. . . something mysterious is at work in the universe
– Physicsworld.com.
When I had a real job (at a company called Itek Optical Systems), I had the privilege of working on the Hubble Space Telescope studies and the W. M. Keck telescope primary mirror fabrication proposal. As a young engineer, I was so excited to be involved, even in a small way, in the initial stages of these new “eyes on the sky”. I wondered what great mysteries these instruments might some day reveal. Now I know.
In 1998, using data from Hubble and Keck (and other telescopes), physicists Saul Perlmutter of Lawrence Berkeley National Laboratory, Brian Schmidt of the Australian National University, and Adam G. Riess of Johns Hopkins University made the most significant cosmological discovery since the Cosmic Microwave Background in 1965. For this, they were awarded the 2011 Nobel Prize in physics.
Like so many, their discovery came as a complete surprise.
Per cosmological models, physicists in the late 20th century believed gravity should be slowing the expansion of the universe. What was needed was observational data to confirm this theoretical prediction.
So in 1998, two independent teams led by Perlmutter (the Supernova Cosmology Project), and Schmidt/ Riess (the High-Z Supernova Search) tried to measure exactly how much the expansion of the universe is being slowed by gravity. And by measuring this “deceleration parameter” they hoped to also get a better indication of the total amount of matter/energy in the universe.
The idea was to utilize Hubble, Keck, and other telescopes to examine the brightness and redshift of some one hundred or so type 1a supernovae:
Hubble and supernova brightness – The 2.4 meter Hubble Space Telescope in low Earth orbit was involved in the measurement of the brightness of these type 1a supernovae. Why type 1a? Because physics predicts these supernovae all produce about the same luminosity. (Luminosity is the amount of light an object gives off.)
Just like car headlights, a supernova appears dimmer the further away it is. So by measuring its apparent brightness as seen from Earth and comparing it to its luminosity, the physicists determined how far away it was, i.e. its distance from us.
And since light from a supernova explosion travels at the speed of light c, knowing its distance from us told them how long ago the supernova light was emitted — or when each supernova explosion occurred.
Keck and supernova redshift – 13,600 feet high on Mauna Kea in the big island of Hawaii, the ten-meter Keck telescope and its LRES spectrometer were involved in the determination of the redshift of these type 1a supernovae. This told the physicists how much the universe has expanded since the supernova explosions.
Say a supernova is 10 billion light-years away. This means it takes 10 billion years for its light to reach us here on Earth. During that time, the universe has expanded — stretching the wavelength of the supernova light as it makes its journey to us. So the supernova light arrives at Earth at a lower frequency — shifted towards the red end of the spectrum.
Thus the amount of redshift reveals how much the universe has expanded in the time it took the supernova’s light to travel to the Earth.
Together, the data from Keck, Hubble, and other telescopes told Perlmutter, Schmidt, and Riess when each supernova was born and how much the universe has expanded since.
The results were shocking, to say the least. Distant supernovae were dimmer than expected. This meant they were further away than initially thought. So their light must have taken longer to get here. Thus these supernovae were born earlier in time than expected.
Therefore the measured expansion amounts (redshifts) associated with these distant supernovae occurred earlier than expected. So the universe must be expanding faster than expected.
To see this, imagine the surface of a balloon expanding as you blow it up. You expect it to be a certain size at a certain time. But when you measure it, you find it reaches that size sooner than you thought. So, you conclude, the balloon must be expanding faster than you thought.
From all this, Perlmutter and Schmidt/Riess concluded the expansion of the universe did slow at first — but for the past 5 to 7 billion years, the expansion of our universe has been speeding up!
Later observations of over two hundred type 1a supernovae confirmed this analysis — the expansion of the universe is indeed accelerating. Data from the Wilkinson Anisotropic Probe (WMAP), the Two-Degree Field (2DF), and the Sloan Digital Sky Survey (SDSS) support this conclusion.
What causes this acceleration? Physicists have no idea. Some postulate our entire universe is filled with a kind of repulsive energy dubbed “dark energy”. Exactly what this so-called dark energy is remains one of the great unsolved mysteries of modern physics.
For details, see Perlmutter’s article: http://supernova.lbl.gov/PhysicsTodayArticle.pdf
I welcome all comments — pro and con.
My website: marksmodernphysics.com
References for this article:
1) Imagine the Universe! http://imagine.gsfc.nasa.gov/docs/science/mysteries_l1/dark_energy.html
2) B. Rosenblum, F. Kuttner, Quantum Enigma, p. 196.
3) B. Greene, The Fabric of the Cosmos, p. 297.
4) “Dark Energy”, Physicsworld.com, May 30, 2004. http://physicsworld.com/cws/article/print/19419
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