Nobel Prize in Physics 2011: The Accelerating Universe and Dark Energy

This note covers the Nobel Prize in Physics 2011: who won it, how exploding stars called supernovae revealed that the universe's expansion is speeding up rather than slowing down, how the discovery unfolded, what dark energy is, why the finding matters, and quick facts for exams.

What was the Nobel Prize in Physics 2011 awarded for?

The official citation reads: "for the discovery of the accelerating expansion of the Universe through observations of distant supernovae".

In plain words, the laureates found that galaxies are not just moving apart from each other, which astronomers already knew, but that the rate at which they are moving apart is increasing over time. This was the opposite of what almost every physicist had expected.

The award is formally called the Nobel Prize in Physics, given by the Royal Swedish Academy of Sciences.

Think of the universe as a cake full of raisins baking in an oven: as the cake (space itself) expands, every raisin (galaxy) moves away from every other raisin. Before this prize, scientists assumed gravity would slowly brake this expansion.

Instead, something is pushing space apart faster and faster, and that "something" is now called dark energy.

Who are the laureates?

Saul Perlmutter

Saul Perlmutter was born in 1959 in Champaign-Urbana, IL, USA. At the time of the award he was affiliated with the Lawrence Berkeley National Laboratory and the University of California, Berkeley, both in California, USA.

He received one half of the prize. Perlmutter led the Supernova Cosmology Project, a research team he had set up in 1988, which searched the sky for distant exploding stars and eventually analysed 42 type Ia supernovae to measure how the universe's expansion was changing.

Perlmutter grew up outside Philadelphia, Pennsylvania. His parents were professors, one in chemical and biomolecular engineering and the other in social work administration. He studied at Harvard University before earning his doctorate from the University of California, Berkeley, in 1986, and he carried out his prize-winning research at Lawrence Berkeley National Laboratory as a co-founder of the Supernova Cosmology Project.

Brian P. Schmidt

Brian P. Schmidt was born on 24 February 1967 in Missoula, MT, USA. At the time of the award he was affiliated with the Australian National University, Weston Creek, Australia. He received one quarter of the prize.

Schmidt headed a rival project called the High-z Supernova Search Team, launched at the end of 1994, which independently found the same surprising result as Perlmutter's group.

Schmidt was raised in Missoula, Montana; his father worked there as a fisheries biologist, and the family later relocated to Anchorage, Alaska. He received his doctorate from Harvard University in 1993 and moved to Australia the following year, where he helped build the High-z Supernova Search Team with backing from the renowned scientist Robert Kirshner.

Adam G. Riess

Adam G. Riess was born on 16 December 1969 in Washington, D.C., USA. At the time of the award he was affiliated with Johns Hopkins University and the Space Telescope Science Institute, both in Baltimore, MD, USA.

He received one quarter of the prize. Riess played a crucial role within the High-z Supernova Search Team, and he was the lead author of the team's key 1998 paper, which drew on observations of sixteen type Ia supernovae.

Riess grew up in Warren, New Jersey. After earning his doctorate from Harvard University in 1996, he took up a position at the University of California, Berkeley, where he became part of the High-z Supernova Search Team and carried out this prize-winning analysis, before moving to the Space Telescope Science Institute in 1999 and later taking a professorship at Johns Hopkins University.

What problem were the laureates trying to solve?

Astronomers have known for about a century that the universe is expanding. In the 1920s, observations using redshift (the stretching of light's wavelength as its source moves away) showed that distant galaxies are rushing away from us, and the farther away a galaxy is, the faster it recedes.

This is called Hubble's law, and it was one of the clues that led to the idea of the Big Bang, a single explosive beginning of space and time about 14 billion years ago.

Because every clump of matter pulls on every other clump through gravity, physicists assumed that this mutual attraction must be slowing the expansion down over time, the way the pull of the Earth slows down a ball thrown upward.

The big unanswered question was simply: by how much is the expansion decelerating, and will that deceleration ever reverse the expansion into a "Big Crunch"? If the universe is "closed", gravity would eventually halt the expansion and pull everything back together in a hot, violent collapse. If it is "open", matter would simply dilute into an ever colder, ever emptier space. Many physicists hoped for a "flat" universe in between, where the expansion gradually declines without ever quite reversing.

To measure this deceleration, scientists needed a reliable way to tell how far away and how fast very distant objects were moving, far beyond the range of earlier methods such as measuring Cepheid variable stars, pulsating stars whose brightness reveals their distance but which fade from view at the huge distances needed, many billions of light years away.

The two teams led by Perlmutter and by Schmidt and Riess set out, independently, to measure this deceleration using a special kind of exploding star as a "standard candle": an object whose true brightness is known, so that how dim it looks tells you how far away it is.

Before supernovae could be used this way, earlier teams had shown the method was workable in principle. A Danish-British collaboration made the first systematic search for distant type Ia supernovae in the late 1980s, and the Calán/Tololo Supernova Survey measured nearby examples carefully enough to show that, once corrected for how quickly their brightness faded, type Ia supernovae were uniform enough to serve as reliable cosmic yardsticks.

How do type Ia supernovae work as cosmic rulers?

A type Ia supernova is the explosion of a white dwarf, an old, extremely compact star about the mass of the Sun packed into a volume the size of the Earth.

White dwarfs form when a star has burned up its hydrogen and helium fuel and only carbon and oxygen remain at its core.

Many white dwarfs exist in binary systems, orbiting a companion star, and such a white dwarf can slowly pull gas away from its neighbour using its own strong gravity.

When the white dwarf's mass grows to about 1.4 solar masses, it becomes unstable, heats up internally, and undergoes a runaway nuclear fusion reaction that tears the star apart within seconds.

Because this explosion always happens at almost the same mass limit, the light released is remarkably uniform from one type Ia supernova to the next, which is exactly what makes them useful as standard candles.

For a few weeks, a single such supernova can shine as brightly as an entire galaxy of hundreds of billions of stars, before fading again over the following months.

Even so, the match between supernovae is not perfect. Brighter explosions were found to fade more slowly than fainter ones, a relation that let researchers recalibrate each supernova's light curve to a common standard and filter out the rare abnormal cases. Scientists also had to check that intergalactic dust was not dimming distant supernovae in a way that could be mistaken for genuine recession, and that supernovae from the early universe were not simply different in nature from nearby ones; both checks supported the conclusion.

Across the whole visible universe such supernovae are common, with roughly ten occurring every minute, yet within any single typical galaxy only one or two happen in a thousand years, which is why both teams had to search enormous numbers of galaxies at once.

1. Photograph a patch of sky just after the new moon, when the sky is darkest, using a sensitive digital imaging sensor (a CCD).
2. Photograph the same patch of sky again roughly three weeks later, before the next bright moonlight washes out faint light.
3. Compare the two images pixel by pixel to spot a new point of light that was not there before, a possible supernova.
4. Confirm the object is a type Ia supernova by studying how its brightness rises and falls over the following weeks (its light curve), and measure its redshift.
5. Use the known true brightness of type Ia supernovae and the observed faintness of the light reaching Earth to calculate the supernova's distance, and compare that with its redshift to see how the expansion rate has changed over time.

Draw two identical square grids representing the same patch of sky, one labelled "new moon" and a second labelled "three weeks later", with one extra bright dot appearing in the second grid, an arrow pointing to that dot labelled "candidate supernova".

Draw a small dense star pulling a stream of gas from a larger companion star, with a second panel showing the small star bursting apart into an expanding shell of debris once it reaches 1.4 times the Sun's mass.

What did the observations actually show?

Both teams expected the supernovae at great distances to look slightly brighter than a simple, non-decelerating expansion would predict, because gravity should have been slowing the expansion down earlier in cosmic history, which would push the supernovae closer to us than otherwise expected.

Instead, when all the measurements were combined, the distant supernovae turned out to be fainter than expected.

A fainter supernova at a given redshift means it is farther away than a steadily or decelerating expanding universe would place it.

Altogether the two teams found more than 50 distant supernovae whose light was weaker than predicted, a pattern that only makes sense if the expansion of the universe has been speeding up, not slowing down.

Compared with an unrealistic, empty universe expanding at a constant rate, the distant supernovae appeared roughly 10 to 15 per cent fainter than predicted. If the universe had instead been dominated entirely by matter, as most physicists expected, the supernovae should have looked about 25 per cent brighter than what was actually observed. The gap between these predictions and the real data pointed firmly towards an accelerating expansion.

Because the two independent teams, using different telescopes and techniques, reached the same extraordinary conclusion, each result reassured the other that the surprising finding was real rather than a shared mistake.

The accelerating expansion is attributed to an unknown form of energy that fills space, called dark energy.

Dark energy makes up roughly 73 per cent of the total energy content of the universe, with around 23 per cent being an unrelated unknown substance called dark matter, and only about 4 per cent being ordinary matter such as atoms.

What dark energy actually is remains unsolved; it remains perhaps the greatest unsolved enigma in physics today.

Later studies of even more distant supernovae, dating from when the universe was much younger and denser, suggest that gravity did slow the expansion down in that earlier era, and that the acceleration only began roughly five to six billion years ago, once matter had thinned out enough for the pushing effect of dark energy to take over.

How did the discovery unfold?

Why does this discovery matter?

The discovery forced scientists to accept that roughly 95 per cent of the universe, the combination of dark energy and dark matter, is made of something science does not yet understand, leaving only about 5 per cent as the ordinary matter that forms galaxies, stars and people.

It also rehabilitated Einstein's cosmological constant, a mathematical term he had added to his equations in 1917 to keep the universe static and later regretted, reportedly calling it his "greatest mistake".

This once-rejected idea now looks surprisingly useful for describing dark energy.

The finding reshaped predictions about the universe's ultimate fate. If the acceleration continues indefinitely, galaxies will keep flying apart ever faster into cold darkness, an ending that, in the words of Robert Frost's poem "Fire and Ice", is the universe ending "in ice" rather than in a fiery collapse.

The nature of dark energy, whether it is truly constant or changes over time (an idea physicists call quintessence), remains an open research question that current and future experiments are trying to answer.

The result has since been checked by entirely different methods. Precise measurements of the faint cosmic microwave background radiation left over from the Big Bang, and large surveys mapping how galaxies cluster together across space, have independently confirmed that the expansion of the universe really is accelerating, strengthening confidence in the original supernova evidence.

The discovery also sits alongside other scientific and technological advances that made it possible: the digital imaging sensors (CCDs) used to photograph faint supernovae earned their own Nobel Prize in Physics, and the earlier mapping of the expanding universe in the 1920s, and Einstein's General Theory of Relativity from 1915, supplied the theoretical framework within which the new, accelerating universe had to be understood.

How does this connect to what you study?

This discovery builds directly on ideas found in school physics: gravity, light and waves. The redshift used to track supernovae resembles the stretching of light waves you meet when studying the Doppler effect, just applied to galaxies instead of ambulance sirens or passing trains. When a source of light moves away, its wavelength stretches and shifts towards the red end of the spectrum, and the faster it recedes, the greater the shift, much as with sound, though at cosmological distances the stretching comes from the expansion of space itself rather than motion through it.

The idea that gravity should slow the expansion of the universe is a direct extension of Newton's law of gravitation, which states that every mass attracts every other mass, a rule learnt in basic mechanics. Here, that same attractive pull was expected to act on the entire cosmos, pulling galaxies back together and braking their outward flight, until the supernova data showed the opposite was happening.

Students studying the life cycle of stars will also recognise the white dwarf stage described here as the fate that lies ahead for stars like our own Sun once they exhaust their nuclear fuel of hydrogen and helium. The idea of a standard candle, an object of known true brightness used to measure distance, also connects to the inverse square law for light intensity taught in optics, where brightness falls away with the square of distance from the source.

Quick facts for exams

The Nobel Prize in Physics 2011 was announced on 4 October 2011 by the Royal Swedish Academy of Sciences. It was shared between Saul Perlmutter (one half), Brian P. Schmidt (one quarter) and Adam G. Riess (one quarter) "for the discovery of the accelerating expansion of the Universe through observations of distant supernovae".

All three laureates were born in the United States, though Schmidt also held Australian citizenship and worked in Australia at the time of the award.

The discovery, based on studying type Ia supernovae, showed that the universe's expansion is speeding up rather than slowing down, a result attributed to dark energy. The total prize amount that year was 10,000,000 Swedish kronor.

Note: Source. The prize facts in this note are from the Nobel Prize's official site, nobelprize.org.

Glossary

• Supernova — the extremely bright explosion of a dying star, briefly outshining an entire galaxy.
• Type Ia supernova — the explosion of a white dwarf that has grown to about 1.4 solar masses, used as a standard candle because its brightness is very consistent.
• White dwarf — a small, extremely dense star left behind after an ordinary star has used up its hydrogen and helium fuel.
• Standard candle — an astronomical object whose true brightness is known, so its distance can be worked out from how dim it appears.
• Redshift — the stretching of light's wavelength towards the red end of the spectrum as its source moves away from the observer.
• Hubble's law — the rule that a galaxy's distance from us is roughly proportional to the speed at which it is receding.
• Big Bang — the extremely dense, hot origin of the universe roughly 14 billion years ago, from which space has been expanding ever since.
• Dark energy — an unexplained form of energy filling space that appears to be causing the expansion of the universe to accelerate.
• Dark matter — an unknown form of matter, invisible and detected only through its gravitational pull, distinct from dark energy.
• Cosmological constant — a term Einstein added to his equations in 1917 to keep the universe static; later discarded, now linked to dark energy.
• Deceleration parameter — a quantity describing how quickly the expansion of the universe is slowing down (or, as found here, speeding up).
• Light curve — the graph of how a supernova's brightness rises and then fades over the weeks following its explosion.
• Supernova Cosmology Project — the research team, led by Saul Perlmutter, that searched for and studied distant type Ia supernovae from 1988 onward.
• High-z Supernova Search Team — the rival research team, led by Brian Schmidt with Adam Riess playing a crucial role, launched at the end of 1994.

Common errors and misconceptions

• Misconception: The prize was for discovering that the universe is expanding. Correct: The expansion of the universe was already known since the 1920s; this prize was for discovering that the expansion is accelerating.
• Misconception: Dark energy and dark matter are the same thing. Correct: They are different unexplained components; dark energy pushes space apart while dark matter pulls through gravity, and together they make up about 95 per cent of the universe.
• Misconception: A type Ia supernova can occur in any star. Correct: It specifically involves a white dwarf in a binary system that grows to about 1.4 solar masses before exploding.
• Misconception: Both research teams worked together from the start. Correct: The Supernova Cosmology Project and the High-z Supernova Search Team were independent, competing teams that happened to reach the same conclusion.
• Misconception: Einstein's cosmological constant was simply wrong and useless. Correct: Though Einstein regretted introducing it and called it his greatest mistake, the concept is now used to help describe dark energy.
• Misconception: The faintness of distant supernovae was expected from the start. Correct: Both teams initially expected the opposite, that distant supernovae would appear brighter, consistent with a decelerating universe.

Exam-style questions with model answers

Q1. For which discovery was the Nobel Prize in Physics 2011 awarded? [2 marks]

2. It was awarded for the discovery of the accelerating expansion of the universe.
3. This was found through observations of distant type Ia supernovae.

Q2. Name the three laureates of the Nobel Prize in Physics 2011 and state how the prize was shared among them. [2 marks]

2. The three laureates were Saul Perlmutter, Brian P. Schmidt and Adam G. Riess.
3. Saul Perlmutter received one half of the prize, while Brian P. Schmidt and Adam G. Riess each received one quarter.

Q3. Explain why type Ia supernovae are useful as "standard candles" in astronomy. [4 marks]

1. A type Ia supernova is the explosion of a white dwarf that has grown to about 1.4 solar masses by pulling gas from a companion star.
2. Because this explosion always happens at nearly the same mass, the amount of light released is remarkably consistent from one such supernova to another.
3. This means astronomers know, approximately, how bright the explosion truly is, its intrinsic luminosity.
4. By comparing this known true brightness to how faint the supernova appears from Earth, astronomers can calculate its distance, making it act like a reliable cosmic ruler or candle.

Q4. Describe the method the two research teams used to find and study distant supernovae. [4 marks]

1. The teams photographed the same patch of sky twice, once just after the new moon and again about three weeks later, using digital imaging sensors called CCDs.
2. Comparing the two images allowed them to spot new points of light that might be supernovae.
3. They confirmed a candidate was a type Ia supernova by studying its light curve, how its brightness rose and faded over weeks, and by measuring its redshift.
4. Follow-up observations on large telescopes measured the supernova's peak brightness, which, combined with its redshift, revealed how the expansion rate had changed over cosmic history.

Q5. Discuss what the 2011 Nobel Prize discovery revealed about the composition and likely fate of the universe. [6 marks]

1. Before 1998, most physicists expected that the mutual gravitational pull of all matter in the universe would gradually slow down its expansion, possibly even reversing it into a Big Crunch.
2. Instead, the two independent teams led by Perlmutter, and by Schmidt and Riess, found that distant type Ia supernovae appeared fainter than expected, meaning they were farther away than a decelerating or steady expansion would predict.
3. The conclusion was that the expansion of the universe is accelerating, driven by an unknown form of energy named dark energy, which makes up about 73 per cent of the universe's total energy content.
4. A further roughly 23 per cent is an unrelated unknown substance called dark matter, leaving only about 4 per cent as the ordinary matter that makes up stars, planets and people.
5. This revived interest in Einstein's cosmological constant, a term he had introduced in 1917 and later abandoned, which turns out to describe the effect of dark energy mathematically.
6. If the acceleration continues, in the words of the poet Robert Frost, the universe's fate is to end "in ice", with galaxies flying apart into cold, ever more diluted darkness, though the true nature of dark energy remains an open scientific question.

Q6. What role did Robert Kirshner play in supporting the discovery, and which team was it associated with? [2 marks]

2. Robert Kirshner was a renowned scientist who backed the project with his support.
3. It was the High-z Supernova Search Team, headed by Brian Schmidt with Adam Riess playing a crucial role.

Key takeaways

• The Nobel Prize in Physics 2011 was awarded for discovering that the universe's expansion is accelerating, not slowing down.
• Saul Perlmutter received one half of the prize; Brian P. Schmidt and Adam G. Riess each received one quarter.
• The discovery relied on observing type Ia supernovae, exploding white dwarfs that act as reliable standard candles.
• Two independent, competing research teams reached the same surprising conclusion, which strengthened confidence in the result.
• The accelerating expansion is attributed to dark energy, thought to make up about 73 per cent of the universe's total energy.
• The discovery revived interest in Einstein's cosmological constant, which he had earlier called his "greatest mistake".
• Only about 4 per cent of the universe is made of ordinary matter; the rest is dark energy and dark matter, both still poorly understood.
• The nature of dark energy, and whether it changes over time, remains an unanswered question in physics.

Test yourself

What did Perlmutter, Schmidt and Riess discover about the universe's expansion?

They discovered that the expansion of the universe is accelerating over time, rather than slowing down as gravity alone would predict.

What kind of star explosion did the laureates study?

They studied type Ia supernovae, explosions of white dwarf stars that have grown to about 1.4 solar masses.

Where was Saul Perlmutter working at the time of the award?

Saul Perlmutter was affiliated with the Lawrence Berkeley National Laboratory and the University of California, Berkeley, both in California, USA.

Where was Brian P. Schmidt affiliated at the time of the award?

Brian P. Schmidt was affiliated with the Australian National University in Weston Creek, Australia.

What is dark energy believed to do?

Dark energy is an unexplained form of energy filling space that is believed to be pushing the universe apart, causing its expansion to accelerate.

Why did Einstein call the cosmological constant his "greatest mistake"?

He introduced it in 1917 to keep the universe static, but later evidence showed the universe is expanding, making the idea seem unnecessary at the time.

What fraction of the prize did Adam G. Riess receive?

Adam G. Riess received one quarter of the Nobel Prize in Physics 2011.

How did the two research teams confirm their surprising result was reliable?

Two independent teams, using different telescopes and methods, both found the same accelerating expansion, reassuring each other that the result was genuine.