Lichtgedanken 04

S C HW E R P U N K T 42 Someday it is going to be perfectly dark In addition to space, time and matter, the Big Bang, which occurred some 13.8 billion years ago, created elec­ tromagnetic radiation and, in doing so, created visible light. This light could not be seen for the first 300,000 years, however. In this interview, physicists Holger Gies and Martin Ammon explain why the universe was initially opaque; what happens to light when it is absorbed by a black hole; and when the light will once again go out in the universe. Was everything dark before the Big Bang? Ammon: No-one knows what existed before the Big Bang and even if there was a »before«. According to the latest cosmological models, the Big Bang cre- ated the universe, the laws of nature and time. Prior to the Big Bang, neither time nor space existed. Since the Big Bang, the universe has been expanding to its current size in line with Einstein's general theory of relativity. Gies: Nowadays we assume, howev- er, that Einstein's theory alone cannot fully describe the process of the Big Bang. It represents the beginning of the universe as a so-called singularity: matter and space-time were focused in an infinitesimal point; the density of particles and radiation was infinitely large. Quantum physical effects are required to describe this constellation. At present we, along with physicists across the globe, are working to com- bine Einstein’s theory of relativity with quantum theory. A unified theory— that of quantum gravitation—would allow us to pose reasonable questions about the »before« and to seek answers. So, to put it another way: did the Big Bang ignite the light in the current universe? Ammon: Yes, the natural forces that we know today, including the electro- magnetic force and thus light, emerged just a blink after the Big Bang. Despite this, the universe remained opaque for a while. There were light particles up to around 300,000 years after the Big Bang, but the light was not able to ex- pand freely. Why not? Gies: Think of it as a glass of milk that you cannot see through. During this early phase, the matter lay in very hot plasma, in which the atomic nuclei and electrons were separate from one an- other. It was only later on, after around 300,000 years, that the atomic nuclei and free electrons came together to form the atoms and the light particles were no longer absorbed. It was only then that the universe became trans- parent. Ammon: And this does not just apply to the range of visible light; it also applies to all electromagnetic waves. The old- est sign of electromagnetic radiation that we have been able to receive—es- sentially the oldest light in the world— is microwave radiation from the cos- mic background. This comes from precisely this period, when the young universe was around 300,000 years old. Amongst other things, your research grapples with celestial bodies, which could be described as the natural ene­ mies of light: the black holes. Why do black holes absorb light? Ammon: A black hole is a massive, highly compact object, which deforms the surrounding space-time to such a degree that, according to Einstein’s theory of relativity, neither matter nor light can leave the object. Black holes are created at the end of the life cycle INTERVIEW: UTE SCHÖNFELDER CF E AT U R E