Last week I attended the Eighth Harvard-Smithsonian Conference on Theoretical Astrophysics, entitled “Debates on the Nature of Dark Matter”, which brought together leading figures in astronomy, astrophysics, cosmology and particle physics. Although there wasn’t much that was particularly new, it was a very useful conference for taking stock of where we are. I thought I’d bring you a few selected highlights that particularly caught my eye. (more…)
Author: Matt Strassler
A scientific controversy has been brewing concerning the results of BICEP2, the experiment that measured polarized microwaves coming from a patch of the sky, and whose measurement has been widely interpreted as a discovery of gravitational waves, probably from cosmic inflation. (Here’s my post about the discovery, here’s some background so you can understand it more easily. Here are some of my articles about the early universe.) On the day of the announcement, some elements of the media hailed it as a great discovery without reminding readers of something very important: it’s provisional!
From the very beginning of the BICEP2 story, I’ve been reminding you (here and here) that it is very common for claims of great scientific discoveries to disappear after further scrutiny, and that a declaration of victory by the scientific community comes much more slowly and deliberately than it often does in the press. Every scientist knows that while science, as a collective process viewed over time, very rarely makes mistakes, individual experiments and experimenters are often wrong. (To its credit, the New York Times article contained some cautionary statements in its prose, and also quoted scientists making cautionary statements. Other media outlets forgot.)
Doing forefront science is extremely difficult, because it requires near-perfection. A single unfortunate mistake in a very complex experiment can create an effect that appears similar to what the experimenters were looking for, but is a fake. Scientists are all well-aware of this; we’ve all seen examples, some of which took years to diagnose. And so, as with any claim of a big discovery, you should view the BICEP2 result as provisional, until checked thoroughly by outside experts, and until confirmed by other experiments.
What could go wrong with BICEP2? On purely logical grounds, the BICEP2 result, interpreted as evidence for cosmic inflation, could be problematic if any one of the following four things is true:
1) The experiment itself has a technical problem, and the polarized microwaves they observe actually don’t exist.
2) The polarized microwaves are real, but they aren’t coming from ancient gravitational waves; they are instead coming from dust (very small grains of material) that is distributed around the galaxy between the stars, and that can radiate polarized microwaves.
3) The polarization really is coming from the cosmic microwave background (the leftover glow from the Big Bang), but it is not coming from gravitational waves; instead it comes from some other unknown source.
4) The polarization is really coming from gravitational waves, but these waves are not due to cosmic inflation but to some other source in the early universe.
The current controversy concerns point 2. (more…)
POSTED BY Matt Strassler
ON May 19, 2014
One of the concepts that’s playing a big role in contemporary discussions of the laws of nature is the notion of “vacua”, the plural of the word “vacuum”. I’ve just completed an article about what vacua are, and what it means for a universe to have multiple vacua, or for a theory that purports to describe a universe to predict that it has multiple vacua. In case you don’t want to plunge right in to that article, here’s a brief summary of why this is interesting and important.
Outside of physics, most people think of a vacuum as being the absence of air. For physicists thinking about the laws of nature, “vacuum” means space that has been emptied of everything — at least, emptied of everything that can actually be removed. That certainly means removing all particles from it. But even though vacuum implies emptiness, it turns out that empty space isn’t really that empty. There are always fields in that space, fields like the electric and magnetic fields, the electron field, the quark field, the Higgs field. And those fields are always up to something.
First, all of the fields are subject to “quantum fluctuations” — a sort of unstoppable jitter that nothing in our quantum world can avoid. [Sometimes these fluctuations are referred to as “virtual particles”; but despite the name, those aren’t particles. Real particles are well-behaved, long-lived ripples in those fields; fluctuations are much more random.] These fluctuations are always present, in any form of empty space.
Second, and more important for our current discussion, some of the fields may have average values that aren’t zero. [In our own familiar form of empty space, the Higgs field has a non-zero average value, one that causes many of the known elementary particles to acquire a mass (i.e. a rest mass).] And it’s because of this that the notion of vacuum can have a plural: forms of empty space can differ, even for a single universe, if the fields of that universe can take different possible average values in empty space. If a given universe can have more than one form of empty space, we say that “it has more than one vacuum”.
There are reasons to think our own universe might have more than one form of vacuum — more than just the one we’re familiar with. It is possible that the Standard Model (the equations used to describe all of the known elementary particles, and all the known forces except gravity) is a good description of our world, even up to much higher energies than our current particle physics experiments can probe. Physicists can predict, using those equations, how many forms of empty space our world would have. And their calculations show that our world would have (at least) two vacua: the one we know, along with a second, exotic one, with a much larger average value for the Higgs field. (Remember, this prediction is based on the assumption that the Standard Model’s equations apply in the first place.) An electron in empty space would have a much larger mass than the electrons we know and love (and need!)
The future of the universe, and our understanding of how the universe came to be, might crucially depend on this second, exotic vacuum. Today’s article sets the stage for future articles, which will provide an explanation of why the vacua of the universe play such a central role in our understanding of nature at its most elemental.
POSTED BY Matt Strassler
ON May 8, 2014
For those who haven’t heard: Professor Gerry Guralnik died. Here’s the New York Times obituary, which contains a few physics imperfections (though the most serious mistake in an earlier version was corrected, thankfully), but hopefully avoids any errors about Guralnik’s life. Here’s another press release, from Brown University.
Guralnik, with Tom Kibble and Carl Hagen, wrote one of the four 1964 papers which represent the birth of the idea of the “Higgs” field, now understood as the source of mass for the known elementary particles — an idea that was confirmed by the discovery of a type of “Higgs” particle in 2012 at the Large Hadron Collider. (I find it sad that the obituary is sullied with a headline that contains the words “God Particle” — a term that no physicist involved in the relevant research ever used, and which was invented in the 1990s, not as science or even as religion, but for $$$… by someone who was trying to sell his book.) The other three papers — the first by Robert Brout and Francois Englert, and the second and third by Peter Higgs, were rewarded with a Nobel Prize in 2013; it was given just to Englert and Higgs, Brout having died too early, in 2011. Though Guralnik, Hagen and Kibble won many other prizes, they were not awarded a Nobel for their work, a decision that will remain forever controversial.
But at least Guralnik lived long enough to learn, as Brout sadly did not, that his ideas were realized in nature, and to see the consequences of these ideas in real data. In the end, that’s the real prize, and one that no human can award.
POSTED BY Matt Strassler
ON May 6, 2014
To fill in another important detail that will be important later, I added a short section to the end of my article about quantum tunneling. Specifically, suppose you have an electron, placed in one of two traps, such that the electron can tunnel from one trap to the other. What happens if one of the traps is deeper than the second?
This difference between the traps leads to a bias — the electron tends to end up in the deeper trap, because it is harder for it to tunnel back to the shallow trap than it is to tunnel into the deeper one. This simple fact has implications for the entire universe, as I’ll describe in a few days.
POSTED BY Matt Strassler
ON May 5, 2014
A few weeks ago there was (justified) hullabaloo following the release of results from the BICEP2 experiment, which (if correct as an experiment, and if correctly interpreted) may indicate the detection of gravitational waves that were generated at an extremely early stage in the universe (or at least in its current phase)… during a (still hypothetical but increasingly plausible) stage known as cosmic inflation. (Here’s my description of the history of the early universe as we currently understand it, and my cautionary tale on which parts of the history are well understood (and why) and which parts are not.)
During that wild day or two following the announcement, a number of scientists stated that this was “the first direct observation of gravitational waves”. Others, including me, emphasized that this was an “indirect observation of gravitational waves.” I’m sure many readers noticed this discrepancy. Who was right?
No one was wrong, not on this point anyway. It was a matter of perspective. Since I think some readers would be interested to understand this point, here’s the story, and you can make your own judgment. (more…)
POSTED BY Matt Strassler
ON April 30, 2014
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