Is Dark Energy Causing the Universe's Expansion?

[Marcus1122]

Is dark energy the reason for the expansion of the universe?

 

[DaveC426913]

Is dark energy the reason for the expansion of the universe?

Caveat: my knowledge may be out-of-date.

Dark energy is a placeholder to explain an observed phenomenon we do not yet fully understand, but have no reason to doubt.

We observe the expansion of the universe.
The only way it could be expanding, in spite of gravity, is if there is some phenomenon overcoming gravity.
We don't know yet the nature of this phenomenon, but it must apply an expansive component of force.

Whatever form this force takes it will be describable in terms of energy. We can work with it, even without knowing its nature, thus the placeholder.

 

[Marcus1122]

My thinking is that the explosion of the big bang is still happening and the future has already happened because of this and we are moving to catch up with this.

 

[Drakkith]

My thinking is that the explosion of the big bang is still happening and the future has already happened because of this and we are moving to catch up with this.

The expansion process that started with the big bang is of course still going on but note that this is not an explosion in the sense of a bomb going off. The rest of your statement makes no sense. Please see PF rules regarding personal theories/speculation here: https://www.physicsforums.com/help/terms-of-service/

 

[Marcus1122]

The expansion process that started with the big bang is of course still going on but note that this is not an explosion in the sense of a bomb going off. The rest of your statement makes no sense. Please see PF rules regarding personal theories/speculation here: https://www.physicsforums.com/help/terms-of-service/

 

[Marcus1122]

Ok so explain the big bang

 

[Drakkith]

Ok so explain the big bang

That's a broad topic. What would you like to know about it?

 

[DaveC426913]

Ok so explain the big bang

What do you need explaining from what you have read/researched already?

The executive summary: The Big Bang is seen - not as an expansion in space - but an expansion of space.

The initial expansion and inflation periods are not necessarily the same phenomenon as the current expansion of the universe. It expanded very rapidly at first, then slowed for a while, then started increasing.

 

[Marcus1122]

The model describes how the universe expanded from a very high-density and high-temperature state and quatumq physics helps us on this from nothing something can happen.

 

[Drakkith]

Do you have a question?

 

[Marcus1122]

Im not in the right place to succeed in the unknown we must be in the frontier I'm in the already known.

 

[Drakkith]

Im not in the right place to succeed in the unknown we must be in the frontier I'm in the already known.

I have no idea what you're trying to get at. If you want to have a meaningful conversation you're going to need to put more effort into your posts.

 

[DaveC426913]

Perhaps we would do well to remain within the bounds of the opening post.

Marcus, if you want to discuss another subject, a new thread would be the right way.

 

[PeterDonis]

Is dark energy the reason for the expansion of the universe?

It's not the reason for the expansion itself; the reason for that is the Big Bang--the universe is expanding now because it was expanding in the past.

Dark energy is the reason why the expansion of the universe has been accelerating for the past few billion years.

so explain the big bang

This is much too broad a question for a PF discussion. You should spend some time working through a good textbook presentation of our best current model, and then ask more specific questions if there are things you are having trouble with.

The Wikipedia article on the Lambda-CDM model (which is the best current model) gives a decent overview and some good references for further reading:

https://en.wikipedia.org/wiki/Lambda-CDM_model

 

[kimbyd]

These questions are just way, way too vague for any significant useful discussion. Please do some research on your own and come back if you have more specific questions.

 

[BearY]

How about this:
You will tell me what you know about Friedmann's solutions. And I will see if I know enough GR to help you.

 

[rootone]

Is dark energy the reason for the expansion of the universe?

Something is causing space to expand and the expansion is accelerating.
Any accelerating requires a form of energy.
We call it dark energy because so far we have very little idea what it could be.

 

[PeterDonis]

Something is causing space to expand

No, space is not expanding. Objects in the universe are flying apart. (Pop science articles that use the "space expanding" terminology are not good sources for understanding the actual science.)

Any accelerating requires a form of energy.

This is not correct when applied to the "acceleration" of the universe's expansion, since that "acceleration" is not felt; accelerometers attached to comoving objects in our universe read zero.

 

[rootone]

Isn't the idea of expanding space, the same idea as objects in space being seen to become more distant from each other?

 

[PeterDonis]

Isn't the idea of expanding space, the same idea as objects in space being seen to become more distant from each other?

Not if you think expanding space requires "something causing" it. It doesn't.

 

[rootone]

OK, I think what you are saying is that what is called dark energy is actually metric expansion.
I can just about get my head around that, but then doesn't that make the whole concept of dark energy irrelevant ?
No?
Yes?

 

[PeterDonis]

I think what you are saying is that what is called dark energy is actually metric expansion.

No. Metric expansion is a description (which is OK if used with care) of the spacetime geometry of the universe. Dark energy is a description of the form of stress-energy--the "source" that determines the spacetime geometry--that became dominant a few billion years ago.

doesn't that make the whole concept of dark energy irrelevant ?

No. See above.

 

[Ibix]

OK, I think what you are saying is that what is called dark energy is actually metric expansion.

In a universe with only normal matter, metric expansion (or contraction) happens, according to general relativity. But when we look out into the universe, thanks to the finite speed of light, we see the history of expansion. That let's us estimate the density of matter in the universe.

But the experimental data doesn't quite fit the model. The expansion curve cannot be explained by any amount of matter - the curve is the wrong shape for a universe containing only normal matter (I'm including dark matter under the umbrella or normal matter here). It does fit, however, if you add a small amount of something which has constant density even as the universe expands. That's dark energy.

 

[bhobba]

what if it is time itself? Time as a force as well as an energy? Time as dark energy?

We do not allow personal theories. But we know from very general considerations time by itself does not exist:
http://www2.physics.umd.edu/~yakovenk/teaching/Lorentz.pdf

If you want to come up with a personal theory and discuss it here you are going to have to have it published and peer reviewed. Sorry, but those are the rules. You will however during that journey learn a lot.

Thanks
Bill

 

[DaveC426913]

Isn't the idea of expanding space, the same idea as objects in space being seen to become more distant from each other?

There's some popular ideas out there about space being a thing. It's often analogized to a "fabric" that can be stretched, curved and even ripped. This leads to misconceptions about how the universe works.

 

[Viopia]

Einstein's relativity should be taken into account when determining the size and density. The muon experiences length contraction allowing it to reach the Earth even though it is too short lived from it's creation to cover the necessary distance from the Earth's inertial frame of reference.

 

[Ibix]

Einstein's relativity should be taken into account when determining the size and density. The muon experiences length contraction allowing it to reach the Earth even though it is too short lived from it's creation to cover the necessary distance from the Earth's inertial frame of reference.

Statements about "the universe" generally adopt the point of view of an FLRW co-moving observer, one who sees the CMB as isotropic. They never say so for the same reason that road signs don't say that the speed limit is 30mph relative to the ground, but that's the convention. If you want to pick an observer moving fast with respect to co-moving observers, go ahead. I rather suspect the description of anything in such a choice of coordinates is horribly involved, but you can do it if you wish.

 

[Viopia]

Does this mean that Einstein's relativity has no role in determining the size and density of the Universe? If time dilation becomes infinite (time is stopped) at the event horizon of a black hole from an outside observer (eg: from the Earth) how old is this black hole from the Earth's inertial frame of reference? In other words, how long would we have to wait for something to enter this black hole's event horizon from an observer on the Earth?

 

[Ibix]

Does this mean that Einstein's relativity has no role in determining the size and density of the Universe?

I don't know how you read that from my post. The whole model of cosmology is based on general relativity.

Edit: the point I was trying to make is this. There is one, and only one, family of observers who see the universe as isotropic. Only they can say that the universe has a single well-defined density. Observers in motion with respect to these will see the density of the universe varying with distance in their direction of motion, since their notion of "now" is not the same as that of co-moving observers. All of this is consistent with Einstein's general relativity.

The rest of your post is about black holes. If you want to discuss them, I suggest starting a separate thread. They have nothing to do with the topic of this thread.

 

[StandardsGuy]

No, space is not expanding. Objects in the universe are flying apart. (Pop science articles that use the "space expanding" terminology are not good sources for understanding the actual science.)

Post #8 disagrees with you on this point and so do I. I like the rest of your post. If it is not expansion IN space, and not expansion OF space, then what is it?

 

[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.