Skip to main content

Cosmologist reflects on 20 years of Hubble’s discoveries

Colorful nebula image from Hubble.
April 26, 2010

For 20 years, scientists, astronomers and countless others around the world have enjoyed the beauty and brilliance of the heavens thanks to the Hubble Space Telescope (HST). To mark the 20th anniversary of the April 24 launch of the world’s first space-based optical telescope, Rogier Windhorst, a Regents’ Professor in the School of Earth and Space Exploration at Arizona State University, reflects on Hubble’s operation and the era of unprecedented discoveries that it initiated. 

With Hubble’s help, Windhorst has contributed significantly to our understanding of the universe, specifically the structure of the universe and the formation and evolution of distant galaxies. He has had his hands on Hubble since its inception, and over the past two decades he has been involved with more than 45 HST projects. Windhorst is one of the most consistently funded users of the telescope. Currently, he is co-investigator of the HST Wide Field Camera 3 Early Release Science program, which images nearby and distant galaxies in 10 filters from the ultraviolet to the near-infrared, using the new camera that the space shuttle astronauts installed in Hubble in May 2009.

What’s the long-term significance of HST?

Hubble has been arguably the most profound and productive telescope and science instrument launched since Galileo first looked at the skies through a telescope in the early 1600s. Despite the initial issues with Hubble’s mirrors, which were successfully fixed by the astronauts during the first shuttle servicing mission in December 1993, Hubble has exceeded all expectations, and then some. It has truly revolutionized astronomy in the last two decades. You can hardly find a school or university in the nation that does not have some spectacular Hubble images posted somewhere.

What top three things changed your research focus or knowledge in cosmology because of Hubble?

First, the exact expansion rate of the universe, which is now known to well within 10 percent thanks to Hubble. As a corollary, we now know the age of the universe to be 13.67 billion years, to which Hubble made a great and essential contribution.

Second, the way galaxies form and evolve over cosmic time, and how supermassive black holes grow in their centers subsequently. Both are a major piece of the puzzle of how stars and galaxies formed throughout cosmic time.

And, third, how young stars in dwarf galaxies likely finished the so-called epoch of Cosmic Reionization, which is the last major phase transition in the universe. Hubble showed that this process was completed about a billion years after the Big Bang, when almost all remaining hydrogen gas in the universe was split into its individual protons and electrons by the intense ultraviolet light from hot stars in these young galaxies. This event turned the universe from opaque to transparent, and enabled Hubble to look back into cosmic history to redshifts of seven to eight, when the universe was eight to nine times smaller than today and only about 700 million years old. [For the record, the James Webb Space Telescope to be launched by NASA in 2014 is designed to see another factor of three further back into cosmic time and redshift space.]

What are your favorite Hubble images?

I have three: they all were made at ASU, I am a little biased:

First, the Eagle Nebula:

Second, the Dawn of Galaxies from the Hubble Ultra Deep Field:

Third, the new very colorful Hubble Wide Field Camera 3 images:

Thanks to Hubble’s new WFC3 camera, we can see a wide variety of stages in the galaxy assembly process. In these new Hubble images, the ultraviolet light shows the blue glow of hot, young stars in galaxies teeming with star-birth. The orange light reveals the nearly final assembly stages of massive galaxies about eight-to-10 billion years ago. The near infrared reveals the red glow of very distant galaxies – in a few cases as far as 12-to-13 billion light years away – whose light has been stretched from ultraviolet light to longer-wavelength infrared light due to the enormous expansion of the Universe since then.

What is the biggest change in science pre- and post-Hubble?

We had no idea that faint and distant galaxies were so numerous and small. They are full of active compact regions of star-formation, that are prevalent throughout the universe, and in which the heavy elements were made throughout cosmic time. Hubble has revealed to us in detail most of this epoch of galaxy assembly and supermassive black hole growth.

Before Hubble, galaxy evolution was thought of as a neat, slow and steady process, like cars moving slowly off a factory assembly line. After Hubble, we know this process to be chaotic, random, and sometimes very sudden, full of spectacular galaxy collisions, like watching the Indy 500 or the Daytona 500. Hubble has shown us that life in the cosmic fast lane never gets boring.

Did you ever imagine Hubble would last 20 years?

Not initially, but then after I saw NASA successfully pull off the first shuttle servicing mission to Hubble in December 1993 (and then SM2 in February 1997, SM3A in December 1999, SM3B in March 2002, and SM4 in May 2009), I thought, well, this dream may stay alive forever, or at least go on for the length of a human career. It has been truly unprecedented, life-changing, with never one boring day. It’s like being thirsty in the desert and being able to sip from a fire hydrant.

Windhorst, who also is a co-director of ASU’s Cosmology Initiative in the College of Liberal Arts and Sciences, has been involved in upgrades to the Hubble, as well as the development of its successor, the James Webb Space Telescope (JWST). He is one of the world’s six Interdisciplinary scientists for JWST, which is scheduled to be launched in 2014. With JWST, his group at ASU plans to use JWST to map the epoch of first light in detail.

Students and postdoctoral researchers working at ASU on the new Hubble images include: Seth Cohen, Rolf Jansen, Kazuyuki Tamura, Hwihyun Kim, Teresa Ashcraft, Katie Kaleida, Matt Mechtley and Mike Rutkowski, as well as former ASU graduate students Steve Finkelstein (now at Texas A&M), Nimish Hathi (now at University of California, Riverside), Russell Ryan (now at University of California, Davis), Amber Straughn (now at NASA Goddard Space Flight Center), and Haojing Yan (now at Ohio State University).

Hubble images from the last 20 years are online at:

Image files and additional information about the Hubble Space Telescope are at:

The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. NASA’s Goddard Space Flight Center manages the telescope. The Space Telescope Science Institute conducts Hubble science operations. The institute is operated for NASA by the Association of Universities for Research in Astronomy, Inc., in Washington D.C.