CapitalistImperialistPig

Dark Matter and Dark Energy are inventions intended to solve certain bedevilling problems in cosmology and galaxy dynamics. The experimental evidence for them is somewhat circumstantial, but has generally been considered compelling. Dark matter, in particular particulate dark matter is generally considered to be on somewhat more solid ground, both because there are plausible additions to the standard model that seemingly fit the bill, but also because of its apparently crucial roles in the early universe and galaxy formation. The hypothetical particles that constitute it have remained stubbornly unobserved, however, despite extensive searches. This has led to new interest in so-called Modified Newtonian Dynamics, or MOND. This old idea tinkers with gravity itself, but has not convinced many because it's not so hot at dealing with the early universe, and, IMHO, ugly as a bullfrog flattened by a semi.. Anyway, Sabine of Backreaction has a new post out arguing that a classic ...

I was at a performance of William Missouri Burroughs' play Mad Gravity last night. It turns out the play involves a bit of audience participation, so at one point a character asks the audience if anyone knows the difference between an asteroid and a comet. My date pointed to me, so I offered that asteroids were rocky denizens of the inner Solar System while comets were icy wanderers of its distant outskirts. It seems that the plot involved a close approach and possible impact by a comet. I didn't get around to mentioning it, but overall comets are a heck of a lot more menacing than asteroids. Orbiting mostly in our near neighborhood, asteroids are pretty familiar, and the big ones on nearby trajectories are pretty well cataloged. If a really big one (major extinction class event) has our number, modern astronomy should see it coming for a few hundred years before impact, probably giving us enough time to persuade even the dumbest Republican climate denialist that we ough...

The light we receive from the stars originates in the stellar atmosphere - the outer regions of the star. The stellar atmosphere is usually measured in terms of optical depth, where: Optical depth is defined as the negative natural logarithm of the fraction of radiation (e.g., light) that is not scattered or absorbed on a path....... Wikipedia . Typically the stellar atmosphere is considered to extend to optical depths of roughly 100-1000, but most of the star's light comes from an optical depth of about 2/3. Stellar atmospheres are very tenuous at those depths, with densities 5 orders of magnitude or so less than the density of Earth's atmosphere at the surface.

So far as we know, Mars never had any flowers, but it did have water, and fair amounts of it, at least for a while. My current favorite MOOC is Mike Brown's Caltech Solar System class via Coursera. The first third of the course is devoted to Mars, and it's fascinating. The oldest terrain on Mars, called the Noachian, shows evidence of rain, flowing streams, and lakes. There are also lots of craters, suggesting that this may have occurred during the so-called Late Heavy Bombardment. Not a whole lot of rain, but something comparable to the deserts and semi-deserts of modern Earth. Obviously this would have required a warmer and wetter Mars than that of today. How could it have been that warm, especially with the Sun perhaps 70% as bright as today? The most plausible answer is greenhouse gas, probably mostly CO2, supplemented by some water vapor. After a few hundred millions of years, by the Hesperian period, evidence of rain goes away, but evidence of water doesn't...

Suppose, for example, that you are one of those 100 nanometer sized particles of rocky materials that populate the galaxy as a result of Supernovae and other major Stellar events. It's lonely out there in the cosmic void, so you would like to get together with friends to form a planet. How does one go about this? First thing is, you've got to hang where the cool crowd does - in a large, cool (say 10 C above absolute zero), cloud of gas and dust. There's usually a bunch of them around the plane of the disc of any respectable spiral galaxy. You and your cloud may need a little push to get started - a nearby supernova or a density wave in the spiral galaxy, for example, but once your local cloud is gravitationally bound, you're on your way. Once the cloud starts contracting, it begins to break up into small pieces. It's important at this point to stay in the thick of things, since the early stars get the gas - and dust. Be cool though, and hang on to some of you...

The metallicity of a star, we might recall, measures how much of its material consists of elements heavier than hydrogen and helium, that is, those elements that were manufactured in earlier generations of stars that subsequently spewed their contents across the cosmos. Our Sun, for example, has about 1 iron atom for every 20,000 hydrogen atoms. Metallicities of other stars are measured on a logarithmic scale on which the Sun is defined to have metallicity zero. Thus, a star with metallicity -4 has an iron fraction only 1/10000 that of the Sun (1 iron atom for every 200 million hydrogen atoms). The most metal poor stars of all seem to have metallicities of about -4.5, while the most metal rich are near 1, with ten times more iron than the Sun. Since the very first generation of stars had no metals at all, these would have been off the scale in the negative direction, but none of these so-called Population III stars seem to have survived to the present. The kinds of planets a st...

I'm currently taking three MOOC courses, plus doing some Spanish.  The MOOCs all have an Astro flavor: Astrobiology from Edinburg , Relativistic Astro from Cornell, and Galaxies and Cosmology from Caltech.  The first two might be a bit easy, the last maybe too hard - actually I have probably already flunked since I started a few weeks late, and some deadlines have already passed. I had tried the G&C once before and dropped out.  It looks much better prepared this time.

The curvature of spacetime produces non-euclidean behavior in light rays, and consequently multipathing and lensing effects. Einstein worked this out early - actually before he had the correct final form for the equations of General Relativity - but didn't publish it until 1936, and only then because he was pestered to by Rudi Mandl, a pesky Czech engineer. Einstein didn't consider the effect interesting or important. Fritz Zwicky knew better and recognized the potential immediately (1937). Almost another half-century had to pass before experiment caught up with theory. These are a few of the tidbits I have picked up in Evalyn Gates popular book Einstein's Telescope: The Hunt for Dark Matter and Dark Energy in the Universe . Viewed through a normal telescope, a quasar looks like a point of light, much like a star. (Hence the name quasi-stellar object, which is abbreviated to quasar.) However, if a massive galaxy lies directly between the quasar and Earth, what we observe...

The total gravitational potential energy of the Earth, that is, the amount of energy necessary to disperse the planet to dust at infinity, is a bit more than 2.2x10^32 Joules. That's roughly the energy in an earthquake of magnitude 18.37 on the steeply logarithmic Richter scale (2.25 x 10^32 Joules). Phil Plait of Bad Astronomy reports that about 8 years ago, the planet got hit with the after effects of a Richter Scale magnitude 23 event. Fortunately, it wasn't terribly close (50,000 light years): Eight years ago today—on Dec. 27, 2004—the Earth was rocked by a cosmic blast so epic its scale is nearly impossible to exaggerate. The flood of gamma and X-rays that washed over the Earth was detected by several satellites designed to observe the high-energy skies. RHESSI, which observes the Sun, saw this blast. INTEGRAL, used to look for gamma rays from monster black holes, saw this blast. The newly-launched Swift satellite, which was designed and built to detect bursts of gam...

The usually quiescent black hole at the center of our galaxy appears about ready to produce some dramatic fireworks. A modest sized cloud of interstellar gas - about three Earth masses worth - seems destined to approach the black hole, be ripped apart, and largely swallowed by our BH. The event should heat it to millions of degrees and make it a very powerful x-ray emitter. The results should start appearing in our instruments in a couple of years. From another point of view, all this happened 27,000 years ago, but we should get the news soon.

The Hubble Telescope is now old enough to vote even in Rick Parry's universe. They have collected up some pretty pictures. Here's One:

Via Technology Review : Can very large scale gravitational waves explain (apparent) cosmic acceleration and other exotic and funky features of the Cosmos? Edmund R. Schluessel says yes: http://arxiv.org/abs/1109.4315 Strong long-scale gravitational waves can explain cosmic acceleration within the context of general relativity without resorting to the assumption of exotic forms of matter such as quintessence. The existence of these gravitational waves in sufficient strength to cause observed acceleration can be compatible with the cosmic microwave background under reasonable physical circumstances. An instance of the Bianchi IX cosmology is demonstrated which also explains the alignment of low-order multipoles observed in the CMB. The model requires a closed cosmology but is otherwise not strongly constrained. Recommendations are made for further observations to verify and better constrain the model.

Hmmm, a white dwarf that seems to have condensed into a giant diamond planet? A team of astronomers dotted across the globe have discovered an unusual planet circling an old, dead star about 4,000 light years away that appears to be a gigantic, solid diamond. The planet is about five times the diameter of the Earth, and circles the dead star of about 12-miles in diameter every two hours and 10 minutes. From the size and mass (1.4 Sols), it's not a planet, but the junior partner in a twin star system.

Water is pretty essential to life, so we probably need to find some wherever we go. First the Good News: Looking from a distance of 30 billion trillion miles away into a quasar—one of the brightest and most violent objects in the cosmos—the researchers, led by scientists at the California Institute of Technology (Caltech), have found a mass of water vapour that’s at least 140 trillion times that of all the water in the world’s oceans combined, and 100,000 times more massive than the sun. Aside from the fact that it's pretty far away (and twelve billion years ago], there is another reason you shouldn't expect to find any of it in Perrier bottles soon. ...In this particular quasar, the water vapour is distributed around the black hole in a gaseous region spanning hundreds of light-years (a light-year is about six trillion miles), and its presence indicates that the gas is unusually warm and dense by astronomical standards. Although the gas is a chilly –53 degrees Celsius (–63...

The Hubble space telescope lost a key capalility recently. The Hubble Space Telescope is flying partially blind across the heavens because of a short circuit in its most popular instrument, the advanced camera for surveys. The story indicates that the capability was lost permanently, but this quote seems to indicate that a new camera being installed by an upcoming spacewalk could repace some of the functionality. Adam Riess of space telescope institute, who has used Hubble to search for supernova explosions in the distant universe in order to gauge the effects of dark energy on cosmic history, said these explosions would now be out of reach until the new camera was installed.