Observing the Expansion of the Universe Using the Hubble Space Telescope Shoko Sakai Ph.D. Jet Propulsion Laboratory, Pasadena CA Can you figure out how old the Universe is? How would you carry out such an experiment? Even more, what does the Universe look like? How did it begin? And most of all, what is going to happen in the future, a very distant future? If you are remotely curious, you have probably wondered at some point where everything in the world came from. Cosmology deals precisely with questions as those, and we, astronomers, attempt to solve these puzzles by collecting data using powerful telescopes and by developing theories. Observational cosmology has seen enormous changes in the past two decades, thanks to the dramatic improvement in technology, allowing us to work with an amount of the data that was unthinkable just a decade ago. The Universe started out from a primordial explosion, the Big Bang. This is perhaps the most important scientific discovery of the 20th century. During the 1920s, Edwin Hubble discovered that the galaxies are indeed receding from us, in a sense that galaxies located further away are moving away at a faster speed. This is a direct evidence of the expanding Universe. A stunning confirmation of the Big Bang theory was the discovery of the cosmic microwave background radiation (CMB) by Arno Penzias and Robert Wilson in the 1960s, which is a remnant of the Big Bang fireball. Today, 70 years after Edwin Hubble's first discovery of the expansion of the Universe, the basic picture of the history of the Universe has been verified. However, perhaps to a 1 surprise to most of the readers, the most fundamental questions such as the scale and ages of the Universe are only now being settled. Answering these questions is closely connected to determining the expansion rate of the Universe, which is represented by a number called the Hubble constant. And appropriately enough, a powerful telescope named after Edwin Hubble himself, Hubble Space Telescope (HST), plays the key role here. In June 1984, a conference on Hubble constant was held at the Aspen Center for Physics in Colorado, during which a group of astronomers gathered ideas together to form a project, that later developed into one of the HST Key Projects. In fact, determining the value of the Hubble constant was one of the major objectives for building the HST. The goal of the Key Project is to observe 25 to 30 spiral galaxies and estimate their distances accurately using stars called Cepheid variable stars. The project moved into full gear after the HST refurbishment mission in December 1993, and today, it has grown to a large group consisting of 27 astronomers, including quite a few postdoctoral associates and graduate students, and is led by three co-principal investigators: Robert Kennicutt Jr. of Steward Observatory, Wendy Freedman of Carnegie Observatories and Jeremy Mould of Mount Stromlo Siding Spring Observatory. As a postdoctoral fellow at Jet Propulsion Laboratory, I joined this team in fall of 1994. Although the team members are spread out on three continents, the internet keeps us updated on a day-to-day basis. In this Science Series, I will describe how we are carrying out this experiment to measure the age of the Universe and what the impacts of HST are with regard to our project. Expansion of the Universe During the 1920s, using the 2.5-meter telescope on Mount Wilson near Los Angeles, Edwin Hubble and Milton Humason carried out the observations of nearby galaxies that led to the discovery of the expansion of the Universe. Gas in general emits and absorbs 2 light from stars at certain wavelengths. When the light from these galaxies are observed through a prism, we see a “spectrum” of light, consisting of various emission and absorption lines. Different elements show different spectral lines; in fact, they are analogous to fingerprints and astronomers can obtain enormous amount of information about gases and stars by examining their spectral features. Using these data, Hubble and Humason found that the absorption lines observed in nearby galaxies were shifted to longer wavelengths. This shift is referred to as Doppler-shift or redshift, and the size of the shift in wavelength is correlated with the recession velocity. They also found that the redshifts for galaxies are correlated well with their distances. velocity = H0 x distance (Hubble Law) This correlation between the distances and recession velocities is known as the Hubble Law, and H0 here is the Hubble constant (Figure 1). This equation suggests that the faster moving galaxies travel further in a given time. This is analogous to a bomb explosion in which thousands of pieces fly away from the same point; those that travel faster end up further away from the starting point. Imagine a movie showing the galaxies separating further and further away from each other with time. If you played this film backwards, the galaxies would come closer to each other, eventually into a single point. This indicates the beginning of the Universe in the distant past, the Big Bang. The time that has elapsed since the Big Bang is related to the Hubble constant, so if we can measure its value accurately, we can actually determine the age of the Universe itself. The Hubble Law is one of the direct evidences confirming the expansion of the Universe. Here, the expansion means that the space itself is expanding. This may be a difficult concept to grasp at first, but it can rather easily be demonstrated by an analogy illustrated in Figure 2, in which the expanding Universe is compared to an inflating balloon. On the surface of the balloon, the coins are placed which could be thought as the galaxies. As the balloon expands, distance between the coins become larger, just as the galaxies recede from each other while maintaining the constant size themselves in an expanding Universe. 3 It is important to determine the Hubble constant accurately. It leads to knowing a correct scale of the Universe, and thereby distances of objects such as the galaxies. Distances are extremely important as without their good measurements, characteristics such as the mass or energy output of galaxies cannot be estimated accurately. It is also important to have a good estimate of the Hubble constant because many of the fundamental quantities, such as the age of the Universe, is embedded in knowing accurately the expansion rate of the Universe. The age of the Universe is expressed in terms of the Hubble constant; it is inversely proportional to H0 for flat Universe. That is, the larger the Hubble constant is, the smaller the age becomes (see Figure 3). Here, the ‘flat’ Universe refers to the shape of the Universe itself. This topic itself is beyond the scope of this article, but is one of the fundamental questions that need to be answered in cosmology. I will discuss briefly towards the end of this article what effects the shape has on the determination of the age of the Universe. Structure of the Universe and Peculiar Velocities As in any other scientific problem, measuring the expansion rate and the age of the Universe is similar to solving a puzzle. You need to first identify what pieces should be collected, and then you need to solve how you put all of them together. For the Hubble constant, these pieces are the expansion velocities of the galaxies and their distances. We have now learned that H0 is a constant that connects velocities and distances to galaxies, and also that knowing the value of H0 would help in estimating how old the Universe is. Recession velocities can be measured from spectra of galaxies. Thus, what we are missing is a tool to accurately measure distances. However, there is yet one more complication that needs to be considered before measuring H0. The real world is much more sophisticated, mainly because the Hubble Law does not fully explain the velocities of the galaxies, in particular the way they interact with their immediate neighborhood. As we know, the gravitational force plays an important role in the physical world, and it also has a significant effect on the motions of galaxies and on the structure of the 4 Universe, making the distribution of galaxies far from smooth and homogeneous. Shortly after the Big Bang, the Universe was relatively smooth, but there were small unevenness in its mass distribution. The gravitational perturbation from a small mass concentration amplified the clumping, which later grew to form clusters and superclusters of galaxies. The gravitiy forces from these galaxy ‘clumps’ subsequently produce motions of galaxies, usually referred to as peculiar motion, that do not follow the smooth Hubble expansion. On average, the peculiar velocities amount to few hundreds of km s-1 (the recession velocities of galaxies range from thousands up to tens of thousands of km s-1), but the accurate measurement of these motions is extremely difficult, as that would require a precise knowledge of the matter distribution in the clusters and superclusters, which is hindered by the presence of large amounts of dark matter on these scales. Fortunately, the typical peculiar velocities are small when compared to the cosmological expansion if you observe distant enough galaxies. But for very nearby galaxies, the peculiar velocities can be comparable to or even larger than the expansion speeds. For example, one of our nearest neighbor galaxies, such as the Andromeda Galaxy (M31), are known to be moving towards us, instead of receding from us as expected by the Hubble Law. This is not because the Unierse behaves differently locally, but because of the significant gravitational pull among nearby galaxies.