This is an overview of ‘dark matter’ and ‘dark energy’, that was started back in 2016 but published until I was recently asked again. The TLDR; is that no-one knows what dark matter or dark energy are, and it is not even absolutely certain that either exists. In fact, my addition to the official version, is that neither need exist. Both dark matter and energy are possible explanations for for things we do not understand, but not the only possible explanations. That being said, a significant majority of physicists do believe dark matter does exist, and many are looking to find it.
- Dark Matter:
- What does ‘dark matter’ mean?
- Why? Because known matter does not explain observed gravity.
- So there must be matter we don’t know about: Dark Matter.
- Its hiding. Dark matter is just matter we have not found.
- Or, It is matter, but not as we know it.
- Or, maybe we have gravity wrong!
- So there must be matter we don’t know about: Dark Matter.
- Dark Energy.
- What is dark energy?
- Why? Something is accelerating the expansion of the universe?
- The unofficial explanation: food for thought?
What does dark matter mean?
The name is misleading, and a better name would be ‘undiscovered matter’. Dark matter is not dark in colour, and the assumption is that dark matter does not block or absorb light. There are different possibilities as to what dark matter is, but it is just matter that has so far, evaded all attempts to find it. We are looking for why galaxies have gravity that should require more mass than we can detect.
Why? Because known matter does not explain observed gravity.
Consider the planets of the solar system. The is a simple equation for the speed of a planet orbiting the sun. The further away the planet, the slower it needs to be moving to avoid falling into the sun, and there is a formulae for that speed. Every planet moves at the speed we expect, proving we definitely know exactly how gravity works. Perfect.
Now consider stars moving around the galaxy. The calculation of expected speed is more complex, as unlike the solar system where almost all matter is at the centre and there is one planet in each orbit, in the galaxy, mass is scattered amongst the stars themselves.
Again we have a formulae to predict the speed of stars orbiting the galaxy, but this time, what we see does not match what we expect. We expect to need a lower speed near the galaxy centre, as almost as much of the galaxy is pulling ‘out’ as ‘in’. But we also expect that once you move outside the most dense area, the force pulling inwards will again trail off. On the basis of what we know, stars out where our Sun is, are moving way to fast and should fly outwards by their centrifugal force. There should not be enough gravity pulling them inwards for the stars to stay in orbit.
Theory 1: There must be matter we don’t know about? ‘Dark’ Matter?
The first theory is that, given we can’t see enough matter to explain the gravitational forces, there also be matter we can’t see. In fact, there has to be a lot of matter we can’t see, and we need 4x as much matter we can’t see as the matter we can see to explain the motion of stars and other observed effects.
Its hiding. Dark matter is just matter we have not found.
All this matter hiding somewhere requires either matter formed from the building blocks we already understand somehow going undetected, or new building blocks we do not know how to detect. If using the building blocks or fundamental particles we already know how to detect, then all this matter has to have some way of hiding. Ways of ‘hiding’ ordinary matter considered are:
- neutrinos – but that is ruled out as there simply cannot be enough of them and behaviour is wrong.
- dark stars, dead planets etc – but that is also ruled out as they just cannot be enough.
- black holes – the real option, with most being tiny (asteroid mass) black holes could be possible.
Or It is matter, but not as we know it.
The other option to most matter being hidden in very small undiscovered black holes, is that there is at least one undiscovered particle type that interacts only in a very weak way with known types of matter. This the current preferred theory to explain the additional gravity: WIMPs (Weakly Interactive Massive Particles). The only problem being, we have not found them yet. There is a formulae for what portion of the universe should be these particles, based on particle interaction level and rate of decay:
The formulae leaves a wide range of possibilities, although the most glaringly obvious choices have already been explored without success, this is still the favourite approach to find an answer to the puzzle.
The mystery of additional particles not only has to discover the ‘secret’ particles, but also explain their distribution. If the only interaction with normal matter is gravity, then why are these particles not distributed like the rest of the mass in the universe? The observed graph above not only requires additional matter, it requires a specific distribution of that additional matter, different from the distribution of normal matter. Note the graph is not the same shape as the expected values.
Theory 2: Maybe we have gravity wrong!
Back when Einstein was working on relativity and the implications for gravity, there was a mystery somewhat similar to the current dark matter problem. At the time, it was that the orbit of Mercury did not follow predictions, and the favourite theory was that this had to be due to a ‘dark planet’ also exerting gravitational force on Mercury. No one could find this ‘dark’ planet. However, by using Einstein’s theories to calculate gravity in place of only those of Newton, no dark planet was needed. It was the understanding of gravity that was insufficient to explain the data, not some ‘dark’ object.
What if there is still more to gravity than we are so far aware? Perhaps the only problem is our calculations of ‘expected’. Of course, it possible that both are a work, a most likely far, far smaller amount of ‘dark’ matter, and a need to improve our calculation of the effects of gravity at the scale of a galaxy, and perhaps as the effects of gravity traverse time.
A key point is that gravity is not a force in that it does not require energy.
What does ‘dark energy’ mean?
Just as ‘dark’ matter hypothetical matter that would explain our observations of gravity, ‘dark energy’ is a type of energy needed to explain our observations of the universe expanding.
Dark energy is even more of a hypothetical than is dark matter. We just know that with current theories, we need a lot of energy we have not discovered. Given E=mc2, energy can be compared to mass, and so much energy is needed to explain what is going on, that this energy is equivalent to more than double the amount ‘everything’, even including the huge amount of dark matter that we do not even know for certain exists.
Why? Something is accelerating the expansion of the universe?
The ‘big bang’ started the universe expanding. We can accept that. The universe could even still be expanding almost 14 billion years later. But given gravity exists and attracts all matter together, the expansion must have the force of gravity to eventually stop the expansion. Just as black holes ‘suck in’ all matter that gets close enough, it seems the entire universe could one day become a giant black hole.
Except Professors Brian Smidt, Saul Perlmutter and Adam Riess discovered, to everyone’s surprise, the expansion of the universe is accelerating. No one knows why, or what energy behind this acceleration. I mean, how much energy is needed to make the entire universe expand faster?
So don’t know what this energy is, or how it works, but we do have a name and know they must be a lot of this energy.
The first thing to under
The unofficial explanation: food for thought?
Now for the unofficial version. A proposal that, while there may be some ‘dark matter’ or a least matter not yet detected, there need be nothing like the amount proposed in dark matter theories, as dark matter may not be needed to explain observed gravity at a galactic level.
Gravity warps space time, in a way that effectively creates a force attracting matter together. Note this gravitational affect does not require huge amounts of energy to produce gravitational attraction, and instead is a property of spacetime. Now consider, what if the true nature spacetime in the absence of mass, is effectively the reverse of the warp created by matter. That when there is sufficiently low mass, objects don’t attract, they repel. An effect that is ‘smother’ or with a lesser gradient than gravity, such that to observe the effect requires comparison of space at different distances from the centre of galaxies. A comparison of the space at our point in the galaxy with space at the outer rim of galaxy before the effect of the ‘repulsive distortion’ or negative gravity can be observed. This affect would assist in the ‘clumping’ of matter in the universe and assist the formation of globular clusters of lower total mass than other galaxies as they would be pushed together by the surrounding space.
It is then this repulsion that causes the universe to accelerate expansion. Rather than dark energy, the repulsion is part of the fabric of space-time.
Not only would galaxies be repelled from each other, but the outer areas of galaxies would be repelled from the empty space between galaxies, creating an additional force that would act in the same manner as additional gravity, but with increased effect as object position increases from the galactic centre.
This same effect would explain both ‘dark matter’ and ‘dark energy’ without the need for either to actually exist.