Some news on the Higgs particle from the ATLAS and CMS experiments, the two general purpose experiments at the Large Hadron Collider. I just mention a few highlights. (more…)
Author: Matt Strassler
The controversy continues to develop over the interpretation of the results from BICEP2, the experiment that detected “B-mode” polarization in the sky, and was hailed as potential evidence of gravitational waves from the early universe, presumably generated during cosmic inflation. [Here’s some background info about the measurement].
Two papers this week (here and here) gave more detailed voice to the opinion that the BICEP2 team may have systematically underestimated the possible impact of polarized dust on their measurement. These papers raise (but cannot settle) the question as to whether the B-mode polarization seen by BICEP2 might be entirely due to this dust — dust which is found throughout our galaxy, but is rather tenuous in the direction of the sky in which BICEP2 was looking.
I’m not going to drag my readers into the mud of the current discussion, both because it’s very technical and because it’s rather vague and highly speculative. Even the authors of the two papers admit they leave the situation completely unsettled. But to summarize, the main purpose and effect of these papers seems to be this:
POSTED BY Matt Strassler
ON May 30, 2014
NOTE ADDED: FALSE ALARM! DISREGARD! HERE’S SWIFT’S DETAILED EXPLANATION AS TO WHY! with my own brief summary below.
Comparable in size to the Milky Way, our host galaxy, the Andromeda galaxy is the most distant object easily visible (in dark skies) to the naked eye; it lies 2.5 million light-years away. About 2.5 million years ago, something in this distant star city went “boom”. And in doing so it flashed, brightly, in high-energy photons — particles of light (or, more precisely, particles of electromagnetic radiation, of which visible light is just an example) — photons that carry many thousands of times more energy than do the photons that our eyes are designed to see.
Some of these photons, after traveling for millions of years across space, arrived at Earth this afternoon. They showed up in the Swift satellite’s telescopes, which are designed precisely to notice these things. And Swift’s telescopes identified these photons as arriving from a location somewhere within Andromeda… within a globular cluster of stars, a tightly-knit neighborhood within the city that is Andromeda. NOTE ADDED: Actually, a combination of low-probability events caused
- a false alarm, of a sort that’s rare but not unexpected: a known object in Andromeda that emits X-rays appeared to brighten, as a result of electronic noise in Swift’s instruments (such noise is always present, in all scientific instruments, and it is normal to occasionally get a strong burst of it)
- followed (
due perhaps to a poorly-timed computer problem at Swift’s data center, which slowed the arrival of more complete informationthe Swift people know why but haven’t explained it in detail) by a delay in identifying this apparent brightening as a false alarm;
all of which is explained here. The apparent brightening, which was rather mild, would in fact have been completely disregarded if it hadn’t occurred in Andromeda; for relatively nearby objects like Andromeda, the Swift team sets a low threshold for false alarms, because something real would be so amazingly important and exciting that we can’t afford to miss it.
What caused this colossal explosion, perhaps the nearest of its type ever observed by modern astronomers? That is the burning question that astronomers, and their friends in gravitational physics and particle physics, are aching to know. It is likely that by tomorrow morning, and certainly within the next couple of days, we’ll know much more… perhaps we’ll even learn something of great importance. NOTE ADDED: And indeed, we know.
For the moment, though, there’s lots of guessing, most all of which will turned out to be wrong. (Maybe, some are speculating, this is a gamma-ray burst, perhaps caused by a merger of two neutron stars, with consequent bursts of neutrinos and gravitational waves that we might detect; but right now there’s no evidence for this, so don’t get your hopes up.) You can read many breathless articles by following the Twitter hashtag #GRBm31. Admittedly you might be better off without it. NOTE ADDED: Yep.
But do stay tuned as the facts emerge. The opportunity to observe such a nearby explosion is rare. So this is certainly going to be interesting… and maybe, if we’re very lucky, it will be more than merely interesting…
NOTE ADDED: Actually, we were very unlucky, and it was completely uninteresting —except as an illustration that it can be very difficult, in the heat of a moment when data is sparse, to distinguish between something scientifically fascinating and a weird fluke. Scientists do expect these things to happen sometimes. Fortunately, science is self-correcting. Even if Swift’s team hadn’t identified this signal as a fluke in their data, other telescopes would have been unable to find the object they’d identified, and doubts would quickly have emerged as a result. If something’s real, everyone will see it.
The lesson, in my view, is that when new scientific results are announced, be patient. Give the experts a little time to check things, and don’t do science the way Twitter does.
And finally: if you are inclined to criticize the Swift team, you’re making a big mistake. On the contrary, they did exactly what they were supposed to do, as quickly as they could. Gamma-ray bursts [GRBs] are extremely rare and extremely valuable and extremely brief; Swift’s job is to let the scientific community know, as quickly as possible, that one may have been seen, so that others may look at it. Inevitably, someone with such a job will occasionally give a false alarm. Swift has discovered so many GRB’s, and made so many direct and indirect contributions to our knowledge about them and about other objects in the sky, that scientists, while disappointed that this was a false alarm, will certainly not view Swift as irresponsible.
A week ago, regarding BICEP2’s results coming into question, Seth Zenz wrote a nice, short article on Why, in Science, it’s OK to be Wrong. I recommend it.
POSTED BY Matt Strassler
ON May 27, 2014
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…)
POSTED BY Matt Strassler
ON May 27, 2014
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
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