“The first unequivocal indication of the inflation of the universe”
Interview with Max Planck Director Karsten Danzmann on the indirect observation of gravitational waves from the birth of our universe
It is very rare that a discovery excites media and scientists alike. This was the case with the Bicep2 experiment: the antenna is installed 2800 metres above sea level at the South Pole and receives microwave radiation which originates from the very birth of the universe. Researchers have found something like the fingerprints of gravitational waves in this cosmic baby photograph. They seem to confirm inflation theory, which states that the universe inflated abruptly immediately after the Big Bang 13.8 billion years ago from the size of an atom to that of a football; it is this inflation which gave the model its name. We discussed the new findings with Karsten Danzmann, Director at the Max Planck Institute for Gravitational Physics in Hanover.
Interview: Helmut Hornung
Mr. Danzmann, how do you assess the value of your American colleagues’ discovery on a scale from 0 to 10?
Karsten Danzmann: Definitely a 10! This is the first unequivocal experimental indication for an inflationary expansion immediately after the Big Bang.
Are there no other observed indications for the inflation model?
There are actually only indirect indications. The inflation was invented to provide this very explanation for the uniformity and flatness of the universe, and this is what it does, of course. Because it doesn’t matter what direction we look in, the universe at large always looks the same. Moreover, it seems that space does not display any curvature – as if inflation has smoothed it. And finally, inflation explains the presence of galaxy clusters, which emerged from density fluctuations. And these, in turn, were caused by quantum fluctuations which inflation abruptly increased to cosmic scales.
The universe was opaque immediately after it came into being, of course, because the dense primordial soup meant the photons were continually colliding with other particles and so did not get through. The radiation was only able to set off on its travels when the fog lifted some 400 000 years later. So everything we record today with our instruments originates from this later era. The gravitational waves observed were generated as early as the inflation period, however....
Indeed, the inflation took place long before the point in time when the universe became transparent. It is therefore quite remarkable that a signature of the inflation was found in the microwave background, nevertheless. This signature has apparently somehow survived the initial 400,000 years of the universe - stored in the primordial gravitational waves.
The Bicep2 researchers have observed the gravitational waves indirectly. This phenomenon was predicted 100 years ago by Albert Einstein - has no-one seen it yet?
Gravitational waves are always generated when masses move. They speed away with the velocity of light, causing space to compress and stretch. Direct detection is difficult and has not yet been successful. But our colleagues Russell Hulse and Joseph Taylor were awarded the Nobel Prize for the indirect proof in 1993. They were able to prove that the binary pulsar system PSR1913+16, which consists of two stars orbiting each other very rapidly, loses exactly as much energy as is expected by the emission of gravitational waves.
Does this mean that the new Bicep2 data are not really anything special?
Of course they are! These measurements go one step further. They make it clear that not only does the emission of gravitational waves take place as the General Theory of Relativity predicts, but also that the interaction with matter happens precisely as the theory assumes: the impression which the gravitational waves stamped into the cosmic microwave background radiation 13.8 billion years ago looks exactly as has been assumed.
What exactly do you mean by that?
The measurements record the polarisation – a quantity which indicates the degree to which waves oscillate in the same direction. Now the patterns observed in the polarisation appear sort of turbulent. They must therefore be caused by wave movements which caused space-time to tremble. And really, the only waves which could do this are gravitational waves. Incidentally, this makes us very confident that any traces which the gravitational waves leave behind today in our detectors will also follow the predictions of the theory of relativity.
Would you have any chance of detecting primordial gravitational waves with your GEO600 detector?
That depends very much on what exactly happened back then in the early universe. Directly, the Bicep2 data tell us only something about how strong primordial gravitational waves are at extremely low frequencies of 10-16 Hertz. If the standard model of inflation describes everything correctly, then waves from the Big Bang will be too weak for the current generation of detectors on Earth. And even the planned Lisa satellites stationed in space would not be able to detect them. But the birth of the universe was probably much more complicated than this. There are therefore a large number of possible processes which all generate much stronger signals at the higher frequencies for detectors on Earth and in space.
When do you expect the first direct proof of gravitational waves?
The first generation of ground-based Ligo and Virgo detectors are currently being modified to achieve a significantly higher sensitivity. Only our GEO600 installation in Ruthe near Hanover is still on the lookout for occasional events in our neighbourhood. When the detectors on Earth have reached their design sensitivity in around 2019, it would be very surprising if we did not detect quite a few events within a very short time.