Thursday, March 16, 2006

WMAP for Dummies

WMAP is a small NASA satellite mission that is measuring the microwave background radiation, a diffuse electromagnetic glow that peaks at long wavelengths (down around millimeter) and is pervasive in the universe.
This radiation is the cooled afterglow of the Big Bang, the origin of the observable universe from a hot, dense initial configuration a finite time (about 13.7 billion years ago) in the past.

The primary task of the satellite is to measure the intensity of the radiation as a function of position on the sky at several different wavelengths, and to a lesser extent measure the polarization of the radiation (WMAP has some sensitivity to this, but is not designed to measure polarization).

First task is to subtract the "foreground" radiation, which is the irregular infrared radiation emitted primarily by cool dust in the Milky Way galaxy (this is a narrow ribbon of emission on the sky) and to a lesser extent the "zodiacal emission" of dust within the solar system.

The residual radiation has a very uniform characteristic temperature across the sky, an almost perfect "black body" radiation, with a temperature of about 2.73 K.
There are deviations from the mean temperature of about a part in million, these fluctuations, seen on scales ranging from a quarter of the sky, to degree scales (about the width of the Moon on the sky) code most of the detailed physics of formation and evolution in the early universe.

The WMAP data is reduced to a set of six primary parameters, and some secondary inferred parameters describing these physics. Data is considered in two ways - the constraints on these parameters from the WMAP data alone, and the constraints given data from other observations; and separately whether the different observations are consistent. (They pretty much are).
Formally, you can make different "prior" assumptions about what you know about the universe and then see what that tells you about the unknown parameters...

So, what do the results mean?

First of all, they are consistent with the standard model Big Bang theory for cosmogenesis.

The density of the universe is consistent with being exactly the critical density (and the allowed deviation from this is at most a few percent).
72% (or 74% depending on which prior you take in) of the universe consists of "dark energy", unknown something with an unusual characteristic - the pressure of dark energy is negative. This is characterised by the "w" parameter. w=-1 is consistent with a "cosmological constant".
w < -1 is often consider unphysical, but is it were so, the universe would be heading for a Big Rip in a finite time in the future. This seems now to be excluded.
0 > w > -1 implies that there is some (possibly dynamical or varying) equation of state for some dark energy stuff, sometimes labeled "quintessence". That would be interesting.
Current data suggests w is very close to being exactly -1, which strongly suggests it is a cosmological constant type of thing. This will disappoint a lot of particle theorists who would prefer a more exciting dynamical entity.

Dark matter is there at about 24% (22% with different priors folded in) of total universe content and the normal "baryonic" matter we are made of is about 4% of the universe. This dark matter is consistent with being "cold" dark matter.
Neutrinos make up about 0.1% of the mass of the universe, which constrains the neutrino mass in interesting ways.

If anyone cares, Hubble's constant, H0 = 73 km/s/Mpc +/- 3 .

The first stars formed early, at redshift of about 1512 (few hundred million years after the Big Bang (365 million)).
This was hinted at in the previous data and now looks quite solid. This is a bit surprising but consistent with interesting theories. (τ ~ 0.08, or 0.1 if you allow running power spectrum). The early reionization due to "first stars" is needed with the current parameters, but need not have been complete, data looks consistent with patchy ionization with maybe some "neutral zones" remaining down to redshift 7 or so.

The "spectrum of density fluctuations" in the universe is close to being "scale invariant" (crudely speaking fluctuations are independent of their size at the beginning). Which is parametrised by a number n=1. This is consistent with the inflation model for the Big Bang. Inflation theory in general predicts a small deviation from exact n=1, and the amplitude of this deviation (parametrised by the α and r parameters) is known to be small now, and small enough that some variants of the basic inflation theory are in trouble (but the basic inflation scenario is in very good shape, this tests competing theories of the exact mechanism for inflation).
WMAP is now claiming n is slightly less than unity, at some significant level, and that this is consistent with some small contribution from tensor modes. They like n=0.95 when combined with prior data, at about 3 σ from unity.
They favour dn/dlnk ~ -0.09 and tensor modes present (-0.06 without tensor modes), data is marginally inconsistent with a perfectly flat spectrum.

The fluctuations on very large scales due to gravitational radiation are small, small enough to start also testing some alternatives to the standard Big Bang. (eg Ekpyrotic theory looks to be in some trouble, shame its a cute theory)

There are some interesting technical issues: one is the anomalously low amplitude of fluctuation on the largest scales (l=2 mode), which has been suggested to imply the universe loops back on itself (has a non-trivial topology). Another is the "third acoustic peak" is now measured in position and amplitude, which constrains the physics of dark matter. Another is the deviation from scale invariance may be showing, but it is degenerate with whether the radiation field has a net "curl" (whether the temperature fluctuations are smooth or form vortexes with little whirls in them).

No big surprises, but lots of interesting constraints.

Next mission to tackle this will be ESA's Planck mission

Update: see Sean's comments and Phil's intro
I don't think the shift in the best estimate of τ is profoundly significant, and it is consistent with the 1st year WMAP range. The essential point is that there is early ionization with substantial energy input from a Pop III epoch at z > 10, and NOT that reionization just happens at z ~ 6-7. It is interesting to see whether early reionization was z=11, 15 or 20 but it doesn't change the essential physics of the argument for a Pop III contribution.


Anonymous Anonymous said...

I appreciate the clarity of your post, but I want to be sure I understand this , since my son asked me a question about it that I could not answer. He read in Yahoo News that the universe expanded from "...the size of a marble to billions of light-years across...) in the first trillionth of a second after the Big Bang. This implies (though I may well be confused here) that SOMETHING, either matter or energy, was occupying that space, and if so, must have travelled hugely in excess of the speed of light. I'm a pediatrician, not an astrophysicist, so I am stumped for an answer. Can you help?

11:16 AM  
Anonymous Anonymous said...

Hi db,
You and your son have brought up one of the most interesting parts of relativity. It turns out there is no paradox, and that two different points can recede from each other faster than the speed of light. In fact, this defines the edge of the "observable universe", since we cannot see any light from points that recede from us faster than the speed of light. The key element above is "two different points". Relativity says that nothing passing through a point can be measureed by an observer there (at the same point) to be faster than the speed of light. In other words, the speed limit is a local property. The distance between two different points, on the other hand, is another thing altogether, without the same speed limit. It is a prpoerty of the global space. If you will, as any driver knows, speed limits are local. There are places elsewhere that allow you to move beyond the local speed limit, but you can't get there from here.

11:35 AM  
Blogger Steinn said...

What he said.

The "pop" way of saying this, is that you can't traverse space faster than light, but two points in space can move apart faster than light (the distance between them may expand at a rate faster than the speed of light).

Good question.

5:06 PM  
Anonymous Anonymous said...

As explained on my page,
the WMAP data analysis is flawed, with details of the resultant angular power spectrum being artefacts of the differential method being used.


1:00 PM  

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