Gravitational waves: the media has been clamoring about the recent discovery and importance of these phenomena, saying that their discovery is the breakthrough of the century. But what exactly are these waves, and why are they so important?
To better understand these waves, we have to first start with the Big Bang–– the start of our universe. First, the universe was a singularity, which had an infinite density and contained all the mass and spacetime of the universe. Then, suddenly, rapid expansion took place. The temperature and density were both very high at first, but the universe soon started to rapidly cool off while it expanded. But, the very first few moments of the Big Bang are more complicated though. What happened then?
The theory of inflation describes the exponential expansion of space of the early universe, from 10^-36 seconds after the Big Bang to around 10^-33 or 10^-32 seconds, during a period called the Inflationary Epoch. During exponential expansion, distances between two points increase faster than the speed of light. This does not violate Einstein's theory of special relativity, which states that nothing can travel faster than light, because inflation is the expansion of space itself, and no matter or information traveled faster than light. After this period, the universe started to expand less rapidly.
So why does the Theory of Inflation make sense? Well, this rapid exponential expansion explains why, for instance, the Cosmic Microwave Background Radiation (CMB), thermal radiation left over from the Big Bang, we detect is almost completely uniform (with very small fluctuations), as if it had all came from one tiny region. In a universe without exponential expansion, scientists predict that CMB would vary significantly in different regions. Thus, proving the theory of inflation is true would, among other things, explain the mystery of CMB Radiation.
Though this theory of inflation is widely accepted by many astrophysicists as a reasonable explanation for the earliest stages of the Big Bang, scientists need to find sufficient evidence to support this theory. This theory hasn’t had much evidence supporting it, but one discovery could lead to the confirmation of the theory: gravitational waves.
To better understand gravitational waves, we need to understand Einstein’s Theory of General Relativity, which predicts the existence of gravitational waves. First, Einstein’s Theory of General Relativity states that massive objects warp the spacetime around them. To make it easy to visualize this warping around very massive objects, we can take a sheet that is flat when there are no masses in it. This represents space-time. When there is a mass present, the sheet is warped from the mass’s gravitational field.
The warping of space changes the motion of objects near these masses. For example, the Earth orbits the Sun because of the Sun’s warping of space. Similarly, light can also be bent because of the warping.
The warping of time means that near a large mass, time runs “slower” relative to someone isn’t near that mass. This is called time dilation.
One way gravitational waves could be formed is if two masses orbit each other. As the masses orbited, there would be ripples in the grid. These ripples in spacetime would then propagate outwards as gravitational waves.
Most students learn in introductory physics courses that gravity is a force of attraction between masses. According to Einstein’s theory, however, gravity is not a force. Instead, it is a warping of space and time that acts similarly to a force.
These gravitational waves have many interesting properties. For example, Einstein predicted that they would transport energy in the form of gravitational radiation. Another interesting property that makes them easy to detect is that they can’t be blocked or scattered by objects; they pass right through them at the speed of light.
Now how does this answer the question of how to prove the theory of inflation? Well, during the Big Bang, there were many small gravitational and quantum fluctuations. On their own, these fluctuations would be too weak to detect now. However, with exponential expansion, they would have been amplified to become very powerful gravitational waves, potentially making them detectable. These waves in particular are called primordial gravitational waves. Therefore, if we manage to detect gravitational waves and we figure out that they came from the Big Bang, the theory of inflation will be proven.
After many rumors and false “discoveries”, it seems that we have finally confirmed the existence of gravitational waves. On September 14, 2015, LIGO (The Laser Interferometer Gravitational-Wave Observatory) facilities in Washington State and Louisiana both detected waves from a supermassive black hole collision that took place around 1.3 billion years ago. These black holes were each 1.3 billion light-years away, approximately 150 km in diameter, and 30 times the mass of our sun. This collision created extremely powerful gravitational waves. However, even though these waves were very powerful, the ripples were only the size of around 1/1000 of the size of a proton. And after months of analysis, on February 11, 2016, LIGO announced that the existence of gravitational waves had been confirmed.
So does this solve our problem about the theory of inflation? Unfortunately, no. The existence of gravitational waves does help a little with showing that the theory of inflation is true, but it is still not sufficient evidence. The gravitational waves detected from LIGO were from a supermassive black hole collision, not primordial gravitational waves from the Big Bang itself. This discovery is still very important, however, as it proved another aspect of General Relativity. It has also helped in our understanding of black holes and other objects that don’t emit light.
But if the discovery by LIGO doesn’t solve our Inflationary Epoch problem, then what will? LIGO was focusing on waves that formed from violent events such as collisions, but not on the Big Bang itself. A different facility, BICEP2, has been targeting waves from the Big Bang.
BICEP2, also known as the Background Imaging of Cosmic Extragalactic Polarization 2 experiment, tries to detect primordial gravitational waves directly from the Big Bang, which exist in the edges of the observable universe. BICEP2 detects gravitational waves in a more indirect manner. Shortly after the rapid inflationary period, the universe started to cool very quickly, which would have left some small temperature fluctuations in the CMB. BICEP2 analyzes this CMB and tries to find evidence of gravitational waves from these fluctuations. To analyze the CMB, they look at the polarization patterns of the CMB, and try to find B-Modes, which would have originated from the Inflationary Epoch.
So, has BICEP2 detected these primordial gravitational waves yet? Well, in 2014, BICEP2, reported the detection of these waves before the results were actually reviewed. After great congratulations and celebration, however, it turned out that what they detected actually just came from magnetically aligned space dust.
So no, BICEP2 hasn’t succeeded in detecting these primordial gravitational waves yet, but the experiment is still working on it. If they are discovered, however, the theory of inflation will be confirmed, helping us understand what the universe was really like at the Big Bang. Such a discovery could also help serve as a link between two conflicting branches of science: quantum physics (which the theory of inflation is part of) and classical physics (which gravitational waves is part of). The most recent LIGO confirmation of gravitational waves is a small but crucial step in confirming the Theory of Inflation.