Democritus, ancient Greek philosopher who is credited with the birth of atomic theory. Credit: phil-fak.uni-duesseldorf.de
As the philosopher Nietzsche famously said “He who would learn to fly one day must first learn to stand and walk and run and climb and dance; one cannot fly into flying.” This is certainly true when it comes to humanity’s understanding of the universe, something which has evolved over many thousands of years and been the subject of ongoing discovery.
And along the way, many names stand out as examples of people who achieved breakthroughs and helped lay the foundations of our modern understanding. One such person is Democritus, an ancient Greek philosopher who is viewed by many as being the “father of modern science”. This is due to his theory of universe that is made up of tiny “atoms”, which bears a striking resemblance to modern atomic theory.
Though he is typically viewed as one of Greece’s many pre-Socratic natural philosopher, many historians have argued that he is more rightly classified as a scientist, at least when compared to his contemporaries. There has also been significant controversy – particularly in Germany during the 19th century – over whether or not Democritus deserves credit for atomic theory.
This argument is based on the relationship Democritus had with contemporary philosopher Leucippus, who is renowned for sharing his theory about atoms with him. However, their theories came down to a different basis, a distinction that allows Democritus to be given credit for a theory that would go on to become a staple of the modern scientific tradition.
Democritus, by Hendrik ter Brugghen – Heraclitus, 1628. Credit: rijksmuseum.nl
Birth and Early Life:
The precise date and location of Democritus birth is the subject the debate. While most sources claim he was born in Abdera, located in the northern Greek province of Thrace, around 460 BCE. However, other sources claim he was born in Miletus, a coastal city of ancient Anatolia and modern-day Turkey, and that he was born in 490 BCE.
It has been said that Democritus’ father was from a noble family and so wealthy that he received the Persian king Xerxes on the latter’s march through Abdera during the Second Persian War (480–479 BC). It is further argued that as a reward for his service, the Persian monarch gave his father and other Abderites gifts, and left several Magi among them. Democritus was apparently instructed by these Magi in astronomy and theology.
After his father had died, Democritus used his inheritance to finance a series of travels to distant countries. Desiring to feed his thirst for knowledge, Democritus traveled extensively across the known world, traveling to Asia, Egypt and (according to some sources) venturing as far as India and Ethiopia. His writings include descriptions of the the cities of Babylon and Meroe (in modern-day Sudan).
Upon returning to his native land, he occupied himself with the study of natural philosophy. He also traveled throughout Greece to acquire a better knowledge of its cultures and learned from many of Greece’s famous philosophers. His wealth allowed him to purchase their writings, and he wrote of them in his own works. In time, he would become one of the most famous of the pre-Socratic philosophers.
The ruins of the ancient Greek city of Abdera, with the west gate shown. Credit: Wikipedia Commons/Marysas
Leucippus of Miletus had the greatest influence on him, becoming his mentor and sharing his theory of atomism with him. Democritus is also said to have known Anaxagoras, Hippocrates and even Socrates himself (though this remains unproven). During his time in Egypt, he learned from Egyptian mathematicians, and is said to have become acquainted with the Chaldean magi in Assyria.
In the tradition of the atomists, Democritus was a thoroughgoing materialists who viewed the world in terms of natural laws and causes. This differentiated him from other Greek philosophers like Plato and Aristotle, for whom philosophy was more teleological in nature – i.e. more concerned with the purpose of events rather than the causes, as well things like essence, the soul, and final causes.
According to the many descriptions and anecdotes about Democritus, he was known for his modesty, simplicity, and commitment to his studies. One story claims he blinded himself on purpose in order to be less distracted by worldly affairs (which is believed to be apocryphal). He was also known for his sense of humor and is commonly referred to as the “Laughing Philosopher” – for his capacity to laugh at human folly. To his fellow citizens, he was also known as “The Mocker”.
Scientific Contributions:
Democritus is renowned for being a pioneer of mathematics and geometry. He was among the first Greek philosophers to observe that a cone or pyramid has one-third the volume of a cylinder or prism with the same base and height. While none of his works on the subject survived the Middle Ages, his mathematical proofs are derived from other works with contain extensive citations to titles like On Numbers, On Geometrics, On Tangencies, On Mapping, and On Irrationals.
Right circular and oblique circular cones. Credit: Dominique Toussaint
Democritus is also known for having spent much of his life experimenting with and examining plants and minerals. Similar to his work in mathematics and geometry, citations from existing works are used to infer the existence of works on the subject. These include On the Nature of Man, the two-volume collection On Flesh, On Mind, On the Senses, On Flavors, On Colors, Causes concerned with Seeds and Plants and Fruits, and to the three-volume collection Causes concerned with Animals.
From his examination of nature, Democritus developed what could be considered some of the first anthropological theories. According to him, human beings lived short lives in archaic times, forced to forage like animals until fear of wild animals then drove them into communities. He theorized that such humans had no language, and only developed it through the need to articulate thoughts and ideas.
Through a process of trial and error, human beings developed not only verbal language, but also symbols with which to communicate (i.e. written language), clothing, fire, the domestication of animals, and agriculture. Each step in this process led to more discoveries, more complex behaviors, and the many things that came to characterize civilized society.
In terms of astronomy and cosmology, Democritus was a proponent of the spherical Earth hypothesis. He believed that in the original chaos from which the universe sprang, the universe was composed of nothing but tiny atoms that came together to form larger units (a theory which bears a striking resemblance to The Big Bang Theory and Nebular Theory). He also believed in the existence of many worlds, which were either in state of growth or decay.
In a similar vein, Democritus advanced a theory of void which challenged the paradoxes raised by his fellow Greek philosophers, Parmenides and Zeno – the founders of metaphysical logic. According to these men, movement cannot exist because such a thing requires there to be a void – which is nothing, and therefore cannot exist. And a void cannot be termed as such if it is in fact a definable, existing thing.
To this, Democritus and other atomists argued that since movement is an observable phenomena, there must be a void. This idea previewed Newton’s theory of absolute space, in which space exists independently of any observer or anything external to it. Einstein’s theory of relativity also provided a resolution to the paradoxes raised by Parmenides and Zeno, where he asserted that space itself is relative and cannot be separated from time.
Democritus’ thoughts on the nature of truth also previewed the development of the modern scientific method. According to Democritus, truth is difficult, because it can only be perceived through senses-impressions which are subjective. Because of this, Aristotle claimed in his Metaphysicsthat Democritus was of the opinion that “either there is no truth or to us at least it is not evident.”
However, as Diogenes Laertius quoted in his 3rd century CE tract, Lives and Opinions of Eminent Philosophers: “By convention hot, by convention cold, but in reality atoms and void, and also in reality we know nothing, since the truth is at bottom.”
Diogenes Laertius, Lives and Opinions of Eminent Philosophers, makes mention of Democritus and his theories. Credit: Public Domain
Ultimately, Democritus’ opinion on truth came down to a distinction between two kinds of knowledge – “legitimate” (or “genuine”) and bastard (or “secret”). The latter is concerned with perception through the senses, which is subjective by nature. This is due to the fact that our sense-perception are influence by the shape and nature of atoms as they flow out from the object in question and make an impression on our senses.
“Legitimate” knowledge, by contrast, is achieved through the intellect, where sense-data is elaborated through reasoning. In this way, one can get from “bastard” impressions to the point where things like connections, patterns and causality can be determined. This is consistent with the inductive reasoning method later elaborated by Renee Descartes, and is a prime example of why Democritus is considered to be an early scientific thinker.
Atomic Theory:
However, Democritus greatest contribution to modern science was arguably the atomic theory he elucidated. According to Democritus’ atomic theory, the universe and all matter obey the following principles:
Everything is composed of “atoms”, which are physically, but not geometrically, indivisible
Between atoms, there lies empty space
Atoms are indestructible
Atoms have always been, and always will be, in motion
There are an infinite number of atoms, and kinds of atoms, which differ in shape, and size.
He was not alone in proposing atomic theory, as both his mentor Leucippus and Epicurus are believed to have proposed the earliest views on the shapes and connectivity of atoms. Like Democritus, they believed that the solidity of a material corresponded to the shape of the atoms involved – i.e. iron atoms are hard, water atoms are smooth and slippery, fire atoms are light and sharp, and air atoms are light and whirling.
Democritus’ model of an atom was one of an inert solid that interacted mechanically with other atoms. Credit: .science.edu.sg
However, Democritus is credited with illustrating and popularizing the concept, and for his descriptions of atoms which survived classical antiquity to influence later philosophers. Using analogies from our sense experiences, Democritus gave a picture or an image of an atom that distinguished them from each other by their shape, size, and the arrangement of their parts.
In essence, this model was one of an inert solid that excluded other bodies from its volume, and which interacted with other atoms mechanically. As such, his model included physical links (i.e. hooks and eyes, balls and sockets) that explained how connections occurred between them. While this bears little resemblance to modern atomic theory (where atoms are not inert and interact electromagnetically), it is more closely aligned with that of modern science than any other theory of antiquity.
While there is no clear explanation as to how scholars of classical antiquity came to theorize the existence of atoms, the concept proved to be influential, being picked up by Roman philosopher Lucretius in the 1st century CE and again during the Scientific Revolution. In addition to being indispensable to modern molecular and atomic theory, it also provided an explanation as to why the concept of a void was necessary in nature.
If all matter was composed of tiny, indivisible atoms, then there must also be a great deal of open space between them. This reasoning has also gone on to inform out notions of cosmology and astronomy, where Einstein’s theory of special relativity was able to do away with the concept of a “luminiferous aether” in explaining the behavior of light.
Early atomic theory stated that different materials had differently shaped atoms. Credit: github.com
Diogenes Laertius summarized Democritus atomic theory as follows in Lives and Opinions of Eminent Philosophers:
“That atoms and the vacuum were the beginning of the universe; and that everything else existed only in opinion. That the worlds were infinite, created, and perishable. But that nothing was created out of nothing, and that nothing was destroyed so as to become nothing. That the atoms were infinite both in magnitude and number, and were borne about through the universe in endless revolutions. And that thus they produced all the combinations that exist; fire, water, air, and earth; for that all these things are only combinations of certain atoms; which combinations are incapable of being affected by external circumstances, and are unchangeable by reason of their solidity.”
Death and Legacy:
Democritus died at the age of ninety, which would place his death at around 370 BCE; though some writers disagree, with some claiming he lived to 104 or even 109. According to Marcus Aurelius’ book Meditations, Democritus was eaten by lice or vermin, although in the same passage he writes that “other lice killed Socrates”, implying that this was meant metaphorically. Since Socrates died at the hands of the Athenian government who condemned him, it is possible that Aurelius attributed Democritus death to human folly or politics.
While Democritus was highly esteemed amongst his contemporaries, there were also those who resented him. This included Plato who, according to some accounts, disliked him so much that he wished that all his books would be burned. However, Plato’s pupil Aristotle was familiar with the works of Democritus and mentioned him in both Metaphysics and Physics, where he described him as a “physicist” who did not concern himself with the ideals of form or essence.
Democritus meditating on the seat of the soul, by Léon-Alexandre Delhomme (1868). Credit: Pubic Domain
Ultimately, Democritus is credited as being one of the founders of the modern science because his methods and theories closely resemble those of modern astronomers and physicists. And while his version of the atomic model differs greatly from our modern conceptions, his work was of undoubted value, and was a step in an ongoing process that included such scientists as John Dalton, Neils Bohr and even Albert Einstein.
As always, science is an process of continuing discovery, where new breakthroughs are built upon the foundations of the old and every generations attempts to see a little farther by standing on the shoulders of those who came before.
In honor of Dr. Stephen Hawking, the COSMOS center will be creating the most detailed 3D mapping effort of the Universe to date. Credit: BBC, Illus.: T.Reyes
When we think of major figures in the history of science, many names come to mind. Einstein, Newton, Kepler, Galileo – all great theorists and thinkers who left an indelible mark during their lifetime. In many cases, the full extent of their contributions would not be appreciated until after their death. But those of us that are alive today are fortunate to have a great scientist among us who made considerable contributions – Dr. Stephen Hawking.
Considered by many to be the “modern Einstein”, Hawking’s work in cosmology and theoretical physics was unmatched among his contemporaries. In addition to his work on gravitational singularities and quantum mechanics, he was also responsible for discovering that black holes emit radiation. On top of that, Hawking was a cultural icon, endorsing countless causes, appearing on many television shows as himself, and penning several books that have made science accessible to a wider audience.
Early Life:
Hawking was born on January 8th, 1942 (the 300th anniversary of the death of Galileo) in Oxford, England. His parents, Frank and Isobel Hawking, were both students at Oxford University, where Frank studied medicine and Isobel studied philosophy, politics and economics. The couple originally lived in Highgate, a suburb of London, but moved to Oxford to get away from the bombings during World War II and give birth to their child in safety. The two would go on to have two daughters, Philippa and Mary, and one adopted son, Edward.
The family moved again in 1950, this time to St. Albans, Hertfordshire, because Stephen’s father became the head of parasitology at the National Institute for Medical Research (now part of the Francis Crick Institute). While there, the family gained the reputation for being highly intelligent, if somewhat eccentric. They lived frugally, living in a large, cluttered and poorly maintained house, driving around in a converted taxicab, and constantly reading (even at the dinner table).
Stephen Hawking as a young man. Credit: gazettereview.com
Education:
Hawking began his schooling at the Byron House School, where he experienced difficulty in learning to read (which he later blamed on the school’s “progressive methods”.) While in St. Albans, the eight-year-old Hawking attended St. Albans High School for Girls for a few months (which was permitted at the time for younger boys). In September of 1952, he was enrolled at Radlett School for a year, but would remain at St. Albans for the majority of his teen years due the family’s financial constraints.
While there, Hawking made many friends, with whom he played board games, manufactured fireworks, model airplanes and boats, and had long discussions with on subjects ranging from religion to extrasensory perception. From 1958, and with the help of the mathematics teacher Dikran Tahta, Hawking and his friends built a computer from clock parts, an old telephone switchboard and other recycled components.
Though he was not initially academically successfully, Hawking showed considerable aptitude for scientific subjects and was nicknamed “Einstein”. Inspired by his teacher Tahta, he decided to study mathematics at university. His father had hoped that his son would attend Oxford and study medicine, but since it was not possible to study math there at the time, Hawking chose to study physics and chemistry.
Stephen Hawking (holding the handkerchief) and the Oxford Boat Club. Credit: focusfeatures.com
In 1959, when he was just 17, Hawking took the Oxford entrance exam and was awarded a scholarship. For the first 18 months, he was bored and lonely, owing to the fact that he was younger than his peers and found the work “ridiculously easy”. During his second and third year, Hawking made greater attempts to bond with his peers and developed into a popular student, joining the Oxford Boat Club and developing an interest in classical music and science fiction.
When it came time for his final exam, Hawking’s performance was lackluster. Instead of answering all the questions, he chose to focus on theoretical physics questions and avoided any that required factual knowledge. The result was a score that put him on the borderline between first- and second-class honors. Needing a first-class honors for his planned graduate studies in cosmology at Cambridge, he was forced to take a via (oral exam).
Concerned that he was viewed as a lazy and difficult student, Hawking described his future plans as follows during the viva: “If you award me a First, I will go to Cambridge. If I receive a Second, I shall stay in Oxford, so I expect you will give me a First.” However, Hawking was held in higher regard than he believed, and received a first-class BA (Hons.) degree, thus allowing him to pursue graduate work at Cambridge University in October 1962.
Hawking on graduation day in 1962. Credit: telegraph.co.uk
Hawking experienced some initial difficulty during his first year of doctoral studies. He found his background in mathematics inadequate for work in general relativity and cosmology, and was assigned Dennis William Sciama (one of the founders of modern cosmology) as his supervisor, rather than noted astronomer Fred Hoyle (whom he had been hoping for).
In addition, it was during his graduate studies that Hawking was diagnosed with early-onset amyotrophic lateral sclerosis (ALS). During his final year at Oxford, he had experienced an accident where he fell down a flight of stairs, and also began experiencing difficulties when rowing and incidents of slurred speech. When the diagnosis came in 1963, he fell into a state of depression and felt there was little point in continuing his studies.
However, his outlook soon changed, as the disease progressed more slowly than the doctors had predicted – initially, he was given two years to live. Then, with the encouragement of Sciama, he returned to his work, and quickly gained a reputation for brilliance and brashness. This was demonstrated when he publicly challenged the work of noted astronomer Fred Hoyle, who was famous for rejecting the Big Bang theory, at a lecture in June of 1964.
Stephen Hawking and Jane Wilde on their wedding day, July 14, 1966. Credit: telegraph.co.uk
When Hawking began his graduate studies, there was much debate in the physics community about the prevailing theories of the creation of the universe: the Big Bang and the Steady State theories. In the former, the universe was conceived in a gigantic explosion, in which all matter in the known universe was created. In the latter, new matter is constantly created as the universe expands. Hawking quickly joined the debate.
Hawking became inspired by Roger Penrose’s theorem that a spacetime singularity – a point where the quantities used to measure the gravitational field of a celestial body become infinite – exists at the center of a black hole. Hawking applied the same thinking to the entire universe, and wrote his 1965 thesis on the topic. He went on to receive a research fellowship at Gonville and Caius College and obtained his PhD degree in cosmology in 1966.
It was also during this time that Hawking met his first wife, Jane Wilde. Though he had met her shortly before his diagnosis with ALS, their relationship continued to grow as he returned to complete his studies. The two became engaged in October of 1964 and were married on July 14th, 1966. Hawking would later say that his relationship with Wilde gave him “something to live for”.
Scientific Achievements:
In his doctoral thesis, which he wrote in collaboration with Penrose, Hawking extended the existence of singularities to the notion that the universe might have started as a singularity. Their joint essay – entitled, “Singularities and the Geometry of Space-Time” – was the runner-up in the 1968 Gravity Research Foundation competition and shared top honors with one by Penrose to win Cambridge’s most prestigious Adams Prize for that year.
In 1970, Hawking became part of the Sherman Fairchild Distinguished Scholars visiting professorship program, which allowed him to lecture at the California Institute of Technology (Caltech). It was during this time that he and Penrose published a proof that incorporated the theories of General Relativity and the physical cosmology developed by Alexander Freidmann.
Based on Einstein’s equations, Freidmann asserted that the universe was dynamic and changed in size over time. He also asserted that space-time had geometry, which is determined by its overall mass/energy density. If equal to the critical density, the universe has zero curvature (i.e. flat configuration); if it is less than critical, the universe has negative curvature (open configuration); and if greater than critical, the universe has a positive curvature (closed configuration)
According to the Hawking-Penrose singularity theorem, if the universe truly obeyed the models of general relativity, then it must have begun as a singularity. This essentially meant that, prior to the Big Bang, the entire universe existed as a point of infinite density that contained all of the mass and space-time of the universe, before quantum fluctuations caused it to rapidly expand.
Per the Friedmann equations, the geometry of the universe is determined by its overall mass/energy density, and can have either flat, negative, or positive curvature. Credit: NASA/GSFC
Also in 1970, Hawking postulated what became known as the second law of black hole dynamics. With James M. Bardeen and Brandon Carter, he proposed the four laws of black hole mechanics, drawing an analogy with the four laws of thermodynamics.
These four laws stated that – for a stationary black hole, the horizon has constant surface gravity; for perturbations of stationary black holes, the change of energy is related to change of area, angular momentum, and electric charge; the horizon area is, assuming the weak energy condition, a non-decreasing function of time; and that it is not possible to form a black hole with vanishing surface gravity.
In 1971, Hawking released an essay titled “Black Holes in General Relativity” in which he conjectured that the surface area of black holes can never decrease, and therefore certain limits can be placed on the amount of energy they emit. This essay won Hawking the Gravity Research Foundation Award in January of that year.
In 1973, Hawking’s first book, which he wrote during his post-doc studies with George Ellis, was published. Titled, The Large Scale Structure of Space-Time, the book describes the foundation of space itself and the nature of its infinite expansion, using differential geometry to examine the consequences of Einstein’s General Theory of Relativity.
Hawking was elected a Fellow of the Royal Society (FRS) in 1974, a few weeks after the announcement of Hawking radiation (see below). In 1975, he returned to Cambridge and was given a new position as Reader, which is reserved for senior academics with a distinguished international reputation in research or scholarship.
The mid-to-late 1970s was a time of growing interest in black holes, as well as the researchers associated with them. As such, Hawking’s public profile began to grow and he received increased academic and public recognition, appearing in print and television interviews and receiving numerous honorary positions and awards.
In the late 1970s, Hawking was elected Lucasian Professor of Mathematics at the University of Cambridge, an honorary position created in 1663 which is considered one of the most prestigious academic posts in the world. Prior to Hawking, its former holders included such scientific greats as Sir Isaac Newton, Joseph Larmor, Charles Babbage, George Stokes, and Paul Dirac.
His inaugural lecture as Lucasian Professor of Mathematics was titled: “Is the end in sight for Theoretical Physics”. During the speech, he proposed N=8 Supergravity – a quantum field theory which involves gravity in 8 supersymmetries – as the leading theory to solve many of the outstanding problems physicists were studying.
Hawking’s promotion coincided with a health crisis which led to Hawking being forced to accept some nursing services at home. At the same time, he began making a transition in his approach to physics, becoming more intuitive and speculative rather than insisting on mathematical proofs. By 1981, this saw Hawking begin to focus his attention on cosmological inflation theory and the origins of the universe.
Inflation theory – which had been proposed by Alan Guth that same year – posits that following the Big Bang, the universe initially expanded very rapidly before settling into to a slower rate of expansion. In response, Hawking presented work at the Vatican conference that year, where he suggested that their might be no boundary or beginning to the universe.
During the summer of 1982, he and his colleague Gary Gibbons organized a three-week workshop on the subject titled “The Very Early Universe” at Cambridge University. With Jim Hartle, an American physicist and professor of physics at the University of California, he proposed that during the earliest period of the universe (aka. the Planck epoch) the universe had no boundary in space time.
In 1983, they published this model, known as the Hartle-Hawking state. Among other things, it asserted that before the Big Bang, time did not exist, and the concept of the beginning of the universe is therefore meaningless. It also replaced the initial singularity of the Big Bang with a region akin to the North Pole which (similar to the real North Pole) one cannot travel north of because it is a point where lines meet that has no boundary.
This proposal predicted a closed universe, which had many existential implications, particularly about the existence of God. At no point did Hawking rule out the existence of God, choosing to use God in a metaphorical sense when explaining the mysteries of the universe. However, he would often suggest that the existence of God was unnecessary to explain the origin of the universe, or the existence of a unified field theory.
In 1982, he also began work on a book that would explain the nature of the universe, relativity and quantum mechanics in a way that would be accessible to the general public. This led him to sign a contract with Bantam Books for the sake of publishing A Brief History of Time, the first draft of which he completed in 1984.
After multiple revisions, the final draft was published in 1988, and was met with much critical acclaim. The book was translated into multiple languages, remained at the top of bestseller lists in both the US and UK for months, and ultimately sold an estimated 9 million copies. Media attention was intense, and Newsweek magazine cover and a television special both described him as “Master of the Universe”.
Further work by Hawking in the area of arrows of time led to the 1985 publication of a paper theorizing that if the no-boundary proposition were correct, then when the universe stopped expanding and eventually collapsed, time would run backwards. He would later withdraw this concept after independent calculations disputed it, but the theory did provide valuable insight into the possible connections between time and cosmic expansion.
During the 1990’s, Hawking continued to publish and lecture on his theories regarding physics, black holes and the Big Bang. In 1993, he co-edited a book with Gary Gibbons on on Euclidean quantum gravity, a theory they had been working on together in the late 70s. According to this theory, a section of a gravitational field in a black hole can be evaluated using a functional integral approach, such that it can avoid the singularities.
It was also in 1990s that major developments happened in Hawking’s personal life. In 1990, he and Jane Hawking commenced divorce proceedings after many years of strained relations, owing to his disability, the constant presence of care-givers, and his celebrity status. Hawking remarried in 1995 to Elaine Mason, his caregiver of many years.
Stephen Hawking lectured regularly throughout the 90s, many of which were collected and published in “The Nature of Space and Time” in 1996. Credit: educatinghumanity.com
In the 2000s, Hawking produced many new books and new editions of older ones. These included The Universe in a Nutshell (2001), A Briefer History of Time (2005), and God Created the Integers (2006). He also began collaborating with Jim Hartle of the University of California, Santa Barbara, and the European Organization for Nuclear Research (CERN) to produce new cosmological theories.
Foremost of these was Hawking’s “top-down cosmology”, which states that the universe had not one unique initial state but many different ones, and that predicting the universe’s current state from a single initial state is therefore inappropriate. Consistent with quantum mechanics, top-down cosmology posits that the present “selects” the past from a superposition of many possible histories.
In so doing, the theory also offered a possible resolution of the “fine-tuning question”, which addresses the possibility that life can only exist when certain physical constraints lie within a narrow range. By offering this new model of cosmology, Hawking opened up the possibility that life may not be bound by such restrictions and could be much more plentiful than previously thought.
In 2006, Hawking and his second wife, Elaine Mason, quietly divorced, and Hawking resumed closer relationships with his first wife Jane, his children (Robert, Lucy and Timothy), and grandchildren. In 2009, he retired as Lucasian Professor of Mathematics, which was required by Cambridge University regulations. Hawking has continued to work as director of research at the Cambridge University Department of Applied Mathematics and Theoretical Physics ever since, and has made no indication of retiring.
“Hawking Radiation” and the “Black Hole Information Paradox”:
In the early 1970s, Hawking’s began working on what is known as the “no-hair theorem”. Based on the Einstein-Maxwell equations of gravitation and electromagnetism in general relativity, the theorem stated that all black holes can be completely characterized by only three externally observable classical parameters: mass, electric charge, and angular momentum.
In this scenario, all other information about the matter which formed a black hole or is falling into it (for which “hair’ is used as a metaphor), “disappears” behind the black-hole event horizon, and is therefore preserved but permanently inaccessible to external observers.
In 1973, Hawking traveled to Moscow and met with Soviet scientists Yakov Borisovich Zel’dovich and Alexei Starobinsky. During his discussions with them about their work, they showed him how the uncertainty principle demonstrated that black holes should emit particles. This contradicted Hawking’ second law of black hole thermodynamics (i.e. black holes can’t get smaller) since it meant that by losing energy they must be losing mass.
What’s more, it supported a theory advanced by Jacob Bekenstein, a graduate student of John Wheeler University, that black holes should have a finite, non-zero temperature and entropy. All of this contradicted the “no-hair theorem” about black boles. Hawking revised this theorem shortly thereafter, showing that when quantum mechanical effects are taken into account, one finds that black holes emit thermal radiation at a temperature.
From 1974 onward, Hawking presented Bekenstein’s results, which showed that black holes emit radiation. This came to be known as “Hawking radiation”, and was initially controversial. However, by the late 1970s and following the publication of further research, the discovery was widely accepted as a significant breakthrough in theoretical physics.
However, one of the outgrowths of this theory was the likelihood that black holes gradually lose mass and energy. Because of this, black holes that lose more mass than they gain through other means are expected to shrink and ultimately vanish – a phenomena which is known as black hole “evaporation”.
In 1981, Hawking proposed that information in a black hole is irretrievably lost when a black hole evaporates, which came to be known as the “Black Hole Information Paradox”. This states that physical information could permanently disappear in a black hole, allowing many physical states to devolve into the same state.
This was controversial because it violated two fundamental tenets of quantum physics. In principle, quantum physics tells us that complete information about a physical system – i.e. the state of its matter (mass, position, spin, temperature, etc.) – is encoded in its wave function up to the point when that wave function collapses. This in turn gives rise to two other principles.
The first is Quantum Determinism, which states that – given a present wave function – future changes are uniquely determined by the evolution operator. The second is Reversibility, which states that the evolution operator has an inverse, meaning that the past wave functions are similarly unique. The combination of these means that the information about the quantum state of matter must always be preserved.
By proposing that this information disappears once a black evaporates, Hawking essentially created a fundamental paradox. If a black hole can evaporate, which causes all the information about a quantum wave function to disappear, than information can in fact be lost forever. This has been the subject of ongoing debate among scientists, one which has remained largely unresolved.
However, by 2003, the growing consensus among physicists was that Hawking was wrong about the loss of information in a black hole. In a 2004 lecture in Dublin, he conceded his bet with fellow John Preskill of Caltech (which he made in 1997), but described his own, somewhat controversial solution to the paradox problem – that black holes may have more than one topology.
In the 2005 paper he published on the subject – “Information Loss in Black Holes” – he argued that the information paradox was explained by examining all the alternative histories of universes, with the information loss in those with black holes being cancelled out by those without. As of January 2014, Hawking has described the Black Hole Information Paradox as his “biggest blunder”.
Other Accomplishments:
In addition to advancing our understanding of black holes and cosmology through the application of general relativity and quantum mechanics, Stephen Hawking has also been pivotal in bringing science to a wider audience. Over the course of his career, he has published many popular books, traveled and lectured extensively, and has made numerous appearances and done voice-over work for television shows, movies and even provided narration for the Pink Floyd song, “Keep Talking”.
Stephen Hawking’s theories on black holes became the subject of television specials, such as “Stephen Hawking’s Universe” on PBS. Credit: discovery.com
A film version of A Brief History of Time, directed by Errol Morris and produced by Steven Spielberg, premiered in 1992. Hawking had wanted the film to be scientific rather than biographical, but he was persuaded otherwise. In 1997, a six-part television series Stephen Hawking’s Universe premiered on PBS, with a companion book also being released.
In 2007, Hawking and his daughter Lucy published George’s Secret Key to the Universe, a children’s book designed to explain theoretical physics in an accessible fashion and featuring characters similar to those in the Hawking family. The book was followed by three sequels – George’s Cosmic Treasure Hunt (2009), George and the Big Bang (2011), George and the Unbreakable Code (2014).
Since the 1990s, Hawking has also been a major role model for people dealing with disabilities and degenerative illnesses, and his outreach for disability awareness and research has been unparalleled. At the turn of the century, he and eleven other luminaries joined with Rehabilitation International to sign the Charter for the Third Millennium on Disability, which called on governments around the world to prevent disabilities and protect disability rights.
Professor Stephen Hawking participating in a zero-gravity flight (aka. the “Vomit Comet”) in 2007. Credit: gozerog.com
Motivated by the desire to increase public interest in spaceflight and to show the potential of people with disabilities, in 2007 he participated in zero-gravity flight in a “Vomit Comet” – a specially fitted aircraft that dips and climbs through the air to simulate the feeling of weightlessness – courtesy of Zero Gravity Corporation, during which he experienced weightlessness eight times.
In August 2012, Hawking narrated the “Enlightenment” segment of the 2012 Summer Paralympics opening ceremony. In September of 2013, he expressed support for the legalization of assisted suicide for the terminally ill. In August of 2014, Hawking accepted the Ice Bucket Challenge to promote ALS/MND awareness and raise contributions for research. As he had pneumonia in 2013, he was advised not to have ice poured over him, but his children volunteered to accept the challenge on his behalf.
During his career, Hawking has also been a committed educator, having personally supervised 39 successful PhD students.He has also lent his name to the ongoing search for extra-terrestrial intelligence and the debate regarding the development of robots and artificial intelligence. On July 20th, 2015, Stephen Hawking helped launch Breakthrough Initiatives, an effort to search for extraterrestrial life in the universe.
Also in 2015, Hawking lent his voice and celebrity status to the promotion of The Global Goals, a series of 17 goals adopted by the United Nations Sustainable Development Summit to end extreme poverty, social inequality, and fixing climate change over the course of the next 15 years.
President Barack Obama talks with Stephen Hawking in the Blue Room of the White House before a ceremony presenting him and 15 others the Presidential Medal of Freedom, August 12th, 2009. Credit: Pete Souza/White House photo stream
Honors and Legacy:
As already noted, in 1974, Hawking was elected a Fellow of the Royal Society (FRS), and was one of the youngest scientists to become a Fellow. At that time, his nomination read:
Hawking has made major contributions to the field of general relativity. These derive from a deep understanding of what is relevant to physics and astronomy, and especially from a mastery of wholly new mathematical techniques. Following the pioneering work of Penrose he established, partly alone and partly in collaboration with Penrose, a series of successively stronger theorems establishing the fundamental result that all realistic cosmological models must possess singularities. Using similar techniques, Hawking has proved the basic theorems on the laws governing black holes: that stationary solutions of Einstein’s equations with smooth event horizons must necessarily be axisymmetric; and that in the evolution and interaction of black holes, the total surface area of the event horizons must increase. In collaboration with G. Ellis, Hawking is the author of an impressive and original treatise on “Space-time in the Large.
Other important work by Hawking relates to the interpretation of cosmological observations and to the design of gravitational wave detectors.
Peter Higgs and Stephen Hawking visiting the “Collider” exhibition at London’s Science Museum in 2013, in honor of the discovery of the Higgs Boson. Credit: sciencemuseum.org.uk
In 1975, he was awarded both the Eddington Medal and the Pius XI Gold Medal, and in 1976 the Dannie Heineman Prize, the Maxwell Prize and the Hughes Medal. In 1977, he was appointed a professor with a chair in gravitational physics, and received the Albert Einstein Medal and an honorary doctorate from the University of Oxford by the following year.
In 1981, Hawking was awarded the American Franklin Medal, followed by a Commander of the Order of the British Empire (CBE) medal the following year. For the remainder of the decade, he was honored three times, first with the Gold Medal of the Royal Astronomical Society in 1985, the Paul Dirac Medal in 1987 and, jointly with Penrose, with the prestigious Wolf Prize in 1988. In 1989, he was appointed Member of the Order of the Companions of Honour (CH), but reportedly declined a knighthood.
In 1999, Hawking was awarded the Julius Edgar Lilienfeld Prize of the American Physical Society. In 2002, following a UK-wide vote, the BBC included him in their list of the 100 Greatest Britons. More recently, Hawking has been awarded the Copley Medal from the Royal Society (2006), the Presidential Medal of Freedom, America’s highest civilian honor (2009), and the Russian Special Fundamental Physics Prize (2013).
Stephen Hawking being presented by his daughter Lucy Hawking at the lecture he gave for NASA’s 50th anniversary. Credit: NASA/Paul Alers
Also in 2008, while traveling to Spain, Hawking received the Fonseca Prize – an annual award created by the University of Santiago de Compostela which is awarded to those for outstanding achievement in science communication. Hawking was singled out for the award because of his “exceptional mastery in the popularization of complex concepts in Physics at the very edge of our current understanding of the Universe, combined with the highest scientific excellence, and for becoming a public reference of science worldwide.”
Multiple films have been made about Stephen Hawking over the years as well. These include the previously mentioned A Brief History of Time, the 1991 biopic film directed by Errol Morris and Stephen Spielberg; Hawking, a 2004 BBC drama starring Benedict Cumberbatch in the title role; the 2013 documentary titled “Hawking”, by Stephen Finnigan.
Most recently, there was the 2014 film The Theory of Everything that chronicled the life of Stephen Hawking and his wife Jane. Directed by James Marsh, the movie stars Eddie Redmayne as Professor Hawking and Felicity Jones as Jane Hawking.
Death:
Dr. Stephen Hawking passed away in the early hours of Wednesday, March 14th, 2018 at his home in Cambridge. According to a statement made by his family, he died peacefully. He was 76 years old, and is survived by his first wife, Jane Wilde, and their three children – Lucy, Robert and Tim.
When all is said and done, Stephen Hawking was the arguably the most famous scientist alive in the modern era. His work in the field of astrophysics and quantum mechanics has led to a breakthrough in our understanding of time and space, and will likely be poured over by scientists for decades. In addition, he has done more than any living scientist to make science accessible and interesting to the general public.
Stephen Hawking holding a public lecture at the Stockholm Waterfront congress center, 24 August 2015. Credit: Public Domain/photo by Alexandar Vujadinovic
To top it off, he traveled all over the world and lectured on topics ranging from science and cosmology to human rights, artificial intelligence, and the future of the human race. He also used the celebrity status afforded him to advance the causes of scientific research, space exploration, disability awareness, and humanitarian causes wherever possible.
In all of these respects, he was very much like his predecessor, Albert Einstein – another influential scientist-turned celebrity who was sure to use his powers to combat ignorance and promote humanitarian causes. But what was especially impressive in all of this is that Hawking has managed to maintain his commitment to science and a very busy schedule while dealing with a degenerative disease.
For over 50 years, Hawking lived with a disease that doctor’s initially thought would take his life within just two. And yet, he not only managed to make his greatest scientific contributions while dealing with ever-increasing problems of mobility and speech, he also became a jet-setting personality who travelled all around the world to address audiences and inspire people.
His passing was mourned by millions worldwide and, in the worlds of famed scientist and science communicator Neil DeGrasse Tyson , “left an intellectual vacuum in its wake”. Without a doubt, history will place Dr. Hawking among such luminaries as Einstein, Newton, Galileo and Curie as one of the greatest scientific minds that ever lived.
A simulation of galaxies during the era of deionization in the early Universe. Credit: M. Alvarez, R. Kaehler, and T. AbelCredit: M. Alvarez, R. Kaehler, and T. Abel
In the beginning, there was chaos.
Hot, dense, and packed with energetic particles, the early Universe was a turbulent, bustling place. It wasn’t until about 300,000 years after the Big Bang that the nascent cosmic soup had cooled enough for atoms to form and light to travel freely. This landmark event, known as recombination, gave rise to the famous cosmic microwave background (CMB), a signature glow that pervades the entire sky.
Now, a new analysis of this glow suggests the presence of a pronounced bruise in the background — evidence that, sometime around recombination, a parallel universe may have bumped into our own.
Although they are often the stuff of science fiction, parallel universes play a large part in our understanding of the cosmos. According to the theory of eternal inflation, bubble universes apart from our own are theorized to be constantly forming, driven by the energy inherent to space itself.
Like soap bubbles, bubble universes that grow too close to one another can and do stick together, if only for a moment. Such temporary mergers could make it possible for one universe to deposit some of its material into the other, leaving a kind of fingerprint at the point of collision.
Ranga-Ram Chary, a cosmologist at the California Institute of Technology, believes that the CMB is the perfect place to look for such a fingerprint.
The cosmic microwave background (CMB), a pervasive glow made of light from the Universe’s infancy, as seen by the Planck satellite in 2013. Tiny deviations in average temperature are represented by color. Credit: ESA and the Planck Collaboration.
After careful analysis of the spectrum of the CMB, Chary found a signal that was about 4500x brighter than it should have been, based on the number of protons and electrons scientists believe existed in the very early Universe. Indeed, this particular signal — an emission line that arose from the formation of atoms during the era of recombination — is more consistent with a Universe whose ratio of matter particles to photons is about 65x greater than our own.
There is a 30% chance that this mysterious signal is just noise, and not really a signal at all; however, it is also possible that it is real, and exists because a parallel universe dumped some of its matter particles into our own Universe.
After all, if additional protons and electrons had been added to our Universe during recombination, more atoms would have formed. More photons would have been emitted during their formation. And the signature line that arose from all of these emissions would be greatly enhanced.
Chary himself is wisely skeptical.
“Unusual claims like evidence for alternate Universes require a very high burden of proof,” he writes.
Indeed, the signature that Chary has isolated may instead be a consequence of incoming light from distant galaxies, or even from clouds of dust surrounding our own galaxy.
NGC 2419 as imaged by the Hubble Space Telescope. Image credit: NASA/STScl
Turns out, we may not know our extragalactic neighbors as well as we thought.
One of the promises held forth with the purchase of our first GoTo telescope way back in the late 1990s was the ability to quickly and easily hunt down ever fainter deep sky fuzzies. No more juggling star charts and red headlamps, no more star-hopping. Heck, it was fun to just aim the scope at a favorable target field, hit ‘identify,’ and see what it turned up.
One of our more interesting ‘discoveries’ on these expeditions was NGC 2419, a globular cluster that my AstroMaster GoTo controller (featuring a 10K memory database!) triumphantly announced was an ‘Intergalactic Wanderer…’
Or is it? The case for NGC 2419 as a lonely globular wandering the cosmic void between the galaxies is a romantic and intriguing notion, and one you see repeated around the echo chamber that is the modern web. First observed by Sir William Herschel in 1788 and re-observed by his son John in 1833, the debate has swung back and forth as to whether NGC 2419 is a true globular or—as has been also suggested of the magnificent southern sky cluster Omega Centauri—the remnant of a dwarf spheroidal galaxy torn apart by our Milky Way. Lord Rosse also observed NGC 2419 with the 72-inch Leviathan of Parsonstown, and Harlow Shapley made a distance estimate of about 163,000 light years to NGC 2419 in 1922.
The relative distances of NGC 2419, the LMC, SMC and M31. Created by author using NASA graphics.
Today, we know that NGC 2419 is about 270,000 light years from the Sun, and about 300,000 light years from the core of our galaxy. Think of this: we actually see NGC 2419 as it appeared back in the middle of the Pleistocene Epoch, a time when modern homo sapiens were still the new hipsters on the evolutionary scene of life on Earth. What’s more, photometric studies over the past decade suggest there is a true gravitational link between NGC 2419 and the Milky Way. This would mean at its current distance, NGC 2419 would orbit our galaxy once every 3 billion years, about 75% the age of the Earth itself.
NGC 2419 and the nearby +7 magnitude star HIP 37133. Image credit and copyright: Joseph Brimacombe
This hands down makes NGC 2419 the distant of the more than 150 globular clusters known to orbit our galaxy.
At an apparent magnitude of +9 and 6 arc minutes in size, NGC 2419 occupies an area of the sky otherwise devoid of globulars. Most tend to lie towards the galactic core as seen from our solar vantage point, and in fact, there are no bright globulars within 60 degrees of NGC 2419. The cluster sits 7 degrees north of the bright star Castor just across the border of Gemini in the constellation of the Lynx at Right Ascension 7 Hours, 38 minutes and 9 seconds and declination +38 degrees, 52 minutes and 55 seconds. Mid-January is the best time to spy NGC 2419 when it sits roughly opposite to the Sun , though June still sees the cluster 20 degrees above the western horizon at dusk before solar conjunction in mid-July.
The location of NGC 2419 in the night sky. Image credit: Starry Night Education software
We know globular clusters (say ‘globe’ -ular, not “glob’ -ular) are some of the most ancient structures in the universe due to their abundance of metal poor, first generation stars. In fact, it was a major mystery up until about a decade ago as to just how these clusters could appear to be older than the universe they inhabit. Today, we know that NGC 2419 is about 12.3 billion years old, and we’ve refined the age of the Universe as per data from the Planck spacecraft down to 13.73 (+/-0.12) billion years.
What would the skies look like from a planet inside NGC 2419? Well, in addition to the swarm of hundreds of thousands of nearby stars, the Milky Way galaxy itself would be a conspicuous object extending about 30 degrees across and shining at magnitude -2. Move NGC 2419 up to 10 parsecs distant, and it would rival the brightness of our First Quarter Moon and be visible in the daytime shining at magnitude -9.5.
The view of the Milky Way galaxy as seen from NGC 2419. Image Credit; Starry Night Education Software
And ironically, another 2007 study has suggested that the relative velocity of Large and Small Magellanic Clouds suggest that they may not be bound to our galaxy, but are instead first time visitors passing by.
And speaking of passing by, yet another study suggests that the Milky Way and the Andromeda galaxy set on a collision course billions of years hence may be in contact… now.
The view of the Andromeda galaxy as seen from NGC 2419. Image credit: Starry Night Education software
Mind not blown yet?
A 2014 study looking at extragalactic background light during a mission known as CIBER suggests that there may actually be more stars wandering the universe than are bound to galaxies…
But that’s enough paradigm-shifting for one day. Be sure to check out NGC 2419 and friends and remember, everything you learned about the universe as a kid, is likely to be false.
Could there be a mirror universe, where everything is backwards – and everybody has goatees? How badly do you need to bend the laws of physics to make this happen?
One of the great mysteries in cosmology is why the Universe is mostly matter and not antimatter. If you want to learn more about that specific subject, you can click here and watch an episode all about that.
During the Big Bang, nearly equal amounts of matter and antimatter were created, and subsequently annihilated. Nearly equal. And so we’re left with a Universe made of matter.
But could there be antimatter stars out there? With antimatter planets in orbit. Could there be a backwards Universe that operates just like our regular Universe, but everything’s made of antimatter? And if it’s out there, does it have to be evil? Do they only know how to conquer? Does everyone, even the antimatter babies and ladies, have handsome goatees? How about sashes? I hear they’re big on sashes. OOH and daggers. Gold daggers with little teensy antimatter emeralds and rubies.
Antimatter, without the goatee, was theorized in 1928 by Paul Dirac, who realized that one implication of quantum physics was that you could get electrons that had a positive charge instead of a negative charge. They were discovered by Carl D. Anderson just 4 years later, which he named “positron” for positive electron.
We believe he was clearly snubbing Dirac, by not naming them the “Diracitron”, alternately they were saving that name for a giant Japanese robot.
These antiparticles are created through high energy particle collisions happening naturally in the Universe, or unnaturally inside our “laugh in the face of God and nature” particle accelerators. We can even detect the annihilation out there in the Universe where matter and antimatter crash into each other.
Physicists have discovered a range of anti-particles. Anti-protons, anti-neutrons, anti-hydrogen, anti-helium. To date, there’s been no evidence of any goatees or sashes. Naturally, they wondered what might happen if the balance of the Universe was flipped. What if we had a Universe made out of mostly antimatter? Would it still… you know, work? Could you have antimatter stars, antimatter planets, and even those antimatter people we mentioned?
The Large Hadron Collider (CERN/LHC)
When physics swap out matter for anti-matter in their equations, they call it charge conjugation. It turns out, no. If you reversed the charge of all the particles in the Universe, it wouldn’t evolve in the same way as our “plain old non-sashed” Universe.
To fix this problem, physicists considered the implications if you had an actual mirror Universe, where all the particles behaved as if they were mirror images of themselves. This sounds a little more in line with our “Through a mirror, darkly” goatee and sash every day festival universe. This is all the bits backwards. Spin, charge, velocity, the works. They called this parity inversion. So, would this work?
Again, it turns out that the answer is no. It would almost work out, but there’s a tendency for the weak nuclear force, the one the governs nuclear decay to violate this idea of parity inversion. Even in a mirror Universe, the weak nuclear force is left-handed. Dammit, weak nuclear force, get your act together, if not just for the sake of the costumes and cooler bridge lighting.
What matter and antimatter might look like annihilating one another. Credit: NASA/CXC/M. Weiss
What if you reversed both the charge and the parity at the same time? What if you had antimatter in a mirror Universe? Physicists called this charge-parity symmetry, or CP symmetry.
In a dazzling experiment and absolute “what if” one-upmanship exercise by James Cronin and Val Fitch in 1964. They demonstrated that no, you can’t have a mirror-antimatter Universe evolve with our physical laws. This experiment won the Nobel Prize in 1980.
Physicists had one last trick up their sleeves. It turns out that if you reverse time itself as well as making everything out of antimatter and holding it up to a mirror, you get true symmetry. All the physical lays are preserved, and you’d get a Universe that would look exactly like our own.
It turns out we could live in a mirror Universe, as long as you were willing to reverse the charge of every particle and run time backwards. And if you did, it would be indistinguishable from the Universe we actually live in. Now, if you’ll excuse me, I think I need to call my tailor, I hear sashes are going to be huge this year.
So what do you think, do we live in the real Universe or the mirror Universe? Tell us in the comments below.
Cosmologists are intellectual time travelers. Looking back over billions of years, these scientists are able to trace the evolution of our Universe in astonishing detail. 13.8 billion years ago, the Big Bang occurred. Fractions of a second later, the fledgling Universe expanded exponentially during an incredibly brief period of time called inflation. Over the ensuing eons, our cosmos has grown to such an enormous size that we can no longer see the other side of it.
But how can this be? If light’s velocity marks a cosmic speed limit, how can there possibly be regions of spacetime whose photons are forever out of our reach? And even if there are, how do we know that they exist at all?
The Expanding Universe
Like everything else in physics, our Universe strives to exist in the lowest possible energy state possible. But around 10-36 seconds after the Big Bang, inflationary cosmologists believe that the cosmos found itself resting instead at a “false vacuum energy” – a low-point that wasn’t really a low-point. Seeking the true nadir of vacuum energy, over a minute fraction of a moment, the Universe is thought to have ballooned by a factor of 1050.
Since that time, our Universe has continued to expand, but at a much slower pace. We see evidence of this expansion in the light from distant objects. As photons emitted by a star or galaxy propagate across the Universe, the stretching of space causes them to lose energy. Once the photons reach us, their wavelengths have been redshifted in accordance with the distance they have traveled.
Two sources of redshift: Doppler and cosmological expansion; modeled after Koupelis & Kuhn. Bottom: Detectors catch the light that is emitted by a central star. This light is stretched, or redshifted, as space expands in between. Credit: Brews Ohare.
This is why cosmologists speak of redshift as a function of distance in both space and time. The light from these distant objects has been traveling for so long that, when we finally see it, we are seeing the objects as they were billions of years ago.
The Hubble Volume
Redshifted light allows us to see objects like galaxies as they existed in the distant past; but we cannot see all events that occurred in our Universe during its history. Because our cosmos is expanding, the light from some objects is simply too far away for us ever to see.
The physics of that boundary rely, in part, on a chunk of surrounding spacetime called the Hubble volume. Here on Earth, we define the Hubble volume by measuring something called the Hubble parameter (H0), a value that relates the apparent recession speed of distant objects to their redshift. It was first calculated in 1929, when Edwin Hubble discovered that faraway galaxies appeared to be moving away from us at a rate that was proportional to the redshift of their light.
Fit of redshift velocities to Hubble’s law. Credit: Brews Ohare
Dividing the speed of light by H0, we get the Hubble volume. This spherical bubble encloses a region where all objects move away from a central observer at speeds less than the speed of light. Correspondingly, all objects outside of the Hubble volume move away from the center faster than the speed of light.
Yes, “faster than the speed of light.” How is this possible?
The Magic of Relativity
The answer has to do with the difference between special relativity and general relativity. Special relativity requires what is called an “inertial reference frame” – more simply, a backdrop. According to this theory, the speed of light is the same when compared in all inertial reference frames. Whether an observer is sitting still on a park bench on planet Earth or zooming past Neptune in a futuristic high-velocity rocketship, the speed of light is always the same. A photon always travels away from the observer at 300,000,000 meters per second, and he or she will never catch up.
General relativity, however, describes the fabric of spacetime itself. In this theory, there is no inertial reference frame. Spacetime is not expanding with respect to anything outside of itself, so the the speed of light as a limit on its velocity doesn’t apply. Yes, galaxies outside of our Hubble sphere are receding from us faster than the speed of light. But the galaxies themselves aren’t breaking any cosmic speed limits. To an observer within one of those galaxies, nothing violates special relativity at all. It is the space in between us and those galaxies that is rapidly proliferating and stretching exponentially.
The Observable Universe
Now for the next bombshell: The Hubble volume is not the same thing as the observable Universe.
To understand this, consider that as the Universe gets older, distant light has more time to reach our detectors here on Earth. We can see objects that have accelerated beyond our current Hubble volume because the light we see today was emitted when they were within it.
Strictly speaking, our observable Universe coincides with something called the particle horizon. The particle horizon marks the distance to the farthest light that we can possibly see at this moment in time – photons that have had enough time to either remain within, or catch up to, our gently expanding Hubble sphere.
And just what is this distance? A little more than 46 billion light years in every direction – giving our observable Universe a diameter of approximately 93 billion light years, or more than 500 billion trillion miles.
The observable universe, more technically known as the particle horizon.
(A quick note: the particle horizon is not the same thing as the cosmological event horizon. The particle horizon encompasses all the events in the past that we can currently see. The cosmological event horizon, on the other hand, defines a distance within which a future observer will be able to see the then-ancient light our little corner of spacetime is emitting today.
In other words, the particle horizon deals with the distance to past objects whose ancient light that we can see today; the cosmological event horizon deals with the distance that our present-day light that will be able to travel as faraway regions of the Universe accelerate away from us.)
Dark Energy
Thanks to the expansion of the Universe, there are regions of the cosmos that we will never see, even if we could wait an infinite amount of time for their light to reach us. But what about those areas just beyond the reaches of our present-day Hubble volume? If that sphere is also expanding, will we ever be able to see those boundary objects?
This depends on which region is expanding faster – the Hubble volume or the parts of the Universe just outside of it. And the answer to that question depends on two things: 1) whether H0 is increasing or decreasing, and 2) whether the Universe is accelerating or decelerating. These two rates are intimately related, but they are not the same.
In fact, cosmologists believe that we are actually living at a time when H0 is decreasing; but because of dark energy, the velocity of the Universe’s expansion is increasing.
That may sound counterintuitive, but as long as H0 decreases at a slower rate than that at which the Universe’s expansion velocity is increasing, the overall movement of galaxies away from us still occurs at an accelerated pace. And at this moment in time, cosmologists believe that the Universe’s expansion will outpace the more modest growth of the Hubble volume.
So even though our Hubble volume is expanding, the influence of dark energy appears to provide a hard limit to the ever-increasing observable Universe.
Our Earthly Limitations
Cosmologists seem to have a good handle on deep questions like what our observable Universe will someday look like and how the expansion of the cosmos will change. But ultimately, scientists can only theorize the answers to questions about the future based on their present-day understanding of the Universe. Cosmological timescales are so unimaginably long that it is impossible to say much of anything concrete about how the Universe will behave in the future. Today’s models fit the current data remarkably well, but the truth is that none of us will live long enough to see whether the predictions truly match all of the outcomes.
Disappointing? Sure. But totally worth the effort to help our puny brains consider such mind-bloggling science – a reality that, as usual, is just plain stranger than fiction.
Planck view of BICEP2 field. Credit: ESA/Planck Collaboration. Acknowledgment: M.-A. Miville-Deschênes, CNRS – Institut d'Astrophysique Spatiale, Université Paris-XI, Orsay, France.
Last March, international researchers from the Background Imaging of Cosmic Extragalactic Polarization (BICEP2) telescope at the South Pole claimed that they detected primordial “B-mode” polarization of the cosmic microwave background (CMB) radiation. If confirmed, this would have been an incredibly important discovery for astrophysics, as it would constitute evidence of gravitational waves due to cosmic inflation in the first moments of the universe. Nevertheless, as often happens in science, the situation turns out to be more complicated than it initially appeared.
In a joint analysis of data from BICEP2/Keck Array in the South Pole and the space-based Planck telescope, scientists from both collaborations now have a more complete picture and argue that the interpretation of the evidence is muddier than they had previously thought. Their paper will appear in the arXiv pre-print server in a few days and is submitted for publication in the journal Physical Review Letters. [Update: the paper is now available on the arXiv.] The European Space Agency issued a press release about the paper on Friday after a summary of it was leaked and briefly posted on a French website.
Courtesy: Jet Propulsion Laboratory
According to inflationary theory, the universe expanded for a brief period at an exponential rate 10-36 seconds after the Big Bang. As a result, models of inflation predict that this rapid acceleration would create ripples in space, generating gravitational waves that would remain energetic enough to leave an imprint on the last-scattered photons, the CMB radiation, approximately 380,000 years later. The CMB spectrum, the “afterglow of the hot Big Bang,” has rich structure in it and has been measured to a “ridiculous level of precision,” according to Professor Martin White (University of California, Berkeley), who gave a plenary talk on cosmology results from Planck at the recent American Astronomical Society meeting.
The twists in the polarization signal of the CMB, known as B-modes (shown below) and quantified by a nonzero tensor-to-scalar ratio r, would be evidence in favor of inflation but they are much more difficult to detect. Scientists are trying to decipher a signal on the level of parts per trillion of ambient temperature, mere fractions of a nano-degree! Since inflation would explain why the universe appears to have no overall curvature, why it approximately appears the same in all directions, and why it has structures of galaxies in it, BICEP2’s result last year—the first claimed detection of cosmic inflation—excited physicists around the world. But last summer, Planck scientists presented a map of polarized light from interstellar dust grains and argued that the polarization signal BICEP2 detected could be due to “foreground” dust in our own Milky Way galaxy rather than due to primordial gravitational waves in the distant universe. The hotly debated controversy remained unresolved and led to the new joint analysis by scientists from both teams.
Courtesy: BICEP2
BICEP2 is sensitive to low frequencies (150 GHz) while Planck is more sensitive to higher ones (353 GHz). As Professor Brian Keating (University of California, San Diego), a member of the BICEP2 collaboration, puts it, “it’s as if you’re listening to an opera, but BICEP2 could only hear the tenors and Planck could only hear the sopranos.” Unfortunately, the joint analysis produced only an upper limit to the value of r, meaning that the evidence for B-mode polarization due to inflation remains elusive for now. “It’s probably at best an admixture of Milky Way dust and gravitational waves,” says Keating. [Full disclosure: until last year, Ramin Skibba was a research scientist in the same department but in a different field as Keating at UC San Diego.]
This result must seem disappointing to BICEP2 scientists, but science often works this way, especially for such a difficult phenomenon to study. The signal is strong, but the interpretation is more complicated than it first appeared. On a positive note, the analysis shows that CMB researchers are faced with a foreground challenge rather than one due to the Earth’s atmosphere or to their detectors.
Illustration by Andy Freeberg, SLAC / South Pole Telescope photo by Keith Vanderlinde
Although Planck will have additional polarization measurements and more assessments of systematic uncertainties in a later data release, they will not be able to settle this debate for now. But new experiments will come online soon, including a BICEP3, and they will produce more precise measurements that could effectively remove the contribution from dust. The signal is tractable, and scientists are looking forward to the day when they can declare with strong statistical significance that they have finally discovered evidence of inflation.
Distribution of galaxies and quasars in a slice of BOSS out to a redshift of 3, or 11 billion years in the past. Credit: SDSS-III
Astrophysicists studying the expansion of the Universe with the largest galaxy catalogs ever assembled are ushering in an exciting era of precision cosmology. Last week, the Sloan Digital Sky Survey (SDSS) issued its final public data release, and scientists working in its largest program, the Baryon Oscillation Spectroscopic Survey (BOSS) also presented their final results at the American Astronomical Society meeting in Seattle, Washington.
By mapping over 10,000 square degrees — 25% of the sky — BOSS is “measuring our universe’s accelerated expansion with the world’s largest extragalactic redshift survey,” according to SDSS-III Director Daniel Eisenstein of the Harvard-Smithsonian Center for Astrophysics. The BOSS results include new and precise measurements of the universe’s expansion rate (called the “Hubble constant”) and matter density, which includes dark matter, stars, gas, and dust.
BOSS conducted its observations at 2.5-meter Sloan Foundation Telescope at Apache Point Observatory in New Mexico, producing spectra and spatial positions for 1.5 million galaxies and 300,000 quasars in a volume equivalent to a cube with length 8.5 billion light-years on a side (see image above). Astronomers used this rich dataset to map the objects’ distributions and to detect the characteristic scale imprinted by baryon acoustic oscillations in the early universe. Sound waves propagate outward with time, like ripples spreading in a pond, and are indicated by a large-scale clustering signal in the positions of galaxies relative to each other (see illustration below). By analyzing this signal at different times, it is possible to study the behavior of the mysterious “dark energy” causing the accelerating expansion of the universe.
An illustration of the concept of baryon acoustic oscillations, imprinted in the early universe and seen today in galaxy surveys. (courtesy: Chris Blake and Sam Moorfield)
In BOSS’s final results, hundreds of scientists in the international collaboration measured this scale with unprecedented precision. In particular, Ashley Ross from Ohio State University presented results that demonstrated the power of combining an analysis of the transverse and line-of-sight distributions of galaxies. In a paper by Eric Aubourg and collaborators, BOSS astronomers measured the cosmic distance scale of galaxies in the “local” universe and of quasars in the distance universe with impressively small systematic errors—at less than the 1% level—when combined with cosmic microwave background constraints. Their cosmological analysis yields a measurement of the Hubble constant and of the matter density of the universe consistent with a “flat” cold dark matter cosmology with a cosmological constant (see below). Cosmological models including curvature, evolving dark energy, or massive neutrinos are not completely ruled out but are less supported by the data than before. Other results from the collaboration will be submitted for publication in the coming months.
Cosmological constraints on the Hubble parameter h, matter density Ωm, and curvature parameter Ωk from BOSS’s baryon acoustic oscillations (BAO) combined with supernovae (SN) and Planck results. (Courtesy: Aubourg et al. 2014)
The BOSS dataset “represents the gold standard in mapping out the network of galaxies that comprises the large-scale structure of the Universe…The data enables us to trace, with greater precision than ever before, the presence of dark energy, the behaviour of gravity on cosmic scales, and the effect of massive neutrinos,” says Chris Blake of Swinburne University, not affiliated with the collaboration.
Where will the BOSS team go from here? The collaboration has begun work on SDSS-IV, whose six-year mission includes an ambitious extended BOSS (eBOSS) survey. According to eBOSS Targeting Coordinator Jeremy Tinker of New York University, eBOSS observations of over 700,000 quasars will precisely measure the distance scale “at a much higher redshift regime that is not covered by current large-scale surveys.”
You can read more about BOSS and updates about the three other componenets of the SDSS in our previous article here. SDSS website
(Full disclosure: Ramin Skibba had been a member of the BOSS collaboration during 2010-2012.)
The dark nebula LDN 483 imaged by ESO's La Silla Observatory in Chile (ESO)
What may appear at first glance to be an eerie, empty void in an otherwise star-filled scene is really a cloud of cold, dark dust and molecular gas, so dense and opaque that it obscures the distant stars that lie beyond it from our point of view.
Similar to the more well-known Barnard 68, “dark nebula” LDN 483 is seen above in an image taken by the MPG/ESO 2.2-meter telescope’s Wide Field Imager at the La Silla Observatory in Chile.
While it might seem like a cosmic no-man’s-land, no stars were harmed in the making of this image – on the contrary, dark nebulae like LDN 483 are veritable maternity wards for stars. As their cold gas and dust contracts and collapses new stars form inside them, remaining cool until they build up enough density and gravity to ignite fusion within their cores. Then, shining brightly, the young stars will gradually blast away the remaining material with their outpouring wind and radiation to reveal themselves to the galaxy.
The process may take several million years, but that’s just a brief flash in the age of the Universe. Until then, gestating stars within LDN 483 and many other clouds like it remain dim and hidden but keep growing strong.
Wide-field view of the LDN 483 region. (Credit: ESO and Digitized Sky Survey 2)
