News from the Frontiers of Cosmology: A companion to the book The Edge of Physics
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Hubble’s 20th anniversary: Not just pretty pictures


IT WAS TWENTY YEARS AGO, TODAY (well 24 April), that the Hubble Space Telescope was launched, making it the first optical telescope to be operated from space.

We have all been amazed and astounded by the images of galaxies and nebulae that have poured out of Hubble’s amazing vision.

But it’s important to highlight Hubble’s role in what has been the seminal discovery of the 1990s: Dark Energy.

From The Edge of Physics:

In the 1990s, two separate teams of astronomers had trained their telescopes to look deep into the nearby universe. One was led by Saul Perlmutter of Lawrence Berkeley National Laboratory and the other by Brian Schmidt of the Australian National University and Adam Riess, who was then at UC Berkeley. They were looking at individual stars in galaxies hundreds of millions, even billions, of light-years away. Normally, it’s impossible to see individual stars so far away. But these astronomers were after stars that were in their death throes—the exploding stars known as supernovae. Supernovae, in their final moments, can outshine their host galaxies and act as beacons, lighting our way to distant parts of our universe. The astronomers were hoping to confirm that the expansion of the universe was slowing down with age.

Both teams had been monitoring a special type of exploding star known as a type-Ia supernova. Such stellar explosions have become astronomy’s “standard candle,” meaning that we can determine, within limits, their absolute—that is, intrinsic—brightness and, as a result, their distance from us. This calculation is based on the relationship between how long such a supernova takes to reach peak brightness and then wane. This period can be accurately measured and is strongly correlated with the supernova’s absolute brightness. Then astronomers measure its apparent brightness—its brightness as seen from Earth—and its redshift. The apparent brightness, when compared with the absolute brightness, tells us the distance to the supernova, and its redshift is a measure of how much the universe has expanded since the supernova detonated. The idea was to gather data both from nearby and distant supernovae and then compare them to see how the expansion of the universe had changed over time.

And what they found stunned the world of physics: The expanding universe, instead of slowing down with age, or even just coasting along, is actually speeding up.

Einstein (him again) had shown that space could have an inherent energy and that this energy counters gravity. The supernova studies indicate that something akin to this is indeed happening. Some energy is countering gravity and causing the universe’s expansion to accelerate. Along with dark matter, there is now another puzzle: dark energy (sometimes referred to as the cosmological constant). Together they form the bulk of the universe.

But when the announcement was first made in 1998, there was still concern that there was something wrong with the observations.  Were the supernovae appearing faint because of dust in the galaxies?

That’s when astronomers turned to the Hubble Space Telescope. Perlmutter’s team was the first to use the Hubble in 2000 to study a dozen galaxies. No dust. The results were good.

Then in 2002, the Hubble was fitted with the stunning Advanced Camera for Surveys (ACS), and a new team led by Riess found 25 more supernovae. The results were conclusive: the exploding stars were indeed fainter. The observations concluded that about 73 % of the universe is indeed made of dark energy.

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