Once Before Time - Martin Bojowald [64]
Thanks to this weak reaction rate, neutrinos can reach us from much earlier times than light. While light could freely penetrate space only thousands of years after the big bang, neutrinos were allowed to do so after about just a second! With a neutrino telescope, one could thus watch much earlier times in the universe, but the weak interaction of neutrinos with matter is a double-edged blade: for this very reason, neutrinos are difficult to detect on Earth. A neutrino telescope allowing us to see a significant fraction of all neutrinos, and even to determine the direction of their origin, remains a fantasy with existing technologies.
As a second alternative, we have gravitational waves: small perturbations of space-time that propagate with the same speed as light. They are generated, for instance, in collisions of heavy masses such as neutron stars or black holes; masses influence space-time and its curvature. Some of the curvature can split off from the collision range and advance into outer space like light leaving a star. The passage of a gravitational wave would announce itself by tiny periodic changes in distances between objects, in accordance with gravitational waves being propagating disturbances of space-time. If one could measure distances very precisely, one would be able to detect gravitational waves. Such detectors are currently being constructed in several places: the LIGO observatories in Louisiana and Washington, Virgo in Italy, GEO600 in Germany, and TAMA in Japan.
Very large masses are necessary to generate sufficiently strong gravitational waves. A direct detection of typically expected signals on Earth would require a precision length measurement of just a thousandth of the proton radius. As impossible as this may sound, clever constructions are already within reach of this goal, and they are being refined steadily. A direct detection is indeed expected within the next few years.10 While this has not yet been achieved, there is little doubt as to the existence of gravitational waves. As described in the chapter on general relativity, the energy loss of double pulsars in close orbit can be explained precisely by an emission of gravitational waves as it follows from Einstein’s equations. This is one of the closest agreements between theory and observation in all of physics. But the way to a gravitational wave telescope will be long and arduous. There are plans for a satellite system, LISA, which could fulfill such a purpose, and also for an underground telescope called ET—the Einstein Telescope. Realizing these plans will have to wait several years, and for a direct view deep into the big bang, even these systems will not be sensitive enough. Still, the prospect of a new kind of astronomy, entirely independent of the conventional one based on electromagnetic radiation such as light or radio waves, is an impressive one.
In the cosmology of the early universe, the big bang’s high energies pose strict limits even to the propagation of neutrinos and gravitational waves. At such high energy densities as prevailed at those times, the universe is opaque even for those weakly interacting messengers. We are still denied a direct glance at the universe before the big bang, but an indirect investigation is not precluded. An earlier collapse phase, in combination with the repulsive forces of quantum gravity active during the bounce phase, should have weak implications for the ensuing expansion. Such indirect evidence may leave traces in the late phases of the big bang, which can be computed with theoretical models and possibly confirmed by comparison with observations.
Sensitive indirect effects require a detailed understanding of the theory and precise solutions of its equations. At present, even computer programs are not advanced enough to provide reliable values for the expected effects. But with further progress of the theory, combined with ever more precise measurements, indirect