# What does Dark Energy explain?

#### JinChang

It appears that the Dark Energy concept was born out of a need to explain the two major observational phenomenons (increasing rate of expansion of the Universe and the faster-than-expectated-rotation of the galactic outer-rim). If I'm not mistaken, this also has to do with gravity seemingly trending towards 1/r rather than 1/r2 with more distance.

That said, I'm curious as to what other phenomenons it explains?

For example, does it also explain (better) the Pioneer Anomaly and the precession of the perihelion of Mercury?

Also, has anyone considered the possibility that the Dark Energy IS THE ONLY SOURCE of Gravity rather than separating the Matter/Dark-Matter as the other sources? For example, Is it wrong to speculate that the Dark Energy, with it's negative-pressure, is the source of gravity with the Matter simply being a region of space-time (due to presence of localized energy) where such gravity is more concentrated (i.e. gravity well)? I ask this question without having any means to formulate it given that it's been a long-long-time since I took Calculus in college.

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#### mathman

It appears that the Dark Energy concept was born out of a need to explain the two major observational phenomenons (increasing rate of expansion of the Universe and the faster-than-expectated-rotation of the galactic outer-rim).
You seem have mixed up two weird concepts, dark energy and dark matter. Dark energy is used to explain the increased rate of expansion (discovered late 1990's). Dark matter is used to explain the faster rotation (middle 20th century) and other gravity anomalies.

According to current theory, dark energy is about 70% of the universe, dark (non-baryonic) matter about 25%, and ordinary (baryonic) matter about 5%.

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#### cesiumfrog

has anyone considered the possibility that [normal matter is not a source of gravity]?
Refuted by Cavendish, 1790's.

#### JinChang

You seem have mixed up two weird concepts, dark energy and dark matter. Dark energy is used to explain the increased rate of expansion (discovered late 1990's). Dark matter is used to explain the faster rotation (middle 20th century) and other gravity anomalies.
I probably did, and I'm sure it won't be the last time...

That said, what other phenomenon could these Dark Energy and Dark Matter be instrumental in explaining?

#### JinChang

Refuted by Cavendish, 1790's.
I'm in no way suggesting that Matter has nothing to do with gravity. Instead, I'm trying to grasp the relationship between gravity and matter where one could be explained using the other. Isn't it equally valid to say that gravity realizes matter as we know it versus matter causing gravity? If so, I'm asking if gravity itself is more fundamental than the matter given its role in matter, dark matter, and dark energy. Be it the cosmological constant, quintessence, vacuum energy, or dark energy... they all seem to imply something fundamental tied to explaining away any deficiencies in the way we understood gravity until recently. Why not start with gravity to explain other things instead, including the very definition of matter.

Just wondering out aloud.

#### Ich

That http://en.wikipedia.org/wiki/Geon_(physics)" [Broken], but without success.

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#### JinChang

If I read it correctly, it seems that Wheeler's concept of Geon tries to break down the elementary particles into smaller constituents -- mainly, Geons ("gravitational electromagnetic entity"). If so, it still attributes Mass as the source of gravity (which in all likelihood is correct).

What I was suggesting, however, is to start with gravity, which is universally prevalent (call it quintessence, dark energy, cosmological constant, or whatever). This stress-energy-tensor then gets distorted in regions of space-time where what we call Mass is present. Thus, Mass is no longer the cause of gravity, but rather the cause of distortion of gravity. I'm not sure if this view leads to different results when it comes to equations governing the way we calculate gravitational effects, but doesn't it explain the 1/r (rather than 1/r2) behavior expected by some for long-range gravitational effects?

Mind you, I could be (probably) totally off base on this.

#### Ich

If so, it still attributes Mass as the source of gravity (which in all likelihood is correct).
It is nothing but its own field energy, kind of bootstrap mechanism. No a priori mass involved.

#### JinChang

It is nothing but its own field energy, kind of bootstrap mechanism. No a priori mass involved.
That is absolutely correct, sort of like the Ying-and-Yang where one cannot exist without the other. If so, I'm wondering if the current model of gravity (GR) takes this into account. Is it a "pull" model or a "push" model? Are there differences between the two in terms of how the gravity is viewed or the phenomenons they can explain?

#### Ich

I'm wondering if the current model of gravity (GR) takes this into account.
This geon-concept is based entirely on GR, there are no new forces postulated (as far as I understood, my knowledge is nothing more than wikipedia and what wheeler wrote in MTW).
Is it a "pull" model or a "push" model?
What, GR? It's neither, it's a geometric model with no forces.

#### JinChang

What, GR? It's neither, it's a geometric model with no forces.
I understand that it's a geometric mode, but does that really mean no forces are involved? Maybe it is since the objects in a gravitational "field" simply takes the geodesic path.

#### Ich

I understand that it's a geometric mode, but does that really mean no forces are involved?
Yes, no forces, no acceleration.
Maybe it is since the objects in a gravitational "field" simply takes the geodesic path.
Exactly.

#### JinChang

Certainly makes sense when discussing GR, but what about other theories and researches going on related to gravity? For example, if GR is the true representation of gravity, then are the concept/search for graviton and/or Higgs (God) particle futile? Another related question might be: if space-time fabric warping is real, then is there a fundamental unit of space-time or is it infinitely malleable?

Sorry for raising so many questions, but you know how confusing trying to reconcile various theories can be...

#### Ich

For example, if GR is the true representation of gravity, then are the concept/search for graviton and/or Higgs (God) particle futile? Another related question might be: if space-time fabric warping is real, then is there a fundamental unit of space-time or is it infinitely malleable?
You're asking about quantum gravitation and a theory of everything. I don't know how a TOE would look like, and whether a modified GR will be a part of it or whether GR will end up in the waste bin.

#### Chronos

Gold Member
I'm still attached to GR. It has never been refuted. However close they may be, no alternative theory can make this claim.

#### mathman

GR and quantum theory are in conflict when both have to be used at the same time (black hole, big bang). Neither theory has been refuted, so physics theorists still have work to do.

#### Phrak

I'm in no way suggesting that Matter has nothing to do with gravity. Instead, I'm trying to grasp the relationship between gravity and matter where one could be explained using the other. Isn't it equally valid to say that gravity realizes matter as we know it versus matter causing gravity? If so, I'm asking if gravity itself is more fundamental than the matter given its role in matter, dark matter, and dark energy. Be it the cosmological constant, quintessence, vacuum energy, or dark energy... they all seem to imply something fundamental tied to explaining away any deficiencies in the way we understood gravity until recently. Why not start with gravity to explain other things instead, including the very definition of matter.

Just wondering out aloud.
Backing up to this above statement of yours.

I understand your notion, I think. The standard reply is that mass (and energy and momentum) is the source of gravity). "Mass gives rise to gravity. Gravity acts back on matter." And so everything is neatly taken care of, or so it would seem.

Are you asking "Could it be that gravity gives rise to matter?"

I think this is a legitimate question.

Einstein's equation of gravity--gravity on the left, stuff on the right,

$$R_{\mu\nu} - \frac{1}{2} g_{\mu\nu}R = \frac{8 \pi G}{c^4} T_{\mu\nu} \ ,$$

is not expressed as

$$R_{\mu\nu} - \frac{1}{2} g_{\mu\nu}R \leftarrow \frac{8 \pi G}{c^4} T_{\mu\nu}\ ,$$

to indicate that "matter gives rise to gravity".

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#### JinChang

Backing up to this above statement of yours.

I understand your notion, I think. The standard reply is that mass (and energy and momentum) is the source of gravity). "Mass gives rise to gravity. Gravity acts back on matter." And so everything is neatly taken care of, or so it would seem.

Are you asking "Could it be that gravity gives rise to matter?"

I think this is a legitimate question.

Einstein's equation of gravity--gravity on the left, stuff on the right,

$$R_{\mu\nu} - \frac{1}{2} g_{\mu\nu}R = \frac{8 \pi G}{c^4} T_{\mu\nu} \ ,$$

is not expressed as

$$R_{\mu\nu} - \frac{1}{2} g_{\mu\nu}R \leftarrow \frac{8 \pi G}{c^4} T_{\mu\nu}\ ,$$

to indicate that "matter gives rise to gravity".
That's an interesting way of putting it, Phrak.

I'm going to spill the bean and ask the real question that I wanted to ask, but have been avoiding due to my lack of various concepts. It may appear amateurish, but please bear with me and cut me some slack.

Few years ago, I was trying to reconcile (in my mind) the expanding universe (inflation) and gravity. Just wondering if they are somehow related or if the inflation can actually effect gravity. Then came the concept of Dark Energy with negative pressure that may possibly explain why the universe was expanding ever faster. Such concept, if true, could possibly tie the Dark Energy to that of Gravity rather than supplementing it (so I thought).

It was also around the same time that I thought of an Isotropic source of gravity, which if viewed in similar ways to quintessence (filling the void of vacuum), could possibly explain the relationship between matter and gravity. It wasn't difficult to imagine such concept given other esoteric theories such as gravity leaking into our universe from the neighboring "multiverses" (branes) which sounded even stranger. If such Isotropic source of gravity pervade our universe, be it the Dark Energy or something else, then Mass could be defined as a region of space (space-time) that either is devoid of such phenomenon or resists it. So, we have your normal Vacuum, which is no longer "empty" since gravitational force is constantly emanating isotropically, and your Mass which lacks such phenomenon. Don't ask me how Mass would behave that way since I haven't thought it through, but it could be due to presence of localized Energy (E=Mc2, right?) that is causing it, or something totally different altogether...

Given this premise, the Gravitational Field can be viewed as Pressure relative to the surrounding Isotropic Gravity Source (space itself). The greater the Mass, the higher the Pressure. It would also mean that Gravitational Field is dependent on the uniformity of the Isotropic Gravity Source (relative gravity?) rather than depending only on the Mass. Also, it would mean that there's some sort of ground-state for gravity that pervades the universe, which may have changed over time.

Chances are good that someone already had a similar theory, and probably proven false, but in terms of visualization, this is as elegant as I can get given little what I know. But, this visualization has helped me a great deal in grasping the phenomenons like the double-slit experiment where the "pilot-wave" concept distills down to the Isotropic Gravity Pressure Wave (similar to geodesic) as well as perhaps explaining the Galactic-Rim-Rotation issues (possibly explained by gravity being a constant flux towards mass from empty space, possibly trending to 1/r rather than 1/r2 behavior in long ranges). My lack of Calculus prevents me from pursuing these things beyond imagination, which explains my many questions.

#### Chronos

Gold Member
what observation has caused a disconnect? I am well aware they are discordant, just unaware of any observation that affirms the disconnect.

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