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The Royal Swedish Academy of Sciences awarded the 2011 Nobel Prize in Physics to three astronomers for discovering the accelerating expansion of our universe. This was one of the most dramatic and surprising discoveries of the 20th century, found independently by two groups who set out to measure the universe’s deceleration — the exact opposite of what they discovered.
This major discovery seems to settle a long-standing cosmic question: “Will the universe end infire or ice?” By fire, we mean the universe eventually re-collapsing to a tiny point with an immense temperature. By ice, we mean the universe continuing to expand forever and becoming ever colder. If the observed expansion rate continues to accelerate, the universe will end in ice. Either alternative would play out over hundreds of billions of years. There’s no rush to stock up on arctic clothing.
It’s gratifying that the Nobel committee has begun recognizing the contribution of observational astronomy. None of the first 76 Nobel Prizes in Physics went to astronomers, not even to Edwin Hubble or Henrietta Leavitt. Even now, the total number of Nobel Prizes awarded to astronomers can be counted on one hand, with fingers to spare.
Saul Perlmutter of the Lawrence Berkeley National Laboratory led one group that began its work in 1988. Brian Schmidt, born and educated in the US and now at the Mount Stromlo Observatory in Australia, launched a second group in 1994; their data was mainly analyzed by Adam Riess at UC Berkeley.
The Nobel committee awarded half the prize money to Perlmutter (about $700,000) and one-quarter each to Schmidt and Riess (about $350,000 each). (The prize money is exempt from income tax.) About 20 astronomers participated in each group, but Nobel Prize rules mandate no more than three awardees.
Both groups searched for exploding stars of a specific kind — type 1a supernova—and measured their distance and velocity relative to Earth. This allowed them to measure the expansion rate of the universe at various times during the 13.75 billion years since the Big Bang. Contrary to virtually every scientist’s expectations, they found that the expansion rate is now increasing — the expansion is accelerating.
The primary scientific data are shown below (I reorganized and relabeled it to improve clarity):
In the above chart, the expansion rate of the universe (how rapidly the universe is growing) is shown on the vertical axis — faster expansion is at the top and slower expansion is at the bottom. Time is shown on the horizontal axis — today is all the way to the left, and 11 billion years ago is to the right — this is called “Lookback Time.” The data point with the highest measured expansion rate is at the upper right, between 10 and 11 billion years ago. The dots indicate the measured values and the vertical bars denote the measurement uncertainties. Because supernovae are very far away and even our best telescopes collect only a modest amount of their light, measurement precision is limited. Properly determining the uncertainty in measurements is an essential part of science.
Beginning at the right, the chart shows what everyone expected — the expansion rate was initially very high and gradually decreased from 11 billion to 6 billion years ago. This makes sense. As galaxies fly apart from the Big Bang, each one pulls on all the others with the force of gravity from the mass of its billions of stars. As every galaxy pulls on every other one, the expansion rate will slow down. The dashed curve shows the overall trend of the data. It clearly decreases from 11 billion years ago to 6 billion years (“6G YRS”) ago.
The great surprise is what has happened since 6 billion years ago. The expansion rate reached a minimum at that time, and has increased ever more rapidly to the present (“NOW”) — the expansion rate is accelerating.
For the expansion rate to increase, something must now be pushing the universe apart more forcefully than the gravity of all the galaxies pulling it together — there must now be a dominating source of “negative” gravity. Scientists have named this mysterious phenomenon “dark energy.”
We’ll explore dark energy later. Let’s talk first about how Perlmutter, Schmidt, Riess, and their colleagues made their measurements.
Type 1a supernovae are a very special class of exploding stars. A beautiful example taken by Hubble is shown below — supernova SN1994D on the outskirts of spiral galaxy NGC 4526.
Three characteristics make them particularly useful for this project. Firstly, since they are extremely bright, we can see them at even great distances and therefore even from the distant past — a supernova that we see now and that exploded 8 billion light-years away actually exploded 8 billion years ago and its light is just reaching us now (one light-year is the distance light travels in one year). Thus, supernovae tell us what the universe was like long ago. Secondly, because type 1a supernovae always occur in the same way, they all explode with the same amount of energy, emitting the same amount of light. This allows us to measure how far away they are/were because light intensity decreases in a well-known way as it expands throughout space. Astronomers describe this by saying that type 1a supernovae are outstanding “standard candles” (a term that reflects how old the science of astronomy is). And thirdly, type 1a supernovae have distinctive signatures that allow astronomers to reliably distinguish them from other cosmic explosions.
The origin of type 1a supernovae is quite special and very interesting. Our Sun is a bit unusual in its solitude — it has no companion stars — enabling its planets, including Earth, to have stable orbits, vital for life. However, most stars have nearby partners, and two-star systems, called “binaries”, are common. Stellar partners can have quite different masses and heavier stars evolve more rapidly. In a binary system with two ordinary stars (not super-massive), the heavier star will reach the end of its life first, becoming a red giant and then a white dwarf. White dwarfs are collapsed stellar cores about the size of Earth with masses up to 140% of our Sun’s. Their surface gravity can be one million times greater than Earth’s. When its lighter companion becomes a red giant, a white dwarf can capture gas from the red giant’s loosely held outer layers. The white dwarf grows more massive at its partner’s expense, in a form of stellar cannibalism. When the white dwarf grows to 140% of our Sun’s mass, a cataclysmic explosion utterly destroys the star. These explosions all have the same origin and all explode at the same mass, making type 1a supernovae superb “standard candles.”
The new Nobel Laureates used smaller telescopes, which are more readily available, to repeatedly image specific areas of the sky. Supernovae stood out as extremely bright spots that appeared suddenly. They then used larger and more expensive telescopes, including Hubble and Keck, to make higher precision measurements of the supernovae’s intensity, spectrum, and redshift. Type 1a’s are distinguished by their spectrum and by how their intensity varies over time.
Their peak intensity observed on Earth determines how far away the supernovae are, and their redshift helps determine the universe’s expansion rate at the time of the explosion.
That both groups independently came to the same shocking conclusion made their results much more convincing. Anyone can occasionally make a mistake, but it is far less likely that two groups make exactly the same mistake.
So what does it all mean? What is this mysterious dark energy?
While we have ideas and some important observations, the bottom line is that understanding dark energy is cutting-edge science, or perhaps better described as bleeding-edge science. Analysis of the cosmic microwave background radiation (CMB) shows that 73% of all the energy in our observable universe is now in the form of dark energy — it’s the vast majority of what’s out there.
Our best idea comes from a combination of Quantum Mechanics and Einstein’s Theory of General Relativity. Quantum Mechanics says that empty space is never truly empty. Due to Heisenberg’s Uncertainty Principle, neither man nor nature can detect very small energy changes that persist for very short time periods — the smaller the energy change, the longer it can persist. And as Murray Gell-Mann, one of my Caltech professors, famously said, “Whatever isn’t forbidden is mandatory.” Thus, “virtual” particles can spontaneously appear anywhere, anytime, and quickly disappear back into nothingness. They borrow energy from the Bank Of Heisenberg on a very short-term loan.
While these virtual particles “exist”, they must have positive energy, however miniscule and fleeting. Since they occur everywhere and always, they give even “empty” space some amount of positive energy. And to conserve energy, virtual particles must give space negative pressure. (The gas law, P dV = –dE, says that as space expands, dV>0, and the amount of dark energy increases, dE>0, the pressure, P, must be negative). General Relativity says that an entity with energy and pressure exerts a gravitational attraction proportional to E+3P, which in this case is negative. This means virtual particles give space negative gravity, gravity that pushes things apart rather than pulling things together as normal gravity does. Several effects of virtual particles are precisely confirmed by lab experiments.
If all this is indeed true, Perlmutter, Schmidt, and Riess have shown us a brave new world. When the universe was young and much smaller, there was less space and less dark energy. The mutual gravitational attraction of the galaxies dominated, and the expansion gradually decreased. As the universe became larger, there was more space and thus more dark energy. Also, the mutual attraction of the galaxies diminished as their separations expanded. At about 6 billion years ago, the balance of power shifted; the repulsive gravity of dark energy began to exceed the attractive gravity of matter (including dark matter), causing the expansion rate to increase. As the universe continues growing, dark energy becomes ever more dominant, and the universe will accelerate ever faster.
That is the large-scale picture for the universe. Galaxy clusters will move apart from one another ever faster. But the clusters themselves will not expand. On this smaller scale, the mutual gravity of all the massive objects in a cluster still dominates its dark energy. If the cluster doesn’t expand, the amount of its space and dark energy will not increase, and thus the balance will not shift. Smaller structures will also not expand — our galaxy will not expand, Earth will not expand, and we will not expand (except at Thanksgiving).
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