Each galaxy lives within its own three-dimensional halo of dark matter whose gravitational field corrals the stars within it. Without the stars, the halo would still be there, albeit invisible to our eyes; but if the halo vanished, the stars would scatter into the depths of the universe – just as a Christmas tree remains a tree with or without the pretty lights. Whereas without the tree, the lights would merely be a puddle of colour on the snowy ground.
As you can guess from the title, The First Three Minutes tells the story of the moments following the Big Bang. The early universe sets the stage for the development of the cosmos we observe today, and the Cosmic Microwave Background is a key link between the distant past and the present day.
Books could and probably will be written about this saga (science writers, call your agents): there is ambition, drama, excitement, Nobel fever, science-by-media, a telescope at the South Pole, and astrophysicists so hungry for data that they analysed images lifted from in Powerpoint slides when the originals were unreleased.
The next batch of Planck data was released yesterday (very early in the morning here in New Zealand), and this time the big news is that Planck has accurately measured how long it took for the first stars to light up after the Big Bang. The headline story is that the dark ages – the time before stars – lasted roughly 550 million years, 100 million years longer than Planck's previous estimate. (This is a long time, but a fraction of the 13.8 billion years since the Big Bang itself.)
When the news started to unravel, it struck me that the cosmology community was in the same position as someone waking up in an unfamiliar Las Vegas hotel room with a throbbing headache, hazy memories of the night before, and a fresh tattoo reading "r=0.2".
Unfortunately, once the initial excitement died away, a number of voices asked whether BICEP2's signal had a more humble origin -- dust in our own galaxy. Dust can mimic a gravitational wave signal if it interacts with the galaxy's magnetic field. From a cosmic perspective, anything inside our galaxy is a "foreground" – dirt on the window through which we peer at the microwave background, the fossil light from the big bang coming to us from the furthest reaches of space.
When the news of the BICEP2 result broke, the mood was euphoric. There was open speculation about Nobel Prizes, a certain video went viral and cosmologists spoke of a radical transformation in our understanding of the early universe. And all of this may still come to pass. But the wave of doubt blowing through the cosmological community in the last week is growing into a consensus that the BICEP2 team has overstated the case for a discovery.
It's possible that the cosmology community is slowly waking up to find itself in an unfamiliar Las Vegas hotel room with a throbbing headache, hazy memories of the night before, and a fresh tattoo reading "r=0.2".
For theoretical physicists, ambulance chasing involves getting papers out quickly after a major data release. Some ambulance chasers make significant contributions, some are just trying to draw attention to their earlier work, while others are banging out insubstantial papers in the hope that they will be cited by their slower colleagues. But whatever their motives, cosmologists have certainly been busy: the BICEP2 discovery paper has been cited 188 times on the Arxiv, all in "preprints" written within a month of the original announcement. I am pretty sure this is a world record, and you can always check the current tally.
Cosmologists don't give tips to newbie universe-builders, but we do ask how our universe evolved. It was quickly discovered that a simple Big Bang needed special and apparently arbitrary initial conditions in order to grow into the universe we now inhabit. But In 1980, physicist Alan Guth, then a post-doc at SLAC, realised that a mechanism he dubbed inflation made these "initial conditions problems" manageable, even if it didn't solve them completely.
When it comes to the details, the stories diverge. Some claim a (relatively) large B-mode that could be hard to square with other datasets, or would imply that the early universe is weirder than we imagine. Other rumours tell of a signal that is consistent with everything else we know, but might permit only a more tentative detection.
Planning to live blog the Planck live blog data release tonight. In the meantime, read Renee Hlozek and Shaun Hotchkiss's blogposts which give good discussions of what is at stake, or watch Ed Copeland giving a quick survey of cosmology.
The story has been told many times. The detector had an annoying and remarkably intransigent "hiss" and Penzias and Wilson knew it was the detector, since the hiss didn't change as they pointed their antenna at different places in the sky. A radio hiss can be converted into a temperature: a red-hot coal has a temperature of a few thousand degrees Celsius but this hiss was microwave-hot, putting it just a few degrees above absolute zero.
Header image: the microwave background as measured by the CoBE and WMAP satellites; the transition shows the evolving precision of these measurements from the early 1990s through to 2010. Original video