[This post is a bit long and diffuse… I may hack it up into bite-sized pieces later…]
Just because my job has ‘astro’ in the title, doesn’t mean I know enough to comment on whether or not Pluto is a planet. And there’s plenty of other science-in-the-news…
The International Astronomical Union (IAU) has decided that a planet is anything with enough gravitational pull to make itself round. As a scientific organization, the IAU probably had to go for something like this — a more or less physical definition — along with the sociological desideratum of preserving Pluto’s planetary status. The other — perhaps more sensible — option would have been to declare “planet” to be a category something like “race” or “pornography”: not actually well-defined by some set of principles, but nonetheless “we know it when we see it”. We could then just declare the same old 9 planets, including tiny but venerable Pluto, and move on. Instead, with the current definition there are 12 planets, and astronomers will probably find a lot more over the coming years. I’m not sure if we should bother changing the textbooks quite yet.
More meaningful to me is the age of the Universe. Astronomers at the Carnegie Institution and elsewhere have observed eclipsing binary stars in a nearby galaxy, and thereby determined the stars’ masses. With careful modelling, they’ve then been able to predict how luminous those stars should be, and by comparing that to the stars’ observed brightness, determine the distance to the galaxy. Their result puts the galaxy about 15% further away than previous (less direct) measurements; if correct — and if we can distinguish the galaxy’s cosmological “motion” from its attraction to other nearby galaxies (such as our own) — this wouldn’t impact merely the distance to this one object, but would revamp the entire cosmic distance scale, lowering the Hubble Constant which measures the expansion rate of the Universe, and finally making the Universe about 15% older than we thought.
On the one hand, 15% isn’t that big a change in a quantity, the Hubble Constant, that used to be uncertain to about 50% as recently as a decade ago. On the other hand, recent measurements from a variety of quite disparate sources have confirmed its higher value to better than 10% or so. But it’s an intriguing possibility that could push the details of the Hot Big Bang model in intriguing ways, but almost certainly without getting rid of the weirdest features of the models, such as the unexplained, exciting, and increasingly solidly measured Dark Energy. (As usual, Ned Wright’s Cosmology Tutorial is an excellent starting point if you’re perplexed by my jargon.)
In other cosmology news, the lucrative and prestigious Gruber Prize in Cosmology has been awarded to the COBE team, which first measured the fluctuations in the Cosmic Microwave Background that’s since enabled us to absolutely confirm the hot Big Bang theory, measure the curvature of the Universe and the mass of its contents.
I’m spending the week at the Workshop on Nongaussianity in Cosmology at the Abdus Salam International Centre for Theoretical Physics, just outside of Trieste, Italy.
When I’ve got more time, I’ll try to explain what we’re talking about here. The basic idea is that our baseline theories for the way structure formed in the early Universe say that it is likely to have a statistically isotropic and very nearly Gaussian distribution, which in turn means that it can be described by a single function called the two-point function, ξ(
I’m quoted today in article in New Scientist, “Universe Weighs in Surprisingly Light”. I spoke to the author, Zeeya Merali, last week about a recent article by Hans Fahr and Martin Heyl, in which they define a “radius of the universe” and therefore the amount of mass inside that radius. Because their calculation is done in the context of Einstein’s theory of General Relativity, we have to be very careful when we define any coordinate system — and that’s just what a definition of “radius” is.
Unfortunately, as far as I can tell, their definition seems to be purely an artifact of the way they’ve decided to write down their coordinates. This is in contrast to a more standard definition — such as the so-called “particle horizon”, which is the distance a photon could travel from the big bang until some particular time. But their definition has the interesting property that it is proportional to (and just a little smaller than) the radius of a black hole that has the same density as the average density of the Universe at a particular time.
A subscription is required to get the whole article, but I suppose I can reproduce my own words:
Stating the obvious: “Our universe is supposed to be infinite and expanding.”
A meagre attempt at cross-disciplinary profundity: “What’s interesting is that they have managed to reproduce this physical correspondence with black holes just by starting from an essentially philosophical question: what does it even mean to ask about the mass of the universe?”
And back to the obvious (or is it profound?): “This is just one way to define the mass of the universe…. There’s room for different definitions — and they may not give the same answer.”
It is … paradox that the earth moves round the sun, and that water consists of two highly inflammable gases. Scientific truth is always paradox, if judged by everyday experience, which catches only the delusive nature of things.
—Karl Marx, Das Kapital, quoted by Francis Wheen in The Guardian.
Or that objects in motion tend to stay in motion: It took an extraordinary leap of imagination, to traverse the nearly two millennia from Aristotle, who thought otherwise, to Newton, who realized this, despite no one ever having, say, thrown a ball that didn’t stop moving (it took another couple of centuries before we could do that — launch a satellite into orbit, that is).
Once you accept these supposed paradoxes, does that help you understand that the universe might have had a beginning, and no end, starting from hot and dense 15 billion years ago? Or that proto-apes may have begat proto-humans?
Congratulations to my colleagues Saul Perlmutter, Adam Riess and Brian Schmidt! They are sharing the rather lucrative Shaw Prize for their leadership in the late 1990s discovery that the Universe seems to be accelerating in its expansion. In particular, through painstaking observational campaigns over many years, they observed that distant supernovae — exploding stars whose intrinsic brightness are all roughly the same — seem to be dimmer than would be expected in the simplest universe. Dimmer, hence further away than expected; further away, then, due to an accelerated expansion of the Universe as a whole.
The most obvious mechanism for this expansion is Einstein’s Cosmological Constant. The problem is that, although we don’t have any precise way to calculate its value, the best guess is either strictly zero (so no acceleration at all) or that it’s something like 122 orders of magnitude (that’s 10122) times larger than it’s observed to be (the Planck density, for aficianados). Another possibility is that the acceleration is due to something not quite as fundamental as a cosmological constant, some sort of field pervading the Universe, often called Dark Energy or Quintessence. Unfortunately, there are no particularly compelling ways to calculate its value in that case — these are more like paradigms in search of a detailed theory. A final possibility is the so-called anthropic argument: the cosmological constant has the value that it does because, if it didn’t, we wouldn’t be able to be here to observe it. Unfortunately, any version of that argument that isn’t a tautology is unpredictive, useless, or silly, at least for now.
So let’s hope a future version of the Shaw prize goes to someone clever enough to explain the acceleration.
Thanks to Sean for pointing this out, and for noting that the ideas behind the discovery were already in the air (especially since that allows me a little self-aggrandizement). Also, in the interests of equal-time, I should point out that there are reputable astrophysicists out there who aren’t quite convinced by the Supernovae data.
Like fellow-blogger Mark Trodden , I’ve just spent the week at scientific meetings in Ischia, an island off the coast of Naples. The first half of the week was for the yearly consortium meeting of the Planck Surveyor satellite. Although still endangered by further delays, we expect the satellite to be launched in early or mid 2008, and by then we have to be ready to analyze the data from Planck as it gets transmitted, just a few bits at a time, from the satellite at the “L2” point, 1.5 million kilometers from the Earth, a place where the sun, earth and moon will all be in a small area of the sky — so it’s easier to shield the satellite, which is measuring temperature differences of a few parts in a hundred thousand on top of a background just three degrees above absolute zero.
Of course, at an experts-only meeting like this, we didn’t discuss the exciting scientific prospects so much as the details confronting us today: planning how the mission is going to scan the sky, how we’re going to measure the instrument’s properties
After the detailed work of the consortium meeting, we turned to the scientific side of cosmology as it is today, hearing about details of early universe physics, dark matter, and, especially, Planck’s predecessor, WMAP, from Mike Nolta.
I even got some time free at the end to spend a day at Pompeii, and at the National Archaeological Museum. Coming from a country only a couple of centuries old, walking through two-thousand year-old streets, it was remarkably easy to imagine the ancient Romans peddling their wares, living their lives, eating and drinking, just like us (except for the slaves, of course…). (More pictures here.)
To top it all off, I returned to find Spring finally arrived in London, my favorite plants in bloom at last. But now, no rest for the weary: after about a day and a half back home, it’s off to another meeting. But that’s an entry for later.
As promised, the team behind the WMAP (Wilkinson Microwave Anisotropy Probe) satellite have released their lovely new results. WMAP measures fluctuations in the CMB (which I’ve already written about a lot), and in 2003 they released high-resolution, high-sensitivity maps of the CMB over the whole sky. Today, they updated those maps, and also released new maps of the CMB polarization: the CMB is made of photons (light) and light can be characterised not just by its intensity (the color in the map below), but also by its polarization, which you can think of as a little arrow that travels along with it (normally, those lines average to zero, but if you have polarized sunglasses, you already know that sometimes, when light reflects off of something like the surface of a pool of water, it gets polarized). When the photons that become the CMB scatter off of electrons in the early universe, they too can end up with a small net polarization, due to the same physics that produces the temperature fluctuations in the first place (there are other, even more exciting, ways of producing polarization that may let us probe even earlier times and the epoch of cosmological inflation that we believe may be responsible for the large-scale structure of the Universe today, but WMAP doesn’t have the sensitivity to see that sort of polarization signal).
[Courtesy of the NASA/WMAP Science Team]
Of course there’s much more science to discuss: what does it mean? What’s changed since the last data release? (For one thing, some of the most interesting results, such as the epoch of formation of the first bright objects, seem to have been softened somewhat.) More later.
Update: It’s too late for me to expound much on the results, but the NY Times lets the great and the good (of the cosmology set) have their say, and for more real commentary, please check out Sean Carroll and Steinn Sigurdsson (twice!), and, as we say, references therein.
The ΛCDM Model requires two pieces of unknown physics. One is 'Λ' and the other is 'CDM'
I'll explain later.
A couple of weeks ago, my colleagues in the Boomerang Collaboration, spread out over the US, Italy, Canada, France and the UK, released five papers analyzing the data from the latest flight of the Boomerang instrument, over Antarctica in January 2003 (check this out for information from my fellow Boomerangers on what it's like down near the South Pole).
Boomerang is an experiment which measures the Cosmic Microwave Background, or CMB (there are some tutorials on the physics behind the CMB here, here and here). Boomerang, along with the Maxima experiment (with which I'm also fortunate to be involved) produced the first high-resolution maps of the CMB back in 2000, measuring the tiny -- one part in 100,000 -- fluctuations around the mean temperature of about 2.72 degrees above absolute zero. The pattern of these fluctuations on the sky lets us measure the geometry of the Universe (which is equivalent to asking the question, What happens to light rays that start off parallel to one another? Do they converge, in which case the universe is said to be "closed" -- curved like a ball? Do they diverge, in which case the universe is "open" -- curved like a saddle, or a Pringle's potato chip? Or, do they stay parallel, making the universe flat? In 2000, these experiments proved what had already been surmised -- the Universe is indeed flat, so parallel lines are indeed parallel forever. Since then, experiments have measured the CMB at higher and higher resolution and accuracy, and allowed us to understand even more about the origins of the Universe.
So why is our new data so exciting? First, we've made some of the most sensitive measurements of the CMB ever made -- similar to our 2000 measurements, but with error bars many times smaller.
Second, and even more exciting, we've measured the polarization of the CMB. In addition to the CMB temperature measured by the 2000 experiments, the CMB is also polarized, which means that the CMB photons tend to point more in some direction on the sky than others. This reflects some of the same properties of the Universe as the temperature, but is a completely independent (and much harder!) measurement. Although the CMB polarization has been detected by the DASI, CBI and CAPMAP experiments, these new results are the full maps of the CMB polarization (although I must disclose that they are still mostly noise; we can only see the cosmic signal by taking averages).
Links to the Boomerang papers here, and on astro-ph:
- The Instrument and Maps
- Temperature power Spectrum
- Polarization Power Spectrum
- The Temperature/Polarization Cross Power Spectrum
- Cosmological Parameters