Cosmic Microwave Whatnow?

8th June 2018

For the last 20 years, I’ve been developing pictures of the Big Bang.

The photons created in the Big Bang have been stretched by the expansion of space and now form a faint radiation field we call the Cosmic (since it is omnipresent) Microwave (since its intensity peaks at microwave frequencies) Background (since it comes from behind all astrophysical sources). It was accidentally discovered using a huge radio telescope, and indeed about a percent of the noise you can hear between FM radio stations is this echo of the Big Bang.

To measure the CMB, my instrument-building colleagues sweep telescopes with arrays of microwave detectors back and forth across the sky for years at a time recording one of the most uniform – and at the same time faintest – signals in the Universe. Brought back from the quietest (and remotest) observing sites available, these enormous, noisy, datasets are passed to us data analysts, and we turn them into pictures of the Big Bang.

These pictures are scientific treasure troves. Within them are the tracers of Big Bang physics, laid down as micro- to nano-Kelvin fluctuations in the temperature and polarization of that first light. And while the spatial distribution of these hot and cold spots that we see has a randomness to it (being our particular realization of Universal physics), there is a statistical underpinning in their 1-D angular distribution. Since different model cosmologies produce different distributions of power on long and short wavelengths, this gives us predictions that we can test with measurements.

And what predictions! The geometry, age, and surprising composition of the Universe. The epoch of reionization, when the first stars lit up the infant Universe. The mass of the neutrinos – the ghostly particle whose sheer abundance gives them a key role in the formation of the largest scale structures in the Universe. Whirlpools spun from quantum gravitational waves from inflation, writ large across the sky.

CMB Power Spectra

And then the rest of history happens. Stars light, galaxies coalesce and cluster, huge filaments and voids form in the dark matter, dragging the normal stuff with it. To all of this the CMB is a backlight, and now its uniformity becomes a blessing. Different evolutionary histories leave different secondary distortions in the CMB, letting us measure the distribution of matter throughout our Hubble volume.


But we pay a heavy price for this. The primary CMB signals are now heavily contaminated by extragalactic distortion and galactic emission, potentially fatally confusing the cosmological and astrophysical signals. Fortunately, the CMB – unlike the contaminants – is a near-perfect black body. If we observe at enough different frequencies we can hope to distinguish between it and the various foregrounds based on the way their brightness changes with frequency – with the primary galactic contaminants, thermal dust and synchrotron radiation, getting relatively brighter and fainter than the CMB respectively with increasing frequency.

The challenge for us today is to detect the faintest CMB signals coming from gravitational waves generated during Inflation – the brief period of exponential expansion in the early Universe. To do this we’re planning new observing campaigns from the ends of the earth, the edge of the atmosphere, and deep space. It’s a great time to be a CMB scientist!