Hubble Space Telescope Contributes to Nobel Prize in Physics

These snapshots, taken by NASA's Hubble Space Telescope, reveal five supernovae, or exploding stars, and their host galaxies. The arrows in the top row of images point to the supernovae. The bottom row shows the host galaxies before or after the stars exploded. The supernovae exploded between 3.5 and 10 billion years ago.

Observations made by NASA’s Hubble Space Telescope of a special type of supernovae contributed to research on the expansion of the universe that today was honored with the 2011 Nobel Prize in Physics.

Adam Riess, an astronomer at the Space Telescope Science Institute and Krieger-Eisenhower professor in physics and astronomy at The Johns Hopkins University in Baltimore, was a member of a team awarded the Nobel Prize in Physics by the Royal Swedish Academy of Sciences. The academy recognized him for leadership in the High-z Team’s 1998 discovery that the expansion rate of the universe is accelerating, a phenomenon widely attributed to a mysterious, unexplained “dark energy” filling the universe. Critical parts of the work were done with NASA’s Hubble Space Telescope.

Riess shares the prize with Saul Perlmutter, an astrophysicist at the University of California, Berkeley, and the Lawrence Berkeley National Laboratory, whose Supernova Cosmology Project team published similar results shortly after those published by Riess and High-z teammate Brian Schmidt of the Australian National University. Both teams shared the Peter Gruber Foundation’s 2007 Cosmology Prize — a gold medal and $500,000 — for the discovery of dark energy, which Science Magazine called “The Breakthrough Discovery of the Year” in 1998.

“The work of Reiss and others has completely transformed our understanding of the universe,” said Waleed Abdalati, NASA chief scientist. “This award also recognizes the tremendous contributions of the technological community that engineered, deployed, and serviced the Hubble Space Telescope, which continues to open new doors to discovery after more than 20 years of peering deep into the universe. With the future launch of the even more powerful James Webb Space Telescope, NASA is ensuring more revolutionary science discoveries like these in our future.”

Space Telescope Science Institute director Matt Mountain added, “The power of this discovery is that NASA has kept Hubble going for 20 years. This meant that Adam was able to track the history of the universe using science instruments that were upgraded from one servicing mission to the next. That is why this work has been recognized with the Nobel Prize.”

Riess led the study for the High-z Supernova Search Team of highly difficult and precise measurements of objects that spanned 7 billion light years that resulted in the 1998 discovery that many believe has changed astrophysics forever: an accelerated expansion of the universe propelled by dark energy….. Continue reading Hubble Space Telescope Contributes to Nobel Prize in Physics

Supernovae, Dark Energy, and the Accelerating Universe

Using very distant supernovae as standard candles, one can trace the history of cosmic expansion and try to find out what’s currently speeding it up.

Saul Perlmutter

 For millennia, cosmology has been a theorist’s domain, where elegant theory was only occasionally endangered by inconvenient facts. Early in the 20th century, Albert Einstein gave us new conceptual tools to rigorously address the questions of the origins, evolution, and fate of the universe. In recent years, technology has developed to the point where these concepts from general relativity can be substantiated and elaborated by measurements. For example, measurement of the remnant glow from the hot, dense beginnings of the expanding universe–the cosmic microwave background–is yielding increasingly detailed data about the first half-million years and the overall geometry of the cosmos (see the news story on page 21 of this issue).

The standard model of particle physics has also begun to play a prominent role in cosmology. The widely accepted idea of exponential inflation in the immediate aftermath of the Big Bang was built on the predicted effect of certain putative particle fields and potentials on the cosmic expansion. Measuring the history of cosmic expansion is no easy task, but in recent years, a specific variety of supernovae, type Ia, has given us a first glimpse at that history–and surprised us with an unexpected plot twist.

Searching for a standard candle

In principle, the expansion history of the cosmos can be determined quite easily, using as a “standard candle” any distinguishable class of astronomical objects of known intrinsic brightness that can be identified over a wide distance range. As the light from such beacons travels to Earth through an expanding universe, the cosmic expansion stretches not only the distances between galaxy clusters, but also the very wavelengths of the photons en route. By the time the light reaches us, the spectral wavelength λ has thus been redshifted by precisely the same incremental factor z ≡ Δλ/λ by which the cosmos has been stretched in the time interval since the light left its source. That time interval is the speed of light times the object’s distance from Earth, which can be determined by comparing its apparent brightness to a nearby standard of the same class of astrophysical objects.

The recorded redshift and brightness of each such object thus provide a measurement of the total integrated expansion of the universe since the time the light was emitted. A collection of such measurements, over a sufficient range of distances, would yield an entire historical record of the universe’s expansion.

Conceptually, this scheme is a remarkably straightforward means to a profound prize: an empirical account of the growth of our universe. A spectroscopically distinguishable class of objects with determinable intrinsic brightness would do the trick. In Edwin Hubble’s discovery of the cosmic expansion in the 1920s, he used entire galaxies as standard candles. But galaxies, coming in many shapes and sizes, are difficult to match against a standard brightness. They can grow fainter with time, or brighter–by merging with other galaxies. In the 1970s, it was suggested that the brightest member of a galaxy cluster might serve as a reliable standard candle. But in the end, all proposed distant galactic candidates were too susceptible to evolutionary change.

As early as 1938, Walter Baade, working closely with Fritz Zwicky, pointed out that supernovae were extremely promising candidates for measuring the cosmic expansion. Their peak brightness seemed to be quite uniform, and they were bright enough to be seen at extremely large distances.1 In fact, a supernova can, for a few weeks, be as bright as an entire galaxy. Over the years, however, as more and more supernovae were measured, it became clear that they were a rather heterogeneous group with a wide range of intrinsic peak brightnesses.

In the early 1980s, a new subclassification of supernovae emerged. Supernovae with no hydrogen features in their spectra had previously all been classified simply as type I. Now this class was subdivided into types Ia and Ib, depending on the presence or absence of a silicon absorption feature at 6150 Å in the supernova’s spectrum.2 With that minor improvement in typology, an amazing consistency among the type Ia supernovae became evident. Their spectra matched feature-by-feature, as did their “light curves”–the plots of waxing and waning brightness in the weeks following a supernova explosion.3,4

The uniformity of the type Ia supernovae became even more striking when their spectra were studied in detail as they brightened and then faded. First, the outermost parts of the exploding star emit a spectrum that’s the same for all typical type Ia supernovae, indicating the same elemental densities, excitation states, velocities, and so forth. Then, as the exploding ball of gas expands, the outermost layers thin out and become transparent, letting us see the spectral signatures of conditions further inside. Eventually, if we watch the entire time series of spectra, we get to see indicators that probe almost the entire explosive event. It is impressive that the type Ia supernovae exhibit so much uniformity down to this level of detail. Such a “supernova CAT-scan” can be difficult to interpret. But it’s clear that essentially the same physical processes are occurring in all of these explosions.

The detailed uniformity of the type Ia supernovae implies that they must have some common triggering mechanism . Equally important, this uniformity provides standard spectral and light-curve templates that offer the possibility of singling out those supernovae that deviate slightly from the norm. The complex natural histories of galaxies had made them difficult to standardize. With type Ia supernovae, however, we saw the chance to avoid such problems. We could examine the rich stream of observational data from each individual explosion and match spectral and light-curve fingerprints to recognize those that had the same peak brightness.

Within a few years of their classification, type Ia supernovae began to bear out that expectation. First, David Branch and coworkers at the University of Oklahoma showed that the few type Ia outliers–those with peak brightness significantly different from the norm–could generally be identified and screened out.4Either their spectra or their “colors” (the ratios of intensity seen through two broadband filters) deviated from the templates. The anomalously fainter supernovae were typically redder or found in highly inclined spiral galaxies (or both). Many of these were presumably dimmed by dust, which absorbs more blue light than red.

Figure 1 Light curves of nearby, low-redshift type Ia supernovae measured by Mario Hamuy and coworkers.7 (a) Absolute magnitude, an inverse logarithmic measure of intrinsic brightness, is plotted against time (in the star's rest frame) before and after peak brightness. The great majority (not all of them shown) fall neatly onto the yellow band. The Figure emphasizes the relatively rare outliers whose peak brightness or duration differs noticeably from the norm. The nesting of the light curves suggests that one can deduce the intrinsic brightness of an outlier from its time scale. The brightest supernovae wax and wane more slowly than the faintest. (b) Simply by stretching the time scales of individual light curves to fit the norm, and then scaling the brightness by an amount determined by the required time stretch, one gets all the type Ia light curves to match.5,8

Soon after Branch’s work, Mark Phillips at the Cerro Tololo Interamerican Observatory in Chile showed that the type Ia brightness outliers also deviated from the template light curve–and in a very predictable way.5 The supernovae that faded faster than the norm were fainter at their peak, and the slower ones were brighter (see Figure 1). In fact, one could use the light curve’s time scale to predict peak brightness and thus slightly recalibrate each supernova. But the great majority of type Ia supernovae, as Branch’s group showed, passed the screening tests and were, in fact, excellent standard candles that needed no such recalibration.6

Cosmological distances

When the veteran Swiss researcher Gustav Tammann and his student Bruno Leibundgut first reported the amazing uniformity of type Ia supernovae, there was immediate interest in trying to use them to determine the Hubble constant, H0, which measures the present expansion rate of the cosmos…….. Continue reading Supernovae, Dark Energy, and the Accelerating Universe

Physics Nobel will attract controversy

Assigning credit for a scientific discovery is never easy, especially when two rival, interacting teams of scientists are involved. That is exactly the problem that the Nobel committee must have grappled with before awarding this year’s physics prize to Saul Perlmutter, Adam Riess and Brian Schmidt.

Perlmutter led the Supernova Cosmology Project, while Schmidt and Riess were involved with the High-Z Supernovae programme. Both groups came to the surprising conclusion in 1998 that the rate of expansion of the universe is increasing, not decreasing as had been thought. So a shared prize seems fair enough.

Or is it? In 2007 Bob Crease wrote an extensive article about the same discovery that proved controversial – to say the least. Some members from both teams had been particularly worried about Crease’s article, which went through more than 20 drafts.

At issue was the fact that the teams were rivals using different techniques – as well as the question of who reported and published their work first. What Bob’s article reveals is how deeply scientific progress is indebted to ambition, desire, pride, rivalry, suspicion and other perfectly ordinary human passions.

You can read the article here.
By Hamish Johnston –