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Wed April 17, 2013
There's Trouble Brewing At The Birth Of The Universe
Originally published on Tue April 16, 2013 5:59 pm
Scientists can't just agree to disagree. It's not because we are stubborn or ornery (OK, maybe we are). It's because the whole point of science is to establish "public knowledge" — an understanding of the cosmos on which we can all agree. That is why there is trouble brewing at the beginning of the Universe.
There is a number, the Hubble Constant, that's fundamental to the study of the cosmos. The problem is, different folks are finding different values for that number and no one yet knows what that means.
Two weeks ago the scientific team running the Planck satellite announced the most comprehensive analysis of their data to date. The Planck mission was designed to study radiation left over from just a few-hundred-thousand years after the Big Bang. This is the famous cosmic microwave background radiation (or CMB) and it's a kind of fossil light, imprinted with all kinds of details about how the Universe evolved. These details include parameters describing the proportions of material in the Universe, like dark matter and dark energy.
One of the parameters that fall out of their analysis is the Hubble Constant (written as Ho) that describes the rate of the Universe's expansion in the current epoch of cosmic history. The Hubble constant is directly tied to finding the Universe's age (though other parameters are needed as well). The Planck analysis yielded a value of Ho = 67.11 kilometers per second per Megaparsec (km/s/Mpc: yes those units are weird but don't worry about them for now). Put it all together and the Planck team finds the Universe to be 13.89 billion years old give or take a few hundred million years. You gotta admit, that kind of precision is pretty impressive.
So what's the problem?
Other teams using a very different set of methods get a very different answer for the Hubble Constant. When Edwin Hubble (you know, that guy with the constant named for him) first discovered the Universe was expanding back in 1928, he did it by measuring how fast galaxies were flying away from us versus their distance from us. The CMB wasn't even a gleam in scientist's eye back then. This direct method — measure the distance and recession velocity of lots of galaxies, then plot up the result — has been getting more refined ever since. These days the best values for the Hubble Constant that come from this approach find something around Ho = 74 km/s/Mpc.
These values come from folks like recent Nobel winner Adam Riess's SHOES project or Wendy Freedman's team using the Spitzer Space Telescope. They are excellent scientists and they know what they are doing, just like the folks working with the Planck data. But they get a different value for Ho than the Planck team. The numbers don't agree. Worse, the error bars don't even overlap.
Something is going on.
Now you might think "Hey, what's a difference of seven, or so, between friends? Can't we all just get along?" But the cool thing is science doesn't work that way. It can't work that way. The Hubble Constant is so crucial to understanding cosmic history that we have to find as many ways as possible to directly measure its value. If the values don't line up, then astronomers MUST figure what lies behind the discrepancy.
It could be new physics hiding in the CMB that we have not thought of — an indication of some unknown force or process. It could be something more mundane about the way measurements from the local Universe (the direct method) differ from measurements from the early Universe (the CMB). Time will tell. But hiding in this anomaly is a deep lesson for us all about the very nature of science and culture.
Every measurement is a direct interrogation of nature. It's a chance to sit down with the Universe and ask, "Did we get this right?" and "Do we really understand?"
This learned intimacy with shared experience is what that makes the discovery of science itself such a milestone in the history of human culture. It gives us a way to base our claims about the world in the world's own actions. It takes us beyond the "demon haunted world" into a public space, a collective commons, where all claims about nature must rest on public evidence.
The power of this approach should be apparent to us all every time we make a cell-phone call or step into an airplane. And it is exactly this public domain of knowledge that has come under attack in the strange world of science denial we find ourselves in today when long-established understanding in a field like climate studies can be treated like whims or hoaxes.
The trouble with the Hubble Constant will, I am sure, be resolved one way or another (that is why we are not changing the name of the blog, yet). I am eagerly waiting to see how it gets sorted out. But in the meantime we can take it in as another example of what makes science so valuable to us as a culture and why it needs to be respected and defended.
(Many thanks to my fellow University of Rochester professor Dan Watson for his help with this topic.)