Astronomy and Cosmology

Do you think dark matter and/or dark energy are expanding like the universe, or is their size more of a definite nature like the atom?

The Vector Potential, generated by an individual charge, is [math]\vec{A_{q}} \approx \frac{q}{r} \times \vec{v}[/math]. And, the Dirac expression, for the Vector Potential [math]\vec{A}[/math], of a magnetic monopole, in spherical coordinates, is: [math]\vec{A_{m}} \approx \frac{1}{r} \times tan \left( \frac{\theta}{2} \right) \hat{\phi}[/math] (Jackson. Classical Electro-Dynamics, p. 290) Thus, qualitatively, a spherical distribution of charged plasma, rotating differentially, with "slowly spinning poles" and a "fast rotating equator", would have an azimuthal plasma velocity profile, comparable to [math]\vec{v} \approx tan \left( \frac{\theta}{2} \right) \hat{…

A question inspired by another thread on the same subject sprang to mind that has puzzled me for about 10 years. Round about late December of 2000 there was an occultation of Venus by the Moon. The atmospheric conditions that evening were crisp and clear with Venus in sharp view which I knew from the news somewhere was due to pass behind the Moon through its exact centre. The Moon was in fine crescent phase (might have been a New Moon.). Venus passed behind the Moon into the lit side and passed out the the unlit side such that the Moon and Venus looked like the Islamic symbol. The size and intensity of Venus was pretty good before it passed behind but as it came…

Planetary Nebula (PNe) consist of an increasingly exposed "pre-WD" star core, surrounded by the increasingly distant, diffuse, and (often) ionized shed outer layers, of said star. After the failure of He fusion, the post-fusion, "pre-WD" star core remnant quickly cools down, from ~100k K, to ~30k K, in ~10 kyr: [math]L = 4 \pi R^2 \times \sigma T^4 = -\frac{dE}{dt}[/math] [math]E \approx N k_B T \approx \frac{M}{m_H} k_B T[/math] [math]\therefore \frac{dT}{dt} \approx -\frac{4 \pi R^2 \, \sigma}{N k_B}T^4[/math] [math]\therefore \frac{1}{T_i^3} - \frac{1}{T_f^3} \approx \frac{4 \pi R^2 \, \sigma}{3 N k_B} \times t[/math] Plugging in characteristic va…

On the HR diagram, the 'Super-Giant Branch' represents an "outgrowth", of super-massive MS stars ([math]\approx 30-60 M_{\odot}[/math]), at (roughly) constant Luminosity. Such suggests, that the fusing-and-radiating star-core is (largely) unaffected, by the outward swelling, of its outer envelope layers, as the star "puffs up". And, the onset of this stellar "super swelling" coincides closely, with (initial) star surface temperatures above the H-ionization threshold (i.e., Tsurf> 1 eV). Moreover, such stellar super-swelling may be much more extensive. For, super-massive stars, like Betelgeuse & Rigel, "are surrounded by shells of expelled gas", associated wi…

Please ponder a co-moving volume, which winds up expanding to become 1 cubic cm, today, z=0. That co-moving volume today contains ~400 CMBR photons (Maoz. Astrophys. in a Nutshell, eq. 9.25) ([math]n_{\gamma}[/math]), and about a billionth as many baryons ([math]n_e \approx n_B[/math]). Since cosmic re-ionization, back at z ~ 10, our space-time has been filled with a diffuse, and nearly completely ionized, plasma. Thus, if the 'Look-back Time' to the re-ionization epoch is ~12 Gyr ([math]t_{LB_{10}}[/math]), then the number of photon-free-electron Thompson scatterings, occurring in our comoving cubic volume, since that ancient eon, has been (roughly): [math]N = …

Suppose you look at the planet Saturn in the night-sky, with your naked eye. All your eye shows you, is a point of light. This point of light, reveals nothing of Saturn's disc, the spots and cloud-belts on the disc, or the rings encircling the disc. These features can't be discerned, because Saturn is so far from the Earth. Say around 800 million miles away. If you wanted to see these features, you could get on a rocketship, take off from Earth, and journey to within say, 8 million miles of Saturn. Then you'd get a view of the disc, belts and rings. Since you'd be 100X closer. But getting this closer view, would require a lot of energy. In the form of …

Back at the Big Bang, the expansion of the universe was infinitely rapid, [math]H(t=0) \rightarrow \infty[/math]. This is visualizable, as the vertical, 'infinitely steep', asymptote, of the Cosmic Scale Factor, plotted as a function of time, simply in 1D: or, as a more attention-grabbing 'surface-of-revolution': Thus, at the "bottom of space-time", back at the Big Bang, the fabric of space-time becomes asymptotically flat, with a flat, planar, tangent surface. Is that a way of "visually explaining" the cosmological 'Flatness Problem' ??

This is a fantastic DVD, about 3 hours of Stephen Hawking narrating 3 episodes. One about ETIs, an episode about Time Travel, and a longer episode, The History of Everything. There is also about 15 minutes of questions and answers, even what he likes on TV. He likes crime dramas. This has fantastic graphics and is excellent for the novice. However I have a few questions. Can anyone confirm the following statements Mr. Hawking makes in these episodes? 1. One Trillionth of a second after the Big Bang happened, the universe (I assume he means observable universe) grew from smaller than an atom to the size of an orange, after inflation. 2. After 100 seconds th…

Trying to put explicit units back into this source (eqs. 121-125) could conceivably yield: [math]A \equiv \frac{4 \pi G \rho_0 a_0^3}{3 c^2} = \frac{H_0^2 a_0^3}{2 c^2}\frac{\rho_0}{\rho_c} \equiv \frac{H_0^2 a_0^3}{2 c^2} \Omega_m[/math] [math]a = A (1 - cos(\eta) )[/math] [math]c t = A (\eta - sin(\eta) )[/math] If so, one would presumably set a = a0 in the first equation, to solve for the conformal time [math]\eta (d \eta \equiv c dt / a)[/math], and then plug the same, back into the second equation, to calculate the age of the current (closed) cosmos, given the original choice of (matter-only) density. EDIT -- No, that wouldn't work. There are t…

Ever since I read about Lee Smolin's fecund universes theory in Richard Dawkins' book The Ancestor's Tale I've been enthralled with the idea that our universe evolved by a process which shares a certain degree of similarity to the biological process of natural selection. Unfortunately, I later discovered, at least in the context of Smolin's theory, that there is no selection mechanism. Rather, fecund universes work more like evolution prior to sexual reproduction, a diffuse cloud of self-replicating universes with no real selection mechanism. Well, apparently I'm wrong: cosmological natural selection can involve a selection mechanism: http://arxiv.org/abs/070…

According to the Borexino Princeton Group, Neutrino Oscillations behave differently, in high density media, at high energies: What effects would this have had, on Neutrinos, at ultra-early epochs, at ultra-high red-shifts, associated with Primordial Nucleo-Synthesis ?

The following is this writer's understanding, of the mechanism, by which a core-collapsing hyper-massive star generates, firstly, a central stellar-massed BH ('polar pancake phase'); and, secondly, a short-lived Accretion Disk, around that BH, whose 'consumption' powers the GRB jets, parallel to the rotation axis ('equatorial consumption phase'). Is such the conventional explanation ?

I read the suns poles reverse every solar cycle and the nest one is due 2012. This appears to happen without effecting the earth, but periodically the earths poles also shift, and I have read we are due for a pole shift and the climate change could be part of this shift. What are the thoughts on this?

I was reading the latest issue of Astronomy today. There was a nice article on antimatter and a short article about a simulation with two neutron stars that merged into a black hole. Very interesting stuff, as usual! I started thinking about the ramifications of anti-matter and black holes and wondered if the matter-antimatter assymetry could be explained by it. For example, if an anti-matter particle enters an event horizon, it will produce a corresponding amount of high-energy photons when it inevitably collides with normal matter. However, these photons are trapped within the EH and ultimately increase the mass of the black hole as they are pulled back to its c…

whats the cosmos and everything we know and have thought of even in? and were does it exsist in?

There seem to be a few similarities between the universe and black holes. There's some differences too obviously. Not sure if there is any significance to this. Similarities: * Both have an "event horizon". For the black hole, its a spherical (ish) surface where anything that goes in doesn't come back out, ever. With the universe (if expanding), there is an event horizon where anything going past is regressing faster than the speed of light and so can't come back "in" (it's still in the universe, of course, just forever separated from a different point to which it was previously connected). * Stuff may always be visible as it passes through the event horizon. It…

Hey Im a Physics Student doing Ma Edexcel GCE Als. I came accross a problem under cosmology In the txt book they hav given a equation for the age of the universe using the hubble constant . By rearranging hubbles law (V=Hod) they have come up with the equation 1/Ho =time of the universe what I want to know is whether this correct if it is correct then Ho is a constant Ho= 2.3 * 10^-18. If we calculate the age of the universe using this equation today and after 1000 years if we do the same there want be a age difference how could this be is the hubble con…

Hi guys, not to put too fine a point on it, I suspect I've probably gone wrong somewhere, but I'm a curious chimp and I'd love to know where... It seems to me that if the Big Bang shoots out energy randomly in all directions, this energy will coalesce into particles that are also moving randomly in all directions, so it would be overwhelmingly probable that the universe would start out in a high entropy state and no (or very little) work could be done because everything would be the same temperature. However, this is obviously not the case. Since the Second Law of Thermodynamics is a mathematical law, not a physical law (it is based on statistics in statistica…

There was an article in a recent issue of Discover magazine, or some similar magazine, perhaps Popular Science or Sky and Telescope, but I'm pretty sure it was Discover. Maybe the June issue. But I'm going crazy trying to find it. It covered the Kepler project from its inception to now. I can't find it, and it's driving me INSANE! Can anyone help? I don't need a copy. If just need to know where it appears and I can get a copy.

"Optically Cloaked" dark cloud in Virgo Virgo-HI-21 (VH21) seems to be a "dark spiral galaxy", with a "dark disc" spanning some 50 thousand light-years across. VH21 resides in the Virgo galaxy cluster, about 50 million light-years away: VH21 appears to possess a flattened, rotating galactic disc, typical of spiral galaxies (jb). Seemingly seen from the side, or 'edge-on', that disc is full of the diffuse, cold-and-un-ionized Hydrogen gas, otherwise common in the discs of ordinary, luminous, star-spangled spiral galaxies (register). But, sans stars, VH21 is only observable, from the faint, radio-frequency emissions (with a wave-length of 21 cm), of those c…

In light travel distances this would mean that if we consider three points aligned in space, the distance between the two extremes is not the sum of the two sub-segments.. Let me explain. The idea is that the Hubble coefficient in the light travel distance (LTD) framework is expressed as a function of LTD: the recession speed is Ho*c*T where T is the distance in LTD and c the celerity of light. The Hubble coefficient is in the Euclidean framework is expressed as a function of Eclidean distances: the recession speed is He*y where y is the Euclidean distance. The rationale is to find a time-varying Hubble coefficient in the Euclidean framework, such that Ho*c*T = He(…

Our universe is 13.7 billion years old. But what was going on before that? And what caused the Big Bang of this universe? Both the Big Bang and black holes are singularities which are points with infinite density and the radius of zero, and where the laws of physics break down. We have now directly observed a super-massive black hole at the center of our Milky Way Galaxy and astrophysicists have concluded that there is a super-massive black hole at the center of every galaxy. Is is possible, perhaps even likely, that these two singularities - a super-massive black hole and a Big Bang or super-massive white hole - are linked in a death-birth relationship?! This would be a …

I agree that it is undeniable that the universe is expanding. What I don't understand is why one would conclude that the universe has always been expanding. Could it not be possible that the universe is eternal and that the expansion is something that started happening recently? Could it be possible that some outside force may be pulling on the universe from the outside? Another idea is that maybe the big bang did happen but the universe hasn't been expanding at a constant rate. Think about a rip in your shirt, or a crack in your windshield and how they can go through spurts of expansion due to outside forces causing the crack or rip to become wider or longer. Maybe …

Now it is believed that: It reviews the results of extra-solar planet finding, with over 200 planets so far discovered in a little under 200 star systems. The surprise finding is that nothing like our solar system has yet been discovered. Our solar system is atypical. We have a system with small rocky planets close to the sun, and large gas giants further out. All are in almost circular orbits, moving in a well behaved, stately way, around the sun. And of course, we have Earth in the liquid water belt, also in a beautiful, stable, almost circular orbit. Other stellar systems have all kinds of different systems. Giant planets orbiting very close to their parent st…