Gravitational waves in the cosmic microwave background
Last Monday an extremely important discovery was announced. The BICEP2 collaboration with its telescope at the Admundsen Scott station at the South Pole has discovered indirect signs of gravitational waves in the polarisation of the Cosmic Microwave Background (CMB). This is the second time that indirect indications have been found for the existence of gravitational waves. The first indirect indication came from the energy loss of a binary neutron star and Hulse and Taylor received the Nobel Prize for this in 1993. The new results are not yet published in a peer-reviewed journal.
Reaction from Nikhef scientists
"It is a fantastic achievement to detect the fingerprint of gravitational waves in the cosmic microwave background that originates from cosmological disruptions immediately after the Big Bang," says Prof.dr.ing. Jo van den Brand, leader of the 'Gravitational physics' programme of FOM and professor at the VU University Amsterdam.
BICEP2 has found a surprisingly large gravitational wave signal which points to the fact that the energy scale during inflation was extremely high. If this result is confirmed then theoretical physicists can really get their teeth stuck into this.
"The most spectacular discovery that LIGO and Virgo can make is finding gravitational waves from immediately after the Big Bang or another gravitational wave background such as that caused by cosmic strings," says Marieke Postma, theoretical physicist at Nikhef. LIGO and Virgo are interferometer experiments that could detect gravitational waves signals directly.
Fom Institute Nikhef participates in experiments that will soon detect gravitational waves directly
Prof.dr.ing. Jo van den Brand and his team from Nikhef are trying to detect gravitational waves directly (not indirectly via a polarisation effect in the electromagnetic CMB). For this, interferometers with arms several kilometres long are being used, including Virgo near Pisa in Italy that Nikhef is participating in and the LIGO detectors in the US. This year new equipment is being installed to improve the sensitivity of the instruments. Next year Virgo (and also the LIGO detectors in the US) will be switched on. The first joint measurement will take place in 2016.
Future gravitational wave experiments
Scientists are also working on a space mission for gravitational wave physics. On 28 November 2013, ESA decided to select eLISA as an L3 mission. eLISA consists of three satellites that will jointly form an interferometer in space with arms of one million kilometres in length. The launch is planned for 2034 and with this a space-based observatory for gravitational waves will be realised. In addition, studies are being made for the Einstein Telescope: a gigantic underground infrastructure for which six partially cryogenic interferometers with ten-kilometre-long arms will be placed several hundreds of metres underground so as to minimise the effect of seismic vibrations.
BICEP2 result
The BICEP2 collaboration discovered signs of gravitational waves in the polarisation of the cosmic microwave background. This CMB is the residual radiation from the Big Bang. During the Big Bang, the temperature of this radiation was immensely high, but due to the continuous expansion of the universe the temperature has now decreased to about 2.7 Kelvin. The crucial implication of the BICEP2 result is that it gives strong experimental indications that the process of inflation has played a role in the evolution of our universe. The inflation theory proposes that the ultra-young universe, at about 10-36 seconds after the Planck time experienced a brief period (about 10-32 s) of rapid expansion as result of which its size increased by about a factor of 1026. This theory was conceived by the US cosmologist Alan Guth with important contributions from various scientists, including the Russian American physicist Andrei Linde. The theory of inflation offers an explanation as to why the universe now seems so homogenous and isotropic at such a large scale.
The temperature distribution of the CMB has been accurately measured with the Planck satellite, for example, and only exhibits small fluctuations (about one part in one hundred thousand). According to cosmologists these small temperature fluctuations are the direct consequence of quantum fluctuations in the so-called inflaton field (quantum field that is responsible for inflation; quantum excitations of this field are called inflatons). These small temperature differences developed into stars, galaxies and clusters.
Besides the above-mentioned 'scalar' fluctuations there was also a second quantum field that fluctuated: the gravitational field. The so-called 'tensor' fluctuations in the structure of spacetime are called gravitational waves. These stretch and contract objects as the waves propagate at the speed of light. BICEP2 has succeeded in observing the effects of gravitational waves that arose during the Big Bang in the form of special patterns (so-called B-modes) in the polarisation of the CMB.
Contact
Vanessa Mexner, Science Communication Department Nikhef, +31 (0)20 592 50 75.
Prof.dr.ing. Jo van den Brand (Nikhef and VU University Amsterdam)
Dr. Marieke Postma (Nikhef)
Further information
Website Nikhef gravitational waves programme
Website Virgo
Website BICEP2