Dark Matter Could Make Interstellar Travel Possible

We have ever imagined to live on a distant star colony. Our Neanderthal ancestor were also seeing to be on distant star planet, I think. So, lets start a  brand new advanced method of propulsion which was presented by Jia Liu, professor at Center for Cosmology and Particle Physics. He speculated that we could attain near light speed if we could use dark matter engine in our rockets. Our existing technology is not yet far in future to travel vast intergalactic distances. So, if  you really want to meet aliens  you have to leave old propulsion methods try something new exotic propulsion methods. A while back Louis Crane has suggested that black could be used as propellant and these black hole craft could get at near light speed. 

Let’s start with me on the possibility of dark matter propulsion. So how that will work? Probably you know how jets fly? We usually never put oxidants in a typical Jet engine because it takes oxygen from air to completely combust fuel. Universe is filled with dark matter as our general assumptions[although ther are many questionable assumptions, click here to read] say. So it would be the dark matter which could detract heavy fuel and can reduce weight of rocket easily.  Here we assume the DM particle and the annihilation products can not pass through the wall of the box. In picture A, the space ship moves very fast from right to left. The DM particles, which are assumed to be static, go into the box and are absorbed in the picture B. In the picture C, we compress the box and raise the number density of the DM for annihilation, where we assume the annihilation process happens immediately. In the picture D, only the wall on the right side is open. The annihilation products, for example Standard Model (SM) particles, are all going to the right direction. The processes from A to D are the working cycle for the engine. Thus, the spaceship is boosted by the recoil of these SM particles. Note the spaceship can decelerate by the same system when it reaches the destination, by opening the left wall in the picture D.Even taking he mass of spaceship to be 100ton it could travel as fast as equals  to 10-³c.  This is an example of DM engine using DM annihilation products as propulsion. The acceleration is proportional to the velocity, which makes the velocity increase exponentially with time in the non-relativistic region. The important points for the acceleration are how dense is the DM density and how large is the saturation region[which significantly depends upon dark matter  density in the space]. The parameters of the spaceship also have great influence on the results.  
  

 For example, the velocity will increase if  S/M increases, where S/M is area of space ship per unit mass, so that it can collect most of the dark matter.  The paper shows that the (sub)halos can accelerate the spaceship to velocity 10−5c ∼ 10−3c under the reasonable parameters of spaceship. Moreover, in case there is a central black hole in the halo, like galactic center, the core radius of DM can be large enough to accelerate the spaceship close to the speed of light). Once we know the velocity distribution of DM, it can be solved by programming the direction of the spaceship when speed is low. An analogue in our daily life is airplanes work well in both headwind and tailwind. Second, it has assumed the DM particles and the annihilation products can not pass through the wall of the engine. For the annihilation products, they may be SM fermions which have electric charges. Thus we can make them go into certain direction by the electromagnetic force. The most serious problem comes from DM which are weakly interacting with matter. Current direct searches of DM have given stringent bound on cross-section of DM and matter. It may be difficult using matter to build the containers for the DM, because the cross-section is very small. However, the dark sector may be as complex as our baryon world, for example the mirror world. Thus the material from dark sector may build the container, since the interactions between particles in dark sector can be large. Third, the annihilation process is assumed to happen immediately in the picture C. This is the second serious problem we should pay attention to.To make the annihilation process efficient, we have to compress the volume of the engine to raise the annihilation speed. Whether it can be achieved in the future is not clear. Nevertheless, the engine works in the vacuum where the baryonic matter is dilute, which means we do not need to worry about the pressure from the baryonic matter. Sometimes, when looking at the N-body simulation pictures of DM, I think it may describe the future human transportation in some sense. In the picture, there are bright big points which stand for large dense halos, and the dim small points for small sparse halos. 

  
Interestingly, these halos have some common features with the cities on the Earth. The dense halos can accelerate the spaceship to higher speed which make it the important nodes for the transportation. However, the sparse halos can not accelerate the spaceship to very high speed, so the spaceship there would better go to the nearby dense halo to get higher speed if its destination is quite far from the sparse halos. Similarly, if we want to take international flight, we should go to the nearby big cities. The small cities usually only have flights to the nearby big cities, but no international flights. Thus we can understand the dense halos may be very important nodes in the future transportation, like the big cities on the Earth.