Is the meaning of "comoving wordlines" in the invariant definition you gave in the first sentence different from the meaning of "comoving worldlines" in the context of FRW coordinates? It doesn't seem so as you are mentioning "constant spatial coordinates". So why is this definition coordinate independent?

 

[PeterDonis]

Is the meaning of "comoving wordlines" in the invariant definition you gave in the first sentence different from the meaning of "comoving worldlines" in the context of FRW coordinates?

No. Whether worldlines are "comoving" or not is invariant; it doesn't depend on your choice of coordinates. FRW coordinates are useful because they are chosen in such a way that comoving worldlines have constant spatial coordinates.

 

[timmdeeg]

No. Whether worldlines are "comoving" or not is invariant; it doesn't depend on your choice of coordinates. FRW coordinates are useful because they are chosen in such a way that comoving worldlines have constant spatial coordinates.

Ok, thanks for clarifying.