Ok, fair point - there are a lot of little, dense things like brown dwarfs and planets that we can't currently observe. However, these can only be a tiny component of the "dark" stuff that we don't see. If brown dwarfs or planets comprised a significant chunk of the dark matter, it would be detected by gravitational microlensing events, and those observations suggest that dense baryonic objects (such as stars, brown dwarves, etc.) aren't a big (which is to say, dynamically important) component of the galactic halo.
Also, with respect to dust, it's actually quite easy to detect it in the interstellar medium, in both emission and absorption. It doesn't ALWAYS emit radiation, and doesn't do it spontaneously, but when dust is bombarded by light from nearby stars, it tends to re-emit in the infrared and radio. So it's incredibly easy to detect it in both of those bands, and use it to learn things about galaxies. It blocks optical light, of course, so you can see it in nearby (and not so nearby) gas-rich galaxies.
I see by the link below your name that you're from Berkeley, or at least probably have some berkeley ties. You should go talk to Chris McKee in the astronomy department if you think dust is more of a pain than it's worth - he'll set you straight!
Radiant energy (which is to say photons) is actually a very tiny fraction of the total energy density of the universe - something less than 0.1%. We ignore it in our simulations, but it's safe to do so.
You are mostly correct - what we're talking about are the filaments that link galaxy groups and clusters. However, this gas isn't actually all that cold - its temperature is generally between 100,000 and 1,000,000 Kelvin, making it emit in the ultraviolet. That particular waveband is very hard to observe, and the filaments are also quite diffuse - so it hasn't been seen because it emits in an inconvenient energy band (that can only be seen by orbital telescopes), and is also very, very dim.
Actually, if it was dust, we'd be able to see it with radio telescopes, since it emit very low frequency radiation. The dark matter can't emit any radiation whatsoever.
Right. That's exactly my point. Nobody wants to have to be constantly rewriting and revalidating their code - they want to be doing science with it. Unless Microsoft learns that serious supercomputing users demand stability, they'll never make it very far.
One thing that may be a serious hindrance to Microsoft edging into the supercomputing market is that people who do serious supercomputing are fairly reactionary. Note that I'm referring to people who burn the vast majority of the CPU time at the US's national supercomputing centers - astrophysicists, plasma physicists, molecular dynamicists, people who run QCD (quantum chromodynamics) simulation - and also those who work at government labs doing simulations of nuclear bombs and such. Take a look at the various supercomputing center websites - NCSA, SDSC and PSC - and look up the amount of computer times various groups use. Those doing the most computing, and getting the most science done, are doing truly old-school supercomputing
One of the main reasons for this that that these people (I'm one of them) write and use simulation codes that have a VERY long lifetime - in astrophysics there are codes that are 20-30 years old and still in wide use. This is because these codes first and foremost have to solve whatever equations you're interested in CORRECTLY, and second off, solve them FAST. People base their academic reputations on the results of these codes, and are very interested in making sure that they get the right answer. In some fields (astrophysics being the one I know the most about) people can spend 10 years developing and adding science to a code.
Now, this is a reasonable thing on a unix machine. From the user's point of view, one supercomputer really isn't all that different than another. You just need to figure out where the various libraries and compilers are, but once you do that, you type 'make' and are up and running. So if Microsoft wants to break into the traditional supercomputing market, in order to entice hard-core computational scientists into trying their products they'll have to make it so that codes written for unix systems can be ported over essentially transparently - have the same libraries, the same types of compilers, etc. etc. Frankly, that doesn't seem like a likely thing to me. But then again, I'm one of the crusty old school big-iron computational physicists, so my opinion might not be all that forward looking. All I really care about is what platforms let me get my job done the easiest, and that seems to be the various unix and unix-like systems out there right now.
Just a quick reply to this. I'm a graduate student doing computational astrophysics - in particular, cosmological structure formation (galaxies and such). The law of conservation of energy is only valid in closed systems. If the universe isn't a closed system - if there's something 'outside the universe' which is adding/subtracting energy - then energy doesn't necessarily have to be conserved. Also, there are some cosmologists that believe that energy is not conserved on cosmological scales, so the law of conservation of energy is not valid on all scales. I suppose it's fair to say that as of right now, dark energy appears to result in the non-conservation of energy on very large scales, given our current understanding of particle physics. However, there is almost certainly a lot going on that we don't really understand, so it's an open problem.
It's unsurprising that you had problems with your car up on Mt. Wilson. One of my friends works up there (I'm an astrophysics grad student at UC San Diego) and has told me some amusing stories about the antenna array. Apparently the observatory gets so much interference due to it that all of the networks on the mountain are fiber optic, and they have to use faraday cage-type enclosures to shield a lot of the electronics. Many of the staff and long-term visitors to the observatory have noticed similar problems with their cars and various wireless gadgets.
Basic research is fine, but I wish that the money poured into it would go towards immediate business applications.
Now that's just silly. Basic research is incredibly important, and it is vital to the economic health of the country (and the world, for that matter) that money is spent on it. Where did the transistors that your nice new Intel chip is made up of come from? Basic research. How about lots of medical technology like MRI machines and x-rays? Basic research. And there are lots of indirect benefits to basic research as well. How about those snazzy digital cameras? The need for high-quality CCDs for astronomy (*cough* hubble *cough*) and for other research applications pushes that. Do you like the world wide web? Thank a bunch of physicists who put it together so they could share their data.
The point of basic research isn't the small, immediate payoff - it's the hope that somewhere along the line, some scientist is going to come up with something that will revolutionize the world - just like the transistor! So I respectfully disagree with you - while it's important for companies to be concerned with their quarterly earnings reports, in the long term, basic research is most certainly worth the investment.
When RHIC was being designed a lot of possible disaster scenarios came up. A very serious study was done by some rather well-respected physicists, with the results in a paper that can be found here. They found that the possibility of a black hole forming was very small, and that all of the other disaster scenarios were also very unlikely.
One of the main reasons that RHIC was developed was to study the quark-gluon plasma. Though there are more energetic acclerators (Fermilab's Tevatron, for example), RHIC is unique because it collides gold nuclei together instead of single atoms or leptons. Even so, most of a gold nucleus is empty space, so when they collide, the nuclei basically pass through each other, but at such a high energy that the bonds that hold the quarks together are temporarily broken, creating a very hot, dense quark-gluon plasma. (BTW, gluons are the carriers of the 'strong' force and hold quarks together to make hadrons such as protons and neutrons). This allows physicists to study the properties of very hot, dense matter, such as the stuff that existed shortly after the big bang, before the era where protons and neutrons were formed. A paper describing the potential physics of it can be found here.
In relativity, mass and energy are equivalent. We know that E^2 = m^2*c^4 + p^2*c^2 (E=energy, p=momentum, m=mass, c=light speed) where E=MC^2 is the case for a massive particle at rest and E=PC for a massless particle such has a photon, since m=0. As long as you have enough energy in a small enough volume, a black hole can form. This works because, as stated before, mass and energy are equivalent in relativity. Photons are massless but have energy, so in theory one could create a black hole just by having enough light in one place! It's awfully hard to do in practice, though, as the energy density needed is very, very high.
Interestingly enough, one can also generate a black hole by having constructive interference between very strong gravity waves. I saw a video from a simulation that showed this happening at a relativity conference I was at (I'm an astrophysics grad student) and it was very cool. I can't seem to find the link and google is on the fritz, but I believe that it was a simulation done by the Virgo Consortium (a bunch of European universities).
Speaking as a physics graduate student whose PhD thesis will be in cosmology, I would consider it a strong overstatement to say that this paper "casts doubts" on our theories of galaxy formation. The paper (available here) describes the possible effects that magnetic fields could have on inflation. Basically, the tension in cosmic magnetic field lines (which act like big rubber bands - the more you stretch the field line, the stronger the tension in it) tends to accelerate spatially closed regions and decelerate open regions. What's important about this is that even weak fields can have a very strong impact in an open universe, suppressing the acceleration phase of inflation. The addition of the magnetic terms to the FRW metric (which describes how the universe curves) causes makes the universe tend towards flatness in an effect which is potentially much stronger than the amount of matter and dark energy in the universe, to a degree.
However, it would be rather difficult to have a significant net magnetic field in the inflationary era. The universe was basically an incredibly hot, dense soup of plasma at this era and most importantly, is almost totally homogenous and isotropic (ie there are no preferred directions or places in the universe at this time) which would serve to keep magnetic fields somewhat local and randomized. The net result is that there probably weren't large-scale magnetic fields in the early universe.
All in all, it's a very interesting paper, and makes a very valid point - namely, that magnetic fields have the potential to be very influential in a cosmological model! However, it's important to realize that the author wasn't trying to say that the inflationary theory was wrong, just that in theory things could have turned out much, much differently.
where Bart and Lisa go to military school. The commandant's closing address is:
"The wars of the future will not be fought on the battlefield or at sea.
They will be fought in space, or possibly on top of a very tall
mountain. In either case, most of the actual fighting will be done by
small robots. And as you go forth today remember always your duty is
clear: To build and maintain those robots. Thank you."
I'd have to agree. I bought a pair of 10" DCM's at Circuit City (one of my friends works in the audio dept. so I got them cheap.) The sound quality is excellent and doesn't decline much at high volumes (important if you throw parties).
but Irix has a couple of really handy debugging utilities, Purify and TotalView Debugger. I haven't used Purify much, but TotalView is incredibly handy.
-fnord
I'm not a PhD yet, but I am an astrophysics graduate student - and my research is in galaxy formation. As one of the other responses said, this is a pretty complicated (and not totally solved problem), but the general idea is known, thanks to some really interesting theories and lots and lots of computer simulations.
It all goes back to the big bang. After recombination (when quarks and other fundamental particles recombined to make hydrogen, helium, etc.), the distribution of matter in space wasn't completely uniform (ie some parts were denser than others), and as the universe expanded and things cooled off, these denser areas became centers of gravitational attraction and became the first stars and clumps of stars. In turn, these clumps attracted each other and formed galaxies, and so on and so forth, giving the really cool hierarchical structure that we observe today. This is known as the bottom-up theory of galaxy formation, by the way. For more information, and if you like math, check out _Physical Cosmology_ by Peebles. Another excellent book is _A Short History of the Universe_ by Joseph Silk. It's at the level of Stephen Hawking's popular books, and really interesting.
Anyhow, things that are relatively close together (such as our galaxy and those in our local group, and other clusters of galaxies) will stay near each other, since the gravitational potential holding them together is much, much stronger than the expansion of the universe. Since all of the galaxies in a cluster are moving around as well as being attracted to all of the other galaxies, their orbits are generally very complicated and can't be modelled analytically. Probability dictates that it is practically inevitable that a few will hit each other. As a matter of fact, the general consensus in the cosmology community is that most large galaxies (such as Andromeda) were created when smaller galaxies collided.
As far as the fates of the colliding galaxies, individual stars are generally unaffected since, after all, there's a lot of empty space in a galaxy. However, tidal forces typically distort or completely destroy at least one of the galaxies, or make them into one larger galaxy. Another interesting effect is that the hydrogen gas clouds in the galaxies are disturbed, which causes lots of new stars to form during or right after the collisions. A huge singularity wouldn't form because the density of stars, gas, etc. isn't high enough (by many orders of magnitude) for that to happen. If two big stars happen to collide, it is entirely possible that a black hole will form, though I don't know how probable that is.
Of course, what I have said is merely the "prevailing wisdom" of cosmologists. Computer simulations (including my own) support this theory, but the debate certainly isn't over.
One of the most critical questions that we should be asking ourselves is this: Once we get there, is it a good idea to immediately start terraforming the red planet?
One of the most interesting things about Mars is that understanding how Mars formed and its weather systems will help us to understand how things work here on Earth, through what Ames and the Mars Society crew like to call "comparative planetology." However, if humans dump greenhouse gases into the atmosphere and the planet gets hotter, that changes the weather patterns, so Mars would be less useful for understanding Earth.
And, of course, there is the ever-present debate about life on Mars. If the atmosphere gets thicker and the planet gets warmer, Earth-born fungi and bacteria will flourish, "contaminating" the planet and making it very difficult to conclusively prove (or disprove) whether there is or was life on Mars.
It is most likely that it doesn't accrete much matter (ie eat stars, gas, etc.) because it has already sucked in everything in its immediate neighborhood. Black holes warp space, causing everything within a certain radius (called the ISCO, or innermost stable circular orbit) to eventually plunge into the black hole. If you are outside the ISCO, then matter is pretty much safe from getting eaten, at least for a while. So, it is most likely that all sorts of stuff is whizzing around right outside the ISCO (and they get the mass by examining the speeds of those objects) but very little is right near the black hole.
When I was an undergrad (I'm an astrophysics grad student now), I spent 1 1/2 years doing research for a radio astronomer who was a member of the International Astronomical Union (as most astronomers are). A significant part of his time was spent lobbying for the SKA, and I got the opportunity to learn a lot about the device.
The main thrust of the Square Kilometer Array is NOT to detect extraterrestrial life. That happens to be one of the flashier goals, but it isn't the most interesting to most astronomers. Since more collecting area=more sensitivity and larger baseline = more resolution for a telescope, the SKA will be the most sensitive telescope ever built, though not necessarily capable of the highest resolution (the USA's Very Large Baseline Interferometer currently holds that record). Some of the many topics of interest that will be examined with the SKA include the large scale structure of the universe, first galaxy formation, the intergalactic medium, figuring out what powers quasars and radio galaxies, pulsars, and the radio properties of main-sequence stars. And, of course, looking for other technological civilizations.
It takes a long time to build huge telescopes, not because they are incredibly complex (which they are) or because they are incredibly expensive (they are, but not prohibitively so) but because governments are the ones funding them, and also because astronomers need to make compromises to that the telescope can serve the most users possible as well and as efficiently as possible. Fifteen years is a long time, but the telescope won't be out of date when it is constructed! Most of that 15 years will be spent lobbying for funds, finding a suitable location, getting the necessary permits, doing feasability studies, developing technology (coordinating radio interferometry between 2 dishes is difficult - thousands even more so!), etc. etc. And THEN it gets built, near the end of that 15 years. And, like the Hubble Space Telescope, the Very Large Array, the Very Long Baseline Array and the Chandresekhar X-Ray Observatory (all of which took 15-20+ years to lobby, fund, design and build), the SKA will once again radically change the way we look at the sky.
Slashdot Mirror
Ok, fair point - there are a lot of little, dense things like brown dwarfs and planets that we can't currently observe. However, these can only be a tiny component of the "dark" stuff that we don't see. If brown dwarfs or planets comprised a significant chunk of the dark matter, it would be detected by gravitational microlensing events, and those observations suggest that dense baryonic objects (such as stars, brown dwarves, etc.) aren't a big (which is to say, dynamically important) component of the galactic halo.
Also, with respect to dust, it's actually quite easy to detect it in the interstellar medium, in both emission and absorption. It doesn't ALWAYS emit radiation, and doesn't do it spontaneously, but when dust is bombarded by light from nearby stars, it tends to re-emit in the infrared and radio. So it's incredibly easy to detect it in both of those bands, and use it to learn things about galaxies. It blocks optical light, of course, so you can see it in nearby (and not so nearby) gas-rich galaxies.
I see by the link below your name that you're from Berkeley, or at least probably have some berkeley ties. You should go talk to Chris McKee in the astronomy department if you think dust is more of a pain than it's worth - he'll set you straight!
Radiant energy (which is to say photons) is actually a very tiny fraction of the total energy density of the universe - something less than 0.1%. We ignore it in our simulations, but it's safe to do so.
You are mostly correct - what we're talking about are the filaments that link galaxy groups and clusters. However, this gas isn't actually all that cold - its temperature is generally between 100,000 and 1,000,000 Kelvin, making it emit in the ultraviolet. That particular waveband is very hard to observe, and the filaments are also quite diffuse - so it hasn't been seen because it emits in an inconvenient energy band (that can only be seen by orbital telescopes), and is also very, very dim.
Actually, if it was dust, we'd be able to see it with radio telescopes, since it emit very low frequency radiation. The dark matter can't emit any radiation whatsoever.
That's not correct. The article discusses matter that should be there as pointed out by the http://en.wikipedia.org/wiki/Friedmann_equations/Friedmann equations and similar equations that describe the http://en.wikipedia.org/wiki/Cosmic_microwave_background_radiation cosmic microwave background. The Dirac equation doesn't say anything about the composition of the universe.
Right. That's exactly my point. Nobody wants to have to be constantly rewriting and revalidating their code - they want to be doing science with it. Unless Microsoft learns that serious supercomputing users demand stability, they'll never make it very far.
One thing that may be a serious hindrance to Microsoft edging into the supercomputing market is that people who do serious supercomputing are fairly reactionary. Note that I'm referring to people who burn the vast majority of the CPU time at the US's national supercomputing centers - astrophysicists, plasma physicists, molecular dynamicists, people who run QCD (quantum chromodynamics) simulation - and also those who work at government labs doing simulations of nuclear bombs and such. Take a look at the various supercomputing center websites - NCSA, SDSC and PSC - and look up the amount of computer times various groups use. Those doing the most computing, and getting the most science done, are doing truly old-school supercomputing
One of the main reasons for this that that these people (I'm one of them) write and use simulation codes that have a VERY long lifetime - in astrophysics there are codes that are 20-30 years old and still in wide use. This is because these codes first and foremost have to solve whatever equations you're interested in CORRECTLY, and second off, solve them FAST. People base their academic reputations on the results of these codes, and are very interested in making sure that they get the right answer. In some fields (astrophysics being the one I know the most about) people can spend 10 years developing and adding science to a code.
Now, this is a reasonable thing on a unix machine. From the user's point of view, one supercomputer really isn't all that different than another. You just need to figure out where the various libraries and compilers are, but once you do that, you type 'make' and are up and running. So if Microsoft wants to break into the traditional supercomputing market, in order to entice hard-core computational scientists into trying their products they'll have to make it so that codes written for unix systems can be ported over essentially transparently - have the same libraries, the same types of compilers, etc. etc. Frankly, that doesn't seem like a likely thing to me. But then again, I'm one of the crusty old school big-iron computational physicists, so my opinion might not be all that forward looking. All I really care about is what platforms let me get my job done the easiest, and that seems to be the various unix and unix-like systems out there right now.
Just a quick reply to this. I'm a graduate student doing computational astrophysics - in particular, cosmological structure formation (galaxies and such). The law of conservation of energy is only valid in closed systems. If the universe isn't a closed system - if there's something 'outside the universe' which is adding/subtracting energy - then energy doesn't necessarily have to be conserved. Also, there are some cosmologists that believe that energy is not conserved on cosmological scales, so the law of conservation of energy is not valid on all scales. I suppose it's fair to say that as of right now, dark energy appears to result in the non-conservation of energy on very large scales, given our current understanding of particle physics. However, there is almost certainly a lot going on that we don't really understand, so it's an open problem.
I hope that helps!
It's unsurprising that you had problems with your car up on Mt. Wilson. One of my friends works up there (I'm an astrophysics grad student at UC San Diego) and has told me some amusing stories about the antenna array. Apparently the observatory gets so much interference due to it that all of the networks on the mountain are fiber optic, and they have to use faraday cage-type enclosures to shield a lot of the electronics. Many of the staff and long-term visitors to the observatory have noticed similar problems with their cars and various wireless gadgets.
Basic research is fine, but I wish that the money poured into it would go towards immediate business applications.
Now that's just silly. Basic research is incredibly important, and it is vital to the economic health of the country (and the world, for that matter) that money is spent on it. Where did the transistors that your nice new Intel chip is made up of come from? Basic research. How about lots of medical technology like MRI machines and x-rays? Basic research. And there are lots of indirect benefits to basic research as well. How about those snazzy digital cameras? The need for high-quality CCDs for astronomy (*cough* hubble *cough*) and for other research applications pushes that. Do you like the world wide web? Thank a bunch of physicists who put it together so they could share their data.
The point of basic research isn't the small, immediate payoff - it's the hope that somewhere along the line, some scientist is going to come up with something that will revolutionize the world - just like the transistor! So I respectfully disagree with you - while it's important for companies to be concerned with their quarterly earnings reports, in the long term, basic research is most certainly worth the investment.
When RHIC was being designed a lot of possible disaster scenarios came up. A very serious study was done by some rather well-respected physicists, with the results in a paper that can be found here. They found that the possibility of a black hole forming was very small, and that all of the other disaster scenarios were also very unlikely.
One of the main reasons that RHIC was developed was to study the quark-gluon plasma. Though there are more energetic acclerators (Fermilab's Tevatron, for example), RHIC is unique because it collides gold nuclei together instead of single atoms or leptons. Even so, most of a gold nucleus is empty space, so when they collide, the nuclei basically pass through each other, but at such a high energy that the bonds that hold the quarks together are temporarily broken, creating a very hot, dense quark-gluon plasma. (BTW, gluons are the carriers of the 'strong' force and hold quarks together to make hadrons such as protons and neutrons). This allows physicists to study the properties of very hot, dense matter, such as the stuff that existed shortly after the big bang, before the era where protons and neutrons were formed. A paper describing the potential physics of it can be found here.
In relativity, mass and energy are equivalent. We know that E^2 = m^2*c^4 + p^2*c^2 (E=energy, p=momentum, m=mass, c=light speed) where E=MC^2 is the case for a massive particle at rest and E=PC for a massless particle such has a photon, since m=0. As long as you have enough energy in a small enough volume, a black hole can form. This works because, as stated before, mass and energy are equivalent in relativity. Photons are massless but have energy, so in theory one could create a black hole just by having enough light in one place! It's awfully hard to do in practice, though, as the energy density needed is very, very high.
Interestingly enough, one can also generate a black hole by having constructive interference between very strong gravity waves. I saw a video from a simulation that showed this happening at a relativity conference I was at (I'm an astrophysics grad student) and it was very cool. I can't seem to find the link and google is on the fritz, but I believe that it was a simulation done by the Virgo Consortium (a bunch of European universities).
Speaking as a physics graduate student whose PhD thesis will be in cosmology, I would consider it a strong overstatement to say that this paper "casts doubts" on our theories of galaxy formation. The paper (available here) describes the possible effects that magnetic fields could have on inflation. Basically, the tension in cosmic magnetic field lines (which act like big rubber bands - the more you stretch the field line, the stronger the tension in it) tends to accelerate spatially closed regions and decelerate open regions. What's important about this is that even weak fields can have a very strong impact in an open universe, suppressing the acceleration phase of inflation. The addition of the magnetic terms to the FRW metric (which describes how the universe curves) causes makes the universe tend towards flatness in an effect which is potentially much stronger than the amount of matter and dark energy in the universe, to a degree.
However, it would be rather difficult to have a significant net magnetic field in the inflationary era. The universe was basically an incredibly hot, dense soup of plasma at this era and most importantly, is almost totally homogenous and isotropic (ie there are no preferred directions or places in the universe at this time) which would serve to keep magnetic fields somewhat local and randomized. The net result is that there probably weren't large-scale magnetic fields in the early universe.
All in all, it's a very interesting paper, and makes a very valid point - namely, that magnetic fields have the potential to be very influential in a cosmological model! However, it's important to realize that the author wasn't trying to say that the inflationary theory was wrong, just that in theory things could have turned out much, much differently.
where Bart and Lisa go to military school. The commandant's closing address is:
"The wars of the future will not be fought on the battlefield or at sea. They will be fought in space, or possibly on top of a very tall mountain. In either case, most of the actual fighting will be done by small robots. And as you go forth today remember always your duty is clear: To build and maintain those robots. Thank you."
fnord
I'd have to agree. I bought a pair of 10" DCM's at Circuit City (one of my friends works in the audio dept. so I got them cheap.) The sound quality is excellent and doesn't decline much at high volumes (important if you throw parties).
-Brianbut Irix has a couple of really handy debugging utilities, Purify and TotalView Debugger. I haven't used Purify much, but TotalView is incredibly handy.
-fnord
I'm not a PhD yet, but I am an astrophysics graduate student - and my research is in galaxy formation. As one of the other responses said, this is a pretty complicated (and not totally solved problem), but the general idea is known, thanks to some really interesting theories and lots and lots of computer simulations.
It all goes back to the big bang. After recombination (when quarks and other fundamental particles recombined to make hydrogen, helium, etc.), the distribution of matter in space wasn't completely uniform (ie some parts were denser than others), and as the universe expanded and things cooled off, these denser areas became centers of gravitational attraction and became the first stars and clumps of stars. In turn, these clumps attracted each other and formed galaxies, and so on and so forth, giving the really cool hierarchical structure that we observe today. This is known as the bottom-up theory of galaxy formation, by the way. For more information, and if you like math, check out _Physical Cosmology_ by Peebles. Another excellent book is _A Short History of the Universe_ by Joseph Silk. It's at the level of Stephen Hawking's popular books, and really interesting.
Anyhow, things that are relatively close together (such as our galaxy and those in our local group, and other clusters of galaxies) will stay near each other, since the gravitational potential holding them together is much, much stronger than the expansion of the universe. Since all of the galaxies in a cluster are moving around as well as being attracted to all of the other galaxies, their orbits are generally very complicated and can't be modelled analytically. Probability dictates that it is practically inevitable that a few will hit each other. As a matter of fact, the general consensus in the cosmology community is that most large galaxies (such as Andromeda) were created when smaller galaxies collided.
As far as the fates of the colliding galaxies, individual stars are generally unaffected since, after all, there's a lot of empty space in a galaxy. However, tidal forces typically distort or completely destroy at least one of the galaxies, or make them into one larger galaxy. Another interesting effect is that the hydrogen gas clouds in the galaxies are disturbed, which causes lots of new stars to form during or right after the collisions. A huge singularity wouldn't form because the density of stars, gas, etc. isn't high enough (by many orders of magnitude) for that to happen. If two big stars happen to collide, it is entirely possible that a black hole will form, though I don't know how probable that is.
Of course, what I have said is merely the "prevailing wisdom" of cosmologists. Computer simulations (including my own) support this theory, but the debate certainly isn't over.
-Brian
One of the most critical questions that we should be asking ourselves is this: Once we get there, is it a good idea to immediately start terraforming the red planet?
One of the most interesting things about Mars is that understanding how Mars formed and its weather systems will help us to understand how things work here on Earth, through what Ames and the Mars Society crew like to call "comparative planetology." However, if humans dump greenhouse gases into the atmosphere and the planet gets hotter, that changes the weather patterns, so Mars would be less useful for understanding Earth.
And, of course, there is the ever-present debate about life on Mars. If the atmosphere gets thicker and the planet gets warmer, Earth-born fungi and bacteria will flourish, "contaminating" the planet and making it very difficult to conclusively prove (or disprove) whether there is or was life on Mars.
It is most likely that it doesn't accrete much matter (ie eat stars, gas, etc.) because it has already sucked in everything in its immediate neighborhood. Black holes warp space, causing everything within a certain radius (called the ISCO, or innermost stable circular orbit) to eventually plunge into the black hole. If you are outside the ISCO, then matter is pretty much safe from getting eaten, at least for a while. So, it is most likely that all sorts of stuff is whizzing around right outside the ISCO (and they get the mass by examining the speeds of those objects) but very little is right near the black hole.
When I was an undergrad (I'm an astrophysics grad student now), I spent 1 1/2 years doing research for a radio astronomer who was a member of the International Astronomical Union (as most astronomers are). A significant part of his time was spent lobbying for the SKA, and I got the opportunity to learn a lot about the device.
The main thrust of the Square Kilometer Array is NOT to detect extraterrestrial life. That happens to be one of the flashier goals, but it isn't the most interesting to most astronomers. Since more collecting area=more sensitivity and larger baseline = more resolution for a telescope, the SKA will be the most sensitive telescope ever built, though not necessarily capable of the highest resolution (the USA's Very Large Baseline Interferometer currently holds that record). Some of the many topics of interest that will be examined with the SKA include the large scale structure of the universe, first galaxy formation, the intergalactic medium, figuring out what powers quasars and radio galaxies, pulsars, and the radio properties of main-sequence stars. And, of course, looking for other technological civilizations.
It takes a long time to build huge telescopes, not because they are incredibly complex (which they are) or because they are incredibly expensive (they are, but not prohibitively so) but because governments are the ones funding them, and also because astronomers need to make compromises to that the telescope can serve the most users possible as well and as efficiently as possible. Fifteen years is a long time, but the telescope won't be out of date when it is constructed! Most of that 15 years will be spent lobbying for funds, finding a suitable location, getting the necessary permits, doing feasability studies, developing technology (coordinating radio interferometry between 2 dishes is difficult - thousands even more so!), etc. etc. And THEN it gets built, near the end of that 15 years. And, like the Hubble Space Telescope, the Very Large Array, the Very Long Baseline Array and the Chandresekhar X-Ray Observatory (all of which took 15-20+ years to lobby, fund, design and build), the SKA will once again radically change the way we look at the sky.
Here are a few links for interested parties:
Homepage of the Square Kilometer Array
Nat'l Radio Astronomy Observatories The home of the VLA, VLBI and Arecibo
International Astronomical Union Homepage