What is All of That Dark Matter/Energy Anyway?

<...nobody else seems to have any answers that make sense...> Guilty, as charged.

It begs the question, how is it possible to detect gravitational waves?

Not everyone is convinced that dark matter exists. The competing theory is that the law of gravity needs to be modified.

https://www.forbes.com/sites/startswithabang/2016/11/01/why-are-dark-matter-and-modified-gravity-in-such-conflict/#6a8980136112

Seems to me it's all about density....space is full of dust, it's constantly being created and constantly aggregating in clumps....the scale is hard to imagine because it's so vast..

Gravity does store energy by warping space-time, but it's negative energy. If you took the earth apart and scattered the pieces infinitely far apart, the gravitational field would disappear. But it would take energy to do this, so the energy stored in a gravitational field is negative.

It's been estimated that the positive energy in the universe associated with mass (E=mc²) is equal to the negative energy in gravitational fields, yielding a net energy of zero.

"The zero-energy universe hypothesis proposes that the total amount of energy in the universe is exactly zero: its amount of positive energy in the form of matter is exactly canceled out by its negative energy in the form of gravity.^[1][2]"

^{https://en.wikipedia.org/wiki/Zero-energy_universe}

If the earth were scattered infinitely far apart, would not the sum of the gravitational fields of all the particles still be equal to that of the previous body called earth?

I could be dead wrong here as this ain't my area.

Yes.

Dark matter..

The neutrino of the day.

Hi Rixter, one should caution that "negative energy" for the gravitational field is just a convention for convenience. In Newton's time, they have chosen an object that is infinitely far from a gravitating mass as having zero potential energy (relative to the primary mass). Then the potential energy goes negative for smaller separations. This is so that for simplicity, one can keep the potential energy + kinetic energy constant for a free-falling object. It is actually only the change of potential energy that is important - one could choose the zero point anywhere you like.

I general relativity, the gravitational field energy of the primary mass is added to the mass-energy in a positive form. That means that the field causes extra gravity ("gravity gravitates") and the gravitational field has a positive energy value. The effective energy of the primary+field is given by a simple equation at distance r:

Effective gravitational energy = Mc²/√[1-2GM/(rc²)], where the divisor is always positive and less than unity.

"But if it were as plentiful as the equations imply then surely there would be all kinds of eclipses of distant stars. In fact, it should make it difficult to distinguish between detecting planets in orbit around distant stars and space debris."

The dark matter particles that would fit the majority of observations do not have the sort of properties that you have described above. They don't interact with anything electromagnetically, they only interact through gravity. So it can't reflect, absorb, scatter, or any of the usual things that matter does with light, except that it can curve spacetime and hence 'bend' light.

This means that it must have mass-energy, in fact 25% of the energy density needed to make space (not spacetime) as 'flat' as what we observe it to be. This is 5 times the average density of ordinary matter.

It is understood today that space has a lot of energy in itself - in fact 3 times more than the dark matter has to be the "energy of space itself" - the so-called 'dark energy'. We have even less of a clue what this is, although it is almost certain that there must be something like it going on. Without it, we simply cannot explain observations.

BTW, modified Newtonian gravity that has been proposed, falls far short of explaining what we observe, so it is no longer considered a serious candidate for replacing dark matter/energy.

How does the black hole fit this discussion - is dark energy different or is the stuff in a black whole part of the dark matter "universe".

Could someone please enlighten me.

Black holes come in two classes. Those that were made before normal matter was made (primordial) and those that formed later, by collecting normal (baryonic) matter until they (partially) collapsed into 'normal' black holes. The prior could be part of dark matter and the latter not. There were simply not enough baryons 'manufactured' in the BB to make up the dark matter density that we observe through gravity. It must be non-baryonic.

'Dark energy' has no real connection to matter. It could perhaps just be a negative intrinsic curvature of spacetime that happened to be there from the beginning - no particle required for that. Just Einstein's cosmological constant, which has the same status as Newton's gravitational constant, i.e. we don't know how they came about, but we observe their values and we are happy.

Although theorists continue to rack their brains on the idea that maybe both Newton's and Einstein's constants are perhaps not constants... And that they may find a cause, perhaps a deeper, unifying principally below them in quantum theory.

Good luck to them, but I hope that it will be after my time when they have to be talking "dark gravity" together with "dark energy". Fortunately, there is no evidence that anyone of the universal constants are changing.

Jorrie, When the words "non-baryonic" and "dark matter density" are used together, I just want to go have a drink.

Now I'm a regular schmoe who spends his days dealing with materials, electronics, physics, you know stuff you can either touch, build, or predictaby measure and repeat.

How can there possibly be something with massive density out there that was created before matter existed, and is not made of matter?

I understand that a universe where a bunch of atoms can coalesce into a big ball, and further develop to the extent that some of those atoms even become self aware, is a very strange thing (mind boggling to me).

But massive density made of something other than matter?

But massive density made of something other than matter? Here's something to think about:

Information comes in many forms. One such form is books, or files on a computer. The "form/carrier" is not the info, and the info is not the form/carrier. Both have their own discrete properties. Therefore, not only can the books/files be dense, but also the info itself (which is not matter) can be dense. The unknown would be, how can info be massive? In the abstract/figure-of-speech, we say that all the time.

"Jorrie, When the words "non-baryonic" and "dark matter density" are used together, I just want to go have a drink."

Invite me, and we can have long and sobering discussion on it!

Because we think we understand chemistry, we 'know' that only 25% of the matter needed to explain the gravity that we observe on the large scales were made soon after the BB (nucleosynthesis). So, what are the rest?

Must be dark (not reacting electromagnetically) and must be non-ordinary matter (non-baryons). Ideas abound, but no firm explanation yet.

This is totally unscientific! I submit that we have not yet found the proper "simple" theory. I call the correct theory "elegant;" that is, it makes sense and covers the observations without a bunch of exceptions. What little I have read about string theory (and understand even less), it is overly complex. I imagine other theories are similarly more complex than the "correct" theory will be when we discover it. This means there is a better theory "out there" waiting to be discovered--similar to relativity in the early 1900s.

Maybe such a theory already exists - see: http://onlyspacetime.com/

It is simple, elegant and explains (among many other things) what is the root cause of inertia, of gravity and of the expansion of the universe. I find it a very good candidate to what you are looking for.

Download the book and read it (I did it in less than a day and then returned to better "digest" its content). And maybe you can share some thoughts about the ideas found there.

I don't know where the theory of dark matter/energy came from, but I have an opinion that might explain it. First, I want point out that quantum physicists think they have discovered a new "fundamental particle". A lot of physicists say that "The standard wisdom to finding new particles, is to create higher & higher energies.". Well, the only thing that we "know" about energy itself is that it seems to always come inside of a "carrier". That is probably where/when/why Quantum physics ever got started in the first place. But if we can imagine pure energy without any carrier at all, we might be able to explain more & more things.

Let me say in a nutshell why I have a hard time believing any quantum physics at all. The law that says that energy/matter can neither be created nor destroyed but only changes form, has a conflict of interest with quantum physics. Quantum physics seems to be finding more and more "fundamental" quanta that never ends. But with relativity/analog theories, that can be explained by just by saying that there are an unknown (infinite?) number of forms of energy. That makes energy itself the only "fundamental particle" that you'll ever need.
"The standard wisdom to finding new particles, is to create higher & higher energies."? Well, duh. when E=mc squared, the higher the energy, the more likely you'll CREATE a new particle. What came first, the chicken or the egg? You do know that some of the elements on the Periodic Chart are man-made?

Someday, someone will come up with a simpler explanation that will be as plain as the ass on a goat and everyone else will wonder why they didn't think of it.

Personally, I suspect the mass is tied up in black holes and the reason that things look like they are going faster farther away is that what we see is what the thing was doing long ago and has since changed. I also suspect that space is growing because there is energy being injected into it to inflate it, but I don't know what method one would use to directly inject potential energy into a system which is probably not closed.

Well, I have an opinion about that, too. A lot of people won't like it, tho. The whole theory of black holes depends on the fact that the distance between two masses can occupy the same space at the same time, thus the distance between the two become 0, and therefore the mass becomes a singularity with infinite mass/gravity. But when two masses collide, their individual particles mingle, and never really become a singularity anywhere. They seem to ignore that the center of gravity is not a real "thing", but rather a mathematical average/approximation of all the individual intermingled particles. This black-hole thing, seems to me, to be just a mathematical illusion. Since when can two of anything occupy the same space/time?

I don't follow your reasoning at all.

The way the formation of a black-hole is explained is two ways. #1: A huge massive star collapses in on itself with such big gravitational force that it becomes a "singularity" with an even more powerful gravity. #2 Two different masses (each with their own gravity) collide and combine their mass/gravity with the assumption that the distance between each masses' center of gravity become zero to form a "singularity" with an infinite gravity. I don't believe that either can be true. I don't believe that any sub-atomic/atomic/elemental/molecular/mixture collision will result in any sub-atomic/atomic/elemental/molecular/mixture's particles ever occupying the same space-time as any other sub-atomic/atomic/elemental/molecular/mixture's particles, creating any singularity of any sort resulting in an infinite gravity.

On any of those levels, no boson, quark, electron , proton, neutron, photon, graviton, element, molecule, mixture particles can occupy the same space-time as another particle. If you had a jar of sand and poured water into it, the water will simply "fill in the gaps" of the other particles. If a neutron collides with an atom, it will only add itself to the first nucleus/energy. If two massless particles collide, even tho the have no gravity at all, they will still never occupy the same space-time, rendering the whole concept/formula for black-holes invalid.

Upon further explanation, I disagree; let's see what others comment.

I am far from knowledgeable on black holes, but it seems we do not say the particles occupy the SAME space-time, but that they are very closely packed--that is, dense. Also, gravity cannot be infinite--very large, but not infinite. If it were infinite it would suck in the whole universe!

Well, I suppose. But that would mean that the term "singularity" is just a figure of speech, and the formula for gravitational pull would never put the value for distance between two masses at zero. I've never seen an explanation that didn't.

Something else bothers me about black-holes. Looking at the earth, its center of gravity consists of the accumulation of all its particles/mass/gravity. From far away, it looks like a point source. But as you get closer, some of its particles/mass/gravity come to to your right side, and some come to the left, and starts to change away from a point source. Even tho its center of gravity doesn't change, some of its gravity is to your right or left and not strictly from the center. If you start digging, then some of its particles/mass/gravity become above you and pulls you upward right along with the particles/mass/gravity to the right & left pulling you in that direction. In other words, as you become surrounded by the particles/mass/gravity, you start to lose sense of up/down, because, in the center of gravity, you are actually being pulled in all directions and feel weightless. So by this, the pressure of water at the bottom of the ocean does follow the strictly with the laws of gravity as a point source because there is also some particles/mass/gravity above canceling out the particles/mass/gravity below as well as the particles/mass/gravity to the left & right are not adding to the weight of the water. Extrapolating this to the cosmos, I can't even imagine gravity causing fusion to occur. All the particles would merely drift together in a loose pack. There must be something else going on there.

What if the law of Newtonian gravity wasn't a universal constant? Suppose it's varied over time since the BB and thus still varies to the observer over great distances as it continues to dampen. Not in waves mind you, but in very slow undulating and perhaps predicable manner that we have yet to directly observe. Seems entirely possible that many of the physical laws we assume are constant are still ringing on a very large time scale from that bang. After all the natural frequency of the universe is probably quite low. You see it's just too much for me to accept that a pico-second or less after the bang it all reached a steady state and stayed there. My theory is we live in a universe of transients and just haven't come to realize that. It's my 'black' space time transient theory.

Nice.

There is a paper I cannot find by a South American female physicist that postulates that a slow change in as I remember it, the gravitational constant, can account for red shift and maybe other things in the big picture. Not sure sure if she has any connection with the De Aquino stuff

As I recall, the red shift is perfectly accounted for already, without changes in the gravitational constant.

I agree with Tornado: no need for changes of any of the constants of nature to account for all observed phenomena on the large scales. This even includes Einstein's cosmological constant, which perfectly accounts for the observed accelerating expansion of the universe.

The issue is not the constancy of the constants - just "where did they come from and why do they have their observed values"? This is the origin of the terms "dark matter" and "dark energy". However, theoretical physicists love them, because they can toy with all sorts of origins and/or changeability and see if they can perhaps discover a new observable effect. Confirmation by observation might then just bring a Nobel...

Interestingly, "black holes" are not normally included in the "dark" category, because what we can observe of them, including gravitational waves, is fully explained by general relativity. What's going on at or near the central "singularity" is not explained yet, but AFAIK there are in any case no observations connected with that region so far.

I'd have to agree with you that the universal constant varies with time and continues to do so at a very slow rate. I don't think I've ever heard anyone speak of the "steady state" of the universe shortly after the BB. In fact, I don't think of the universe as being in a steady state even now. It might look that way because of the distance between objects but we are a long way from being in steady state.

Our perspective is so limited mostly because we are confined to this planet. But simulations of the liquid or solid core of Juipiter have revealed that chemistry as we know it changes under the extremes of pressure and temperature. So, I submit that we are also a long way from knowing even what is possible. The assumed nature of moons and planets have shocked us in to realizing that there is a lot we couldn't even speculate accurately about until we got a closer look. It is a shame that closed minded people over the centuries have suppressed ideas because they wanted to believe something else from tradition. At least it isn't so dangerous to be a free thinker.

Getting back to the central theme, it seems that the concept of space time is this web that has neither mass or energy. But what if that is wrong. What if it is a conduit instead. That is, could space-time be responsible for the accumulation of forces or energy that throws off our calculations. And since gravity affects space-time by bending light, i.e. changing the direction of photons traveling in space-time, couldn't that also be explained as the conduit being distorted by gravity? If that were true, perhaps our calculations are not taking into account the cumulative effect of the wrinkles in space-time. It is apparent that space-time is not constant in the presence of gravity. Consequently, gravity may play an unexpected role in what we believe is either mass or energy that can not be observed. And Space-Time is the conduit that allows it to distort our perspective. OK, my brain is starting to hurt now.

The term "dark" has historical origins which basically mean "we don't know squat about this." "Darkest Africa" was used to describe parts of that continent which had not yet been mapped (yes, some may have been talking about skin color, I am not.) Another phrase one hears about singularities in black holes is "the laws of physics break down" means the same thing, basically we do not understand it. Stephen Hawking has been credited with an early model of black holes as toroids. Just for example, if there are toroidal BH's then the center of mass is in the doughnut hole and like a hurricane eye, little of significance is happening there. So, this mysterious "singularity" in the case of a toroidal BH would not strain the laws of physics in the least and is a lot of noise about nothing. I fear that modern physics has joined the hippies of the sixty's wrt hallucinogenic ten or eleven dimensional models which will probably never have a chance of relating to a tangible experiment without extreme interpretation. They may be geniuses but how useful would Einstein have been on LSD? Universe expansion is probably an oxymoron and universe expansion faster than the speed of light is more raisin bread than I intend to swallow. The BB started out as a joke, and it may well come full circle. Doppler is not the only way red shift could happen: What if all that "dark" stuff out there leaches some energy out of light passing through it ? What if light spontaneously splits into a photon of red shifted visible and a photon indistinguishable from "microwave background": poof, all you need to claim a fantasy BB. I find it hard to believe that we can do cosmic scale delta distance/delta time to insure that our one-shot "standard candles" are *actually* traveling at the velocity we ostensibly perceive via red shift. The skeptic rests...

thewildotter