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.