Date: August 16, 2011
Title: On the Success of Big Bang Cosmology
Podcaster: Rob Knop
Organization: Quest University Canada
Links: My home page : http://www.questu.ca/academics/faculty/rob_knop.php
Description: The Big Bang model is one of the foundations of modern astronomy. Although sometimes people will believe or claim that modern cosmology is on a shaky footing, in fact the Big Bang model is extremely succesful. Not only are there a wide range of observations that directly support the picture of a Universe that has expanded from a hot and dense state, but a large fraction of everything astronomers do implicitly confirm it by fitting so well within the framework that it provides for understanding our Universe.
Bio: Rob Knop obtained a PhD in Physics from Caltech in 1997. He then worked with the Supernova Cosmology Project and was part of the discovery that the expansion of the Universe is accelerating. After six years as an assistant professor at Vanderbilt University, he worked in the computer industry for two years. He now teaches physics the new college Quest Unviersity in British Columbia. He gives regular astronomy talks in Second Life in association with the Meta-Institute of Computational Astronomy.
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Transcript:
ON THE SUCCESS OF BIG BANG COSMOLOGY
This is Rob Knop, professor of physical science at Quest University Canada. Thank you for listening to 365 Days of Astronomy!
For my podcast today, I want to take something of a philosophical aside. My topic today, about the success of Big Bang cosmology, is related to a podcast I made in June of 2010 comparing our modern idea of Dark Matter with the turn-of-the-20th-century idea of the luminiferous ether. (A short summary: they don’t compare very well. We have many lines of evidence that Dark Matter is real, whereas the luminiferous ether in fact did not stand up to tests for its existence.)
When I give public outreach astronomy talks, and when I interact with people, I am sometimes struck that people don’t realize just how successful the Big Bang model really is. Even that statement, however, raises questions. What am I talking about when I say the Big Bang “model”? And what do I mean to say that it is successful?
Science is the process of trying to understand the natural world. One of the primary ways in which science progresses is to build models that describe the natural world. When I say models, I’m not talking about things like model airplanes. Rather, a scientific model is a conceptual framework, usually with associated equations and numbers, that makes predictions about how some system will behave, and how future observations or experiments associated with that system will turn out. Sometimes, if a model is broad enough, good enough, or based well enough on what seems to be fundamental principles, we’ll actually call the model a theory. It would not be inaccurate to say that General Relativity is a model for how gravity works, but it’s also entirely correct to refer to the theory of General Relativity as our modern theory of gravity.
What exactly is the Big Bang model? The Big Bang model is a picture of our Universe as a whole, but only on the largest scales. The theory of the Big Bang itself doesn’t really say anything about what happens on small scales, such as what happens to individual stars, or what happens to the price of tea in China. It is, however, a model of how the Universe as a whole has evolved over time. The fundamental core of the Big Bang model is that the Universe was once much smaller, hotter, and denser; over time, the Universe expanded and cooled to the state we see today. Despite the name, and despite how it’s often presented, the extremely successful Big Bang model that astronomers use today does not actually include anything about an initial explosion! Indeed, the truth of our understanding of fundamental physics is such that we can make calculations going back in time and estimate what the Universe was like only up to a point; we reach an early period where our physics breaks down, and we cannot calculate earlier. So, the Big Bang model, if by which you mean the general model that has done a great job of describing our real Universe, doesn’t start with a moment of beginning.
The current version of the Big Bang model starts at an early period in the Universe, where all the galaxies that we are able to see today (which is a subset of all the ones that are out there!) were squeezed into a volume something like the size of a pea; it was unimaginably dense, extremely hot, and composed of fundamental particles sloshing about and interacting with each other in ways that physicists are only vaguely beginning to understand. Outside of this pea-size there was more Universe, pretty much the same, just as today outside of the volume that we can see (because light has had time to reach us from it) there are more galaxies, pretty much like the ones we can see.
In the Big Bang model, the Universe expands and cools off. First particles like protons and neutrons form from quarks, and then some light nuclei form from protons and neutrons. Then, for a few hundred thousand years, the Universe is a plasma: protons, some light nuclei, electrons, and photons– a photon being the particle of light– are bouncing around interacting with each other. (There is also Dark Matter present, but it’s only interacting with the other stuff via gravity; Dark Matter particles don’t bounce off of, say, electrons the way that light does.) Because photons interact with other particles the Universe is opaque; light can’t travel very far before being absorbed or being scattered. A few hundred thousand years later, though, the Universe gets thin enough, and cools off enough, for two things to happen. First, protons and other light nuclei can capture electrons, forming atoms. Second, the Universe is no longer so thick that light rays bounce off of the other stuff. There is a period of the Universe, a period we call “last scattering”, when light rays stopped bouncing off of things. The photons scatter one last time at around this epoch, and then pretty much fly freely through the Universe thereafter. Indeed, this is one of the strong predictions of the Big Bang model: if we look at the right wavelengths, we should still see these light rays streaming through the now-transparent Universe, still carrying the signature left over from the last time they bounced off the plasma of the Universe when it was still opaque.
As with any large and important theory in science, the development of the Big Bang was complicated and involved a lot of people. However, when it comes to the beginnings of our modern conception of the Universe, I want to highlight two observational astronomers. The first is Henrietta Leavitt. She was the astronomer who recognized that a certain class of variable star, known as Cepheid variables, has a tight correlation between its intrinsic brightness and its period of variation. If you see one of these stars, and you measure how long it takes to vary, then you will know how much light it is putting out. You can then measure how bright it looks to you. Because you know how much light it’s putting out, you can figure out how far away it is. The dimmer it looks, the farther away it must be. You can only do this calculation if you know intrinsically how luminous an object is; otherwise, you can’t be sure it it’s dim because it’s far away, or just because it’s not putting out as much light.
Edwin Hubble was the astronomer who used these Cepheid variables to provide us with the basis of our modern view of the Universe. First, he observed these stars in the Andromeda galaxy. At the time, it wasn’t certain whether the Andromeda galaxy was another galaxy like our own, or a nebula inside our own Galaxy. Hubble found that the variable stars in the Andromeda nebula (as it was called) were too dim to be close enough to be inside our own Galaxy, thereby proving that the Andromeda galaxy was another galaxy outside our own. This settled the debate; our Galaxy wasn’t the whole Universe, but just one galaxy among many.
The second thing Hubble did was compare distances to galaxies, as measured from these Cepheid variables, to the redshifts of those galaxies. He discovered that most other galaxies show a positive redshift, meaning that they’re getting more distant from us. What’s more, the more distant they are right now, the faster they’re moving away. This is the signature of an expanding Universe.
Although many of us today consider something like the Big Bang model the natural extension of an observation of an expanding Universe, in fact there are other kinds of Universes in which we might exist and in which we’d observe an expansion. The Steady-State model, which was popular in the mid-twentieth century, held that the Universe today is pretty much like it has always been. As it expands, and the galaxies get more spread out, new material is formed, creating new galaxies and filling in the spaces.
So why today do we believe the Big Bang model is a better description of the Universe? There are a lot of reasons, but one big one is the discovery of the Cosmic Microwave Background. In the early 1960’s, we discovered that if you look at any area of the sky, you see microwave light, showing the signature of a thermal object just a few degrees above absolute zero. This is, in fact the light left over from the hot and dense early Universe when light was bouncing around within the plasma that filled the Universe at the time. This Cosmic Microwave Background is the light rays from the “last scattering” that I was talking about earlier.
Although this observation, predicted by the Big Bang model, is perhaps its single greatest success, the overall model rests on more than this one observation. Further observations of the Cosmic Microwave Background have given us not just its existence, but detailed measurements about it that fit mathematically with predictions of what we would expect to see given the Big Bang model. And there are a host of other observations that both directly and indirectly support the Big Bang model as a good description of the nature of our Universe.
There exist some in the general public who think that modern cosmology is on a shaky foundation. This belief comes from not really understanding everything that astronomers do today. There are mysterious things such as Dark Matter and Dark Energy– both things that astronomers believe exist (although that belief is much more assured for Dark Matter!), but which are made up of particles that we can’t identify. People hear about this, and think that cosmology must be in trouble. After all, if we’re making up and giving crazy names to things that we can’t even see in order to explain all of our observations, surely the model must be falling apart!
But, in fact, that is not the case. One alternate proposed model, which a few decades ago received some work from some serious scientists but which has since fallen into the realm of fringe pseudoscience, is what’s called Plasma Cosmology. Plasma Cosmology holds that on the largest scale, electromagnetic forces are more important than the Big Bang model gives them credit for. More extreme versions of Plasma Cosmology, sometimes called the Electric Universe, verge upon really crazy ideas, such as the notion that Venus was emitted from Saturn within the last few thousand years, and its migration through the Solar System is the ultimate source of disaster stories in human history and mythology. One tenet of Plasma Cosmology is that redshift isn’t caused by the expansion of the Universe, but rather by plasma effects as light propagates through the Universe.
This is not a crazy idea. Indeed, it might be rather attractive; it doesn’t resort to an expanding Universe from an exotic state that is hard to imagine and that we can’t observe directly, and it doesn’t require Dark Matter made of particles we don’t know. However, every day astronomers make observations that implicitly confirm the Big Bang model. Astronomers observe objects at high redshift, and the results of those observations fit with the description of high redshift provided by standard Big Bang cosmology. For example, Big Bang cosmology predicts that objects at high redshift aren’t just far away, but should also appear to have time slowed down; this is called cosmological time dilation. And, indeed, observations of the rate at which supernovae get dimmer have shown us that the higher the redshift of a supernova, the more time it takes for that supernova to get dimmer– the “clock” represented by the timing of the supernova’s observed decay really is running slow. What’s more, the match is in numerical agreement with the quantitative predictions of the cosmology underlying the Big Bang model.
Next time you’re thinking it sounds like astronomers are building a house of cards when they talk about Dark Matter, or next time it sounds like somebody talking about Plasma Cosmology has a viable alternative to the standard cosmology, remember that huge numbers of astronomers every day are making observations and calculations that fit within the framework of Big Bang cosmology. It’s not just one accounting error that makes us believe Dark Matter, nor is it just the fact that this is the current paradigm that makes everybody blindly accept the Big Bang model. Rather, it’s that the Big Bang model is so successful that all of our observations naturally fit within it. For an alternate picture to be a serious challenger to our standard model, it would have to be a compelling enough picture in which all of our current observations fit. It can’t just replace one element that people find aesthetically unpleasant, nor can it even just explain one thing that isn’t explained by the standard model; it has to make sense with everything else, everything else that already makes sense in the Big Bang model. Specifically with regard to Plasma Cosmology, I would refer you to Tom Bridgman’s excellent blog, “Dealing with Creationism in Astronomy”. On July 2 of this year, his blog pointed to a page he created, “Challenges for Electric Universe Theorists.” In this, he points out a large number of very basic problems with the Electric Universe, problems that would need to be addressed before that idea could even be seriously considered as a challenger to the standard Big Bang model. My recommendation is that you don’t hold your breath….
The Big Bang model is one of the foundation paradigms of astronomy today. It is as central to our understanding of our Universe as the Theory of Evolution is to our understanding of biological systems. It’s not something that explains a few ideas, and that the high priests of astronomy have chosen for the paradigm that the rest of us must follow. Rather, it’s a model that is explicitly supported by a wide range of observations, and that is implicitly supported by its success in providing a framework for a huge fraction of everything that astronomers do every day. Indeed, in the last ten or fifteen years, the observations have gotten so good that astronomers have started talking about the era of “precision cosmology”– we now know that we don’t just have the basic picture right, but we’re able to start making precise measurements, such as the measurement that the age of the Universe is within a few percent of 13.7 billion years. It really is a tremendously successful model, not because we make things fit with it, but because everything we do naturally does fit with it.
End of podcast:
365 Days of Astronomy
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