Since it's the end of the quarter/finals week, I'd like to make some comments about what I've learned throughout roughly ten weeks of astrophysics. First, to be completely honest, my opinions haven't changed all that drastically regarding what an astrophysicist does. However, what the class did do was give me the opportunity to see (to a small degree) the inner-workings of the things that I find most interesting about our universe. Needless to say, the actual work of an astrophysicist seems more difficult than simply looking at twinkling stars in the sky. Besides that, these blogs gave me a great excuse to be a nerd and look up information about things I found particularly interesting. So, since this is in all likelihood my last post for the time being, I wanted to pose a question for whoever might be reading this (probably nobody...). There's so many complex systems that comprise just the observable parts of the Universe. So far, the big bang is pretty much the accepted theory... but what came before that? I've tried to look into it, but I'm honestly having trouble finding any concrete evidence for anything in particular... Curse you Phys111 for making me think.

I came across this on yahoo today and thought it was somewhat interesting: http://news.yahoo.com/see-earths-moon-evolved-nasa-videos-140012773.html

While looking into information on black holes, I stumbled across a few interesting things. First is the existence of what are known as stellar black holes, or stellar mass black holes, which are formed by the collapse of extremely large and massive stars. Going off of this, I also read a bit about hypothetical primordial black holes. While formation of black holes requires a great deal of mass that can typically only be found in stars in the current state of the universe, certain conditions in the early universe following the big bang could have allowed for what are known as primordial black holes. In theory, due to the extreme density of matter during the early formation of the universe, potentially, a primordial black hole could be formed without the collapse of a star. The theoretical formation of primordial black holes has been proposed as an explanation for dark matter. This then leads into discussion of the big bang theory (not the tv show.)

Using general relativity, with observations of the various aspects of the universe (including expansion rate, the amount of matter, etc.), theoretically, at some point in time (around 13.7 billion years ago) the universe consisted of a singularity - a point of infinite density and temperature. Following this, during the Plancks Epoch (the moment of the big bang, about time = 0 to 10e-43 seconds) was a period consisting of extremely high energy and density, where quantum effects of gravity were as significant as other forces, leading to the possibility of a unification of forces. The transitions between various phases of the initial expansion occurred very rapidly, with the temperature dropping drastically as expansion continued, leading to the conversion of matter. During the exponential expansion period, an excess of quarks and leptons began to emerge over antiquarks and antileptons, leading to the current state of the universe in which matter far outnumbers antimatter. A few minutes into the expansion came the moment when matter combined to form hydrogen, the most abundant element in the universe. Following this, after hundreds of thousands of years, denser regions of material began to combine, eventually leading to the formation of stars, galaxies, etc. From that point on, the universe began to "settle down" and eventually, along came earth. Of course, this is all theoretical and based on observations and understanding of physics. Still, I think it's pretty cool, and kinda makes sense. I wonder what came before the big bang then...?

I don't think it's really necessary to say, but when people think about space, especially for those of us who aren't exactly experts in the field, things like supernovas, black holes, etc. seem particularly interesting. The primary topic I'd like to talk about right now is black holes - something that, due to its nature, isn't particularly easy to study. You can't see the black hole itself, nor are we exactly capable of approaching one to study in person. Although the idea of an object of sufficient mass and density have a powerful enough gravitational field to affect light itself was first proposed in the 18th century by the geologist John Michell, the idea of the black hole did not gain much ground until Albert Einstein developed his theories on general relativity in 1915. The combined work of Karl Schwarzchild and independently, Johannes Droste, gave way to the Schwarzchild Radius, the radius at which an objects compressed mass would create a gravitational field from which the escape speed is equal to the speed of light. As a result, black holes are in essence, objects which have been compressed beyond the Schwarzchild Radius. While it is impossible to observe an object once it falls into the black hole, from the interaction with matter around it, it is possible to observe mass ,charge, and angular momentum. The information-loss associated with objects falling into a black hole is something that can be described by both quantum mechanics and general relativity, and is known as the Black-hole information paradox (maybe I'll talk about that later...). The aspects of a black hole can be described in 4 pieces, the event horizon, singularity, photon sphere, and ergosphere. The event horizon is so name due to the fact that at the point of the event horizon is essentially the point of no return, as any information that passes through the point is no longer able to escape the gravitational pull of the black hole. Usually, this is observed as the glowing ring around a very dark region of space that marks a black hole. The singularity is considered to be the very center of a black hole where, theoretically, according to general relativity, the entirety of the black holes mass is contained in zero volume. The implication of this is a single point of infinite gravitational field and spacetime curvature. A photon sphere is the point at a certain radius from the center of a black hole where photons moving tangent to the black hole are permanently trapped in a circular orbit. While it is possible for light to escape from within the photon sphere, due to the exact properties of a black hole, the light escaping and observed must come from an object trapped within the photon sphere but outside the event horizon. According to general relativity, due to the gravitational field of a black hole, a certain region of spacetime right outside the radius of a black hole will be permanently "dragged" along. This region of perpetual motion is what is known as the ergosphere - a region surrounding the event horizon from which an escaping object can emerge with greater energy than it entered, taking with it some of the energy of the black hole. At this point, it would be pretty wordy to get into more detail with black holes, so I may save this for another post... but in a nutshell, black holes form through gravitational collapse, typically when stars run out of fuel to sustain its size. Here's a pretty picture to distract people again.

Something that I find pretty mind-boggling is the idea of Dark Matter. What is it? How does it work? etc. etc. Well, to start off, I did what every good Physics student would do, and did plenty of research (all of ten minutes on Wikipedia. Man I'm hardcore). It turns out that "dark matter" for better or worse, is basically unexplained, unobservable stuff that's believed to comprise the majority of the matter in the universe. Because it doesn't interact with any form of electromagnetic radiation in any way (i.e. doesn't scatter, reflect, or emit any light), it isn't possible with current telescope technology to observe it, and therefore, we don't even know whether it exists. The existence of dark matter was first proposed by Jan Oort, in order to explain the missing mass that should have been there when observing the orbital and rotational speeds of galaxies, as well as a few other weird thingies like strong gravitational lensing:

Because of the particular complexities of dark matter, I'd rather not get ahead of myself by trying to explain too much detail, since, to be honest, I'm still trying to wrap my mind around some of this stuff myself. But dark matter can be broken into cold, warm, and hot dark matter. Cold dark matter is basically dark matter moving at classical velocities, while warm is any relativistic dark matter, and hot is "ultrarelativistic", or speeds extremely close to the speed of light. Much of the evidence for dark matter comes from observation of the movement of galaxies, and dark matter detection experiments are still being run today. Of course, because of the theoretical nature of dark matter, there are alternative theories such as modifying the laws of gravity, etc. Who knows? I'm sure the astrophysicists working on this stuff are pretty smart people, and we'll see (hopefully before we die) what "dark matter" really is

Another pretty picture from the internet to distract people... it's a 3D map of the distribution of dark matter, from Hubble's measurements of weak gravitational lensing.

Just a quick thought about something that I find particularly interesting - Quasars. First, I'm obviously not an authority on these things, nor am I exactly the best person to come to regarding astrophysics knowledge, but let's face it, space stuff is pretty awesome. So, first, to my understanding, Quasars are basically highly energetic, nuclei of very distant galaxies which are some of the most luminous objects in the universe, and consequently, were some of the earliest examples of redshift. It's believed that quasars are formed by accretion of material in supermassive black holes, or in other words, stuff getting pulled by the very powerful gravitational field around supermassive black holes, and the very bright light is what energy is immediately outside the event horizon (as anything past the event horizon is impossible to see - hence, black holes). Now, one may ask "what the is redshift, and why is it important? Before getting into that, I'd like to quickly highlight some of the more interesting bits of history about quasars. Early on, quasars were first "seen" in the 1950's with radio telescopes as radio sources, but without a visible image attached to them. It wasn't until a decade later that, with really big telescopes (like the Hale Telescope at Palomar) that redshifted spectral lines were observed. Fastforward a few years, allow for some of us to be born, and now, over 200,000 quasars have been observed, no just as spectral lines, but as actual, resolved objects, thanks to the construction of a lot of really big telescopes. So now, redshift - Why's it important, what the is it, and what significance does it hold? Redshift is, essentially, the Doppler effect happening with light, rather than sound. In a nutshell, the Doppler effect is observed when a wave-emitting source is moving either towards or away from a (relatively) stationary object, and because of the motion either shortens or lengthens the frequency (and consequently, the wavelength) of the wave, whether it be sound or light. Now, in the case of quasars, the redshift implies that the quasars, being extremely luminous and very very far away, emit electromagnetic waves which are detected by whatever instruments we use here on earth. The spectral lines detected by us turn out to be shifted further into the red region that they should be, hence the term redshift. The significance of this lies in the fact that the shift towards red implies a Doppler effect, and by extension, that the quasars are in fact moving further and further away from us over time. This would then imply that the universe is not stationary, but is in fact expanding. This further holds some implications related to special relativity, which coincidentally, falls into quantum mechanics and I feel a little lazy about trying to talk about in this post. Maybe next time? Yeah probably. But anyways, pretty neat stuff. Here's a cool picture to distract whoever is reading this (artist rendering of what a quasar might look like):

OOOOH PRETTY PICTURE

So, I know I haven't been posting much lately, but I stumbled across this, and felt compelled to share this with everyone. It's mind-boggling to think about the scale of all the things in the universe... how big things can be, and how imperceptibly small... and this provides some pretty cool perspective


http://onemorelevel.com/game/scale_of_the_universe_2012