Origin of Gravitational Waves
When Einstein was formulating the general theory of relativity, he considered this simple question: ‘If the Sun suddenly disappeared right now, when would we found out about it on Earth?’ To understand why this is an interesting problem, one must know one of the most important consequences of the Special Theory of Relativity – nothing can travel faster than light! This concept, follows from the Lorentz invariance that requires the laws of nature to be independent of orientations and velocity boosts of the laboratory in space in which experiments are carried out. This had to be present in the general theory of relativity.
The distance between the Earth and the Sun is about 150 million kilometres, which means that it takes about 8 minutes for the light to travel from the Sun to Earth. So, we know that if Sun suddenly disappeared, we would definitely know so in about 8 minutes because in that time it would get dark. But what about gravity? If we look at Newton’s theory of gravity, the answer is that if the Sun was removed suddenly from the Solar System we would know instantaneously because the ‘speed of gravity’ is infinite and in the absence of the Sun’s gravitational pull Earth would fly out into the freezing cold space and we would descend into the darkest and coldest ice age ever.
However, the gravitational force cannot be allowed to propagate faster than the speed of light in Einstein’s gravitational theory. Newton was wrong, so to correct this the theory of general relativity predicts that the gravitational force travels in waves that we call gravitational waves that travel at the speed of light. Therefore, if the Sun disappeared just as you started reading this article, you have exactly 8 minutes and 20 seconds to finish it.
What are Gravitational Waves?
In general relativity, one considers a (3+1)-dimensional space, that is 3 space dimensions and a time dimension, hence the name 4-dimensional spacetime. This formulation simplifies the mathematics of the physical theory. To qualitatively understand gravitational waves, one can imagine 4-dimensional spacetime to be like a surface of a swimming pool. If two people were to get into the pool and start and started walking around in a circle, like two supermassive black holes orbiting a common center of mass in spacetime, they would create ripples on the surface of the water analogous to the gravitational waves generated by the two black holes.
Anyone can generate gravitational waves by simply wave their arm in a circle, but their frequency would have extremely low intensity and impossible to detect with current technology. As mentioned before, gravitational waves travel at the speed of light, but they are quite different to light waves. One very unique and important property is that they propagate through matter undisturbed, therefore they contain undisrupted information about their source. This is not true for a light wave which can easily be scattered or absorbed. However, just like light waves, they can be redshifted, i.e. their apparent frequency can be less than their frequency at the source, either by the Doppler effect or gravity.
The Big Bang Theory and Cosmic Inflation
The Big Bang is the term used for the moment of creation of our Universe. There are number of theories that attempt to model the evolution of the universe after the Big Bang and one of the most popular is the cosmic inflation theory. This model assumes that after the Big Bang the universe expanded exponentially, i.e. at an extremely high speed. This rapid expansion would have generated the first gravitational waves. The detection of these primordial gravitational waves would be one form of supporting evidence for cosmic inflation. In 2014, Stanford scientists published their analysis of the Cosmic Microwave Background (CMB) gathered from their BICEP2 telescope.
CMB is a map of the radiation left over from the time when the universe cooled down to a level so that protons and electrons could combine to make hydrogen. By studying the tiny fluctuations in the intensity of this radiation, the scientists were able to find evidence for those primordial gravitational waves. This is because these waves left a distinct pattern on the cosmic microwave background due to their squeezing of space.
Direct detection of gravitational waves
After the promising results from Stanford, scientists are running and designing experiments to try to directly detect gravitational waves. One very promising project operated by Caltech and MIT is the Laser Interferometer Gravitational-wave Observatory (LIGO). LIGO has two L-shaped interferometers, with each arm measuring 4km in length. This allows for the detection of high frequency gravitational waves, from 30 Hz to 7000 Hz. Larger interferometers are impossible to build on the ground. The interferometers are separated by 3000 km. In September 2015, the two interferometers began their search for gravitational waves, after 5 years of renovation.
About two weeks ago, rumours sprung up on Twitter about LIGO’s successful detection of gravitational waves. This was a very exciting news for the scientific community and cosmology fans. The spokesperson for LIGO, Gabriela Gonzalez said that they stopped taking data now, after only four months of taking measurements. She has not confirmed any of the rumours. At the moment the data is reviewed and analysed. It may take a few months before the scientists fully analyse and understand the data, but an announcement can be expected sometime this year.
Future of Gravitational Wave Detection
To explore gravitational waves at low frequencies, from 0.03 mHz to 0.1 Hz, the National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA) joined forces to build the Laser Interferometer Space Antenna (LISA). This will be a giant triangular-shaped interferometer in outer space, with each triangle side 5 million km in length! It is expected that the gravitational waves in this range will be very intense and will allow a very detailed study of the systems that generate them. Some such systems can be the supermassive black hole at the center of our galaxy and some star binaries, i.e. two stars orbiting each other in an elliptical or quasi-elliptical orbit.
A New Tool to Study the Universe
The experimental discovery of gravitational waves will be another strong piece of evidence to support the general theory of relativity and will very likely be awarded the Nobel Prize in physics. Most importantly, it will provide unique insights into many astronomical phenomena. Examples include the formation and growth of massive black holes and galaxies, dynamics of galactic nuclei and the populations of stars in our galaxy. Furthermore, scientists will gain data to test general relativity in extreme conditions and perhaps discover new physics. This is why now is such an exciting time in cosmology. Technology has allowed science to uncover more secrets of the universe.
LIGO is doing pretty well so far and will surely yield many interesting results. Cosmologists are also looking forward with great anticipation for LISA to be built and start running to expand the frequency range and our knowledge further.
In conclusion, the discovery of gravitational waves is not the end, but only the beginning of learning more about the universe and finding new physics.
Illustration by Musbashir Sultan.