Research

Black holes and neutron stars gather a lot of mass at very little space. Both can result from the collapse of a massive star at the end of its lifetime. During such a collapse, several processes that would keep matter in shape are overcome and a super-dense configuration of matter is formed. Neutron stars can be said to be the last stable (and most dense) configuration of matter known. They are about as massive as our sun but measure only tens of kilometers in diameter. In other cases, massive stars collapse all the way to black holes.

In black holes, the entire mass is found in one "singularity". The "size" of a black hole is pure definition and not the size of any structure - it is the (gravitational) radius at which nothing, not even light can escape the steep trough of space-time formed by the super-dense black hole. We call both neutron stars and black holes "compact objects".

When we neglect time, an analogy of space is a stretched sheet. Now, let's compare two objects of the same mass: a heavy marble (the black hole) and porous Dutch bread (our sun). The marble will curve the sheet deeper than the bread. Still, both will curve the sheet the same way towards its outer edge. A lighter marble thrown onto the sheet will therefore draw circles the same way around both central objects. Curvature of space is exactly what causes gravitational attraction. Likewise, Earth will orbit our sun the same way it would orbit a black hole of the same mass. The story changes only when an observer approaches the black hole very closely and experiences the localized and much deeper (and steeper) curvature of space-time (our sheet plus time as fourth dimension). Black holes therefore only have a very small area of influence. Here, all the fascinating effects come to play. Light, for example, has to follow space-time just as a marble on a curved sheet. Close to a black hole, light from your flashlight may come back to you and you may also see yourself from behind. But be aware of the extreme tidal forces that will stretch everything like a spaghetti. The steeper the sheet (or space) is curved, the stronger the tidal force.

Only the environment of black holes (and neutron stars) makes them the most efficient and energetic power plants in the universe. Everything that formed anywhere in the Universe has angular momentum and follows trajectories around a center of mass. If there is matter close to a black hole with angular momentum, it will thus spiral around the heavy central mass just like planets around their central star(s). This is true for black holes at all sizes and masses. Matter around the largest black holes of millions to billions of solar masses forms the centers of huge galaxies like our Milky Way. Matter around stellar-sized black holes may have originated from gas remaining after the explosion of the foregoing massive star.

Very close to the black hole, gas is assumed to be very dense. Friction can lead to an outward transport of angular momentum and therefore the formation of a disk. This disk can continuously feed the inner object with mass. Einstein's famous equation predicts that this mass is proportional to energy. During the "accretion of matter", this gravitational energy is released via radiation across the electromagnetic spectrum and strong outflows of matter. The efficiency at which radiation is produced exceeds nuclear fusion by a factor of more than 14 and we can certainly call it the most efficient process to produce energy across the Universe. On Earth, however, we have limited possibilities to squeeze the energy contained in pure mass. For example, we burn it (one of our biggest problems), throw it somewhere, try to extract the kinetic energy carried by winds, or let reactions do their thing.

Accretion onto compact objects is an ubiquitous process in the Universe at all mass scales. It is a process of gradual in-spiral and not the consequence of black holes sucking in their environment in free-fall. Galaxies, where their central black hole happens to accrete matter and thus releases heaps of energy, are called "Active Galaxies". Our Milky Way is not an Active Galaxy. Note that, on smaller scales, neutron stars can also attract matter from their surroundings. For them, this process can not last forever. After accumulating a certain mass, an explosion is triggered that may lead to the formation of a black hole. Black holes, however, can merge and accrete matter over cosmological time scales and reach extreme masses. Merging black holes (and neutron stars) were just recently found by detecting gravitational wave signals from these mergers with Earth itself as detector material.

My research focuses on a better understanding of compact objects as cosmic power plants. To do so, me and my colleagues try to picture the invisible. The size of a supermassive black hole inside a distant galaxy is comparable to the size of a bacterium on the moon, which is much smaller than the best telescope could resolve. We therefore use some tricks. We study all forms of the released energy to infer the energizing processes very close to the black hole. We observe the light being emitted from these systems from the lowest to the highest energies (from the radio to the gamma-rays). The energy dependency of the light encodes valuable information on the processes we try to understand. Matter that is thrown out of these systems (the "exhaust"), however, can be observed and imaged in many cases. The most extreme of these outflows are "jets". They emerge from the centers of Active Galaxies and the environment of stellar-sized black holes inside galaxies. Jets are remaining focused at extremely large distances. They accelerate matter to nearly the speed of light, which also produces radiation. We can for example observe their strong radio light with large radio antennas on Earth.