The discovery of dark energy (1998)*
What is this mysterious entity called dark energy which is driving the universe’s expansion?[1]
Scientists have been aware for some time that the universe is expanding - for the last 7 billion years in fact. (That's how long it has been expanding, not how long scientists have known about it). This was confirmed observationally in 1929, when Edwin Hubble discovered that whichever way he looked, every distant galaxy was moving away from us and from each other, rather like raisins in rising dough. Furthermore, the rate of the galaxies’ recession was proportional to their distance. In other words, the further the distance, the faster the rate of recession. Since 1998, it has been realised that the rate of this expansion is accelerating, and that galaxies are receding from us and from one another at a faster rate than ever before. What is causing this expansion? The name given to this phenomenon is dark energy, but what actually is it?
The answer is we don't know, but there are various theories. The first is something akin to Einstein’s cosmological constant[2], according to which the vacuum of space has an inherent energy brought about by particles and antiparticles popping into and out of existence as they annihilate one another in the minutest of time frames. This vacuum energy in empty space is endowed with a negative pressure which is the source of the repulsive gravity driving the universe apart. This hypothesis holds that the density of dark matter is constant and unvarying over space and time. The astrophysical evidence currently available accords with this theory. This is the subject of elaboration below.
General relativity has a lot going for it. All its predictions have thus far been proved correct, the latest being the recent discovery of gravitational waves. If the cosmological constant is correct – that dark energy is indeed the energy of empty space – the acceleration will continue forever, and about a trillion years from now, all the galaxies beyond our Local Group will have long since receded from our view, along with the afterglow of the Big Bang which will be stretched to wavelengths longer than the size of the visible universe.
An alternative hypothesis trades under the name of quintessence. According to this theory, dark energy is a dynamic quantum field, not unlike an electrical or magnetic field, the pressure of which is also negative and which pervades the universe, imbuing every point in space with a property that counteracts the pull of gravity. If dark energy is a field, it will not be a constant and might change over time, either increasing to eventually rip all structures in space apart, or decreasing and changing directions to allow the universe to contract in a ‘big crunch’[3].
A third option is that there is in fact no such thing as dark energy, and the quickening expansion of the universe results from physics not explained by Einstein’s theory of relativity, which is therefore incomplete. In other words, gravity operates differently than we think on extremely large scales. On the scale of galaxies and clusters, gravity behaves as general relativity predicts, but on the scale of the universe as a whole, gravity grossly diverges from general relativity and the universe appears to accelerate. This is the subject of further elaboration below.
How significant is it?
Taking the amount of matter, both visible and dark, to be about 28% of the universe's critical density (4 or 5% accounting for the things we can see with our own eyes, such as stars, galaxies, ourselves and the things we see about us, and 24% dark matter) it is currently estimated that the accelerated expansion of the universe accounts for an outward push whose dark energy contributes about 72 % of its critical density, so because dark energy is denser in space than any other constituent of the universe, it exerts the dominant influence on the cosmos and will therefore control its fate. If its density increases, it may become so powerful that it rips apart all structures in space.
How is this outward expansion measured?: 1 - generally[4]
One method is by measuring its equation of state parameter (w), that is the ratio of its pressure to its density. If w is constant and = -1, that accords with dark energy being the energy of the vacuum (the cosmological constant). If it accords with a field that varies over time (quintessence), its value should differ from -1 and be evolving throughout cosmic history, and if the observed acceleration rate necessitates a modification of Einstein’s theory of gravity for extreme distances (the no dark matter hypothesis), there should be an inconsistency between the value of w we find at different scales in the universe.
One unresolved problem is that according to the cosmological constant, dark energy should be stronger than it actually is, so why is it so small? Other methods of detection involve studying how studying how large scale galaxy clusters grew over time in order to ascertain how strong dark matter was at various points in its history by means of gravitational lensing. Scientists have also been able to study how the universe’s expansion has changed over time by observing objects and measuring their redshift. This is dealt with below.
[1] This is an edited summary of "The puzzle of dark energy" by Adam Riess and Mario Livio in Scientific American, March 2016, 30 at 32, 33.
[2] See /inflation-and-negative-pressure.html
[3] As to quintessence, see also /inflation-and-negative-pressure.html, especially the graphic at the end.
[4] Ibid, 34-35.
How is this outward expansion measured?: 2 - Type 1a supernovae, the inverse square law and the cosmological constant revisited.
Until the 1990s, scientists were attempting to answer this question by measuring the extent to which ordinary attractive gravity had been slowing the expansion of space: the so-called decentralisation parameter.
However, since 1998, with the aid of new technology (stronger telescopes, more refined digital detectors and more inspired techniques) they have been able to measure the recession speeds of certain kinds of exploding star called type 1a supernovae [1], also known as cosmic or standard candles, because they give off a steady amount of brightness. Type 1a supernovae occur in a binary system consisting of two stars orbiting one another. One of the stars in the system must be a white dwarf star - the dense, carbon remains of a star that was about the size of our sun, which, having insufficient mass to ignite a supernova explosion of its own, sucks the surface material from its nearby companion star, possibly a red giant, or a main sequence star like our sun: The white dwarf will begin to pull material off its companion star, adding that matter to itself. It grows and grows until it cannot get any bigger.
When it reaches 1.4 solar masses, or about 40 percent more massive than our sun, a nuclear chain reaction occurs, causing the white dwarf to explode, giving off so much energy that it outshines whole galaxies. The resulting light is 5 billion times brighter than the sun. Because the chain reaction always happens in the same way, and at the same mass, the brightness of these type 1a supernovae are consistent and always the same, and last for 44 days. The explosion point is known as the Chandrasekhar limit, after Subrahmanyan Chandrasekhar, the astronomer who discovered it. With stars that initially have a mass of more than about 8 times that of our Sun, if the mass of the star’s core at the end of its life exceeds the Chandrasekhar limit, the core will produce a neutron star or black hole. [1.1]
Because the brightness of these supernova explosions is virtually the same across the board – that is why they are referred to as cosmic candles or standard yardsticks, astrophysical objects that always shine with the same brightness no matter where they are in the universe - any differences in how bright they appear to us stems derives solely from their distance and those distances are a constant: those that look dimmer are farther away. Thus if you see two identical lighthouses but one appears a quarter as bright, then pursuant to the inverse square law explained below, you know the fainter one is two times further away[2], and a type 1a supernova 100 times as faint as another means that it is 10 times further away[3]. Examples can include varying types of pulsating or exploding stars, or even massive galaxies as first proposed by R Brent Tully and the astronomer J Richard Fisher in 1977, though the distance estimates they yield have uncertainties of up to 20%.[4]
Recent studies have indicated that only a small minority of type 1a supernovae stem from material being torn from main sequence or giant stars, in fact fewer than 20% of the classically assumed scenario. The most likely scenario now points to a major role for a lesser known mechanism – pairings of two white dwarfs in which one cannibalises its orbital companion before exploding into a supernova[5]. Scientists have searched unsuccessfully for remnants of a victim star that precipitated the type 1a supernovae seen from earth in the year 1006, without success. This is significant because a main sequence star would have been expected to leave traces after the event, whereas a normal sized star would not, pointing to another white dwarf as the likely victim[6].
In a typical galaxy, type 1a supernovae occur only once every 250 years, hence scientists have to literally comb the cosmos with their telescopes looking for examples in far flung galaxies elsewhere. When one is detected, to find the distance to the galaxy that contains the supernova, scientists just have to compare how bright they know the explosion should be with how bright the explosion appears. Using the inverse square law, they can compute the distance to the supernova and thus to the supernova's home galaxy[7]:
Scientists have been aware for some time that the universe is expanding - for the last 7 billion years in fact. (That's how long it has been expanding, not how long scientists have known about it). This was confirmed observationally in 1929, when Edwin Hubble discovered that whichever way he looked, every distant galaxy was moving away from us and from each other, rather like raisins in rising dough. Furthermore, the rate of the galaxies’ recession was proportional to their distance. In other words, the further the distance, the faster the rate of recession. Since 1998, it has been realised that the rate of this expansion is accelerating, and that galaxies are receding from us and from one another at a faster rate than ever before. What is causing this expansion? The name given to this phenomenon is dark energy, but what actually is it?
The answer is we don't know, but there are various theories. The first is something akin to Einstein’s cosmological constant[2], according to which the vacuum of space has an inherent energy brought about by particles and antiparticles popping into and out of existence as they annihilate one another in the minutest of time frames. This vacuum energy in empty space is endowed with a negative pressure which is the source of the repulsive gravity driving the universe apart. This hypothesis holds that the density of dark matter is constant and unvarying over space and time. The astrophysical evidence currently available accords with this theory. This is the subject of elaboration below.
General relativity has a lot going for it. All its predictions have thus far been proved correct, the latest being the recent discovery of gravitational waves. If the cosmological constant is correct – that dark energy is indeed the energy of empty space – the acceleration will continue forever, and about a trillion years from now, all the galaxies beyond our Local Group will have long since receded from our view, along with the afterglow of the Big Bang which will be stretched to wavelengths longer than the size of the visible universe.
An alternative hypothesis trades under the name of quintessence. According to this theory, dark energy is a dynamic quantum field, not unlike an electrical or magnetic field, the pressure of which is also negative and which pervades the universe, imbuing every point in space with a property that counteracts the pull of gravity. If dark energy is a field, it will not be a constant and might change over time, either increasing to eventually rip all structures in space apart, or decreasing and changing directions to allow the universe to contract in a ‘big crunch’[3].
A third option is that there is in fact no such thing as dark energy, and the quickening expansion of the universe results from physics not explained by Einstein’s theory of relativity, which is therefore incomplete. In other words, gravity operates differently than we think on extremely large scales. On the scale of galaxies and clusters, gravity behaves as general relativity predicts, but on the scale of the universe as a whole, gravity grossly diverges from general relativity and the universe appears to accelerate. This is the subject of further elaboration below.
How significant is it?
Taking the amount of matter, both visible and dark, to be about 28% of the universe's critical density (4 or 5% accounting for the things we can see with our own eyes, such as stars, galaxies, ourselves and the things we see about us, and 24% dark matter) it is currently estimated that the accelerated expansion of the universe accounts for an outward push whose dark energy contributes about 72 % of its critical density, so because dark energy is denser in space than any other constituent of the universe, it exerts the dominant influence on the cosmos and will therefore control its fate. If its density increases, it may become so powerful that it rips apart all structures in space.
How is this outward expansion measured?: 1 - generally[4]
One method is by measuring its equation of state parameter (w), that is the ratio of its pressure to its density. If w is constant and = -1, that accords with dark energy being the energy of the vacuum (the cosmological constant). If it accords with a field that varies over time (quintessence), its value should differ from -1 and be evolving throughout cosmic history, and if the observed acceleration rate necessitates a modification of Einstein’s theory of gravity for extreme distances (the no dark matter hypothesis), there should be an inconsistency between the value of w we find at different scales in the universe.
One unresolved problem is that according to the cosmological constant, dark energy should be stronger than it actually is, so why is it so small? Other methods of detection involve studying how studying how large scale galaxy clusters grew over time in order to ascertain how strong dark matter was at various points in its history by means of gravitational lensing. Scientists have also been able to study how the universe’s expansion has changed over time by observing objects and measuring their redshift. This is dealt with below.
[1] This is an edited summary of "The puzzle of dark energy" by Adam Riess and Mario Livio in Scientific American, March 2016, 30 at 32, 33.
[2] See /inflation-and-negative-pressure.html
[3] As to quintessence, see also /inflation-and-negative-pressure.html, especially the graphic at the end.
[4] Ibid, 34-35.
How is this outward expansion measured?: 2 - Type 1a supernovae, the inverse square law and the cosmological constant revisited.
Until the 1990s, scientists were attempting to answer this question by measuring the extent to which ordinary attractive gravity had been slowing the expansion of space: the so-called decentralisation parameter.
However, since 1998, with the aid of new technology (stronger telescopes, more refined digital detectors and more inspired techniques) they have been able to measure the recession speeds of certain kinds of exploding star called type 1a supernovae [1], also known as cosmic or standard candles, because they give off a steady amount of brightness. Type 1a supernovae occur in a binary system consisting of two stars orbiting one another. One of the stars in the system must be a white dwarf star - the dense, carbon remains of a star that was about the size of our sun, which, having insufficient mass to ignite a supernova explosion of its own, sucks the surface material from its nearby companion star, possibly a red giant, or a main sequence star like our sun: The white dwarf will begin to pull material off its companion star, adding that matter to itself. It grows and grows until it cannot get any bigger.
When it reaches 1.4 solar masses, or about 40 percent more massive than our sun, a nuclear chain reaction occurs, causing the white dwarf to explode, giving off so much energy that it outshines whole galaxies. The resulting light is 5 billion times brighter than the sun. Because the chain reaction always happens in the same way, and at the same mass, the brightness of these type 1a supernovae are consistent and always the same, and last for 44 days. The explosion point is known as the Chandrasekhar limit, after Subrahmanyan Chandrasekhar, the astronomer who discovered it. With stars that initially have a mass of more than about 8 times that of our Sun, if the mass of the star’s core at the end of its life exceeds the Chandrasekhar limit, the core will produce a neutron star or black hole. [1.1]
Because the brightness of these supernova explosions is virtually the same across the board – that is why they are referred to as cosmic candles or standard yardsticks, astrophysical objects that always shine with the same brightness no matter where they are in the universe - any differences in how bright they appear to us stems derives solely from their distance and those distances are a constant: those that look dimmer are farther away. Thus if you see two identical lighthouses but one appears a quarter as bright, then pursuant to the inverse square law explained below, you know the fainter one is two times further away[2], and a type 1a supernova 100 times as faint as another means that it is 10 times further away[3]. Examples can include varying types of pulsating or exploding stars, or even massive galaxies as first proposed by R Brent Tully and the astronomer J Richard Fisher in 1977, though the distance estimates they yield have uncertainties of up to 20%.[4]
Recent studies have indicated that only a small minority of type 1a supernovae stem from material being torn from main sequence or giant stars, in fact fewer than 20% of the classically assumed scenario. The most likely scenario now points to a major role for a lesser known mechanism – pairings of two white dwarfs in which one cannibalises its orbital companion before exploding into a supernova[5]. Scientists have searched unsuccessfully for remnants of a victim star that precipitated the type 1a supernovae seen from earth in the year 1006, without success. This is significant because a main sequence star would have been expected to leave traces after the event, whereas a normal sized star would not, pointing to another white dwarf as the likely victim[6].
In a typical galaxy, type 1a supernovae occur only once every 250 years, hence scientists have to literally comb the cosmos with their telescopes looking for examples in far flung galaxies elsewhere. When one is detected, to find the distance to the galaxy that contains the supernova, scientists just have to compare how bright they know the explosion should be with how bright the explosion appears. Using the inverse square law, they can compute the distance to the supernova and thus to the supernova's home galaxy[7]:
* The inverse-square law derives from purely geometric considerations and generally applies when some force, energy, or other conserved quantity is radiated outward radially in three-dimensional space from a point source. Since the surface area of a sphere (which is 4π r2 ) is proportional to the square of the radius, as the emitted radiation gets farther from the source, it is spread out over an area that is increasing in proportion to the square of the distance from the source. Hence, the intensity of radiation passing through any unit area (directly facing the point source) is inversely proportional to the square of the distance from the point source.
Source: http://hyperphysics.phy-astr.gsu.edu/hbase/forces/isq.html
Since 1998, two teams of astronomers working separately – the Supernova Cosmology Project and the High-Z team – have been fortunate enough to identify nearly four dozen type 1a supernovae at various distances from earth, and after painstakingly determining the distances and recessional velocities of each, each group independently came to an unexpected conclusion for which the members of each team were awarded the Nobel Prize. An Australian, Brian Schmidt, was a leading member of the High-Z team. Both teams’ calculations revealed that ever since the universe was about 7 billion years old, its expansion rate has in fact been speeding up, and that something with negative mass and negative gravity - dark energy, the cosmological constant, something akin to Hubble’s constant, call it what you will – is the culprit[8]. (my emphasis)
For at least the last seven billion years, this repulsive push - whose strength does not change as matter spreads out - has gradually gained the upper hand, and the era of decelerated spatial expansion gave way to a new era of accelerated expansion. Taking the amount of matter, both visible and dark, to be about 28% of the critical density, the supernova researchers concluded that the accelerated expansion they had observed required an outward push of a cosmological constant whose dark energy contributes about 72 % of the universe’s critical density, and as space continues to expand at an ever increasing rate, the likely end result is that the universe will either die of heat death (with most galaxies racing away from one another at accelerating speeds until a final darkness descends on the universe as every star in every galaxy dies and all matter cools to absolute zero), or in a “Big Rip” if the rate of accelerated expansion is fast enough - but the good news is: not for another 30 to 40 billion years or so, by which time our sun, our planet and our species will long since have departed the scene, and not even be faded memories[9].
Meanwhile, what is happening in our own little corner of the cosmos?
Recently (2016), there have emerged two lines of research producing findings which, to my untrained eye at least, appear to be conflicting. One line of research, reproduced in an article in the Scientific American by Libeskind and Tully, appears to indicate that our local galaxy clusters are being drawn closer together[10]. Another, to be published in the Astrophysical Journal, opines that that they are being propelled further apart[11].
Things are moving together…
The Libeskind and Tully research has identified a number of galaxy clusters in our wider region which may be identified as “Local”. The Local Group itself may be defined as the galaxy group that includes the Milky Way, and comprises more than 54 galaxies, most of them dwarf galaxies. Spanning some seven million light years of space, its gravitational centre is located somewhere between the Milky Way and the Andromeda Galaxy[12]. Zooming out from the Milky Way we find the Small and Large Magellanic Clouds some 180,000 to 220,000 light-years away, and Andromeda, 2 ½ million light-years away, which is projected to collide with the Milky way in roughly 4 billion years.
The Local Group itself exists at the outskirts of the Virgo Cluster, a 50-million-light-year-distant cluster of more than 1,000 galaxies that is itself a small part of the Local Supercluster, a collection of hundreds of galaxy groups sprawled across more than 100 million light-years. The Local Supercluster is itself but one lobe of a much larger supercluster, a collection of 100,000 large galaxies stretching across 400 million light-years, recently named Laniakea, a Hawaiian word meaning “immeasurable heaven”.
Libeskind and Tully postulate that clusters of galaxies across a span of more than 400 million light-years have all moved together within a local “basin of attraction”, akin to water pooling at the lowest point of a landscape’s topography, and that local and flow measurements have shown that “the Local Group is falling along a 50-million-light-year-long filament of dark matter toward the Virgo Cluster, a gathering of more than 1,000 galaxies squeezed into a volume of 13 million light-years”. Were it not for the universe’s incessant expansion, these galaxies would eventually coalesce into one compact, gravitationally bound structure: the Laniakea supercluster.
The total density of visible and dark matter within Laniakea lends support to the view that, just as dark energy theorists thought, the universe is destined for a cold death of ever accelerating expansion. However, so far as the immediate cosmic landscape is concerned, the authors suggest that the Milky Way, Andromeda and four dozen other galaxies comprising the Local Group, form a region where gravity has won the battle against cosmic expansion and is undergoing collapse. The entire Local Group, including the Milky Way, is in fact hurrying towards “a mysterious concentration of mass” in the direction of Centaurus at more than 600 kilometres per second, and the greater region around the Virgo Cluster that extends to our location – the Local Supercluster - is moving towards Centaurus along with the Milky Way at hundreds of kilometres per second.
The mysterious mass pulling all these galaxies together is known as the Great Attractor, where the density of matter is high because it contains seven clusters comparable to the Virgo Cluster lying within a sphere 100 million light years wide including Norma, Centaurus and Hydra. (Less so beyond in the so-called Local Sheet where galaxies are flying apart at the rate of cosmic expansion with only small peculiar velocities caused by local interactions[13].
So what is causing our Local Supercluster's peculiar velocity of 600 kilometres per second? Various culprits have been identified, such as the Great Attractor itself, the Shapley Supercluster, and the peculiar velocities of another 8,000 galaxies that demonstrate a coherent flow toward the Shapley Supercluster, and perhaps more…
.. or are they rather moving apart?
Libeskind and Tully’s attractive scenario (in the gravitational sense) is to be contrasted with the recent research of an international team of 15 astrophysicists which has calculated the most accurate measurement yet for the expansion rate of the “nearby universe”, and have found that everything is moving apart much more quickly than previously thought. Using the Hubble Space Telescope, they measured the movements of 2,400 Cepheid variable stars[14] and 300 type 1a supernovae in 22 nearby galaxies which showed that the local universe is expanding at the rate of 73.2 kilometres a second for every megaparsec of distance. (A megaparsec is 3.26 million light years and is an accepted limit for the "local" universe). The Hubble constant for the whole universe, based on calculations from the remnant energy of the Big Bang, is 69.3 kilometres a second for every megaparsec of distance, according to NASA.
Parenthetically, Libeskind and Tully explain here in their article that a galaxy measured to be 3.25 million light years away should have a velocity of about 70 kilometres per second which accords with the above, but if instead the galaxy’s redshift yields a velocity of 60 kilometres per second, astronomers could infer that matter concentrations near that galaxy are giving it a peculiar velocity of 10 kilometres per second[15].
The higher Hubble constant for the local universe could indicate a lower density of matter, but it is not known why. It could be due to an unproved force called "dark radiation", which would be connected to dark matter and dark energy, a new force similar to dark energy, or a new particle ... or it could be that dark energy itself has changed over time. Or it could mean our assumptions about supernovae and Cepheid variables as the universe's standard candles could be wrong[16].
The astrophysical team’s research, which can be read on the arXiver site [17], will be published in the Astrophysical Journal. The discrepancy between the two group’s findings, if there is one, may be purely a matter of definition as to variant meanings of local and Local Group. I have perused the international team’s findings on the arXiver site and cannot see the term defined, although frequently used.
The measure of our insignificance
Local factors apart and looking at things on the grander scale, we, the inhabitants of this tiny planet and our planet itself have been demonstrated to be even more insignificant than we really thought. As Brian Greene ruefully notes, “If these ideas are right (dark matter and dark energy), they dramatically extend the Copernican revolution: not only are we not the centre of the universe, but the stuff of which we are made is like flotsam on the cosmic ocean. If protons, neutrons and electrons had been left out of the grand design, the total mass/energy of the universe would hardly have been diminished”[18].
Another consequence of dark energy, postulated by the Nobel laureate Steven Weinberg, is that we may live in a multiverse – one in which the cosmological constant is not uniquely determined from the basic laws of physics, but rather a random variable that assumes different values in different members of a huge ensemble of universes, in other words, a multiverse.[19]
But does dark energy really exist at all?
As in the case of dark matter, the concept of dark energy also has its sceptics. Recent cosmological observations have found that for a given redshift - the stretching of light waves by the expansion of light - distant supernova explosions appear dimmer than anticipated. This has led to another hypothesis: that perhaps expansion is decelerating, but at different rates at different places, and if our neighbourhood is emptier than other areas (in a ‘void’, as it were) it has less matter to retard the expansion and decelerates less quickly. As light from a supernova spreads out, it enters zones of increasingly rapid expansion having the same effect as cosmic acceleration, but without any need for dark energy.
The jury is still out on this. For one thing, it would appear to fly in the face of the CMBR which is uniform to one part in 100,000, not to mention the apparently uniform distribution of galaxies. However, this relic radiation of the big bang merely requires the universe to look nearly the same in every direction. If a void is roughly spherical and if we lie reasonably close to the centre, these observations do not necessarily preclude it. Observational tests may help distinguish between dark energy and the void models. The European Space Agency’s Planck spacecraft, launched in May 2009, and a variety of ground-based and balloon- borne instruments are mapping out the CMBR in even greater detail
The observation and measurement of additional supernova explosions will also help to pin down the expansion rate and check whether it varies with position, as a void model predicts[20]. If the density of the universe does in fact vary on large scales, and Earth lies at or near the centre of a relatively less dense region or “void”, perhaps our location may be a bit special after all[21].
… or, contrariwise, is the ISW effect (the cooling of light photons as they emerge from a supervoid) in fact a smoking gun for dark energy?[22]
Speculation about a vast supervoid containing relatively few galaxies had been entertained since 2007 and had come to nothing, but recently (2015) astronomers discovered one about 3 billion light years from earth which may coincide with a mysterious “cold spot” discovered in the CMBR about a decade ago. When some of the CMB light passed through the supervoid, so the theory goes, it lost not speed (it can’t because the speed of light is a constant), but energy via a process known as the integrated Sachs-Wolfe (ISW) effect, causing it to cool. This “cold spot” was first identified in data from NASA’s Wilkinson Anisotropy Probe (WMAP) in 2004.[23]
The universe is expanding, and, so the theory goes, as the light enters the void containing minimal matter the photons lose energy which is not totally regained upon exit so they emerge cooler. Temperature dips on the CMBR near low-density regions should therefore be evident, even though tiny. (On WMAP the warmer (more energetic) light appears red and the cooler (less energetic) light is blue). In the instance of a truly large void, the difference in temperature variations may be sufficient to identify a cold spot.
In their search for a supervoid that could explain the cold spot, astronomers analysed a catalogue of galaxies created by the Wide-field Infrared Survey Explorer (WISE) satellite, the Two Micron All Sky Survey (2MASS) and the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) which consists of astronomical cameras, telescopes and a computing facility that is surveying the sky for moving objects on a continual basis. Many galaxies were discovered and their redshift, a by-product of the universe’s expansion, identified. Remember, the greater the redshift, the further the galaxy lies from earth. Using this method, scientists were able to create a three-dimensional map of the density of galaxies in the region of the CMB’s cold spot, though the search was not entirely concentrated there.
More data is yet needed to determine if this supervoid is in fact responsible for the cooling of temperatures which has been identified. If a coincidence of these phenomena is identified, this may have something to say about the forces behind dark energy, that “putative kind of negative pressure throughout space”[24] that is counteracting the inward pull of gravity and driving the universe’s accelerated expansion, for if it could be demonstrated that the supervoid is the driving force behind the temperature anomaly, this may also “offer a smoking gun for dark energy because the ISW effect can only occur if dark energy is operating on the universe, accelerating its expansion”[25].
A new player on the scene: the Dark Energy Survey (DES)[26]
Since August 2013, a new player arrived on the scene in the ongoing quest to explain why the universe continues to expand at an ever faster pace: the Dark Energy Survey. The DES takes the form of a four meter diameter telescope at the Cerro Tololo Inter-American Observatory, a US facility high in the Andes Mountains of Northern Chile. Over 5 years, it aims to produce a deep high-resolution map of about 200 million galaxies spread over one-eighth of the sky as well as a catalogue of stellar explosions that can be used to track cosmic expansion, and in so doing to bring to fruition a thorough record of the history of cosmic expansion and the rate of growth of the vast conglomeration of galaxies spread across the universe with unprecedented precision.
In doing so, the DES aims to resolve two competing theories explaining the reasons for the Universe’s expansion, and help clarify the controversy concerning what dark matter actually is by investigating four phenomena that are particularly sensitive to whatever is pulling the universe apart: type 1 supernovae, signatures of primary sound waves, gravitational lensing (the bending of light by gravity) and the make-up of galaxy clusters.
The first of the two theories which would seek to explain the ongoing and accelerating expansion of the universe is sometimes referred to as ‘modified gravity’. In contradistinction to the gravitational attraction between bodies here on Earth and in the solar system, when it comes to ‘the vast distances of intergalactic space’ where there is little or no matter, it is conjectured that gravity instead acts as a repulsive force prompting the universe to expand at an ever increasing rate. The other theory is that it is this mysterious entity called dark matter that counteracts the operation of gravity as we know it, making objects repel instead of attract one another, which is driving the universe’s expansion.
The second issue revolves around the nature of dark energy itself, if indeed it exists at all. The first hypothesis is that it comprises the energy of empty space itself. As we have seen when considering the concept of a universe from nothing, according to quantum physics even empty space carries energy, which derives from virtual particles – particle and antiparticle, which spontaneously appear for a very brief instant, then mutually annihilate one another and disappear back into the vacuum. This process, so it is thought, produces the energy which is driving the universe’s expansion and causing it to speed up. But there is a problem, and that is that the amount of vacuum energy in space should be 120 orders of magnitude (10120) greater than what it seems to be if it responsible for dark energy.
The alternative is that dark energy takes the form of an undetected particle which could be a relation of the Higgs boson and would have the properties of the Higgs but would be 44 orders of magnitude lighter. This is sometimes referred to as ‘quintessence’ (see above) with both kinetic energy (deriving from its motion) and potential energy sufficient to drive cosmic expansion at its present rate.
In order to shed some light on these mysteries, the DES is utilising the four probes mentioned above:
Since its official start in August 2013, the DES has covered nearly 5,000 square degrees of sky, obtaining colour images of about 100 million galaxies, and the supernova probe has discovered more than 1,000 type 1a supernovae in the nearby and distant universe so far, nearly 100 times as many as were used in the 1998 discovery of cosmic acceleration.
Other telescopes are measuring how much the wavelength of the light from has been stretched by the expansion of the cosmos between the time it was emitted and now. Comparing the distances of supernovae with their redshift - a measurement of how fast they are receding from us - reveals how fast the universe was expanding at different epochs, and in this way the DES will be able to reconstruct with great precision the past 10 billion years of the expansion history of the universe.
Gravitational lensing is another arm of the probe. The DES will measure the slight distortions of many galaxies to create a map of how mass is spread throughout space, and the degree to which galaxies at different distances from us are gravitationally lensed will reveal the clumpiness of matter at different epochs of the universe. The evolution of this clumpinesss over time, reflecting as it does the competition between gravity and dark energy, can help tell us what is causing the universe to accelerate.
The signatures of primary sound waves are also being analysed and tens of thousands of clusters out to billions of light-years away examined and comparison made between the numbers of clusters seen nearby, corresponding to recent times, and those far away, corresponding to long ago, to learn how fast galaxies have clumped together over time, all in an effort to find the answers to these questions: is dark energy or modified gravity the culprit behind the universe’s expansion; and is vacuum energy or quintessence the key to understanding dark energy? The first phase of analysis should be complete in about a year (from November 2015).
Other technological innovations assisting in the clarification of these significant issues include the Rapid Response System (Pan-STARRS) (which suggest a value for w that is more negative than -1), and the DES Large Synoptic Survey Telescope (LSST) due to open around 2021.
* Header source: https://www.techexplorist.com/dark-matter-dark-energy-really-exist/8853/ Credit: André Maeder, UNIGE
[1] For some qualifications to the standard concept of type 1a supernovae, examples of some which exceed the Chandrasekhar limit, and others that are "ridiculously, anomalously dim", see Nadia Drake, "Type 1a supernovae: Why our Standard Candle isn't really Standard" (28 Aug 2014): http://phenomena.nationalgeographic.com/2014/08/28/type-1a-supernovas-cosmic-candle-mystery/
[1.1] Adam Hart-Davis, Science – the definitive visual guide, DK, London, 2009, 331.
[2] Noam I Libeskind and R Brent Tully, “Our place in the cosmos”, Scientific American, July 2016, 29 at 34.
[3] Joshua Frieman, "Seeing in the dark - the riddle of why space is expanding at an ever increasing rate", Scientific American, November 2015, 30 at 35.
[4] Libeskind and Tully, op cit, 35.
[5] John Matson ‘No star left behind – what fuels a white dwarf’s nuclear detonation’, Scientific American, December 2012, 10.
[6] Ibid.
[7] New Scientist, March 2012, 14; Also, in the same edition, “The far, far future of stars”, Donald Goldsmith, 26 at 32-33.
[8] MoCA Public Lecture Series, Monash Centre for Astrophysical Physics, 15 February 2011, Lecture by Brian P Schmidt, 2011 Noble Laureate for Physics; “Universe could end in Big Rip”, SMH 19 July 2012, with accompanying video from a public talk by Professor Schmidt at the Australian Astronomical Observatory, Sydney, on 18 July 2012. See also “From the Big Bang to the Big Rip”. SMH. 14 September 2010, an analysis of the WiggleZ survey into the Universe’s expansion since it was 7 billion years old being conducted at the Australian Astronomical Observatory at Coonabarabran, part of the SMH “Cosmic Sleuths” series.
[9] Adam Riess and Mario Livio say a trillion in their article "The puzzle of dark energy", Scientific American, March 2016, 30 at 34, but from our perspective the difference is largely academic.
[10] Noam I Libeskind and R Brent Tully, “Our place in the cosmos”, Scientific American, July 2016, 29 ff.
[11] Marcus Strom, “New Hubble’s constant shows local universe is expanding even faster than we thought”, Sydney Morning Herald, 3 June 2016.
[12] https://en.wikipedia.org/wiki/Local_Group
[13] Peculiar velocity is the difference between a galaxy’s motion from cosmic expansion and the motion from its local environment
[14] For short distances in space - within our galaxy or within our local group of nearby galaxies - astronomers use cepheid variables as standard candles. These young stars pulse with a brightness that tightly relates to the time between pulses. By observing the way the star pulses, astronomers can calculate its actual brightness: Hubble site: http://hubblesite.org/hubble_discoveries/dark_energy/de-type_ia_supernovae.php; http://www.atnf.csiro.au/outreach/education/senior/astrophysics/variable_cepheids.html
[15] Op cit, p 31.
[16] Strom, op cit.cold spot appears
[17] https://arxiv.org/pdf/1604.01424.pdf
[18] Greene (2005), 300-301.
[19] Riess and Livio, op cit, 34.
[20] For the material on the void hypothesis, see Timothy Clifton and Pedro G Ferreira, “Does dark energy really exist?” Scientific American, Special Collector’s Edition – Extreme Physics: Probing the Mysteries of the Cosmos, August 2013, 58-65. The article was originally written in 2009.
[21] Ibid at 59, 62.
[22] This is an abridgment of Istvan Szapudi's article "The emptiest place in Space", Scientific American, August 2016, 22-29.
[23] For a graphic of WMAP, see Hubble's constant: the age of the universe and the consequences of unbridled expansion The cold spot appears as the blue area close to the bottom edge of the graphic at about 4 pm. What is not apparent is why other much larger blue areas shown as existing elsewhere in the universe are not also regarded as "cold spots".
[24] Szapudi, op cit, 25.
[25] Ibid, 25-26.
[26] This is an abridged version of Joshua Frieman's article in the Scientific American referred to in footnote [3] above.
[27] Considered also in the page on dark matter.
Source: http://hyperphysics.phy-astr.gsu.edu/hbase/forces/isq.html
Since 1998, two teams of astronomers working separately – the Supernova Cosmology Project and the High-Z team – have been fortunate enough to identify nearly four dozen type 1a supernovae at various distances from earth, and after painstakingly determining the distances and recessional velocities of each, each group independently came to an unexpected conclusion for which the members of each team were awarded the Nobel Prize. An Australian, Brian Schmidt, was a leading member of the High-Z team. Both teams’ calculations revealed that ever since the universe was about 7 billion years old, its expansion rate has in fact been speeding up, and that something with negative mass and negative gravity - dark energy, the cosmological constant, something akin to Hubble’s constant, call it what you will – is the culprit[8]. (my emphasis)
For at least the last seven billion years, this repulsive push - whose strength does not change as matter spreads out - has gradually gained the upper hand, and the era of decelerated spatial expansion gave way to a new era of accelerated expansion. Taking the amount of matter, both visible and dark, to be about 28% of the critical density, the supernova researchers concluded that the accelerated expansion they had observed required an outward push of a cosmological constant whose dark energy contributes about 72 % of the universe’s critical density, and as space continues to expand at an ever increasing rate, the likely end result is that the universe will either die of heat death (with most galaxies racing away from one another at accelerating speeds until a final darkness descends on the universe as every star in every galaxy dies and all matter cools to absolute zero), or in a “Big Rip” if the rate of accelerated expansion is fast enough - but the good news is: not for another 30 to 40 billion years or so, by which time our sun, our planet and our species will long since have departed the scene, and not even be faded memories[9].
Meanwhile, what is happening in our own little corner of the cosmos?
Recently (2016), there have emerged two lines of research producing findings which, to my untrained eye at least, appear to be conflicting. One line of research, reproduced in an article in the Scientific American by Libeskind and Tully, appears to indicate that our local galaxy clusters are being drawn closer together[10]. Another, to be published in the Astrophysical Journal, opines that that they are being propelled further apart[11].
Things are moving together…
The Libeskind and Tully research has identified a number of galaxy clusters in our wider region which may be identified as “Local”. The Local Group itself may be defined as the galaxy group that includes the Milky Way, and comprises more than 54 galaxies, most of them dwarf galaxies. Spanning some seven million light years of space, its gravitational centre is located somewhere between the Milky Way and the Andromeda Galaxy[12]. Zooming out from the Milky Way we find the Small and Large Magellanic Clouds some 180,000 to 220,000 light-years away, and Andromeda, 2 ½ million light-years away, which is projected to collide with the Milky way in roughly 4 billion years.
The Local Group itself exists at the outskirts of the Virgo Cluster, a 50-million-light-year-distant cluster of more than 1,000 galaxies that is itself a small part of the Local Supercluster, a collection of hundreds of galaxy groups sprawled across more than 100 million light-years. The Local Supercluster is itself but one lobe of a much larger supercluster, a collection of 100,000 large galaxies stretching across 400 million light-years, recently named Laniakea, a Hawaiian word meaning “immeasurable heaven”.
Libeskind and Tully postulate that clusters of galaxies across a span of more than 400 million light-years have all moved together within a local “basin of attraction”, akin to water pooling at the lowest point of a landscape’s topography, and that local and flow measurements have shown that “the Local Group is falling along a 50-million-light-year-long filament of dark matter toward the Virgo Cluster, a gathering of more than 1,000 galaxies squeezed into a volume of 13 million light-years”. Were it not for the universe’s incessant expansion, these galaxies would eventually coalesce into one compact, gravitationally bound structure: the Laniakea supercluster.
The total density of visible and dark matter within Laniakea lends support to the view that, just as dark energy theorists thought, the universe is destined for a cold death of ever accelerating expansion. However, so far as the immediate cosmic landscape is concerned, the authors suggest that the Milky Way, Andromeda and four dozen other galaxies comprising the Local Group, form a region where gravity has won the battle against cosmic expansion and is undergoing collapse. The entire Local Group, including the Milky Way, is in fact hurrying towards “a mysterious concentration of mass” in the direction of Centaurus at more than 600 kilometres per second, and the greater region around the Virgo Cluster that extends to our location – the Local Supercluster - is moving towards Centaurus along with the Milky Way at hundreds of kilometres per second.
The mysterious mass pulling all these galaxies together is known as the Great Attractor, where the density of matter is high because it contains seven clusters comparable to the Virgo Cluster lying within a sphere 100 million light years wide including Norma, Centaurus and Hydra. (Less so beyond in the so-called Local Sheet where galaxies are flying apart at the rate of cosmic expansion with only small peculiar velocities caused by local interactions[13].
So what is causing our Local Supercluster's peculiar velocity of 600 kilometres per second? Various culprits have been identified, such as the Great Attractor itself, the Shapley Supercluster, and the peculiar velocities of another 8,000 galaxies that demonstrate a coherent flow toward the Shapley Supercluster, and perhaps more…
.. or are they rather moving apart?
Libeskind and Tully’s attractive scenario (in the gravitational sense) is to be contrasted with the recent research of an international team of 15 astrophysicists which has calculated the most accurate measurement yet for the expansion rate of the “nearby universe”, and have found that everything is moving apart much more quickly than previously thought. Using the Hubble Space Telescope, they measured the movements of 2,400 Cepheid variable stars[14] and 300 type 1a supernovae in 22 nearby galaxies which showed that the local universe is expanding at the rate of 73.2 kilometres a second for every megaparsec of distance. (A megaparsec is 3.26 million light years and is an accepted limit for the "local" universe). The Hubble constant for the whole universe, based on calculations from the remnant energy of the Big Bang, is 69.3 kilometres a second for every megaparsec of distance, according to NASA.
Parenthetically, Libeskind and Tully explain here in their article that a galaxy measured to be 3.25 million light years away should have a velocity of about 70 kilometres per second which accords with the above, but if instead the galaxy’s redshift yields a velocity of 60 kilometres per second, astronomers could infer that matter concentrations near that galaxy are giving it a peculiar velocity of 10 kilometres per second[15].
The higher Hubble constant for the local universe could indicate a lower density of matter, but it is not known why. It could be due to an unproved force called "dark radiation", which would be connected to dark matter and dark energy, a new force similar to dark energy, or a new particle ... or it could be that dark energy itself has changed over time. Or it could mean our assumptions about supernovae and Cepheid variables as the universe's standard candles could be wrong[16].
The astrophysical team’s research, which can be read on the arXiver site [17], will be published in the Astrophysical Journal. The discrepancy between the two group’s findings, if there is one, may be purely a matter of definition as to variant meanings of local and Local Group. I have perused the international team’s findings on the arXiver site and cannot see the term defined, although frequently used.
The measure of our insignificance
Local factors apart and looking at things on the grander scale, we, the inhabitants of this tiny planet and our planet itself have been demonstrated to be even more insignificant than we really thought. As Brian Greene ruefully notes, “If these ideas are right (dark matter and dark energy), they dramatically extend the Copernican revolution: not only are we not the centre of the universe, but the stuff of which we are made is like flotsam on the cosmic ocean. If protons, neutrons and electrons had been left out of the grand design, the total mass/energy of the universe would hardly have been diminished”[18].
Another consequence of dark energy, postulated by the Nobel laureate Steven Weinberg, is that we may live in a multiverse – one in which the cosmological constant is not uniquely determined from the basic laws of physics, but rather a random variable that assumes different values in different members of a huge ensemble of universes, in other words, a multiverse.[19]
But does dark energy really exist at all?
As in the case of dark matter, the concept of dark energy also has its sceptics. Recent cosmological observations have found that for a given redshift - the stretching of light waves by the expansion of light - distant supernova explosions appear dimmer than anticipated. This has led to another hypothesis: that perhaps expansion is decelerating, but at different rates at different places, and if our neighbourhood is emptier than other areas (in a ‘void’, as it were) it has less matter to retard the expansion and decelerates less quickly. As light from a supernova spreads out, it enters zones of increasingly rapid expansion having the same effect as cosmic acceleration, but without any need for dark energy.
The jury is still out on this. For one thing, it would appear to fly in the face of the CMBR which is uniform to one part in 100,000, not to mention the apparently uniform distribution of galaxies. However, this relic radiation of the big bang merely requires the universe to look nearly the same in every direction. If a void is roughly spherical and if we lie reasonably close to the centre, these observations do not necessarily preclude it. Observational tests may help distinguish between dark energy and the void models. The European Space Agency’s Planck spacecraft, launched in May 2009, and a variety of ground-based and balloon- borne instruments are mapping out the CMBR in even greater detail
The observation and measurement of additional supernova explosions will also help to pin down the expansion rate and check whether it varies with position, as a void model predicts[20]. If the density of the universe does in fact vary on large scales, and Earth lies at or near the centre of a relatively less dense region or “void”, perhaps our location may be a bit special after all[21].
… or, contrariwise, is the ISW effect (the cooling of light photons as they emerge from a supervoid) in fact a smoking gun for dark energy?[22]
Speculation about a vast supervoid containing relatively few galaxies had been entertained since 2007 and had come to nothing, but recently (2015) astronomers discovered one about 3 billion light years from earth which may coincide with a mysterious “cold spot” discovered in the CMBR about a decade ago. When some of the CMB light passed through the supervoid, so the theory goes, it lost not speed (it can’t because the speed of light is a constant), but energy via a process known as the integrated Sachs-Wolfe (ISW) effect, causing it to cool. This “cold spot” was first identified in data from NASA’s Wilkinson Anisotropy Probe (WMAP) in 2004.[23]
The universe is expanding, and, so the theory goes, as the light enters the void containing minimal matter the photons lose energy which is not totally regained upon exit so they emerge cooler. Temperature dips on the CMBR near low-density regions should therefore be evident, even though tiny. (On WMAP the warmer (more energetic) light appears red and the cooler (less energetic) light is blue). In the instance of a truly large void, the difference in temperature variations may be sufficient to identify a cold spot.
In their search for a supervoid that could explain the cold spot, astronomers analysed a catalogue of galaxies created by the Wide-field Infrared Survey Explorer (WISE) satellite, the Two Micron All Sky Survey (2MASS) and the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) which consists of astronomical cameras, telescopes and a computing facility that is surveying the sky for moving objects on a continual basis. Many galaxies were discovered and their redshift, a by-product of the universe’s expansion, identified. Remember, the greater the redshift, the further the galaxy lies from earth. Using this method, scientists were able to create a three-dimensional map of the density of galaxies in the region of the CMB’s cold spot, though the search was not entirely concentrated there.
More data is yet needed to determine if this supervoid is in fact responsible for the cooling of temperatures which has been identified. If a coincidence of these phenomena is identified, this may have something to say about the forces behind dark energy, that “putative kind of negative pressure throughout space”[24] that is counteracting the inward pull of gravity and driving the universe’s accelerated expansion, for if it could be demonstrated that the supervoid is the driving force behind the temperature anomaly, this may also “offer a smoking gun for dark energy because the ISW effect can only occur if dark energy is operating on the universe, accelerating its expansion”[25].
A new player on the scene: the Dark Energy Survey (DES)[26]
Since August 2013, a new player arrived on the scene in the ongoing quest to explain why the universe continues to expand at an ever faster pace: the Dark Energy Survey. The DES takes the form of a four meter diameter telescope at the Cerro Tololo Inter-American Observatory, a US facility high in the Andes Mountains of Northern Chile. Over 5 years, it aims to produce a deep high-resolution map of about 200 million galaxies spread over one-eighth of the sky as well as a catalogue of stellar explosions that can be used to track cosmic expansion, and in so doing to bring to fruition a thorough record of the history of cosmic expansion and the rate of growth of the vast conglomeration of galaxies spread across the universe with unprecedented precision.
In doing so, the DES aims to resolve two competing theories explaining the reasons for the Universe’s expansion, and help clarify the controversy concerning what dark matter actually is by investigating four phenomena that are particularly sensitive to whatever is pulling the universe apart: type 1 supernovae, signatures of primary sound waves, gravitational lensing (the bending of light by gravity) and the make-up of galaxy clusters.
The first of the two theories which would seek to explain the ongoing and accelerating expansion of the universe is sometimes referred to as ‘modified gravity’. In contradistinction to the gravitational attraction between bodies here on Earth and in the solar system, when it comes to ‘the vast distances of intergalactic space’ where there is little or no matter, it is conjectured that gravity instead acts as a repulsive force prompting the universe to expand at an ever increasing rate. The other theory is that it is this mysterious entity called dark matter that counteracts the operation of gravity as we know it, making objects repel instead of attract one another, which is driving the universe’s expansion.
The second issue revolves around the nature of dark energy itself, if indeed it exists at all. The first hypothesis is that it comprises the energy of empty space itself. As we have seen when considering the concept of a universe from nothing, according to quantum physics even empty space carries energy, which derives from virtual particles – particle and antiparticle, which spontaneously appear for a very brief instant, then mutually annihilate one another and disappear back into the vacuum. This process, so it is thought, produces the energy which is driving the universe’s expansion and causing it to speed up. But there is a problem, and that is that the amount of vacuum energy in space should be 120 orders of magnitude (10120) greater than what it seems to be if it responsible for dark energy.
The alternative is that dark energy takes the form of an undetected particle which could be a relation of the Higgs boson and would have the properties of the Higgs but would be 44 orders of magnitude lighter. This is sometimes referred to as ‘quintessence’ (see above) with both kinetic energy (deriving from its motion) and potential energy sufficient to drive cosmic expansion at its present rate.
In order to shed some light on these mysteries, the DES is utilising the four probes mentioned above:
- type 1 supernovae;
- the signatures of primary sound waves which travelled through space in the early universe at nearly the speed of light until the cosmos had cooled enough for atoms to form, a period of time corresponding to 480 million light years today;
- gravitational lensing (the bending of light by gravity around large spatial entities such as galaxies and galaxy clusters)[27];
- and the make-up of galaxy clusters themselves.
Since its official start in August 2013, the DES has covered nearly 5,000 square degrees of sky, obtaining colour images of about 100 million galaxies, and the supernova probe has discovered more than 1,000 type 1a supernovae in the nearby and distant universe so far, nearly 100 times as many as were used in the 1998 discovery of cosmic acceleration.
Other telescopes are measuring how much the wavelength of the light from has been stretched by the expansion of the cosmos between the time it was emitted and now. Comparing the distances of supernovae with their redshift - a measurement of how fast they are receding from us - reveals how fast the universe was expanding at different epochs, and in this way the DES will be able to reconstruct with great precision the past 10 billion years of the expansion history of the universe.
Gravitational lensing is another arm of the probe. The DES will measure the slight distortions of many galaxies to create a map of how mass is spread throughout space, and the degree to which galaxies at different distances from us are gravitationally lensed will reveal the clumpiness of matter at different epochs of the universe. The evolution of this clumpinesss over time, reflecting as it does the competition between gravity and dark energy, can help tell us what is causing the universe to accelerate.
The signatures of primary sound waves are also being analysed and tens of thousands of clusters out to billions of light-years away examined and comparison made between the numbers of clusters seen nearby, corresponding to recent times, and those far away, corresponding to long ago, to learn how fast galaxies have clumped together over time, all in an effort to find the answers to these questions: is dark energy or modified gravity the culprit behind the universe’s expansion; and is vacuum energy or quintessence the key to understanding dark energy? The first phase of analysis should be complete in about a year (from November 2015).
Other technological innovations assisting in the clarification of these significant issues include the Rapid Response System (Pan-STARRS) (which suggest a value for w that is more negative than -1), and the DES Large Synoptic Survey Telescope (LSST) due to open around 2021.
* Header source: https://www.techexplorist.com/dark-matter-dark-energy-really-exist/8853/ Credit: André Maeder, UNIGE
[1] For some qualifications to the standard concept of type 1a supernovae, examples of some which exceed the Chandrasekhar limit, and others that are "ridiculously, anomalously dim", see Nadia Drake, "Type 1a supernovae: Why our Standard Candle isn't really Standard" (28 Aug 2014): http://phenomena.nationalgeographic.com/2014/08/28/type-1a-supernovas-cosmic-candle-mystery/
[1.1] Adam Hart-Davis, Science – the definitive visual guide, DK, London, 2009, 331.
[2] Noam I Libeskind and R Brent Tully, “Our place in the cosmos”, Scientific American, July 2016, 29 at 34.
[3] Joshua Frieman, "Seeing in the dark - the riddle of why space is expanding at an ever increasing rate", Scientific American, November 2015, 30 at 35.
[4] Libeskind and Tully, op cit, 35.
[5] John Matson ‘No star left behind – what fuels a white dwarf’s nuclear detonation’, Scientific American, December 2012, 10.
[6] Ibid.
[7] New Scientist, March 2012, 14; Also, in the same edition, “The far, far future of stars”, Donald Goldsmith, 26 at 32-33.
[8] MoCA Public Lecture Series, Monash Centre for Astrophysical Physics, 15 February 2011, Lecture by Brian P Schmidt, 2011 Noble Laureate for Physics; “Universe could end in Big Rip”, SMH 19 July 2012, with accompanying video from a public talk by Professor Schmidt at the Australian Astronomical Observatory, Sydney, on 18 July 2012. See also “From the Big Bang to the Big Rip”. SMH. 14 September 2010, an analysis of the WiggleZ survey into the Universe’s expansion since it was 7 billion years old being conducted at the Australian Astronomical Observatory at Coonabarabran, part of the SMH “Cosmic Sleuths” series.
[9] Adam Riess and Mario Livio say a trillion in their article "The puzzle of dark energy", Scientific American, March 2016, 30 at 34, but from our perspective the difference is largely academic.
[10] Noam I Libeskind and R Brent Tully, “Our place in the cosmos”, Scientific American, July 2016, 29 ff.
[11] Marcus Strom, “New Hubble’s constant shows local universe is expanding even faster than we thought”, Sydney Morning Herald, 3 June 2016.
[12] https://en.wikipedia.org/wiki/Local_Group
[13] Peculiar velocity is the difference between a galaxy’s motion from cosmic expansion and the motion from its local environment
[14] For short distances in space - within our galaxy or within our local group of nearby galaxies - astronomers use cepheid variables as standard candles. These young stars pulse with a brightness that tightly relates to the time between pulses. By observing the way the star pulses, astronomers can calculate its actual brightness: Hubble site: http://hubblesite.org/hubble_discoveries/dark_energy/de-type_ia_supernovae.php; http://www.atnf.csiro.au/outreach/education/senior/astrophysics/variable_cepheids.html
[15] Op cit, p 31.
[16] Strom, op cit.cold spot appears
[17] https://arxiv.org/pdf/1604.01424.pdf
[18] Greene (2005), 300-301.
[19] Riess and Livio, op cit, 34.
[20] For the material on the void hypothesis, see Timothy Clifton and Pedro G Ferreira, “Does dark energy really exist?” Scientific American, Special Collector’s Edition – Extreme Physics: Probing the Mysteries of the Cosmos, August 2013, 58-65. The article was originally written in 2009.
[21] Ibid at 59, 62.
[22] This is an abridgment of Istvan Szapudi's article "The emptiest place in Space", Scientific American, August 2016, 22-29.
[23] For a graphic of WMAP, see Hubble's constant: the age of the universe and the consequences of unbridled expansion The cold spot appears as the blue area close to the bottom edge of the graphic at about 4 pm. What is not apparent is why other much larger blue areas shown as existing elsewhere in the universe are not also regarded as "cold spots".
[24] Szapudi, op cit, 25.
[25] Ibid, 25-26.
[26] This is an abridged version of Joshua Frieman's article in the Scientific American referred to in footnote [3] above.
[27] Considered also in the page on dark matter.