Speculative evolution

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Speculative_evolution 

 

Surviving dinosaurs and Mesozoic creatures are a common theme in alternative evolution. One example is the 2001-2005 Speculative Dinosaur Project and its invention of many speculative animals.

Speculative evolution is a genre of speculative fiction and an artistic movement focused on hypothetical scenarios in the evolution of life, and a significant form of fictional biology. It is also known as speculative biology and it is referred to as speculative zoology in regards to hypothetical animals. Works incorporating speculative evolution may have entirely conceptual species that evolve on a planet other than Earth, or they may be an alternate history focused on an alternate evolution of terrestrial life. Speculative evolution is often considered hard science fiction because of its strong connection to and basis in science, particularly biology.

Speculative evolution is a long-standing trope within science fiction, often recognized as beginning as such with H. G. Wells's 1895 novel The Time Machine, which featured several imaginary future creatures. Although small-scale speculative faunas were a hallmark of science fiction throughout the 20th century, ideas were only rarely well-developed, with some exceptions such as Edgar Rice Burroughs's Barsoom, a fictional rendition of Mars and its ecosystem published through novels from 1912 to 1941, and Gerolf Steiner's Rhinogradentia, a fictional order of mammals created in 1957.

The modern speculative evolution movement is generally agreed to have begun with the publication of Dougal Dixon's 1981 book After Man, which explored a fully realized future Earth with a complete ecosystem of over a hundred hypothetical animals. The success of After Man spawned several "sequels" by Dixon, focusing on different alternate and future scenarios. Dixon's work, like most similar works that came after them, were created with real biological principles in mind and were aimed at exploring real life processes, such as evolution and climate change, through the use of fictional examples.

Speculative evolution's possible use as an educational and scientific tool has been noted and discussed through the decades following the publication of After Man. Speculative evolution can be useful in exploring and showcasing patterns present in the present and in the past. By extrapolating past trends into the future, scientists can research and predict the most likely scenarios of how certain organisms and lineages could respond to ecological changes. In some cases, creatures first imagined within speculative evolution have since been discovered, such as an imaginary filter-feeding anomalocarid illustrated by artist John Meszaros in the 2012 book All Yesterdays by John Conway, C. M. Kosemen and Darren Naish being proven as having existed through fossils discovered in 2014 of the real anomalocarid Tamisiocaris.

History

Early works

The Time Machine (1895) by H. G. Wells is seen by some as an early instance of speculative evolution and has been cited as an inspiration by later creators within the field.

Explorations of hypothetical worlds featuring future, alternate or alien lifeforms is a long-standing trope in science fiction. One of the earliest works usually recognized as representing one of speculative evolution is H. G. Wells's science fiction novel The Time Machine, published in 1895. The Time Machine, set over eight hundred thousand years in the future, features post-human descendants in the form of the beautiful but weak Eloi and the brutish Morlocks. Further into the future, the protagonist of the book finds large crab-monsters and huge butterflies. Science fiction authors who wrote after Wells often used fictional creatures in the same vein, but most such imaginary faunas were small and not very developed.

A four-armed "Green Martian" riding a "throat" from Edgar Rice Burroughs's Barsoom, a fictional version of the planet Mars. Illustration by James Allen St. John (1920).

Edgar Rice Burroughs, who wrote in the early 20th century, can like Wells be considered an early speculative evolution author. Although his fictional ecosystems were still relatively small in scope, they were the settings of many of his novels and as such quite well-developed. In particular, Burroughs's Barsoom, a fictional version of the planet Mars which appeared in ten novels published from 1912 to 1941, featured a Martian ecosystem with a variety of alien creatures and several distinct Martian cultures and ethnic groups.

A mock taxidermy of a rhinograde, using its nasorium to catch fish. Rhinogrades, created by Gerolf Steiner in 1957, are one of the earliest concrete examples of speculative zoology.

In 1930, Olaf Stapledon published a "future history", Last and First Men: A Story of the Near and Far Future, describing the history of humanity from the present onwards, across two billion years and eighteen human species, of which Homo sapiens is the first. The book anticipates the science of genetic engineering, and is an early instance of the fictional group mind idea. Published in 1957, German zoologist Gerolf Steiner's book Bau und Leben der Rhinogradentia (translated into English as The Snouters: The Form and Life of the Rhinogrades) described the fictional evolution, biology and behavior of an imaginary order of mammals, the Rhinogradentia or "rhinogrades". The Rhinogrades are characterized by a nose-like feature called a "nasorium", the form and function of which vary significantly between species, akin to Darwin's finches and their beak specialization. This diverse group of fictional animals inhabits a series of islands in which they have gradually evolved, radiating into most ecological niches. Satirical papers have been published continuing Steiner's imagined world. Although the work does feature an entire speculative ecosystem, its impact is dwarfed by the later works due to its limited scope, only exploring the life of an island archipelago.

In 1976, the Italian author and illustrator Leo Lionni published Parallel Botany, a "field guide to imaginary plants", presented with academic-style mentions of genuine people and places. Parallel Botany has been compared to the 1972 book Invisible Cities by Italo Calvino, in which Marco Polo in a dialogue with Kublai Khan describes 55 cities, which, like Lionni's "parallel" plants, are "only as real as the mind's ability to conceptualize them".

Movement

Author Dougal Dixon with a model of a "Strida", one of the creatures featured in his 2010 book Greenworld.

One of the significant "founding" works of speculative evolution is After Man by Dougal Dixon, published in 1981. To this day, After Man is recognized as the first truly large-scale speculative evolution project involving a whole world and a vast array of species. Furthering its significance is the fact that the book was made very accessible by being published by mainstream publishers and being fully illustrated with color images. As such, After Man is often seen as having firmly established the idea of creating entire speculative worlds. Through the decades following After Man's publication, Dixon remained one of the sole authors of speculative evolution, publishing two more books in the same vein as After Man; The New Dinosaurs in 1988 and Man After Man in 1990. Dixon cited The Time Machine as his primary inspiration, being unaware of Steiner's work, and devised After Man as a popular-level book on the processes of evolution that instead of using the past to tell the story projected the processes into the future. A central idea of After Man, besides a wave of extinction following humans, is convergent evolution as new species bear a close resemblance to their unrelated predecessors.

When designing the various animals of the book, Dixon looked at the different types of biomes on the planet and what adaptations animals living there have, designing new animals descended from modern day ones with the same set of adaptations. The success of After Man inspired Dixon to continue writing books that explained factual scientific processes through fictional examples. The New Dinosaurs was in essence a book about zoogeography, something the general public would be unfamiliar with, using a world in which the non-avian dinosaurs had not gone extinct. Man After Man, explored climate change over the course of the next few million years by showcasing its effects through the eyes of future human descendants.

Today, many artists and writers work on speculative evolution projects online, often in the same vein as Dixon's works. Speculative evolution continues to endure a somewhat mainstream presence through TV shows featuring hypothetical and imaginary creatures, such as The Future is Wild (2002), Primeval (2007–2011) and Terra Nova (2011) and films such as Avatar (2009) and After Earth (2013). The modern explosion of speculative evolution has been termed by British paleontologist Darren Naish as the "Speculative Zoology Movement".

As an educational and scientific tool

Reconstruction of Tamisiocaris (top), an anomalocarid from the Cambrian which was discovered to have been a filter-feeder in 2014. A hypothetical filter-feeding anomalocarid was featured in the book All Yesterdays (2012).

Although primarily characterized as entertainment, speculative evolution can be used as educational tool to explain and illustrate real natural processes through using fictional and imaginary examples. The worlds created are often built on ecological and biological principles inferred from the real evolutionary history of life on Earth and readers can learn from them as such. For example, all of Dixon's speculative works are aimed at exploring real processes, with After Man exploring evolution, The New Dinosaurs zoogeography and both Man After Man and Greenworld (2010) exploring climate change, offering an environmental message.

In some cases, speculative evolution artists have successfully predicted the existence of organisms that were later discovered to resemble something real. Many of the animals featured in Dixon's After Man are still considered plausible ideas, with some of them (such as specialized rodents and semi-aquatic primates) being reinforced with recent biology studies. A creature dubbed "Ceticaris", conceived by artist John Meszaros as a filter-feeding anomalocarid, was published in the 2012 book All Yesterdays and two years later, in 2014, the actual Cambrian anomalocarid Tamisiocaris was discovered to have been a filter-feeder. In honor of Meszaros's prediction, Tamisiocaris was included in a new clade named the Cetiocaridae.

Dougal Dixon's The New Dinosaurs was heavily influenced by paleontological ideas developing during its time, such as the ongoing dinosaur renaissance, and as such many of the dinosaurs in the book are energetic and active creatures rather than sluggish and lumbering. Dixon extrapolated on the ideas of paleontologists such as Robert Bakker and Gregory S. Paul when creating his creatures and also used patterns seen in the actual evolutionary history of the dinosaurs and pushing them to an extreme. Perhaps because of this, many of the animals in the book are similar to actual Mesozoic animals that were later discovered. Many of the dinosaurs in it are feathered, something not widely accepted at the time of its publication but seen as likely today. Similarly, After Man in 1981 represents a sort of time capsule of geological thought before global warming was fully discerned, but Dixon also portrays a sixth mass extinction or Anthropocene before it was commonplace to do so.

Hypothetical restoration of Dromaeosauroides bornholmensis, which is known from two teeth. Its appearance is inferred from related genera.

 

Speculative reconstruction of Sinopliosaurus fusuiensis with generalized spinosaurid morphology, and unique coloration pattern.

Speculative evolution can be useful in exploring and showcasing patterns present in the present and in the past, and there is a useful aspect to hypothesizing on the form of future and alien life. By extrapolating past trends into the future, scientists could research and predict the most likely scenarios of how certain organisms and lineages could respond to ecological changes. As such, speculative evolution facilitates authors and artists to develop realistic hypotheses of the future. In some scientific fields, speculation is essential in understanding what is being studied. Paleontologists apply their own understanding of natural processes and biology to understand the appearances and lifestyles of extinct organisms that are discovered, varying in how far their speculation goes. For instance, All Yesterdays and its sequel All Your Yesterdays (2017) explores highly speculative renditions of real (and in some cases hypothetical) prehistoric animals that do not explicitly contradict any of the recovered fossil material. The speculation undertaken for All Yesterdays and its sequel has been compared to that of Dixon's speculative evolution works, though its objective was to challenge modern conservative perceptions and ideas of how dinosaurs and other prehistoric creatures lived, rather than designing whole new ecosystems. The books have inspired a modern artistic movement of artists going beyond conventional paleoart tropes, expanding into increasingly speculative renditions of prehistoric life.

Additionally, the evolutionary history of fictional organisms has been used as a tool in biology education. Caminalcules, named after Joseph H. Camin, are a group of animal-like lifeforms, consisting of 77 purported extant and fossil species that were invented as a tool for understanding phylogenetics. The classification of Caminalcules, as well as other fictional creatures like dragons and aliens, have been used as analogies to teach concepts in evolution and systematics.

Speculative evolution is sometimes presented in museum exhibitions. For instance, both After Man and The Future is Wild has been presented in exhibition form, educating museum visitors on the principles of biology and evolution through using their own fictional future creatures.

Subsets

Alien life

See also: Astrobiology, Xenobiology, and Extraterrestrials in fiction

 

The "Hellfire wasp", a wasp-like alien creature designed for James Cameron's film Avatar (2009).

A popular subset of speculative evolution is the exploration of possible realistic extraterrestrial life and ecosystems. Speculative evolution writings focusing on extraterrestrial life, like the blog Furahan Biology, use realistic scientific principles to describe the biomechanics of hypothetical alien life. Although commonly identified with terms such as "astrobiology", "xenobiology" or "exobiology", these terms designate actual scientific fields largely unrelated to speculative evolution. Though 20th century work in exobiology sometimes formulated "audacious" ideas about extraterrestrial forms of life. Astrophysicists Carl Sagan and Edwin Salpeter speculated that a "hunters, floaters and sinkers" ecosystem could populate the atmospheres of gas giant planets like Jupiter, and scientifically described it in a 1976 paper.

In extraterrestrial-focused speculative biology, lifeforms are often designed with the intention to populate planets wildly different from Earth, and in such cases concerns like chemistry, astronomy and the laws of physics become just as important to consider as the usual biological principles. Very exotic environments of physical extremes may be explored in such scenarios. For example, Robert Forward's 1980 Dragon's Egg develops a tale of life on a neutron star, and the resulting high-gravity, high-energy environment with an atmosphere of iron vapor and mountains 5-100 millimeters high. Once the star cools down and stable chemistry develops, life evolves extremely quickly, and Forward imagines a civilization of "cheela" that lives a million times faster than humans.

In some cases, artists and writers exploring possible alien life conjure similar ideas independent of each other, often attributed to studying the same biological processes and ideas. Such occasions can be called "convergent speculation", similar to the scientific idea of convergent evolution.

Perhaps the most famous speculative work on a hypothetical alien ecosystem is Wayne Barlowe's 1990 book Expedition, which explores the fictional planet Darwin IV. Expedition was written as a report of a 24th-century expedition that had been led to the planet by a team composed of both humans and intelligent aliens and used paintings and descriptive texts to create and describe a fully realized extraterrestrial ecosystem. Barlowe later served as an executive producer of a TV adaptation of the book, Alien Planet (2005) where exploration of Darwin IV is instead carried out by robotic probes and the segments detailing the ecosystems of the planet are intercut with interviews with scientists, such as Michio Kaku, Jack Horner and James B. Garvin.

Other examples of speculative evolution focused on extraterrestrial life include Dougal Dixon's 2010 book Greenworld, TV programmes such as 1997 the BBC2/Discovery Channel special Natural History of an Alien and the 2005 Channel 4/National Geographic programme Extraterrestrial as well as a variety of personal web-based artistic projects, such as C. M. Kosemen's "Snaiad" and Gert van Dijk's "Furaha", envisioning the biosphere of entire alien worlds.

Through science fiction, the speculative biology of extraterrestrial organisms has a strong presence in popular culture. The eponymous monster of Alien (1979), particularly its life cycle from egg to parasitoid larva to 'Xenomorph', is thought to be based on the real habits of parasitoid wasps in biology. Further, H. R. Giger's design of the Alien incorporated the features of insects, echinoderms and fossil crinoids, while concept artist John Cobb suggested acid blood as a biological defense mechanism. James Cameron's 2009 film Avatar constructed a fictional biosphere full of original, speculative alien species; a team of experts ensured that the lifeforms were scientifically plausible. The creatures of the movie took inspiration from Earth species as diverse as pterosaurs, microraptors, great white sharks, and panthers, and combined their traits to create an alien world.

Alternative evolution

Speculative zoology can examine sometimes overlooked prehistoric animals in an evolutionary context. The Speculative Dinosaur Project focused as much on mammals, squamates, and crocodylomorphs as on dinosaurs. Pictured are metatherian marsupials that have converged on our world's mustelids.

Similar to alternate history, alternative evolution is the exploration of possible alternate scenarios that could have played out in the Earth's past to give rise to alternate lifeforms and ecosystems, popularly the survival of non-avian dinosaurs to the present day. As humanity is often not a part of the worlds envisioned through alternative evolution, it has sometimes been characterized as non-anthropocentric.

Although dinosaurs surviving to the age of humans has been adapted as a plot point in numerous science fiction stories since at least 1912, beginning with Arthur Conan Doyle's The Lost World, the idea of exploring the fully fledged alternate ecosystems that would develop in such a scenario truly began with the publication of Dixon's The New Dinosaurs in 1988, in which dinosaurs were not some lone stragglers of known species that had survived more or less unchanged for the last 66 million years, but diverse animals that had continued to evolve beyond the Cretaceous. In the vein of Dixon's The New Dinosaurs imagination, a now largely defunct, but creatively significant collaborative online project the Speculative Dinosaur Project followed in the same zoological worldbuilding tradition.

Since 1988, alternative evolution has sometimes been applied in popular culture. The creatures in the 2005 film King Kong were fictitious descendants of real animals, with Skull Island being inhabited by dinosaurs and other prehistoric fauna. Inspired by Dougal Dixon's works, the designers imagined what 65 million years or more of isolated evolution might have done to dinosaurs. Concept art for the film was published in the book The World of Kong: A Natural History of Skull Island (2005), which explored the world of the film from a biological perspective, envisioning Skull Island as a surviving fragment of ancient Gondwana. Prehistoric creatures on a declining, eroding island had evolved into "a menagerie of nightmares".

A hypothetical natural history of dragons is a popular subject of speculative zoology, being explored in works such as Peter Dickinson's The Flight of Dragons (1979), the 2004 mockumentary The Last Dragon and the Dragonology series of books.

Future evolution

The evolution of organisms in the Earth's future is a popular subset of speculative evolution. A relatively common theme in future evolution is civilizational collapse and/or humans becoming extinct due to an anthropogenic extinction event caused by environmental degradation. After such a mass extinction event, the remaining non-human fauna and flora evolve into a variety of new forms. Although the foundations of this subset were laid by Wells's The Time Machine already in 1895, it is generally agreed to have been definitely founded through Dixon's After Man in 1981, which explored a fully realized future ecosystem set 50 million years from the present. Dixon's third work on speculative evolution, Man After Man (1990) is also an example of future evolution, this time exploring an imagined future evolutionary path of humanity.

Peter Ward's Future Evolution (2001) makes a scientifically accurate approach to the prediction of patterns of evolution in the future. Ward compares his predictions with those of Dixon and Wells. He tries to understand the mechanism of mass extinctions and the principles of recovery of ecosystems. A key point is that "champion supertaxa" who diversify and speciate at a greater rate, will inherit the world after mass extinctions. Ward quotes the paleontologist Simon Conway Morris, who points out that the fantastical or even whimsical creatures devised by Dougal Dixon, echo nature's tendency to converge on the same body plans. While Ward calls Dixon's visions "semi-whimsical" and compares them to Wells' initial visions in The Time Machine, he nonetheless continues the use of analogous evolution, which is a larger trend in speculative zoology.

Future evolution has also been explored on TV, with the mockumentary series The Future is Wild in 2002, for which Dixon was a consultant (and author of the companion book), and the series Primeval (2007–2011), a drama series in which imagined future animals occasionally appeared. Ideas of future evolution are also frequently explored in science fiction novels, such as in Kurt Vonnegut's 1985 science fiction novel Galápagos, which imagines the evolution of a small surviving group of humans into a sea lion-like species. Stephen Baxter's 2002 science fiction novel Evolution follows 565 million years of human evolution, from shrewlike mammals 65 million years in the past to the ultimate fate of humanity (and its descendants, both biological and non-biological) 500 million years in the future. C. M. Kosemen's 2008 All Tomorrows similarly explores the future evolution of humanity. Speculative biology and the future evolution of the human species are significant in bio art.

Dark energy

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Dark_energy 

In physical cosmology and astronomy, dark energy is an unknown form of energy that affects the universe on the largest scales. The first observational evidence for its existence came from measurements of supernovae, which showed that the universe does not expand at a constant rate; rather, the expansion of the universe is accelerating. Understanding the evolution of the universe requires knowledge of its starting conditions and its composition. Prior to these observations, it was thought that all forms of matter and energy in the universe would only cause the expansion to slow down over time. Measurements of the cosmic microwave background suggest the universe began in a hot Big Bang, from which general relativity explains its evolution and the subsequent large-scale motion. Without introducing a new form of energy, there was no way to explain how an accelerating universe could be measured. Since the 1990s, dark energy has been the most accepted premise to account for the accelerated expansion. As of 2021, there are active areas of cosmology research aimed at understanding the fundamental nature of dark energy.

Assuming that the lambda-CDM model of cosmology is correct, the best current measurements indicate that dark energy contributes 68% of the total energy in the present-day observable universe. The mass–energy of dark matter and ordinary (baryonic) matter contributes 26% and 5%, respectively, and other components such as neutrinos and photons contribute a very small amount. The density of dark energy is very low (~ 7 × 10⁻³⁰ g/cm³), much less than the density of ordinary matter or dark matter within galaxies. However, it dominates the mass–energy of the universe because it is uniform across space.

Two proposed forms of dark energy are the cosmological constant, representing a constant energy density filling space homogeneously, and scalar fields such as quintessence or moduli, dynamic quantities having energy densities that can vary in time and space. Contributions from scalar fields that are constant in space are usually also included in the cosmological constant. The cosmological constant can be formulated to be equivalent to the zero-point radiation of space i.e. the vacuum energy. Scalar fields that change in space can be difficult to distinguish from a cosmological constant because the change may be extremely slow.

Due to the toy model nature of concordance cosmology, some experts believe that a more accurate general relativistic treatment of the structures that exist on all scales in the real universe may do away with the need to invoke dark energy. Inhomogeneous cosmologies, which attempt to account for the back-reaction of structure formation on the metric, generally do not acknowledge any dark energy contribution to the energy density of the Universe.

History of discovery and previous speculation

Einstein's cosmological constant

The "cosmological constant" is a constant term that can be added to Einstein's field equation of general relativity. If considered as a "source term" in the field equation, it can be viewed as equivalent to the mass of empty space (which conceptually could be either positive or negative), or "vacuum energy".

The cosmological constant was first proposed by Einstein as a mechanism to obtain a solution of the gravitational field equation that would lead to a static universe, effectively using dark energy to balance gravity. Einstein gave the cosmological constant the symbol Λ (capital lambda). Einstein stated that the cosmological constant required that 'empty space takes the role of gravitating negative masses which are distributed all over the interstellar space'.

The mechanism was an example of fine-tuning, and it was later realized that Einstein's static universe would not be stable: local inhomogeneities would ultimately lead to either the runaway expansion or contraction of the universe. The equilibrium is unstable: if the universe expands slightly, then the expansion releases vacuum energy, which causes yet more expansion. Likewise, a universe which contracts slightly will continue contracting. These sorts of disturbances are inevitable, due to the uneven distribution of matter throughout the universe. Further, observations made by Edwin Hubble in 1929 showed that the universe appears to be expanding and not static at all. Einstein reportedly referred to his failure to predict the idea of a dynamic universe, in contrast to a static universe, as his greatest blunder.

Inflationary dark energy

Alan Guth and Alexei Starobinsky proposed in 1980 that a negative pressure field, similar in concept to dark energy, could drive cosmic inflation in the very early universe. Inflation postulates that some repulsive force, qualitatively similar to dark energy, resulted in an enormous and exponential expansion of the universe slightly after the Big Bang. Such expansion is an essential feature of most current models of the Big Bang. However, inflation must have occurred at a much higher energy density than the dark energy we observe today and is thought to have completely ended when the universe was just a fraction of a second old. It is unclear what relation, if any, exists between dark energy and inflation. Even after inflationary models became accepted, the cosmological constant was thought to be irrelevant to the current universe.

Nearly all inflation models predict that the total (matter+energy) density of the universe should be very close to the critical density. During the 1980s, most cosmological research focused on models with critical density in matter only, usually 95% cold dark matter (CDM) and 5% ordinary matter (baryons). These models were found to be successful at forming realistic galaxies and clusters, but some problems appeared in the late 1980s: in particular, the model required a value for the Hubble constant lower than preferred by observations, and the model under-predicted observations of large-scale galaxy clustering. These difficulties became stronger after the discovery of anisotropy in the cosmic microwave background by the COBE spacecraft in 1992, and several modified CDM models came under active study through the mid-1990s: these included the Lambda-CDM model and a mixed cold/hot dark matter model. The first direct evidence for dark energy came from supernova observations in 1998 of accelerated expansion in Riess et al. and in Perlmutter et al., and the Lambda-CDM model then became the leading model. Soon after, dark energy was supported by independent observations: in 2000, the BOOMERanG and Maxima cosmic microwave background (CMB) experiments observed the first acoustic peak in the CMB, showing that the total (matter+energy) density is close to 100% of critical density. Then in 2001, the 2dF Galaxy Redshift Survey gave strong evidence that the matter density is around 30% of critical. The large difference between these two supports a smooth component of dark energy making up the difference. Much more precise measurements from WMAP in 2003–2010 have continued to support the standard model and give more accurate measurements of the key parameters.

The term "dark energy", echoing Fritz Zwicky's "dark matter" from the 1930s, was coined by Michael Turner in 1998.

Change in expansion over time

Diagram representing the accelerated expansion of the universe due to dark energy.

High-precision measurements of the expansion of the universe are required to understand how the expansion rate changes over time and space. In general relativity, the evolution of the expansion rate is estimated from the curvature of the universe and the cosmological equation of state (the relationship between temperature, pressure, and combined matter, energy, and vacuum energy density for any region of space). Measuring the equation of state for dark energy is one of the biggest efforts in observational cosmology today. Adding the cosmological constant to cosmology's standard FLRW metric leads to the Lambda-CDM model, which has been referred to as the "standard model of cosmology" because of its precise agreement with observations.

As of 2013, the Lambda-CDM model is consistent with a series of increasingly rigorous cosmological observations, including the Planck spacecraft and the Supernova Legacy Survey. First results from the SNLS reveal that the average behavior (i.e., equation of state) of dark energy behaves like Einstein's cosmological constant to a precision of 10%. Recent results from the Hubble Space Telescope Higher-Z Team indicate that dark energy has been present for at least 9 billion years and during the period preceding cosmic acceleration.

Nature

The nature of dark energy is more hypothetical than that of dark matter, and many things about it remain in the realm of speculation. Dark energy is thought to be very homogeneous and not very dense, and is not known to interact through any of the fundamental forces other than gravity. Since it is quite rarefied and un-massive—roughly 10⁻²⁷ kg/m³—it is unlikely to be detectable in laboratory experiments. The reason dark energy can have such a profound effect on the universe, making up 68% of universal density in spite of being so dilute, is that it uniformly fills otherwise empty space.

Independently of its actual nature, dark energy would need to have a strong negative pressure to explain the observed acceleration of the expansion of the universe. According to general relativity, the pressure within a substance contributes to its gravitational attraction for other objects just as its mass density does. This happens because the physical quantity that causes matter to generate gravitational effects is the stress–energy tensor, which contains both the energy (or matter) density of a substance and its pressure. In the Friedmann–Lemaître–Robertson–Walker metric, it can be shown that a strong constant negative pressure (i.e., tension) in all the universe causes an acceleration in the expansion if the universe is already expanding, or a deceleration in contraction if the universe is already contracting. This accelerating expansion effect is sometimes labeled "gravitational repulsion".

Technical definition

See also: Friedmann equations

In standard cosmology, there are three components of the universe: matter, radiation, and dark energy. Matter is anything whose energy density scales with the inverse cube of the scale factor, i.e., ρ ∝ a⁻³, while radiation is anything which scales to the inverse fourth power of the scale factor (ρ ∝ a⁻⁴). This can be understood intuitively: for an ordinary particle in a cube-shaped box, doubling the length of an edge of the box decreases the density (and hence energy density) by a factor of eight (2³). For radiation, the decrease in energy density is greater, because an increase in spatial distance also causes a redshift.

The final component is dark energy; "dark energy" is anything that is, in its effect, an intrinsic property of space: That has a constant energy density, regardless of the dimensions of the volume under consideration (ρ ∝ a⁰). Thus, unlike ordinary matter, it is not diluted by the expansion of space.

Evidence of existence

The evidence for dark energy is indirect but comes from three independent sources:

• Distance measurements and their relation to redshift, which suggest the universe has expanded more in the latter half of its life.
• The theoretical need for a type of additional energy that is not matter or dark matter to form the observationally flat universe (absence of any detectable global curvature).
• Measures of large-scale wave patterns of mass density in the universe.

Supernovae

A Type Ia supernova (bright spot on the bottom-left) near a galaxy

In 1998, the High-Z Supernova Search Team published observations of Type Ia ("one-A") supernovae. In 1999, the Supernova Cosmology Project followed by suggesting that the expansion of the universe is accelerating. The 2011 Nobel Prize in Physics was awarded to Saul Perlmutter, Brian P. Schmidt, and Adam G. Riess for their leadership in the discovery.

Since then, these observations have been corroborated by several independent sources. Measurements of the cosmic microwave background, gravitational lensing, and the large-scale structure of the cosmos, as well as improved measurements of supernovae, have been consistent with the Lambda-CDM model. Some people argue that the only indications for the existence of dark energy are observations of distance measurements and their associated redshifts. Cosmic microwave background anisotropies and baryon acoustic oscillations serve only to demonstrate that distances to a given redshift are larger than would be expected from a "dusty" Friedmann–Lemaître universe and the local measured Hubble constant.

Supernovae are useful for cosmology because they are excellent standard candles across cosmological distances. They allow researchers to measure the expansion history of the universe by looking at the relationship between the distance to an object and its redshift, which gives how fast it is receding from us. The relationship is roughly linear, according to Hubble's law. It is relatively easy to measure redshift, but finding the distance to an object is more difficult. Usually, astronomers use standard candles: objects for which the intrinsic brightness, or absolute magnitude, is known. This allows the object's distance to be measured from its actual observed brightness, or apparent magnitude. Type Ia supernovae are the best-known standard candles across cosmological distances because of their extreme and consistent luminosity.

Recent observations of supernovae are consistent with a universe made up 71.3% of dark energy and 27.4% of a combination of dark matter and baryonic matter.

Cosmic microwave background

Estimated division of total energy in the universe into matter, dark matter and dark energy based on five years of WMAP data.

The existence of dark energy, in whatever form, is needed to reconcile the measured geometry of space with the total amount of matter in the universe. Measurements of cosmic microwave background (CMB) anisotropies indicate that the universe is close to flat. For the shape of the universe to be flat, the mass–energy density of the universe must be equal to the critical density. The total amount of matter in the universe (including baryons and dark matter), as measured from the CMB spectrum, accounts for only about 30% of the critical density. This implies the existence of an additional form of energy to account for the remaining 70%. The Wilkinson Microwave Anisotropy Probe (WMAP) spacecraft seven-year analysis estimated a universe made up of 72.8% dark energy, 22.7% dark matter, and 4.5% ordinary matter. Work done in 2013 based on the Planck spacecraft observations of the CMB gave a more accurate estimate of 68.3% dark energy, 26.8% dark matter, and 4.9% ordinary matter.

Large-scale structure

The theory of large-scale structure, which governs the formation of structures in the universe (stars, quasars, galaxies and galaxy groups and clusters), also suggests that the density of matter in the universe is only 30% of the critical density.

A 2011 survey, the WiggleZ galaxy survey of more than 200,000 galaxies, provided further evidence towards the existence of dark energy, although the exact physics behind it remains unknown. The WiggleZ survey from the Australian Astronomical Observatory scanned the galaxies to determine their redshift. Then, by exploiting the fact that baryon acoustic oscillations have left voids regularly of ≈150 Mpc diameter, surrounded by the galaxies, the voids were used as standard rulers to estimate distances to galaxies as far as 2,000 Mpc (redshift 0.6), allowing for accurate estimate of the speeds of galaxies from their redshift and distance. The data confirmed cosmic acceleration up to half of the age of the universe (7 billion years) and constrain its inhomogeneity to 1 part in 10. This provides a confirmation to cosmic acceleration independent of supernovae.

Late-time integrated Sachs–Wolfe effect

Accelerated cosmic expansion causes gravitational potential wells and hills to flatten as photons pass through them, producing cold spots and hot spots on the CMB aligned with vast supervoids and superclusters. This so-called late-time Integrated Sachs–Wolfe effect (ISW) is a direct signal of dark energy in a flat universe. It was reported at high significance in 2008 by Ho et al. and Giannantonio et al.

Observational Hubble constant data

A new approach to test evidence of dark energy through observational Hubble constant data (OHD) has gained significant attention in recent years.

The Hubble constant, H(z), is measured as a function of cosmological redshift. OHD directly tracks the expansion history of the universe by taking passively evolving early-type galaxies as “cosmic chronometers”. From this point, this approach provides standard clocks in the universe. The core of this idea is the measurement of the differential age evolution as a function of redshift of these cosmic chronometers. Thus, it provides a direct estimate of the Hubble parameter

${\displaystyle H(z)=-{\frac {1}{1+z}}{\frac {dz}{dt}}\approx -{\frac {1}{1+z}}{\frac {\Delta z}{\Delta t}}.}$

The reliance on a differential quantity, Δz/Δt, brings more information and is appealing for computation: It can minimize many common issues and systematic effects. Analyses of supernovae and baryon acoustic oscillations (BAO) are based on integrals of the Hubble parameter, whereas Δz/Δt measures it directly. For these reasons, this method has been widely used to examine the accelerated cosmic expansion and study properties of dark energy.

Direct observation

An attempt to directly observe dark energy in a laboratory failed to detect a new force. Recently, it has been speculated that the currently unexplained excess observed in the XENON1T detector in Italy may have been caused by a chameleon model of dark energy.

Theories of dark energy

Dark energy's status as a hypothetical force with unknown properties makes it a very active target of research. The problem is attacked from a great variety of angles, such as modifying the prevailing theory of gravity (general relativity), attempting to pin down the properties of dark energy, and finding alternative ways to explain the observational data.

The equation of state of Dark Energy for 4 common models by Redshift.
A: CPL Model,
B: Jassal Model,
C: Barboza & Alcaniz Model,
D: Wetterich Model

Cosmological constant

Main article: Cosmological constant

Further information: Equation of state (cosmology)

Estimated distribution of matter and energy in the universe

The simplest explanation for dark energy is that it is an intrinsic, fundamental energy of space. This is the cosmological constant, usually represented by the Greek letter Λ (Lambda, hence Lambda-CDM model). Since energy and mass are related according to the equation E = mc² , Einstein's theory of general relativity predicts that this energy will have a gravitational effect. It is sometimes called a vacuum energy because it is the energy density of empty space – the vacuum.

A major outstanding problem is that the same quantum field theories predict a huge cosmological constant, about 120 orders of magnitude too large. This would need to be almost, but not exactly, cancelled by an equally large term of the opposite sign.

Some supersymmetric theories require a cosmological constant that is exactly zero. Also, it is unknown if there is a metastable vacuum state in string theory with a positive cosmological constant, and it has been conjectured by Ulf Danielsson et al. that no such state exists. This conjecture would not rule out other models of dark energy, such as quintessence, that could be compatible with string theory.

Quintessence

Main article: Quintessence (physics)

In quintessence models of dark energy, the observed acceleration of the scale factor is caused by the potential energy of a dynamical field, referred to as quintessence field. Quintessence differs from the cosmological constant in that it can vary in space and time. In order for it not to clump and form structure like matter, the field must be very light so that it has a large Compton wavelength.

No evidence of quintessence is yet available, but it has not been ruled out either. It generally predicts a slightly slower acceleration of the expansion of the universe than the cosmological constant. Some scientists think that the best evidence for quintessence would come from violations of Einstein's equivalence principle and variation of the fundamental constants in space or time. Scalar fields are predicted by the Standard Model of particle physics and string theory, but an analogous problem to the cosmological constant problem (or the problem of constructing models of cosmological inflation) occurs: renormalization theory predicts that scalar fields should acquire large masses.

The coincidence problem asks why the acceleration of the Universe began when it did. If acceleration began earlier in the universe, structures such as galaxies would never have had time to form, and life, at least as we know it, would never have had a chance to exist. Proponents of the anthropic principle view this as support for their arguments. However, many models of quintessence have a so-called "tracker" behavior, which solves this problem. In these models, the quintessence field has a density which closely tracks (but is less than) the radiation density until matter–radiation equality, which triggers quintessence to start behaving as dark energy, eventually dominating the universe. This naturally sets the low energy scale of the dark energy.

In 2004, when scientists fit the evolution of dark energy with the cosmological data, they found that the equation of state had possibly crossed the cosmological constant boundary (w = −1) from above to below. A no-go theorem has been proved that this scenario requires models with at least two types of quintessence. This scenario is the so-called Quintom scenario.

Some special cases of quintessence are phantom energy, in which the energy density of quintessence actually increases with time, and k-essence (short for kinetic quintessence) which has a non-standard form of kinetic energy such as a negative kinetic energy. They can have unusual properties: phantom energy, for example, can cause a Big Rip.

Interacting dark energy

This class of theories attempts to come up with an all-encompassing theory of both dark matter and dark energy as a single phenomenon that modifies the laws of gravity at various scales. This could, for example, treat dark energy and dark matter as different facets of the same unknown substance, or postulate that cold dark matter decays into dark energy. Another class of theories that unifies dark matter and dark energy are suggested to be covariant theories of modified gravities. These theories alter the dynamics of the spacetime such that the modified dynamics stems to what have been assigned to the presence of dark energy and dark matter. Dark energy could in principle interact not only with the rest of the dark sector, but also with ordinary matter. However, cosmology alone is not sufficient to effectively constrain the strength of the coupling between dark energy and baryons, so that other indirect techniques or laboratory searches have to be adopted. A recent proposal speculates that the currently unexplained excess observed in the XENON1T detector in Italy may have been caused by a chameleon model of dark energy.

Variable dark energy models

The density of the dark energy might have varied in time during the history of the universe. Modern observational data allow us to estimate the present density of the dark energy. Using baryon acoustic oscillations, it is possible to investigate the effect of dark energy in the history of the Universe, and constrain parameters of the equation of state of dark energy. To that end, several models have been proposed. One of the most popular models is the Chevallier–Polarski–Linder model (CPL). Some other common models are, (Barboza & Alcaniz. 2008), (Jassal et al. 2005), (Wetterich. 2004), (Oztas et al. 2018).

Observational skepticism

Some alternatives to dark energy, such as inhomogeneous cosmology, aim to explain the observational data by a more refined use of established theories. In this scenario, dark energy doesn't actually exist, and is merely a measurement artifact. For example, if we are located in an emptier-than-average region of space, the observed cosmic expansion rate could be mistaken for a variation in time, or acceleration. A different approach uses a cosmological extension of the equivalence principle to show how space might appear to be expanding more rapidly in the voids surrounding our local cluster. While weak, such effects considered cumulatively over billions of years could become significant, creating the illusion of cosmic acceleration, and making it appear as if we live in a Hubble bubble. Yet other possibilities are that the accelerated expansion of the universe is an illusion caused by the relative motion of us to the rest of the universe, or that the statistical methods employed were flawed. It has also been suggested that the anisotropy of the local Universe has been misrepresented as dark energy. This claim was quickly countered by others, including a paper by physicists D. Rubin and J. Heitlauf. A laboratory direct detection attempt failed to detect any force associated with dark energy.

A study published in 2020 questioned the validity of the essential assumption that the luminosity of Type Ia supernovae does not vary with stellar population age, and suggests that dark energy may not actually exist. Lead researcher of the new study, Young-Wook Lee of Yonsei University, said "Our result illustrates that dark energy from SN cosmology, which led to the 2011 Nobel Prize in Physics, might be an artifact of a fragile and false assumption." Multiple issues with this paper were raised by other cosmologists, including Adam Riess, who won the 2011 Nobel Prize for the discovery of dark energy.

Other mechanism driving acceleration

Modified gravity

See also: Massive gravity

The evidence for dark energy is heavily dependent on the theory of general relativity. Therefore, it is conceivable that a modification to general relativity also eliminates the need for dark energy. There are very many such theories, and research is ongoing. The measurement of the speed of gravity in the first gravitational wave measured by non-gravitational means (GW170817) ruled out many modified gravity theories as explanations to dark energy.

Astrophysicist Ethan Siegel states that, while such alternatives gain a lot of mainstream press coverage, almost all professional astrophysicists are confident that dark energy exists, and that none of the competing theories successfully explain observations to the same level of precision as standard dark energy.

Implications for the fate of the universe

Cosmologists estimate that the acceleration began roughly 5 billion years ago. Before that, it is thought that the expansion was decelerating, due to the attractive influence of matter. The density of dark matter in an expanding universe decreases more quickly than dark energy, and eventually the dark energy dominates. Specifically, when the volume of the universe doubles, the density of dark matter is halved, but the density of dark energy is nearly unchanged (it is exactly constant in the case of a cosmological constant).

Projections into the future can differ radically for different models of dark energy. For a cosmological constant, or any other model that predicts that the acceleration will continue indefinitely, the ultimate result will be that galaxies outside the Local Group will have a line-of-sight velocity that continually increases with time, eventually far exceeding the speed of light. This is not a violation of special relativity because the notion of "velocity" used here is different from that of velocity in a local inertial frame of reference, which is still constrained to be less than the speed of light for any massive object (see Uses of the proper distance for a discussion of the subtleties of defining any notion of relative velocity in cosmology). Because the Hubble parameter is decreasing with time, there can actually be cases where a galaxy that is receding from us faster than light does manage to emit a signal which reaches us eventually.

However, because of the accelerating expansion, it is projected that most galaxies will eventually cross a type of cosmological event horizon where any light they emit past that point will never be able to reach us at any time in the infinite future because the light never reaches a point where its "peculiar velocity" toward us exceeds the expansion velocity away from us (these two notions of velocity are also discussed in Uses of the proper distance). Assuming the dark energy is constant (a cosmological constant), the current distance to this cosmological event horizon is about 16 billion light years, meaning that a signal from an event happening at present would eventually be able to reach us in the future if the event were less than 16 billion light years away, but the signal would never reach us if the event were more than 16 billion light years away.

As galaxies approach the point of crossing this cosmological event horizon, the light from them will become more and more redshifted, to the point where the wavelength becomes too large to detect in practice and the galaxies appear to vanish completely. Planet Earth, the Milky Way, and the Local Group of which the Milky Way is a part, would all remain virtually undisturbed as the rest of the universe recedes and disappears from view. In this scenario, the Local Group would ultimately suffer heat death, just as was hypothesized for the flat, matter-dominated universe before measurements of cosmic acceleration.

There are other, more speculative ideas about the future of the universe. The phantom energy model of dark energy results in divergent expansion, which would imply that the effective force of dark energy continues growing until it dominates all other forces in the universe. Under this scenario, dark energy would ultimately tear apart all gravitationally bound structures, including galaxies and solar systems, and eventually overcome the electrical and nuclear forces to tear apart atoms themselves, ending the universe in a "Big Rip". On the other hand, dark energy might dissipate with time or even become attractive. Such uncertainties leave open the possibility of gravity eventually prevailing and lead to a universe that contracts in on itself in a "Big Crunch", or that there may even be a dark energy cycle, which implies a cyclic model of the universe in which every iteration (Big Bang then eventually a Big Crunch) takes about a trillion (10¹²) years. While none of these are supported by observations, they are not ruled out.

In philosophy of science

In philosophy of science, dark energy is an example of an "auxiliary hypothesis", an ad hoc postulate that is added to a theory in response to observations that falsify it. It has been argued that the dark energy hypothesis is a conventionalist hypothesis, that is, a hypothesis that adds no empirical content and hence is unfalsifiable in the sense defined by Karl Popper.