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Old 05-10-03   #1
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Dark Enough...?

What is dark matter?

There seems to be a halo of mysterious invisible material engulfing galaxies, which is commonly referred to as dark matter. Scientists infer the existence of dark (=invisible) matter from the observation of its gravitational pull, which causes the stars in the outer regions of a galaxy to orbit faster than they would if there was only visible matter present. Another indication is that we see galaxies in our own local cluster moving towards each other.

The Andromeda galaxy -about 2.2 million light years away from the Milky Way- is speeding toward us at 200,000 miles per hour. This motion can only be explained by gravitational attraction, even though the mass we observe is not nearly great enough to exert that kind of pull. It follows there must be a large amount of unseen mass causing the gravitational pull -roughly equivalent to ten times the size of the Milky Way- lying between the two galaxies.

Astronomers have no idea what the dark matter that supposedly makes up 90% to 99% of the mass of the universe is made of. Black holes and massive neutrinos are two possible explanations. Dark matter must have played an important role in galaxy formation during the evolution of the cosmos. Its existence will decide the ultimate fate of the universe, because it depends on the universe's total mass, whether gravitation is strong enough act against the expansion of space and eventually induce a period of contraction, or whether space keeps on expanding forever.

What is dark energy?

Dark energy is perhaps even more mysterious than dark matter. The discovery of dark energy goes back to 1998 when a 10-year study of supernovae took an astonishing turn. A group of scientists had recorded several dozen supernovae, including some so distant that their light had started toward Earth when the universe was only a fraction of its present age. The group's goal was to measure small changes in the expansion rate of the universe, which in turn would yield clues to the origin, structure, and fate of the cosmos. Contrary to their expectation, the scientists found that the expansion of the universe is not slowing, but accelerating.

The acceleration is supposedly due to the anti-gravitational properties of the so-called dark energy. While the exact nature of this energy is presently unknown, scientists agree that dark energy is the dominant constituent of our universe, which means that it is larger than visible and dark matter together. Einstein already postulated an anti-gravitational force at the beginning of the 20th century. He realized that the observed matter would lead to a gravitational collapse, and hence, introduced a cosmological constant to bring relativity into line with observation. After it was discovered by Hubble that the universe is expanding, Einstein called his cosmological constant the greatest blunder of his life.

Yet, at the beginning of the 21st century it seems that anti-gravity is coming back with a vengeance. A possible explanation is that the energy content of a vacuum is non-zero with a negative pressure. This negative pressure of the vacuum would grow in strength as the universe expands and it would cause the expansion to accelerate. If the acceleration does not stop to accelerate, this will lead to the Big Rip scenario suggested by Caldwell, in which the universe will be literally torn apart by the anti-gravitational force in several billion years.

Home did the universe come into being?

Stephen Hawking says in the foreword of The Cosmos Explained (Cambridge, July 28, 1997): "At the Big Bang, the universe and time itself came into existence, so that this is the first cause. If we could understand the Big Bang, we would know why the universe is the way it is. It used to be thought that it was impossible to apply the laws of science to the beginning of the universe, and indeed that it was sacrilegious to try. But recent developments in unifying the two pillars of twentieth-century science, Einstein's General Theory of Relativity and the Quantum Theory, have encouraged us to believe that it may be possible to find laws that hold even at the creation of the universe. In that case, everything in the universe would be determined by the laws of science. So if we understood those laws, we would in a sense be masters of the universe."

It is uncertain whether mankind is able to develop such a theory in the near future, and it may be even more questionable whether this knowledge would indeed help us to become masters of the universe, as Stephen Hawking connotes. Obviously it is difficult to speculate on a theory that has not been developed yet. The theory might as well have no practical value at all. The great 20th century physical theories showed us that complexity and abstraction are growing, while intelligibility and practical applicability seem to decrease. From a unified physical theory we can expect a more complete picture of matter, space, and time and a better understanding of the beginning of the universe. It may satisfy our curiosity in view of some big philosophical questions. Any practical value beyond this is rather uncertain.

Unified theories: How does gravity fit into the big picture?

The theory of gravity as formulated by Einstein is incompatible with the rules of quantum mechanics. Physicists encounter serious difficulties when trying to construct a quantum version of gravity. In the later years of his life, Einstein tried but failed to devise a theory that unifies gravity with quantum theory. In the 1960's, the weak subnuclear force was united with electromagnetism to form the electroweak theory, which was subsequently verified in particle accelerator experiments. The next step is to create a model that unites three of the four basic forces.

Theorists are working on such a model, which they call grand unified theory (GUT). It amalgamates electromagnetism with the weak and strong nuclear interaction, but omits gravity. From GUT we expect the answer to why particles have the masses we observe. Although we observe the masses of electrons, protons, and neutrons generated through what is called "electroweak breaking," we don't know how this breaking mechanism works. GUT should be able to interpret the electroweak breaking process and thus provide an explanation for the mass of a particle.

Beyond GUT, there is a theory that accounts for all four fundamental forces in nature, including gravity. The greatest endeavor of physics is to draw hitherto unrelated and incompatible theories together into a single unified theory. The advantage of such a system is obvious: It would account for all currently known phenomena without leaving theoretical holes and it may point towards future areas of study. It is hypothesized that such a theory could create a new fundamental understanding of nature. String theory, supersymmetry, and M-theory are presently the most promising candidates.

Are quarks and leptons actually fundamental, or are they made up of even more fundamental particles?

Presently it is not known whether quarks and leptons are elementary or compound particles. It seems that physicists have become more careful with announcing the fundamentality of particles after having learned that atoms, atom cores, and finally protons and neutrons are divisible. What is more, quarks and leptons are so small that they may be thought of as geometrical points in space with no spatial extension at all. This is perhaps not as miraculous as it first sounds, because after having learned from Rutherford's model that the volume of an atom is mostly made of "empty" space, it would not be too surprising to find out that matter is in fact nothing but empty space.

While the commonly accepted standard model of matter provides a very good description of the phenomena observed in experiments, the model is still incomplete. It can explain the behavior of particles fairly well, but it cannot explain why some particles exist as they do. For example, it has been impossible to predict the mass of the top quark accurately from theoretical inference until it was determined experimentally. As mentioned before, the standard model of matter does not provide any mathematical model that allows us to calculate the observed mass.

Another question concerns the fact that there are three families of quarks and leptons. Of the three families (or generations) of particles, only the first is stable, namely that of up/down quarks, e-neutrinos, and electrons. There seems to be no need for the other two generations in the natural world, yet they exist. Theoretical physics has no explanation for the existence of the two unstable generations. Likewise, the question why there is hardly any antimatter in the observable universe remains unaccounted for. Since there is an almost perfect symmetry between matter and antimatter, one would expect some regions of the universe to be composed of matter and others of antimatter, yet almost all mass we can observe is composed of conventional matter.

Is our universe unique, or are there many universes?

Andrei Linde at Stanford has brought forward the cosmological model of a multiverse, which he calls the "self-reproducing inflationary universe." The theory is based on Alan Guth's inflation model, and it includes multiple universes woven together in some kind of spacetime foam. Each universe exists in a closed volume of space and time. Linde's model, based on advanced principles of quantum physics, defies easy visualization. Quite simplified, it suggests quantum fluctuations in the universe's inflationary expansion period to have a wavelike character. Linde theorizes that these waves can "freeze" atop one another, thus magnifying their effect.

The stacked-up quantum waves can in turn create such intense disruptions in scalar fields -the underlying fields that determine the behavior of elementary particles- that they exceed a critical mass and start procreating new inflationary domains. The multiverse, Linde contends, is like a growing fractal, sprouting inflationary domains, with each domain spreading and cooling into a new universe.

If Linde is correct, our universe is just one of the sprouts. The theory neatly straddles two ancient ideas about the universe: that it had a definite beginning, and that it had existed forever. In Linde's view, each particular part of the multiverse, including our part, began from a singularity somewhere in the past, but that singularity was just one of an endless series that was spawned before it and will continue after it.

Will a complete physical model of the world help us to understand ultimate reality? Can we understand ultimate reality at all through science?

Some physicists believe that a complete physical model can explain everything we observe. They hold that once the fundamental laws are known and powerful computers allow us to compute models of the world by applying these laws, we can eventually deduce explanations for all phenomena. In other words, physics can lead us to understanding ultimate reality. Is this really possible?

One may doubt it. Even if we give physicists credit for their remarkable discoveries, we have to realize that their research takes place in an isolated field of knowledge. Physics does not concern itself with issues outside its own domain. For example, the subjects of biology, life, and chemistry, as well as the phenomena of mind and consciousness cannot be explained in physical terms. In addition, the following fundamental questions arise:

1. Physics deals only with what can be measured. A complete physical model must therefore necessarily produce a materialistic view of reality. Although materialists usually deny the possibility that phenomena exist which cannot be measured or somehow quantified, they may actually exist.

2. There are limits to what can be measured, as demonstrated by the uncertainty principle.

3. Like any form of knowledge, physics represents not the world, but our ideas of the world. The question arises whether our ideas converge with ultimate reality, or whether this convergence is an illusion.

4. Advanced physical models are abstract to the degree of being unintelligible to most people. Modern physics is based on higher mathematics and can hardly be put into common language, much less can it be imagined. The multidimensional worlds of relativity and string theory, for example, are elusive to plastic imagination. The value of any science depends on how useful its models are for the thoughts and actions of humanity as a whole, hence, its usefulness leans much on predictability and intelligibility.

Last edited by Jobe X; 05-10-03 at 16:15.
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Old 05-10-03   #2
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What is light?

Light is a phenomenon that has particle and wave characteristics. Its carrier particles are called photons, which are not really particles, but massless discrete units of energy.

What is the speed of light?

The speed of light is 299,792,458 m/s in a vacuum. The symbol used in relativity for the speed of light is "c", which probably stands for the Latin word "celeritas", meaning swift.

Is the speed of light really constant?

The speed of light is constant by definition in the sense that it is independent of the reference frame of the observer. Light travels slightly slower in a transparent medium, such as water, glass, and even air.

Can anything travel faster than light?

No. In relativity, c puts an absolute limit to speed at which any object can travel, hence, nothing, no particle, no rocket, no space vehicle can go at faster-than-light (=superluminal) speeds. However, there are some cases where things appear to move at superluminal speeds, such as in the following examples: 1. Consider two spaceships moving each at 0.6c in opposite directions. For a stationary observer, the distance between both ships grows at faster-than-light speed. The same is true for distant galaxies that drift apart in opposite directions of the sky. 2. Another example: Consider pointing a very strong laser on the moon so that it projects a dot on the moon's service and then moving the laser rapidly towards earth, so that it points on the floor in front of you. If you accomplish this in less than one second, the laser dot obviously traveled at superluminal speed, seeing that the average distance between the Earth and the Moon is 384,403 km.

What is matter?

The schoolbook definition would be: Matter is what takes up space and has mass. Matter as we know it is composed of molecules, which themselves are built from individual atoms. Atoms are composed of a core and one or more electrons that spin around the core in an electron cloud. The core is composed of protons and neutrons, the former have a positive electrical charge, the latter are electrically neutral. Protons and neutrons are composed of quarks, of which there are six types: up/down, charm/strange, and top/bottom. Quarks only exist in composite particles, whereas leptons can be seen as independent particles. There are six types of leptons: the electron, the muon, the tau and the three types of neutrinos. The particles that make up an atom could be seen as a stable form of locked up energy. Particles are extremely small, therefore 99.999999999999% (or maybe all) of an atom's volume is just empty space. Almost all visible matter in the universe is made of up/down quarks, electrons and (e-)-neutrinos, because the other particles are very unstable and quickly decay into the former.

How fast does an electron spin?

An electron in an hydrogen atom moves at about 2.2 million m/s. With the circumference of the n=1 state for hydrogen being about 0,33x10-9 m in size, it follows that an n=1 electron for a hydrogen atom revolves around the nucleus 6,569,372 billion times in just one second.

Are quarks and leptons all there is?

Not really. Fist of all, quarks always appear in composite particles, namely hadrons (baryons and mesons), then there is antimatter, and finally there are the four fundamental forces.

What is antimatter?

The existence of antimatter was first predicted in 1928 by Paul Dirac and has been experimentally verified by the artificial creation of the positron (e+) in a laboratory in 1933. The positron, the electron's antiparticle, carries a positive electrical charge. Not unlike a reflection in the mirror, there is exactly one antimatter particle for each known particle and they behave just like their corresponding matter particles, except they have opposite charges and/or spins. When a matter particle and antimatter particle meet, they annihilate each other into a flash of energy. The universe we can observe contains almost no antimatter. Therefore, antimatter particles are likely to meet their fate and collide with matter particles. Recent research suggests that the symmetry between matter and antimatter is less than perfect. Scientists have observed a phenomenon called charge/parity violation, which implies that antimatter presents not quite the reflection image of matter.

What are the four fundamental forces?

The four fundamental forces are gravity, the electromagnetic force, and the weak and strong nuclear forces. Any other force you can think of (magnetism, nuclear decay, friction, adhesion, etc.) is caused by one of these four fundamental forces or by a combination of them.

What is gravity?

Gravity is the force that causes objects on earth to fall down and stars and planets to attract each other. Isaac Newton quantified the gravitational force: F = mass1 * mass2 / distance≤. Gravity is a very weak force when compared with the other fundamental forces. The electrical repulsion between two electrons, for example, is some 10^40 times stronger than their gravitational attraction. Nevertheless, gravity is the dominant force on the large scales of interest in astronomy. Einstein describes gravitation not as a force, but as a consequence of the curvature of spacetime. This means that gravity can be explained in terms of geometry, rather than as interacting forces. The General Relativity model of gravitation is largely compatible with Newton, except that it accounts for certain phenomena such as the bending of light rays correctly, and is therefore more accurate than Newton's formula. According to General Relativity, matter tells space how to curve, while the curvature of space tells matter how to move. The carrier particle of the gravitational force is the graviton.

What is electromagnetism?

Electromagnetism is the force that causes like-charged particles to repel and oppositely-charged particles to attract each other. The carrier particle of the electromagnetic force is the photon. Photons of different energies span the electromagnetic spectrum of x rays, visible light, radio waves, and so forth. Residual electromagnetic force allows atoms to bond and form molecules.

What is the strong nuclear force?

The strong force acts between quarks to form hadrons. The nucleus of an atom is hold together on account of residual strong force, i.e. by quarks of neighboring neutrons and protons interacting with each other. Quarks have an electromagnetic charge and another property that is called color charge, they come in three different color charges. The carrier particles of the strong nuclear force are called gluons. In contrast to photons, gluons have a color charge, while composite particles like hadrons have no color charge.

What is the weak nuclear force?

Weak interactions are responsible for the decay of massive quarks and leptons into lighter quarks and leptons. It is the primary reason why matter is mainly composed of the stable lighter particles, namely up/down quarks and electrons. Radioactivity is due to the weak nuclear force. The carrier particles of the weak force are the W+, W-, and the Z particles.

How are carrier particles different from other particles?

Carrier particles, such as the photon, gluon, and the graviton are hypothetical. They are thought to be massless and having no electrical charge (except W+ and W-). Force carrier particles can only be absorbed or produced by a matter particle which is affected by that particular force. They allow us to explain interactions between matter.

How old is the universe?

Today's most widely accepted cosmology, the Big Bang theory, states that the universe is limited in space and time. The current estimate for the age of the universe is 13.7 billion years. This figure was computed from the cosmic microwave background (CM radiation data that the Wilkinson Microwave Anisotropy Probe (WMAP) captured in 2002.

What came before the Big Bang?

The Big Bang model is singular at the time of the Big Bang. This means that one cannot even define time, since spacetime is singular. In some models like the oscillating universe, suggested by Stephen Hawking, the expanding universe is just one of many phases of expansion and contraction. Other models postulate that our own universe is just one bubble in a spacetime foam containing a multitude of universes. The "multiverse" model of Linde proposes that multiple universes recursively spawn each other, like in a growing fractal. However, until now there is no observational data confirming either theory. It is indeed questionable, whether we will ever be able to gain empirical evidence speaking in favor these theories, because nothing outside our own universe can be observed directly. Hence, the question can currently not be answered by science.

How big is the universe?

The universe is constantly expanding in all directions, therefore its size cannot be stated. Scientists think it contains approximately 100 billion galaxies with each galaxy containing between 100 and 200 billion star systems. Our own galaxy, the Milky Way, is average when compared with other galaxies. It is a disk-shaped spiral galaxy of about 100,000 light-years in diameter.

What is the universe expanding into?

This question is based on the popular misconception that the universe is some curved object embedded in a higher dimensional space, and that the universe is expanding into this space. There is nothing whatsoever that we have measured or can measure that will show us anything about the larger space. Everything that we measure is within the universe, and we see no edge or boundary or center of expansion. Thus the universe is not expanding into anything that we can see, and this is not a profitable thing to think about.

Why is the sky dark at night?

If the universe were infinitely old, and infinite in extent, and stars could shine forever, then every direction you looked would eventually end on the surface of a star, and the whole sky would be as bright as the surface of the Sun. This is known as Olbers's paradox, named after Heinrich Wilhelm Olbers [1757-1840] who wrote about it in 1823-1826. Absorption by interstellar dust does not circumvent this paradox, since dust reradiates whatever radiation it absorbs within a few minutes, which is much less than the age of the universe. However, the universe is not infinitely old, and the expansion of the universe reduces the accumulated energy radiated by distant stars. Either one of these effects acting alone would solve Olbers's paradox, but they both act at once.

If the universe is only 13.7 billion years old, how can we see objects that are 30 billion light-years away?

This question is essentially answered by Special Relativity. When talking about the distance of a moving object, we mean the spatial separation now, with the positions of us and the object specified at the current time. In an expanding universe, this distance is now larger than the speed of light times the light travel time due to the increase of separations between objects, as the universe expands. It does not mean that any object in the universe travels away from us faster than light.

Last edited by Jobe X; 05-10-03 at 16:19.
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Old 05-10-03   #3
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That was a great read. Thanks!
- Karl

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Old 05-10-03   #4
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I thought that seeing as people have a lot of questions on alot of different but also related subjects I thought it might be of use to some.

We tend to ask one question only to find another after we know the answer...

Also I think it fits the whole 'Dark' forum thing
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Old 05-11-03   #5
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Well I always find these types of things interesting. I had never heard of the big rip theory until this post. I might research it more later.
- Karl

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