Is dark energy pushing or is it a mirror concept of gravity?

[Firehanger]

TL;DR Summary

What is the evidence for our theory for the existence of dark energy

Hey peeps,

Ok, so I need some help here. I've just been watching some videos explaining the existence of dark energy and dark matter - something which has always fascinated me.

These videos pose the theory that for the first 9 billion years of the universe dark matter ruled but, for the last 5 billion years, dark energy has now taken over - causing the increase an increase in the expansion of the universe.

I'm hoping someone can help me understand this a little better, as it's suggesting that its the every increasing generation of dark matter, primarily in the cosmic voids, which is pushing normal and dark matter away.

I'm interested to know what evidence we have that dark energy is pushing as opposed to dark and normal matter pulling.

For example, it sounds to my novice brain that it's almost the equivalent of posing an argument to state that, in the case of an object falling into a gravity well, it's actually also being pushed into the denser gravity by another force.

Are we able to measure the effects of dark energy in such as way as to be confident that it exists and is exerting this repulsive force, rather than simply being a mirror concept which is the opposite of the attractive force of gravity?

Another example is with magnetism. We know a metal object will be pulled towards a magnet (or at least one pole of the magnet). Dark energy sounds to be the equivalent of saying that in addition to the magnetic force attracting the metal object, there is also another force pushing the object toward the magnet.

This just doesn't sit well in my layman's mind, and I'm sure I'm just being uneducated! lol

The above makes sense in my head, as I've written it, but so I hope I've explained my conundrum sufficiently for someone to help me out!

:)

 

[mfb]

Dark energy, as far as we can tell, has a constant energy density. Matter - dark and visible - have a constant total energy: They are not created or destroyed in relevant amounts. As the universe expands the density of matter decreases (same mass in a larger volume) while the density of dark energy stays the same. That means dark energy was negligible in the early universe and became more important over time. Today it is the main energy component in the universe.

Dark energy that would "pull" wouldn't lead to the accelerated expansion we see. It would slow the expansion.

 

[PeroK]

I'm interested to know what evidence we have that dark energy is pushing as opposed to dark and normal matter pulling.

If we try to model the universe using matter (and radiation) only, then we should see a decelerating expansion. That is what was assumed until about 1998, when evidence that the expansion was accelerating was found.

It's not possible to construct an accelerating expansion with only matter (dark or otherwise). You need something else. That implies that there must be an additional factor.

One obvious candidate is that the vacuum of space itself has some (dark) energy density. If you include a fixed vacuum energy density in the equations for cosmological evolution, you get an accelerated expansion. And, for the reasons explained above, when the universe was relatively "small" it was the matter density that dominated, but as it expanded, more space means more dark energy and eventually the dark energy dominates and drives an accelerating expansion

The outstanding question, of course, is what causes dark energy and why it has the observed energy density. There is no real problem explaining why dark energy gives us the observed universe expansion; the problem is explaining dark energy itself.

 

[lomidrevo]

Are we able to measure the effects of dark energy in such as way as to be confident that it exists and is exerting this repulsive force, rather than simply being a mirror concept which is the opposite of the attractive force of gravity?

Yes, we have pretty much of convincing evidence that it exists:
https://en.wikipedia.org/wiki/Dark_energy#Evidence_of_existence

 

[Firehanger]

Dark energy, as far as we can tell, has a constant energy density. Matter - dark and visible - have a constant total energy: They are not created or destroyed in relevant amounts. As the universe expands the density of matter decreases (same mass in a larger volume) while the density of dark energy stays the same. That means dark energy was negligible in the early universe and became more important over time. Today it is the main energy component in the universe.

Dark energy that would "pull" wouldn't lead to the accelerated expansion we see. It would slow the expansion.

Ah gotcha. That sounds even more interesting then - constant energy density! Thank you. :)

Is this purely theoretical, or is there evidence that points to the energy density of dark matter then?

 

[Janus]

Ah gotcha. That sounds even more interesting then - constant energy density! Thank you. :)

Is this purely theoretical, or is there evidence that points to the energy density of dark matter then?

I'm not sure what you are asking here. Dark matter and Dark energy are separate and unrelated ( at least as far as we can tell). The only thing they are known to share is the word "dark" in their names.

 

[Firehanger]

I'm not sure what you are asking here. Dark matter and Dark energy are separate and unrelated ( at least as far as we can tell). The only thing they are known to share is the word "dark" in their names.

Ooops, I meant to write dark energy, rather than dark matter in that last post!

 

[lomidrevo]

Here is another intro text where you can find what is the observed value of the density of dark energy and what evidence supports that:
http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/dareng.html

 

[phinds]

Ooops, I meant to write dark energy, rather than dark matter in that last post!

Same as in the middle of your first post

Summary:: What is the evidence for our theory for the existence of dark energy

as it's suggesting that its the every increasing generation of dark matter

 

[Delta2]

when the universe was relatively "small" it was the matter density that dominated, but as it expanded,

What caused it to expand from being small and where the matter density dominated? It was not dark energy since dark energy was small when universe was small. So what was it?

 

[PeterDonis]

What caused it to expand from being small

When the universe was small, it was also hot, dense, and rapidly expanding. So it kept expanding by inertia.

The current front-runner explanation for why the early universe, at the Big Bang, was hot, dense, and rapidly expanding is that the epoch prior to the Big Bang was an epoch of inflation.

 

[Delta2]

When the universe was small, it was also hot, dense, and rapidly expanding. So it kept expanding by inertia.

The current front-runner explanation for why the early universe, at the Big Bang, was hot, dense, and rapidly expanding is that the epoch prior to the Big Bang was an epoch of inflation.

All this according to the Big Bang theory, because according to another theory the universe is (nearly) flat and infinite and the Big Bang never happened, which of the two theories is prevailing among modern astrophysicists/cosmologists?

 

[phinds]

All this according to the Big Bang theory, because according to another theory the universe is (nearly) flat and infinite and the Big Bang never happened, which of the two theories is prevailing among modern astrophysicists/cosmologists?

In the Big Bang theory, the universe is nearly flat (probably all the way) and probably infinite and the "big bang" (the start of the whole thing) is not part of the model. THAT is the prevailing model. The model starts at the end of the hypothesized inflationary period.

 

[PeterDonis]

according to another theory the universe is (nearly) flat and infinite and the Big Bang never happened

Please give a reference for this theory, since I have no idea what you are talking about.

 

[PeterDonis]

the "big bang" (the start of the whole thing) is not part of the model

I was using the term "Big Bang" to refer to the hot, dense, rapidly expanding state that is the earliest state of our universe for which we have good evidence. This state is certainly part of our best current cosmological model. What is not part of that model is an "initial singularity" state; the term "big bang" is sometimes used to refer to that state but that usage is not really correct.

 

[Delta2]

Please give a reference for this theory, since I have no idea what you are talking about.

The reference is here Physics Forums, some posts say that the universe was always infinite and always expanding and that the Big Bang didn't actually happen.

I was using the term "Big Bang" to refer to the hot, dense, rapidly expanding state that is the earliest state of our universe for which we have good evidence

If it is infinite now then it s should have always been infinite, even back then when it was at the hot dense state, am I right on this? You are right that by the term Big Bang I refer to the initial singularity state which you and phinds say now that it is not part of the model.

 

[phinds]

What is not part of that model is an "initial singularity" state; the term "big bang" is sometimes used to refer to that state but that usage is not really correct.

Which is what I meant. I should have been more clear, but I DID say "the start of it all".

 

[mfb]

The reference is here Physics Forums, some posts say that the universe was always infinite and always expanding and that the Big Bang didn't actually happen.

Where?
PF is not a scientific reference on its own, but I'm still quite sure that you just misunderstand these posts.
If the universe is infinite now then it was always infinite for as long as it existed. That doesn't rule out a beginning that doesn't have a "before".
Eternal inflation is a popular model that doesn't have a unique beginning, but it still has the Big Bang. It's what happens after inflation.

You are right that by the term Big Bang I refer to the initial singularity state

If you use scientific terms and redefine them to mean completely different things in your posts then confusion is inevitable.

 

[PeterDonis]

The reference is here Physics Forums

Then please provide a link. You have been here long enough to know better than to give vague references without a link to back them up.

 

[PeterDonis]

If it is infinite now then it s should have always been infinite, even back then when it was at the hot dense state, am I right on this?

Yes. In our best current model, our universe is spatially infinite and always has been. But we cannot directly observe the entire universe; we can only directly observe our observable universe, which is finite in size and always has been.

You are right that by the term Big Bang I refer to the initial singularity state which you and phinds say now that it is not part of the model.

Correct. And, as I said, I was not using the term "Big Bang" in that way.

 

[PeterDonis]

My model is that the big bang singularity existed and it was the moment that God decided to create the universe.

And you should definitely know better than to mention this, since you know it is off topic and out of bounds for PF. And you have been told this explicitly about this particular type of statement before.

You have just earned yourself a warning and a thread ban.

 

[artis]

. As the universe expands the density of matter decreases (same mass in a larger volume) while the density of dark energy stays the same. That means dark energy was negligible in the early universe and became more important over time

There is something I don't understand here, if the density of dark energy/vacuum energy stays the same while the universe is expanding doesn't that mean that it's energy total has to increase with time?
Also I assume you say it's energy was negligible in the early universe because all the other fields and energies were much higher/denser so dark energy was small compared to them?

 

[lomidrevo]

if the density of dark energy/vacuum energy stays the same while the universe is expanding doesn't that mean that it's energy total has to increase with time?

Correct. As universe expands, it is more and more filled by vacuum. Dark energy is very probably the vacuum energy. However, as the universe is very probably spatially infinite, it doesn't really make sense to talk about total energy. Energy densities are much better defined

 

[Ibix]

There is something I don't understand here, if the density of dark energy/vacuum energy stays the same while the universe is expanding doesn't that mean that it's energy total has to increase with time?

Defining a "total energy of the universe" turns out to be extremely difficult. It can be done for a spacetime that is asymptotically flat or one that has a timelike Killing vector field. FLRW spacetimes are not asymptotically flat and do not have a timelike Killing vector field. So we don't actually know how to get an answer this question.

Also I assume you say it's energy was negligible in the early universe because all the other fields and energies were much higher/denser so dark energy was small compared to them?

Yes. The energy density in matter, radiation, and dark energy changes at different rates as the universe expands, so different components can be the most important at different times.

 

[lomidrevo]

Here is a graph showing how the energy densities evolved for each of the constituents: radiation, matter and dark energy. One can recognize three eras. Very early universe was dominated by radiation. Matter was dominant till about 9 billion years after big bang. Then, dark energy took over and the expansion of the universe accelerated.

https://pages.uoregon.edu/jimbrau/BrauImNew/Chap27/6th/27_01Figure-F.jpg

 

[artis]

As far as I can recall dark energy/vacuum energy seems to be the only field that can keep a fixed energy density while space is expanding as all the other fields decrease in strength as they expand, this seems very counterintuitive because we are used to having an explanation for a source of energy and it seems unclear where dark energy could have it's source.

 

[PeterDonis]

if the density of dark energy/vacuum energy stays the same while the universe is expanding doesn't that mean that it's energy total has to increase with time?

There is no global conserved energy in an expanding universe. See here:

https://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/

Strictly speaking, since in our best current model our universe is spatially infinite, the "total energy" of our universe would be infinite as well. But even in a finite universe with dark energy (for example, a spatially closed FRW spacetime with a positive cosmological constant), the "total energy" (dark energy density integrated over spatial volume) would change with time, and that's to be expected.

 

[lomidrevo]

it seems unclear where dark energy could have it's source

Sure, the origin of dark energy is an open question. I think the prevailing opinion in mainstream is that it has to be explained as energy of vacuum. According to quantum field theory, even in vacuum there exist field fluctuations due to Heisenberg uncertainty principle. So naturally, one would think that energy of these fluctuations could be the source of dark energy observed in cosmology. However, there is enormous discrepancy between the observed value of dark energy (a.k.a. cosmological constant) and the theoretical value coming from QFT. The predicted value asi higher by 120 orders when compared to observed value.
https://en.m.wikipedia.org/wiki/Cosmological_constant_problem

So clearly we don't have satisfactory explanation yet. Maybe we won't know until we have a complete theory of quantum gravity

 

[PeterDonis]

As far as I can recall dark energy/vacuum energy seems to be the only field that can keep a fixed energy density while space is expanding

No, it isn't. It's possible for a scalar field to do this. For example, in inflation models, the inflaton field has a constant (and very large) energy density during inflation; that's what causes the inflationary (exponential) expansion.

 

[mfb]

and the theoretical value coming from QFT.

There is no such thing. QFT is silent about absolute energy densities. There is a term that can be interpreted as absolute energy density, but that term is infinite. It only gets finite if you add an energy cut-off, and then it depends critically on that cut-off. If you take the Planck energy then you are off by 120 orders of magnitude. But this is assuming (a) new physics only happens at the Planck energy, (b) this new physics suddenly stops the growth of the absolute energy of known particles without adding any new terms on its own, (c) there is no reason why the contributions from all the different particles don't cancel each other.
(a) is questionable, (b) is quite absurd in my opinion, and (c) is plausible but unknown. And even if all three are true we still have only a very rough estimate, it could easily be off by a few orders of magnitude.
Calling it a prediction is vastly overselling this handwaving procedure.

 

[artis]

@PeterDonis Ok so you say scalar fields can do this, IIRC examples of known scalar fields would be also the B field (static), E field(static) , EM field , gravity , others?
Can the E, B, EM or gravity fields do anything similar to what is happening to the dark/vacuum energy ?
Let me ask one more thing with regards to the topic at hand, normally we associate a field with a source, like the E field and it's source charge or gravity and mass , so the way I imagine in ordinary conditions if I have a object that has charge and mass it is a source for both E field and gravity
What is the source for dark/vacuum energy?
Is the source known or hypothetical as of now?

It seems that unlike matter and the fields associated with matter the source for dark energy seems to be everywhere at once and fundamentally linked to space time.

 

[mfb]

IIRC examples of known scalar fields would be also the B field (static), E field(static) , EM field , gravity , others?

None of these are scalar fields. They are vector fields or tensor fields, they "have directions".

What is the source for dark/vacuum energy?

It doesn't need one. That's why it is called vacuum energy or cosmological constant or similar. It's there even in vacuum.