Good answere Mr Sebastian

so have i! haha isn't she great?

the compression is the result of gravity, in a black hole gravity is overcoming the repulsion between bits of matter. Whether or not they are "present in a meaningful way" depends on what you mean by that... the fate of matter going into a black hole is the subject of debate, search for black hole firewalls, hawking radiation, etc but essentially no one really knows what's going on inside of them. however, it does seem to be the consensus that at the very least the mass is there, i.e. energy going into the black hole is conserved, which I think is a very meaningful way in which the matter is present =)

The simple answer to your question is no: subatomic particles as we know them cease to exist inside of a black hole, although their mass (and hence, their gravity) remain. Scientists don't have any clear idea what actually happens inside a point of infinite density, but they do have some idea how matter gets there. Consider neutron stars: they are also the products of supernovae, but these objects have masses that are less than ~3 solar masses (after the explosion). During the supernova event, most of their electrons are "captured" by the protons due to the overwhelming crush of gravity, forming an extremely dense ball of neutrons (hence the name). So even before we reach a black hole, most electrons cease to exist as such. When the mass of the supernova remnant is greater than 3 solar masses, even the neutrons can't hold each other up and the object continues collapsing until its volume approaches zero.

Thank you both for your answers, they are very enlightening.

Hi, I will attempt to answer your question. Scientists call the expansionary force "dark energy" (which simply denotes that they have no idea what it really is or where it comes from). One thing we know about this force is that it is quite weak, but it adds up over extremely large distances. Meanwhile, gravity's effect decreases with distance. Of course, the universe is unimaginably huge, so the effect of dark energy ends up being easily observable (easy for scientists), but only at great distances. The scale required is well beyond the scale of galaxies and even clusters of galaxies, so at the relatively small scale of clusters (huge for us, but small for the universe) gravity dominates, and local galaxies are more or less bound to one another, often colliding/merging as you mentioned. The expansionary force is still there, but it only amounts to roughly 1 km/s per 45,000 lightyears of distance. Andromeda, the nearest galaxy to our own, should therefore be receding at about 55 km/s (because it's 2.5 million lightyears away), but this velocity is actually rather slow astronomically speaking. The Earth orbits the Sun at about 30 km/s, while our own galaxy rotates at over 200 km/s. But if you looked at a galaxy that's, say, a billion lightyears away, you would find it to be receding at over 22,000 km/s, or 7% the speed of light. It may have neighbours to which it is bound by gravity, but relative to us it will appear to be speeding away at 22,000 km/s. I hope I have answered your question!

David Kraemer Leonard Susskind explains how homogeneity and isotropic properties of space renders "no center" of expansion. Does it not also mean what is approaching and what is receding does not have any meaning?

Homogeneity and isotropy are also very large-scale concepts, which are more theoretical in nature than observational. When we look at the nearby universe, things are clearly not homogeneous nor isotropic, because we see a galaxy in one place but not in another, and we see one galaxy moving toward us and another moving away from us. Averaging over very large distances, however, things become generally homogeneous and isotropic. But it is still meaningful to talk about recession, because every galaxy is being carried away from every other galaxy by the expansion or "stretching" of space itself. Because space itself is doing the expanding, the more space you have between two galaxies, the faster they will appear to be separating. So the apparent velocity of recession is proportional to the distance to the object you are observing, given by the equation v=HD where H is Hubble's parameter and D is distance. This holds true no matter where you observe from, whether it's here in the Milky Way or in some galaxy 1 Gly away: any galaxy you look at will appear to be receding faster when it is increasingly further away from you. It isn't erroneous to place oneself at some arbitrary center, because all motion is relative. We don't feel like we're moving, so it is natural to pretend that we are stationary because that's how the universe looks from our point of view.

David Kraemer Recently uploaded video named Laniakea seems more acceptable. Suspicious Observer comments that electrical charges plays a more important role than gravitation, but does not explain. What do you think?

Okay, I have presented you only with observational facts. Laniakea is our home supercluster, and this video does a brilliant job of mapping it for us. But I don't understand why it should be seen to disagree with what I've told you or be "more acceptable." Gravity still acts amid the expansion: its range is technically infinite. Now I couldn't find the commenter you mentioned, but I know that electrical charges or magnetic fields do not act between galaxies, or even stellar systems. Gravity is an additive force, meaning it only increases as you add mass, while electrical charges are both positive and negative and therefore tend to cancel each other over short distances. Gravity is the dominant force acting in superclusters.

I should point out that I have no qualifications, but I think I can answer your main questions... The interesting thing about the cosmic microwave background (CMB) is that it is almost perfectly uniform in all directions. Gamma ray bursts occur from definite individual sources throughout both space and time, so even if there were several orders of magnitude more of them, the resulting wash of radiation would be much more patchy than the evenly distributed CMB and would not exhibit the perfect blackbody spectrum shown in the video. To really get at the source of the CMB, we need to think about how matter behaves at high energies. If you were to heat a transparent gas such as hydrogen to a temperature of 3000 K, the electrons will dissociate from their atomic nuclei and form a plasma (the 4th state of matter), which has the property of being completely opaque to light. Stars do this. The light you see from a star is only that which escapes the surface: the rest of the body is actually opaque. Scientists believe that when the universe was ~380,000 years old, the temperature of the universe fell below 3000 K for the very first time in history, i.e. the universe's first stable atoms with protons, neutrons, and electrons formed into a gas (mostly hydrogen and helium, which are transparent). Since this occurred everywhere in the universe at more or less the same time, today we still see this evenly distributed light coming from every direction, and as Professor Crawford discusses, the redshift of this light matches exactly with what one would expect given the known expansion of the universe since that early time. As for matter and antimatter annihilation, I believe that this occurred immediately after the Big Bang--probably in less than a second. I hope this answers your questions!

Wednesday, 5 March 2014 - 1:00pm. That would be before the announcement of the polarization results.