Why Dark Energy and Dark Matter?

The simple relationship of gravitational force being inversely related to the square of the separation distance is completely based on experimental observations. Balls rolling down old ramps, things falling past measuring sticks on high-speed video...Gravity is always an attractive force in those experiments as well as in observations of the solar system.If observations of more distant moving objects, made possible by better and better telescopes, show that gravitational force is NOT smoothly decreasing as predicted at the edge of a galaxy or a galaxy cluster, once the force has decreased to a particularly weak force, and beyond that distance gravity SEEMS to convert to a slight repulsive force between distant galaxies...Why is the physics community convinced all of this expansion between galaxy clusters and the higher orbiting speeds of fringe galaxies in clusters is NOT just the real nature of gravity?
- Don (age 63)
Panama City FL, USA

There are two questions here.

One is why the accelerating expansion is attributed to dark energy rather than to some long-range property of gravity. One possibility is that the acceleration is due to a cosmological constant that has no connection with any other part of physics. It would just be a part of the law of gravity. That's sort of along  the lines you're considering, except that it's not a repulsion between objects, which would lead to a very different history (the "w" parameter would be different) than seen. There's more interest in connecting this acceleration effect up with some dynamical field, but no guarantee that will work out.

The other question concerns whether the gravitational effects within galaxies and galactic clusters that have been attributed to dark matter could just be due to a modification of the interaction law between objects. That idea is called "MOND", and you can read about it here: https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics. We've hinted elsewhere why most physicists prefer the dark matter approach. (http://van.physics.illinois.edu/qa/listing.php?id=29410&t=why-think-theres-dark-matter, http://van.physics.illinois.edu/QA/listing.php?id=1267) Partly, it's because our current form of gravity (general relativity) has a simple mathematical expression via a local field equation, not some complicated law with weird changes in form at intermediate distances. GR works well on the scale for which there are precision measurements, and it fits cosmological models. A breakdown of GR in between isn't impossible, but would be more surprising than just having some other particle types around.

Mike W.

(published on 03/30/2015)

Thanks ! Now, regarding the first question you answered... I understand Gravity is, in fact, a "dynamic field" as we visualize it to be a quantum field modified dynamically by the presence and location of masses in that field. Why create or visualize an "additional" dynamic, yet undiscovered, field rather than attributing the long distance observed effects to the gravitational (dynamic) field?
- Don (age 63)
Panama City, FL, USA

Nice question. We already know there are several types of dynamic fields, each fundamental at our current level of understanding: gravity, electroweak, Higgs, chromodynamic. Each is described by a local field equation. Unless there's some compelling reason, it seems odd to switch to an entirely different type of equation. Adding another type of field, perhaps as part of a more unified picture, seems more along the lines that have proved successful so far.

Mike W.

(published on 04/01/2015)

I take your answer to be somewhat a question to me... Why should we choose to attribute the observed long-range forces on masses to gravity since that would complicate our safe, simple local formula?My answer... It would not in everyday practice. Just as we add to our beloved F=mA terms to complete the description if the masses are moving at relativistic speeds, in the interest of depicting the true nature of gravity, a more (complicated) complete formula for the effects of mass on the entire gravitational field would only be used when the effects of masses VERY distant from the point in the field we show interest.Classically, yes, gravity problems are worked knowing that THIS m1 is the primary driver for the effects on m2 with m3, m4, m5, etc being insignificant. The complication at vast range from m2 is that MANY m3, m4, and m5's exist that cannot be ignored. That complication will not be avoided either way we go, gravity or dark energy field.The goal of science and physics is to describe nature as completely, accurately, and as simply as possible. Please don't emphasize simplicity at the expense of accuracy.
- Don (age 63)
Panama City, FL, USA

So far, the cold dark matter + dark energy modification of the standard model has done very well on accuracy. It's not my area of expertise, but the consensus in the area seems to be that it does better than MOND for the overall fitting of data of all types. So at best, right now MOND has to try to compete on simplicity. 

Mixing up equations of radically different types, with basic principles like locality popping in and out ad hoc strikes most of us as far less simple than adding in another couple fields resembling the fields we already know.

What happened to F=ma is very far from the adding in of a variety of correction terms. It was reformulated, along with everything else, on the basis of extremely simple symmetry principles. Nevertheless, if the dark matter particles never show up, there will be strong motivation to look for some other explanation. Maybe MOND, despite its ugly unmotivated current form, would help hint at what to look for. For example, I've heard people mention that some peculiar gravitational effects might arise from interactions with other 3+1 D branes. 

Mike W.

(published on 04/02/2015)

Just want to express my appreciation for your time and carefully thought out comments. I feel like I got all my thoughts addressed expertly and my original questions answered fully. Having graduated so long ago, (1975 Purdue, BSEE, minor in Physics) I didn't know then to ask much about what has begun to interest me now. And even if I had, concepts about the nature of matter and fundamental forces have evolved dramatically in the meantime. Thanks for sharing your University environment, in a sense, with me.
- Don (age 63)
Panama City, FL, USA

Don- We really appreciate these thanks.

Other readers- We didn't make this up, honest. 

Mike W.

(published on 04/03/2015)