Archive for the 'Cosmology' category

"Big Bang" : A terrible name for a great theory [from the archives]

Aug 21 2009 Published by under Cosmology, Physics & Astronomy, Science

[This was originally posted in June of 2006 on an earlier incarnation of this blog that's no longer available. It was collected in "The Open Laboratory: The Best Writing on Science Blogs 2006" I'm reposting it here so that it will again be available to the broader net.]

Perhaps, from a marketing point of view, "Big Bang" is a great name. It's short, it's punchy, it evokes memorable images, and it's easy to remember. But, marketing is something I really don't understand; as we all know, it's easy to fear what we don't understand (particularly when it has a huge amount of control over us and the world around us), and to hate what we fear, and as such I'm very convinced in my opinion that Marketing is Evil.

So why is the Big Bang a terrible name? Because the name itself evokes and supports many misconceptions about the theory. Read on for my musings on the matter.

The Big Bang was not an explosion.

The name suggests an explosion: some point that blew up, and everything outside it is now rushing away from it due to the force of the explosion. It's an analogy that does provide some useful intuition (without Dark Energy, the Universe is expanding because of the left over expansion that started way back when and hasn't been stopped by gravity), but there are a lot of problems with it. Most notably, galaxies are not in fact flying away from each other! A much more modern way to look at it, and a way that matches the mathematics that physicists and astronomers really use when they work with the model of the expanding Universe, is that galaxies (except for little local motions) are pretty much fixed, but space itself expands. Galaxies get farther apart from each other because, as space expands, there's more space between them. (Note that the galaxies themselves are not expanding; they are held together by their own forces.) An analogy is rising raisin bread: the bread expands, and the raisins get farther apart, but the raisins aren't moving through the bread. Another analogy would be pennies pasted on the surface of a balloon.

[Addendum added 2009-08-21: there is some debate in the astronomical community as to whether "galaxies flying apart" or "space expands" is a better qualitative description of the mathematics of the Big Bang. Obviously, I'm in the "space expands" camp. I'll write more about this in a later post.]

The Universe didn't all start from one point.

Sometimes you will hear people say this. The classical Big Bang model (but see below) has an initial singularity; at that beginning, any two points which are separated by a finite distance now were separated by zero distance. That sure seems like things begin at an initial point. It is, however, much better to say that the initial singularity is a point where our models break down, and that all we can really say is that densities in the extremely early universe, just "after" the "bang", were extremely high. The problem with thinking of it all starting from a point is that it suggests that there is a center that everything is rushing away from. In fact, however, the Big Bang happened everywhere. If you must think about it as that point, that point has itself expanded to the whole infinite Universe... and we, right here, are within that point, as is a distant galaxy a billion light-years away. In the Big Bang theory, the Universe has no center. Any point is as good as any other, and indeed, an observer looking from any point would see things around her as if she were at the center of the expansion! Which leads me to the T-shirt I want to have made one day. On the front: "Yes, in fact, I am at the center of the Universe!..." ; on the back: "...but so are you."

We know little or nothing about the moment of "Bang" itself.

This is perhaps the most egregious. The Big Bang theory is tremendously successful, tremendously well supported by observations, yet... the thing that the theory is named after is something that the theory as it exists right now really can't address. Sad, huh? What the Big Bang theory tells us is that the Universe evolved from a hot and dense state to what we have now. There are various different epochs we have been able to probe using different techniques, and the whole picture hangs together very well using these very different techniques. Right now, we have the expansion of the Universe. At a moment a few hundred thousand years after the "beginning", we have the cosmic microwave background observed by the WMAP satellite and large numbers of other ground, balloon-borne, and space experiments. A few minutes after the Big Bang, we have the time when the Universe was so hot and dense that nuclear reactions routinely happened, at which the primordial elements were created... in proportions that match what we observe. And, finally, a tiny fraction of a second after the beginning, we have Inflation, which isn't as well supported as those other bits, but which does explain a lot and even has stood up to one test from recent WMAP results. (Indeed, many people, myself included, are attracted to the notion of calling the end of Inflation "the beginning", as subtracting that tiny fraction of a second from any later epoch won't make any difference in the time we quote, and to talk about things before that you need Physics we don't know how to do.)

All those different epochs, tied together by one expanding Universe theory that started from an extremely hot and dense state, all supported by a vast array of different observations. Pretty cool.

But.

The classical Big Bang-- what you get if you just look at the Universe with Einstein's General Relativity, ignoring Quantum Mechanics-- gives a well-defined "beginning", that time we call t=0 and measure all the other times from. That beginning is the singularity, where things diverge, where densities go infinite. "What caused it" is something we don't even know how to address (unless you're a string theorist talking about branes, but that's a whole nuther issue).

The problem is, the classical Big Bang ignores Quantum Mechanics, which is a bad idea: Quantum Mechanics is another extremely well-supported theory in Physics we know to be right. It's the theory of the very small (speaking very roughly). At some time a tiny fraction after the moment of the classical Big Bang, the Universe got large enough that it became reasonable to do your Quantum Mechanics (e.g. for the nuclear reactions a few minutes after the beginning) without worrying about gravity, and to do General Relativity (which describes gravity) without worrying about Quantum Mechanics. However, before that moment, you have to worry about both at once. Here's the rub: the two theories don't work together. At the moment, we don't have a working theory that can handle gravity and quantum mechanics at the same time, although the string theorists keep telling us that they're working on it.

What the previous paragraph says is that our extremely successful physical theories are incapable of predicting what happened during the first tiny time interval after the moment of the classical big bang. Indeed, if you think about it in reverse-- start now and go back in time-- we reach an early period before which we can say almost nothing. We can go back through the present day expansion, through the epoch when galaxies first formed, through the epoch when the Universe was a plasma and the Cosmic Microwave Background was emitted, through the epoch when the elements formed, through the epoch when quarks and gluons were all mushed about in one big continuous mess, to Inflation... but we can't go before that.

We don't know what happened before that. Was there even a "t=0" beginning? Dunno. Was there really a singularity? Dunno. What happened before Inflation? Dunno. What was the deal with the actual Big Bang itself? Dunno.

Kind of sad that this extremely successful, extremely well-tested theory, a theory so good that we can call it right and true, doesn't really address the moment it seems to be named after.

That's why, marketing reasons aside, Big Bang is a terrible name for that theory. But I sure as hell can't come up with a better name that doesn't violate all sorts of marketing, so I guess we're stuck with it.

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This Saturday at 10AM PDT in SL : "How We Know Dark Matter Exists"

As part of my regular Dr. Knop Talks Astronomy series of public astronomy lectures in Second Life, this coming Saturday I'll be talking about how we know that Dark Matter really exists.

The talk will be at the Large Ampitheater on StellaNova. Note that a Second Life account is completely free! You can register for an account here. This is the "SciLands" entry portal, which will put you at the SciLands own orientation island. (StellaNova, the sim of MICA, is part of the SciLands.)

Here's a description of the talk:

Modern cosmology tells us that the majority of the Universe is made up of stuff whose nature is largely unknown to us. Two thirds of it is Dark Energy; most of the rest is Dark Matter, the subject of this talk. Dark Matter interacts with normal matter through gravity, but otherwise it interacts hardly at all. Yet, we have very high confidence that this mysterious Dark Matter really does exist. Because it doesn't interact with light, we haven't seen it glowing, nor have we observed it absorbing background light as we've seen with dust clouds. All of the evidence we have for Dark Matter comes from its gravitational interaction with other matter, and with light. Yet, this evidence is extremely compelling. In this talk, I will attempt to convince you that there is no reasonable doubt that Dark Matter exists.

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This Saturday : "The Discovery of the Accelerating Universe" in Second Life

“Dr. Knop Talks Astronomy”, in association with MICA (www.mica-vw.org)

Saturday, April 4, 10AM SLT

http://slurl.com/secondlife/StellaNova/213/210/32

The Discovery of the Accelerating Universe

In 1998, two teams of astronomers observing supernovae discovered that the expansion of the Universe is accelerating. The speaker, Dr. Knop, was on one of the two teams, working with Saul Perlmutter. In this talk, I will describe just how it is that you can measure the expansion history of the Universe by observing distant exploding stars, and what surprising things we saw in our results that indicated to us that the expansion of the Universe was in fact accelerating. At the end, I'll briefly mention some things about "dark energy," the mysterious substance that is causing this surprising universal acceleration.

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The Big Bang and Evolution : when does a theory evolve so much that it deserves a new name

I am currently visiting Colgate University, giving a physics colloquium about dark energy. I'm hosted by my friend Jeff Bary (who's a first year professor there). Yesterday evening, his class gave presentations about discoveries that they'd researched. A few of the talks touched on the Big Bang. Afterwards, I was sitting around musing with Jeff and the departmental chair, Thomas Balonek. Thom was saying that it's disingenuous for us to claim that we're still talking about the Big Bang as it being the same theory that we had all those decades ago. What with the introduction of inflation, cold dark matter, dark energy, it's changed so much that really it's not entirely the same theory any more. I argued that the basic picture is the same-- the Universe expanded from a very hot, very dense state to its current form-- that it warrants having the same name.

I then asked the question: which theory has evolved more, the Big Bang or Biological Evolution? To point a finer point on it, let's go back to the (say) 1950's or early 1960's, when people were arguing about Big Bang vs. Steady State cosmology, before the discovery of the Cosmic Microwave Background, well before the introduction of inflation to solve the flatness and horizon problems. Take what people were talking about then as the Big Bang, and compare to what we talk about today. Has that changed more or less than the Theory of Evolution has changed from what Darwin originally envisaged when he wrote the Origin of the Species?

To be sure, the theory of Evolution is better understood and understood in better detail than the Big Bang theory. They both share the feature that they are theories describing the evolution of a system, not it's origin (although both the name of the cosmological theory, and the title of the work that started Evolution, both would seem to indicate that they do). We know a whole lot more about both today than we did then. Both have features today that people in the early days couldn't have anticipated. (I understand the cosmology better, of course, but know, for instance, that DNA and the genetics gives us an actual mechanism for Darwin's Evolution.)

So, what do you think? Which one has changed more? And is either theory similar enough to what was originally proposed that it deserves the same name, or should we have changed the name by now?

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Randall Munroe and the Size of the Observable Universe

Oct 04 2008 Published by under Cosmology, Physics & Astronomy, Science

Randall Munroe of the fabulous webcomic xkcd has a great logarithmic height poster showing the size of everything from folks all the way up through the edge of the Solar System, on to the radius of the observable Universe. As a logarithmic plot, each gap of the same size vertically on the plot represents a doubling of the distance from the surface of the Earth; this is why he can show things of such vastly different scales as people and the whole Universe in the same plot.

But, wait, I thought you cosmologists kept saying that the Universe was infinite! How can this picture show the whole Universe then?

It doesn't... it shows the observable Universe. Because the Universe is only 14 billion years old, and the speed of light is finite, we can only see things that are as far away as light has had time to reach us from. There is more Universe beyond that, but the light hasn't reached us from it yet; the part of the Universe beyond our horizon is not the observable Universe.

But wait... if the Universe is only 14 billion years old, then, we should only be able to see things that are 14 billion light-years away... yet the xkcd pictures says the top of the Universe is 46 billion years away. What's up with that?

Continue Reading »

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Technical talk on Dark Energy and Vacuum Fluctuations in Second Life this Friday

In addition to the popular-level astronomy talks I've started doing as part of MICA, there will also be regular "journal clubs." This is something you'll see in physics and astronomy departments sometimes: people take an interesting paper from the recent literature (really, from preprints nowadays), and lead a discussion about it with their colleagues. It's one way of trying to keep up with some of what's going on in the literature, and it's also a way that one scientist can share his particular area of interest with his immediate colleagues.

This Friday, George Djorgovski will be leading a journal club in Second Life at 8AM SLT (pactific time) on the paper Dark Energy from Vacuum Fluctuations by Djorgovski and Gurzadyan. Note that while in this case, the author of the paper is talking about his own paper, that's not always the case. In journal clubs I've been too, sometimes people talk about their own work, and sometimes they talk about other interesting papers from the literature. For instance, when I was at Vanderbilt and still working with the Supernova Cosmology Project (which used optical and infrared astronomical data), I gave a journal club about the WMAP 3-year results -- very releavant to cosmology and my work, but not something in which I was involved.

The journal club will be where all of the MICA meetings currently happen, at the ISM Workshop in Spaceport Bravo.

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Dr. Knop Talks Astronomy

The Meta Institute for Computational Astrophysics is a new venture being spearheaded by Piet Hut, also known as Pema Pera in Second Life. Piet and collaborators have already done some actual scientfic collaboration a virutal world, through Qwaq. However, he— and others, to the point that I can start using the pronoune "we"— are starting to do more in Second Life in an attempt both to bring in more astronomers, professional and amateur, as well as interested community members. Activities are going to include a regular journal club, as well as a monthly outreach talk at the popular level. I will write more about MICA later.

Right now, though, I want to (at the last minute) publicize that I will be giving tomorrow's in-world popular talk. I'll be talking on the title "The Power of the Dark Side: How Dark Matter and Dark Energy dominate our Universe". This talk will be in The Galaxy Dome in the ISM's Spaceport Bravo. This is going to be a version of a talk that I've given before, last Spring when I was still at Vanderbilt and going around giving AAS Seyfert Shapley Lectures. (I think I gave five of those last year.) The talk will be in-world using Second Life Voice, and will be at a level accessible to all (although I always do try to challenge your minds when I give these sorts of talks).

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