Hubble's Exciting Universe: Measuring the Universe's Expansion Rate
Before the Hubble telescope was launched, there was a huge uncertainty over the expansion rate of the universe. This value is needed to calculate the age of the universe, estimate its evolution over billions of years, and understand the forces driving it. At first, astronomers were delighted to narrow the expansion estimate to 10 percent accuracy. Now, with a lot of perseverance and precise observations, they are approaching one percent accuracy.
In 1929, Edwin Hubble provided the first observational evidence for the universe having a finite age. Using the largest telescope of the time, he discovered that the more distant a galaxy is from us, the faster it appears to be receding into space. This means that the universe is expanding uniformly in all directions. Hubble noted that light from faraway galaxies appeared to be stretched to longer wavelengths, or reddened, a phenomenon called redshift.
By precisely determining the expansion rate, called the Hubble constant, the cosmic clock can be rewound and the age of the universe calculated. However, Edwin Hubble's estimates of the expansion implied that the universe was younger than the age of the Earth and the Sun. Hubble, therefore, concluded that the redshift phenomenon was an unknown property of space and not a measurement of true space velocity. Astronomers later realized that redshift was a consequence of the expansion of space itself, as predicted in Einstein's theory of special relativity.
However, the age estimate is only as reliable as the accuracy of the distance measurements. A precise value for the Hubble constant is a critical anchor point for calibrating other fundamental cosmological parameters for the universe.
When the Hubble Space Telescope was launched, the uncertainly over the universe’s expansion rate was off by a factor of two. This meant that the universe could be as young as 9.7 billion years or as old as 19.5 billion years. The younger value presented a huge problem; it would mean the universe was younger than the oldest known stars.
In 1994, astronomers began refining the Hubble constant by making precise distance measurements out to the Virgo cluster of galaxies, located 56 million light-years away. This allowed astronomers to begin refining distance measurements that are needed to calculate a more precise value for the Hubble constant. They made observations of a class of star called Cepheid Variables. These stars go through rhythmic pulsations where they slightly rise and fall in brightness. The period of this oscillation is directly linked to the Cepheid’s intrinsic brightness. Once the star’s true brightness is known, astronomers can calculate a precise distance to it.
By the late 1990s, the refined value of the Hubble constant was reduced to an error of only about 10 percent. Another team of astronomers continues to streamline and strengthen this by calibrating more Cepheids ever more distant than the local universe. These data were cross-correlated with even farther milepost measurements of exploding stars, supernovas, to build a cosmic “distance ladder.” The measurement of the Hubble constant improved from 10 percent uncertainty at the start of the 2000s to less than 2 percent by 2019.
A variety of other observing strategies have been applied to look at other milepost markers such as red giants star. A novel technique uses Hubble to look at where the gravity of a foreground galaxy acts like a giant magnifying lens, amplifying and distorting light from background objects such as quasars. Astronomers next reliably deduce the distances from the galaxy to the quasar, and from Earth to the galaxy and to the background quasar. By comparing these distance values, the researchers measured the universe's expansion rate that is completely independent of the “distance ladder” techniques.
However, there is a troubling disagreement between the collective programs arriving at values for the Hubble constant in the nearby universe as compared with those of the early universe. The present rate of the universe’s expansion can be predicted from the cosmological model using measurements of the early universe, as encoded in the cosmic microwave background (CMB). The CMB is a snapshot of the cosmos as it looked only 360,000 years after the big bang (as made by the Planck space observatory). The value from the Planck data is in disagreement with more direct measurements of the nearby universe made with Hubble and other observatories.
According to standard cosmological models, the values from the early and local universe should be the same. Because they are not, it presents a major challenge to theorists by implying that there is an incomplete understanding of the physical underpinnings of the universe. This may require revising current astrophysics theories.
A century after the discovery of the expanding universe, the Hubble Space Telescope has allowed astronomers to enter the realm of precision astronomy, nailing down the expansion rate to extraordinary precision through several complementary observing strategies. Future Hubble telescope observations may help settle the discrepancy between two independent approaches that measure the early universe versus the late universe’s expansion. This may open up a whole new frontier in our understanding of the evolving universe.