## What Time Is It

1 Martin Gorst, Measuring Eternity, Broadway Books, New York 2001,

2. To compute the Hubble time, we need to gel all the quantities in the same units. /„ = 1/Hln but H„ is in units of kilometers per second per megaparsec.

Since 1 kilometer = 101 meters and 1 megaparsec = 3 08 x 10" meters, then a Hubble constant of 70 kilometers per second per megaparsec = 70 x 10A meter/kilometer x 1/(3 x 10JJ) meter/megaparsec = 2.27 x 10 second""1.

Strictly speaking, this is the correct way to express the Hubble constant. It means that the local patch of the universe stretches out by 2.27 x 10~IK of its present size each second Once the units are straightened out, the computation of the Hubble time is simple: fn = 1/(2.27 x 10"lK second-1) = 4 40 x 1017 seconds So that's the age of the Universe, based on the present expansion rate. Since a year has 3-16 x 107 seconds, this can be expressed as A, = 4.40 x 1017 seconds/3.16 x 10T seconds/year = 13 9 x 10^ years. Fourteen billion is close enough Since we're not completely sure this is the final value for the Hubble constant, for some other value of Ht> you would get t,t = 13.9 billion years x (70 kilometers/second/megaparsec / //„) This ""Hubble time" would be the age of the universe if the rate of expansion does not change. Gravity from dark matter slows expansion and dark energy accelerates it.

3- The legend is that Eddington was approached by a reporter who asked whether it was true that there were only three people in the world who understood general relativity. Eddington didn't answer "Come, come, sir, don't be mildest."

"I was just trying to think who the third might be."

4. A S. Sharov and I D. Novikov, Edwin Hubble, the Discoverer of the Big Bang Universe p. 67, Cambridge University Press, Cambridge, UK, 1993-

5. A. S Eddington, The Expanding Universe, p. 65.

6. Amazingly, Hans Bethe is still making contributions to astrophysics-he has been working effectively on the mechanisms of supernova explosions and published a paper on that topic in 2001.

7. In 1999, confusion between English units and metric units led to the Climate Orbiter burning up in the Martian atmosphere.

8. An amusing chart compiled by my colleague John Huchra shows the quoted values of the Hubble constant from 1929 to the present: http //cfa-www. h a rva rd. ed u/~h uchra

9 N Panagia, R Gilmozzi, F Macchetto, I l.-M. Adorf, and K. P. Kirsh-ner, "Properties of the SN 19H7A Circumstellar Ring and the Distance to the Large Magellanic Cloud/ The Astrophysical Journal Letters 380, L23 (1991).

10. R. G. Eastman, and R. P. Kirshner, "Model Atmospheres for SN 1987A and the Distance to the Large Magellanic Cloud," The Astrophysical Journal 347, 771 (1989).

Chapter 7: A Hot Day in Holmdel

1. Astronomers are familiar with electrons being ripped off hydrogen atoms by ultraviolet photons near hot stars, and then emitting visible light as the atoms reassemble themselves. In the trade, this is called "recombination." The light from recombining hydrogen atoms makes gas clouds in star-forming regions glow. It isn't quite logical, but we also call the era when hydrogen atoms first formed in the cooling aftermath of the Big Bang, "recombination." It isn't logical because it isn't "re" combination when it's the first time. But that's we call it. It would be couth to call it combination, but our advertant speech is maculate.

2. When we were in Munich not long ago, Jayne and 1 wandered through the endless halls of the Deutches Museum—a giant and thorough museum of science and technology. There were zeppelin parts and full-sized ships, a spark-crackling van de Graff generator, and didactic exhibits on elecrromagnetism that would take about a year to assimilate But up in the astronomy hall, [ was astonished to see the original receiver that Pen-zias and Wilson had used. There, on its chart recorder, was the actual signature of the hot Big Bang. What was it doing in Munich' Penzias, born in Munich in 1933, moved with his family to the United States in 1940. I guess he had good memories of the Deutches Museum Perhaps that is where he learned about elecrromagnetism1

3 This is not the whole story The motion of the Milky Way through the photons of the microwave background does create a fore-and-aft effect of about 2 parts in 100 Once this simple motion is removed, the small-scale roughness is of order ] part in 100,000

4. Guth kept a diary, which was open to this page in an exhibit at the Adler Planetarium in Chicago All of this is recounted in his book The inflationary Universe, Helix Books, Reading, MA, 1997.

5. There are versions of inflation that lead to a universe where £2 is not exactly one. Some of this is described in chapter 4 of J Richard Gott's brilliantly hued Time Travel in Einstein '\$ Universe, Houghton Mifflin, Boston, 2001.

6. Firsthand accounts are in George Smoot and Keay Davidson's Wrinkles in Time, Avon Books, New York, 1993, and 1he Very First Light by John Mather and John Boslough, New York, Basic Books, 1996.

7. The temperature of Hell is reported by Dante and others to be the temperature at which brimstone (sulfur) melts. This is 718 kelvins. So we're talking about the universe when it was 60 million times hotter than Hell

8 This subject is elegantly discussed by Steven Weinberg in his classic book, Ihe First 'Ihree Minutes, Basic Books, New York, 1993.

9 I lelium in birthday balloons is not exactly straight from the Big Bang It comes from the radioactive decay in the Earth of more complex elements that formed in stars. The world's biggest source of helium is a natural gas field near Amarillo, Texas. At the Helium Monument, helium's indifference to combining chemically with other atoms is illustrated by a piece of apple pic in a container filled with helium It looks as fresh as the day it was baked t>y the infrared photons of an oven. It is 33 years old (www.dhdc org/beliu mmon u ment.htm).

10. Zwicky's initial work was published in Helvetica Physica Acta 6, 110 (1933) His book Morphological Astronomy, SpringerVerlag, Berlin, 1957, contains a longer English version A recent review by Sidney van den Bergh in Publications of the Astronomical Society of the Pacific 111, 657 (1999) is very entertaining

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