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 ...

New measurements of the local rate of expansion of the universe are creating a stir . The measurements found that the current value does not seem to agree with data from the Cosmic Microwave Background and the so-called Lambda CDM model. The second link, to a Scientific American news story, discusses potential implications in a bit more speculative manner than the paper. From SA: The Hubble constant discrepancy, though, suggests that dark energy might actually change over space and time, potentially causing an increasing acceleration of the cosmos instead of a constant outward force. One theory proposing this type of dark energy is called quintessence, which posits that dark energy results not from the vacuum of space but from a field that pervades spacetime and can take on different values at different points. An alternative explanation for the discrepancy, however, is that the universe contains an additional fundamental particle beyond the ones we know about. In particular, a new...

How Did the First Stars and Galaxies Form? By Abraham Loeb This is intended to be a short introduction to the interface between cosmology and astrophysics governing the first galaxies and stars. Allegedly it is aimed at the undergraduate with a science background or the non-specialist scientist, but I found the level of presentation rather uneven. What kind of student, I wondered, would need to have the terms "star" and "galaxy" defined but still be able to decipher "Polarization is produced when free electrons scatter a radiation field with quadrupole anisotropy Q?" This short book packs a lot of information into it, though I'm not sure that I agree with the title. It's mostly not about those events but rather about the modeling of the growth of cosmological density perturbations which ultimately gave rise to those stars and galaxies. This is a technical book, with lots of equations, but derivations of those equations are mostly absent or ex...

Dennis Overbye, writing in the NYT, has a nice article on the significance of the BICEP2 measurements. He makes a point that I had not understood (I'm assuming that he is right). The gravitons themselves, theory says, are produced by the same process by which black holes leak. It is known as Hawking radiation, after Stephen Hawking of Cambridge University, the renowned black hole theorist, best-selling author and avatar of cosmic mystery who discovered it in a prodigious calculation in 1973. Shortly thereafter, William Unruh, now at the University of British Columbia, showed that you didn’t need black holes to see this radiation, just acceleration in space. In this case, the role of the black hole you can’t get out of is played by the rapidly retreating horizon you can’t reach in the inflating universe. Hawking radiation has been part of the physics firmament for decades; it’s the best-known prediction of quantum gravity. “Now it seems that Hawking and Unruh were right!” sai...

If you haven't seen this video yet, you should.  Note that the furthest direct view of the origin of the Universe from the CMB on takes us back to t = 300,000 years or so.  The BICEP2 result goes back to phenomena many, many orders of magnitude earlier (via the imprint of primordial gravitational waves on the CMB).

BICEP2 UPDATE: It's unsurprising, I suppose, but still disappointing, that much of the press coverage seems to confuse Cosmic Inflation (which the new data would seem to strongly support) with the Big Bang, which has long been on solid ground.

Physics rumordom is rife with rumors of something big in the Cosmic Microwave Background (CMB). In particular, the suspicion is that so-called primordial B-modes (of polarization) might have been observed by the BICEP2 South Polar experiment. For reasons beyond my (zero) pay grade, these are expected to show evidence of the characteristic energy scale of cosmic inflation - or at least give hints about that very early age of the Universe. The link is to Lumo's account, which contains explanatory material as well as numerous links to other speculations and explanations.

Black holes seem to be pretty messy eaters. Roughly 10% of the potential energy of mass consumed seems to get radiated by the accretion disc and polar jets. Bright quasars radiate up to 10 trillion times Solar luminosity - perhaps 1000 times the brightness of a galaxy. That requires a yearly consumption of a couple of Solar masses of gas and or stars. You wouldn't want to be in the immediate neighborhood. Of course most quasars were creatures of the early universe. Lately they have become as scarce as Ents.

Metals, in Astro speak, are any elements other than hydrogen and helium. Since they weren't synthesized in the big bang (except for traces of lithium) the cosmos started out without any of them. The metals (in the astronomical use of the word) out of which we and our planet are made were created inside of stars and later dispersed in the interstellar medium through stellar winds and supernova explosions. The oldest stars in our current universe, some of them over twelve billion years old, have much smaller metallicities (metal contents) than our own star, the Sun, which wasn't formed until the Universe was already about 9.4 billion years old. None of those stars, however, have the zero metallicities expected from the first born stars, so it looks like none of those survived to the present. Since a star with a mass of 85% of that of the Sun (or less) would be expected to live longer than the present age of the Universe, that suggests that no such stars were made in that fir...

Via the Lumonator , some interesting news about the search for dark matter. Dark matter, you may recall, makes up about 80% of the matter content of the universe and about 25% of the total energy content. It's the second most mysterious component of that universe, trailing behind its bigger, stranger, brother, dark energy, thought to make up the other 70% of the cosmos. It is clumped by gravitation, but appears to interact scarcely at all with ordinary matter via the electromagnetism or the strong force. Its condensation under gravitation seeded the cosmic web and galaxy formation. The most popular theory of its nature is the so-called WIMP hypothesis, for Weakly Interacting Massive Particle. Since nothing is known about WIMPs, this doesn't pin them down much, but presumably that arose when the universe was hot enough in approximately equal numbers of particles and antiparticles. After the temperature cooled enough, they went their separate ways, influenced only by gravi...

Actually, I want to talk about the so-called Lyman-alpha forest. Suppose we look at a distant Quasar. It puts out a broad continuum in the UV, which gets red-shifted depending on how far it is away. On its way, it is extremely likely to encounter clouds of ionized intergalactic hydrogen, as well as more minor ingredients. Even at a cosmically "modest" red shift of 1, corresponding to a look back time of about 7.5 billion years, back in the strapping youth of the cosmos (5.5 Gyr.) there will be a forest of absorption lines in that emission. That forest consists almost entirely of differently red shifted versions of the Lyman-alpha line. The various incarnations are due to differently red shifted clouds of hydrogen along the path from quasar to Earth, with the least red shifted due to nearby clouds and the more red shifted to clouds farther away. The farther the source is away, the more intervening gas clouds until the "forest" becomes an impassible jungle of t...

Lumo has an interesting comment on a new measurement of the cosmological equation of state . That equation, we might recall, relates pressure to energy density in the universe considered as a perfect fluid: w = pressure/energy density. The new measurement suggests that w may be less than -1, which apparently argues against the most natural interpretation of dark energy as Einstein's cosmological constant. Lumo is appropriately skeptical, both because of the statistics and because |w| > 1 implies a speed of sound greater than that of light.

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.

Dennis Overbye, writing in the New York Times, has a nice article today on the so-called Boltzmann brain problem in cosmology. It's a new incarnation of a familiar problem in statistical mechanics and the physics of time, the problem that pure statistics seems to imply that entropy should increase in the past as well as the future: it's a priori more probable that the present is a momentary fluctuation rather than part of a long history extending into the past. Overbye, like cosmologists, recognizes that this way madness lies - the question is what to do about it. It could be the weirdest and most embarrassing prediction in the history of cosmology, if not science. If true, it would mean that you yourself reading this article are more likely to be some momentary fluctuation in a field of matter and energy out in space than a person with a real past born through billions of years of evolution in an orderly star-spangled cosmos. Your memories and the world you think you see ...