Galaxy Filaments

When we look at a map of the galaxies, we can find that they are not randomly distributed across the universe. Instead, they form in large clusters, which are part of even larger superclusters, of which they are around 10 million. Nevertheless, due to careful analysis of mass distribution in galaxies, astrophysicists have determined that there are dark matter filaments that hold these clusters together. They are some of the largest structures out there. Let’s explore how they form and some of their properties.

A visualisation of a galactic filament. Credit: Rich Murray.

The filaments are said to have formed in the Big Bang, due to primordial density fluctuations due to fluctuations in the quantum field. What that means is that small differences in mass distribution a few microseconds after the formation of the universe, created these large superstructures over time through expansion. It’s kind of like the butterfly effect from chaos theory, where small changes in initial conditions result in huge implications after a while.

Structures larger than superclusters begun getting discovered in the 1980s. R. Brent Tully, working at the University of Hawaii Institute of Astronomy discovered the Pisces–Cetus Supercluster Complex, which houses 60 smaller clusters. Subsequent discoveries distinguished two types of filaments, flat horizontal ones and thin vertical ones. The latter are called galaxy walls and we have discovered a larger amount of them. One such wall, the Hercules–Corona Borealis Great Wall, is a largest structure in the observable universe. It was discovered in 2012 and named by a Filipino teenager on Wikipedia! It is so massive that there have been numerous papers that attempted to disprove it, nevertheless the authors keep successfully responding to the doubters.

A map of the nearest galaxy walls. Credit: Richard Powell.

Recently, it was discovered by a team of scientists working on a 3D model of galaxy filaments using data from the Sloan Digital Sky survey, that these superstructures spin. This is interesting, as the current model of the formation of the universe suggests that matter flowed from dense to less dense regions, however with no torque associated with it. Therefore, a spin on such a large scale means that something might be wrong with our idea of the Big Bang, as these structures had to get the angular momentum from somewhere. The authors make it clear that not all filaments spin, however thinking about the universe as a collection of spinning clusters definitely plays with the mind.

It is interesting to imagine our universe as having a hidden system of connections made of dark matter. I have always viewed it as a ballon or a void that just houses little randomly distributed specks of galaxies and stars. Nevertheless, it seems like there is a lot more than meets the eye. Recent research showed that quasars are a possible way of “illuminating” these dark matter filaments, therefore with better equipment we might be able to deduce an even bigger structure. Who knows, perhaps everything is connected with one giant filament?

References

Galactic Filaments | COSMOS. (n.d.). Cosmos. https://astronomy.swin.edu.au/cosmos/g/galactic+filaments

Redfern, S. (2014, January 23). Filaments that bind galaxies together illuminated by a quasar. The Conversation. https://theconversation.com/filaments-that-bind-galaxies-together-illuminated-by-a-quasar-22146

Wikipedia contributors. (2021a). Galaxy filament. Wikipedia. https://en.wikipedia.org/wiki/Galaxy_filament

Wikipedia contributors. (2021b). Hercules–Corona Borealis Great Wall. Wikipedia. https://en.wikipedia.org/wiki/Hercules%E2%80%93Corona_Borealis_Great_Wall

(2021, June 22). The Largest Rotating Objects in the Universe: Galactic Filaments Hundreds of Millions of Light-Years Long. Universe Today. https://www.universetoday.com/151553/the-largest-rotating-objects-in-the-universe-galactic-filaments-hundreds-of-millions-of-light-years-long/

Hidden Figures – The Story of Katherine Johnson

When we think of the Apollo program, most of us imagine the story of brave white male military test pilots, engineers and politicians. Nevertheless, the real heroes of the first Moon landing remain under-appreciated to this day. These women challenged the patriarchal structure of NASA. They faced racial discrimination and sexism on their way to performing crucial calculations for the Apollo 11 mission. Their bravery was portrayed in the 2016 film Hidden Figures. I would like to focus specifically on one of them, Katherine Johnson.

Katherine Johnson (1918-2020) working with her adding machine in 1962 at the Langley Research Center. Credit: NASA.

She was born in West Virginia, in 1918. Katherine was a brilliant student in school and skipped a few grades, enrolling in high school at the age of 13. She enrolled at the HBCU West Virginia State College at 18, and earned her degree there. As there were no opportunities for brilliant black students to pursue further education in graduate school, she went to teach in a public school after graduating with highest honors in 1937. Only in 1939, graduate schools were integrated in Virginia and she was chosen by the State’s president, along with two black men, to attend West Virginia University. She studied math, but after a year decided to abandon her studies in order to start a family with her husband.

After WW2, in 1952, she applied for a position in the computing unit of the Langley Aeronautical Laboratory. It was staffed solely by black women. Before the Apollo program, she mostly worked on wake turbulence and other engineering projects for the military. After the Soviets launched the Sputnik satellite, the focus at NASA shifted to the space race. She contributed to Glenn’s manned space flight in 1962, by working out orbital trajectories and co-authoring the report “Determination of Azimuth Angle at Burnout for Placing a Satellite Over a Selected Earth Position” in years prior.

John Glenn standing in front of the Friendship 7 on February 20th, 1962 preparing for his flight. Credit:.AP Photo.

By 1962, IBM computers were being used more and more across the facility, and “human computers” were beginning to become obsolete. Nevertheless, John Glenn told Johnson a few days before to check the trajectory calculations, which were input into the IBM 7090 computer. She worked at NASA until the 1980s, mostly developing the space shuttle, until her retirement. She then visited various schools to inspire children to pursue a STEM career.

The situation at NASA in the 1960s and 70s was appalling. The highest ranking woman at the agency, Ruth Bates Harris, was fired for submitting a report that criticised the equal opportunity program for being a complete failure. At the time, NASA employed less racial minorities and women than any other federal agency. Furthermore, they were often relegated to low-level bureaucratic jobs, compared to males with equal qualifications. Fortunately, things have began changing in the 1980s and the latter half of the 1970s, with changes to the government. Read more about it an article by Kim McQuaid. In 2019, the first all-female spacewalk was conducted, which shows that things definitely moved in the right direction and we should remain optimistic for the future.

Johnson passed away in February 2020, at the ripe age of 101. To this day, she serves as a symbol of pursuing a career in STEM despite stereotypes, racial discrimination and sexism. Let’s hope that in years to come, the male dominance in STEM will begin to fade even more, as diversity brings innovation and creativity.

References

Davenport, C. (2019, December 9). At Nasa, women are still facing outdated workplace sexism. The Independent. https://www.independent.co.uk/news/long_reads/women/nasa-women-workplace-sexism-spacex-space-station-a9230801.html

Deb Kiner, dkiner@pennlive.com. (2020, February 20). ‘A real fireball of a ride’: John Glenn became the first American to orbit Earth in 1962. Pennlive. https://www.pennlive.com/life/2020/02/a-real-fireball-of-a-ride-john-glenn-became-the-first-american-to-orbit-earth-in-1962.html

MCQUAID, K. (2007). Race, Gender, and Space Exploration: A Chapter in the Social History of the Space Age. Journal of American Studies, 41(2), 405–434. https://doi.org/10.1017/s0021875807003532

NASA. (2020, February). Katherine Johnson Biography. https://www.nasa.gov/content/katherine-johnson-biography/

Nature Editorial. (2020, March 12). Katherine Johnson (1918–2020). Nature. https://www.nature.com/articles/d41586-020-00749-3?error=cookies_not_supported&code=5a8e9bba-6790-46c9-92ee-b96f500b7cd0

st. Fleur, N. (n.d.). Katherine Johnson. History. https://kids.nationalgeographic.com/history/article/katherine-johnson

Surely You’re Joking, Mr. Feynman!

I love the works of Richard Feynman, and I especially love biographies of famous scientist. But this one is surely the best one I have read so far. It shows the human, funny and artistic side to one of the most prominent physicists of all time. There are so many things to love about this biography, so let me briefly outline the main things I learned.

Richard Feynman (1918-1988) was a leading American physicist educated at MIT and Princeton. He received the 1965 Nobel Prize for the development of quantum electrodynamics. He is known for his famous series of lectures and the popularisation of physics. Credit: Caltech.

The author discusses his rejection of the invitation to work at the Institute for Advanced Study in Princeton. Work in the institute allows scientists and mathematicians to forget about teaching and focus solely on research. Nonetheless, Feynman said that many of his ideas came from questions asked by students and from reorganising knowledge in his head every time he had to teach a course. I have certainly experienced this myself when explaining a concept to a friend while preparing for a test or just casually talking about physics. Getting yourself locked in an echo chamber with people at the same educational level and with a similar background as you can only lead to stagnation. Therefore, I’m quite excited to be a TA once in university, because the students might teach me something too.

Secondly, you need to balance science and art. Feynman discusses his adventures of playing bongo drums while delivering guest lectures in Brazil and becoming obsessed with samba music. Drums became his passion for the rest of his life and he sometimes gave performances. Furthermore, he also got interested in painting despite very limited skill. He actually got quite good, at least according to his account. Nevertheless, this goes to show that balance is tremendously important for the mind, as one of the greatest scientists in modern history played bongos in the evenings instead of reading the newest edition of Nature. Personally, I try to achieve balance by making beats and playing guitar. While indirect, I feel like it helps me organise ideas and just puts me in a more creative headspace.

Thirdly, experiment. Being able to apply textbook knowledge to unfamiliar situation sounds like a school cliche, but it is crucial to actually get a grasp on science. The author discusses his tinkering with radios and electronics as a teenager and Feynman diagrams are a product of wanting to achieve visual clarity. He played many pranks on his colleagues at Los Alamos when picking locks, applying combinatorics and an understanding of mechanisms. He illustrates the importance of understanding physics in context through another story about teaching in Brazil. One of his students could perfectly answer questions on refraction if they were presented with textbook terminology, i.e. a plate with a certain refractive index. Nonetheless, when told to show an example of such a plate, which of course could be glass or plastic, the students were clueless. This might just be a funny hyperbole, but still we should not memorise equations but actually visually and conceptually understand what is going on. Youtube is a great place for that.

This book is hilarious and informative, without at all trying to be the latter. This is a must read for any future physicist or for anyone wanting to read a good biography. It isn’t very much tied to astrophysics, but it’s too great to not be talked about on this blog.

References

Feynman, R. P., Leighton, R., Hutchings, E., & Hibbs, A. R. (1997). Surely You’re Joking, Mr. Feynman! (Adventures of a Curious Character) (Reprint ed.). W. W. Norton & Company.

Surely you’re joking, Mr Feynman by Richard Feynman – review. (2012, March 18). The Guardian. https://www.theguardian.com/childrens-books-site/2012/mar/18/review-surely-youre-joking-richard-feynman

Gravitational Lensing

The world of general relativity is a full of valleys, undulations and holes. Light, as any type of matter, gets affected by the curvature of spacetime and bends around objects with a large gravitational field. We see this effect when we look at the sky, some distant galaxies become distorted and unrecognisable. This also acts like a cosmic magnifying glass, allowing us to see galaxies too far away to be observed directly with current technology. Let’s explore some examples of gravitational lensing, its applications and Einstein rings.

A diagram of the mechanics of gravitational lensing. Credit: NASA, ESA.

But wait, aren’t photons supposed to be massless? From Newton’s law of gravitation, we multiply two masses, so photons should not be affected by a gravitational force. Nonetheless, we must remember Einstein’s general relativity, which states that spacetime is curved according to the mass and energy that exert stress on it. In that picture, photons do get bent when they travel along the grooves of space. This was tested during a solar eclipse in 1919, when the light from the star Regulus was determined to be bent. It was bent so significantly that the explanation had to be from general relativity and not simply photons having mass.

An explanation for the observation of light bending during the 1919 solar eclipse from the Eddington expedition to Brazil. Credit: The Illustrated London News.

An interesting phenomenon occurs when light is bent symmetrically around a center of mass and the observer is aligned with the source. The light bends in a circular shape known as an Einstein ring. The properties of an Einstein ring can be described mathematically. The size of the ring in radians is given by the equation

where DLS is the distance between the lens and the source, DL is the distance between the lens and the observer and DS is the distance from the observer to the source.

This is what you can see in images of black holes and when blue halos can be observed around galaxies and stars.

A diagram showing the variables in the equation for gravitational lensing. Credit: Amitchell125.

More interestingly, a double ring has been found, although these are exceptionally rare at 1 in 10000. These special rings provide tremendous insight into dark matter and energy distribution, as well as the curvature of space.

Analysing gravitational lensing also provides insight into the distribution of matter in galaxies. Sometimes, lensing occurs in region where there is little visible stars of gas clouds, which implies that there is dark matter present. Images of lensing taken by the Hubble Space Telescope allowed scientists to create maps of dark matter distribution in galaxy clusters.

A 3D gradient map of dark matter distribution in nearby galaxy clusters. X denotes the Milky Way. Credit: Hong et al.

I personally love gravitational lensing due to the beautiful halos of light it produces around objects. Check out this gallery of Einstein rings from the Hubble website. The blue auroras and the warm yellow starlight combined with the grainy quality make me feel strangely cozy. And spooked.

Selected Einstein ring images. Credit: NASA and ESA.

References

Gravitational Lensing. (2019, May 30). HubbleSite.Org. https://hubblesite.org/contents/articles/gravitational-lensing

Nave, R. (n.d.). Einstein Ring. HyperPhysics. http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/einring.html

Siegel, E. (2018, December 1). Ask Ethan: How Do Massless Particles Experience Gravity? Forbes. https://www.forbes.com/sites/startswithabang/2018/12/01/ask-ethan-how-do-massless-particles-experience-gravity/

Sutter, P. (2018, March 16). Nature’s Lens: How Gravity Can Bend Light Like a Telescope. Space.Com. https://www.space.com/39999-how-gravitational-lenses-work.html

Caroline Herschel

She was the first woman to discover a comet, receive a salary as a full-time scientist, hold a government office in England, publish in the Philosophical Transactions of the Royal Society, receive a Gold Medal of the Royal Astronomical Society, and be named a honorary member of the Royal Astronomical Society. A trailblazer and a brilliant Astronomer, Herschel made a name for herself recording the telescope observations of her brother. She also made a great deal of her own discoveries. Let’s look at her biography for inspiration, and to understand what it took to be appreciated as a woman in science a couple hundred years ago.

A portrait of Caroline Herschel from the 1800s. Credit: Science History Images/Alamy.

Caroline Herschel was born in the German city Hanover in 1750, to a family of 10. Her family was very musical, as her father was a self-taught oboist. When she was 10, she caught typhus, which stumped her growth and damaging her left eye. Her mother thought that she would never marry and that it would be best to train her as a servant, despite her father’s wishes to educate her. He tutored her in his free time during the absence of the mother. She was briefly allowed to learn dress-making from the neighbour, although this was often interrupted with loads of chores. She also was prevented from learning advanced French and needlework, to prevent her from becoming a governess. So, right off the bat she faced tremendous obstacles in her education.

After her father’s death, she decided to join her musical brother William in Bath, England as a soprano singer in 1772. She ran William’s household and in her free time finally was able to learn arithmetic, English, harpsichord and dance. She performed alongside William at many festivals, nonetheless she began getting replaced by other musicians as William became more focused on astronomy. She also supported his efforts in this field. When William began developing his own lenses to improve his astronomical observations, she fed and read to him. In 1781 William discovered Uranus, which brought him to fame, but Caroline remained largely unacknowledged. She claimed to have felt like his dog at the time.

Caroline and William polishing a mirror for a telescope. A lithograph from 1894. Credit: Wellcome Collection gallery.

When William was developing his high precision telescopes, she meticulously polished the mirrors. She learned how to organise his observations and catalogued them with speed and accuracy. Eventually, she got tired of being just an assistant and decided to start her own catalogue, where she recorded her observations. In February 1783, she made her first discovery of a nebula. She then discovered a variety of comets and other objects, With her brother, they discovered a total of 2400 astronomical objects.

Her catalogue of observations was published in The Philosophical Transactions of the Royal Society under William’s name, which listed 500 new Nebulae. She then expanded on the catalogue later in her life. After William’s death in 1822, she moved back to Hanover to continue her observations and catalogue, but this was difficult due to the architecture of the city. She was awarded the Gold medal of the Royal Society for her work in 1828. She introduced her nephew to astronomy, and he would continue her work and expanding the catalogue. Herschel remained active and healthy in her old age, dying peacefully in 1848.

Her story is of inspiring perseverance in fighting stereotypes and wanting to pursue an education, despite her mother’s attempts to make her into a house servant. She paved the way for many other women to pursue a career in STEM. We should remember her both for her discoveries, but also for the circumstances she conducted them in.

References

Caroline Herschel. (n.d.). Maths History. https://mathshistory.st-andrews.ac.uk/Biographies/Herschel_Caroline/

Caroline Herschel | Biography, Discoveries, & Facts. (n.d.). Encyclopedia Britannica. https://www.britannica.com/biography/Caroline-Lucretia-Herschel

Tillman, N. T. (2012, September 4). Caroline Herschel Biography. Space.Com. https://www.space.com/17439-caroline-herschel.html

Wikipedia contributors. (2021). Caroline Herschel. Wikipedia. https://en.wikipedia.org/wiki/Caroline_Herschel

Winterburn, E. (2015). Learned modesty and the first lady’s comet: a commentary on Caroline Herschel (1787) ‘An account of a new comet.’ Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 373(2039), 20140210. https://doi.org/10.1098/rsta.2014.0210

The Magnetic Field of the Earth

If you have ever seen the northern lights (aurora borealis), you have probably wondered where they originate from. It turns out, they are a stunning visual demonstration of the Earth’s magnetic field interacting with cosmic rays. The magnetic field is estimated to a have originated around 3.5 billion years ago based on analysis of Australian soil. They protect the Earth from solar wind, which allows all life to exist. Moreover, it is what makes compasses work. In fact, the south pole is actually the north side of the “bar magnet” of the magnetic field! Let’s find out where it originates from and how it works.

Northern lights over the Gulf of Finland. Credit: Shutterstock.

The main scientific consensus about how the field is generated is called the dynamo theory. It was first proposed by Joseph Larmor in 1919. For an object to become magnetised, the domains in it need to be aligned. A magnetic field is generated through the movement of charge. The Earth has a liquid core made out of iron, so its movement causes a magnetic field. However, it is so hot, reaching temperatures greater than the Curie point temperature of iron at 1043K, that the domains become disorganised. So, how come the Earth has a magnetic field? Scientists propose that it is maintained by a self-sustaining dynamo. The Coriolis effect due to the rotation of the planet creates an initial magnetic field. In turn, this field induces a current in the liquid core, which generates another magnetic field that supports the existing one. Researchers are now refining their computer models to show this theory holds in practice.

Interestingly enough, the field is not even across the planet’s surface. It is measured at around 30 microteslas in the region of South America and South Africa, and at around 60 microteslas in northern Canada and Siberia. Furthermore, there are daily variations due to the field’s interaction with Sun rays on the scale of 25 nanoteslas. Using magnetometers, it is possible to detect variations in the magnetic field due to the composition of the ocean floor. The volcanic rock basalt, which contains large amounts of the magnetic mineral magnetite, can distort the direction of the compass needle. This phenomenon was already observed by Icelandic sailors in the 18th century. It now offers an interesting way to map out the composition of the oceanic floor. Moreover, since this rock cools rapidly, it can create a historical record of how the magnetic field varied over time.

A computer simulation of the Earth’s magnetic field. Notice how messy it is compared to a bar magnet, due to the fluctuations in it. Credit: Dr. Gary A. Glatzmaier.

You might have heard about a prediction of the apocalypse where the magnetic poles of the Earth flip. Based on basalt samples, scientists have noticed that the magnetic field of the Earth reversed polarity at supposedly random intervals, ranging from millions to thousands of years, with an average period of 300,000 years. The last time this occurred was 781,000 years ago, so you might imagine we are due for one. Nonetheless, these are probably not as abrupt as portrayed in popular culture. The duration of such flip ranges from several thousand years to a human lifetime. Nonetheless, this is still a point of contention of researchers, as examination of the lava flow on the Steens mountain in Oregon suggest that the rate of change of the field could have been as rapid as 6 degrees per day.

What causes the magnetic field to reverse? We don’t have a clear answer for that either. Some computer simulations produce a field that is not exactly a stable dynamo as we said before, and it spontaneously switches due to the messy nature of the currents or perhaps the cooling of the core. Another theory is that large comet impacts trigger the reversal, nevertheless the age of large craters does not exactly correlate with the dates of these flips.

A visual representation of the switch in polarity of the Earth’s magnetic field. Credit: Steffen Wiers.

Looking at the Earth’s magnetic field and comparing it to the fields of other extraterrestrial bodies, is a fascinating way of studying them. Weiss, a professor of Planetary Sciences at MIT, suggests that determining the presence of a magnetic field is evidence for the presence of a metallic core. Furthermore, the history of the field on a planet, suggests about the climate change processes that happened on it. For instance, Mars is thought to have had a magnetic field in the past, but lost it at a similar time to the vanishing of the thicker atmosphere and the warm climate, which supported the presence of liquid water.

Magnetic field analysis of planets is a fascinating research field to go into if you are interested in developing detailed computer simulations and combining geology with astrophysics. I recommend checking out Researchgate and some journals if you want to find out more, as this is a constantly changing field with a lot of complexities that I don’t have space for here.

References

Brunhes–Matuyama reversal. (2021). Wikipedia. https://en.wikipedia.org/wiki/Brunhes%E2%80%93Matuyama_reversal

dynamo theory | geophysics. (n.d.). Encyclopedia Britannica. https://www.britannica.com/science/dynamo-theory

Earth’s magnetic field. (n.d.). Earth’s Magnetic Field. https://web.ua.es/docivis/magnet/earths_magnetic_field2.html

Explained: Dynamo theory. (2010, March 25). MIT News | Massachusetts Institute of Technology. https://news.mit.edu/2010/explained-dynamo-0325

Vejayan, V. V. R., & V.V. (2017, May 1). What creates Earth’s magnetic field? Cosmos Magazine. https://cosmosmagazine.com/earth/earth-sciences/what-creates-earths-magnetic-field/

Very Large Telescope

The Very Large Telescope is a beautiful facility in northern Chile. The ESO ran facility started operation in 1999. It consists of four optical telescopes that operate at infrared and visible wavelengths. The facility is extremely productive, with only the Hubble telescope producing more research papers during its operation. I just think it’s a dream place to work at. It looks like a rendering of a Mars colony.

This observatory is located on the Cerro Paranal mountain in the Atacama Desert. The four telescopes are named Antu, Kueyen, Melipal, and Yepun, since these are words for astronomical objects in the Mapuche language.

The results from the observatory lead to more than one research paper per day on average. For example, 600 papers were published in 2017 based on results from this lab. One of its coolest discoveries is first ever direct imaging of an extrasolar planet: Beta Pictoris b. This planet is located 63 light-years away from Earth in the constellation of Pictor. Like many other exoplanets, it is a super-Jupiter. This means that is has a mass and radius greater than that of Jupiter. It’s also pretty hot at 1,451 °C. The direct imagining technique is very promising for the future search for exoplanets.

Another great discovery is tracking stars around the supermassive black hole at the centre of the Milky Way. They have furthered used this data to test Einstein’s general relativity on the movement of a star near a black hole. This was the first experiment of its kind. They did it by measuring gravitational redshift. It’s an interesting consequence of the Doppler effect, when photons travelling out of a gravitational well lose energy. This loss of energy corresponds to an increase in wavelength, since E=hf. The greater the wavelength, the more redshifted the waves are.

A diagram representing gravitational redshift. Observe how the frequency of the light decreases as it gets further away from the potential well.

The ESO is building another telescope in Chile, which began construction in 2017. It is called the Extremely Large Telescope (a very original name). Some of its main goals are searching for more exoplanets. It will be able to look for more Earth size planets and study the atmospheres of large planets through direct imaging. It will also attempt to directly measure the rate of the acceleration of the universe’s expansion. This would be a major leap in understanding dark energy. Another crazy idea is figuring out if physical constants change with time. This would dramatically change physics as we know it, and also be a pain for physics students. I think investing in these observatories is very much worth it. What else can we desire as a civilisation other than knowledge about the space we inhabit?

An artist’s render of the ELT.

References

OpenStax. (n.d.). Physics. Lumen. https://courses.lumenlearning.com/physics/chapter/29-3-photon-energies-and-the-electromagnetic-spectrum/.

Wikimedia Foundation. (n.d.). Beta pictoris b. Wikipedia. https://en.wikipedia.org/wiki/Beta_Pictoris_b.

Wikimedia Foundation. (n.d.). Extremely large telescope. Wikipedia. https://en.wikipedia.org/wiki/Extremely_Large_Telescope#Science_goals.

Wikimedia Foundation. (n.d.). Very large telescope. Wikipedia. https://en.wikipedia.org/wiki/Very_Large_Telescope.

YouTube. (2017, September 22). Elementary introduction to gravitational redshift. YouTube. https://www.youtube.com/watch?v=6tbCk_4Tk10.

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Detecting Gravitational Waves

The detection of gravitational waves by the LIGO and Virgo groups announced in 2016 was one of the biggest experimental breakthroughs in recent physics. Not only was it direct evidence for a prediction made all the way back in 1916 by Einstein, but also was the first observation of a binary black hole merger. But first, what are gravitational waves and why do we care so much about them?

When massive objects that exert tremendous gravitational fields orbit each other, they send faint ripples through the fabric of spacetime. Gravitational waves are a way of exerting gravity on other objects. According to Newton’s laws of motion, the information that a gravitational force is being exerted on an object is sent instantaneously. However, from Einstein’s special relativity we know that no information can travel at the speed of light. Therefore he predicted that gravity acts on other object through waves, same as light from the Sun travels to Earth in the form of light waves. Gravity is a very weak force, on the relative scale of 10-39 compared to the strong force, therefore these undulations are faint and difficult to detect.

These waves exist on a spectrum, and their frequencies are determined by the mass and acceleration of the body. They are emitted when an object experiences acceleration, provided it is not symmetrical such as the expansion and contraction of a sphere.

The gravitational wave spectrum. Credit: NASA Goddard Space Flight Center

How does LIGO detect such faint signals? The basic principle is quite simple. Two light beams are split down 4km arms and reflected off mirrors. The presence of a gravitational wave alters the length of the arms, which would change the time taken for one beam to travel back and forward, desynchronising them. The beams travel in a vacuum to ensure that no gas particles disturb them, and there is painstaking maintenance and measurement work done to ensure perfection, as the detectors run months at a time collecting data points. The merger of the black holes referred to as GW150914 happened a billion light years away, so it changed the length of a LIGO arm that spans 4km by a thousand of a proton. Therefore, any noise due to weather, seismic events or equipment malfunctions is detrimental to the result.

The LIGO detector at Hanford. Credit: Caltech.

During the detection of the event, the experimental data was compared with the theoretical prediction to see if there was any correlation and if this was not simply noise. As you can see, the data fit very well, but despite that debate raged over this result. Nonetheless, several more detections of binary neutron star gravitational waves in 2017 and 2018 put an end to the debate.

The data from various LIGO observatories during the first detection of gravitational waves. Credit; LIGO collaboration.

Why should we care? Not only is this another piece of evidence for general relativity, but it also showed that smaller black holes can merge to form bigger ones. The LIGO experiment pushed the envelope for what is possible in large scientific collaborations. Similar to the discovery of the Higgs Boson, the detection of gravitational waves shows that sky is the limit in terms of building very precise instruments. Perhaps, this will pave the wave to detecting gravitons and developing experimental evidence for string theory. For now, we should be proud of what these scientists have accomplished.

References

Daw, E. (2016, February 11). Gravitational waves discovered: how did the experiment at LIGO actually work? The Conversation. https://theconversation.com/gravitational-waves-discovered-how-did-the-experiment-at-ligo-actually-work-54510

Stierwalt, S., PhD. (2016, February 15). 5 Reasons You Should Care About the Discovery of Gravitational Waves. Quick and Dirty Tips. https://www.quickanddirtytips.com/education/science/5-reasons-you-should-care-about-the-discovery-of-gravitational-waves

Tate, K. (2014, April 11). Hunting Gravitational Waves with Lasers: How Project LIGO Works (Infographic). Space.Com. https://www.space.com/25445-how-ligo-lasers-hunt-gravitational-waves-infographic.html

What are Gravitational Waves? (n.d.). LIGO Lab | Caltech. https://www.ligo.caltech.edu/page/what-are-gw

The Sun

Our Sun is the source of all useful energy on Earth. It is absolutely necessary for life. It’s a gigantic nuclear fusion reactor that uses hydrogen as fuel. It is the center of the Solar System, around which all the other planets rotate. But, how does it really work?

Structure

Most scientists divide the Sun into six layers. The innermost layer is the core, which is the hottest and densest part. It is the source of the fuel for nuclear reactions. It has a temperature of 15 million K and makes up about 20% of the star’s interior.

A diagram of the Sun’s layers.

The layer above the core is the radiative zone. It is named after the mode of energy transfer in this region, which is radiation. The energy is transported very slowly, since the photons do a ‘drunkard’s walk’. Matter is so dense in this region, that a photon does not travel very far before encountering a particle. It is like getting through a giant maze.

The outermost layer of the interior is the convective zone: energy is transported using convection. Convection is the process where hot material rises and cooler material falls. The plasma ‘bubbles’ to the surface, releasing energy to space and then sinking back down.

The first of the outer layers of the sun is the photosphere. It is the layer that appears opaque to us. It is not very dense, as it is only about 10% of Earth’s pressure at sea level. What’s interesting is that the surface exhibits a granulation pattern. They are 700-1000 km in diameter and last just a few minutes. By studying the Doppler shifts in the gas spectra, we can see that the bright granules are rapidly rising material. As the gas cools down, it falls down into the Sun leaving the dark spots between the granules.

A high quality image of the granules taken by the DKIST.

The second layer of the atmosphere is the chromosphere. It is about 2000-3000km thick. Its spectrum consists of strong emission lines in the red region. It gives this layer its reddish color and it is caused by hydrogen. This layer gave way to the discovery of helium in 1868. Scientists discovered. The spectrum had a distinct yellow line in it that didn’t correspond to a known element. They realised that it must be a new element, and they named it helium (after the Sun helios). It took until 1895 for the element to be discovered on Earth.

The outermost region of the Sun is called the corona. It was first observed during solar eclipses. It extends millions of kilometers above the photosphere. It thins out dramatically with altitude and it even extends to Earth’s orbit! Technically, we are living inside the Sun’s atmosphere, but it is so thin that we don’t really feel it. Before we get to the corona, there is a transition region, where temperature rises dramatically. It’s only a few kilometers thick but it corresponds with this tremendous temperature spike.

Graph of temperature in Kelvin vs. the altitude in the Sun’s atmosphere. Pay attention to the rapid temperature spike in the transition region.

Nuclear fusion

Nuclear fusion is the mode of energy production in the Sun. It fuses hydrogen nuclei into helium, releasing tremendous amounts of energy. The basic model for hydrogen fusion is this

Two of the protons change into neutrons. You might be wondering, where the energy comes from. This is because of the mass defect. It is the difference between the mass of the particle and the sum of its components. This mass gets directly converted to energy using Einstein’s famous E = mc2 . The speed of light is a huge number, so even with a tiny mass you can still have lots of energy released. Nuclear fusion seems like a great way to generate electricity on Earth, however it requires tremendous temperature and pressure. However, we are getting better at creating these extreme conditions. ITER is the world’s biggest fusion experiment and it is looking very promising. In my opinion, we can expect these reactors to be functional in the 2050s.

The Joint European Torus fusion experiment in 1991.

References

Calculate the average temperature needed for hydrogen fusion reaction. Physics Stack Exchange. (n.d.). https://physics.stackexchange.com/questions/107113/calculate-the-average-temperature-needed-for-hydrogen-fusion-reaction.

Mott, V. (n.d.). Introduction to chemistry. Lumen. https://courses.lumenlearning.com/introchem/chapter/nuclear-binding-energy-and-mass-defect/.

OpenStax. (n.d.). Astronomy. Lumen. https://courses.lumenlearning.com/astronomy/chapter/the-structure-and-composition-of-the-sun/.

The way to new energy. ITER. (n.d.). https://www.iter.org/.

Wikimedia Foundation. (n.d.). Granule (solar physics). Wikipedia. https://en.wikipedia.org/wiki/Granule_(solar_physics).

Wikimedia Foundation. (n.d.). Nuclear binding energy. Wikipedia. https://en.wikipedia.org/wiki/Nuclear_binding_energy.

Wikimedia Foundation. (n.d.). Sun. Wikipedia. https://en.wikipedia.org/wiki/Sun.

Zell, H. (2015, March 2). Layers of the sun. NASA. https://www.nasa.gov/mission_pages/iris/multimedia/layerzoo.html.

Dark Matter and Dark Energy

Dark matter and energy are fascinating concepts that capture the imagination of every astrophysicist. They have not yet been observed directly, but they impact a lot of the processes we take for granted. Without them, galaxies would fly apart and possibly would not have formed. They account for 95% of the total mass-energy composition of the universe, with the rest being ordinary matter, so it is vital to talk about them. While some scientist proposed alternative explanations such as modifications to general relativity, the majority of the community agree that there is enough observational evidence to conclude the existence of dark matter and energy.

The mass energy distribution of the universe. Notice the tiny fraction that visible matter consists of. Credit: NASA.

The first mentions of dark matter appear as early as 1884, when Lord Kelvin proposed that there must be dark bodies in the Milky Way, since the mass of the galaxy calculated using velocity dispersion of stars did not match up with the mass of visible stars. There were multiple other similar calculations that confirmed the disparity of the mass-light ratio. One of them was that the outside regions of galaxies were not supposed to be rotating as fast as they were if there was only ordinary matter. The name dark matter comes from the fact that this type of matter did not interact with light.

One of the major sources of evidence comes from gravitational lensing. This is when light rays coming from distant object get bent and distorted when passing over a gravitational well. Sometimes, rays get bet when passing over seemingly nothing, which indicates the presence of dark matter. This was observed in Abell 1689 and MACS J0416.1-2403 galaxy clusters. While a possible explanation would be the presence of brown dwarfs, neutron stars and supermassive black holes that are hard to detect, these would not account for enough of the missing matter.

What is it made of? It does not interact with baryonic matter other than through gravity. Therefore scientist hypothesise that it is a new elemental particle that has not been discovered yet. There are efforts at particle accelerators to detect it. A proposed particle is a sterile neutrino. The search for these weakly interacting massive particles is conducted on the Alpha Magnetic Spectrometer on the ISS and through the LUX detector.

Dark matter distribution simulation created using a supercomputer. Credit: Volker Springel.

In the 1920s and 30s, Edwin Hubble showed that the universe expands through the analysis of red shift in galaxies. Initially we thought that this rate is constant. Nonetheless the observations of distant supernovae in 1998 using the Hubble space telescope showed that the expansion was accelerating. This made no sense in the current model of physics, since we could reasonably expect that the rate would even slow down due to galaxies being gravitationally attracted to each other and eventually coming together in the center of mass of the universe. Therefore, there must have been a force acting against gravity and we have called it dark energy. While some scientist claim that this can be accounted for with Einstein’s cosmological constant from general relativity, others claim that it arises from a negative pressure field. We are yet to find out.

Dark matter and dark energy remain mysteries and are the main focus of cosmological research today. I believe we are closer to finding dark matter, nonetheless the search might require new technologies that we simply don’t have yet, or just a lot of luck. Studying these concepts is an exciting time in astrophysics, and hopefully we will have more than just computational answers soon.

References

Dark Matter and Dark Energy. (n.d.). Science. https://www.nationalgeographic.com/science/article/dark-matter

The Hubble Expansion. (n.d.). Berkeley Astronomy. https://w.astro.berkeley.edu/%7Emwhite/darkmatter/hubble.html

Magazine, S. (2010, April 1). Dark Energy: The Biggest Mystery in the Universe. Smithsonian Magazine. https://www.smithsonianmag.com/science-nature/dark-energy-the-biggest-mystery-in-the-universe-9482130/

Tillman, N. T. (2019, July 19). What Is Dark Matter? Space.Com. https://www.space.com/20930-dark-matter.html