CMB Sky

What to watch for

The two views are the same physics seen two ways. The blobs in the map are about a degree across precisely because the first acoustic peak sits at $\ell \approx 220$; that angle is the loudest note the early universe could play.

In the spectrum, watch how the sliders move the peaks: each does something physically distinct:

• Baryon density loads the oscillator. Baryons add inertia to the photon–baryon fluid, so it falls deeper into the wells before pressure can stop it. That deepens the compression phases and shallows the rarefaction phases, which boosts the odd peaks (first, third) and suppresses the even peaks (second). The first-to-second peak height ratio is therefore a direct baryometer: drag $\Omega_b h^2$ up and watch the ratio climb. This is how we weigh the ordinary matter in the universe without ever seeing it.

• Matter density sets the distance to last scattering, and so the angle the sound horizon subtends. Lower $\Omega_m$ pushes the whole comb of peaks to higher $\ell$ (smaller angles); raise it and they march left. The peak positions are how the CMB measures the geometry of space and finds it flat.

• Tilt, the scalar spectral index $n_s$, sets the slope of the primordial fluctuations laid down by inflation. A "red" tilt ($n_s < 1$, as observed) means slightly less power on small scales: the right-hand peaks sag relative to the left. A perfectly scale-invariant input would be $n_s = 1$; the measured departure is one of inflation's cleaner predictions.

Past $\ell \approx 1000$ the spectrum falls away exponentially. That is Silk damping: on small scales photons random-walk out of the over-dense regions during recombination and smear the contrast away, erasing the smallest ripples.

Knobs

• Baryon density Ω_b h²: the physical density of ordinary (baryonic) matter. Raising it boosts the odd compression peaks and suppresses the even rarefaction peaks, lifting the first-to-second peak ratio. The single most legible effect in the panel.
• Matter density Ω_m: the total matter density today. It fixes the distance to the surface of last scattering and so the angular scale of the peaks: lower values slide the whole peak comb to higher $\ell$, higher values slide it lower.
• Tilt n_s — the spectral index of the primordial fluctuations. Below 1 it tilts power away from small scales, dropping the higher peaks; at 1 the input is scale-invariant.
• View — switch between the mottled temperature map (the sky as you would image it) and the power spectrum (the same ripples sorted by angular size). The two are a Fourier pair: one is what you measure, the other is what you fit.