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Big Bang

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The "Big Bang" marks the beginning point of the universe from which all time, space, matter, and energy originated. The term "Big Bang" was coined by Fred Hoyle in an attempt to mock the theory; however, the term caught on and was quickly adopted by proponents of the theory. Unfortunately, the adopted name often gives the mistaken impression of an explosion of matter into empty space, whereas the actual theory describes the rapid expansion of all space with no external reference frame.

Contents

History : What led us to consider the Big Bang?

  1. In 1916 Einstein proposed the Theory of General Relativity. This theory uses the model that Gravitational Force is a result of curvature of space-time. This model has since been considered as a successful model for large-scale behaviour of the universe.
  2. It was realised that Einstein's theory of General Relativity, as is, did not have 'static solutions', in other words, according to the theory, the universe would always either expand or contract. This was initially thought of as a problem. Einstein, himself, in order to explain the notion of a 'static universe' proposed the notion of a Cosmological Constant.
  3. However, in 1929 Hubble observed, using 'red shift' method, that galaxies were moving away from the earth at a speed which was proportional to their distance from the earth. An elementary mathematical analysis of this leads us to the conclusion that everthing converges to one point as we go backwards in time, a phenomenon now known as Big Bang.
  4. Even after indications from Einstein's Relativity and Hubble's observation, the idea of Big Bang met opposition when Fritz Zwicky proposed that a static universe was still possible, if 'light lost energy as it travelled'. This model is called the "Tired Light" model. However, subsequent advances in cosmological theory proved this option to be inviable, and the Big Bang is now considered to be the most probable explanation of Hubble's observations.

Facts and Misconceptions about the Big Bang

Singularity in the Big Bang
  1. The Big Bang theory is not just a 'belief' but is a well tested Scientific Theory. Like any other Scientific Theory it has been critically considered. It is the evidence which makes the Big Bang seem the most probable explaination.
  2. The Big Bang theory is unable to say anything about the exact point at which all matter seems to converge to. This point in space-time is sometiems referred to a 'singularity' and we currently have no satisfactory explanation about this. Thus the popular picture of the Big Bang of "explosion of a point of infinite density" is simply wrong.
  3. Big Bang is an explanation of the universe "after" the singularity. In particular the Big Bang does not say anything about how the universe was created.
  4. There is another concept called Big Crunch, related to the Big Bang. It is a point where the entire universe converges ( as opposed to diverges in the case of Big Bang). It has been proposed that the universe contracts and expands in a cyclic manner. However this is NOT a tested theory and is not a part of the standard Big Bang theory.

Evidence in support of Big Bang

  1. Hubble's Red Shift Observation.
  2. Large Scale Homogenity.
  3. Abundance of light elements.
  4. Cosmic Background Microwave Radiation
  5. Tolman Tests

Timeline

Going backwards in time from the present day to when the Big Bang occurred (13.7 billion years ago):

Quasars common; most of them have now burned out, but we see many active ones because their light has taken over 10 billion years to arrive at the Earth.

Around then: decoupling, when matter (slow particles) becomes more dense than radiation (particles traveling at the speed of light). This permits overdense regions to collapse, though that only happens later.

Around that time: supersymmetry, if it exists, would become broken. Supersymmetry-related integer-spin and half-odd-spin particles would acquire masses different by a few hundred GeV or more.

The "Particle Desert"; nothing much is expected to happen

What Is the Universe Made Of?

At first thought, it might be all the familiar "baryonic" matter and energy that we can observe and detect. But galaxies and galaxy clusters must have more mass in them than is contained in their stars, because otherwise they would fall apart on account of their internal motions. This extra mass, about 10 times the stars and interstellar gas, has been named "dark matter", and there have been numerous hypotheses as to what it might possibly be. Constraints from Big Bang nucleosynthesis suggest that only about 6% of the Universe's matter is baryonic matter; this means that nearly all of the dark matter is nonbaryonic, most likely Weakly Interacting Massive Particles (WIMP's). I note in passing that another candidate is substellar "Brown Dwarfs": Massive Compact Halo Objects (MACHO's), but they appear to be relatively insignificant.

WIMP's would be left over from early in the Big Bang, where various extensions of the Standard Model predict extra stable particles that the Big Bang could produce. One favorite such theory is supersymmetry, in which each known elementary particle has a counterpart with different spin and a mass of at least a few hundred GeV. The lightest one of these will not be able to decay into any other particle, and will thus survive.

There are experiments underway for detecting WIMP's by measuring the energy they impart by bouncing off of a nucleus in a supercold crystal, and upcoming particle accelerators like the Large Hadron Collider should be able to see some of these supersymmetric counterparts -- if they exist at some reasonable mass like a few hundred to a few thousand GeV.

More exotic than dark matter is "dark energy", which is inferred from recent measurements of the Universe's expansion -- it seems to be accelerating. This requires that some of the Universe's mass have a negative pressure comparable to its mass-energy density. Such a negative pressure is a potential byproduct of certain possible elementary-particle fields (spin-0 ones), but the nature of dark energy is otherwise a mystery.

How Far Is It?

Since we cannot stretch a measuring tape between the Earth and the Moon, let alone the Earth and a quasar, we have to use other methods.

A simple one is parallax. It is easy to demonstrate by closing one eye and then the other. A nearby object will jump around more than a distant one. It was used in the Solar System before interplanetary radar and spacecraft became practical, and it is the basis of distance scales outside of the Solar System.

But even with a baseline or "eye separation" the size of the Earth's orbit, parallax is useful only on the nearest stars, though the improved precision offered by the astrometric satellite Hipparcos has certainly been helpful. Beyond that, one can use a technique called "statistical parallax", in which one uses the Sun's motion relative to the nearby stars to generate a long baseline. But the stars being observed have their own motions, making this technique a statistical one.

To get beyond a few hundred light years / parsecs, it is necessary to use some version of "standard candles", where the "candle" is some object who luminosity one knows or has a reasonable guess of. For the more distant parts of our Galaxy, one looks at a star cluster, finds its "main sequence" stars, and then compares them to nearby main-sequence stars. with known distance. Such clusters may also contain relatively-bright "Cepheid variables", which are useful for measuring nearby galaxies.

Supernovae are the brightest of all such "candles", but they are also very rare; one has to look at lots of galaxies to have a chance of seeing one in a reasonable amount of time. But the nearest observed supernovae overlap with the farthest observable Cepheid variables, enabling calibration of their luminosity. The subset of supernovae known as Type 1a is especially useful, since they have a nearly-constant maximum luminosity of around 4 billion times that of the Sun. They are thought to be produced when a white dwarf star in a binary system has accreted a critical amount of material from its companion star, causing it to collapse and act like a giant nuclear bomb. This circumstance would explain why Type 1a supernovae are much alike.

Despite their great brightness, very little of their light reaches the Earth from cosmological distances, and distant-supernova searches have become feasible only in the last decade. But they are already producing interesting cosmological results and are helping to get improved figures for the Universe's expansion rate and its rate of variation.

This expansion rate can be used to calibrate the ultimate distance method -- distance as a function of redshift. And it can be used to work out the Universe's age; the best figure at present is 13.7 billion years (+/- 1%). This is over 9 billion years more than the Earth's age (4.5 billion years), and much greater than the ~6,000 years claimed by proponents of YEC.

Creationists have had several responses to that strong evidence of great age. Hugh Ross has rejected YEC in favor of OEC, while several others have adopted a Philip Henry Gosse-like solution of in-flight creation of starlight. But many creationists have preferred to believe that we are, in fact, observing the distant stars and galaxies.

A favorite solution along those lines has been to suppose that the speed of light in a vacuum (c) was much faster when the Universe was young, thus allowing the light to quickly traverse great distances. Barry Setterfield has advocated that solution, claiming that c has declined with time. However, the older measurements he has cited tend to be less accurate, and he has selected the high values and ignored the low values. When one uses both, there is no evidence of a decline. The c-decay hypothesis has the additional problem of what is the decline of c relative to, since in special relativity, c is essentially a units-conversion constant. In fact, in recognition of that, is now officially defined to be constant: 299,792,458 m/s.

An ambitious alternative is proposed in Russell Humphreys's Starlight and Time; he proposes that we are living inside of a "white hole", meaning that general-relativistic time dilation has operated on the rest of the Universe, so it goes through billions of years as only a few thousand years elapse on Earth. However, it is rather difficult to make that scenario be consistent with general relativity without grotesque ad-hoc additions.

Are Quasars Near Or Far?

An important part of the aforementioned distance scale has been calibration by association; Cepheid variables among main-sequence stars, supernovae among Cepheid variables. Inspired by this, the astronomer Halton Arp has proposed measuring the distances of quasars by finding quasars associated with galaxies and using that association to conclude that quasars are only as far away as those galaxies. And he has found several examples of seeming quasar-galaxy associations.

However, Arp's "results" are contrary to the much-greater distances inferred from their redshifts, meaning that there must be some other source of their redshifts if Arp is right. But has Arp only discovered line-of-sight coincidences? Most other astronomers think so.

And adding to the controversy has been the denial to Arp of some telescope time; this has made him seem like a Galileo-ish martyr to some.

Not surprisingly, some creationists appreciate Arp's work. But his quasar-associated galaxies are far enough for their light to need several million years to travel here, meaning that his claimed associations offer little comfort for proponents of YEC.

What is Inflation?

This is a hypothesized period of exponential expansion around the GUT era that can produce its remarkable flatness. During that period, quantum fluctuations became imprinted and redshifted, forming an approximately scale-free spectrum of fluctuations. And the spectrum of fluctuations observed in the CMB is approximately scale-free, thus being consistent with inflation.

Inflation was caused by a sort of "dark energy" that got damped toward the end of it, letting the Universe's dynamics be dominated by its matter-radiation content. However, as with present-day dark energy, it is not clear how inflation-era dark energy fits in with known particle physics, though it does require parameters that are relatively reasonable for a GUT-era particle.

Why Is There Matter?

The Universe is apparently all composed of the sort of matter familiar on Earth, and not of its mirror image, antimatter. That mirror-image matter has the properties of ordinary matter with the same composition, making it next to impossible to distinguish from ordinary matter by remote observation. However, antimatter cosmic rays would be recognizable as such, and ordinary matter and antimatter have a tendency to annihilate when they come into contact, producing lots of gamma rays and the like. But there is no evidence of either antimatter cosmic rays or annihilation zones anywhere in the observable Universe.

However, a solution to this cosmic-asymmetry conundrum was proposed by the physicist Andrei Sakharov in the 1960's. He proposed that there are elementary particles whose decay rates are antisymmetric in time -- that they produce more ordinary matter than antimatter when they decay. If there are such particles, then they would be produced in abundance in the high temperatures of the early Big Bang. But as the Universe expands and the Big Bang cools, these particles would stop being produced and would decay, producing a Universe with the matter-antimatter imbalance that we see today.

But is there any reason to suppose that such particles exist? The answer lies in what symmetries that elementary particles and their interactions may have. There are three types of symmetry of interest here:

C: charge (matter-antimatter)
P: parity (space reflection)
T: time (time reflection)
B: conservation of "baryon number", positive for ordinary matter and negative for antimatter

Using some very reasonable assumptions, it's possible to show that some elementary-particle theory must obey C, P, and T together (CPT symmetry), even if it breaks them separately. There is no comparable theoretical principle that mandates B, but all known interactions preserve B.

The strong nuclear, electromagnetic, and gravitational interactions obey C, P, and T separately, and it was taken for granted that a reasonable physical theory ought to do that until the 1950's, when it was discovered that weak interactions violate P. However, they were shown to violate C enough to keep CP symmetry, and thus T symmetry. And in the 1960's, neutral kaon decays were discovered to violate CP, meaning that they had to violate T in order to preserve CPT. Thus, the weak interaction violates C, P, and T.

According to the Sakharov scenario, there must exist particles whose interactions violate both CP and B. But violations of B are expected from many Grand Unified Theories, and since GUT's must explain the weak interactions, such GUT's will also violate CP, meaning that they satisfy the Sakharov conditions by violating both B and T. Beyond that, however, it has been difficult to make precise predictions.

Alternative models?

Just like any other scientific theory, the Big Bang theory has questions which are still under investigation and not completely understood. In principle, since no Scientific Theory is ever "the Final explanation", it is concievable that the Big Bang is corrected to included in a bigger theory.

Here is a list of scientific models proposed as an alternative to the Big Bang. However none of them can explain all observations which Big Bang can explain.

  1. Steady State Model (cannot explain Cosmic Microwave Background Radiation)
  2. Tired Light Model (cannot explain Time dilation of supernova light curves)


References /External Links

  1. Talkorigins.org/bigbang.html.
  2. http://en.wikipedia.org/wiki/Big_Bang
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