Big Bang
The Big Bang is the cosmological model of the universe that is best supported by all lines of scientific evidence and observation. The essential idea is that the universe has expanded from a primordial hot and denseinitial condition at some finite time in the past and continues to expand to this day. Georges Lemaître proposed what became known as the Big Bang theory of the origin of the Universe, although he called it his 'hypothesis of the primeval atom'.
History
Main article: History of the Big Bang theory
See also: Timeline of cosmology and History of astronomy
The Big Bang theory developed from observations of the structure of the universe and from theoretical considerations. In 1912 Vesto Slipher measured the first Doppler shift of a "spiral nebula" (spiral nebula is the obsolete term for spiral galaxies), and soon discovered that almost all such nebulae were receding from Earth. He did not grasp the cosmological implications of this fact, and indeed at the time it was highly controversialwhether or not these nebulae were "island universes" outside our Milky Way. Ten years later, Alexander Friedmann, a Russian cosmologist and mathematician, derived the Friedmann equations from Albert Einstein'sequations of general relativity, showing that the universe might be expanding in contrast to the static universemodel advocated by Einstein. In 1924, Edwin Hubble's measurement of the great distance to the nearest spiral nebulae showed that these systems were indeed other galaxies. Independently deriving Friedmann's equations in 1927, Georges Lemaître, a Belgian physicist and Roman Catholic priest, predicted that the recession of the nebulae was due to the expansion of the universe.
Big bang theory assumptions
The Big Bang theory depends on two major assumptions: the universality of physical laws, and theCosmological Principle. The cosmological principle states that on large scales the universe is homogeneousand isotropic.
These ideas were initially taken as postulates, but today there are efforts to test each of them. For example, the first assumption has been tested by observations showing that largest possible deviation of the fine structure constant over much of the age of the universe is of order 10−5. Also, General Relativity has passed stringent tests on the scale of the solar system and binary stars while extrapolation to cosmological scales has been validated by the empirical successes of various aspects of the Big Bang theory.
If the large-scale universe appears isotropic as viewed from Earth, the cosmological principle can be derived from the simpler Copernican Principle, which states that there is no preferred (or special) observer or vantage point. To this end, the cosmological principle has been confirmed to a level of 10−5 via observations of the CMB. The universe has been measured to be homogeneous on the largest scales at the 10% level.
Dark matter
A pie chart indicating the proportional composition of different energy-density components of the universe, according to the best ΛCDM model fits. Roughly ninety-five percent is in the exotic forms of dark matter and dark energyDuring the 1970s and 1980s, various observations showed that there is not sufficient visible matter in the universe to account for the apparent strength of gravitational forces within and between galaxies. This led to the idea that up to 90% of the matter in the universe is dark matter that does not emit light or interact with normal baryonic matter. In addition, the assumption that the universe is mostly normal matter led to predictions that were strongly inconsistent with observations. In particular, the universe today is far more lumpy and contains far less deuterium than can be accounted for without dark matter. While dark matter was initially controversial, it is now indicated by numerous observations: the anisotropies in the CMB, galaxy cluster velocity dispersions, large-scale structure distributions, gravitational lensing studies, andX-ray measurements of galaxy clusters.The evidence for dark matter comes from its gravitational influence on other matter, and no dark matter particles have been observed in laboratories. Many particle physics candidates for dark matter have been proposed, and several projects to detect them directly are underway.[49]
Dark energy
Main article: Dark energy
Measurements of the redshift–magnitude relation for type Ia supernovae have revealed that the expansion of the universe has been accelerating since the universe was about half its present age. To explain this acceleration,general relativity requires that much of the energy in the universe consists of a component with large negative pressure, dubbed "dark energy". Dark energy is indicated by several other lines of evidence. Measurements of the cosmic microwave background indicate that the universe is very nearly spatially flat, and therefore according to general relativity the universe must have almost exactly the critical density of mass/energy. But the mass density of the universe can be measured from its gravitational clustering, and is found to have only about 30% of the critical density. Since dark energy does not cluster in the usual way it is the best explanation for the "missing" energy density. Dark energy is also required by two geometrical measures of the overall curvature of the universe, one using the frequency of gravitational lenses, and the other using the characteristic pattern of the large-scale structure as a cosmic ruler.
Negative pressure is a property of vacuum energy, but the exact nature of dark energy remains one of the great mysteries of the Big Bang. Possible candidates include a cosmological constant and quintessence. Results from the WMAP team in 2008, which combined data from the CMB and other sources, indicate that the universe today is 72% dark energy, 23% dark matter, 4.6% regular matter and less then 1% of neutrinos. The energy density in matter decreases with the expansion of the universe, but the dark energy density remains constant (or nearly so) as the universe expands. Therefore matter made up a larger fraction of the total energy of the universe in the past than it does today, but its fractional contribution will fall in the far future as dark energy becomes even more dominant.
In the ΛCDM, the best current model of the Big Bang, dark energy is explained by the presence of acosmological constant in the general theory of relativity. However, the size of the constant that properly explains dark energy is surprisingly small relative to naive estimates based on ideas about quantum gravity. Distinguishing between the cosmological constant and other explanations of dark energy is an active area of current research.
Black hole
A black hole is a theoretical region of space in which the gravitational field is so powerful that nothing, not even electromagnetic radiation (e.g.visible light), can escape its pull after having fallen past its event horizon. The term derives from the fact that the absorbsion of visible light renders the hole's interior invisible, and indistinguishable from the blackspace around it.
What makes it impossible to escape from black holes?
Popular accounts commonly try to explain the black hole phenomenon by using the concept of escape velocity, the speed needed for a vessel starting at the surface of a massive object to completely clear the object's gravitational field. It follows from Newton's law of gravitythat a sufficiently dense object's escape velocity will equal or even exceed the speed of light. Citing that nothing can exceed the speed of light they then infer that nothing would be able to escape such a dense object. However, the argument has a flaw in that it doesn't explain why light would be affected by a gravitating body or why it would not be able to escape. Nor does it give a satisfactory explanation for why a powered spaceship would not be able to break free.
Two concepts introduced by Albert Einstein are needed to explain the phenomenon. The first is that time and space are not two independent concepts, but are interrelated forming a single continuum, spacetime. This continuum has some special properties. An object is not free to move around spacetime at will, instead it must always move forwards in time, and not only must an object move forwards in time, it also cannot change its position faster than the speed of light. This is the main result of the theory of special relativity.
The second concept is the base of general relativity: mass deforms the structure of this spacetime. The effect of a mass on spacetime can informally be described as tilting the direction of time towards the mass. As a result, objects tend to move towards masses. This is experienced as gravity. This tilting effect becomes more pronounced as the distance to the mass becomes smaller. At some point close to the mass the tilting becomes so strong that all the possible paths an object can take lead towards the mass. This implies that any object that crosses this point can no longer get further away from the mass, not even using powered flight. This point is called the event horizon.
According to the "No Hair" theorem a black hole has only three independent physical properties: mass, chargeand angular momentum. Any two black holes that share the same values for these properties are completely indistinguishable. This contrasts with other astrophysical objects such as stars, which have very many—possibly infinitely many—parameters. Consequently, a great deal of information is lost when a star collapses to form a black hole. Since in most physical theories information is (in some sense) preserved, this loss of information in black holes is puzzling. Physicists refer to this as the black hole information paradox.
Effects of falling into a black hole
Spaghettification
Main article: spaghettification
An object in any very strong gravitational field feels a tidal force stretching it in the direction of the object generating the gravitational field. This is because the inverse square law causes nearer parts of the stretched object to feel a stronger attraction than farther parts. Near black holes, the tidal force is expected to be strong enough to deform any object falling into it, even atoms or composite nucleons; this is called spaghettification. The process of spaghettification is as follows. First, the object that is falling into the black hole splits in two. Then the two pieces each split themselves, rendering a total of four pieces. Then the four pieces split to form eight. This process of bifurcation continues up to and past the point in which the split-up pieces of the original object are at the order of magnitude of the constituents of atoms. At the end of the spaghettification process, the object is a string of elementary particles.
Evaporation
If Hawking's theory of black hole radiation is correct then black holes are expected to emit a thermal spectrum of radiation, and thereby lose mass, because according to the theory of relativity mass is just highly condensed energy (e = mc2). Black holes will thus shrink and evaporate over time. The temperature of this spectrum (Hawking temperature) is proportional to the surface gravity of the black hole, which in turn is inversely proportional to the mass. Large black holes thus emit less radiation than small black holes.
Micro black holes
In theory there is no smallest size for a black hole. Once created, it has the properties of a black hole. Stephen Hawking theorized thatprimordial black holes could evaporate and become even tinier, i.e. micro black holes. Searches for evaporating primordial black holes are proposed for the GLAST satellite to be launched in 2008. However, if micro black holes can be created by other means, such as by cosmic ray impacts or in colliders, that does not imply that they must evaporate.
The formation of black hole analogs on Earth in particle accelerators has been reported. These black hole analogs are not the same as gravitational black holes, but they are vital testing grounds for quantum theories of gravity.
They act like black holes because of the correspondence between the theory of the strong nuclear force, which has nothing to do with gravity, and the quantum theory of gravity. They are similar because both are described by string theory. So the formation and disintegration of a fireball in quark gluon plasma can be interpreted in black hole language. The fireball at the Relativistic Heavy Ion Collider [RHIC] is a phenomenon which is closely analogous to a black hole, and many of its physical properties can be correctly predicted using this analogy. The fireball, however, is not a gravitational object. It is presently unknown whether the much more energeticLarge Hadron Collider [LHC] would be capable of producing the speculative large extra dimension micro black hole, as many theorists have suggested.
Hitting the singularity
As an infalling object approaches the singularity, tidal forces acting on it approach infinity. All components of the object, including atoms and subatomic particles, are torn away from each other before striking the singularity. At the singularity itself, effects are unknown; it is believed that a theory of quantum gravity is needed to accurately describe events near it.
Large Hadron Collider
The Large Hadron Collider (LHC) is the world's largest and highest-energy particle acceleratorcomplex, intended to collide opposing beams ofprotons charged with approximately 7 TeV of energy. Its main purpose is to explore the validity and limitations of the Standard Model, the current theoretical picture for particle physics. It is theorized that the collider will produce the Higgs boson, theobservation of which could confirm the predictions and missing links in the Standard Model, and could explain how other elementary particles acquire properties such as mass.
Safety issues
Safety of particle collisions
Main article: Safety of the Large Hadron Collider
Although some individuals, including some scientists, have questioned the safety of the planned experiments in the media and through the courts, the consensus in the scientific community is that there is no basis for any conceivable threat from the LHC particle collisions.
Operational safety
The size of the LHC constitutes an exceptional engineering challenge with unique operational issues on account of the huge energy stored in the magnets and the beams. While operating, the total energy stored in the magnets is 10 GJ (equivalent to 2.4 tons of TNT) and the total energy carried by the two beams reaches 724 MJ.
Loss of only one ten-millionth part (10−7) of the beam is sufficient to quench a superconducting magnet, while the beam dump must absorb an energy equivalent to that of a typical air-dropped bomb. These immense energies are even more impressive when one considers how little matter is carrying it. Under nominal operating conditions (2,808 bunches per beam, 1.15×1011 protons per bunch), the beam pipes contain 1.0×10-9 gram of hydrogen, which, in standard conditions for temperature and pressure, would fill the volume of one grain of fine sand.
Safety of the Large Hadron Collider
Concerns have been raised in the media and through the courts about the safety of the particle physicsexperiments planned to take place at the Large Hadron Collider (LHC), the world's largest and most powerfulparticle accelerator to date, built by the European Organization for Nuclear Research (CERN) near Geneva, inSwitzerland. The claimed dangers of the LHC particle collisions, which are scheduled to begin on 21 October 2008,] include doomsday scenarios involving the production of stable micro black holes or the creation of hypothetical particles called strangelets. The potential risks of these unprecedented experiments were reviewed in 2003 by the LHC Safety Study Group, a group of independent scientists, who concluded that, like current particle experiments such as the Relativistic Heavy Ion Collider (RHIC), the LHC particle collisions pose no conceivable threat. A second review of the evidence commissioned by CERN in 2008 reaffirmed the safety of the LHC collisions in light of further research conducted since the 2003 assessment. The 2008 report was reviewed and endorsed by CERN's governing body and by the Division of Particles & Fields of theAmerican Physical Society and was published in the Journal of Physics G. It concludes that any doomsday scenarios at the LHC are ruled out because the physical conditions and events that will be created in the LHC experiments occur naturally in the universe without hazardous consequences.
Safety concerns
In the run up to the commissioning of the LHC, with the first high energy collisions scheduled for 21 October 2008, a group of individuals led by Walter L. Wagner (a former nuclear safety officer and an original opponent of the RHIC) and Otto Rössler, a German biochemist, have expressed concerns over the safety of the LHC, and have attempted to halt the beginning of the experiments through petitions to the American and European Courts. Opponents assert that the LHC experiments have the potential to create low velocity micro black holes that could grow in mass or release dangerous radiation leading to doomsday scenarios, such as the destruction of the Earth. Other claimed potential risks include the creation of theoretical particles calledstrangelets, magnetic monopoles and vacuum bubbles. The claims made about the safety of the LHC have attracted widespread media attention. This has raised fears among the public that the Earth will be destroyed by the experiments at the LHC, and scientists associated with the project have received many protests. The Large Hadron Collider team revealed they had received death threats and threatening emails and phone calls demanding the experiment be halted.
Safety reviews
Fears similar to those for the LHC were raised in connection with the Relativistic Heavy Ion Collider (RHIC) particle accelerator. After detailed studies, scientists reached such conclusions as "beyond reasonable doubt, heavy-ion experiments at RHIC will not endanger our planet" and that there is "powerful empirical evidence against the possibility of dangerous strangelet production."
CERN-commissioned reports
Drawing from research performed to assess the safety of the RHIC collisions, the LHC Safety Study Group, a group of independent scientists, performed a safety analysis of the LHC, and released their findings in the 2003 report Study of Potentially Dangerous Events During Heavy-Ion Collisions at the LHC. The report concluded that there is "no basis for any conceivable threat". Several of its arguments were based on the accepted theoretical but unobserved existence of Hawking radiation and the theoretical predictions of the Standard Model with regard to the outcome of events to be studied in the LHC. One argument raised against doomsday fears was that collisions at energies equivalent to and higher than those of the LHC have been happening in nature for billions of years apparently without hazardous effects, as ultra-high-energy cosmic rays impact Earth's atmosphere and other bodies in the universe.
On 20 June 2008, in light of new experimental data and theoretical understanding, the LHC Safety Assessment Group (LSAG) issued a report updating the 2003 safety review, in which they reaffirmed and extended its conclusions that "LHC collisions present no danger and that there are no reasons for concern". The LSAG report was then reviewed and endorsed by CERN’s Scientific Policy Committee, a group of external scientists that advises CERN’s governing body, its Council. On 5 September 2008, the LSAG's "Review of the safety of LHC collisions" was published in the Journal of Physics G: Nuclear and Particle Physics.
Micro black holes
A major concern amongst CERN opponents is that any micro black holes produced by the LHC particle collisions may have a very low velocity and so, unlike any created by natural cosmic ray collisions, they will not escape the Earth's gravitational pull. If the black holes do not immediately decay, they may remain within the Earth and begin accreting (collecting) matter in the planet. Walter L. Wagner has argued that if micro black holes are produced at the LHC, they might not decay as predicted by CERN, since Hawking radiation is not an experimentally-tested or naturally observed phenomenon and might not exist.
Although the Standard Model of particle physics predicts that LHC energies are far too low to create black holes, some extensions of the Standard Model posit the existence of extra spatial dimensions, in which it would be possible to create micro black holes at the LHC at a rate on the order of one per second. According to the standard calculations these are harmless because they would quickly decay by Hawking radiation. The LHC Safety Assessment Group (LSAG) indicates that "there is broad consensus among physicists on the reality of Hawking radiation, but so far no experiment has had the sensitivity required to find direct evidence for it." According to the LSAG, even if micro black holes were produced by the LHC and were stable, "they would be unable to accrete matter in a manner dangerous for the Earth. [...] they would also have been produced by cosmic rays and have stopped in the Earth or some other astronomical body, and the stability of these astronomical bodies means that they cannot be dangerous." The LSAG argues that:
| “ | Stable black holes could be either electrically charged or neutral. [...] If stable microscopic black holes had no electric charge, their interactions with the Earth would be very weak. Those produced by cosmic rays would pass harmlessly through the Earth into space, whereas those produced by the LHC could remain on Earth. However, there are much larger and denser astronomical bodies than the Earth in the Universe. Black holes produced in cosmic-ray collisions with bodies such as neutron stars and white dwarf stars would be brought to rest. The continued existence of such dense bodies, as well as the Earth, rules out the possibility of the LHC producing any dangerous black holes. | ” |
