the fundamental forces of nature

8/8/2019 The Fundamental Forces of Nature

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Acknowledgement

To many individuals , I am indebted good connsel and

assistance in various ways. In this respect one of my

sincerest thanks to Mr. Avneesh (lecturer) of Lovely

Professional University, Phagwara for their kind

cooperation and guidance

I owe a deep since of indebtedness of my

pureness that have been source of inspiration of every

work of my life. I deeply express our ineptness and

thanks to all my faculty members of B.Techintg.

M.Tech IT for their valuable guidance which enable

me to presentable manners.


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CONTENTS :

y INTRODUCTIONy MAIN FOUR FORCESyNUCLEAR FORCESy WEAKFORCESy GRAVITATIONAL FORCESy ELECTRO MAGNETIC FORCESy THE GEOMETRY OF THE UNIVERSEy

LIFE

IN THE UN

IVERSE

y THE PROBLEMS WITH THE BIG BANGy REFRENCE

The Fundamental Forces of Nature


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There are 4 fundamental forces that have been identified. In our present Universe they haverather different properties.

Properties of the Fundamental Forces

y The strong interaction is very strong, but very short-ranged. It acts only over ranges oforder 10-13 centimetres and is responsible for holding the nuclei of atoms together. Itis basically attractive, but can be effectively repulsive in some circumstances.

y The electromagnetic force causes electric and magnetic effects such as the repulsionbetween like electrical charges or the interaction of bar magnets. It is long-ranged, but

much weaker than the strong force. It can be attractive or repulsive, and acts onlybetween pieces of matter carrying electrical charge.

y The weak force is responsible for radioactive decay and neutrino interactions. It has avery short range and, as its name indicates, it is very weak.

y The gravitational force is weak, but very long ranged. Furthermore, it is alwaysattractive, and acts between any two pieces of matter in the Universe since mass is its

source.

The Tortoise and the Hare: GravityAlways Wins

The four fundamental forces all play central roles in making the Universe what it is today, butwith respect to the large-scale issues that are of interest to cosmology it is gravitation that is

most important. This is because of two of its basic properties that set it apart from the otherforces: (1) it is long-ranged and thus can act over cosmological distances, and (2) it always

supplies an attractive force between any two pieces of matter in the Universe.

Thus, although gravitation is extremely weak, it always wins over cosmological distances and

therefore is the most important force for the understanding of the large scale structure andevolution of the Universe.

Unification of the Forces of Nature

Although the above discussion indicates that the fundamental forces in our present Universe

are distinct and have very different characteristics, the current thinking in theoretical physics

is that this was not always so. There is a rather strong belief (although it is yet to be

confirmed experimentally) that in the very early Universe when temperatures were very high

compared with today, the weak, electromagnetic, and strong forces were unified into a single

force. Only when the temperature dropped did these forces separate from each other, with the

strong force separating first and then at a still lower temperature the electromagnetic and

weak forces separating to leave us with the 4 distinct forces that we see in our present

Universe. The process of the forces separating from each other is called spontaneous

symmetry breaking. There is further speculation, which is even less firm than that above, that

at even higher temperatures (the Planck Scale) all four forces were unified into a single force.


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Then, as the temperature dropped, gravitation separated first and then the other 3 forcesseparated as described above. The time and temperature scales for this proposed sequential

loss of unification are illustrated in the following table.

Loss of Unity in the Forces of Nature

Characterization Forces UnifiedTime Since

Beginning

Temperature

(GeV)*

All 4 forces unifiedGravity, Strong,Electromagnetic, Weak ~0 ~infinite

Gravity separates (PlanckScale)

Strong, Electromagnetic,Weak

10-43 s 1019

Strong force separates(GUTs Scale)

Electromagnetic, Weak 10-35s 1014

Split of weak andelectromagnetic forces

None 10-11 s 100

Present Universe None 1010y 10-12*Temperature Conversion: 1 GeV = 1.2 x 1013 K

Theories that postulate the unification of the strong, weak, and electromagnetic forces are

called Grand Unified Theories (often known by the acronym GUTs). Theories that add

gravity to the mix and try to unify all four fundamental forces into a single force are calledSuperunified Theories. The theory that describes the unified electromagnetic and weak

interactions is called the Standard Electroweak Theory, or sometimes just the StandardModel.

Grand Unified and Superunified Theories remain theoretical speculations that are as yet

unproven, but there is strong experimental evidence for the unification of the electromagnetic

and weak interactions in the Standard Electroweak Theory. Furthermore, although GUTs are

not proven experimentally, there is strong circumstantial evidence to suggest that a theory atleast like a Grand Unified Theory is required to make sense of the Universe.

Gravitation and the General Theory of

RelativityAs we have discussed in an earlier section, the theoretical physicist Albert Einstein

introduced his Special Theory of Relativity in 1905 and his General Theory of Relativity in


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1915. The fi t showed that Newton's Three Laws of Motion were onl approxi atel correct breaking down when velocities approached that of light. The second showed that

Newton's Law of ravitation was also onl approxi atel correct breaking down whengravitation becames very strong.

SpecialRelativity

Einstein's Special Theory ofRelativity is valid for systems that are not accelerating. Since

from Newton's second lawan acceleration implies a force, special relativity is valid only

when no forces act. Thus, it cannot be used generally when there is a gravitational field

present (as we shall see below in conjunction with the Principle of Equivalence, it can be

used over a sufficiently localized region of spacetime).

e have already discussed some of the important implications of the Special Theory of

Relativity. For example, the most famous is probably the relationshi p between mass and

energy. Other striking consequences are associated with the dependence of space and time on

velocity: at speeds near that of light, space itself becomes contracted in the direction ofmotion and the passage of time slows. Although these seem bizarre ideas (because our

everyday experience typically does notinclude speeds near that oflight), many experimentsindicate that the Special Theory of Relativity is correct and our "common sense" (and

Newton's laws) are incorrect nearthe speed oflight.

eneralRelativity

The eneral Theory ofRelativity was Einstein's stupendous effort to remove the restriction

on Special Relativity that no accelerations (and therefore no forces) be present, so that hecould apply his ideas to the gravitational force. Itis a measure ofthe difficulty ofthe problem

thatittook even the great Einstein approximately 10 years to fully understand how to do this.

Thus, the eneral Theory of Relativity is a new theory of gravitation proposed in place of

Newtonian gravitation.

Tests ofthe Theory of eneralRelativity

eneralRelativity and Newton's gravitationaltheory make essentially identical predictions aslong as the strength of the gravitational field is weak, which is our usual experience.

However, there are several crucial predictions where the two theories diverge, and thus canbe tested with careful experiments.

1. The orientation ofMercury's orbitis found toprecess in space over time, as indicated inthe adjacent figure (the magnitude of the

effect is greatly exaggerated for purposes ofillustration). This is commonly called the

"precession of the perihelion", because it

causes the position ofthe perihelion to move

around the center of mass. Only part ofthis

can be accounted for by perturbations in Newton's theory. There is an extra 43

seconds of arc per century in this precession that is predicted by the Theory of

eneralRelativity and observed to occur (recallthat a second of arc is 1/3600 of an


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angular degree). This effect is extremely small, but the measurements are very preciseand can detect such small effects very well.

2. Einstein's theory predicts that the direction of light propagation should be changed ina gravitational field. Precise observations indicate that Einstein is right, both about the

effect and its magnitude. We have already seen a spectacular consequence of the

deflection of light in a gravitational field: gravitational lensing.

3. The General Theory of Relativity predicts that light coming from a stronggravitational field should have its wavelength shifted to larger values (a redshift).

Once again, detailed observations indicate such a redshift, and that its magnitude is

correctly given by Einstein's theory.

4. The electromagnetic field can have waves in it that carry energy and that we call light.Likewise, the gravitational field can have waves that carry energy and are called

gravitational waves. These may be thought of as ripples in the curvature of spacetime

that travel at the speed of light.

Just as accelerating charges can emit electromagnetic waves, accelerating masses can

emit gravitational waves. However gravitational waves are difficult to detect becausethey are very weak and no conclusive evidence has yet been reported for their direct

observation.

They have been observed indirectly in the binary pulsar. Because the arrival time ofpulses from the pulsar can be measured very precisely, it can be determined that the

period of the binary system is gradually decreasing. It is found that the rate of periodchange (about 75 millionths of a second each year) is what would be expected for

energy being lost to gravitational radiation, as predicted by the Theory of GeneralRelativity.

The Modern Theory ofGravitation

Our best current theory of gravitation is the General Theory of Relativity. However, only ifvelocities are comparable to that of light, or gravitational fields are much larger than those

encountered on the Earth, do the Relativity theory and Newton's theories differ in their

predictions. Under most conditions Newton's three laws and his theory of gravitation are

adequate.

The Principle of Equivalence

The General Theory of Relativity is formulated in terms of mathematics well beyond the

scope of our survey course in astronomy (primarily in fields of mathematics that go by the

names of tensor analysis and Riemannian geometry). Nevertheless, many of the basic ideas

can be understood without extensive mathematics.

General Relativity: the Principle of Equivalence


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One of the most important of these is the Principle of Equivalence, which can be used toderive important results without having to solve the full equations ofGeneral Relativity.

There are several ways to formulate the Principle of Equivalence, but one of the simplest is

Einstein's original insight: he suddenly reali ed, while sitting in his office in Bern,

Swit erland, in 1907, that if he were to fall freely in a gravitational field (think of a sky diver

before she opens her parachute, or an unfortunate elevator if its cable breaks), he would beunable to feel his own weight. Einstein later recounted that this reali ation was the "happiest

moment in his life", for he understood that this idea was the key to how to extend the Special

Theory of Relativity to include the effect of gravitation. We are used to seeing astrononauts

in free fall as their spacecraft circles the Earth these days, but we should appreciate that in

1907 this was a rather remarkable insight.

Importance of the Equivalence Principle

An equivalent formulation of the Principle of Equivalence is that at any local (that is,

sufficiently small) region in space time it is possible to formulate the equations governing

physical laws such that the effect of gravitation can be neglected. This in turn means that the

Special Theory of Relativity is valid for that particular situation, and this in turn allows a

number of things to be deduced because the solution of the equations for the Special Theory

of Relativity is beyond the scope of our course, but is not particularly difficult for those

trained in the required mathematics.

Consequences of the Principle of Equivalence

For example, by considering the Principle of Equivalence applied to light travelling across a

freely falling elevator, it is possible to conclude that light will follow a curved path in a

gravitational field. See this discussion to understand how. Likewise, by considering light

travelling upwards in an elevator in free fall, it is possible to conclude that light will be

redshifted in a gravitational field.

The Geometry of the Universe

The most profound insight of General Relativity was the conclusion that the effect of

gravitation could be reduced to a statement about the geometry of spacetime. In particular,

Einstein showed that in General Relativity mass caused space to curve, and objects travelling

in that curved space have their paths deflected, exactly as if a force had acted on them.

Curvature of Space in Two Dimensions


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The idea of a curved surface is not an unfamiliar one since we live on the surface of a sphere.

More generally, mathematicians distinguish 3 qualitatively different classes of curvature, as

illustrated in the following image (Source):

These are examples of surfaces that have two dimensions. For example, the left surface can

be described by a coordinate system having two variables (x and y, say); likewise, the other

two surfaces are each described by two independent coordinates. The flatsurface atthe leftissaid to have zero curvature, the spherical surface is said to have positive curvature, and the

saddle-shaped surface is said to have negative curvature.

Curvature of 4-Dimensional Spacetime

The preceding is nottoo difficultto visualize, but eneralRelativity asserts that space itself

(notjust an objectin space) can be curved, and furthermore, the space of eneralRelativity

has 3 space-like dimensions and one time dimension, notjusttwo as in our example above.

This IS difficult to visualize! Nevertheless, it can be descri bed mathematically by the samemethods that mathematicians use to describe the 2-dimensional surfaces that we can visualize

easily.

The Large-Scale eometry ofthe Universe

Since space itself is curved, there are three general possibilities for the geometry of theUniverse. Each ofthese possibilites is tied intimately to the amount of mass (and thus to the

total strength of gravitation) in the Universe, and each implies a different past and future forthe Universe:

y If space has negative curvature, there is insufficient mass to cause the expansion ofthe Universe to stop. The Universe in that case has no bounds, and will expandforever. This is termed an open universe.

y If space has no curvature (it is flat), there is exactly enough mass to cause theexpansion to stop, but only after an infinite amount oftime. Thus, the Universe has no

bounds in that case and will also expand forever, but with the rate of expansiongradually approaching zero after an infinite amount of time. This is termed a flat

universe or a Euclidian universe (because the usual geometry of non-curved surfaces

that we learn in high schoolis called Euclidian geometry).


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y If space has positive curvature, there is more than enough mass to stop the presentexpansion of the Universe. The Universe in this case is not infinite, but it has no end

(just as the area on the surface of a sphere is not infinite but there is no point on thesphere that could be called the "end"). The expansion will eventually stop and turn

into a contraction. Thus, at some point in the future the galaxies will stop receding

from each other and begin approaching each other as the Universe collapses on itself.

This is called a closed universe.

Which of these scenarios is correct is still unknown because we have been unable to

determine exactly how much mass is in the Universe.

Is the Universe Open, Flat, or Closed?

The geometry of the Universe is often

expressed in terms of the density parameter,which is defined to the the ratio of the actual

density of the Universe to the critical density

that would just be required to cause the

expansion to stop. Thus, if the Universe is

flat (contains just the amount of mass to

close it) the density parameter is exactly 1, if

the Universe is open with negative curvature

the density parameter lies between 0 and 1,

and if the Universe is closed with positive

curvature the density parameter is greater

than 1.

The density parameter determined from

various methods is summari ed in the adjacent table. In this table, BB nucleosynthesis refers

to constraints coming from the synthesis of the light elements in the big bang, +/- denotes anexperimental uncertainty in a quantity, and the parameter h lies in the range 0.5 to 0.85 and

measures the uncertainty in the value of the Hubble parameter.

Although most of these methods (which we will not discuss in detail) yield values of the

density parameter far below the critical value of 1, we must remember that they have likely

not detected all matter in the Universe yet. The current theoretical prejudice (because it is

predicted by the theory of cosmic inflation) is that the Universe is flat, with exactly the

amount of mass required to stop the expansion (the corresponding average critical density

that would just stop the is called the closure density), but this is not yet confirmed. Therefore,

the value of the density parameter and thus the ultimate fate of the Universe remains one of

the major unsolved problems in modern cosmology.

LIFE IN THE UNIVERSE

The VoyagerI and II spacecraft launched in 1977 are traveling out of the Solar System. The

adjacent image shows a plaque that is attached to each, intended as a greeting to any

The Density Parameter of the Universe

Source Value

Baryons (BB nucleosynthesis) (0.013 +/- 0.005) h-2

Stars in Galaxies 0.004

Intergalactic Stars 30 h-1Mpc) ~0.05 - 1

Source: P. J. E. Peebles, Principles of Physical Cosmology


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extraterrestrialcivilation that might find them (Ref). Given the vastness of interstellar space, itis admittedly very unlikely that the Voyagers would be found by an extraterrestrial

civili ation but the content of the plaque is a useful exercise in the issue of how we woulddeal with an encounter with intelligent living things from beyond our own planet. The content

of the plaque is discussed here.

In this material we have surveyed a few of the violent processes that are taking place in our

universe. These processes have played a central role in shaping our universe. The heavy

elements would not exist but for stars, and they would not be distributed through the galaxies

except for cataclysmic explosions such as novae and supernovae. Furthermore, there is

extensive evidence that the stars themselves are often born from the aftermath of violent

events: supernova blast waves triggering gravitational collapse in surrounding nubulae or

riotous star formation in the debris of colliding galaxies. The Universe itself, and all the

matter that it contains, seems to have been born in the Mother ofAll Explosions, and its

continuing evolution makes abundant use of explosive processes having magnitudes that defy

imagination.

It has been said that we are star-stuff; many of the atoms in our very bodies were almostcertainly forged in the furnaces of supernovae or novae in the distant past, and we may well

owe our present existence to star and element production that can be traced to exploding orcolliding galaxies in earlier epochs of the Universe. Thus we find the supreme irony that the

staid universe of the Middle Ages would likely be barren of life in the forms that we knowbecause it would preclude the formation of elements essential to that life, while the violent

universe of the modern astronomer has produced life in rich variety, at least in this corner ofan average solar system in an average galaxy of 100 billion stars. Though we cannot know

for certain, it is a reasonable assumption that many other life-forms in the Universe owe a

similar debt to exploding galaxies and stars.

Problems with the Big Bang

The hot big bang theory has been extremely successful in correlating the observableproperties of our Universe. However, there are some difficulties associated with the big bang

theory. These difficulties are not so much errors as they are assumptions that are necessarybut that do not have a fundamental justification. The required discussion is technical, so we

will be content with a rather superficial statement of the three basic problems that areassociated with the big bang and how they might be cured by a new idea that arises from

considering the implications of elementary particle physics for cosmology.

The Hori on Problem

We have already encountered the hori on problem in conjunction with the discussion of thecosmic microwave background: when we look at the microwave background radiation

coming from widely separated parts of the sky, it can be shown that these regions are tooseparated to have been able to have ever communicated with each other even with signals

travelling at light velocity. Thus, how did they know to have almost exactly the sametemperature? This general problem is called the hori on problem, because the inability to


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have received a signal from some distant source because of the finite speed of light is termeda hori on in cosmology. Thus, in the standard big bang theory we must simply assume the

required level of uniformity.

The Flatness Problem

The experimental evidence is that the present Universe has very low geometrical curvature inits spacetime (it is nearly flat). Theoretical arguments that are well established but too

complex to go into here suggest that this is a very unlikely result of the evolution of theUniverse from the big bang, unless the initial curvature is confined to an incredibly narrow

range of possibilities. While this is not impossible, it does not seem very natural.

The Monopole ProblemThe only plausible theory in elementary particle physics for how nuclei in the present

universe were created in the big bang requires the use of what are called Grand UnifiedTheories (GUTs). In these theories, at very high temperatures such as those found in the

instants after the Universe was created the strong, weak, and electromagnetic forces were(contrary to the situation today) indistinguishable from each other. We say that they were

unified into a single force.

Although there is as yet no certain evidence for the validity of such theories, there is strong

theoretical reason to believe that they will eventually turn out to be essentially correct. Our

current understanding of elementary particle physics indicates that such theories shouldproduce very massive particles called magnetic monopoles, and that there should be many

such monopoles in the Universe today. However, no one has ever found such a particle. So

the final problem is: where are the monopoles?

The Cosmic Background Radiation

In every direction,

thereis a very low energy and very uniform radiation that we see filling the Universe. This iscalled the 3 Degree Kelvin Background Radiation, or the Cosmic Background Radiation, or

the Microwave Background. These names come about because this radiation is essentially ablack body with temperature slightly less than 3 degrees Kelvin (about 2.76 K), which peaks

in the microwave portion of the spectrum. This radiation is the strongest evidence for the

validity of the hot big bang model. The adjacent figure shows the essentially perfect

blackbody spectrum obtained by NASA's Cosmic Background Explorer (COBE) satellite.

The following image was taken by COBE. It shows the temperature of the cosmic

background radiation plotted in galactic coordinates, with red cooler and blue and violet

hotter (Ref). This dipole anisotropy is because of the Doppler effect. If the Earth moves with

respect to the microwave background, it will be blue shifted to a higher effective temperature

in the direction of the Earth's motion and red shifted to a lower effective temperature in the

direction opposite the Earth's motion.


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The indication of the above image is that the local group of galaxies, to which the Earthbelongs, is moving at about 600 km/s with respect to the background radiation. It is not know

why the Earth is moving with such a highvelocity relative to the background radiation.

Evidence for the Big Bang

The cosmic background radiation (sometimes called the CBR), is the afterglow of the big

bang, cooled to a faint whisper in the microwave spectrum by the expansion of the Universefor 15 billion years (which causes the radiation originally produced in the big bang to redshift

to longer wavelengths). As shown in the adjacent intensity map of the background radiationin different directions taken by the Differential Microwave Radiometer on NASA's COBE

satellite, it is not completely uniform, though it is very nearly so (Ref). To obtain this image,the average dipole anisotropy exhibited in the image above has been subtracted out, since it

represents a Doppler shift due to the Earth's motion. Thus, what remains should represent true

variations in the temperature of the background radiation.

In this image, red denotes hotter fluctuations and blue and black denote cooler fluctuations

around the average. These fluctuations are extremely small, representing deviations from the

average of only about 1/100,000 of the average temperature of the observed background

radiation.

Problems with the Uniformity

The highly isotropic nature of the cosmic background radiation indicates that the early stages

of the Universe were almost completely uniform. This raises two problems for the big bang

theory.

First, when we look at the microwave background coming from widely separated parts of the

sky it can be shown that these regions are too separated to have been able to communicatewith each other even with signals travelling at light velocity. Thus, how did they know to

have almost exactly the same temperature? This general problem is called the hori onproblem.

Second, the present Universe is homogenous and isotropic, but only on very large scales. For

scales the si e of superclusters and smaller the luminous matter in the universe is quitelumpy, as illustrated in the following figure.

Thus, the discovery of small deviations from smoothness (anisotopies) in the cosmic

microwave background is welcome, for it provides at least the possibility for the seeds

around which structure formed in the later Universe. However, as we shall see, we are still far

from a quantitative understanding of how this came to be.

The Hot Big Bang

The big bang starts off with a state of extremely high density and pressure for the Universe.

Under those conditions, the Universe is dominated by radiation. This means that the majority


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of the energy is in the form of photons and other massless or nearly massless particles (likeneutrinos) that move at near the speed of light. As the big bang evolves in time, the

temperature drops rapidly as the Universe expands and the average velocity of particlesdecreases.

Finally, one reaches a state where the energy of the Universe is primarily contained in non-

relativistic matter (matter sufficiently massive that its average velocity is very much less thanthe speed of light). This is called a matter dominated universe. The early Universe was

radiation dominated, but the present Universe is matter dominated. Let us now give a brief

description of the most important events in the big bang.

The Cast of Characters for the Big Bang

The primary cast of characters includes:

1. Photons ("particles" of light)2. Protons and neutrons3. Electrons and their antiparticles the positrons4. Neutrinos and their antiparticles the antineutrinos

Because of the equivalence of mass and energy in the Special Theory of Relativity, in a

radiation dominated era the particles and their antiparticles are continuously undergoingreactions in which they annihilate each other, and photons can collide and create particle and

antiparticle pairs. One says that under these conditions the radiation and the matter are inthermal equilibrium because they can freely convert back and forth.

Let us now follow the approximate sequence of events that took place in the big bang in

terms of the time since the expansion begins.

At this stage the temperature is about 100 billion

Kelvin and the density is more than a billiontimes that of water. The Universe is expanding rapidly and is very hot; it consists of an

undifferentiated soup of matter and radiation in thermal equilibrium. This temperature

corresponds to an average energy of the particles of about 8.6 MeV (million electron-Volts).The electrons and positrons are in equilibrium with the photons, the neutrinos and

antineutrinos are in equilibrium with the photons, antineutrinos are combining with protons toform positrons and neutrons, and neutrinos are combining with neutrons to form electrons

and protons. At this stage the number of protons is about equal to the number of neutrons.Now the temperature has dropped to several times 10 billion Kelvin and the density is a little

over 10 million times that of water as the Universe continues to expand. Because a freeneutron is slightly less stable than a free proton, neutrons beta decay to protons plus electrons

plus neutrinos with a half-life of approximately 17 minutes. Thus, the initial approximately

equal balance between neutrons and protons begins to be tipped in favor of protons. By thistime about 62% of the nucleons are protons and 38% are neutrons.

The free neutron is unstable, but neutrons in composite nuclei can be stable, so the decay of

neutrons will continue until the simplest nucleus (deuterium, the mass-2 isotope of hydrogen)

can form. But no composite nuclei can form yet because the temperature implies an average

energy for particles in the gas of about 2.6 MeV, and deuterium has a binding energy of only

2.2 MeV and so cannot hold together at these temperatures. This barrier to production of


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composite nuclei, which allows the free neutrons to be steadily converted to protons, is calledthe deuterium bottleneck.

The temperature has dropped to about 10 billion Kas the Universe continues to expand, andthe density is now down to about 400,000 times that of water. At this temperature the

neutrinos cease to play a role in the continuing evolution, but the deuterium bottleneck still

exists so there are no composite nuclei and the neutrons continue to beta decay to protons. At

this stage the protons abundance is up to 76% and the neutron abundance has fallen to 24%.The temperature has now fallen to about 3 billion K. The average energy of the particles in

the gas has fallen to about 0.25MeV. This is too low for photons to produce electron-positron

pairs so they fall out of thermal equilibrium and the free electrons begin to annihilate all the

positrons to form photons. The deuterium bottleneck still keeps appreciable deuterium from

forming and the neutrons continue to decay to protons. At this stage the abundance of

neutrons has fallen to about 13% and the abundance of protons has risen to about 87%.

Finally the temperature drops sufficiently low (about 1 billion K) that deuterium nuclei can

hold together. The deuterium bottleneck is thus broken and a rapid sequence of nuclear

reactions combines neutrons and protons to form deuterium, and the resulting deuterium withneutrons and protons to form the mass-4 isotope of helium (alpha particles). Thus, all

remaining free neutrons are rapidly "cooked" into helium. Elements beyond helium-4 cannotbe formed because of the peculiarity that there are no stable mass-5 or mass-8 isotopes in our

Universe and the next steps in the most likely reactions to form heavier elements would formmass-5 or mass-8 isotopes.

The temperature is now about 300 million Kand the Universe consists of protons, the excesselectrons that did not annihilate with the positrons, helium-4 (26% abundance by mass),

photons, neutrinos, and antineutrinos. There are no atoms yet because the temperature is still

too high for the protons and electrons to bind together.

The temperature has fallen to several thousand K, which is sufficiently low that electrons and

protons can hold together to begin forming hydrogen atoms. Until this point, matter and

radiation have been in thermal equilibrium, but now they decouple. As the free electrons are

bound up in atoms the primary cross section leading to the scattering of photons (interaction

with the free electrons) is removed and the Universe (which has been very opaque until this

point) becomes transparent: light can now travel large distances before being absorbed.

Production of the Light Elements in the Big Bang

One important success of the big bang model has been in describing the abundance of light

elements such as hydrogen, helium, and lithium in the Universe. These elements are produced

in the big bang, and to some degree in stars. Analysis of the oldest stars, which contain

material that is the least altered from that produced originally in the big bang, indicate

abundances that are in very good agreement with the predictions of the hot big bang.

One particularly sensitive test involves the abundance of deuterium. Because deuterium has anucleus that is very weakly bound compared with most nuclei, it is very sensitive to the

conditions in which it is formed (as we have just seen): if the temperatures are too high,deuterium breaks apart, and it can only be formed when there are free neutrons to combine

with protons. Detailed analysis of the deuterium abundance gives very strong support to the

hot big bang picture.

The Steady State Model


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The big bang model had an early challenger that was called the steady state model. Thesteady state model did the cosmological principle one better by invoking what has been

termed the perfect cosmological principle: Not only is the Universe the same at all places andin all directions when averaged over a large enough volume; it is the same for all time too.

Since the Universe was known to be expanding, the steady state model had to postulate

continuous creation of matter in the space between the stars and galaxies to maintain thesame density over time and thus satisfy the perfect cosmological principle of a universe

unchanging in time on large scales. This violates the law of mass-energy conservation, but

the rate of mass creation that is required is far too small to be detectable by any conceivable

experiment, so it cannot be ruled out experimentally (the rate that is required is to create

approximately 1 hydrogen atom per cubic centimeter every 1015

years).

The Triumph of the Big Bang

For a time, the steady state theory and the big bang theory competed with each other, but

eventually observations all but ruled out the steady state theory while providing strong

support for the big bang. Probably the two most important observations were

1. Deep space radio telescope observations (which therefore peered far back in time because of the finite speed of light) indicating that the early Universe looked very

different from the present Universe. For example, there appear to be more quasars atgreat distances, implying that there were more quasars in the early Universe than the

present one. This contradicted the steady state hypothesis that the Universe wasunchanging over time on large scales.

2. The discovery of the cosmic microwave background to be discussed shortly, thatappeared to permeate all of space. This was an expected consquence of the big bang

model, but was very difficult to explain in any simple way in the steady state theory.

As a consequence of these and other findings, the steady state theory is no longer

considered viable by most astronomers.

REFRENCES:

y Fundamental forces in physicsy www.amezon.com


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the fundamental forces of nature

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