BOOMERanG presents the first high resolution maps of the cosmic microwave background (CMB) that span a significant portion of the sky. This is important because it allows us to measure the temperature ripples precisely across a wide range of angular scales in a single set of data.
I believe that with this result we have entered a new era of precision cosmology. We are witnessing the beginnings of a revolution that will ultimately be more far-reaching than the COBE discovery. Through their precise measurements of the first peak in the CMB they have made a strong case that the sound waves we've long expected to be visible in the CMB sky are actually there and that they may be used as a kind of cosmic ultrasound to image the conditions of the early Universe.
All interpretations of previous measurements of the degree scale fluctuations in the CMB temperature relied on the assumption that what we were seeing was sound waves. I think the case now is reasonably secure due to their precise measurements of the shape of the first peak and the fact that there is significant structure on the scales where we expect the second peak.
As with any scientific theory, the case for inflation is made by testing its predictions against those of its alternatives. Implausible complications can always be introduced into the alternatives to make them agree with the data. The BOOMERanG results provide a crucial test of one prediction of inflation. Inflation predicts the large angular scale and small width for the first peak that is now seen in the observations. These features are simply inconsistent with the alternate models as we know them today (mainly topological defect models) in which the ripples are formed late and over a long period of time.
In my mind, the most convincing test is the measurement of the higher (second and third) peaks which must be present if the above interpretation is correct. They must lie at angular scales that are predicted by the location of the first peak by a harmonic series, much like the overtones of a musical instrument. They must also have a distinct form where the second peak is lower in amplitude than the first and the third is comparable to or larger than the second.
Although it was a surprise to me that the fluctuations at the location predicted for the second peak are as small as they are, it is not out of line with the inflationary paradigm. Higher precision measurements will determine whether what is seen is indeed a peak of the right form. In fact, the second peak may be low in part because of the physics of inflation. We expect in many inflationary models that the small scale density ripples are slightly smaller in amplitude than the large scale ripples. This "tilt" in the primeval spectrum would help suppress the second and higher peaks.
The higher peaks will also tell us about conditions in the Universe when it was in its infancy. These patterns formed by the sound waves can be used much like ultrasound. It may already be telling us that the amount of ordinary matter in the Universe is significantly higher (20-50%) than previously expected. Ordinary matter loads down the oscillations of the photons, like putting weight on a spring. The result is that the second peak is suppressed relative to the first and third.
There are other possible explanations of the low second peak but in my mind the most likely one is a combination of the above two effects. This situation will be resolved if and when we see the third peak.
In summary: I know who's on first. I've seen what's on second. I-don't-know is on third.
The location of the first peak in the power spectrum provides the best measure of the curvature of the Universe, and hence the total amount of matter in the Universe. Einstein told us that matter curves space: the familiar force of gravity is no more than the curvature of space-time. To see this fact, consider the surface of the Earth. Two people travelling due north from the Equator on different lines of longitude will nonetheless meet at the North Pole. Ignorant of the curvature of the earth, they might attribute this fact to a strange attractive force. The same thing happens to CMB photons on their way to the observer if the Universe is spatially curved. The intervening matter and energy acts as a giant (de)magnifying glass that bends the photon trajectories. The BOOMERanG result supports a flat Universe, which means that the total mass and energy density of the Universe is equal to the so-called critical density. A perfectly flat Universe will remain at the critical density and keep on expanding forever, because there is not enough matter to make it recollapse in a 'big crunch'.
So this argument would make it seem like we already know the fate of the Universe from the results. Unfortunately we don't know that it is perfectly flat. We only know that across the 10-20 billion light years that the CMB photons have travelled there is no detectable curvature. In the earth analogy, it may be that on a clear day we can see for several miles. Over those several miles the earth appears flat. But the earth is not flat! Our observation simply tells us that the radius of the earth is much larger than the distance across which we looked.
Likewise the BOOMERanG results say that the Universe is nearly flat across the observable Universe. Over time, more and more of the Universe becomes visible as light from more distant locations reach us. It may be that we will eventually discover the Universe is closed and will recollapse. We just know that it won't do so in the next few tens of billions of years!
This comes back to the question of "nearly flat". The first peak as observed by BOOMERanG is at about a 10% larger angular scale than we expected from previous measurements and predictions of our previous "best guess" cosmology. This is a small shift and there are uncertainties associated with the measurement so there is no cause for alarm yet.
There are several effects that can shift the peak in that direction by 10% or so. It is true that one of them is to keep the dark matter density low but make the Universe closed by the cosmological constant. Another is to make the dark matter density in physical units higher. This can be achieved by either raising its density compared with the critical density higher than the standard 1/3 value or raising the Hubble constant from 65 to >75 km/s/Mpc. Yet another solution is to slow the acceleration of the Universe by replacing the cosmological constant with a new form of dark energy called quintessence. A combination of all these effects can make the data consistent with a flat Universe without large deviations from the standard parameters.
The higher peaks are "overtones" only in the sense that they are multiples of a fundamental frequency (or equivalently wavelength) that form a harmonic series (approximately 1:2:3...).
In the musical instrument analogy, what replaces the spatial size of the instrument is the amount of time from the Big Bang to the release of the CMB photons: approximately 300,000 years. The peaks correspond to the frequencies of the sound waves that "just fit" in this time period. More specifically, they are the frequencies of the waves that perfectly hit their maxima or minima at 300,000 years. Since you can convert frequency into wavelength using the speed of sound you can think of this as sound in a cavity and get the right result for the frequencies of peaks but you should not think that you can "hear" a coherent sound from the CMB (even if you were present at that time and had an enormous and sensitive eardrum!) The peaks are broad and the coherence is temporal not spatial: what you will hear is low frequency noise.
The physical length scale is the distance sound can travel in the time between the Big Bang and the release of the photons by the formation of atomic hydrogen (recombination). The two things that control the scale are the age of the universe at that epoch and the speed of sound in the fluid. There is a sizeable uncertainty associated with the former as it amounts to knowing the Hubble parameter (expansion rate) at that epoch. Fortunately, this uncertainty mainly drops out of the problem since it is essentially the ratio of the age of the universe then to the age of the universe now that matters when considering angles on the sky and that is much better known. Still uncertainties at the 10% level remain mainly due to comparable uncertainties in the Hubble parameter today. As for the speed of sound, there is little uncertainty there since (CMB) radiation has a well-defined sound speed and ordinary matter simply reduces it by a small amount.
The rough location of the second peak can be predicted by that of the first peak independently of cosmological parameters. It should be close to 1/2 the angular scale. There is some dependence of this ratio on the age of the universe at recombination. In the standard cold dark matter model with a cosmological constant it is closer to 1/2.4; in a critical cold dark matter universe it is approximately 1/2.3. This gives you a sense of the uncertainty in the prediction of where the second peak lies.
Baryons weigh down the photon fluid before recombination just as would happen if you add mass on a spring. This has the effect of suppressing the second peak relative to the first. Remember that the first peak is determined by how far into the potential wells the fluid can fall. The second peak by how far it can spring back. Adding mass enhances how far it can fall leading to a relative suppression of how far it can spring back.
The Microwave Anisotropy Probe is a NASA mission to map the whole CMB sky. There is a comprehensive description of its capabilities on their web site.
The latest schedule (revised this week 4/24/00) gives the launch date as April-May of 2001, a short delay from the November launch originally reported.
I can be reached at (609) 734 8077