What is a gravitational wave?

What is a gravitational wave?

A gravitational wave* is a concept predicted by Einstein's theory of general relativity. General relativity states that mass distorts both space and time in the same way a heavy bowling ball will distort a trampoline.

When an object accelerates, it creates ripples in space-time, just like a boat causes ripples in a pond (and also similarly an accelerating electrical charge produces an electromagnetic wave). These space-time ripples are gravitational waves. They are extremely weak so are very difficult to detect. Missions like LISA or LIGO hope to spot gravitation waves detecting small changes in the distances between objects at set distances; satellites for LISA and mirrors for LIGO. As the strength of the wave depends on the mass of the object our best hope of detecting gravitational waves comes from detecting two black holes or pulsars collapsing into each other.

Gravitational waves have been inferred from watching two pulsars spinning and noticing they are slowing down, due to losing energy from emitting gravitational waves.

Gravitational waves are important in telling us about the early universe. The cosmic microwave background gives us a snapshot of the universe about 380,000 years after the start of the universe. Looking very closely at the cosmic microwave background there are patterns seen which can are also be measured in the large scale structure of the universe (so galaxies and clusters) today. These patterns in the cosmic microwave background were caused by very tiny random perturbations from the time when the universe expanded rapidly, known as inflation.

Inflation should also generate gravitational waves. These waves affect the polarization (the way the wave oscillates) of the cosmic microwave background. Measuring the strength of the polarization due to gravitational waves gives us a ballpark figure of the amount of energy involved at the time of inflation and helps pin down when inflation occurred.

*Not to be confused with a gravity wave (which is a wave driven by the force of gravity).

If scientists can't see dark matter, how do they know it exists?

Q.If scientists can't see dark matter, how do they know it exists?

Ans. Scientists calculate the mass of large objects in space by studying their motion. Astronomers examining spiral galaxies in the 1950s expected to see material in the center moving faster than on the outer edges. Instead, they found the stars in both locations traveled at the same velocity, indicating the galaxies contained more mass than could be seen. Studies of the gas within elliptical galaxies also indicated a need for more mass than found in visible objects. Clusters of galaxies would fly apart if the only mass they contained were visible to conventional astronomical measurements.

Albert Einstein showed that massive objects in the universe bend and distort light, allowing them to be used as lenses. By studying how light is distorted by galaxy clusters, astronomers have been able to create a map of dark matter in the universe.

Although dark matter makes up most of the matter of the universe, it only makes up about a quarter of the composition. The universe is dominated by dark energy.

After the Big Bang, the universe began expanding outward. Scientists once thought that it would eventually run out of the energy, slowing down as gravity pulled the objects inside it together. But studies of distant supernovae revealed that the universe today is expanding faster than it was in the past, not slower, indicating that the expansion is accelerating. This would only be possible if the universe contained enough energy to overcome gravity — dark energy.

All of these methods provide a strong indication that the most of the matter in the universe is something yet unseen.

Why does aurora take different shapes?

Scientists are still trying to answer this question.  The shape of the aurora depends on where in the magnetosphere the electrons came from and on what caused them to precipitate into the atmosphere. Dramatically different auroral shapes can be seen in a single night.

Types Of Galaxies

Many different types of galaxies exist. The different types of galaxies not only appear different, but have different evolutionary histories as well. The three fundamental classes of galaxies are elliptical, spiral, and irregular. These categories are further broken down into subclasses, often illustrated using a Hubble tuning fork diagram. Originally, scientists thought this diagram may have represented an evolutionary sequence for galaxies, but today we know that this is not true. The formation and evolution of galaxies is a complex process that is poorly understood.

Ellipticals

Elliptical galaxies are so named because they have elliptical shapes: they look like fat, fuzzy eggs or footballs. Stars in ellipticals do not spread out into a thin disk, as they do in spiral galaxies; instead, they wrap evenly around the galaxy's center in all directions. Ellipticals have smoothly varying brightnesses, with the degree of brightness steadily decreasing outward from the center. If you look at an ellipse-shaped surface that surrounds the center of an elliptical, all the stars on that surface will have similar brightnesses. Elliptical galaxies are also nearly all the same color: somewhat redder than the Sun. On the tuning fork diagram, they are classified as E, followed by a number indicating how elliptical a given galaxy is. The higher the number, the more elliptical the galaxy; that is, the longer the galaxy is with respect to its width.

The reddish color of ellipticals (as well as other more detailed observations) tells us something important their histories. The galaxies' red color comes from older, cooler stars. The fact that most of the light comes from old stars suggests that most ellipticals formed long ago. The fact that the color of an elliptical is more or less the same throughout the galaxy suggests that most of the stars in these galaxies formed at about the same time.

In addition, most elliptical galaxies in the universe are found near other elliptical galaxies, in galaxy clusters. In these clusters, some 75% of the galaxies are elliptical. This clustering also suggests that they formed a long time ago, because galaxies are likely to have formed first in high-density regions like galaxy clusters.

The largest galaxies in the universe are giant elliptical galaxies. They can contain a trillion stars or more, and span as much as two million light-years - about 20 times the width of the Milky Way. Some of them appear to contain supermassive black holes at their hearts - star-gobbling monsters that are as much as three billion times as heavy as the Sun. These giant ellipticals are often found in the hearts of galaxy clusters.

Spirals

Spiral galaxies like the one to the left have flat disks of stars with bright bulges called nuclei in their centers. Spiral arms wrap around these bulges. An extended spherical halo of stars envelops the nuclei and arms. Spiral arms probably form as the result of waves that sweep through the galactic disk. Like the waves on the ocean, these so-called "density waves" don't carry any material with them - they move by disrupting the material they pass through. In the case of galaxies, density waves squeeze clouds of interstellar gas, causing new stars to form inside the clouds. Some newborn stars are massive, hot, and bright, so they make the spiral arms appear bright. These massive stars are blue or white, so the spiral arms look blue-white, too. When viewed edge-on, the spiral arms often appear as dark lanes, because they contain lots of interstellar dust that blocks the light from the bulge. The gaps between the arms contain older stars, which are not as bright. However, the bulges of spirals are often red, like elliptical galaxies, suggesting that they are composed of older stars.

In some spirals, the density wave organizes the stars in the center into a bar. The arms of barred spiral galaxies spiral outward from the ends of the bar. The Milky Way may fall into this class of spirals, called barred spirals.

In the Hubble tuning fork system, normal spirals are designated "S" and the barred varieties "SB." Each of these classes is subclassified into three types according to the size of the nucleus and the degree to which the spiral arms are coiled. The three subclasses are denoted with the lowercase letters "a," "b," and "c." Some galaxies are also intermediate between ellipticals and spirals. These intermediate galaxies have the disk shape characteristic of spirals, but have no spiral arms. These intermediate forms bear the designation "S0." Three spiral galaxies are shown below.

Irregulars

The final class of galaxies, "irregulars," contains a hodge-podge of shapes - anything that looks neither spiral nor elliptical. Any galaxy with no identifiable form - whose stars, gas, and dust are spread randomly - is classified as irregular. Irregulars are the smallest galaxies, and they may contain as few as one million stars. They may be the "building blocks" that came together to form the first large galaxies. Many small irregular galaxies orbit the Milky Way, including the Large and Small Magellanic Clouds.

Hubble recognized two types of irregular galaxies, Irr I and Irr II. Irr I is the most common type of irregular galaxy. This type and seems to be an extension of the spiral classes, beyond Sc, into galaxies with no discernible spiral structure. Irr I galaxies are blue, highly resolved, and have little or no nucleus. Irr II galaxies are rare. This type includes various kinds of chaotic galaxies, which appear to have formed in many different ways.

Quasars

Quasars were first discovered in the early 1960s when radio astronomers identified a small star designated 3C 48 that emitted powerful radio waves. When they measured the spectrum of the star, they found something completely unexpected: the spectrum was flat with several unexpected, and totally unexplainable, emission lines. The object remained a mystery until similar but brighter object, 3C 273, was discovered in 1963. Astronomers noticed that 3C 273 had a normal spectrum with the same emission lines as observed in radio galaxies, but the spectrum had been greatly redshifted (that is, spectral lines were found at longer wavelengths than expected). This observation explained the mystery of 3C 48's spectrum: it was an ordinary spectrum from a radio galaxy, but it was so redshifted that familiar spectral lines were so far from where they should have been that no one recognized them. When an object moves away from us, its spectral lines are redshifted; the faster it moves, the greater the redshift. If 3C 273's redshift were to be due to its velocity, however, its velocity would have to be faster than the speed of light - which is impossible. Many more such objects were found, and they came to be known as quasi-stellar radio sources, abbreviated as quasars.

Today, we know that quasars are galaxies with extremely energetic nuclei. The amount of radiation emitted by such a nucleus overwhelms the light from the rest of the galaxy, so that only special observational techniques can reveal the rest of the galaxy's existence. The nucleus explains why quasars appear starlike - all we can see is the bright central engine.

Although the nucleus of a quasar is extremely small - only the size of the Solar System - it emits up to 100 times as much radiation as an entire galaxy. The galaxy underlying the brilliant image of a quasar is probably fairly normal, except for the superficial large-scale effects of the quasar at its center. Quasars are thought to be powered by supermassive black holes at the centers of galaxies. The powerful radiation we see comes from matter swirling around and falling into the black hole.

The SDSS (and sky surveys that use visible light) can find distant quasars at redshifts of 4-6, or 90% as old as the universe itself, because quasars look like stars but have peculiar colors. By searching for faint starlike objects and taking their spectra, the SDSS is expected to find thousands of quasars at redshifts greater than 4. The most distant quasar yet discovered, at a redshift of 6.4, was seen by the SDSS in January 2003.

Is there any difference between antimatter and dark matter?

Anonymous asked :Is there any difference between antimatter and dark matter?

Answer:Anti matter is just like normal matter only the sign of certain properties is different.  The classic case would be the electron, which has as it's anti particle the anti-electron also known as the positron.  Electrons are negatively charged, and Positrons are positively charged.  Yet they are identical in every other way.    Then their are particles like neutrons and protons which are made of even smaller particles called quarks.  Quarks interact via the strong atomic force, and electromagnetism.  Anti Quarks have opposite charges to Quarks in those two forces.

Dark matter on the other hand only interacts by way of gravity and the weak atomic force.  Dark matter does not interact via either the strong atomic force or electromagnetism hence dark matter cannot be seen and is hard to detect.  It only interacts via the weak force which is what keeps neutrons and protons inside the nucleus of atoms together.    Such is why experiments to detect dark matter directly rely on a particle of dark matter bumping into a particle of matter dead bang on the nucleus of an atom of normal matter.

Most of the reason we think dark matter exists has to do with the fact that it solves problems in cosmology in a very expedient way without us having to alter General Relativity.  It is widely agreed that dark matter whatever it turns out to be quantifies how much we really don't know about the matter in the universe.

Negative matter is a hypothetical type of matter which if it exist will have negative mass and negative energy.  It will in essence have a negative gravitational charge and repel normal matter.  Yet it will interact just like any other matter in every other way.

	Normal Matter	Anti-	Dark-	Negative-
Gravity	As usual	As usual*	As usual	Opposite sign
Electromagnetism	As usual	Opposite sign	No charge	?
Strong Force	As usual	Opposite sign	No charge	?
Weak Force	As usual	Opposite sign	As usual	?

*We assume that antimatter behaves as normal matter under gravity.  The truth is we have never seen a large enough mass of it to know for certain it behaves the same.  When it comes to Negative matter we know nothing and it may not even exist outside of certain theories.  Dark matter is on the edge of being a confirmed real entity.

Quranic Explanation of Pulsars & Black Holes

Neutron stars are remnants of exploded stars. As more matter falls into a neutron star its mass increases; and as its mass increases its gravity increases. A point will be reached where gravity would have grown so much that not even light could escape, thus a black hole forms. Most neutron stars discovered today are in the form of radio pulsars. They are called radio pulsars because they emit radio waves. We can simply connect a radio telescope to a loud speaker and hear a pulsar. Pulsars sound like someone persistently knocking. Click here and listen to a slow knocking pulsar. Click here and listen to a fast knocking pulsar. So in short, we can hear a pulsar knock; and if matter continues to fall into this pulsar a black hole will eventually form. Moslems say that this is what Allah says. The Quran describes a star by "The one who knocks" and says that it is "The one who makes a hole".

[Quran 86.1-3] And the heaven and the "Knocker" (Tarek in Arabic) How could you know about the "Knocker"? The piercing star (Thakeb in Arabic).

The Arabic word "Thukb" means a hole; "Thakeb" means the one who makes the hole. The Quran is describing a knocking star that makes a hole.

How could an illiterate man who lived 1400 years ago have known that a star can make a hole?

God swears by celestial bodies that are invisible, that move, that sweep:

(Quran 81.15-16) I swear by those that are invisible (Khunnas خنس), that move, that sweep.

Satan is invisible and is described in Arabic as "Khannas" (Quran 114.4). The Quran uses the same word "Khunnas" to describe the celestial bodies that are invisible. All celestial bodies have gravity and can sweep everything in their way but the only ones that are invisible are blackholes.

(Our sun is a very small star. Actually our solar system (including the atoms that you are made of) came from a dying star 100 times more massive than our sun. Some stars are even bigger than our solar system, so you can imagine how large stars are.

Quranic Explanation of Big Bang and Big Crunch

 

After the Big Bang the universe was primarily Hydrogen, Helium and a tiny bit of Lithium. However when a gas is too hot it becomes ionized (loses the electrons) and becomes opaque (like today's smoke). In the beginning the universe was opaque to visible light (non-transparent). After 380,000 years the universe cooled enough and it became transparent to visible light. For other wavelengths it was opaque for a billion years. So "Let there be light" turned out to be false. However the Quran correctly said that at the beginning it was SMOKE, that is, a hot non-transparent gas:

[Quran 41.11] Then He directed himself to the Heaven when it was SMOKE, and then said to it and to Earth: "Come willingly or by force" they said "We do come willingly"

How could an illiterate man who lived 1400 years ago have known that it was just smoke after the Big Bang?

The universe was created in a Big Bang 13.7 billion years ago, is still expanding today, and billions of years from now it might recollapse by its own gravity with a Big Crunch (or continue expanding forever, Big Chill). Moslems say that this is what Allah says. The Quran says that on the first day of creation, God made the heavens and the Earth meshed together, tight and compact (Big Bang), continues to expand it into the universe we know today and at the last day God will recompress it into its original state (Big Crunch). About the first day (Big Bang), Allah says:

[Quran 21.30] Do not those who disbelieve see that the heavens and the Earth were meshed together then We ripped them apart? And then We made of water everything living? Would they still not believe?

In the Quran the Heavens expanded from a single point.

Cosmologists just confirmed the existence of "Dark Energy", a mysterious repulsive force that acts in opposite to gravity. As the distance increases, the attractive gravitational force decreases but this mysterious repulsive force increases. This repulsive force is pushing galaxies apart; the greater the distance the greater the repulsion. Scientists today do not know what this "Dark Energy" is, but they know that it is causing the entire universe to expand at an increasing rate.

[Quran 51.47] And the heaven, We built it with craftsmanship and We are still expanding.

The Quran told us about this expansion 1400 years before it was discovered.

There are three possibilities to how the universe could end: Big Crunch (Quran), Big Chill and the Big Rip. NASA recently ruled out the third scenario (No Big Rip; see also: Universe Today). This leaves the universe with only two possible endings: Big Chill or Big Crunch, depending on what this Dark Energy turns out to be. Learn more: Expansion of Universe in Quran. In the Quran God promises to make the Big Crunch:

[Quran 21.104] On the day when We will fold the heaven, like the folder compacts the books, and as We originated the first creation We shall return it; a promise (binding on Us); surely We will deliver.

Here God promises to make this Big Crunch however not by gravity but rather by folding the Heavens like a book, that is, by the same mechanism He controls wormholes. This means that this Big Crunch can begin and end in a fraction of a second!

Also after this Big Crunch is over, God promises to recreate those heavens and Earth once more before Judgment Day:

[Quran 14.48] On the day when Earth will be swapped by another Earth and so will be the heavens; and all (creatures) will resurrect before the One Dominant God.

[Quran 36.81] Is He not, who created the heavens and the Earth, capable of creating others like them? Yes, indeed! He is the All-Knowing Creator. His command, if He wanted a thing, is that He only says to it, "BE" and it becomes! So glorified is He in whose hands is the dominion of all things, and to Him you shall be returned.

Quranic Explanation of Singularity

All stars will eventually die. They will either directly collapse on themselves and become black holes, or their remnants will merge with other stars that will eventually become black holes, or if external mass falls into those remnants they will blow up and leave black holes... So black holes are the destiny of most stars. Actually our solar system formed out of debris of a star 100 times more massive than our sun that exploded and became a black hole.

At the center of black holes (collapsed stars) lies a location called the Singularity. An observer far away from a black hole sees the events near a back hole in slow motion. If he shines a beam of light into this black hole he will have to wait forever but still this beam of light will never reach the singularity. The singularity is a location in the future of stars where gravity goes so mad that space and time become indistinguishable. From general relativity we know that this is a location where the structure of spacetime becomes singular (hence the name singularity). However singular (Ahad in Arabic) is one of God's 99 names. In the Quran God swears by the locations of stars which turned out to carry His own name:

[Quran 56.75-77] I swear by the locations of stars, it is a great swear if you knew, it is a noble Quran...

Here God swears not by the stars themselves but rather by their locations (mawakeh in Arabic). Today we know that the future of those locations is singular, that is, they carry God's own name: "Ahad".

Q: What is dark matter and why is it important to our understanding of the universe? - Predrag

Dark matter is matter that is invisible to our normal methods of observing matter in galaxies, yet it still has gravitational effects on visible matter. No one is quite sure what dark matter consists of and it's a topic of a lot of ongoing research.

Dark matter was originally theorized by an astronomer by the name of Fritz Zwicky who saw a large discrepancy between the observed mass of galaxies (the visible matter) and their computed masses (computed from gravitational effects). This discrepancy is known as the "missing mass problem." This missing mass is termed dark matter -- it is "dark" because it's not visually observable.

Understanding dark matter helps us understand the history of the universe. Our current knowledge of the formation of galaxies is not consistent with theories that do not involve dark matter, so the more we know about dark matter, the more we understand how these galaxies originally formed. Also, it helps us understand current behavior of galaxies. Without dark matter, objects further away from the center of a galaxy should move slower, but experimental evidence shows that instead, this motion is constant after a certain radius. Dark matter explains this anomaly.

An important concept related to dark matter is dark energy. Dark energy helps us understand the universe's expansion. More information on dark matter and dark energy can be found here and here.

Q: I have a question which has bothered me for quite a number of years and I just have no one to turn to ask. I hope that you can take a few minutes to help me understand. My question has to do with the basic structure of the universe. My understanding is that most physicists buy into the big bang theory - as supported by Hubble's observations of the red light shift indicating that the universe is constantly expanding. However, to put this concept in terms that I can understand, this would mean that from the moment of the big bang to now, the universe would constantly expand from a given point and that all matter would travel outward from that point at some particular speed - as with all explosions. After several billion years, this would create something of a balloon-like structure - relatively empty at the center - but with most matter falling within a certain zone at a constantly expanding radius from that central point - and allowing for differential speeds, collisions, etc. - a fuzzy balloon perhaps. This is the only mechanism I can understand that proves the red light shift is universal. However, it's also my understanding that when the galaxies have been mapped, they suggest the structure of the universe is actually more of a non-ending sponge-like structure with strings and clusters of galaxies linked together with gaps in between. This structure would seem to indicate a steady-state universe over a big bang/fuzzy balloon universe. The obvious conclusion is that the observable universe is completely at odds with the theoretical concept of the big bang. Is there something that ties these two concepts together? Or, if the observation of the sponge-like nature of the universe is correct, is there something fundamentally wrong with the big bang concept? - jeffrey

Yes, it's hard to initially grasp this, but the particular issues you're concerned about actually work out fine in the current picture. Let's look at the key points.

In the BB picture, the universe expands out not from one "given point" but from any given point. Stand anywhere. You'll see the stuff near you moving away, the farther the faster. Think of how things look from somebody else's point of view. They see the same thing.

One illustration often used is a raisin muffin expanding as it cooks. From any raisin's point of view, the other raisins are moving away. There's no particular place that gets especially empty, so there's nothing balloon-like about it. The red light shift is approximately proportional to distance, so it covers a huge range.
As for the current structure of the galaxies, on a fairly large scale it is indeed more spongy and irregular than one would get from well-stirred muffin dough. However, on a very large scale it looks quite uniform.

There is indeed a close tie-in between the BB picture and the current distribution of matter. At one stage, there were just small ripples in the density. These are still visible as small ripples in the cosmic microwave background (CMB) coming in from different directions. Over time, those ripples would tend to grow because regions with a little extra mass pull more mass in via gravity. That process can be simulated on a computer using ordinary gravitational dynamics. It turns out that the slight ripples in the CMB imply that now the matter distribution should have unevenness very close to what we see in the galaxies. So it really does all tie together.
The major remaining uncertainties concern what happened at even earlier stages. There the evidence starts getting thinner. For example, although the ripples in the CMB are close to what's expected from quantum fluctuations, we don't know for sure what was going on when those fluctuations set in. The main picture has been "inflation", a period of rapid exponential expansion, expected from General Relativity for certain types of transient physical states. (We're currently in a period of much slower inflation.) Problems with that picture have led to alternatives, including collisions between entire 3D spaces in some higher dimension. Weird as all that may sound, the Planck satellite is currently measuring details of the CMB ripples, in an attempt to sort out those possibilities.

F.A.Q's:Cosmology and Astrophysics

1. What happened at or before the Big Bang?  Was there really an initial singularity?  Does the history of the Universe go back in time forever, or only a finite amount?  Of course, these questions might not make sense, but they might.
2. Are there really three dimensions of space and one of time?  If so, why?  Or is spacetime higher-dimensional, or perhaps not really a manifold at all when examined on a short enough distance scale?  If so, why does it appear to have three dimensions of space and one of time?  Or are these unanswerable questions?
3. Is the Universe infinite in spatial extent?  More generally: what is the topology of space?
   

   We still don't know, but in 2003 some important work was done on this issue:

   • Neil J. Cornish, David N. Spergel, Glenn D. Starkman and Eiichiro Komatsu, Constraining the Topology of the Universe.

   Briefly, the Wilkinson Microwave Anisotropy Probe (WMAP) was used to rule out nontrivial topology within a distance of 78,000 million light years—at least for a large class of models.  For the precise details, you'll have to read the article!

4. Why is there an arrow of time; that is, why is the future so much different from the past?
   

   Here are two pieces of required reading for anyone interested in this tough question:

   • Huw Price, Time's Arrow and Archimedes' Point: New Directions for a Physics of Time, Oxford University Press, Oxford, 1996.
   • H. D. Zeh, The Physical Basis of the Direction of Time, second edition, Springer Verlag, Berlin, 1992.

5. Will the future of the Universe go on forever or not?  Will there be a "big crunch" at some future time, will the Universe keep on expanding forever, or what?
   

   There's been some progress on this one recently.  Starting in the late 1990s, a bunch of evidence has accumulated suggesting that the universe is not slowing down enough to recollapse in a so-called "big crunch".  In fact, it seems that some form of "dark energy" is making the expansion speed up!  We know very little about dark energy; it's really just a name for any invisible stuff that has enough negative pressure compared to its energy density that it tends to make the expansion of the universe tend to accelerate, rather than slow down.  (In general relativity, energy density tends to make the expansion slow down, but negative pressure has the opposite effect.)Einstein introduced dark energy to physics under the name of "the cosmological constant" when he was trying to explain how a static universe could fail to collapse.  This constant simply said what the density dark energy was supposed to be, without providing any explanation for its origin.  When Hubble observed the redshift of light from distant galaxies, and people concluded the universe was expanding, the idea of a cosmological constant fell out of fashion and Einstein called it his "greatest blunder".  But now that the expansion of the universe seems to be accelerating, a cosmological constant or some other form of dark energy seems plausible.
   For an examination of what an ever-accelerating expansion might mean for our universe, see:
   
   

   • John Baez, The End of the Universe.

   
   But, we still can't be sure the universe will expand forever, because the possibility remains that at some point the dark energy will go away, switch sign, or get bigger!  Here's a respectable paper suggesting that the dark energy will change sign and make the universe recollapse in a big crunch:
   

   • Renata Kallosh and Andrei Linde, Dark Energy and the Fate of the Universe.

   
   But here's a respectable paper suggesting the exact opposite: that the dark energy will get bigger and tear apart the universe in a "big rip":
   
   

   • Robert R. Caldwell, Marc Kamionkowski, and Nevin N. Weinberg, Phantom Energy and Cosmic Doomsday.

   
   In short, the ultimate fate of the universe remains an open question!
   But, before you launch into wild speculations, it's worth emphasizing that the late 1990s and early 2000s have seen a real revolution in experimental cosmology, which answered many open questions (for example: "how long ago was the Big Bang?") in shockingly precise ways (about 13,700 million years).  For good introduction to this material, try:
   

   • Ned Wright, Cosmology Tutorial and Frequently Asked Questions in Cosmology. 

   Our evidence concerning the expansion of the universe, dark energy, and dark matter now comes from a wide variety of sources, and what makes us confident we're on the right track is how nicely all this data agrees.  People are getting this data from various sources including:
   

   1. Distant Supernovae.  See especially these two experimental groups:
      • The High-z Supernova Search Team.
        See also their big paper.
      • The Supernova Cosmology Project.
        See also their big paper.
   2. The Cosmic Microwave Background (CMB).  There have been lots of great experiments measuring little ripples in the background radiation left over from the Big Bang.  For example:
      • Balloon Observations of Millimetric Extragalactic Radiation and Geophysics (BOOMERanG)
      • The Millimeter Anisotropy Experiment Imaging Array (MAXIMA)
      • The Wilkinson Microwave Anisotropy Probe (WMAP).
      • The Digital All-Sky Imager (DASI).
      • The Cosmic Background Imager (CBI).
   3. Large-Scale Structure.  Detailed studies of galactic clustering and how it changes with time give us lots of information about dark matter.  Here's the 800-pound gorilla in this field:
      
      • The Sloan Digital Sky Survey.

6. Is the universe really full of "dark energy"?  If so, what causes it?
   

   As mentioned above, evidence has been coming in that suggests the universe is full of some sort of "dark energy" with negative pressure.  For example, an analysis of data from the Wilkinson Microwave Anisotropy Probe in 2003 suggested that 73% of the energy density of the universe is in this form!  But even this is right and dark energy exists, we're still in the dark about what it is.The simplest model is a cosmological constant, meaning that so-called "empty" space actually has a negative pressure and positive energy density, with the pressure exactly equal to minus the energy density in units where the speed of light is 1.  However, nobody has had much luck explaining why empty space should be like this, especially with an energy density as small as what we seem to be observing: about 6 × 10⁻³⁰ grams per cubic centimeter if we use Einstein's E = mc² to convert it into a mass density.  Other widely studied possibilities for dark matter include various forms of "quintessence".  But, this term means little more than "some mysterious field with negative pressure", and there's little understanding of why such a field should exist.
   For more details, try these:
   

   • Ned Wright, Vacuum Energy Density, or: How Can Nothing Weigh Something?
   • John Baez, What's the Energy Density of the Vacuum?
   • Sean Carroll, The Cosmological Constant.

   The third is the most detailed, and it has lots of good references for further study.

7. Why does it seem like the gravitational mass of galaxies exceeds the mass of all the stuff we can see, even taking into account our best bets about invisible stuff like brown dwarfs, "Jupiters", and so on?  Is there some missing "dark matter"?  If so, is it ordinary matter, neutrinos, or something more exotic?  If not, is there some problem with our understanding of gravity, or what?
   

   Since the late 1990s, a consensus has emerged that some sort of "cold dark matter" is needed to explain all sorts of things we see.  For example, in 2003 an analysis of data from the Wilkinson Microwave Anisotropy Probe suggested that the energy density of the universe consists of about 23% cold dark matter, as compared to only 4% ordinary matter.  (The rest is dark energy.)Unfortunately nobody knows what this cold dark matter is!  It probably can't be ordinary matter we've neglected, or neutrinos, since these wouldn't have been sufficiently "cold" in the early universe to collapse into the lumps needed for galaxy formation.  There are many theories about what it might be.  There's also still a possibility that we are very confused about something, like our theory of gravity.
   For details, try these:
   

   • Jonathan Dursi, Dark Matter Tutorial.
   • Martin White, Dark Matter.
   • Timothy J. Sumner, Experimental Searches for Dark Matter.

   The last of these three is the most detailed, and it has lots of references for further study.

8. The Horizon Problem: why is the Universe almost, but not quite, homogeneous on the very largest distance scales?  Is this the result of an "inflationary epoch"—a period of rapid expansion in very early history of the universe, which could flatten out inhomogeneities?  If so, what caused this inflation?
   

   In 2003 the case for inflation was bolstered by the Wilkinson Microwave Anisotropy Probe, which made detailed measurements of "anisotropies" (slight deviations from perfect evenness) in the cosmic microwave background radiation.  The resulting "cosmic microwave background power spectrum" shows peaks and troughs whose precise features should be sensitive to many details of the very early history of the Universe.  Models that include inflation seem to fit this data very well, while those that don't, don't.However, the mechanism behind inflation remains somewhat mysterious.  Inflation can be nicely explained using quantum field theory by positing the existence of a special particle called the "inflaton", which gave rise to extremely high negative pressure before it decayed into other particles.  This may sound wacky, but it's really not.  The only problem is that nobody has any idea how this particle fits into known physics.  For example, it's not part of the Standard Model.
   For details, try:
   

   • Charles H. Lineweaver, Inflation and the Cosmic Microwave Background.
     Also available on the astrophysics arXiv.

9. Why are the galaxies distributed in clumps and filaments?
10. When were the first stars formed, and what were they like?
    

    As of 2004 this was quite a hot topic in astrophysics.  See for example:

    • Volker Bromm and Richard B. Larson, The First Stars.

11. What are Gamma Ray Bursters?
    

    Gamma ray bursters (GRBs) appear as bursts of gamma rays coming from points randomly scattered in the sky.  These bursts are very brief, lasting between a few milliseconds to a few hundred seconds.  For a long time there were hundreds of theories about what caused them, but very little evidence for any of these theories, since nothing was ever seen at the location where one of these bursts occurred.  Their random distribution eventually made a convincing case that they occurred not within our solar system or within our galaxy, but much farther away.  Given this, it was clear that they must be extraordinarily powerful.Starting in the late 1990s, astronomers made a concerted effort to catch gamma ray bursters in the act, focusing powerful telescopes to observe them in the visible and ultraviolet spectrum moments after a burst was detected.  These efforts paid off in 1999 when one was seen to emit visible light for as long as a day after the burst occurred.  A redshift measurement of z = 1.6 indicated that the gamma ray burster was about 10,000 million light years away.  If the burst of gamma rays was omnidirectional, this would mean that its power was about 10¹⁶ times that of our sun—for a very short time.  For details on this discovery, see:
    

    • Burst and Transient Source Experiment (BATSE), GOTCHA!  The Big One That Didn't Get Away, Gamma Ray Burst Headlines, January 27, 1999.

    A more detailed observation of a burst on March 3, 2003 convinced many astrophysicists that at least some gamma-ray bursters are so-called "hypernovae".  A hypernova is an exceptionally large supernova formed by the nearly instantaneous collapse of the core of a very large star, at least 10 times the mass of the sun, which has already blown off most of its hydrogen.  Such stars are called Wolf-Rayet stars.  The collapse of such a star need not be spherically symmetric, so the gamma ray burst could be directional, reducing the total power needed to explain the brightness we see here (if the burst happened to point towards us).  For more, try:
    

    • European Southern Observatory (ESO), Cosmological Gamma-Ray Bursts and Hypernovae Conclusively Linked, Press Release, June 18, 2003.

    It's hard to resist quoting the theory described here:
    
    

    Here is the complete story about GRB 030329, as the astronomers now read it.
    Thousands of years prior to this explosion, a very massive star, running out of hydrogen fuel, let loose much of its outer envelope, transforming itself into a bluish Wolf-Rayet star.  The remains of the star contained about 10 solar masses worth of helium, oxygen and heavier elements.
    In the years before the explosion, the Wolf-Rayet star rapidly depleted its remaining fuel.  At some moment, this suddenly triggered the hypernova/gamma-ray burst event.  The core collapsed, without the outer part of the star knowing.  A black hole formed inside, surrounded by a disk of accreting matter.  Within a few seconds, a jet of matter was launched away from that black hole.
    The jet passed through the outer shell of the star and, in conjunction with vigorous winds of newly formed radioactive nickel-56 blowing off the disk inside, shattered the star.  This shattering, the hypernova, shines brightly because of the presence of nickel.  Meanwhile, the jet plowed into material in the vicinity of the star, and created the gamma-ray burst which was recorded some 2,650 million years later by the astronomers on Earth.  The detailed mechanism for the production of gamma rays is still a matter of debate but it is either linked to interactions between the jet and matter previously ejected from the star, or to internal collisions inside the jet itself.
    This scenario represents the "collapsar" model, introduced by American astronomer Stan Woosley (University of California, Santa Cruz) in 1993 and a member of the current team, and best explains the observations of GRB 030329.
    "This does not mean that the gamma-ray burst mystery is now solved", says Woosley.  "We are confident now that long bursts involve a core collapse and a hypernova, likely creating a black hole.  We have convinced most skeptics.  We cannot reach any conclusion yet, however, on what causes the short gamma-ray bursts, those under two seconds long."

    Indeed, there seem to be at least two kinds of gamma-ray bursters, the "long" and "short" ones.  Nobody has caught the short ones in time to see their afterglows, so they are more mysterious.  For more information, try these:
    

    • NASA, Gamma Ray Bursts.
    • Edo Berger, Gamma-ray Burst FAQ.
    • Wikipedia, Gamma Ray Bursters.
    • Peter Mészáros, Gamma-ray Burst Physics.

    At the time this was written, NASA was scheduled to launch a satellite called "Swift", specially devoted to gamma-ray burst detection, in September 2004.  For details, see:
    

    • NASA, Swift: Catching Gamma-Ray Bursts on the Fly.

12. What is the origin and nature of ultra-high-energy cosmic rays?
    

    Cosmic rays are high-energy particles, mainly protons and alpha particles, which come from outer space and hit the Earth's atmosphere producing a shower of other particles.  Most of these are believed to have picked up their energy by interacting with shock waves in the interstellar medium.  But the highest-energy ones remain mysterious—nobody knows how they could have acquired such high energies.The record is a 1994 event detected by the Fly's Eye in Utah, which recorded a shower of particles produced by a cosmic ray of about 300 EeV.  A similar event has been detected by the Japanese scintillation array AGASA.  An EeV is an "exa-electron-volt", which is the energy an electron picks up going through a potential of 10¹⁸ volts.  300 EeV is about 50 joules—the energy of a one-kilogram mass moving at 10 meters/second, presumably all packed into one particle!
    Nobody knows how such high energies are attained—perhaps as a side effect of the shock made by a supernova or gamma-ray burster?  The puzzle is especially acute because because particles with energies like these are expected to interact with the cosmic microwave background radiation and lose energy after travelling only moderate extragalactic distances, say 100 mega light years.  This effect is called the Greisen-Zatsepin-Kuz'min (or GZK) cutoff.  So, either our understanding of the GZK cutoff is mistaken, or ultra-high-energy cosmic rays come from relatively nearby—in cosmological terms, that is.
    Right now the data is confusing, because two major experiments on ultra-high-energy cosmic rays have yielded conflicting results.  The Fly's Eye seems to see a sharp drop-off in the number of cosmic rays above 100 EeV, while the AGASA detector does not.  People hope that the Pierre Auger cosmic ray observatory, being built in western Argentina, will settle the question.
    For more information, try these:
    

    • HiRes—the High Resolution Fly's Eye experiment.
    • AGASA—the Akeno Giant Air Shower Array.
    • Pierre Auger Observatory.
    • A. A. Watson, Observations of Ultra-High Energy Cosmic Rays.
      
    • D. J. Bird et al., Detection of a cosmic ray with measured energy well beyond the expected spectral cutoff due to cosmic microwave radiation.

13. Do gravitational waves really exist?  If so, can we detect them?  If so, what will they teach us about the universe?  Will they mainly come from expected sources, or will they surprise us?
    

    Perhaps the most ambitious physics experiments of our age are the attempts to detect gravitational waves.  Right now the largest detector is LIGO—the the Laser Interferometer Gravitational-Wave Observatory.  This consists of two facilities: one in Livingston, Louisiana, and one in Hanford, Washington.  Each facility consists of laser beams bouncing back and forth along two 4-kilometer-long tubes arranged in an L shape.  As a gravitational wave passes by, the tubes should alternately stretch and squash—very slightly, but hopefully enough to be detected via changing interference patterns in the laser beam.LIGO is coming into operation in stages.  The first stage, called LIGO I, is supposed to allow detection of gravitational waves made by binary neutron stars within 65 mega light years of us.  These binaries emit lots of gravitational radiation, spiral into each other, and eventually merge.  In the last few minutes of this process you've got two objects heavier than the sun whipping around each other about 100 times a second, faster and faster, and they should emit a "chirp" of gravitational waves increasing in amplitude and frequency until the final merger.  It's these "chirps" that LIGO is optimized for detecting.  Later, in LIGO II, they'll try to boost the sensitivity to allow detection of in-spiralling binary neutron stars within 1000 mega light years of us.
    To give you an idea of what these distances are like: the radius of the Milky Way is about 50,000 light years.  The distance to the Andromeda galaxy is about 2.3 mega light years.  The radius of the "Local Group" consisting of three dozen nearby galaxies is about 6 mega light years.  The distance to the "Virgo Cluster", the nearest large cluster of galaxies, is about 50 mega light years.  The radius of the observable universe is roughly 10,000 mega light years.  So, if everything works as planned, we'll be able to see quite far with gravitational waves.
    However, binary neutron stars don't merge very often!  The current best guess is that with LIGO I we will be able to see such an event somewhere between once every 3000 years and once every 3 years.  I know, that's not a very precise estimate!  Luckily, the volume of space we survey grows as the cube of the distance we can see out to, so LIGO II should see between 1 and 1000 events per year.
    The really scary thing is how good LIGO needs to be to work as planned.  Roughly speaking, LIGO I aims to detect gravitational waves that distort distances by about 1 part in 10²¹.  Since the laser bounces back and forth between the mirrors about 50 times, the effective length of the detector is 200 kilometers.  Multiply this by 10⁻²¹ and you get 2 x 10⁻¹⁶ meters.  By comparison, the radius of a proton is 8 x 10⁻¹⁶ meters!  So, we're talking about measuring distances to within a quarter of a proton radius!  And that's just LIGO I.  LIGO II aims to detect waves that distort distances by a mere 2 parts in 10²³, so it needs to do 50 times better.
    Actually all this is a bit misleading.  The goal is not really to measure distances, but really vibrations with a given frequency.  However, it will still be an amazing feat... if it works.
    Getting LIGO to work has been a heroic endeavor: so far two earthquakes have caused damage to the equipment, and problems from tree logging in Livingston to wind-blown tumbleweeds in Hanford have made life more difficult than expected.  To keep up with the latest news, try the "LIGO Web Newsletter" here:
    

    • Laser Interferometer Gravitational Wave Observatory (LIGO) home page.

    LIGO is working in collaboration with the British/German GEO 600 detector in Hanover, Germany, a smaller detector that tests out lots of new technology.  Other gravitational wave detectors include the French/Italian collaboration VIRGO, the Japanese TAMA 300 project, and ACIGA in Australia.  For information on these and others try:
    

    • Other Gravitational Wave Detection Projects.

    But, the coolest gravitational wave detector of all—if it gets funded and gets off the ground—will be LISA, the Laser Interferometric Space Antenna:
    

    • NASA's LISA home page.
    • European Space Agency's LISA home page.

    The idea is to orbit 3 satellites in an equilateral triangle with sides 5 million kilometers long, and constantly measure the distance between them to an accuracy of a tenth of an angstrom (10⁻¹¹ meters) using laser interferometry.  The big distances would make it possible to detect gravitational waves with frequencies of 0.0001 to 0.1 hertz, much lower than the frequencies for which the ground-based detectors are optimized.  The plan involves a really neat technical trick to keep the satellites from being pushed around by solar wind and the like: each satellite will have a free-falling metal cube floating inside it, and if the satellite gets pushed to one side relative to this mass, sensors will detect this and thrusters will push the satellite back on course.
    For more details on what people hope to see with all these detectors, try this:
    
    

    • Curt Cutler and Kip Thorne, An Overview of Gravitational-Wave Sources.

14. Do black holes really exist?  (It sure seems like it.)  Do they really radiate energy and evaporate the way Hawking predicts?  If so, what happens when, after a finite amount of time, they radiate completely away?  What's left?  Do black holes really violate all conservation laws except conservation of energy, momentum, angular momentum and electric charge?  What happens to the information contained in an object that falls into a black hole?  Is it lost when the black hole evaporates?  Does this require a modification of quantum mechanics?
15. Is the Cosmic Censorship Hypothesis true?  Roughly, for generic collapsing isolated gravitational systems are the singularities that might develop guaranteed to be hidden beyond a smooth event horizon?  If Cosmic Censorship fails, what are these naked singularities like?  That is, what weird physical consequences would they have?
    

    Proving the Cosmic Censorship Hypothesis is a matter of mathematical physics rather than physics per se, but doing so would increase our understanding of general relativity.  There are actually at least two versions: Penrose formulated the "Strong Cosmic Censorship Conjecture" in 1986, and the "Weak Cosmic Censorship Hypothesis" in 1988.  A fairly precise mathematical version of the former one states:

    Every maximal Hausdorff development of generic initial data for Einstein's equations, compact or asymptotically flat, is globally hyperbolic.

    That's quite a mouthful, but roughly speaking, "globally hyperbolic" spacetimes are those for which causality is well-behaved, in the sense that there are no closed timelike curves or other pathologies.  Thus this conjecture states that for generic initial conditions, Einstein's equations lead to a spacetime in which causality is well-behaved.
    The conjecture has not been proved, but there are a lot of interesting partial results so far.  For a nice review of this work see:
    

    • Piotr Chrusciel, On the uniqueness in the large of solutions of Einstein's equations ("Strong cosmic censorship"), in Mathematical Aspects of Classical Field Theory, Contemp. Math. 132, American Mathematical Society, 1992.