Ever since their discovery in February 2016, gravitational waves have been making huge ripples in the physics community. These waves are essentially ripples in the fabric of spacetime, caused by intense gravitational interactions. An example is the merging of two black holes, which is actually the event that generated the waves we detected back in February 2016. After the detection of that first black whole merger, three other occurrences have been identified and confirmed in the past two years. You might have also heard that the Nobel Prize in Physics was awarded to three scientists who were tremendously involved in developing the interferometersI to detect gravitational waves. What used to be uncharted territory, theorized by Einstein over a hundred years ago, is now being heavily explored.
Just a couple weeks ago, another bombshell dropped. After extensive calculations, LIGO and Virgo (two gravitational wave detectors) confirmed that the fifth and most recent blip in their readings, one that did not match the profile of a black whole merger, was actually from the cataclysmic collision of two neutron starsII. Why is this such a big deal, you ask? For starters, this is the first time gravitational waves have been detected from a source other than a black hole merger. By getting a glimpse at new interactions that have long been shrouded in mystery, we’re standing the very frontiers of current scientific knowledge! Second, this observation has given us great insight into two longstanding hypotheses in astrophysics. It has long been hypothesized that the collision of neutron stars is the cause of high-intensity gamma-ray bursts, an event that unleashes “more energy in a fraction of a second than the sun will pump out in ten billion years.” A mere 1.7 seconds after the gravitational waves were detected, NASA’s Fermi satellite observed gamma rays from the same general area in space, making the relationship between the two events crystal clear. For the following weeks, an arsenal of telescopes were aimed at the patch of the sky from which the gamma-ray burst was observed. By analyzing the spectral linesIII of the embers of the explosion, it was found that heavy radioactive elements were formed, which then decayed into a superabundance of precious metals like gold, silver and platinum. In fact, the gamma-ray burst “could have produced as much gold as the entire mass of the Earth,” upturning the previously popular theory that the majority of the universe’s gold is forged in supernovas. So when you exchange gold rings with your significant other during your wedding ceremony, remember that it was made in the glowing remnants of a catastrophic neutron star collision. Now isn't that romantic?
This gamma-ray burst, given the rather lovely name of GW170817, has given us answers to two prevailing mysteries in astrophysics, made possible only at the intersection of gravitational wave and electromagnetic astronomy.
Gravitational wave astronomy has also given us a richer understanding of the expansion of space. Traditionally, the expansion of the universe has been calculated by measuring the redshiftIV of cosmic objects moving away from us. However, to measure this expansion we need to know the distance between the object and the Earth. Now, this is much easier said than done, requiring extensive distance calibrations with the use of techniques such as ‘standard candle’V observation. However, gravitational waves make things a whole lot easier - the amplitude of the waves received at Earth contain a built-in indication of distance. The arriving waves from GW170817 signified that the event was 130 million lightyears away. Compounding this with redshift measurements allowed astronomers to calculate the Hubble constant, a measure of rate of expansion of the universe, to a greater degree of precision. Electromagnetic measurements calculated Hubble’s constant to be somewhere between 67 and 72 kilometers per second per megaparsec. Gravitational wave astronomers placed this constant right in the middle, at 70 kilometers per second per megaparsec. This value is in full agreement with our existing measurements, while being totally independent of them. This is amazing! Not only does this give us more confidence in our existing astronomic techniques, but also shows the power of gravitational waves - we attained a more precise measurement of the expansion of space through an inherently simpler method. This is the very epitome of scientific advancement.
Gravitational wave astronomy is shedding light (excuse the pun) on the most violent, mysterious high-energy phenomena that traditional electromagnetic techniques have not been able to fully comprehend. It’s giving us a new way to look at interactions in the universe, which in turn gives us new insight. Not only does it give us confirmation of existing observations, but also gives us surprising new knowledge of the universe: black holes are colliding with a greater frequency than we thought; neutron star collisions are the primary causes of gamma-ray bursts and formation of precious metals.
This is only the beginning though. Get wavy - we’re entering a new age in astronomy!
I. Interferometer: an instrument that measures the interference between two parallel beams of light - the difference in phase indicates how much space has been stretched by gravitational waves.
II. Neutron stars: these are extremely dense remnants of supernova explosions which are made up entirely of neutrons. The density of a neutron star is comparable to the entire mass of the Earth squeezed into a stack of 10 pennies.
III. Spectral lines: every element releases characteristic wavelengths of light. By analyzing the light from cosmic objects, we can identify what elements are present.
IV. Redshift: a process in which light gets stretched as a result of the source moving away from us. This is similar to how an ambulance siren sounds lower-pitched as it moves away from you.
V. Standard candles: these are certain cosmic objects that always emit light at a specific rate.
- Hendry, M. (2017). How we discovered gravitational waves from ‘neutron stars’ – and why it’s such a huge deal. [online] The News Minute. Available at: http://www.thenewsminute.com/ article/how-we-discovered-gravitational-waves-neutron-stars-and-why-it-s-such-huge- deal-70324 [Accessed 23 Oct. 2017].
- Drake, N. (2017). Strange Stars Caught Wrinkling Spacetime? Get the Facts. [online] Available at: https://news.nationalgeographic.com/2017/08/new-gravitational-waves-neutron-stars- ligo-space-science/ [Accessed 23 Oct. 2017].
- Scharping, N. (2017). Gravitational Waves Show How Fast The Universe is Expanding. [online] Available at: http://www.astronomy.com/news/2017/10/gravitational-waves-show-how-fast- the-universe-is-expanding [Accessed 23 Oct. 2017].
- Abbot, B. P., et al. (2017). A gravitational-wave standard siren measurement of the Hubble constant. Nature, 551 (7678), pp.85-88.