Over the past thirty years, there has been growing evidence that the expansion rate of the universe has been increasing with time. This result has shocked physics: the equivalent of throwing a ball upward and finding that gravity makes it accelerate away from the point of release. If general relativity is correct, this cosmic acceleration implies that most of the energy in the universe is in the form of dark energy: energy associated with empty space. In the late 1990s, measurements of the relationship between the distance and the redshift to supernova provided the strongest evidence for this strange phenomenon, opines Ashoka, in his erudite weekly column, exclusively for Different Truths.
Our basic model of cosmology rests on Einstein’s nearly century-old theory of general relativity. General relativity consists of two simple ideas: matter tells space how to curve and space tells matter how to move. On the scale of our solar system, the mass of the sun curves space around it, and our Earth moves on a nearly circular orbit in this curved space. On the cosmological scales, the distribution of matter is nearly uniform. General relativity implies that a nearly uniform universe must be either expanding or contracting. Since Edwin Hubble’s observations in the 1920s, we have known that our universe is expanding. Because light travels at a finite speed, when we look out in space, we look back in time. We see the sun as it was eight minutes ago, and we see nearby stars as they were five, ten, or a hundred years ago. It takes light approximately 2.5 million years to travel from the Andromeda galaxy to our eyes, so when we stare at our nearest major neighbour with a telescope, we observe Andromeda as it was back before the dawn of man. The farther out we look, the farther back we look in time. When the Hubble Space Telescope observes a distant galaxy, it sees the galaxy as it was perhaps 12 billion years ago. Our observations of the cosmic microwave background involve the oldest light, photons that formed only one year after the Big Bang and last interacted with atoms just four hundred thou sand years after the Big Bang. This light travels for 13.7 billion years before reaching us, and it brings us our universe’s baby picture.
As the universe expands, the distance between galaxies grows. Today, it takes light roughly fifty million years to travel to the Virgo cluster. Eight billion years ago, the distance between objects was a factor of two smaller, so light would have taken only twenty-five million years to travel from our galaxy to the Virgo cluster. As we go farther back in time, objects get closer and closer together. Thus, the early universe was much denser than today’s universe.
General relativity relates the expansion rate of the universe to the density and geometry of the universe. If the energy in expansion exceeds the self-gravity of the matter in the universe, then the universe is negatively curved and will expand forever, growing increasingly cold and empty. On the other hand, if the energy in expansion is less than the self-gravity of the universe’s matter, the expansion will slow down and reverse, and the universe will collapse in a future big crunch. As Robert Frost prophesied, the universe will end in either fire or ice.
Over the past thirty years, there has been growing evidence that the expansion rate of the universe has been increasing with time. This result has shocked physics: the equivalent of throwing a ball upward and finding that gravity makes it accelerate away from the point of release. If general relativity is correct, this cosmic acceleration implies that most of the energy in the universe is in the form of dark energy: energy associated with empty space. In the late 1990s, measurements of the relationship between the distance and the redshift to supernova – powerful explosions of nearly uniform brightness that can be seen at very large distances – provided the strongest evidence for this strange phenomenon.
Dark energy is different from “dark matter.” Ever since Fritz Zwicky’s work in the 1930s, astronomers have suspected that stars are not the dominant form of matter in galaxies. By the 1970s, astronomers had assembled several independent lines of argument all implying that dark matter was neither gas nor stars. Dark matter appears to be some new type of particle that has not yet been found in our particle accelerators. Dark energy is even stranger: it does not cluster in galaxies, nor does it seem to respond to any of the natural forces. Dark energy affects the universe only through changing its expansion rate.
During the first three hundred thousand years of cosmic history, almost all of the atoms in the universe were ionized into a plasma of electrons, protons, and helium ions. The cosmic background photons were frequently colliding with the electrons in this primordial plasma, so both atomic matter and photons were coupled together in a single fluid. As the universe cooled, the protons and helium ions were able to combine with electrons and form neutral hydrogen and helium atoms. By four hundred thousand years after the Big Bang, most of the electrons had combined with ions, and the universe was mostly neutral. Since the cosmic background photons do not interact with these neutral gases, they were able to propagate freely. The photons that we observe when we look at the cosmic background radiation last interacted with atoms at this very early time. Thus, when we observe the background radiation, we are directly measuring physical conditions at this early moment in the history of the universe.
In 1964, astronomers Arno Penzias and Robert Wilson detected the cosmic background radiation with their horn antenna at Bell Laboratories. Twenty-five years later, the Cosmic Background Explorer (COBE) satellite found that this nearly uniform microwave radiation had exactly the spectral properties predicted by the hot Big Bang model. This measurement of the cosmic background is one of the foundational observations for the hot Big Bang model.
Four hundred thousand years after the Big Bang, the early universe was a simple place. Electron, protons, and photons were bound together into a warm 3000K plasma. Tiny variations in the density of the universe generated sound waves in this plasma. The distance that the sound waves could move in four hundred thousand years imparted a characteristic scale on the universe, and the self-gravity of the plasma and the dark matter determined the height of the peaks. Because these variations were small, cosmologists can use linear theory to accurately predict the relationship between the statistical properties of the fluctuations and the conditions in the early universe.
Cosmologists quantify the properties of these fluctuations by measuring their statistical properties. These fluctuations have very simple statistical properties: they are spatially homogenous and can be characterised almost entirely through measurements of the point correlation function of the data or, equivalently, the angular power spectrum.
Despite the remarkable success of the Big Bang model in describing the evolution of the universe and the growth of fluctuations, the model is incomplete. Intriguingly, inflation – currently the most popular extension of the Big Bang model – not only addresses these problems but also makes predictions that we can test with our cosmic microwave background observations.
The near, but not perfect, uniformity of the early universe is another puzzle in Big Bang cosmology. Different regions of space that were never in causal contact in the Big Bang model have nearly identical densities. The solution to the problem must explain this near, but not perfect, equality; for if the early universe were perfectly uniform, it would still be uniform today.
Over the past few decades, cosmologists have developed a remarkably successful cosmology model that fits a host of astronomical observations. However, while this model addresses many of the previously unsolved questions of cosmology, it raises a new set of questions:
• Why is the universe accelerating? What is the nature of dark energy? Are we seeing the breakdown of gravity on cosmological scales?
• What is the nature of dark matter?
• Did the early universe also undergo a period of acceleration? If so, what was the mechanism that drove this early period of inflation?
There are many different routes toward addressing these questions. Developments in string theory and other attempts at unifying physics may provide new insights into the nature of space and time. Future observations of the geometry of the universe, the statistics of the primordial fluctuations, as well as the gravitational waves predicted in the inflationary scenario will either confirm this basic model or challenge its underlying tenets. Searches for dark matter could reveal the nature of these unknown particles. Astronomical measurements of distances or the growth rate of structure will test the notion that vacuum energy drives cosmic acceleration. Of course, if we can address any of these questions, the answers will likely point toward even deeper and more profound mysteries.
©Ashoka Jahnavi Prasad
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