In 1980, physicist Alan Guth proposed the theory of inflation, the faster-than-light expansion of the universe in the moments after the Big Bang. In March, as the Harvard-Smithsonian Center for Astrophysics reported, “researchers from the BICEP2 collaboration…announced the first direct evidence for this cosmic inflation. Their data also represent the first images of gravitational waves, or ripples in space-time. These waves have been described as the ‘first tremors of the Big Bang.’ Finally, the data confirm a deep connection between quantum mechanics and general relativity.”
“These are exciting results that provide an image of the dynamics that occurred within the first tiny fraction of a second after our universe began,” says Associate Professor of Physics Steven Penn, who, as a member of the Laser Interferometer Gravitational Wave Observatory (LIGO) Scientific Collaboration, is working to detect and observe the universe using gravitational waves.
Albert Einstein predicted the existence of gravitational waves almost 100 years ago. What have been some of the difficulties in directly detecting them?
It is important to make the distinction between a direct and an indirect measurement of gravitational waves. In a direct measurement, the gravitational wave registers a signal on the observer’s detector, whereas in an indirect measurement, the observations indicate that gravitational waves were emitted, although the waves themselves are not detected. Even so, an indirect measurement can provide very strong evidence for gravitational waves. For example, Hulse and Taylor observed a pulsar in a binary star system and calculated the energy lost as the orbit slowly decayed. That energy loss exactly matched the predicted energy that the system should have emitted in gravitational waves. For their work, Hulse and Taylor won the Nobel Prize. Similarly the BICEP2 results are an indirect measurement that provide strong evidence for the gravitational interactions that occurred at the first fractions of a second after the Big Bang. Given the extreme and rapidly changing conditions at the start of the universe, “gravitational interactions” better encompass the range of dynamics rather than the standard definition of gravitational waves.
The difficulty faced by BICEP2 was measuring with very high-resolution the energy and polarization of the cosmic microwave background. They used state-of-the-art, low noise photodetectors that they had to operate near absolute zero while utilizing the clear, water-free skies above Antarctica. Without these low noise detectors, that were only invented with the advent of better semiconductor technology, BICEP2 would not have been able to measure this result.
The direct detection of gravitational waves is a whole different matter. Gravitational wave interferometers have to use state-of-the-art techniques in seismic isolation, extremely stable high-powered lasers, and very low noise, low mechanical loss mirrors. We are at the cutting edge, and often the lead innovators, in each of these areas and still we’re just at the edge of possibly making a detection. Gravitational waves are incredibly weak and hard to detect.
If indeed the BICEP2 telescope‘s results can be confirmed, what do they tell us about the Big Bang and the immediate aftermath?
The strength of the BICEP2 signal yields a prediction for the energy level at the time of inflation. These results indicate that inflation occurred at the energy level predicted for grand unification.
In addition to Gravity, particles are governed by the Strong, Weak, and Electromagnetic forces. The strength of these three forces depend on energy density (temperature). The Grand Unified Theory (GUT) predicts that at very high temperatures (1028 K) these forces are united as a single force, the GUT force. However, as the universe cooled these forces evolved and froze out into the forces we currently observe.
There has always been a question about what initiated the inflationary period. The BICEP2 result suggests that the splitting of the GUT force into the Strong and Electroweak forces may have initiated inflation.
What are the wider-reaching implications and consequences for astrophysics research in general? For understanding the history and future of our universe? For other universes?
Implications are more cosmological rather than astrophysical, meaning it tells us a lot more about the Big Bang and inflation rather than the evolution of universe after that time.
It may provide evidence and restrictions about quantum gravity and multiverse theory but we will have to wait and see.