Someone wanna explain WMAP?

Factual Questions
1

Ok, I’ve read a bit about this, including http://map.gsfc.nasa.gov/m_mm.html, but can’t figure out exactly how this “picture” was generated, or more importantly, how what it pictures represents what happened billions of years ago, or how we derived some of the findings from it.

a) What is this actually a picture of? As near as I can guess, it’s a picture of the background radiation with extreme accuracy. True?

b) Why was it so important that this picture be taken far from the earth? Is it a matter of avoiding extraneous heat from the earth? If so, why is this so much more accurate then what Hubble was able to achieve? does gravity play into it?

c) (assuming #a is accurate,) why would the current state of background radiation be equivalent to a picture 13+ billion years ago?

d) how does this picture tell us how old the universe is?

e) how does this picture tell us the universe will always expand?

A quick explanation of Dark matter and Cold Dark Matter would also be appreciated.

2

You are allowed one question and then one follow-up question, that is all.

[ul]:smiley: [sup]I heard Ari say this at a press conference.[/sup][/ul]

3

That was not a useful response, and neither is this.

4

Fixed link: http://map.gsfc.nasa.gov/m_mm.html

a) You’re right. It’s a map of the CMB temperature plotted over the whole sky.

c) The CMB photons don’t really interact with anything on their way from the last scattering surface to us. Therefore, what we see originated then.

5

Achernar was kind enough to write

So lemme get this straight… There exist these “CMB photons”, which originated from the first light from the first stars. And these CMB photons have been boucing around the universe off of various “scattering surfaces”, and eventually reaching the eye of the WMAP “camera”.

f) is the above true?

g) My guess is that there would be a lot of scattering surfaces involved over those billions of years. True?

h) why aren’t CMB photons affected by hitting a scattering surface? is this just the nature of photon collisions in general?

i) assuming #g is true, why do we have any believe that the direction we’re receiving the CMB photons from is the actual originating direction?

j) How do we differentiate between CMB photons and non-CMB photons, which certainly must be extremely plentiful?

6

Okay, I wasn’t sure how much you already knew about the Cosmic Microwave Background (CMB). Here goes:

Photons were created in the Big Bang. A lot of them. In the early universe, they were scattered and interacted with the matter that was there. As time went on, the matter formed bigger and bigger things. First quarks combined into protons and neutrons. Then these nucleons combined into nuclei, and then these nuclei combined with electrons to form atoms. This formation of atoms is known as recombination, and it occurred at about 380,000 years after the Big Bang.

At this approximate point in the universe’s history, the universe stopped being “opaque” to the background photons. The photons moved freely through the recombined matter, no longer scattering. So this point in the Unvierse’s history is called the last scattering surface. It’s not really a surface, though, more like a point in time.

So these photons originally came from the Big Bang, but when we see them, they’ve been travelling freely through space since the last scattering surface about 13 billion years ago. I think that probably answers most of your questions, but let me go through them, just in case:

f) The photons are not from stars; the last scattering surface was before any stars formed.

g) Surprisingly, no. The photons have managed to get away since that early phase of recombination.

h) They are affected, and in fact, we don’t get any information about what they were like before the last scattering. So when they say that this point is as far back in time as we can look, that’s what they mean. The universe was opaque before that.

i) The CMB is very isotropic (same in all directions). The variations in it are only on the order of 0.01%. So we’re getting it from all directions, as you should expect, if it’s a cosmic phenomenon.

j) There are some non-CMB photons, but not as many as you’d think. These are called the foreground, to distinguish them from the background. The biggest contribution to the foreground is the plane of the galaxy, and this must be subtracted out. I don’t know exactly how they do it, but you can see roughly what the MAP data look like before they subtract out the galactic foreground, in the third image on this page.

I hope that’s at least somewhat clear. I hope that someone with a better understanding of the instrument can answer question b, and someone with a better understanding of the theory can explain questions d and e. And of course, correct any mistakes I made. :slight_smile:

7

Very clear description, Achernar. Fascinating. Thanks.

k) do we know what caused the universe to stop being opaque? Was it because of the newly recombined state? Why would recombination cause the end of opaquity (if that’s a word)?

l) if the CMB photons have been travelling in a straight line outwards from the center of the universe since recombination, why do we see them coming from the opposite direction as well, coming from all directions relatively equally in fact?

8

Just to add to Archenar’s excellent description of j): The CMB has a very specific temperature (about three degrees above absolute zero, actually), and as he said in (i), that temperature only varies only by about 0.01%. There are many, many other photons flying around out there, but only the ones in the narrow window around that exact temperature are from the CMB (plus the small number from sources like the galactic plane, which he also mentioned). So it’s like tuning in a television or radio frequency. Of all the millions and billions of photons flying around, we only “tune in” the ones on channel 3.

b) I don’t believe the Hubble has anything to do with it. It is an optical telescope, and the CMB, as the name says, is microwave radiation. You cannot image microwaves with an optical telescope, just as you can’t recieve visible wavelengths with a television. If I’m not mistaken, the previous best microwave images came from a satellite called COBE (Cosmic Background Explorer).

l) There is no center of the Universe. There never was. This has been discussed many times on the Board (in fact, there’s a current thread about it), so I’m sure you can find stuff about it by searching. Trying to sum it up briefly: the Big Bang occurred everywhere. You must remember that 14 billion years ago, the farthest galaxy you can see in one direction and the farthest galaxy you can see 180 degrees from that direction were in the same place. Everything we can see in the entire Universe was together in the same place at the Big Bang. And all of space was there, as well (i.e., there was no “outside”). After the explosion, everything started moving apart from everything else. It may help to visualize it like this: imagine the Universe at this small size as a ball. Everything that exists is inside the ball. You can’t move outside the ball, you can’t even talk about outside the ball because it doesn’t exist. There is no “there” there. If you’re inside the ball and keep moving in a straight line, you’ll eventually end up where you started, like traveling around the surface of the earth, or like traveling off the screen in Pac-Man. This may not be the exact right geometry, but it gets the point across: there is no center. Everything is just travelling away from everything else, and the Universe is mostly the same in every direction. Just remember that the Universe is not embedded in space of standard Euclidean geometry. The Universe contains the space, and it is curved.

9

In the beginning there was nothing, which exploded.

–Terry Pratchett

10

I thought you were asking us to explain manwithaplan.

That’s something I don’t think anyone will ever know.

11

The WMAP was placed at the L2 Lagrangian point which is one million miles from the earth (in the direction away from the sun). Lagrangian points are special locations where the interaction of gravity from the earth and the sun provide a semi-stable point in space that allows a satelite to orbit the sun with the same period as the earth even though the points are at a different distance from the sun than the earth.

It appears the advantage of the L2 point is that the location is always in the shadow of the earth and would eliminate the problem of solar radiation from affecting the sensitive temperature readings made by WMAP.

Lagrangian Points

12

Thanks very much to brianmcc and rsa for the valuable additions. For those keeping score, my remaining mysteries are:

13

um… the word is “opacity.”

[sub]I can’t answer the other stuff.[/sub]

14

The harder thing to grasp is that not only was there nothing, but there was nowhere for that nothing to not be. There wasn’t any ‘when’, either.

Intellectually, I sort of understand it, but I can’t exactly wrap my mind around it, which is also my reaction to things like

I know it’s true, but I’m stil trying to make the picture work in my head.

Thanks for your excellent explanations, Achernar and bryanmcc.

It impresses the hell out of me that we can see, today, what the Universe looked like at such an astoundingly early point in time.

Bill H. - your questions are also my questions. The Washington Post article gave sketchy answers to some of them, but I’d like to hear more of the details from our more expert posters here.

FWIW, here’s some of what the Post had to say:

How they can see that from the WMAP data is another thing I don’t have a clue about, but would like to.

We fortunately don’t have enough antimatter in the Universe to cancel out the matter, but we have enough antigravity to cancel out all the gravity, plus enough extra to keep the Universe expanding forever. Funky indeed.

But since this was written by a journalist under a deadline, and for a general audience, I’m assuming this explanation lacks precision. Any comments, guys?

15

All right, I’ll take a stab at a couple of these, starting with (k): So prior to recombination, the universe was almost completely ionized. That is, the temperature was high enough that if an electron happened to bind to a proton or heavier nucleus, it would almost immediately get knocked back off. Now, photons interact electromagnetically, and since almost everything they were travelling through prior to recombination was ionized, they were constantly being scattered.

Then, over a relatively short period of time, the temperature fell enough for neutral atoms to form. The charges are all still there, but an atom looks neutral until a photon gets close enough to see the separation between the electron(s) and the nucleus, which is awfully close. Hence the overwhelming majority of the photons that existed at the time of recombination have been “free-streaming” since then, and some of them land on our detectors. There are many things that can slightly alter the CMB spectrum, of course, such as scattering off of hot gases in galaxy clusters, but these make only minor modifications to the spectrum.

Now, how do they measure cosmic parameters (and hence things like the whether the universe will expand forever, its constituents, and so forth)? This information comes from looking at the CMB power spectrum, an example of which you can see here. Essentially this is a measure of how those fluctuations you see on the maps they show are correlated (note that the maps are actually showing deviations from the mean CMB temperature). The spherical harmonics on the x-axis correspond inversely to angular scales; that is, a smaller l is a larger angular separation on the sky.

Where does this kind of structure come from? Structure formation occurs on a range of scales in the universe. The CMB is a picture of the universe at 380,000 years old. Since the big bang, the matter in the universe had been interacting gravitationally, with regions collapsing if the gravitational force was strong enough to overcome thermal-pressure forces. On some scales there had been enough time by recombination for a cloud of matter to contract to its maximum (given its makeup, temperature, etc) density. On some scales matter had not had time to do that. On some scales it had had time to contract, then rebound. And then perhaps contract and rebound again, and so on. The power spectrum shows what the level of correlation, or structure formation, on different angular scales was at the time of recombination.

Now, to fit a curve to the data points requires that one have a cosmological model built from general relativity and quantum mechanics, describing the structure of spacetime, the matter and energy constituents of the universe (baryonic matter, dark matter, dark energy, etc.), the rate of expansion and how quickly that rate is changing, and so on. By fitting a model with those parameters to the data, the WMAP team has come up with the various numbers given for the universe’s makeup, the Hubble parameter, the age of the universe, its geometry and fate, and so on. If you look in the technical papers section, the “Angular Power Spectrum” paper has previous CMB measurements plotted on page 37, and the WMAP results page 38 onward; and the “Determination of Comological Parameters” paper gives their results for parameters.

I don’t know where the result about the first stars comes from–they say it has to do with measurements of CMB polarization, but maybe someone here can explain that.

16

As I understand it, they got the time of star formation from the clumpiness measurements. If you know how clumpy matter is at the time of decoupling (the time seen in the MAP images), then you can project how long it will take for it to become clumpy enough to coalesce into stars.

We can tell that the Universe will expand forever, because it’s flat (has zero curvature) and it has dark energy in it. In a universe with dark energy, you need positive curvature to even have a chance to re-collapse, and even then, it needs to be rather strongly curved to overwhelm the dark energy. The flatness we can derive from geometry: We know how big some of those blobs should be in real size, and we can model how big they should appear, given a value for the curvature. The size that they actually do appear to be matches the value for zero curvature. I don’t know, though, exactly how they get the dark energy from the observations, though (other than to say that it’s a fit to a model of some sort).

17

Hmm…I’m not sure that clumpiness measurements would be reliable for early star formation, given how shaky models of formation even in the modern universe are and that we don’t have a very good knowledge of magnetic fields (a big factor in star formation today) in the early universe.

Looking at the MAP page, it looks like they got their estimate for early star formation from examining the reionization history of the universe using polarization data. Basically, they say that polarization gets damped out when the universe is very opaque, i.e. prior to recombination. When the first very hot stars “turned on”, they ionized regions of space around them, the structure of which left particular polarization patterns on CMB photons scattered off of them. They claim that they can distinguish the reionization signal from early stars from the signals of quasars and other sources, and arrive at their estimate of first star formation around 180 million years after the big bang.

They sound like they’re hedging their claims a bit on this particular result, which makes sense given that the reionization history of the universe is not at all well understood. But I really don’t know much about this, and am just repeating what I think they’re saying, so I could certainly be wrong about some of the above.

18

e) I dont think the picture tells us so much as basic knowledge tells us. As you might know, NOTHING changes velocity unless another force acts upon it. In space, there is no friction and nothing else could act upon it. So in this way, the universe would constantly expand.

l) Are we to assume that the universe is round like the earth? or that it goes on forever?