| Science: WMAP
With Al Kogut
Astrophysicist and WMAP Science Team Member
Wednesday, Feb. 12, 2003; 2 p.m. ET A powerful satellite has captured the best picture ever taken of the infant universe, an image so detailed that scientists said it answers some of the most important questions about the cosmos, including when it was born and how it will probably die. The new picture comes from the first data collected by the Wilkinson Microwave Anisotrophy Probe (WMAP), a small $145 million satellite launched on a Delta 2 rocket June 30, 2001. Astrophysicist and WMAP science team member Al Kogut was online to discuss the new image, the infant universe and the project itself. The transcript follows Editor's Note: Washingtonpost.com moderators retain editorial control over Live Online discussions and choose the most relevant questions for guests and hosts; guests and hosts can decline to answer questions.
Gaithersburg, Md.: Hi, Could you briefly explain why Einstein found the idea of a cosmological constant necessary and why he later regarded the idea to be his "greatest blunder?" Al Kogut: Einstein's introduction of a cosmological constant and later recanting were both heavily influenced by the interplay of theory and observations. When Einstein wrote the equations for general relativity, he realized that if the universe consisted of a large number of galaxies, the mutual gravitational pull from all these galaxies would cause them to move toward each other -- the universe could not be static, but would collapse. He added the "cosmological constant" to the equations (a perfectly valid thing to do mathematically) and chose its sign to oppose gravity. Thus, the "push" from a cosmological constant could exactly balance the "pull" of gravity between galaxies, leaving the universe static.
When Edwin Hubble discovered that distant galaxies are in fact moving AWAY from us, Einstein dropped the idea of a static universe. Since galaxies were observed to be moving away from us, their mutual gravitational attraction would eventually slow the motion. There was no longer any need for a cosmological constant -- the expansion of the universe by itself was enough to keep things from collapsing. The "blunder" was the assumption that the universe was static, not that a cosmological constant might exist.
But even in an expanding universe, Einstein's equations still allowed for a cosmological constant to exist. And now we see that the WMAP data, in fact, provide additional support that the cosmological constant not only exists, but completely dominates the content and motion of the universe.
Washington, D.C.: One of the most frequently used examples of explaining the expansion of the universe is the image of someone inflating a balloon with dots on it. As the balloon expands, the distance between each dot grows. Although I know this is not to be taken literally, I wonder, what happens to the space occupied by matter? As the dots on the balloon grow, do all objects increase in size as the space they occupy expands? Thus, is our meter stick today larger than our meter stick at some point in the past? Al Kogut: Space is expanding, but matter is not. A meter stick consists of a chain of atoms, bound to each other by electrical forces. As the universe expands, the distance from one atom to the next (which depends only on these electrical forces) remains the same. So rigid objects like meter sticks are the same now or in the distant past.
But on very large scales, of course, there are no rigid objects. Cosmologists like to use galaxies as convenient markers. As the universe expends, individual galaxies
(held together by gravity) remain about the same size, but the distance from one galaxy to another gets larger and larger. In the balloon example, the dots don't grow but the space between them does.
Washington, D.C.: How does the WMAP differentiate between light received from the far reaches of space and light from closer sources? Al Kogut: WMAP differentiates between the "fossil" light from the distant universe and light from more local sources (mostly our Milky Way galaxy) by color. Light from the early universe follows a specific pattern of intensity vs wavelength known as a "blackbody spectrum." Emission from the Milky way has a different spectrum. In analogy to the visual spectrum, light from the early universe is "white" (equal intensity vs wavelength), while emission from the Milky Way is either "red" (getting brighter at longer wavelengths) or "blue" (getting brighter at shorter wavelengths). WMAP was designed with this in mind, and measures in 5 different wavelength bands to sort out the origin of the incoming light based on its color.
Dulles, Va.: Why do we need a probe 1 million miles from earth to take these measurements? Al Kogut: We parked WMAP a million miles from Earth to keep the glare down. WMAP is trying to measure small temperature differences (a few millionths of a degree) in a background that's almost at absolute zero. Everything else in the universe is hotter than the signal we're trying to measure. In particular, the Earth is about 100 times hotter than the microwave background. Performing the measurement near the Earth would be like trying to measure the temperature in a room while someone opens a blast furnace right beside you -- possible, but extremely difficult and requiring a LOT of shielding! By moving WMAP far from the Earth, we reduce the competing emission of warm objects so the instrument can make precision measurements of deep space.
Atlanta, Ga.: I wonder if you could comment on any connection between these new microwave background findings and the recent work on dark energy from observations of supernovae. The dark energy calculations tell us that inflation did happen, that something created a negative energy density in the early universe. Can the Wilkinson data shed any light on the nature of dark energy? Aren't there still huge gaps in our understanding, or do the microwave background data give upper and lower bounds on the extent of inflation? Al Kogut: Comparing WMAP to supernova is a good example of finding consistency between very different measurements. Supernova data indicate that the universe is not only expanding, but accelerating -- requiring some "dark energy" to counteract the pull of gravity between galaxies. The WMAP data, too, indicate that the universe is dominated by dark energy. The WMAP dark energy results are in agreement with the supernova results, but have smaller uncertainties. More importantly, they look at the problem at completely different times. Supernovae probe the "nearby" universe (at least in a cosmic sense), while WMAP looks out over much longer distances/earlier times. Combining the two results yields a measurement better than either one alone.
There are two theoretical concepts for what "dark energy" might be. One is the "cosmological constant" first postulated by Einstein, and the other is "quintessence". The WMAP data can not rule out either contender, but the data favor a cosmological constant over quintessence.
Richmond, Va.: Does the WMAP data suggest which inflation theory is most likely? Al Kogut: WMAP provides very firm support for the existence of a period of hyper-expansion (called inflation) very shortly (less than a second) after the big bang. WMAP tests many of the predictions of inflation:
o WMAP finds the geometry of the universe is flat (if you took a balloon and inflated it a billion billion billion ... times, its surface would look really really flat)
o The hot and cold spots in the maps form a Gaussian random field
o The polarization pattern in the sky shows that structure existed in the early universe well before the matter could have moved around on its own.
Previously viable alternatives are now dead. We can now begin the work of probing inflation itself, testing specific theories of inflation instead of the general idea that "something like inflation must have occurred". We've taken some initial steps in this direction. The very simplest types of inflation (powered by a field coupled to itself, so-called "Lambda Phi^4" for the cognoscenti) are heavily disfavored by the data. Other models do just fine. But this is just the beginning on this front...
Bethesda, Md.: Here's what I don't understand: The WMAP image was created from light that has been traveling for many billions of years. That means that it has traversed a distance of many billions of light years to reach Earth. Yet the diameter of the universe when the light started its journey was less than a million light years across. This means that the light was no more than a million light years from Earth's current position when it started out. I understand that the universe has been expanding all this time, but how is it possible that the light took more 13 billion years to travel a distance of less than 1 million light years? It seems that it would have to traveling much slower than "c," the speed of light! What am I missing? Thanks. Al Kogut: When the light started its travel, the universe was much smaller than it is today. But in the time it took the light to travel, say, half-way across this smaller early universe, the universe expanded so that there was now further to go. And while the light was traveling this extra distance, the universe continued to expand, so there's now even further to go.
If you start far enough away, all this extra distance adds up to much more than 13 billion light years. There are parts of the universe that we can't see yet, because the light is still playing "catch-up" against the expansion. We'll have to wait a few billion years. And since we now know that the expansion is getting faster all the time, there are parts of the universe that we will NEVER see, because the extra distance from the expansion will keep piling up faster than the light can overcome it.
Arlington, Va.: Is it technically possible to see back farther in time than 380,000 years after the big bang? (asked another way: is it an engineering problem or a physical one?) If we could see back farther in time, what would you anticipate we would see? Not that time necessarily exists. Al Kogut: It depends on what you mean by "see". Within 380,000 years of the Big Bang, the universe was a dense fog of electrons, protons, and light. This fog blurs out images from earlier times, so you can't show fine spatial details. But there are loopholes.
o The blurring occurs mostly on small scales. Angular scales larger than a few degrees (a few times larger than a full moon) can be seen through the fog, and show us the universe right after the Big Bang.
o Fog blurs shapes but not colors. Scientists can study the spectrum of light from the early universe to learn about conditions up a about a year afer the Big Bang.
o There are other signals than just light. Gravity waves from the early universe (if they exist) zip right through everything. Recording them today would tell us about the early universe almost AT the Big Bang -- nothing else will ever get any closer.
Capitol Hill, Washington, D.C.: Dear Mr. Kogut -- What does the WMAP map say about the formation of galaxies? Can you say what the first galaxies looked like? And when did quasars form? Al Kogut: WMAP is looking at the stuff that would eventually form galaxies, but does not directly observe galaxies as they form (they're much too small). WMAP data does determine that the first stars turned out about 200 million years after the Big Bang, but does not determine whether these stars had formed within galaxies or not (one would presume yes, but the data do not force that conclusion).
Quasars formed much later -- more like a billion years after the Big Bang. Now that we know when the first stars formed, we can turn to figuring out how they formed, and how they grouped into galaxies.
Chevy Chase, Md.: Are there more results to come from this probe? Al Kogut: Yes, there will be more results in the future. WMAP is scheduled to observe for a total of four years. We just presented initial data from the first year of observations. WMAP continues to function well, so we look forward to several more years of data.
washingtonpost.com: That wraps up today's show. Thanks to everyone who joined the discussion. © Copyright 2003 The Washington Post Company | 
