Cosmology Questions

[tuxcat]

Hi all,

I have a few questions about cosmology.

1) What is the cosmic microwave background - how did it form. I've read Wikipedia and a couple of other sites but for the life of me I can't work out what it is. Is it a gigantic ejection of stuff that was formed during the BB and is still lingering to this day?

2) Can anyone explain how to "read" the SOHO images: http://sohowww.nascom.nasa.gov/data/realtime/realtime-update.html I mean what do all the instruments do and what can you infer from the photos? LASCO 2 and 3 are obviously the corona and show events happening. SDO/HMI Continuum seem to be a sunspot locator.

* But what does the SDO/HMI magnetogram do - what can be inferred from the picture and how do you infer it?

* What do the EIT pictures show and what can they allow one to infer?

I have read the explanations but I can't understand them. What I really do want to do is to be able to read the pictures and understand what the sun is doing. Sort of like being a weather forecaster for the sun Is that even possible though without getting a PhD in solar antics

 

[sol invictus]

Hi all,

I have a few questions about cosmology.

1) What is the cosmic microwave background - how did it form. I've read Wikipedia and a couple of other sites but for the life of me I can't work out what it is. Is it a gigantic ejection of stuff that was formed during the BB and is still lingering to this day?

The sun is a ball of plasma - very hot, ionized gas. It emits light because the particles that make up the plasma are moving around very fast, bouncing into each other, and constantly creating and destroying photons in the process. If a photon is created near the surface of the sun and happens to be moving out, it will escape. But if it's created nearer the center, it will never get out, or at any rate take a very long time (something like a million years to get from the center to the edge).

Why is this relevant for the CMB? Until approximately 400,000 years after the big bang, the universe was much like the sun - except the whole universe was like the center of the sun. It was full of very hot plasma everywhere, with nearly uniform density and temperature. There were plenty of photons around, but they could barely move without running into a particle.

Then at about 400,000 years old, the universe cooled to a temperature low enough (about half the sun's surface temperature) that the free electrons in the plasma were captured by the free protons, forming neutral hydrogen (along with a few other light elements).

When that happens, photons become almost perfectly free to propagate (because they scatter strongly off charged particles, but hardly at all of neutral molecules). It's as if the sun suddenly turned into transparent glass, with all the photons inside it at that moment still there. Those photons would then freely propagate in whatever direction they happened to be going.

Since that happened everywhere in the universe, there is radiation everywhere, going in all directions. Because the universe has expanded by a factor of about 1000 since then, all those photons have redshifted by a factor of 1000, so that radiation is now characteristic of a temperature about 1/(2*1000) times colder than the sun: namely, 2.73 degrees Kelvin, or microwave.

 

[tuxcat]

Right I see now - thanks for that

 

[GodMark2]

2) Can anyone explain how to "read" the SOHO images: http://sohowww.nascom.nasa.gov/data/realtime/realtime-update.html I mean what do all the instruments do and what can you infer from the photos? LASCO 2 and 3 are obviously the corona and show events happening. SDO/HMI Continuum seem to be a sunspot locator.

* But what does the SDO/HMI magnetogram do - what can be inferred from the picture and how do you infer it?

* What do the EIT pictures show and what can they allow one to infer?

I have read the explanations but I can't understand them. What I really do want to do is to be able to read the pictures and understand what the sun is doing. Sort of like being a weather forecaster for the sun Is that even possible though without getting a PhD in solar antics

A magnetic field is either coming or going, sometimes called the north or south poles of the field. This is all well and good for simple magnets, with only two ends, but reality is seldom simple.

Imagine you had a 3d compass: a magnet suspended so it could spin in any direction. Near the Earth's equator, it would line up mostly parallel to the ground, with one end pointing north (We call that end the North end: Clever, ain't we?). It does this because the magnet field 'lines' are running parallel to the surface of the planet at this point. As you travel toward the north pole, you notice that the end of this magnet starts to dip down. Again, this is because the field lines are dipping down. Exactly at the (magnetic) pole, the magnet would point straight down.

The Sun, however, works differently. On Earth, the field is made primarily by a spinning iron core. The Sun is governed by plasma. Plasma doesn't simply spin about, it flows, splits, dips up and down, and generally makes a mathematician's brain stagger with it's complexity. As such the field also dips up, down, left, right, and anywhere else it pleases. To show a map of this field, it's easiest to simply show how 'up' or 'down' that 3d compass of ours would point at anywhere on the surface. This is what the map shows: grey is horizontal, black is down, white is up (or maybe vice-versa, but it doesn't matter). A picture of the earth using the same method would be a bit boring, simply being black at the South pole, grey ate the equator, and white at the north pole (there are small local variations, but they are swamped by the main signal at these scales)

 

[tuxcat]

Thanks for that GodMark2 - I can imagine that heliophysics isn't the easiest field given that plasma goes all over the place and is prone to not flowing in nice neat little lines.

I watched an ESA animation and learnt that when the sun ejects a CME or solar flare, it is routed towards the earth like a magnetic railway line by the earth's magnetic field to where the earth's magnetic fields meet at the poles. Hence why the aurora is confined to polar regions.

I know that the granule is related to convection cells a bit like a superheated beaker of orange juice fizzes and bubbles.

Another Q: What is a sunspot - and what is black body radiation? Are the two related?

Thanks for all your patience - I wish I did physics and space at school but as it was I stuck to chemistry and biology. As well as business administration, maths, English, history, etc etc

 

[rjh01]

<snip>

Another Q: What is a sunspot - and what is black body radiation? Are the two related?

Thanks for all your patience - I wish I did physics and space at school but as it was I stuck to chemistry and biology. As well as business administration, maths, English, history, etc etc

A sunspot is an area on the sun that is slightly cooler than the other areas on the sun. It is still hot.

Black body radiation is given off by everything because it is above absolute zero. It is not related to sunspots.

Edit. You can Google for more information.

 

[Delvo]

Because the universe has expanded by a factor of about 1000 since then, all those photons have redshifted by a factor of 1000, so that radiation is now characteristic of a temperature about 1/(2*1000) times colder than the sun: namely, 2.73 degrees Kelvin, or microwave.

what is black body radiation?

Black body radiation is given off by everything because it is above absolute zero.

Another crucial bit about blackbody radiation is alluded to by sol invictus earlier, just not referred to as such because the question hadn't been asked yet: it comes in different frequencies based on temperature. Lower temperatures are associated with lower frequencies and higher temperatures are associated with higher frequencies. In the temperature range that we and everything else around us on the Earth's surface are normally at, the associated frequencies are in the infra-red range, which is why we associate infra-red with heat/warmth. Sometimes when we heat something up a bit, we see it starting to glow because then it's getting to temperatures that happen to be associated with the frequency range that our eyes happen to respond to, starting at the red end of the visible spectrum.

 

[GodMark2]

what is black body radiation?

Imagine you had a small box that absorbed (and emitted) every wavelength of light without preference. It doesn't have an internal source of energy, so it only emits what it absorbs. If it absorbs some energy without emitting any, it will have to raise it's temperature. If it emits without absorbing, it will have to lower it's temperature. This is known as an idealized black body.

Some think and a bit of math later, you'll figure out that is will emit an amount of energy proportional to it's temperature. If it didn't, it would either emit too much and cool off, or not enough and warm up.

If you do a bit more thinking (and rater a bit of math) you'll figure out that, while it does emit every wavelength, it will emit some wavelengths more than others. It will start with low emission at very low wavelengths, coming to a peak somewhere, and falling off again as the wavelengths gets too long.

Some more thinking (and even more math) will show that the amount of energy at each wavelength will raise when the temperature rises.

Even more math gets to the point where you find that the peak emission moves toward the shorter wavelengths when you heat the box up.

So, an ideal black body would have a specific spectrum of emissions for a given temperature, and a specific total energy of emission for that temperature. This is black body radiation.

Now, real objects don't absorb and emit without preference (leaves don't absorb much green). So, their spectrum is not close to this ideal. However, plasma IS very close to this ideal, and the sun closely follows the black-body spectral shape (with a few notable discrepancies). The cosmic background radiation also follows this curve, only for a much lower temperature (and very few anomalies), leading to the conclusion that it was likely emitted from a plasma, but has 'cooled' from space expansion (cosmic redshift) in the meantime.

 

[tuxcat]

Black body rad

Thanks for that you guys - I think I can finally understand what bb radiation is. I had a bit of trouble getting my head around the concept but Delvo and GodMark2 have put it in a way that I can understand.

Sunspots

And of course, sunspots are just cooler areas. From what I can remember from a Beeb documentary "the secret life of the sun", these areas tend to progenitate solar flares and other helioburps for want of a better way of putting it