PGS Dialogue: David Weintraub, author of How Old Is the Universe?

David Weintraub’s most recent book, How Old Is the Universe?, is a readable investigation of the title question that explains how we have arrived at an approximate age of 13.7 billion years for the universe. Weintraub works his way from biblical chronology of the origins of the universe to the high-tech astronomy research taking place today in this accessible and entertaining history. We recently posed some questions to Prof. Weintraub by email and are pleased to present this dialogue.

PUP: I am not an astronomer, so I was relieved to discover I could actually read How Old Is the Universe? You clearly went to great efforts to make the text accessible. How difficult was it to break down these big scientific ideas, terms, and facts for general readers?

Professor David Weintraub: Making sure I was speaking to a non-professional audience in English rather than in the jargon-filled language of astronomy was a constant challenge.  A major goal with this book is to help general-audience readers understand the complicated and unfamiliar concepts described between the covers.  Consequently I focused on this issue quite literally with every word I wrote.  At the risk of being struck down by the gods for hubris, I do think I have done better at this than most astronomers who are trying to communicate with a non-professional audience.  Nevertheless, more than a few of my descriptions passed through my ‘language of the lay reader’ filter unnoticed by me.  Fortunately, Princeton University Press assigned my manuscript to an editor who asked me lots of excellent questions for clarification, and quite often her questions arose when she bumped into a piece of text in which the meaning was unclear to her because of my too-technical word choices.  I do think, in the end, the presentation of difficult concepts in this book is accessible to the general reader because we paid such close attention to language and because I continually reminded myself of whom the readership of the book is intended to be.  My editor was a humanist who knew no astronomy before beginning to edit the book.  So she was my test reader; if she didn’t understand my words, I flunked the test.  When we were done, she felt that she understood every word and had learned and now understood everything in the book.  Fortunately for her and all readers, unless you are in one of my classes, there is no test at the end of each chapter.

(blog editor note: But if you do like homework, you can find lots of extra course materials on Prof. Weintraub’s site:

PUP: One of the scientific principles you describe so clearly in the book is the rate at which the universe is expanding. While we are often told to assume nothing, you note that in this case, there is a lot of evidence to support the assumption that the expansion rate of the universe has been nearly constant. Can you talk a little bit about the expansion of the universe and the role dark energy plays in the process?

DW: The rate at which the universe has been expanding has changed over the lifetime of the universe, but since it first reached its current expansion speed it has not changed by much.  For the first few moments after the universe’s birth, the expansion rate was dramatically higher.  But that period of time (known as the inflationary epoch) lasted only a tiny fraction of a second.  Thereafter, the rate of expansion was nearly constant for ten billion years.  For the last few billion years, the expansion rate has been increasing, but the rate of increase is so slow, it is only barely measureable. Thus, it is safe to say that the expansion rate of the universe has been nearly constant for almost the entire history of the universe.

The actual measurements indicate we now live in an accelerating universe; these words simply mean that the rate of expansion is increasing. Dark energy is the name we give to the physical process that is causing the expansion rate of the universe to increase.  The evidence for the accelerating universe comes from measuring the distances and velocities (away from us, in all cases) of extremely distant galaxies; those measurements come from observing exploding stars called supernovae.  Supernovae are among the brightest objects in the universe; because they are so bright, we can detect them even when they are very far away, and because supernovae are stars that live and die inside galaxies, the supernovae are like flags that wave and tell us the distance to the galaxies in which they reside.

PUP: From exploding to imploding stars…In chapter four, you describe what happens to stars once they have drained the number of protons in their core. The resulting bodies are called “red giants.” What would happen to the Earth if (or when) the Sun turns into a red giant? How would it affect the other planets and bodies in our solar system?

DW: In about three billion years, the Sun will begin to evolve into a red giant.  So the question is not “if” but “when” this will happen.   During this red-giant phase of a star’s lifetime, the star puffs up.  The outer layers of the Sun likely will expand outward and fill up most of the volume of the solar system inside of the Earth’s orbit.  While the surface of the Sun will actually be cooler than it is now, it will still have a temperature of several thousand degrees, and that surface layer will be so close to the Earth that the heat received by the Earth will increase so much that the atmosphere and oceans will literally boil off into space.  The Earth itself might eventually turn molten before it is swallowed by the Sun.  Before the Earth is destroyed by the dying Sun, the planets Mercury and Venus, both of which are closer to the Sun than is the Earth, will suffer similar fates.  Mars is probably far enough away from the Sun that it might survive, but it also will lose what little atmosphere it has.  The outer solar system would be affected in less extreme ways.

PUP: This is the stuff of childhood nightmares, which leads naturally to childhood daydreams, or in this case, star-gazing. A good part of what we know about the universe, we know because of the close observation of identifiable stars. But there are hundreds of millions of stars; how is it possible to keep track of individual stars?

DW: Imagine sitting in the stands at a football game; you look across at the 40,000 fans sitting on the opposite side of the stadium from you.  You make a map of where each fan is sitting and identify each one with a section, row, and seat number.  With your binoculars, you carefully make some observations of each and every fan (long brown hair or bald headed? baseball cap or no hat?  glasses? beard? windbreaker or raincoat?) so that each person can be distinguished from his or her closest neighbors.  Now the rules for movement: no fan is allowed to move more than 1 inch per century.  Yes, per hundred year interval of time.  Now you go home, sleep like Rip van Winkle, eat, work, come back in fifty years and look again across the stadium.  You will be able to identify each and every football fan, both from their locations and their particular characteristics (provided they’ve been fed well and don’t age much). The stars are so far away from us that the rates at which they change positions are almost immeasurable, even in a human lifetime.  The stars do move, and astronomers can measure their motions, but their motions are so small that year after year we are able to easily find and re-find the same stars.

PUP: Another fascinating fact I took away from the book is that stars come in different colors. You note that some stars are blue and others are red, but that they may not be visible to the naked human eye. Why is this?

DW: Most stars emit light in fairly similar amounts across the spectrum of visible light (what our eyes can perceive) and thus appear white.  But red stars and blue stars do exist and are visible to the naked human eye.  Red stars emit light in all colors, but they emit more red light than any other color; similarly, blue stars emit all colors of light, but they emit more blue light than green or yellow or orange or red.

My eyes are not very sensitive to colors — I fail all the color-blindness tests — so to my eyes, red giants are barely different in color from most other stars (that appear white) and blue supergiants are only a bit bluer, as seen by me, than most other stars.  But to observers whose eyes are sensitive to subtle color differences, red stars and blue stars are easily distinguished from the rest of the stars.  Clear skies help; patience in observing the night sky helps; and knowing where to look helps.  But once a red star or blue giant star is pointed out to you, you would immediately recognize their colors.   Most of us simply have not spent enough time looking up at night, under dark skies, to notice the color differences among stars.

PUP: It seems as if any discussion of astronomy almost always leads to the great Hollywood question – “Is there extra-terrestrial life in the universe?” What are your thoughts on the possibility of finding ET?

DW: I find the universe to be incomprehensibly immense and lonely.  I almost long for the olden days — before Copernicus — when humans “knew” that the entire universe was small and we were of central importance to the workings of the universe.

I am not convinced, as most of my professional colleagues and Hollywood producers seem to be, that the universe must be populated with life, including intelligent life.  I also am not convinced that we are alone.  I think we lack the knowledge to make any claims, one way or another, about life beyond the Earth.  But I do find Enrico Fermi’s question “Where are they?” the one I think about the most.  Wherever they are, if someone else is out there, we haven’t found them yet and they haven’t found us yet and we see no evidence of their presence in or impact on at least our part of the Milky Way galaxy.  Our solar system is fairly hostile to life.  Venus and Mars, the most Earthlike planets in our solar system, did not survive for long enough as habitable planets for advanced life forms like us to make them home.  The Earth, in fact, may be a very special place.

This suggests to me that life may be rare, if not unique, at least in our part of the universe and that humanity must take more responsibility for ensuring the survival of life.  Not the survival of life on Earth but life itself.  We may have an incredibly important responsibility.  If we’re it, then if we screw this planet up, the grand experiment of life in the universe might end with us.

I do think that all the wild speculation about life beyond the Earth permits us to be very casual about life on Earth; it permits is to be cavalier about our stewardship of our planet.  Many religious beliefs also lead us to acting selfishly about life and our planet.  I don’t think such carefree attitudes about the health of our planet are good ones.

PUP: That segues somewhat neatly into my next questions. It may surprise some readers that you start your “popular science book” with an account of biblical chronology.  In fact, chapter two (titled “4004 BCE”) opens with a famous quote from The Annals of the World in which James Ussher determines that the universe was born on “the twenty third day of October in the year of the Julian calendar, 710.” Why did you select this quote? And why engage with a religious account that is at odds with scientific research?

DW: The quote, or at least a poor paraphrase thereof, from Bishop Ussher is well-known.  I placed an important quote at the beginning of each chapter that is related to the most important concept presented in that chapter, and I thought it would be of some value to place in front of readers an accurate quote from Bishop Ussher.

As for why the chapter is in the book, I wanted to recognize the role that biblical chronology and scholarship played in understanding the age of the universe.  For a short period of time, in the seventeenth century, scholars like James Ussher and John Lightfoot were at the intellectual forefront in this field.  Most scholars, let alone non-professionals, don’t recognize that fact and often laugh at this quote.  My goals with this chapter were to recognize the important role that biblical chronology played, as an attempt to answer questions about the age of the Earth and the universe, in the seventeenth century, but also to point out that this method of scholarship was quickly found to be flawed and thus was left behind.  As a scholarly discipline, biblical chronology never achieved scholarly success, and in addition, it was supplanted by modern science.  The history of science reveals progress in our knowledge; this chapter presents a good illustration of such progress.  Biblical chronology was cutting edge in 1650, but it was quickly supplanted by better ideas, scientific ideas.

My hope, with this chapter, is that I would engage some readers who might put some credence in biblical chronology as an accurate chronometer for the universe, but to do so without insulting them.

PUP: How did your fascination with astronomy begin?

DW: I have no idea.  I flunked out of boy scouts at age 11 because I failed, multiple times, the test for identifying constellations and simply gave up thinking I could ever achieve the rank of first-class scout.  I know that I was never interested in looking up and memorizing the patterns of stars.  But I was a youth and a young teenager when some big discoveries were made in astronomy — the cosmic background radiation in 1965, pulsars in 1968 — and I grew up in the early days of space exploration — the race to the moon, the Pioneer and Voyager missions to the outer solar system, the Viking mission to Mars.  So my youth was peppered with news and excitement about space.  I also read science fiction, and I know that certain stories (Arthur C. Clarke’s The Nine Billion Names of God, The Star, and 2001, foremost among them) had a big impact on me.  I found myself interested in what stars and galaxies are and how they worked, but mostly I found myself interested in planets and the question “how do planets form?”

PUP: What advice would you offer a person who is thinking about pursuing astronomy?

DW: Astronomy is fun and being an astronomer is a wonderful way to be spending my life.  I would encourage a young person interested in astronomy to follow that interest as far as it takes him or her.  Astronomy is a tremendously successful vehicle for getting our youth interested in science.  With apologies to my friends who are chemists, I think it is obvious that learning about black holes and dark matter is more exciting to most young people, at first blush, than studying about covalent and ionic bonds.  Once youth become excited about astronomy, however, they discover that they need to know some fundamental science and math in order to further pursue their love of astronomy.  As a result, it’s a short hop, skip, and jump into the serious study of mathematics, physics, chemistry, biology, and computer science, all of which are important as foundational disciplines for astronomy.  Most of those who start out interested in astronomy will discover that these or other related disciplines (geology, engineering) or more distant ones that they discover (neuroscience, anthropology) through their studies of these fundamental disciplines become their passion.

PUP: What are your hopes for those who read this book?

DW: I would like readers to share in the wonder of knowing that humans have discovered the age of the universe.  This is a phenomenal, almost outlandish achievement.  Rather than sitting back and saying “Astronomers claim this is so and I guess I should believe them,” I want readers to say “Astronomers have explained to me, step by step, how they obtained the age of the universe, and I understand what they have done and I agree with their answer.”

PUP: Do you have any future projects in mind? Perhaps another book?

DW: Yes, I am working on current projects and have future projects in mind.  My research, primarily on why very young stars produce X-rays and whether those X-rays can tell us anything about the formation of planetary systems around those stars continues.  But like all baseball players, I believe in jinxes.  So I would prefer not to jinx myself by telling you much more than that or by answering the second question.

Watch Prof. Weintraub’s recent lecture series hosted at Vanderbilt (Part 1, 2, 3, 4, 5, 6)