Date: May 5, 2012
Title: Encore: Were the Fundamental Constants of Physics Different in the Early Universe?
Podcaster: Rob Knop
This podcast originally aired on May 23, 2010
Description: The Theory of Relativity is an extremely well-tested theory. However, it does include a couple of parameters, whose values the theory does not predict, but which must be determined through experiment.
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. This semester, he’s teaching physics at Belmont University in Nashville, and next fall will join 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.
Intro Sponsor: “This episode of 365 days of astronomy is sponsored with thanks to all non-US contributors for showing us that the night sky is something we can all share.”
Outro Sponsor: “This episode of 365 days of astronomy was sponsored by the Lake County Astronomical Society: Celebrating 30 years of stellar service to members and the public.”
I am Dr. Rob Knop. This coming fall, I will be joining the faculty of Quest University in Squamish, British Columbia, where I will be teaching physics and related subjects.
We have a number of highly successful theories of physics that describe how the world works. One of these theories is Relativity. Relativity describes how things move, and how distances, times, and speeds are measured relative to each other when you approach the speed of light. It is also our modern theory of gravitation. It’s an extremely well-tested theory. However, the theory does include a couple of parameters, whose values the theory does not predict, but which must be determined through experiment. One of these is the speed of light. We have measured the speed of light and know it to be 3√ó10^8 meters per second, or, more simply, one light-year per year. Another one of these parameters is Newton’s gravitational constant, sometimes called “big G”, which effectively gives the strength of gravity. Why do these constants have the values that they do? We don’t know. They are parameters of our Universe, constants that just are what they are.
Quantum mechanics is another highly successful theory of physics. It includes a constant of its own, Planck’s Constant, sometimes called “h-bar”. h-bar is a very small number, and represents a fundamental scale in quantum mechanics. For example, you have probably heard of Heisenberg’s Uncertainty Principle, which states that you can’t know both the speed and position of a particle perfectly at the same time. The mathematical description of this principle includes h-bar in the limit of the two uncertainties multiplied together. However, just like the speed of light, we don’t know why h-bar has the value that it does; it’s just a constant of our universe, evidently a God-given value, if you will.
There are other constants in physics. The elementary charge, or the charge on the electron, is another important value in physics that seems to be an arbitrary constant of our Universe. Likewise, the masses of the elementary particles.
While we have many great theories in physics with tremendous predictive power, we know that we don’t know everything. There are some regimes– for instance, the exact center of a black hole, or the very beginning of the Universe– where our theories break down, and give nonsensical results. As such, one of the holy grails of theoretical physics right now is a “theory of everything”, a physical theory that would unify our existing theories of physics. It used to be that we hoped that this theory of everything would tell us why the fundamental constants have the values that they do, but it no longer looks like these constants will be required to have uniquely determined values from the theories themselves. However, many of these theories do make another interesting prediction: specifically, that these constants may not be constant at all! Rather, it may be that the speed of light, Planck’s constant, and others may have been slowly changing over time, or at the very least may have been different in the early Universe.
So where does astronomy come into all of this? After all, this is supposed to be a podcast about astronomy! Well, it turns out that astronomers are uniquely positioned to be able to test the idea that the fundamental constants of physics were different billions of years ago. Astronomers have a time machine; they can look into the past. Because light only travels at the rate of one light-year per year, if you look at a galaxy that is millions of light years away, you are seeing it as it was millions of years ago. If you look at one that’s billions of light years away, you may be looking back a substantial fraction of the age of our Universe, which is 13.7 billion years old. Observationally, the Universe seems to be the same everywhere, and the same in all directions. Galaxies that are far away look pretty much like galaxies nearby. Therefore, if you look at a distant galaxy and see it as it was billions of years ago, you’re seeing the same conditions that apply to the Universe as it was right here, billions of years ago. If we can find measurements we can make with telescopes that are sensitive to these fundamental constants of physics, and compare the constants that we derive from those measurements to the laboratory-measured values, we may be able to detect if the constants of physics have been changing over time.
One of the most powerful techniques of astronomy is spectroscopy. Conceptually, spectroscopy is very simple. You put a “dispersive element” at the back of a telescope in order to break the incoming light from an object into separate colors. What the heck is a “dispersive element”? The one you’re probably most familiar with is a prism. If you place a prism in sunlight and allow the light that goes through the prism to shine on a table or a wall, you’ll see a rainbow of colors. What has happened is that the prism has spread out the light by a color. Astronomers use prisms and diffraction gratings, only with much higher resolution, to analyze the colors coming from distant stars and galaxies. One thing that they often see is specific colors that are either absorbed are emitted by specific atoms and molecules. These features are called “absorption lines” and “emission lines”. They arise because atoms have specific energy states, known as orbitals, that electrons can occupy. Atoms can only absorb energy in exactly the amount that corresponds to the difference between two of these states; when the energy is absorbed, the electron will jump from one orbital to another. If an electron jumps from a higher-energy orbital to a lower-energy orbital, the atom will emit exactly that amount of energy, in the form of a photon of a very specific color.
It is the electromagnetic interaction between the nucleus and the electrons that specifies the energy levels in an atom. Some of these energy levels depend on the fundamental physical constants differently, including the speed of light, the charge on the electron, and Planck’s constant. Thus, if the physical constants were a little bit different, the spacing between these levels would change in different ways. Astronomers can measure whether the spacing between the levels is the same in distant galaxies by performing spectroscopy on those galaxies, and measuring the wavelengths of light at which absorption and emission lines in galaxies appear. By comparing these to the wavelengths of light measured in the lab on Earth, we can determine if the physical constants have changed at all in the last few billion years.
For the last ten years or so, a few different groups of astronomers have used the world’s largest telescopes to look for these changes. In the middle of the last decade, one group, using the Keck ten meter optical telescope in Hawaii, reported a detection of a different value for one of these physical constants in galaxies with a lookback time of 7 billion years and higher. The difference they reported was very small, only five parts in a million, and the uncertainty on their measurement was only a little less than a quarter of the measurement itself, indicating that it wasn’t a strong detection. As with all remarkable results in science, we don’t fully believe it until it’s been independently confirmed. In this case, that confirmation hasn’t come.
Another group made similar measurements using the eight meter VLT optical telescope in Chile, and didn’t detect any shift at all. These are very difficult measurements, and the data analysis is very difficult, so at the moment the results remain inconclusive.
In April 29 of this year, a paper from Nissim Kanekar and collaborators appeared in which they reported similar measurements, only using radio telescopes. Because these sorts of measurements are so difficult, it is important to make measurements using different techniques. Different techniques are likely to be subject to different systematic effects, and as such confirmation of results from different techniques lends great confidence to the validity of these results. Of course, the optical results remain controversial, but because this would be such an exciting discovery if it were true, it remains worth pushing the limits of what’s possible to see if we can detect any changes in the fundamental constants. Kanekar’s paper looked at a couple of different galaxies using two different radio telescopes– two telescopes, so that any instrumental effects from one telescope wouldn’t affect the other’s results. In nearby galaxy Centaurus A, they saw no evidence of a shift. However, they did see a very small shift in the relative wavelengths of two lines from galaxy PKS1413+135, which is at a lookback time of nearly three billion years. The shift that they saw, however, is merely suggestive, for the uncertainty on their results was larger than a third of the size of the measurement itself. As such, this result is only marginally statistically significant, and will require both a stronger signal as well as independent confirmation before we can accept it as a true detection.
The question as to whether the fundamental constants of physics have been changing over the age of the Universe, and how much and in what direction they’ve been changing if in fact they are different, is an important one. Answering it will give us pointers to understanding physics beyond the level that we do today. Any measurements about how these parameters have changed, or, just as important, have not changed, will set limits that any “theories of everything” will have to agree with. Astronomy, with its ability to look billions of years into the past, provides the tools necessary to question whether the fundamental constants of physics have changed. Current results have suggested the possibility that we may have detected a very slight change, but the results remain controversial and, at this time, unconfirmed.
End of podcast:
365 Days of Astronomy
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