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Big Bang model

The prevailing Big Bang model accounts for many of the experimental observations described above, such as the correlation of distance and redshift of galaxies, the universal ratio of hydrogen:helium atoms, and the ubiquitous, isotropic microwave radiation background. Sony VAIO VPCEB1AFX Battery

As noted above, the redshift arises from the metric expansion of space; as the space itself expands, the wavelength of a photontraveling through space likewise increases, decreasing its energy. The longer a photon has been traveling, the more expansion it has undergone;Sony VAIO VPCEB1AFX/B Battery

hence, older photons from more distant galaxies are the most red-shifted. Determining the correlation between distance and redshift is an important problem in experimental physical cosmology.

Other experimental observations can be explained by combining the overall expansion of space with nuclear and atomic physics.Sony VAIO VPCEB1AGX Battery

As the universe expands, the energy density of the electromagnetic radiation decreases more quickly than does that of matter, since the energy of a photon decreases with its wavelength. Thus, although the energy density of the universe is now dominated by matter, it was once dominated by radiation; poetically speaking, all was light.Sony VAIO VPCEB1AGX/BI Battery

As the universe expanded, its energy density decreased and it became cooler; as it did so, the elementary particles of matter could associate stably into ever larger combinations. Thus, in the early part of the matter-dominated era, stable protons and neutrons formed, which then associated into atomic nuclei. Sony VAIO VPCEB1BGX Battery

At this stage, the matter in the universe was mainly a hot, dense plasma of negative electrons, neutral neutrinos and positive nuclei. Nuclear reactions among the nuclei led to the present abundances of the lighter nuclei, particularly hydrogen, deuterium, and helium. Sony VAIO VPCEB1BGX/BI Battery

Eventually, the electrons and nuclei combined to form stable atoms, which are transparent to most wavelengths of radiation; at this point, the radiation decoupled from the matter, forming the ubiquitous, isotropic background of microwave radiation observed today.Sony VAIO VPCEB1CGX Battery

Other observations are not answered definitively by known physics. According to the prevailing theory, a slight imbalance ofmatter over antimatter was present in the universe's creation, or developed very shortly thereafter, possibly due to the CP violation that has been observed by particle physicists. Sony VAIO VPCEB1CGX/BI Battery

Although the matter and antimatter mostly annihilated one another, producing photons, a small residue of matter survived, giving the present matter-dominated universe. Several lines of evidence also suggest that a rapidcosmic inflation of the universe occurred very early in its history (roughly 10?35 seconds after its creation). Sony VAIO VPCEB1DGX Battery

Recent observations also suggest that the cosmological constant (?) is not zero and that the net mass-energy content of the universe is dominated by a dark energy and dark matter that have not been characterized scientifically. They differ in their gravitational effects. Sony VAIO VPCEB1DGX/BI Battery

Dark matter gravitates as ordinary matter does, and thus slows the expansion of the universe; by contrast, dark energy serves to accelerate the universe's expansion.

Multiverse theory

Some speculative theories have proposed that this universe is but one of a set of disconnected universes, collectively denoted as the multiverse, altering the concept that the universe encompasses everything.Sony VAIO VPCEB1EGX Battery

By definition, there is no possible way for anything in one universe to affect another; if two "universes" could affect one another, they would be part of a single universe. Thus, although some fictional characters travel between parallel fictional "universes", this is, strictly speaking, an incorrect usage of the term universeSony VAIO VPCEB1EGX/BI Battery

The disconnected universes are conceived as being physical, in the sense that each should have its own space and time, its own matter and energy, and its own physical laws — that also challenges the definition of parallelity as these universes don't exist synchronously (since they have their own time) Sony VAIO VPCEB1FGX Battery

or in a geometrically parallel way (since there's no interpretable relation between spatial positions of the different universes). Such physically disconnected universes should be distinguished from the metaphysical conception of alternate planes of consciousness, which are not thought to be physical places and are connected through the flow of information.Sony VAIO VPCEB1FGX/BI Battery

The concept of a multiverse of disconnected universes is very old; for example, Bishop Étienne Tempier of Paris ruled in 1277 that God could create as many universes as he saw fit, a question that was being hotly debated by the French theologians. Sony VAIO VPCEB1GGX Battery

There are two scientific senses in which multiple universes are discussed. First, disconnected spacetime continua may exist; presumably, all forms of matter and energy are confined to one universe and cannot "tunnel" between them. An example of such a theory is the chaotic inflation model of the early universe.Sony VAIO VPCEB1GGX/BI Battery

Second, according to the many-worlds hypothesis, a parallel universe is born with every quantum measurement; the universe "forks" into parallel copies, each one corresponding to a different outcome of the quantum measurement. However, both senses of the term "multiverse" are speculative and may be considered unscientific; Sony VAIO VPCEB1HGX Battery

no known experimental test in one universe could ever reveal the existence or properties of another non-interacting universe, as the very revealing of the other universe would remove its non-interaction.

Shape of the universe

The shape or geometry of the universe includes both local geometry in the observable universe and global geometry, Sony VAIO VPCEB1HGX/BI Battery

which we may or may not be able to measure. Shape can refer to curvature andtopology. More formally, the subject in practice investigates which 3-manifold corresponds to the spatial section in comoving coordinates of the four-dimensional space-time of the universe.Sony VAIO VPCEB1JFX Battery

Cosmologists normally work with a given space-like slice of spacetime called the comoving coordinates. In terms of observation, the section of spacetime that can be observed is the backward light cone (points within the cosmic light horizon, given time to reach a given observer). Sony VAIO VPCEB1JFX/B Battery

If the observable universe is smaller than the entire universe (in some models it is many orders of magnitude smaller), one cannot determine the global structure by observation: one is limited to a small patch.

Among the Friedmann–Lemaître–Robertson–Walker (FLRW) models, the presently most popular shape of the Universe found to fit observational data according to cosmologists is the infinite flat model, Sony VAIO VPCEB1JFX/G Battery

while other FLRW models include the Poincaré dodecahedral space and the Picard horn. The data fit by these FLRW models of space especially include the Wilkinson Microwave Anisotropy Probe (WMAP) maps of cosmic background radiation. NASA released the first WMAP cosmic background radiation data in February 2003. Sony VAIO VPCEB1JFX/L Battery

In 2009 the Planck observatory was launched to observe the microwave background at higher resolution than WMAP, possibly providing more information on the shape of the Universe. The data should be released in late 2012.Sony VAIO VPCEB1JFX/P Battery

In planetary astronomy and astrobiology, the Rare Earth hypothesis argues that the emergence of complex multicellular life (metazoa) on Earth required an improbable combination of astrophysical and geological events and circumstances. The term "Rare Earth" comes from Rare Earth: Sony VAIO VPCEB1JFX/W Battery

Why Complex Life Is Uncommon in the Universe (2000), a book by Peter Ward, a geologist and paleontologist, and Donald E. Brownlee, an astronomer and astrobiologist. Their book is the source for much of this article.

The rare earth hypothesis is the contrary of the principle of mediocrity (also called the Copernican principle), advocated by Carl Sagan and Frank Drake, among others.Sony VAIO VPCEB1KGX Battery

The principle of mediocrity concludes that the Earth is a typical rocky planet in a typical planetary system, located in an unexceptional region of a common barred-spiral galaxy. Hence it is probable that the universe teems with complex life. Ward and Brownlee argue to the contrary:Sony VAIO VPCEB1KGX/B Battery

planets, planetary systems, and galactic regions that are as friendly to complex life as are the Earth, the solar system, and our region of the Milky Way are very rare.

By concluding that complex life is uncommon, the Rare Earth hypothesis is a possible solution to the Fermi paradox: "If extraterrestrial aliens are common, why aren't they obvious?"Sony VAIO VPCEB1KGX/W Battery

Why complex life may be very rare

The Rare Earth hypothesis argues that the emergence of complex life requires a host of fortuitous circumstances. A number of such circumstances are set out below under the following headings: galactic habitable zone, Sony VAIO VPCEB1LFX Battery

a central star and planetary system having the requisite character, the circumstellar habitable zone, the size of the planet, the advantage of a large satellite, conditions needed to assure the planet has a magnetosphere and plate tectonics, the chemistry of the lithosphere, atmosphere, and oceans,Sony VAIO VPCEB1LFX/BI Battery

the role of "evolutionary pumps" such as massive glaciation and rare bolideimpacts, and whatever led to the still mysterious Cambrian explosion of animal phyla. The emergence of intelligent life may have required yet other rare events.Sony VAIO VPCEB1LFX/WI Battery

In order for a small rocky planet to support complex life, Ward and Brownlee argue, the values of several variables must fall within narrow ranges. The universe is so vast that it could contain many Earth-like planets. But if such planets exist, they are likely to be separated from each other by many thousands of light years.Sony VAIO VPCEB1MFX Battery

Such distances may preclude communication among any intelligent species evolving on such planets, which would solve the Fermi paradox.

The galactic habitable zone

Rare Earth suggests that much of the known universe, including large parts of our galaxy, cannot support complex life; Ward and Brownlee refer to such regions as "dead zones." Sony VAIO VPCEB1MFX/BI Battery

Those parts of a galaxy where complex life is possible make up the galactic habitable zone. This zone is primarily a function of distance from the galactic center. As that distance increases:

  1. The metallicity of stars declines, and metals (which in astronomy means all elements other than hydrogen and helium) are necessary to the formation of terrestrial planets.Sony VAIO VPCEB1NFX Battery
  2. The X-ray and gamma ray radiation from the black hole at the galactic center, and from nearby neutron stars, becomes less intense. Radiation of this nature is considered dangerous to complex life, hence the Rare Earth hypothesis predicts that the early universe,Sony VAIO VPCEB1NFX/B Battery
  3. and likewise regions in the galaxy at present where the stellar density is high and supernovae common, will be unfit for the development of complex life.
  4. Gravitational perturbation of planets and planetesimals by nearby stars becomes less likely as the density of stars decreases.Sony VAIO VPCEB1NFX/L Battery
  5. Hence the further a planet lies from the galactic center, the less likely it is to be struck by a large bolide. A sufficiently large impact may extinguishall complex life on a planet.

(1) rules out the outer reaches of a galaxy; (2) and (3) rule out galactic inner regions, globular clusters, and the spiral arms of spiral galaxies. Sony VAIO VPCEB1NFX/P Battery

These arms are not physical objects, but regions of a galaxy characterized by a higher rate of star formation, moving very slowly through the galaxy in a wave-like manner. As one moves from the center of a galaxy to its furthest extremity, the ability to support life rises then falls. Sony VAIO VPCEB1NFX/W Battery

Hence the galactic habitable zone may be ring-shaped, sandwiched between its uninhabitable center and outer reaches.

While a planetary system may enjoy a location favorable to complex life, it must also maintain that location for a span of time sufficiently long for complex life to evolve.Sony VAIO VPCEB1PFX Battery

Hence a central star with a galactic orbit that steers clear of galactic regions where radiation levels are high, such as the galactic center and the spiral arms, would appear most favourable. If the central star's galactic orbit iseccentric (elliptic or hyperbolic), it will pass through some spiral arms, Sony VAIO VPCEB1QGX Battery

but if the orbit is a near perfect circle and the orbital velocity equals the "rotational" velocity of the spiral arms, the star will drift into a spiral arm region only gradually—if at all. Therefore Rare Earth proponents conclude that a life-bearing star must have a galactic orbit that is nearly circular about the center of its galaxy. Sony VAIO VPCEB1QGX/BI Battery

The required synchronization of the orbital velocity of a central star with the wave velocity of the spiral arms can occur only within a fairly narrow range of distances from the galactic center. This region is termed the "galactic habitable zone". Lineweaver et al.Sony VAIO VPCEB1RGX Battery

calculate that the galactic habitable zone is a ring 7 to 9 kiloparsecs in diameter, that includes no more than 10% of the stars in the Milky Way.  Based on conservative estimates of the total number of stars in the galaxy, this could represent something like 20 to 40 billion stars. Gonzalez et al.Sony VAIO VPCEB1RGX/BI Battery

would halve these numbers; he estimates that at most 5% of stars in the Milky Way fall in the galactic habitable zone.

The orbit of the Sun around the center of the Milky Way is indeed almost perfectly circular, with a period of 226 Ma (1 Ma = 1 million years), one closely matching the rotational period of the galaxy. Sony VAIO VPCEB20 Battery

While the Rare Earth hypothesis predicts that the Sun should rarely, if ever, have passed through a spiral arm since its formation, astronomer Karen Masters has calculated that the orbit of the Sun takes it through a major spiral arm approximately every 100 million years.Sony VAIO VPCEC20 Battery

Some researchers have suggested that several mass extinctions do correspond with previous crossings of the spiral arms.

A central star of the right character

The terrestrial example suggests complex life requires water in the liquid state and its planet must therefore be at an appropriate distance. This is the core of the notion of the habitable zone orGoldilocks Principle.Sony VAIO VPCEE20 Battery

The habitable zone forms a ring around the central star. If a planet orbits its sun too closely or too far away, the surface temperature is incompatible with water being liquid (though sub-surface water, as suggested for Europa, Enceladus, and Ceres, may be possible at varying locations).Sony VAIO VPCEF20 Battery

Kasting et al. (1993) estimate that the habitable zone for the Sun ranges from 0.95 to 1.15 astronomical units.[12]

The habitable zone varies with the type and age of the central star. The habitable zone for a main sequence star very gradually moves out over time until the star becomes a white dwarf, at which time the habitable zone vanishes. Sony VAIO VPCF112FX/B Battery

The habitable zone is closely connected to the greenhouse warming afforded by atmospheric water vapor (H2O), carbon dioxide (CO2), and/or other greenhouse gases. Even though the Earth's atmosphere contains a water vapor concentration from 0% (in arid regions) to 4% (in rain forest and ocean regions) and only 387 parts per million of CO2,Sony VAIO VPCF115FG/B Battery

these small amounts suffice to raise the average surface temperature of the Earth by about 40°C from what it would otherwise be, with the dominant contribution being due to water vapor, which together with clouds makes up between 66% and 85% of Earth's greenhouse effect, with CO2 contributing between 9% and 26% of the effect.Sony VAIO VPCF116FGBI Battery

It is then presumed a star needs to have rocky planets within its habitable zone. While the habitable zone of hot stars such as Sirius or Vega is wide, there are two problems:

  1. Given that rocky planets were (at the time Rare Earth was written) thought to form closer to their central stars, the planet probably forms too close to the star to lie within the habitable zone.Sony VAIO VPCF117FJ/W Battery
  2. This does not rule out life on a moon of a gas giant. Hot stars also emit much more ultraviolet radiation, which will ionize any planetary atmosphere.
  3. Hot stars, as mentioned above, have short lives, becoming red giants in as little as 1 Ga. This may not allow enough time for advanced life to evolve.Sony VAIO VPCF117HG/BI Battery

These considerations rule out the massive and powerful stars of type F6 to O (see stellar classification) as homes to evolved metazoan life.

Small red dwarf stars, on the other hand, have habitable zones with a small radius. Sony VAIO VPCF118FJ/W Battery

This proximity causes one face of the planet to constantly face the star, and the other to always remain dark, a situation known as tidal lock. Tidal locking of a planetary hemisphere to its primary will cause one side of a planet to be extremely hot, while the other will be extremely cold.Sony VAIO VPCF119FC Battery

Planets within a habitable zone with a small radius are also at increased risk of solar flares (see Aurelia), which would tend to ionize the atmosphere and are otherwise inimical to complex life. Rare Earth proponents argue that this rules out the possibility of life in such systems, though some exobiologists have suggested that habitability may exist under the right circumstances. Sony VAIO VPCF119FC/BI Battery

This is a central point of contention for the theory, since these late-K and M category stars make up about 82% of all hydrogen-burning stars.

Rare Earth proponents argue that the stellar type of central stars that are "just right" ranges from F7 to K1. Sony VAIO VPCF119FJ/BI Battery

Such stars are not common: G type stars such as the Sun (between the hotter F and cooler K) comprise only 9%[15] of the hydrogen-burning stars in the Milky Way.

Aged stars, such as red giants and white dwarfs, are also unlikely to support life.Sony VAIO VPCF11AFJ Battery

Red giants are common in globular clusters and elliptical galaxies. White dwarfs are mostly dying stars that have already gone through their red giant phase. The diameter of a red giant has substantially increased from its youth. If a planet was in the habitable zone during a star's youth and middle age,Sony VAIO VPCF11AGJ Battery

it will be fried when its parent star becomes a red giant (though theoretically planets at a much greater distance may become habitable).

The energy output of a star over its lifespan should only change very gradually; variable stars such as Cepheid variables, for instance, are highly unlikely to support life. Sony VAIO VPCF11AHJ Battery

If the central star's energy output suddenly decreases, even for a relatively short while, the planet's water may freeze. Conversely, if the central star's energy output significantly increases, the oceans may evaporate, resulting in a greenhouse effect; this may preclude the oceans from reforming.Sony VAIO VPCF11JFX/B Battery

There is no known way to achieve life without complex chemistry, and such chemistry requires metals, namely elements other than hydrogen or helium. This suggests a condition for life is a solar system rich in metals. The only known mechanism for creating and dispersing metals is a supernova explosion.Sony VAIO VPCF11M1E Battery

The presence of metals in stars is revealed by their absorption spectrum, and studies of stellar spectra reveal that many, perhaps most, stars are poor in metals. Low metallicity characterizes the early universe, globular clusters and other stars formed when the universe was young, stars in most galaxies other than largespirals, and stars in the outer regions of all galaxies. Sony VAIO VPCF11M1E/H Battery

Thus metal-rich central stars capable of supporting complex life are believed most common in the quiet suburbs of the larger spiral galaxies, regions hospitable to complex life for another reason, namely the absence of high radiation.Sony VAIO VPCF11MFX/B Battery

Planetary system

A gas cloud capable of giving birth to a star can also give rise to gas-giant low-metallicity (Jovian) planets like Jupiter and Saturn. But Jovian planets have no hard surface of the kind believed necessary for complex life (their satellites may have hard surfaces, though). Sony VAIO VPCF11S1E Battery

The Ward and Brownlee argument holds that a planetary system capable of sustaining complex life must be structured more or less like our solar system, with small and rocky inner planets, and Jovian outer ones. Recent research calls this line of argument into question, however.Sony VAIO VPCF11S1E/B Battery

Uncertainty over Jupiter

At the time of Ward and Brownlee's book, gas giants were thought to help support life by keeping asteroids away from the life-bearing planet. Hence a gas giant was thought to protect the inner rocky planets from asteroid bombardment. However, recent computer simulations on the matter suggest that the situation is more complex than this: Sony VAIO VPCF11Z1E Battery

Jupiter appears to cause three times as many asteroid impacts as it prevents, while replacing Jupiter with a Saturn-sized body would roughly double the bombardment rate compared to Jupiter.

Disruption of orbit

A gas giant must not be too close to a body upon which life is developing, unless that body is one of its moons.Sony VAIO VPCF11Z1E/BI Battery

Close placement of gas giant(s) could disrupt the orbit of a potential life-bearing planet, either directly or by drifting into the habitable zone.

Newtonian dynamics can produce chaotic planetary orbits, especially in a system having large planets at high orbital eccentricity.Sony VAIO VPCF11ZHJ Battery

The need for stable orbits rules out stars with systems of planets that contain large planets with orbits close to the host star (called "hot Jupiters"). It is believed that hot Jupiters formed much further from their parent stars than they are now, and have migrated inwards to their current orbits. Sony VAIO VPCF127HGBI Battery

In the process, they would have catastrophically disrupted the orbits of any planets in the habitable zone.

Size of planet

A planet that is too small cannot hold much of an atmosphere. Hence the surface temperature becomes more variable and the average temperature drops.Sony VAIO VPCF137HG/BI Battery

Substantial and long-lasting oceans become impossible. A small planet will also tend to have a rough surface, with large mountains and deep canyons. The core will cool faster, and plate tectonics will either not last as long as they would on a larger planet or may not occur at all.Sony VAIO VPC-P111KX/B Battery

Small rocky planets like Earth may be common according to astronomer Michael Meyer of the University of Arizona:

Our observations suggest that between 20% and 60% of Sun-like stars have evidence for the formation of rocky planets not unlike the processes we think led to planet Earth. That is very exciting.Sony VAIO VPC-P111KX/D Battery

Meyer’s team found cosmic dust near recently-formed sun-like stars and sees this as a byproduct of the formation of rocky planets.

Large moon

The Moon is unusual because the other rocky planets in the Solar System either have no satellites (Mercury and Venus), or have tiny satellites that are probably captured asteroids (Mars).Sony VAIO VPC-P111KX/G Battery

The giant impact theory hypothesizes that the Moon resulted from the impact of a Mars-sized body, Theia, with the very young Earth. This giant impact also gave the Earth its axis tilt and velocity of rotation. Rapid rotation reduces the daily variation in temperature and makes photosynthesis viable. Sony VAIO VPC-P111KX/P Battery

The Rare Earth hypothesis further argues that the axis tilt cannot be too large or too small (relative to the orbital plane). A planet with a large tilt will experience extreme seasonal variations in climate, unfriendly to complex life. A planet with little or no tilt will lack the stimulus to evolution that climate variation provides.Sony VAIO VPC-P111KX/W Battery

In this view, the Earth's tilt is "just right". The gravity of a large satellite also stabilises the planet's tilt; without this effect the variation in tilt would be chaotic, probably making complex life forms on land impossible.

If the Earth had no Moon, the ocean tides resulting solely from the Sun's gravity would be very small.Sony VAIO VPC-P112KX/B Battery

A large satellite gives rise to tidal pools, which may be essential for the formation of complex life, though this is far from certain.

A large satellite also increases the likelihood of plate tectonics through the effect of tidal forces on the planet's crust. Sony VAIO VPC-P112KX/D Battery

The impact that formed the Moon may also have initiated plate tectonics, without which the continental crust would cover the entire planet, leaving no room for oceanic crust. It is possible that the large scale mantle convection needed to drive plate tectonics could not have emerged in the absence of crustal inhomogeneity. Sony VAIO VPC-P112KX/G Battery

However, there is strong evidence that plate tectonics existed on Mars in the past, without such a mechanism to initiate it.

If a giant impact is the only way for a rocky inner planet to acquire a large satellite, Sony VAIO VPC-P112KX/P Battery

any planet in the circumstellar habitable zone will need to form as a double planet in order that there be an impacting object sufficiently massive to give rise in due course to a large satellite. An impacting object of this nature is not necessarily improbable.Sony VAIO VPC-P112KX/W Battery

Recent work by Edward Belbruno and J. Richard Gott of Princeton University suggests that a suitable impacting body could form in a planet's trojan points (L4 or L5 Lagrangian point).

Plate tectonics

A planet will not experience plate tectonics unless its chemical composition allows it. Sony VAIO VPCP113KX/B Battery

The only known long-lasting source of the required heat is radioactive decay occurring deep in the planet's interior. Continents must also be made up of less dense granitic rocks that "float" on underlying denser basaltic rock. Taylor  emphasizes that subduction zones (an essential part of plate tectonics) require the lubricating action of ample water;Sony VAIO VPC-P113KX/B Battery

on Earth, such zones exist only at the bottom of oceans.

Oort Cloud

Almost all the comets we see derive from the Oort Cloud and few, if any, have hyperbolic orbits which suggest origins from interstellar space, and thus from other solar systems. Sony VAIO VPCP113KX/D Battery

This is puzzling as the number of comets in our Solar System runs into billions, and many escape into interstellar space, so we would expect to see comets from other solar systems. In the view of J. C. Brandt and R. D. Chapman, although other explanations are possible "Perhaps solar systems like ours are the exception rather than the rule."Sony VAIO VPC-P113KX/D Battery

Rare Earth equation

The following discussion is adapted from Cramer. The Rare Earth equation is Ward and Brownlee's riposte to the Drake equation. It calculates N, the number of Earth-like planets in the Milky Way having complex life forms.Sony VAIO VPCP113KX/G Battery

The Rare Earth equation, unlike the Drake equation, does not factor the probability that complex life evolves into intelligent life that discovers technology (Ward and Brownlee are not evolutionary biologists). Barrow and Tipler  review the consensus among such biologists that the evolutionary path from primitive Cambrian chordates, Sony VAIO VPC-P113KX/G Battery

e.g. Pikaia to Homo sapiens, was a highly improbable event. For example, the large brains of humans have marked adaptive disadvantages, requiring as they do an expensive metabolism, a long gestation period, and a childhood lasting more than 25% of the average total life span. Other improbable features of humans include:Sony VAIO VPCP113KX/P Battery

  • Being the only extant bipedal land (non-avian) vertebrate. Combined with an unusual eye–hand coordination, this permits dextrous manipulations of the physical environment with the hands;
  • A vocal apparatus far more expressive than that of any other mammal, enabling speech. Speech makes it possible for humans to interact cooperatively, to share knowledge, and to acquire a culture;Sony VAIO VPC-P113KX/P Battery
  • The capability of formulating abstractions to a degree permitting the invention of mathematics, and the discovery of science and technology. Only recently did humans acquire anything like their current scientific and technological sophistication.Sony VAIO VPCP113KX/W Battery

Advocates

Books that advocate the Rare Earth hypothesis:

  • Stuart Ross Taylor,  a specialist on the solar system, firmly believes in the hypothesis, but its truth is not central to his purpose, which is to write a short introductory book on the solar system and its formation. Sony VAIO VPC-P113KX/W Battery
  • Taylor concludes that the solar system is probably very unusual, because it resulted from so many chance factors and events.
  • Stephen Webb,[2] a physicist, mainly presents and rejects candidate solutions for the Fermi paradox. The Rare Earth hypothesis emerges as one of the few solutions left standing by the end of the book.Sony VAIO VPC-P114KX/B Battery
  • Simon Conway Morris,[34] a paleontologist, mainly argues that evolution is convergent. Morris devotes chapter 5 to the Rare Earth hypothesis, citing Rare Earth with approval. Yet while Morris agrees that the Earth could well be the only planet in the Milky Way harboring complex life,Sony VAIO VPC-P114KX/D Battery
  • he sees the evolution of complex life into intelligent life as fairly probable, contraErnst Mayr's views as reported in section 3.2 of the following reference.
  • John D. Barrow and Frank J. Tipler (1986. 3.2, 8.7, 9), cosmologists, vigorously defend the hypothesis that humans are likely to be the only intelligent life in the Milky Way, and perhaps the entire universe. Sony VAIO VPC-P114KX/G Battery
  • But this hypothesis is not central to their book, a very thorough study of the anthropic principle, and of how the laws of physics are peculiarly suited to enable the emergence of complexity in nature.Sony VAIO VPC-P114KX/P Battery
  • Ray Kurzweil, a computer pioneer and self-proclaimed Singularitarian, argues in The Singularity Is Near that the coming Singularity requires that Earth be the first planet on which sentient, technology-using life evolved.Sony VAIO VPC-P114KX/W Battery
  • Although other Earth-like planets could exist, Earth must be the most evolutionarily advanced, because otherwise we would have seen evidence that another culture had experienced the Singularity and expanded to harness the full computational capacity of the physical universe.Sony VAIO VPCP115JC Battery

Criticism

Criticisms of the Rare Earth Hypothesis take various forms.

Exoplanets are common

Over 500 exoplanets are known as of early 2011 and more are continually discovered. Sony VAIO VPCP115JC/B Battery

Alan Boss of the Carnegie Institution of Science estimates there may be 100 billion terrestrial planets in ourMilky Way Galaxy alone.[36] Boss believes many could have simple lifeforms and there could be thousands of civilizations in our galaxy. He uses an estimate that each sun-like star has on average one Earth-like planet.Sony VAIO VPCP115JC/D Battery

The discovery of Gliese 581 g, a Goldilocks planet only 20 light-years from Earth, has further called the Rare Earth hypothesis into question. With such proximity to Earth, exoplanetologists now estimate that the likelihood of finding an Earth-like planet in any given system in our galaxy is 10-20%,[37] bringing possible numbers close to Boss's guess.Sony VAIO VPCP115JC/G Battery

Evolutionary biology

Central to the Rare Earth hypothesis is the following claim about evolutionary biology: while microbes of some sort could well be common in the universe, complex life is unlikely to be. Yet to date, the only evolutionary biologist to speak to the hypothesis at any length is Simon Conway Morris (2003). Sony VAIO VPCP115JC/P Battery

The hypothesis concludes, more or less, that complex life is rare because it can evolve only on the surface of an Earth-like planet or on a suitable satellite of a planet. Some biologists, such as Jack Cohen, believe this assumption too restrictive and unimaginative;Sony VAIO VPCP115JC/W Battery

they see it as a form of circular reasoning (see Alternative biochemistry, a speculative biochemistry of alien life forms). Earth-like planets may indeed be very rare, but non carbon-based complex life could possibly emerge in other environments.Sony VAIO VPCP115KG Battery

According to David Darling, the Rare Earth hypothesis is neither hypothesis nor prediction, but merely a description of how life arose on Earth.  In his view Ward and Brownlee have done nothing more than select the factors that best suit their case.Sony VAIO VPCP116KG Battery

What matters is not whether there's anything unusual about the Earth; there's going to be something idiosyncratic about every planet in space. What matters is whether any of Earth's circumstances are not only unusual but also essential for complex life. So far we've seen nothing to suggest there is.Sony VAIO VPC-P116KX/B Battery,Sony VAIO VPC-P116KX/D Battery

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