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Physics for the 21st Century

Dark Energy Interview with Featured Scientist David Spergel

Interviewer: Could you please tell me about the focus of your research?

DAVID: For me, one of the big questions in cosmology is how the universe began, how it evolved, what is it made of and how it will end.

One of the most bizarre results in cosmology and maybe in all of physics is the observation that the universe is accelerating. What we expect is: if you throw a ball up the ball will come back down. If you throw it up fast enough it might escape but it will always decelerate, gravity always slows things down and traps things. What’s really weird is the universe today is accelerating and fundamentally we don’t understand why.

The microwave background provides additional evidence for this acceleration by measuring the total density of the universe and showing us there has to be an additional component. What’s also intriguing about the microwave background is it suggests that this is not the first time the universe accelerated.

Interviewer: What is the history of acceleration in the universe?

DAVID: We believe that during the first moments of the universe the universe underwent a period of acceleration that we call inflation, so that a very tiny portion of a very tiny region in less than a trillionth of a second inflated, accelerated, to become the whole visible universe. When we look at the microwave background what we think we’re seeing are fluctuations in density generated during that first period of acceleration. So, if that region of the sky accelerated for a little bit longer than that region, that region will end up denser and that will then form galaxies. When we look at the microwave sky we’ll see a cold spot in that region because it’s denser. It looks as if we had not one but two periods of acceleration. We don’t understand either. One we inferred directly through measurements like the supernova measurements and confirmed them through measurements like the microwave background measurements, and the other we infer more indirectly by looking at the pattern we see in the microwave background.

Interviewer: What is the cosmic microwave background?

DAVID: The cosmic microwave background is the leftover heat from the Big Bang. It fills all space. In fact, if you’re watching TV, not on cable but with an antenna, and you switch your TV between channels, a couple percent of the static that you pick up on your home television is the cosmic microwave background. It’s the leftover radiation from the Big Bang. We say it’s microwave because it’s a microwave wavelength, so it’s the same wavelength as your microwave cooker. It’s the wavelength you’re looking at as millimeters to centimeters, so relatively long wavelengths compared to optical radiation. This microwave background radiation is everywhere; it fills this room; it fills all space. Today it’s at a temperature of three degrees above absolute zero.

Interviewer: You’re looking at extremely small temperature variations in the CMB.

DAVID: We look at the microwave background; we look at the big picture. It’s nearly uniform. The temperature is close to three degrees everywhere. I think about it as looking at the surface of the earth. If you look at the earth from space the first thing you see is a big, round ball. You have to look very carefully to notice the hills and valleys.

With WMAP, what we’re doing is looking very carefully and seeing these tiny hills and valleys, tiny variations in temperature. So the temperature variations that WMAP measures is one part in 100,000, so that means at that region in the sky the temperature might be 2.73001 Kelvin and it’s 2.73000 over there.

Interviewer: What is the WMAP (Wilkinson Microwave Anisotropy Probe) satellite?

DAVID: WMAP is named after our colleague David Wilkinson. David was one of the pioneers in studying the microwave background. He was part of the first experiment to look for the microwave background that confirmed the Penzias and Wilson result. He was a leader in its study for many, many years and played a very important role in the COBE (Cosmic Background Explorer) that made the first detections of the microwave background, one that earned the Nobel Prize, unfortunately after David’s death.

David passed away from a long struggle with cancer soon after we first analyzed the data and when we announced our first results NASA very generously let us rename the satellite the Wilkinson Microwave Anisotropy Probe to honor David and his contributions to the field.

WMAP is in many ways a very simple experiment. It just sits there at four times the distance to the moon and scans the sky over and over, every two hours, covering roughly a third of the sky. Then, it’s out there beyond the orbit of the moon scanning the sky with the earth and sun always behind it. As the earth moves around in its orbit, the region that the WMAP looks at varies through the year, so over the year it covers the full sky.

Interviewer: How has the experiment evolved?

DAVID: When we started out with WMAP we were focused primarily on the temperature measurement. Now we’re as interested in the polarization measurements. The temperature measurements were hard; we were looking at variations of one part in 100,000. Polarization is a lot harder. The polarization measurements we’re looking at are variations in one part in 10,000,000. So, the polarization signal is 100 times weaker. Because it’s so weak we have to integrate for a very long time to get that information from a polarization signal.

Interviewer: What do you use polarization for?

DAVID: The polarization signals tell us two important new things. By looking at the pattern of temperature and polarization fluctuations what we learn is in that part of the sky its fluctuations were generated on scales that are correlated with this part of the sky. The fact that we see correlations on regions that would normally not have any way of communicating with each other, so in the standard Big Bang model there’s no way to get information at the speed of light or slower from there to there.

You’d expect that there would be no correlations between that region and that region. The only way we know of to produce correlations is somehow bringing those two regions together much more rapidly. That requires one of two things. Either the universe had a rapid period of acceleration in its first moments, something like what we’re seeing today, or it requires that before the Big Bang there was a period of collapse and the universe has actually gone through cycles of collapse and expansion.

That latter possibility is a more exotic one; it’s one that we don’t fully understand mathematically but it’s actually being explored as an alternative to the idea of this early stage acceleration. So, the fact that we seem to need this early stage acceleration or something else like it in a sense is one of the first things we learned from the polarization.

Interviewer: What is the shape of the universe?

DAVID: The prediction of the inflationary universe is that the geometry of the universe should be close to flat. General relativity is very simple. General relativity consists of two ideas; matter tells space how to curve and the curvature of space tells matter and light how to move. So, once I know the geometry of space I know how much matter and energy there is.

If the density of the universe is high, the geometry of the university is positively curved like the surface of a sphere. If the density of the universe is low, the geometry of the universe is negatively curved like a saddle. If the density of the universe is just right, the geometry of the universe is like a piece of paper and the geometry, Euclidean geometry, that you learned in high school that Mrs. Jones taught you, is valid not only in 10th grade geometry class but on the scale of the visible universe.

If the universe is flat, light moves on straight lines and the characteristic size of hot and cold spots in the microwave sky would be a degree. If the universe had only the matter we saw in galaxies, didn’t have any dark energy, then if we add up the matter we see in galaxies the total contribution to the density of the universe turns out to be only about 20 percent, maybe 25, less than 30 percent, of the total amount needed to make the geometry flat.

Interviewer: If the universe expands in all directions, how can it be “flat”?

DAVID: Most of us have a Newtonian notion of space and time. We think of space as being something absolute. What Einstein taught us in general relativity is that all we can truly talk about is the distance between objects. Everything is relative, so all I can really talk about is the distance between here and Tokyo and Tokyo and Paris. If I measure all those distances and realize the way you fly to Tokyo is going up near Alaska—that’s the shortest distance—just by measuring distances locally I can specify the geometry.

When we say the universe is expanding it’s not that everything is being thrown apart in an explosion but rather that the distances between objects, or equivalently the time it takes light to get from point A to point B, just keeps getting larger with time. If you think about the expanding universe you should think about a homogeneous space stretching in all directions.

I find it hard to think about three-dimensional space stretching. It’s easier to think about one or two-dimensional space. Let’s think about two-dimensional space. Let’s think about living on a huge rubber sheet and we’re ants moving around on a rubber sheet and that rubber sheet is being stretched in all directions equally. So, everything is being stretched in all directions and the distance between the ants keeps growing with time as the universe is being stretched. When we talk about the universe expanding it’s that stretching.

What we expected in the Big Bang model was that we should see the universe expanding and because of gravity that expansion slow with time and perhaps even reverse and collapse in a big crunch. What we’re seeing today, which is what’s so bizarre, is that this stretching is not slowing down but accelerating. When we talk about the universe accelerating, that means the distance between objects is not only growing with time but that distance is accelerating.

In a way, it’s very scary because if the distance between objects is accelerating the universe is getting emptier and emptier and getting very empty very quickly. I often think of this Robert Frost poem about: “Some say the world will end in fire, others say in ice.” If I think about a universe that begins in the Big Bang, expands and collapses in a big crunch, that’s the universe that ends in fire as the universe gets hotter and hotter as the big crunch comes.

What the evidence for acceleration seems to suggest is that we get to live in a universe that ends in ice, that the universe accelerates more and more rapidly. As we look towards the future, the microwave background gets colder and colder. Space gets emptier and emptier. The distances between galaxies just grow and grow and the universe eventually dies a fate of cold emptiness. And because of the dark energy the universe dies slowly with the final fate of a cold death. And because of the dark energy it looks like the fate of the universe is to die a cold death.

Interviewer: Why use the CMB?

DAVID: One of the nice things about the microwave background is the universe back then was very simple, so we can make detailed predictions of what we should see depending on the composition of the universe. When we look at the microwave background we’re seeing sound waves produced in the first moments.

The way I think about them is I think about a smooth lake and inflation, as the universe accelerates, throws rocks and pebbles into that lake. Those are the initial fluctuations; they set off sound waves, ripples in the lake. We let the sound wave move for 300,000 years and then we get to see what they look like after 300,000 years. And what they form are ripples that are about 300,000 light years across.

If I know the size of these ripples, of these hot and cold spots, I can measure the geometry of the universe. Geometry tells me density so that gives me a measurement of the total density of the universe. I can then learn more but the kind of ripples I get depend on what the lake is made of. A lake of water will give different ripples than a lake made of mercury.

By looking at the amplitude of the waves, the pattern of hot and cold spots, in more detail I can measure the density of atoms. That’s the second number I get. I can also measure the density of matter. These ripples are affected by their own gravity and how those ripples grow and behave, how they get amplified, depend upon the matter density.

By looking at the amplitude of the fluctuations as a function of scale, how big the hot spots are on this scale and this scale and that scale, I can measure the atomic density and the matter density and the total density, so now I’ve got my three numbers; the density of atoms, the density of matter which we know is about five percent; and the total density. Looking at that we see a universe that’s about four percent atoms, 23 percent dark matter and nearly all the remaining 73 percent is in this form of dark energy, energy associated with empty space.

Interviewer: How surprising is the existence of dark energy?

DAVID: So, when you see something as bizarre as dark energy, when you first hear about the evidence for it, the right reaction as a scientist is to say this is crazy; there must be something wrong with the measurements. That’s why it has been so important to have many lines of evidence pointing in the same direction so we can infer the existence of this accelerating phase now in many ways. A very important way is through the supernova observations. Another way is through our observations of microwave background that tell you the total density of the universe. When you combine that with measurements of galaxies that measure the density of matter, those too imply that there should be more energy.

We can also infer the evidence for this dark energy by looking at measurements of the expansion rate today. We looked at measurements from the Hubble telescope that measure the Hubble Constant, how fast the universe is expanding today, and combine that with our measurements of the geometry of the universe. Those two pieces alone are enough to imply dark energy. So, we have many different ways of combining data that all point to the same thing, the existence of dark energy, with that dark energy making up roughly about 73 percent of the total energy of the universe today.

Interviewer: What was Einstein’s cosmological constant?

DAVID: When Einstein developed his Theory of General Relativity, he realized that in his theory, once you described how matter is distributed, you know how the universe would behave. He recognized that the theory made a prediction that the universe is either expanding or contracting. All Einstein knew about was our own galaxy is that our own galaxy seems to be static. So, Einstein thought that theory must be wrong. I don’t want a theory that predicts the expanding universe. So, he took his theory, and he added a constant term to it, which we now call the cosmological constant, or also now identifies vacuum energy; energy associated with empty space. So, he put this constant into a theory and tuned it just right, so the universe would stay balanced—the expansion would be balanced by this constant contraction. And soon after he did this, Hubble, working out in California started to measure distances to galaxies. And what Hubble found was the further away a galaxy was, the faster it was moving away from us—that the universe was expanding. When Einstein learned about this, he realized this is what his theory had predicted. He took his cosmological constant, set it to zero, and said this was his biggest blunder. Had he had had more confidence, he could have said—the universe may look static—but my theory predicts if we look more carefully, we’ll see the universe is expanding. So, Einstein could have predicted the expanding universe, and he didn’t. Instead he predicted the cosmological constant. History comes around. It now seems that even Einstein’s biggest blunder was an important idea. And while the cosmological constant doesn’t have the value that Einstein wanted to make the universe static, the fact is that you could add a constant just to the same form and beautifully match the acceleration—and the best fit model we have to all our observations is one in which there’s vacuum energy or alternatively a cosmologic constant. So, even Einstein’s biggest blunder may have been a brilliant idea.

Interviewer: Do you have any thoughts about when an answer for “what is dark energy” might be found?

DAVID: This accelerating universe is a tremendous challenge for physicists. It implies there’s a big piece of physics we’re missing. It suggests there are energies at a scale that we didn’t think there was something new and I think it’s actually a wonderful gift. It’s telling us that there’s something new and neat to understand. It’s something that’s probably involved with the connection between quantum mechanics and general relativity. It’s clearly a very important piece of fundamental physics that we don’t understand yet.

The way physics advances, all science advances, is either by having data and observations that push you to something new, or a new theory that helps you interpret things in a deeper way. Very often theories are driven by experimental anomalies and sometimes it goes the other way and theoretical predictions lead experimentalists, or observers to make certain measurements.

Right now, with this accelerating universe, we have this very exciting anomaly that’s motivating theorists to think about what could be going on here and suggesting to observers and experimentalists new ways of getting at this very strange result.

When we talk about the dark energy I suspect we’re using terms and language that physicists 100 years from now will say, oh, those people in 2009 were so naive. They had evidence for this acceleration and they interpreted it in terms of this dark energy but we now know it’s all due to… whatever. We don’t know what that whatever is; that’s what we’re trying to figure out.

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Physics for the 21st Century


Produced by the Harvard-Smithsonian Center for Astrophysics Science Media Group in association with the Harvard University Department of Physics. 2010.
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  • ISBN: 1-57680-891-2