Ever since Edwin Hubble’s discovery of the expansion of the universe in 1929, astronomers have sought an answer to a most basic question: How long ago did the universe begin expanding? To determine an answer, one must know the rate of expansion, a value called the Hubble constant (H0).
Astronomers Wendy Freedman, Robert Kennicutt and Jeremy Mould were awarded the Gruber Foundation 2009 Cosmology Prize for their work toward determining an accurate value for the Hubble constant. The trio, working with a team of more than two dozen other astronomers at 13 different institutions around the world, designed and managed the effort, which was one of the first fundamental or “Key Projects” assigned to the Hubble Space Telescope (HST). In fact, this project was one of the purposes for which the HST had been built.
The official Gruber Prize citation reads in part that the astronomers’ “definitive measurement of the rate of expansion . . . effectively determines the age of the universe at the current time and underpins every other basic cosmological measurement.”
Freedman, now director of the Observatories of the Carnegie Institution of Washington in Pasadena, California, spoke to Vision contributor Dan Cloer about the complexities of measuring the universe and of the human brain that attempts to make sense of it.
DC It is a fascinating experience to stand in one of the Mt. Wilson domes and to imagine the historical moments which are in a sense enshrined there.—Hubble’s work on the motions of the galaxies and Einstein’s visit and discussion of the expansion of the universe are just two that stand out in the observatory’s 100-year history—What is your sense when you visit these places?
WF When I began working as a Carnegie Fellow in the mid-1980s, the Carnegie Institution was no longer funding work at Mt. Wilson so I, too, came essentially as a visitor. When you stop and reflect on what was done there, the significance of the discoveries and that our perception of the universe today was determined there, it is staggering. One can’t help being overwhelmed in some sense by being in that place.
I never got to use the telescopes at Mt. Wilson, but the first telescope that I used as a Carnegie Fellow was the Palomar 200 inch (5 meter). It was hard to walk into that dome and not know the history there, as well; to see that huge machine like a battleship but then to see how it moves and glides—you can literally push it with a finger, it is balanced so well—and to use it almost 50 years later to do first-rate science with something that had such a big impact was great.
I arrived as a young post-doctoral fellow, and the project I was interested in was the further determination of the Hubble constant and setting up a new distance scale using Cepheids that would be more accurate than what was possible before. There is tremendous excitement in laying out a plan that started there and eventually moved to the Hubble Space Telescope [HST].
DC You mention Cepheids. They are a kind of cosmic yardstick, pulsating stars. In the early 1920s Hubble used them to measure distances to other galaxies. How are they different than novas or exploding stars?
WF With Cepheids we can come back and measure them cycle after cycle; these stars have periods that range from a few days to 60. There are longer periods but they are rare. The physics that governs their oscillations is just a mechanical process of the star’s atmosphere moving in and out. Much of the early work that was done from the ground—even Edwin Hubble’s original data—can be combined with more recent work so that there is a cumulative record that shows they oscillate in a regular way. When I came to Carnegie one of my projects was to understand how interstellar dust affects Cepheid brightness.
We also studied how Cepheid chemical composition, what is called “metallicity,” affects their observed properties. One of the aims of the Key Project with the HST was to test how effective as a distance indicator these stars are. Relative to other distance indicators, Cepheids really have been tested quite well.
DC The Key Project was concerned with determining a more exact measure of the expansion of the universe, the Hubble constant. Wasn’t this work controversial at first because it seemed to show that objects in our galaxy were older than the universe itself?
WF Yes, obviously you can’t have stars in the Milky Way (where their ages can be measured) that are older than the universe overall. When we started, there was a controversy that had been going on for a couple decades concerning the expansion rate of the universe itself: a discrepancy of a factor of two between the high and low rates.
The first results that we got from the Key Project—results that others had seen before but could not resolve—indicated a high value for the Hubble constant. The age of the universe that could be calculated from this value based on the density of the universe known at that time was in conflict with the ages of stars measured in our own Galaxy. Our data settled on a high value as well, but what is not widely appreciated is how this lent support to the idea of dark energy. We would still be dealing with the age problem if dark energy had not been discovered.
DC The addition of dark energy to the system makes it possible to have an old universe and a high expansion rate. There must be more in the universe that we are not seeing directly. Is that correct?
WF Yes. One of the things that led to the acceptance of dark energy is this discrepancy between the Hubble constant and the ages of the oldest stars. Another was observations of high redshift supernovas. The distributions of galaxies on the largest scales also contributed to the idea that there must be some missing component. People modeling the distribution with just the visible matter in the universe could not get a match to the observed distribution of galaxies.
DC We are dealing with a strange mix of things when we attribute the acceleration of universe expansion to dark energy and the condensing of galaxies to the gravitational effect of invisible dark matter.
WF Yes, we are dealing with a strange mix of things operating on different scales. For example, the luminous matter in a galaxy does not have enough mass to hold the galaxy together; the outer stars are moving at speeds such that they should be escaping into space if there weren’t more matter to hold them there. But they are not. This effect is seen when we look at stars in a galaxy, the rotation of whole galaxies, and the motions of clusters of galaxies. The first indications of a dark matter source of gravity to hold clusters together were observed in the 1930s. Later X-ray observations showed the same tendency for gases around galaxies. So there is much quantitative evidence pointing to more matter producing a gravitational effect beyond the luminous matter that we can measure. This has now had a long history of testing so that the systematic errors of measurement can be factored out, and it is unambiguous that there is more matter than we can see.
At the moment, the evidence for dark energy is coming primarily from Type 1A Supernovae. Toward the end of the 1990s, two groups set out to measure the deceleration of the universe predicted by the presence of this extra dark matter. The prediction was that more matter must be slowing the expansion even more. But what they found was that the supernovae were much fainter than if they had been moving more slowly over time; in other words, they were actually moving faster than predicted. Both groups found this independently using the same types but not the same individual supernovae. The missing energy factor that accounts for this acceleration is called dark energy.
The effects of dark matter and dark energy cry out in large-scale observations; that’s why astronomers have found them. At present, however, we do not understand what the dark energy is. Quantitatively, the best model of what this may be, based on the standard model of particle physics, is in discrepancy with the observations by about 60 orders of magnitude. For dark matter, theory suggests a particle left over from the big bang that interacts gravitationally but not electromagnetically with ordinary matter (that’s why we don’t see it—it emits no light). Physics experiments around the world are now trying to find these very weakly interacting particles. With the Large Hadron Collider at CERN (European Organization for Nuclear Research) there is hope that these particles can be directly detected. It is extraordinarily challenging.
DC Are cosmologists imagining that this mystery particle is doing both jobs: creating gravity to hold things together and speeding up the expansion rate of the universe?
WF No. Dark matter is governed by gravity, which is an attractive force. Dark energy does not clump with dark matter and visible matter; it is repulsive in nature. Dark energy is distributed smoothly through space; it may not actually be completely smooth but there is not yet enough data to say how smooth. Looking out at these 1A supernovae at different redshifts gives you windows into the universe at different times in the past, so that it is possible to chart the evolution of the expansion with time.
The observed expansion is quite consistent with Einstein’s original formulas that allowed for either attractive or repulsive gravity. He knew the universe was either going to be expanding or contracting, but he added a term to make it steady—that was what he later termed his “biggest blunder.”
What has been measured is something that is quite consistent with repulsive gravity causing the expansion of the universe to speed up. It is quite separate from dark matter. But its explanation remains a complete mystery.
DC This seems somewhat analogous to the geologists’ view of the Earth prior to the unifying theory of plate tectonics. There were many ideas about the cause of mountain ranges and ocean trenches, but they were all trumped by tectonics. Today, we have a great deal of information, evidence and theory about the universe out there, but we don’t really have a core understanding of how the universe works.
WF What makes science interesting in a particular time is the interface between theory, observation and experiment. If a field is only driven by data that can’t be understood, then there is not much progress just as if you have only theory but no data to test it. But when you are at a point where the theory and observations are actually talking to each other and you can put constraints on the theory by the observations, this can motivate understanding until there is a physical explanation. I think we are in that kind of particularly interesting time.
DC Wherever one looks—astronomy, biology, geology—we seem to be gaining a depth of understanding of the intricacies of the moving parts even if we don’t yet know how all the parts mesh together. Do you find any parallels between your interest in the mysteries of cosmos and your early interest in the biophysics of the human mind? Do you still think about human consciousness and where that comes from and your interest in the universe and where it comes from?
WF That’s interesting; I’m not sure I’ve asked myself that question. In a very general sense, absolutely. I find it interesting that in terms of the brain we can do things empirically now that could not have been imagined a decade or so ago. To actually see where the brain is lighting up (using functional magnetic resonance imaging) as people are doing different activities and to be able to narrow in on regions of the brain where different processes take place is amazing. It is interesting that the universe on the largest scale is actually less complex than human consciousness and the brain. You can use mathematics to describe in a general sense the properties of the universe in a very elegant way. But you can’t do that with the brain.
DC Is this just because we don’t know enough yet about the brain?
WF The level of complexity of the brain is going to make it very unlikely that there will ever be a single equation that will describe consciousness.
DC It is strange to think that we can look at all of these billions upon billions of objects in the universe and say that our brains are capable of imagining how that all works, yet we can’t understand the brain that is doing the imagining. Are you saying the human brain is more complex than the universe as a whole?
WF There are pockets of the universe that are very complex and complicated—the physical processes of the explosions of supernovae and gamma ray bursts, regions around black holes, formation of stars and planets that in detail are very complicated for instance—but in its overall structure, and where the law of gravity governs the dynamics of the universe, it has an incredible simplicity to it that can be described mathematically. And it is relatively simple math.
You have to be careful about the specific case you are talking about, but as an overall system, the brain is extraordinarily complex, more complex than the expansion of the universe.