space - Did Big Bang sound as loud as we think?

"Was it as loud as we think" is difficult to answer, since it's opinion-based. But since sound is nothing but longitudal oscillations in a gaseous medium, Big Bang was not at all silent.

If the Universe were completely homogeneous, it would stay like that. But primordial quantum fluctuations ensured that space was a tiny bit more dense in some places, and a tiny bit more dilute in other places.

Gravitation then ensured that the overdensities attracted matter and grew in size, until the pressure thus built up resisted further compression, and waves traveled outward from these overdensities. They then contracted and expanded again a few times, until 380,000 years after Big Bang when the photons decoupled from the gas, relieving the gas of its pressure, and freezing the waves in (comoving) space.

This phenomenon is called baryonic acoustic oscillations (BAOs). You may also be interested in my answer to the question "What is the speed of sound in space?"

However, humans wouldn't be able to hear it, since the wavelength (at decoupling) were roughly half a million lightyears, and the corresponding frequency thus orders of magnitues below the human threshold of ~20 Hz. Due to the expansion of space, the frequency increases as we go back in time, and scaling by a factor of $sim10^{26}$, it is possible to get a notion of what the early Universe sounded like. This has been done e.g. by John Cramer from U. of Washington on the basis of WMAP's observations of the cosmic microwave background which hold information about the BAOs.

You can hear it here. It doesn't really sound nice, though.

What is the temperature of an accretion disc surrounding a supermassive black hole?

It depends on the distance from the central body. This gives the temperature $T$ at a given point as a function of the distance from that point to the center ($R$):
$$T(R)=left[frac{3GM dot{M}}{8 pi sigma R^3} left(1-sqrt{frac{R_{text{inner}}}{R}} right) right]^{frac{1}{4}}$$
where $G$, $pi$, and $sigma$ are the familiar constants, $M$ is the mass of the central body (and $dot{M}$ is the rate of accretion onto the body), and $R_{text{inner}}$ is the inner radius of the disk - possibly (if the object is a black hole) the Schwarzschild radius $R_s$, in which case we can simplify this a little more. So the temperature in the accretion disk is far from constant.

Whether or not there is plasma depends on the exact nature of the disk, the central object and the region around it. For example, a supermassive black hole may have different matter in its disk than that of a stellar-mass black hole. I should think, though, that black holes in binary systems accreting mass from a companion should have plasma in their accretion disks, and supermassive black holes might also have plasma from nearby stars.

Black Hole / Hawking Radiation: Why only capture anti-particle?

I thought that the anti-particle was annihilating with "normal" mass inside the black hole? No?

No. First, both particles and anti-particles have "normal" mass (should they have mass in the first place) and "normal" (positive) energy. The distinction between them is either a matter of convention or a question of which type is more common in the universe. Furthermore, for typical-massed blacked holes, the bulk of Hawking radiation would be made of photons, which properly speaking do not even have anti-particles, though one could also say that they are their own anti-particle.

Shouldn't both particle and anti-particle have equal chance to be the one to fall in, or just manage to escape?

Yes, and uncharged ones do. A smaller black hole would radiate both neutrinos and anti-neutrinos, assuming all neutrinos are massive (otherwise, all black holes would do it already), and a sufficiently small (and thus sufficiently hot) one would radiate both electrons and positrons. Very roughly, a black hole will radiate non-negligible amounts of massive particles when the temperature of the black hole is on the order of the particle mass or greater, in natural units.

It seems that there should be an equal chance of either the particle, or the anti-particle, would be captured while the other "ejected."

Correct, with a minor exception that if a hot black hole has electric charge, it is more likely to radiate particles of the same sign of charge.

So it seems that the black hole should be somewhat steady-state as far as mass change with respect to virtual particles.

If either a particle or an anti-particle falls into a black hole, its mass will go up. It doesn't matter. Fundamentally, the "reason" for Hawking radiation is that the vacuum state in quantum field theory is a state of lowest energy, but different observers can disagree about which state is the vacuum. Thus, since particles are fluctuation on top of the vacuum, they can disagree about whether or not there are particles.

I don't think there is a good way to repair the "antiparticle falls in" story except some roundabout appeal to energy conservation: if the escaping particle is real and have positive energy, the one that fell in must have negative energy, and would therefore decrease the mass of the black hole. Unfortunately, that only shows what must happen for the situation to be consistent, not that it does actually happen.

Although with some knowledge of general relativity, one can motivate this slightly further--e.g., for the Schwarzschild black hole, there is energy conservation given by a Killing vector field, which goes from timelike to spacelike at the horizon--so what an external observer considers time/energy would be space/momentum inside the black hole, and momentum is allowed to be negative.

gravity - If Mars orbited the Earth how distant would it have to be to cause the same tides?

If it were possible to replace the Moon with Mars, how distant would it have to be to essentially create the same oceanic tides as the Moon currently does? Mars seems to be roughly 3 times the mass of the Moon, so does that mean it should be 300% the distance?

Furthermore, would the increased distance cause any issues with Earth's orbit around the Sun or potentially cause some sort of situation like Charon and Pluto where the centre of mass is between them?

gravity - Maximum Amplitude of a Lissajous Orbiting Object in a L4 or L5 Position

I stumbled on The Lagrangian points during some wikipedia reading. After looking at the gravity contours, I naturally come to the conclusion that the L4 & L5 should have a wave pattern and then found the Lissajous orbit page. It states:

Orbits about Lagrangian points L4 and L5 are dynamically stable in theory so long as the ratio of the masses of the two main objects is greater than about 25, meaning the natural dynamics keep the [third object] in the vicinity of the Lagrangian point even when slightly perturbed from equilibrium.

After reading it I started to wonder if there is a maximum possible amplitude (height of the peaks and troughs relative to the orbital plane of the second object) of the pattern?

Also, if there theoretically is none, for cases where it is extremely large, say larger than the 2 times the radius of the second orbiting object, what would the respective object weight ratios have to be to keep such a perturbed orbit stable for any realistic period of time?

FYI, I'm a computer scientist who loves reading about physical cosmology but can be kinda a noob sometimes. Please forgive me if I'm asking for the wrong parameters.

Edit: Here is an animation of 2010 TK7, Earth's first trojan asteroid, showing the wave pattern I'm referring to. Recall that my question is referring to the height of the peaks and troughs relative to Earth's orbital plane. Since the video is a top-down view, the peaks and troughs are going into and out of the screen.

space - Is the expansion accelerating or Decelerating?

The light we see now is not a direct indication of how the galaxy was moving at the time the light was emitted. In the cosmological frame, the galaxies aren't moving (on average, at least); rather, space between them is expanding. The rate at which it is expanding is the Hubble parameter.

If cosmic expansion is homogeneous and isotropic, then all distances on the cosmological scale should be affected in the same manner, proportionally, unless other forces are involved. Thus the distance between some galaxy and us is affected in the same proportion as the wavelength of the light:
$$frac{D_text{now}}{D_text{then}} = frac{lambda_text{now}}{lambda_text{then}}text{.}$$
The light we see now is a direct indication of the scale factor, i.e. by what factor the distance has grown since the time of emission. We can also consider distance $D$ as a function of cosmological time $t$ and define a recession velocity as the rate at which it is changing:
$$v_text{r} equiv frac{mathrm{d}D}{mathrm{d}t} = underbrace{left[{dot{D}}/{D}right]}_{H(t)}Dtext{.}$$
Therefore, the recession velocity now is given by the Hubble parameter $H(t)$ now.

neutrinos - Nuetrino interaction with plasma and electromagnetism

Following question enter link description here

After watching the Thunderbolt Project on Youtube I have a very very very fresh perspective on the universe - the electric universe. From the electric comet theory that pretty much proves water forms at the comet, and comets are not frozen water.

In the solar wind all that hydrogen missing an electron and/or free electrons, fuse/react with oxygen-silicate elements of long orbit bodies that have been long long long holding opposite --- charges +++ inside and at the outside surfaces. When the rocky body nears a sun it gains plasma discharged from the sun, it arch's like a plasma torch, and water is formed, stealing the oxygen long held in the rock formation. A comet is an asteroid that has just had a long long long time to build up a charge.

So with that and deep space showing dancing archs of electric plasma, ie space lightning, across the universe being as, or more influential as a force/energy, than gravity even, and with Einstein's admission that he was missing something in a unified theory, my question is: while neutrinos interact weekly with quote unquote solid matter, in a magnetic electrified universe are neutrinos subject to capture/interaction in Electro-magnetic currents or where voltage potential and capacitance is built up ? A?

I understand particle accelerators speed up sub atomic particles using electromagnetism but does it also translate that due to speed of atomic particles they are more likely to react with neutrinos than as a solid in out environment? B?

Space expansion in layman terms

Basically, if two particles are placed with no other interaction between them, the distance between them will increase.

Imagine living on the surface of a balloon which is being blown up. Your size stays fixed, because you're more or less rigid, but items not attached to you will move further away. Your ruler, another rigid body, stays fixed in size (though it may bend to accommodate the new curvature — this isn't so important). But two rulers (which are not attached to each other) move further away.

...also, if everything, including all our measurement devices expands at the same speed, how can we determine the fact it's expanding? :D

On the outset this seems true, however there are other forces at play here. Our measurement devices are held together by electromagnetic interactions, and the strength of these will not change. So the measurement device will hold itself together.

Imagine two faraway atoms. When space expands, the distance between the two atoms increases. However, the size¹ of the atom does not — this is determined by electrostatic equilibrium (and quantum mechanical considerations), and this remains unaffected. Even if the atom was stretched, it would rebound.

This scales up to measurement devices, so they don't get distorted either. Indeed, the expansion of space only really makes sense when you look at galaxies — these are pretty far away (when not in the same supercluster) and they don't have any interactions maintaining an equilibrium distance between them.

^{1. Whatever closest analog we have to "size" for atoms; eg the area which contains 99% of the charge density; or the nth Bohr radius.}

star - Is the Sun homogeneous?

No it does not have the same composition everywhere. In the core hydrogen is fused into helium, so the fraction of hydrogen (denoted by $X$, between 0 and 1) decreases while the fraction of helium ($Y$) increases as time goes by. There is not much exchange of matter between core and envelope so the envelope will essentially have the same constitution as when the Sun formed.

In other stars the convection zone extends into the core, and for these stars there will be more exchange of the different elements within the star.

Evolved stars (e.g. red giant stars, horizontal branch stars) often have multiple shells where different nuclear fusion processes occur. An Asymptotic branch star, for instance, has a carbon-oxygen core surrounded by a helium burning shell, surrounded by a inert helium shell, surrounded by a hydrogen burning shell, surrounded by a very large envelope consisting mostly of hydrogen.

For non-degenerate matter density depends on the pressure. The deeper you descend into the star, the higher the pressure and therefore the higher the density.

galaxy - Milky Way Formation

Great link in your answer, @LCD3. In addition, I'm going to cover some things about the continuing evolution of the Milky Way:

The Milky Way is currently merging with one or more of its satellite galaxies (I say "one or more" because several objects in its vicinity, such as the Virgo Stellar Stream, may or may not be galaxies). The major merger is with the Sagittarius Dwarf Galaxy, which undergoing a 100 million year+ merger with the Milky Way. It sill eventually be completely torn apart, and become part of the Milky Way.

There is also evidence that this has happened in the past (see this paper) and it could happen in the future. The Milky Way has a lot of satellite galaxies, and even though they are far away, closer satellite galaxies could have been consumed in the past. The eventual merger with Andromeda will also add to the Milky Way's growth, as the two become a new galaxy.

---

Other sources:

List of Milky Way satellite galaxies

Sagittarius Dwarf Spheroidal Galaxy

Note: I am looking for more non-Wikipedia sources, and I will update this answer as soon as I can.

numpy - Python troubles - Stack Overflow

I've been trying to throw together a python program that will align, crop and create an RGB image from HST and VLA .fits data. Unfortunately I've run into a bit of a problem with it continually opening a past file that does not exist in the folder and neither is it opening in the code itself. I've googled and googled and haven't found anything like it, so perhaps it's just common sense to most, but I can't figure it out. Here's the error message:

You can see at the top that the program I'm running has the filename rgbhstvla.py. I'm not sure what the error message means. Here's the python program as well:

import pyfits
import numpy as np
import pylab as py
import img_scale
from pyraf import iraf as ir

fits.open('3c68.fits', readonly)

j_img = pyfits.getdata('230UVIS.fits')
h_img = pyfits.getdata('230IR.fits')
k_img = pyfits.getdata('5GHZ.fits')

jmin,jmax = j_img.mean()+0.75*j_img.std(),j_img.mean()+5*j_img.std()
hmin,hmax = h_img.mean()+0.75*h_img.std(),h_img.mean()+5*h_img.std()
kmin,kmax = k_img.mean()+0.75*k_img.std(),k_img.mean()+5*k_img.std()

img = numpy.zeros((1024,1024,3))
img[:,:,0] = img_scale.asinh(j_img,scale_min=jmin,scale_max=jmax)
img[:,:,1] = img_scale.asinh(h_img,scale_min=hmin,scale_max=hmax)
img[:,:,2] = img_scale.asinh(k_img,scale_min=kmin,scale_max=kmax)

pylab.clf()
pylab.imshow(img)
pylab.show()

(I'm still working on the program since I'm new to python, tips here would be nice as well but they're mostly unnecessary as I'm sure I'll figure it out eventually).

solar system - Galactic Habitable Zone

The zone, you mean, in galaxies would be very unstable - huge changes in electromagnetic flux (something like explosions) because of more dynamic changes of the environment there (than in the outer rim of galaxy: black hole, jets, etc...) - so there is considered lower probability of life in the center of galaxies (or Galaxy). But in fact I think we have to few information about galaxies to seriously think about probability of life on free planets close to the center of the Galaxy (the more other galaxies).

solar system - What would happen if we stepped on the Sun?

Well first thing's first: You would disintegrate. At the temperature of the Sun, most of the molecules that make up our bodies could not even survive, that is why we would not only fry and die, we would really disintegrate (all the molecules breaking apart, leaving only loose atoms).

Let's now pretend heat doesn't exist. This is what would happen. First, we must remember that there is no solid surface to the Sun just like there is no surface to Uranus.

As soon as you reached the Sun itself, you would sink. The sun's density is less than 1 — and 1 is the density of water. So you would be sucked inside kind of. Except that there are convection currents. If we happen to be just above one, we might be kept close to the surface.

But, eventually, the eddies would entrain us sideways until we came to a "downdraft" that would take us downwards.

Anyway, let's talk about gamma rays now. Gamma rays are a form of light.

So even though heat is out of the way, all of these light rays are now trying to tear up whatever remaining molecules we have. This could trigger nuclear reactions.

Eventually, you (not me, I'm out of there) would cycle between two layers, in a convection cell moving you up and down, above and below the depth where the density of the gas is the same as the density of your body.

So I guess there's one sure thing about all of this.

You're going to need some sunscreen.

the sun - How was the core temperature of the Sun estimated?

Hydrodynamic models of the Sun allow one method of estimating its internal properties. To do this, the Mass, radius, surface temperature, and total luminosity (radiative energy emitted)/s of the Sun must be known (determined observationally). Making several assumptions, e.g., that the Sun behaves as a fluid and that local thermodynamic equilibrium applies, the stellar equations of state can be used. Numerical methods are applied to these equations to determine the internal properties of the Sun, such as its central temperature.

A great example for how to work this problem your self can be found in the undergraduate text, 'An Introduction to Modern Astrophysics' by Carroll and Ostlie (Section 10.5). The FORTRAN code to run your own stellar model is included in Appendix H.

A comprehensive review paper on how stars of different masses evolve internally (e.g., with respect to T, P, etc.) that is worth reading is:
http://adsabs.harvard.edu/abs/1967ARA%26A...5..571I

A very interesting historical overview of the development of the Standard Solar Model:
http://arxiv.org/abs/astro-ph/0209080

This (admittedly dry) paper gives you a good idea of how well the 'standard' solar models estimate the internal properties of the Sun using helioseismology and neutrino measurements to help tie down their boundary conditions:
http://adsabs.harvard.edu/abs/1997PhRvL..78..171B
The answer is that they match incredibly well (>0.2% error)

These were the least technical (but still academically published) references I could find.

Here is a comprehensive page on the state-of-the-art in solar modelling and measuring the internal Sun using Helioseismology:
http://www.sns.ias.edu/~jnb/Papers/Preprints/solarmodels.html
(highly technical)

spectroscopy - How are molecules detected in space?

This is a broad question. The energy levels occupied by molecules consist of a mixture of rotational and vibrational energy states. The values of the energies of these states are characteristic of particular molecules - they can be measured in the laboratory or occasionally need to be calculated if the molecules can only be formed in conditions found in space. A molecule may make a transition from a higher to a lower energy level. When it does so, the energy difference is emitted as a photon with a frequency corresponding to the energy level difference: $nu = (E_2 - E_1)/h$.

Generally speaking, the differences in energy between these quantum states is smaller than of electronic transitions in atoms. This means that the photons emitted or absorbed, corresponding to these transitions are in the infrared, microwave or even radio part of the spectrum. What one measures is intensity as a function of frequency. If one is looking at a warm gas, then a pattern of emission lines will be seen at characteristic frequencies depending on what molecules are present.

In the case you mention, the observations were conducted with the Atacama Large Millimetre Array - ALMA, a microwave telescope. The molecules exist in a warm gas (a few hundred Kelvin) which excites certain transitions in the molecules and the emission of mm-wave (100s of GHz) radiation. This is then received by the ALMA instruments.

If you are looking for a treatise on mm-wave astronomical techniques, this is not the place. ALMA is able to detect microwaves with a high angular resolution, but also able to separate the microwaves out into narrow frequency windows of tens of kHz (i.e. it has good spectral resolution too).

the moon - Identification of Lunar mare (maria) from a taken picture

So your image looks more like this, with the red line being the terminator. The illuminated side in your image is the left side of the reference image, turned upside down. I think you've gotten it backwards, the mare that you've labeled "1" is the Sea of Tranquility, and the one you've labeled "2" is the Sea of Serenity.

Expansion of the Universe - Astronomy

The current explanation is that "space" is continuously created in the intergalactic void, "pushing" the galaxies apart. From what i understood ( i am also ignorant ), the current rate at which "space" is created is expressed by the Hubble's constant 67.15 ± 1.2 km/s/Mpc ): for every million parsecs of distance from the observer, the rate of expansion increases by about 67 kilometers per second, which means that the distance between 2 points that are 1 megaparsec apart grows with 67km every second; between 2 points that are 2 megaparsecs apart with 134km/s, and so on so the relative speed between 2 points that are 5000 Mpc apart would be 335000 km/s which is more than the speed of light. In fact, two objects at this distance would not move, but 335000km worth of "extra space" would be "created" between them every second.

However in the last week there was an article depicting the shape of the universe, and it is hard for me to understand how it got to look like this : http://ifreepress.com/wp-content/uploads/2014/09/laniakea.png

http://en.wikipedia.org/wiki/Laniakea_Supercluster

distances - How many light years away is Earth from the closest outer edge of the black hole at the center of the Milky Way?

As @Py-ser already said, there is the very clear information about that, although his link correctly is here.

As we can read there, the distance of this is around 25000 light years, with a precision of 1400 light years.

Normally, the size of most galaxies is around some ten thousands of light years. Compared to the black holes, their size (the diameter of their event horizon) is some kilometers (stellar black holes) or some astronomical units (some millions - hundred millions of kilometers).

Even the size of the event horizon of the greatest known black hole is around the size of the solar system, which is around some thousandth of light years.

You don't need to ask, "how far we are from the nearest edge of Sagittarius A*", you can simply ask "how far we are from Sagittarius A".

How many meteors hit the moon every day or in how many days does a new meteor hit the moon?

We generally like you to check google before posting questions here. I posed your question to google and amongst other links, I got these:

http://science.nasa.gov/science-news/science-at-nasa/2006/13jun_lunarsporadic/
http://www.slate.com/blogs/bad_astronomy/2014/02/24/lunar_impact_video_of_an_asteroid_hitting_the_moon.html

Per Bill cook in the top link:

No one knows exactly how many meteoroids hit the Moon every day. By monitoring the flashes, we can learn how often and how hard the Moon gets hit."

And in the second one:

The Moon is smaller and has less gravity than Earth, so it gets hit less often than we do.

Both articles mention MIDAS (the Moon Impacts Detection and Analysis System). Since the first link is from 2006, perhaps soon we will have a good estimate for you.

astrophysics - How would Jupiter's brightness relative to our sun seem to a remote observer (observing from a remote star)

The absolute magnitude of Jupiter is at best +26 (give or take, depending on how you look at it). The absolute magnitude of the Sun is +4.8. There is a 21 magnitudes difference between them, or a ratio of 2.5 * 10^8 (250 million times brighter), which is huge.

From a 10 parsec distance (basically in our galactic neighborhood, where most visible stars are), the Sun and Jupiter would be approx 0.5 arcsec from each other at best, which is a very tiny angular distance, and there would also be the mentioned 250 mil brightness ratio (or more). It would be very difficult to tell Jupiter from the glare of the Sun with current technology.

At that distance, apparent magnitude and absolute magnitude are equal. So the Sun would be a +5 star (difficult to see with the naked eye, but doable in a dark sky), whereas Jupiter would be a +26 object, impossible to see except in a very large telescope (assuming it would be far from any star, which is NOT the case).

TLDR: Even from not far away, Jupiter would be quickly overwhelmed by Sun's glare, and would be pretty weak anyway.