A Song from the Universe
By Jenna Cammerino
We’ve all wished upon stars or maybe even shouted at the moon, but sometimes it just feels like the Universe never answers our calls. Well, as it turns out, the Universe is actually singing to us.
In late June this past summer, a set of papers was put out by NANOGrav consisting of over 15 years worth of data collection describing the very phenomenon. NANOGrav stands for the North American Nanohertz Observatory for Gravitational Waves and it refers to an international collective of scientists dedicated to studying low-frequency gravitational waves.
What are gravitational waves?
The term refers to a phenomenon that was first conceptualized with Einstein’s theory of general relativity, in which he suggests that our entire world is made up of three spatial dimensions and one temporal dimension; a four dimensional spacetime. This spacetime curves around large masses and it’s that curvature that gives way to orbital motion, and other kinds of gravitational interactions that we can observe.
So when we look at events involving large masses, such as supernovae explosions or binary black hole mergers, the rapid change in motion of these systems will cause fluctuations that propagate through spacetime, much like a sound wave will propagate through the air. It is these “ripples” that we refer to as gravitational waves.
It wouldn’t be until 2015, a hundred years after the publication of Einstein’s theory, that gravitational waves would first be detected by LIGO, the Laser Interferometer Gravitational-Wave Observatory.
How do we detect gravitational waves?
Much like anything in general relativity, the answer to that is with light. The speed of light is invariant; it shouldn’t change regardless of our frame of reference or our speed relative to it. But if spacetime itself were to be stretched or contracted, this would alter the path of light.
This is the philosophy behind laser interferometry, the primary technology utilized by LIGO. The interferometers consist of two, 4 km length arms positioned at a 90 degree angle from one another. A laser is split, sending beams of light along both directions, which are then reflected back by mirrors positioned at the ends of each arm. In theory, these beams should be detected at the same time. So if a discrepancy were to occur, that would be indicative of a gravitational wave passing through at the time of the procedure. To ensure that this is the case, LIGO compares data between both of its detectors, one of which is based in Livingston, Louisiana, and the other near Hanford, Washington.
What about NANOGrav?
The electromagnetic signals that NANOGrav aims to look at, are radio signals from pulsars.
Pulsars are highly magnetized, rapidly rotating, neutron stars. Due to the strength of these stars’ magnetic fields and relativistic speeds, beams of radiation are formed at their poles. As the neutron star rotates, these beams will periodically point directly towards Earth. When it does so, the star will appear to grow brighter in the sky. As the star continues to spin, it will appear to us as a sort of flickering or pulsing, hence the name.
Because of their periodic motion, pulsars are considered to be the Universe’s time clocks, and we can measure this periodicity by detecting radio signals as the beams pass by Earth. If a signal were to be received that were inconsistent with its period, it’d be a good signifier of gravitational waves.
The team at NANOGrav looked specifically at millisecond pulsars, which rotate at periods up to a few milliseconds. The significance of this is that these pulsars are highly sensitive to minute fluctuations in spacetime, thus being able to detect gravitational waves with very low frequencies. For the last 15 years, they used signals from 68 millisecond pulsars to observe these waveforms…
…and what did they find?
Their data is consistent with the notion of a Gravitational Wave Background.
Essentially, everywhere, at all times, the Universe is, ever so slightly, moving.
This terminology might sound familiar if you’ve heard of the Cosmic Microwave Background, which refers to microwave radiation that fills the Universe. Much like the CMB, the gravitational fluctuations of the GWB can be dated all the way back to the Big Bang. What’s even more exciting, is that, while the CMB has allowed us to look within around 380,000 years after the Big Bang, the GWB may be able to provide a look into the first seconds after the Big Bang.
This was right around the time that subatomic particles and nuclei were beginning to form. A further look into these signals may open all kinds of new doors in particle physics and help us to better understand the very building blocks of our being.
So while it may be easy to feel small and insignificant in this vast and endless space, take heed in knowing that the Universe is always waving to you.