I have just finished checking page proofs of an article that will appear in the 2015 Yearbook of Astronomy. The article is entitled “Ripples from the start of time?” and it discusses what had the potential to be one of the most important and exciting cosmological discoveries in decades: B-mode polarization of the cosmic microwave background. Earlier this year, the BICEP2 experiment claimed to have found just such polarization and argued that their measurements could only be explained in terms of primordial gravitational waves – ripples of space made large by inflation, an event that took place when the universe was only a trillionth of a trillionth of a trillionth of a second old.
The BICEP2 team made a bold claim, and bold claims require a lot of solid evidence before they can be accepted by the scientific community. Soon after the team made their announcement of B-mode polarization, scientists raised doubts about the interpretation of the measurements (though not of the measurements themselves: everyone acknowledges that the scientists involved here are extremely capable astronomers). One problem was that the BICEP2 experiment observed the sky at only one frequency: when you have only one data point, any curve can be made to go through it. When you observe the cosmic microwave background you need measurements at several different frequencies before you can be sure that your signal really does come from the distant cosmos and not somewhere nearby. A second, more pernicious, problem was that it was not at all clear that the BICEP2 team had properly accounted for dust.
The issue is that dust grains in the galaxy can polarize light – and in particular it can give rise to a B-mode pattern of polarization. If the BICEP2 team had underestimated the amount of dust emission then their interpretation of their observed signal had to be under suspicion. It’s why I added a question mark to the title of my article: BICEP2 might have been seeing a signal from the dawn of time, but it might not.
A recent paper by the Planck collaboration suggests that the BICEP2 result might well have been the result of dust. It turns out that there is much more dust in the area of sky observed by BICEP2 than was originally thought. That in turn means that the BICEP2 results are entirely consistent with observations of dust. This doesn’t mean that B-mode polarization of the cosmic microwave background does not exist, nor even that BICEP2 didn’t spot such polarization; but it does mean that we don’t need to invoke inflation in order to explain the BICEP2 results.
A joint paper by the Planck and BICEP2 teams, due for publication later this year, should clarify the situation further. But at the moment it seems that our dreams of being able to look back to the very start of the universe must be put on hold. Shame. Those dreams were beautiful while they lasted
This morning an ESA press conference presented results from an analysis of the first 15 months of data from the Planck mission. The results are exquisite, and it’s clear that Planck will be as important for cosmology as its predecessors COBE and WMAP. Cosmologists will be poring over the data for years to come.
I’ll give more detail in future posts, but for the moment here are just two items.
First, the most detailed picture yet of the early universe:
Planck’s stunning new map of the universe (Credit: ESA)
Second, some of the stand-out points from this morning’s presentation:
- The universe is slightly older than we previously thought (about 80 million years older in fact): it’s 13.82 billion years old.
- Planck measures the Hubble constant to be 67 km s-1 Mpc-1. This is slightly smaller than most other recent estimates. Curious!
- The energy inventory of the universe isn’t quite what we thought it was: there’s slightly more dark matter than previously thought and slightly less dark energy. The universe is currently made up of 4.9% normal matter, 26.8% dark matter and 68.3% dark energy.
- On small scales, the standard cosmological model (which includes inflation) agrees supremely well with the observed cosmic microwave background. The standard cosmological model is in good shape.
- There are hints, based on observations of the largest angular scales, of physics beyond our current theories. In particular: (i) the sky in the southern hemisphere is ever so slightly warmer than the sky in the northern hemisphere; (ii) large-scale temperature fluctuations are weaker than expected; and (iii) there’s a cold spot in the universe, in the constellation Eridanus, that’s much larger than our models would predict. Gaining an understanding of these anomalies is going to lead to some really interesting ideas over the next few years.
The Wilkinson Microwave Anisotropy Probe (WMAP) has been perhaps the most spectacularly successful cosmology experiment of all time. WMAP painstakingly mapped the cosmic microwave background and its results have allowed scientists to determine key parameters of the Universe – it’s age, its geometry and so on – with a precision that would have been unthinkable just ten years ago. In 2004, WMAP also found hints of something unusual happening closer to home: it found a weak signal corresponding to haze of microwave radiation, roughly spherical in shape, centred around the centre of our Milky Way galaxy.
If such a haze exists then astronomers have an interesting challenge in trying to explain its origin. But does it exist? Some researchers argued that the haze might be nothing more than an artefact of data analysis, since they doubted whether WMAP had the sensitivity to distinguish such a weak signal from the general microwave background and the strong emission from galactic dust.
Well, the European Planck mission has a sensitivity that exceeds WMAP and it can observe the Universe over a greater range of frequencies. We can expect great things for cosmology when the Planck team releases its full results. One intermediate result of the Planck Collaboration, released recently on the arxiv server, is that the microwave haze found by WMAP does indeed exist. What is particularly interesting, however, is that the haze isn’t spherical; it’s stretched out like a cigar. Furthermore, the sharp edge to the haze suggests that whatever causes it is a sporadic rather than a continuous phenomenon (since a continuous process would lead to a diffuse haze).
Astronomers don’t yet have a good explanation for the haze. One initial suggestion for the WMAP observation was that dark matter annihilation created electrons and positrons which, when they spiralled in the Milky Way galaxy’s magnetic field, generated the observed microwave haze. But the Planck discovery of a squashed rather than spherical haze tends to discount that idea. Planck has found a mystery. I’m hoping that when the Planck Collaboration publish its full results from the mission there’ll be several more discoveries to ponder.